Perovskite Solar Cell Technology

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Introduction To The History Of Perovskite Solar Technology

If you are interested in clean energy or solar energy more specifically, you have probably heard of Perovskite Solar Cell (PSC) technology. Whether you are just learning about them, or are an expert looking for ways to improve test quality and timeline, this article should have you covered. We’ll discuss what perovskites are, how they can be used in solar cells, some considerations needed to build a solar cell with perovskites, how they can fail, and how to test and validate them so they won’t.

Where Did Perovskite Solar Cell Technologies Come From?

The first perovskite material discovered was the compound Copper (II) Titanate (CuTiO3), also known as the mineral Perovskite. It was named by its discoverer Gustav Rose in 1837, in honour of noted Russian mineralogist Lev Aleksevich von Perovski

Later, in 1892, the first synthesis of a cesium lead halide perovskite material in history was successfully performed. This is important because it is the basis for the chemical composition of a modern perovskite solar cell (PSC). However, it wasn’t until 1926, almost 90 years later, that the perovskite crystal structure was theoretically described. It took another 19 years for that theory to be confirmed via X-ray crystallographic investigation of a similar material, Barium Titanate, in 1945

For decades after 1957, perovskite materials found their way into more electric and electrochemical devices and even perovskite-structured superconducting materials. But the next step toward our modern focus in the field of perovskites was not until 1978 with the synthesis of the first organic-inorganic lead halide perovskite material. This new semiconductive material exhibited several interesting optical and electronic properties such as a very high absorption coefficient and high relative permittivity. 

However, the use of an organic lead-halide perovskite as the central component of a solar cell would not happen until 2009 with an experiment meant to attempt to replace low voltage ruthenium-based inks in dye-sensitized solar cells with higher voltage perovskite-based ink.

Perovskite materials are flexible in purpose and ability, able to perform varied tasks depending on their formulation. Below, we’ll discuss the reasons for that flexibility, and many of the ensuing outcomes.

What Is The Structure Of Perovskites And What Makes It Special?

What about the perovskite material made them worthy of so much scientific investigation and application? Compared to other, more traditional solar cell technologies such as silicon or cadmium telluride, perovskites have incredible flexibility in the materials that comprise their structure. Let’s first discuss the nature and reasons for this flexibility before we get into the important implications this has for perovskite behaviour.

Composition And Common Materials Of Perovskite Solar Cells

A perovskite is a three-component system consisting of a central A atom/molecule surrounded by 8 ligands. Each ligand is a B atom surrounded by 6 X atoms in an octahedral or square pyramidal geometry. Due to how atoms are shared between unit cells of a material, this results in a generalized chemical formula of ABX3 for a traditional perovskite material such as the original mineral CuTiO3 (an oxide perovskite).

However, oxide perovskites are not the type of material currently used in photovoltaic (PV) solar cells. Instead, perovskite solar cells primarily use organic-inorganic halides with the most common being methylammonium lead iodide (MAPbI3). However, just because it is the most common does not mean it is the only viable composition. As we’ll discuss later, material researchers continue to find new and interesting ways of making perovskites for the best outcomes.

Below, you can find a table of known alternative component atoms/molecules for their respective positions in the ABX3 structure of perovskite.

A position compounds

B position element

X position element

Methyl Ammonium

Lead

Iodine

Formamidinium

Tin

Bromine

Cesium

Germanium

Chlorine

Rubidium

Bismuth

 

Guanidinium

Antimony

 

Butyl Ammonium (n, t, or iso)

   

Ethyl Ammonium

   

Benzylammonium

   

Acetamidinium

   

Imidazolium

   

Dimethyl Ammonium

   

n-Propyl Ammonium

   

 Reference: https://www.ossila.com/en-ca/collections/perovskite-precursor-materials 

What Makes The ABX3 Perovskite Structure So Special?

With the number of possible components shown above, you can synthesize over 200 different perovskite crystals through varying component combinations without considering fractional mixtures or potential dopants. What’s important to consider about these component possibilities is that they are not all identical in size or structure and that can have interesting effects on the perovskites themselves due to how crystal lattice stresses can affect and alter electronic and even chemical properties.

As we described above, the basic form of perovskite is a cubic crystal comprised of a central component atom/molecule A adjacent to 8 ligands of B and X components. Depending on what those components are, and their relative sizes to each other, it is possible to stress the crystal out of its normal shape into several other alternate perovskite structures.

Caption: Basic Perovskite ABX3 molecular structure
Basic Perovskite ABX3 molecular structure (source: https://www.princeton.edu/~cavalab/tutorials/public/structures/perovskites)

Stressing a crystal by altering a crystal’s components is a well-accepted method of modifying both physical and electronic properties. One example is the alteration or substitution of crystal components to obtain different piezoelectric behaviour.

With so many different possible components of such varying sizes and molecular structures, what novel features can be built into perovskite solar cells?

Changing the X component among the three common halides without changing the other components can cause rotation of the octahedral structures, enforcing a lattice change from cubic to tetragonal by extending one axis of the perovskite crystal compared to the other two as a method to reduce lattice strain, or vice versa depending on substitution. There is a known correlation between crystal structure and semiconductor bandgap, so being able to influence that with a simple chemical substitution is very useful.

Altering the B component can also create what is known as a double perovskite structure if B is fractionally replaced, which allows for even more freedom and possibility in how component replacement can impact the desired structure while still providing useful or novel properties such as removing lead from the structure or producing ferroelectric effects.

A(B1-nCn)X3 Double Perovskite crystal structure
A(B1-nCn)X3 Double Perovskite crystal structure (source: https://www.princeton.edu/~cavalab/tutorials/public/structures/perovskites)

Changing the A component can also shift a perovskite’s structure in fairly significant ways depending on the size of the molecule. If the A component’s diameter is too large compared to the surrounding BX ligands, the larger A components apply stress on the crystal to make room for themselves. This can result in a crystallographic shift from the common 3-dimensional perovskite ABX3 structure to what is known as the Ruddleson-Popper or 2D perovskite structure. This structure consists of 2-dimensional layers of perovskite BX ligands separated by a shifted pattern of A component materials.

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Molecular structure of A2BX4 and A3B2X7 variants of Ruddleson-Popper perovskite structures
Molecular structure of A2BX4 and A3B2X7 variants of Ruddleson-Popper perovskite structures (source: https://www.princeton.edu/~cavalab/tutorials/public/structures/perovskites)

The stress from varying a perovskite’s A component can also alter the chemical composition from ABX3 to A2BX4 or even the A3B2X7 ratio. This occurs by the internal stress of the perovskite cubic crystal being too large to allow the BX ligands to connect as normal and allow other A components to insert themselves into new positions, altering the crystal make-up so that these 2D structures are the most energetically favourable.

Shifting the dimensionality of the crystal can directly impact the electronic properties. Due to higher internal stress on the crystal, a 2D structure can act as a barrier to the formation of defects in the crystal, which is very useful for preventing a common source of reduced power conversion efficiency (PCE) and slowing down the degradation of the cell. With so many options for components, and an understanding of how replacing, substituting, or mixing can alter the structure and thus the electronic landscape of the crystal structure, perovskites can offer a flexible platform for tuning solar cell properties for different applications.

In crystals, defects form and move based on the energy of the bonds between the components. In a single crystal, defect movement is easier due to a crystal being the lowest local energy state for the material meaning it takes less energy for the defect to move through it compared to a more disordered and thus higher energy structure. Perovskites’ ability to be built from mixed and matched components makes it possible to build barriers within the crystal out of the different structures and components so that defects have a harder time forming and can be controlled in their movements according to a researcher’s desires.

Understanding the atomic construction of perovskite crystals is essential for producing high-quality solar cells, however, it is also necessary to understand how these materials can be reliably made at scale if they are to be successfully deployed in solar modules. We’ll discuss perovskite fabrication techniques in the sections below.

How Are Perovskite Solar Cells Made?

To create a perovskite you need to form the final structure from its component precursors. When the general ABX3 perovskite structure is the desired outcome, researchers and fabricators mix AX and BX2 precursors on a surface via one of the following methods for perovskite crystallization to occur.

These methods range from various solution processing, vapour phase or electrodeposition methods, illustrated in the figure below.

Spin-Coating: Benefits And Drawbacks For PSCs

Spin-coating is currently a standard method of perovskite fabrication for researchers. It is a rapid and low-cost fabrication method, allowing rapid iteration and production of small samples in labs.

By rapidly spinning a substrate that has a pool of mixed precursor inks (solutions made of the perovskite precursors AX and BX2 dissolved in volatile solvents) poured on it, it is possible to create a layer of a defined thickness by altering the speed of rotation relative to the viscosity and surface tension of the precursor ink. After spinning, the coated substrate is then heated to finalize the fabrication of the perovskite layer by crystallizing the precursors and driving off the remaining solvent after the crystallization. However, since the quality of the final crystal is governed by the concentration of precursors within the solvent which determines the rate of transport of the perovskite precursors from the liquid phase to the solid phase, if you heat everything too quickly you will drive off too much solvent, reducing the mobility of the precursors, ending up with poor quality crystals.

Spin coating a perovskite precursor onto a defined substrate
Spin coating a perovskite precursor onto a defined substrate

Problems with spin-coating arise when trying to scale up from small samples. Since the thickness of the final coating is determined by balancing the centripetal force of the rotation against the viscosity and surface tension of the perovskite precursor ink, increasing the size of the substrate being spun causes the force applied to the substrate to increase the farther the ink is from the center. This position-varying force on the substrate can lead to a coating that has a non-uniform thickness, because areas of low force will leave a thicker coating on the substrate, and areas of high force will leave a thinner coating or even break the coating up if the centripetal forces exceed the surface tension of the ink.

The other problem is that spin-coating tends to be a very messy process. By spinning the substrate to remove any excess material aside from a desired thin layer, by necessity, you end up wasting a large portion of your precursor ink, potentially up to 50 times more than other less vigorous processes (which we’ll discuss below). 

Roll-To-Roll Fabrication: The Future Of Perovskite Manufacturing?

A scalable alternative to spin-coating for perovskite solar cell manufacturing is roll-to-roll processing which, while still operating in a liquid phase (thus falling under the umbrella of solution processing), has garnered noticeable interest in recent years.

Roll-to-roll processing is a catch-all term for processes that operate continuously via transport on one or more belted surfaces. This type of processing is desired so that the final product is a continuous sheet rather than a line of individual objects, and combines extremely well with the liquid phase solution processing methods used with perovskite solar cells due to the simplicity of transporting and applying liquid phase precursors to any substrate.

“Doctor” Blade deposition is the simplest method of applying thin films to a substrate. You position a blade, or another similar barrier, at a specific desired distance above the substrate surface and apply a pool of the desired material precursor in front of the blade such that the motion of the roll surface pulls the pool into the blade which then scrapes away all but the desired film thickness.

Compared to spin-coating, this is a mechanically simple process but it offers new considerations for stable and homogeneous crystal production. Lacking the same strong forces being applied to the precursor ink of the perovskite solar cell that spin-coating has, inks that are coated via blade deposition tend to retain higher solvent fractions at the heating stage. This is irrespective of the height of the blade above the moving substrate or the speed of the substrate as the blade is only meant to allow only a specific amount of material through to give the desired thickness of the film. This entails different crystallization rates and makes it more difficult to retain control over the formation of grain boundaries (the disordered interfaces within the material between two misaligned zones of single crystal material) and other assorted defects (discussed in more detail later).

Slot die coating is a variation of blade deposition where the precursor ink is applied directly to the substrate by pumping it through channels in a squared-off die nozzle placed directly perpendicular to the substrate. Coating thickness is determined not only by the spacing between the die and the substrate but also by the viscosity of the ink and by the speed of the substrate pulling the ink. This process has the advantage of being more temperature controllable as the die can be heated or cooled to allow for close-to-optimal pumping of the precursor ink, having a more controllable flow rate, and limiting the exposure of the perovskite precursor ink to the atmosphere and any oxygen prior to deposition.

Slot-die-based extrusion of perovskite precursor onto a moving roll-to-roll manufacturing substrate.
Slot-die-based extrusion of perovskite precursor onto a moving roll-to-roll manufacturing substrate.

Because of the high degree of temperature control and lower chance of contamination prior to application in slot die coating, PSCs fabricated with this method have succeeded in joining spin-coated cells in achieving greater than 20% efficiency. However, slot die coating shares the downsides of blade deposition compared to spin-coating in that it retains a much larger fraction of solvent in the ink post-deposition due to the lack of high forces to preferentially remove any excess low-viscosity solvent, resulting in a more complicated and defect-prone crystallization process.

Electrodeposition: New Contender That Could Change The Game For Perovskite Manufacturing

More recently, an effort to provide the benefits of the low cost and rapid rate of solution processing while maintaining a more controllable surface structure has emerged through the use of electrodeposition methods.

Electrodeposition is a very mature manufacturing technology primarily used in the creation of high-purity metals. This deposition method first places electrodes into an electrolyte solution made up of the ionic components desired to be turned into a solid form. Applying an electrical bias across the electrodes forces oxidation-reduction reactions that convert the ions into solids that coat the electrodes. The rate of deposition and quality of the deposited crystals are determined by the voltage and current of the system. The application of materials onto the electrode is sometimes called plating.

A basic showcase of how electrodeposition works for the creation of perovskite films
A basic showcase of how electrodeposition works for the creation of perovskite films

While an electrodeposition system can significantly outperform a vapour deposition system in terms of throughput, it does have its limitations to overcome to achieve high-efficiency perovskites.

Firstly, it is challenging to prevent heterogeneous nucleation upon the substrate electrode, meaning that the perovskite structures are likely to be multi-crystalline and suffer from the known reductions in efficiency that a high density of charge carrier traps (such as grain boundaries) engenders.

Also, since electrodeposition has to take place within a conductive electrolyte that contains ionic forms of the materials you are trying to plate onto the desired substrate, it is difficult to have any success with non-polar organic materials that cannot be ionized and dissolved in solution. Whether this aspect of electrodeposition precludes the use of organic material completely or not is currently unknown but it does give preference to the electrodeposition of wholly inorganic perovskites such as CsPbI3 for now. (For example, two of the most commonly used organic molecules in the A position of an ABX perovskite are the organic molecules methylammonium and formamidinium. Researchers would be very interested in being able to electro-deposit these.)

Small copper electrodeposition facility, imagine this for perovskite solar panels
Small copper electrodeposition facility, imagine this for perovskite solar panels (source: https://www.everestplating.com/)

How To Validate The Success Or Failure Of The Construction Of Perovskite Solar Cells And Panels

So how do researchers know when they have made a viable solar cell?

One common first step is to head in the direction of investigating the solar cell’s electronic properties, testing for the open circuit voltage (Voc) and short circuit current (Isc) to determine the fill factor (FF) and compare it against the predictions for that batch. How the resulting IV curve deviates from ideal behaviour can give insight into how the device might be exhibiting unexpected defects. Depressed max values for Isc or Voc, rapid decays of I values near the max power point (MPP), or even “steps” in the gathered data can all indicate different types of defects or faults during construction.

The crystallinity of the cell is also important to investigate as determining the average size of the crystal grains will give an estimate of the number of grain boundaries and similar defects that exist that could interfere with the motion of charges before they reach the transport layers. To quantify grain boundaries, standard optical and electron microscopy (with associated sample preparation such as cleaving and gold-coating) are tried and true methods but are ultimately slow and labour-intensive. Some research has suggested that X-Ray Diffraction can also be used to determine crystal grain size.

Showcase of visual differences in surface appearance of perovskite materials with average grain sizes ranging from 200 to 390 nm
Showcase of visual differences in surface appearance of perovskite materials with average grain sizes ranging from 200 to 390 nm (source: https://www.researchgate.net/publication/350516490_2D_materials_for_conducting_holes_from_grain_boundaries_in_perovskite_solar_cells )

To quickly determine if there have been faults in production, such as delaminations between layers, for example, you need a process that can create a reliable signal (instead of relying on a trained observer) such as photoluminescence imaging.

But these types of investigations are more generally confined to laboratory settings and early prototype evaluations. When attempting to determine if a specific solar technology is ready for commercialization, the requirements for testing are significantly more involved. The current international regulatory standard for terrestrial solar devices is IEC 61215-2:2021, and it covers all of the requirements for a panel to be considered acceptable as a commercial device.

This standard requires testing and passing of a solar panel through several sets of tests to validate its behaviour and ability to operate even under non-optimal conditions. These tests and their conditions are collected in Table 3 from the IEC standard.

Table 3 from IEC 61215-1:2021 detailing all Module Quality Tests (MQTs) and their parameters
Table 3 from IEC 61215-1:2021 detailing all Module Quality Tests (MQTs) and their parameters IEC. (2021). IEC 61215-1: TERRESTRIAL PHOTOVOLTAIC (PV) MODULES – DESIGN QUALIFICATION AND TYPE APPROVAL. In 61010-1 © Iec:2001.

Suffice it to say, commercializing a solar panel, let alone an entirely new technology such as perovskites, is not a simple endeavour. However, there are a number of tools that can aid in breaking down the testing into manageable, controllable pieces, providing the right level of feedback to facilitate the path to commercialization.

What Are The Limits Of Efficiency For Perovskite Solar Cells?

Once a cell or panel is validated, there is a large array of information available to reference. However, regardless of the different considerations of what is important to different interested parties, one data point spoken of more frequently than any other property of solar technology is Power Conversion Efficiency (PCE). In fact, PCE is spoken of so frequently that it is commonly just called efficiency.

Given that with higher efficiency, a solar panel can generate more power from the same area and create less heat at the same time, it is perfectly understandable that the focus of the most publicly discussed solar research is to find out how to improve the efficiency of any solar cell technology. It’s worth noting,, that, like every other produced good on the international market, additional commercial, bureaucratic and political factors can  impact solar cell feasibility and deployment cost. Nevertheless, efficiency remains of key interest.

Perovskites have captured the imagination of both researchers and the public since their debut due to their rapid growth in efficiency. While the rapid rise of perovskite solar cell efficiency to near parity with crystalline silicon is astounding, perovskites still have to contend with the same limitations as previous generation solar technologies such as dye-sensitized solar cells (DSSCs) or CdTe thin-films.

The Shockley-Queisser limit, for example, is still very much in force for perovskites. While perovskites have some advantages over silicon, such as their variable chemistry that allows for the perovskite band gap to be tuned toward a slightly higher single-junction efficiency, in practice this increase is marginal. In the grand scheme, the difference between a maximum efficiency limit of 34% (for perovskites) and 32% (for silicon) in single-junction cells is not that much.

The laws that govern semiconductor-based solar photovoltaic (PV) technologies are the same for perovskites as they are for all other types of solar cells despite their structural and chemical differences from traditional solar technologies. This is good for us as it means the same tricks that solar PV materials like indium gallium arsenide used to create viable multijunction cells to get around the Shockley-Queisser Limit work just as well for perovskite devices.

How Do Perovskite Materials Make Electricity In A Solar Cell?

Fundamentally all solar cells make electricity in the same way. An incoming photon knocks an electron far enough out of place that it doesn’t immediately fall back in, generating an electron-hole pair. The electron and hole then travel through an electrical circuit to power your fridge, air conditioner, or any other connected electronic device. Alternatively, the energy of the electron-hole pair can be stored in a battery, as shown below.

 

Visual representation of how Charge Transport Layers assist perovskite solar cells in producing electrical currents during operation.
Visual representation of how Charge Transport Layers assist perovskite solar cells in producing electrical currents during operation. (source: https://www.youtube.com/watch?v=TkFf8kRwOo8 )

Granted, this is a dramatically oversimplified version of the physics of the photovoltaic effect. However, with the basics firmly in hand, it is easier to explore the important additional details, such as how the band gap of the material defines the amount of energy it takes for that photon to displace the electron. Or how the construction of a thin-film cell bracketed by its electron transport layer (ETL) and hole transport layer (HTL) on either face of the semiconducting layer creates the electric field to move the electron and hole in the desired directions.

Where things get different and interesting, however, is the method the electron uses to get from where it is separated from the hole, and how these two charges travel to where they are collected and used.

Do Perovskite Materials Exhibit Any Unique Physical Transport Mechanics?

Perovskite PV cells are incredibly malleable in their design. As such, they exhibit several unique properties when it comes to their operation and behaviour. 

A normal crystalline wafer solar cell is tuned to preferentially conduct charges in certain directions in the crystal relative to whether the charges are positive or negative. This is achieved with carefully controlled doping and the intrinsic electrical field caused by said dopants. Perovskites, meanwhile, show several different methods of charge separation and transport through their atomic lattice: traditional lattice conduction, polaron transport, and ion transport.

Lattice conduction, both electronic and ionic, is the most common method of charge transport. It is simply where charges that are boosted into the conduction bands of the material hop from one location to another in the atomic lattice. Ion transport is related to this as it describes when charged atoms (in an ABX metal halide perovskite, these are normally the X position halides) move through the lattice from defect to defect trying to fill imperfections in the lattice to reduce the energy state of the crystal to as low as possible (ie. a perfect flawless crystal).

Where things get interesting is when a charge is forced into a position where it creates a polaron. A polaron is a structure that arises in strongly dielectric materials that have a free charge attempting to move through the lattice. The electric field of the charge attracts oppositely-charged components of the lattice and repels similarly-charged components. The shifted positions of the neighbours nearest to the charge slightly weaken the neighbour’s electric field strength toward their neighbours, causing the local lattice to flex and stress until the local field is suppressed enough to not impact any more of the lattice.

 

Lattice deformation of a polaron in an ionic solid.
Lattice deformation of a polaron in an ionic solid. (source: https://onlinelibrary.wiley.com/doi/abs/10.1002/ijch.201900101)

This process creates a kind of pseudo bond between the free charge and the displaced atoms in the local lattice, significantly increasing the effective mass of the free charge as moving it requires altering the pseudo bond length between it and the locally displaced atoms. This means that it takes noticeably more energy to move the charge but that it is also noticeably longer lived than an equivalent charge moving through a lower relative permittivity material like silicon.

With so many physical transport mechanisms at play and with many possible paths to use their properties for better solar cell performance, the question becomes: is the optimization of these transport mechanisms and other perovskite properties worth the effort to explore?

What Are The Advantages And Disadvantages Of Perovskite Solar Cell Technology?

New technology can have all of the interesting physical behaviours in the world, but if it does not provide any significant benefits it won’t be as widely adopted. In this section, we’ll discuss whether or not the formulations of perovskite semiconductors provide a worthwhile difference to the status quo, as well as what hurdles the technology still has to overcome.

Charge Transport Layers And Junctions: Supporting Materials And Structures Required For Perovskite Solar Cells

So far, our discussion has been focused on the perovskite layer of a perovskite solar cell. However, this layer is not the only important part of a solar cell; there are several accompanying components and structures needed for optimal photovoltaic functionality.

These include structures for creating an internal electric field through the intrinsic semiconductor of the perovskite crystal called charge transport layers (CTLs). Beyond that, you also need current collectors from those transport layers out into the circuits where the devices to be powered are connected, whether that is just a single device or the country’s electrical grid. At the most complex end of the spectrum, there are also junction layers for helping to combine multiple specialized layers of perovskites or other photovoltaic materials to create multijunction cells.

Charge transport layers (CTLs) are very important for the operation of current perovskite solar devices. Without them, perovskites can show a significant reduction in PCE. 

The perovskite absorber layer between the CTLs acts as an uncharged semiconductive material where the incident light can induce charge separation, boosting valence electrons into the conduction band and leaving behind a hole in the valence band, by interacting with the crystal. However, the transport of charges to the respective current collectors is dependent on the electric field, currently created by the charge transport layers (CTLs), just as a silicon cell is dependent on the dopant layers to pull charge carriers out of the depletion zone.

CTLs carry out their transport function by creating favourable junctions with the intrinsic semiconductor of the Perovskite layer. By making it energetically favourable for electrons to enter one CTL and for holes to enter the other, the natural diffusion of free charges from the perovskite into the CTLs creates an intrinsic electric field that helps draw more charges to these layers preferentially. This is similar to how a depletion layer in a traditional pn junction of a silicon solar cell operates.

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Currently, the most common CTLs are titanium dioxide (TiO2) for the electron transport layer and Spiro-OMe-TAD for the hole transport layer. Alternatives of nickel oxide (NiO) and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), respectively, are also fairly commonly used in those roles.

Thanks to perovskites having high carrier mobility and very thin layers, the application of a hole transport layer (HTL) and an electron transport layer (ETL) is sufficient to generate an electric field throughout the perovskite layer capable of promoting charge transport after electron-hole-pair generation. What order these layers are placed in, relative to the incident light on the solar cell, has some effects on the operation of the device but can be generally considered equivalent for purpose. 

Due to the organic and potentially UV-sensitive nature of Spiro-oMeTAD and PTAA, it is more common to find the sunward side of the device to be the ETL, thus defining the orientation of the device as the negative layer (ETL), followed by the intrinsic semiconducting layer (perovskite) and lastly the positive layer (HTL) giving rise to the terminology of “n-i-p construction”.

N-I-P structure thin film example encapsulated between a Transparent Conducting Oxide (TCO) and a metal electrode.
N-I-P structure thin film example encapsulated between a Transparent Conducting Oxide (TCO) and a metal electrode. (source: https://www.researchgate.net/publication/333997916_Recent_progress_concerning_inorganic_hole_transport_layers_for_efficient_perovskite_solar_cells)

Although this construction shares the same nomenclature as n-i-p and p-i-n silicon-based semiconductors, there are important differences to note. Perovskite n-i-p and its reverse p-i-n construction is enticing because of the simplicity of the processing stages required to produce them.

By aligning and joining multiple layers without requiring the high energy costs of thermal diffusion or high vacuum of ion implantation doping of a more traditional semiconductor, processing costs can be noticeably reduced.

A downside of this type of construction is the doubling of what are known as “surface defects.”

The interfaces between the different layers are known as surfaces even if they are not necessarily exposed to open air, and the adhesion between the surfaces of the layers is very important to produce a properly functioning solar cell. Poor connections from mismatched crystal orientations, different coefficients of thermal expansion, and/or different electrical conductivities can create forces that delaminate the layers from each other and can prevent the cell from keeping electron-hole pairs separate.

Electron-hole recombination (when the electron-hole pairs are not kept separate) is a key loss mechanism in solar cells that designers seek to minimize as recombination means the charges do not make it out of the device and therefore can’t do any useful work for us.

With a normal p-n junction-based solar cell like silicon, the only surface defects are the contact between the silicon and the electrical contacts at the front and back that complete the circuit. In this aspect, then, silicon-based solar cells have an advantage over perovskites because they have a lower number of surfaces on which defects can form.

Knowing how to manage these surface defects and ensure good electrical conductivity between layers is important for standard perovskite cells but becomes even more so when perovskite technology is used in the fabrication of multijunction cells. While there are many advantages to using perovskites in multijunction cells (which we’ll discuss a bit later), every subjunction that uses perovskites will also be subjected to the same surface defects we’ve been discussing.

Absorption Properties Of Perovskite Structures

If your solar cell has a low absorption and extinction coefficient (i.e. is more transparent to light), then you will require more material to collect incident light before it exits the back of the device.

Solar cell researchers, therefore, want materials with high absorption and high extinction. One of the biggest benefits that Perovskites have is how efficient they are at converting light into excited charge pairs.

Perovskites, as stated earlier, initially started as specialized materials in dye-sensitized solar cells precisely because they were able to absorb light extremely efficiently within very small distances.

In fact, perovskites have an absorption coefficient over 10 times larger than that of silicon, and while the physical scaling is not perfectly 1:1, this higher absorption coefficient means that a perovskite solar cell can be approximately ten times thinner than a silicon solar cell to capture the same amount of light.

Such proposed solar cell thickness scaling assumes that the devices have the same quantum efficiency, the ratio between the number of excited charge pairs produced for a specific number of incident photons. Perovskites are known for high quantum efficiencies. This is due to their multiple methods of producing excited charge pairs as stated earlier, but some research is working towards improving that value even further, even over 100% due to some very clever manipulation of the properties of the perovskite crystal structure.

Being able to produce more electrons from the same thickness and number of incident photons gives perovskites an impressive advantage in capability over more traditional materials and designs, not just in terms of output but also in terms of device weight. By having such a high quantum efficiency and high absorption, a perovskite cell has no need to be as thick, and therefore as heavy or rigid, as a traditional solar cell. Even compared to other thin films, perovskites come out ahead because they do not need to sacrifice light collection to get down to smaller form factors.

Another aspect of perovskite solar cell performance closely linked to absorption properties is their good performance at lower light intensities. When the sun is not shining directly on the perovskite solar cell, it is still capable of converting ambient indirect light into usable electricity.

This low-intensity behaviour helps to open up several interesting use cases for the use of perovskite solar cells in interior spaces to power or extend the battery life of small items (such as wearables or other low-power devices).

What Impacts Do Bulk And Surface Defects In Perovskite PV Cells Have On Performance?

Quantum efficiency does not directly translate into power conversion efficiency, though. The excited charges still have to be collected and used in the final circuit. For that to happen, the positive and negative charges must be separated and drawn into their respective transport layers. Preventing that from happening perfectly, however, are the various types of defects within the perovskite crystal.

Defects are any deviation from a perfect crystal in the desired stoichiometric ratio or geometric pattern. They come in many forms and when not controlled they can be very detrimental to the operation of any device, not just semiconductors. Broadly they can be separated into two classifications, bulk defects and surface defects.

Bulk defects are errors within a single crystal such as missing atoms, extra atoms, impurities, atoms in the wrong spot, and so on. These types of defects are simple outcomes of being unable to create a crystal perfectly atom by atom.

Some bulk defects are necessary for certain properties to occur. For example, earlier it was mentioned that ion transport is a method for how charges can be shuttled around inside the crystal. If every position for every mobile ion was perfectly filled within a perovskite crystal, the amount of energy required to move ions would be vastly increased since any incoming ion has to first knock out the atom already occupying the space. Compare this required energy to the energy needed to shift an ion into an already empty space, and you can appreciate the magnitude of the drop in ion mobility that a perfect lattice can cause.

Even if some defects are necessary, all defects impact how current flows through the material. It doesn’t matter if the defect is a missing atom, an extra or several extra atoms, or atoms in the wrong position relative to the rest of the crystal, they all impact the local stress on the crystal and the local electric fields. These alterations act like hills in the path of a bicyclist, either requiring more energy to get over (blocking low voltage charges), or more time spent to get around (increasing the required lifespan of the mobile charge to reach a current collector, so fewer charges make it before recombining, lowering the output current).

The good news about bulk defects in perovskites is that they are not very negatively impactful on the operation of the devices. This insensitivity to bulk defects is theorized to be a combined result of the varied methods of charge separation/transport through the bulk materials, and how the mobility of the X position atoms within the lattice cooperate to allow charges to ignore most of their electronic effects during operation. 

By having multiple transport methods you have multiple different interactions with any defects, and by having high mobility of atoms it means that defects are unlikely to stay in one position for very long, giving a statistical equivalence of having the defects “smeared” over a larger volume and having a lower severity of interaction over that volume.

Some examples of bulk defect types in perovskites. Not shown are interstitial defects where extra atoms are wedged in between properly placed atoms.
Some examples of bulk defect types in perovskites. Not shown are interstitial defects where extra atoms are wedged in between properly placed atoms. (source: https://www.researchgate.net/publication/342861512_Compositional_and_Interface_Engineering_of_Organic-Inorganic_Lead_Halide_Perovskite_Solar_Cells)

Surface defects, on the other hand, are defects arising from a mismatch in the atomic lattices either between different materials or within the same material type. “Surface” in this case doesn’t refer to actual external surfaces; surface defects are more practically understood as the contact points between two or more lattice-mismatched crystals or materials.

These are the more detrimental types of defects but are also the ones that have more options for reducing their density and impact. A surface defect, such as a crystal lattice mismatch at the interface between a CTL and the perovskite or a grain boundary between two crystal grains within a perovskite, can act as a trap for charges by influencing the local electric field.

Changes in the local internal electric field divert charges from being properly collected and used in the circuit of the solar cell. This is called non-radiative recombination, or surface recombination, as the charges (either holes or electrons) within the defect attract and combine with their opposite charges in the crystal preventing the charge from making it to the electrode.

While minimizing the density of these defects within devices can improve performance and help prevent the onset of early degradation, the number of combinations and variations possible with perovskite fabrication is both a cause and solution to the problem of surface defects.

The vast array of potential perovskite formulations makes it almost inevitable that a composition and fabrication methodology will provide a solution to the formation of unwanted surface defects, both intrinsic and extrinsic. Whether this is from a formulation improving the self-repair of defects within perovskites, or one that prevents their formation during fabrication is the question. 

Whichever solution becomes the chosen path forward, improving the capability of single-layer perovskites is important not only for single-junction perovskite solar cells but also for the construction of Multijunction cells where both the difficulties and rewards are higher.

Multi-Junctions, Tandem Cells, And Efficiency-Specific Structures For High-Performance Solar Cells

One potential avenue for improving performance comes from the capability of perovskites to be used as layers in multijunction solar cells. To make a multijunction solar cell you need to be able to stack several layers of semiconducting material onto each other with specifically different band gaps so that each layer performs at high efficiency for different spectral sections. This stacking noticeably increases the complexity of the final device and the processing required to fabricate it (without creating a fatal number of new surface defects in between the different layers), but the potential gains can be quite lucrative.

Perovskites make an excellent candidate for producing multijunction cells.

The most important consideration for the construction of a multijunction cell is the ability to have multiple layers operating at different band gaps with correspondingly different spectral selectivities.

Not only can perovskites be fabricated using simple methods conducive to production in multiple layers, but they also have a structure that lends itself to easily adjustable band gaps.

Due to the crystal structure of perovskites being if not identical, then at least similar and controllable, it is also much easier to ensure lattice matching of the contacting layers which helps prevent unwanted device stresses.

Furthermore, if the layers of a multijunction cell do not produce sufficiently equal current densities during operation, a charge can build up at the interface and potentially reverse the internal electric field of a subjunction, rendering it inoperable or damaging it. Thankfully, varying the thickness of your layers can help to control the current densities of the different junctions, a solution well within the capabilities of existing perovskite fabrication.

Normally, tailoring each of the junctions to exact specifications is an imposing barrier to creating multijunction cells. The flexibility of perovskites in how they can be fabricated offers a great deal of potential for overcoming such barriers, especially with how large an effect simple component substitutions in a perovskite can have on the band gap and absorption spectra.

The variability of a perovskite’s composition and the simplicity of some of the available fabrication processes means that tailoring a perovskite cell to exact specifications is far more practical than using other technologies such as III-V semiconductors like Gallium Arsenide and its close relatives that are currently the most common form of multijunction cells.

Example of how absorption spectrum and band gap shifts with different halides in perovskite solar cells.
Example of how absorption spectrum and band gap shifts with different halides in perovskite solar cells. (source: https://www.researchgate.net/publication/280870110_Two-Inch-Sized_Perovskite_CH3_NH3_PbX3_X_Cl_Br_I_Crystals_Growth_and_Characterization)

Multijunction solar cells are an approach to getting around the assumptions used to derive the Shockley-Queisser limit. By adding more junctions, waste heat (from the thermalization of carriers) is minimized and achieves higher efficiencies. There are different approaches and structures for multijunction solar cells that we’ll discuss in another article. Here, we’ll focus on the specific aspects of perovskites applied to multijunction solar cells.

Since a perovskite can reliably have its bandgap altered along with its lattice shape and spacing, we are given flexibility for more variety in lattice-to-lattice material bonding. Perovskite lattice flexibility makes it possible to create multi-material multijunction cells by bonding perovskite layers to silicon cells (crystalline or amorphous) or to Copper Indium Gallium Selenide (CIGS), which can provide their own unique benefits to a solar cell. This allows researchers and designers to obtain better performance than a perovskite solar cell on its own.

Environmental Considerations: Lead Toxicity And The Search For Alternatives In Perovskite PV Technologies

The flexibility of perovskite material compositions has been a major focus of any single or multi-junction design, which has resulted in a wide variety of different formulations in the effort to reach the desired goal of an efficient, stable, and inexpensive solar panel.

However, one problem has remained fairly intractable since the beginning of perovskite solar cell research.

Whether the perovskite du jour is the original methylammonium lead triiodide, or a more complex one like (Cex, Fa+1-x)Pb(Iy, Br1-y)3 there is one element that is uncomfortably consistent throughout the majority of published research.

Lead (Pb).

Lead has had a fairly negative public reputation since we stopped using leaded gasoline, and for good reasons. Being a cause of neurological, reproductive, and physical disorders such as memory or learning problems in adults and children, reduced fertility in men, miscarriages or stillbirths in women, along with general mood disorders, and digestive issues, it feels only reasonable to be wary of lead or similar heavy metals being allowed to enter the environment from any source.

Unfortunately, lead’s use has persisted in perovskite research due to some of its fundamental atomic characteristics. Perovskites require elements with a specific electron coordination number in the B position of the ABX3 structure to maintain the octahedra of halides. Due to the size of most of the organic molecules used in the A positions, having a large B component is beneficial to make sure there is enough room in the crystal for the A components. Lead appears to be the best choice for meeting the requirements of large ionic size and a coordination number of six.

Lead is still considered by most researchers to be a necessary component for high-efficiency perovskite solar PV. As such the best option is finding methods to prevent the lead from having a method of entering the environment in the first place via proper encapsulation of the solar cells. Ensuring good encapsulation is a good general approach for any future solar cell technology intended for large-scale deployment.

Once solar cell encapsulation is achieved, another key requirement for safe and reliable deployment is implementing methods to capture any lead that might get released should the cell or panel get damaged enough to breach the encapsulation. Such considerations will benefit any large deployment of solar cell technology by increasing a solar cell’s lifetime, and may sufficiently mitigate the risks and concerns associated with lead use in perovskites.

However, as we noted above in the ‘Composition And Common Materials Of Perovskite Solar Cells’ table, there are potential alternatives for lead in perovskites: tin and germanium are just two examples.

These elements are in the same column as lead in the periodic table and therefore share the same electron coordination number in their outer shells. Tin even has a similar ionic size to lead. Given these similarities, there is ongoing research investigating the use of these elements to replace lead completely or in part. Some studies have looked at simple replacements with impressive results for stability and efficiency; however, lead remains present to some extent in many of these reports.

Are Perovskite Solar Cells’ Efficiencies Temperature Dependent?

One behavioural aspect of perovskites that needs to be overcome is related to the method of charge transport within the crystal structure. Namely, the movement of polarons as a significant fraction of current conduction leaves perovskite cells vulnerable to fluctuations in temperature. The “effective mass” is higher than the actual charge it forms around.

This increased effective mass means that the energy required to move this charge through the crystal is higher than for a normal charge in the conduction band. But since polarons are also electrically responsive due to being formed by a free charge becoming coupled to the lattice, local electric fields also play a role in their motion.

Research into the influence of these factors  shows that at standard temps (~300K), the strength of the local electric field is a far larger influence on the motion of polarons than temperature if the electric field is strong. Interestingly, at standard temperatures with a weak electric field, thermal motions are more likely to act as a damping force preventing the motion of polarons in any direction.  

This electrical field-differentiated change in the effect of temperature on the conductivity of polarons and the final PCE of the cell raises a concern. While bulk material defects are not large enough to maintain charges of significant enough size to alter the intrinsic electric field of the device, surface defects can act as charge concentrations and become traps for free charges (especially polarons). The reason is that surface defects can create local fields noticeably stronger than the internal field of the intrinsic perovskite layer, thus attracting polarons and preventing them from reaching their charge’s respective CTLs. This sort of non-emissive recombination is theorized to be a major component of the known reduction in PCE for perovskites at higher temperatures.

Fortunately, experiments have also shown that the stabilization of perovskite solar cells against temperature fluctuations have a path forward. The replacement/doping of some of the A components in the perovskite structure of organic halide perovskites with inorganic Cs or Rb is showing promising results in stabilization over larger temperature ranges with only minor reductions in overall PCE.

What Is The Radiation Tolerance Of Perovskite Materials For Space-Based Solar Applications?

Due to their thinness, lightweight, and potential for very high PCE, perovskite photovoltaic technologies are attractive for use in space-based applications, an environment where the transportation of every gram is an expensive proposition.

However, for perovskites to have any chance of succeeding in space, there is a unique consideration that needs to be investigated, namely how well these devices handle exposure to high-energy charged particle radiation.

Initial investigations have given positive results. The inherent mobility of a perovskite’s X components allows for the quick replacement of any atoms that are knocked out of position by incoming radiation. The thinness of the devices prevents incoming high-energy radiation from interacting with the panel simply due to them passing through it with minimal interaction.

Finally, the relatively large size but low mass of the A components combined with the general low density of the entire structure mean that when there are interactions they tend to be purely kinetic (rather than chemical or nuclear) creating new bulk crystal defects which, as mentioned earlier, are not especially detrimental to the operation of a PSC.

The Costs of Perovskites: Sources and Reductions

Technical capabilities, power output, and PCE  inform PSC device performance. However,  there are additional considerations that govern the technologies performance in large-scale deployments.

Materials Costs For Perovskite Solar Cell Technology

No one is going to build a solar panel out of diamonds. To create a valid competitor to current commercial panels there have to be noticeable savings in as many points of the supply and logistics chain as possible to offset the economies of scale enjoyed by the incumbent. Especially one as entrenched as the current crystalline silicon panel industry.

For example, taking into account the preference for very high-purity precursors to limit the potential for defects caused by unwanted elements in the crystal, perovskite precursor inks are quite reasonable in cost. Approximately $250 USD in 2022 will provide enough precursor ink to cover 1 square meter of surface area via a non-spin-coating production method. However, compared to the approximately $100 USD/ m^2 for pure polysilicon in 2022 ($43 USD/kg, 1.16 kg/m^2 for a 0.5mm thick panel plus ~50% wastage from necessary cutting off from the source ingot) perovskites are seemingly at a disadvantage. When you account for the economies of scale, however, adjusting for the relative number of produced silicon solar cells compared to halide perovskite solar cells, it becomes reasonable to posit that there are large savings to be had as perovskite production scales up to match.

Economies Of Scale: Rapid Production Of Perovskite Films For Low-Cost Solar

Where perovskites have the best chance at an early competitive advantage over traditional solar technologies is in production speed. Roll-to-roll manufacturing with solution-based inks has the potential to be rapidly scalable and inexpensive, not only due to the faster production speed but also the amount of production that can be completed under standard atmospheric conditions and temperatures.

Without the need for the controlled vacuum environments of gas/vapour deposition or even worse, high-vacuum for plasma doping, operating costs for perovskite fabrication can be kept low while keeping throughput high. For example, even very unoptimized small batch fabrication costs below $40 USD/ m^2.

Perovskite fabrication is not all benefits, however, since faster production requires less time spent on certain steps such as crystallization and annealing of the crystals to minimize surface defects and grain boundaries that could be detrimental to solar cell operation. To counter the high number of surface defects and hopefully prevent any faulty panels from being sold, there needs to be some form of quality control testing happening on the line itself. This isn’t unique to perovskites, though, as quality assurance testing is common on basically every factory line around the world for every product.

There are many possibilities for the kind of testing that can be done but, understandably, most procedures require the device to be operating under expected conditions, which therefore requires non-destructive testing.

For solar PV this means either sunlight or something as close to sunlight as possible (such as illumination from a solar simulator). Even some non-electrical data collection needs photon illumination, such as photoluminescence scanning to find defects within the cells where poor bonding between layers might have occurred during manufacturing.

These testing methodologies are useful for lab scale cell testing, and doubly so for trying to test any panel scale devices, because as you scale up the size of your devices you want to keep any destructive quality assurance testing to an absolute minimum. This means that a large body of data that links failure modes of panels and cells to certain outputs from the non-destructive methods is required in order to reliably produce high-quality panel-scale perovskite PV. But with the vast field of potential chemical make-ups of perovskite devices, it is extremely time-consuming to characterize the behaviour for every formulation which is why as of yet there is no widely accepted standard for panel testing as a method to predict failure modes of commercial perovskite devices.

As certain formulations become more common in achieving desired properties in the lab, these types of perovskite PV will become the ones most likely to allow proper characterization to inform the creation of acceptable and reliable standards for non-destructive testing for quality assurance and control for commercial perovskite PV panels.

Maximizing Lifespan For Stable Perovskite Solar Cells

Beyond materials and production, however, another important factor in whether a new technology is considered a good investment is its lifespan of operation. The longer a solar cell user can run what they’ve bought, the more time is spent with the device completely paid off and generating free electricity.

Here is where things get difficult for perovskites, however. Historically, perovskite solar cells have been extremely short-lived. Whereas regular silicon solar cells would normally be guaranteed for 20 years and expected to operate likely for 30 years, perovskites in the lab up until very recently could be expected to last for hours or even minutes.

What contributes to this shocking lack of long-term stability in perovskites? A pithy response would be that a shorter list might be what does not contribute. But as we have all been told since childhood, nothing worth doing is easy.

UV Degradation Of Halide Perovskite Solar Cells

Exposure to UV frequencies is known to cause degradation potentially within tens of hours to some perovskite formulations. The degradation occurs most severely at the interface between the perovskite and the TiO2-based electron transport layer (ETL) but reduces when an Al2O3 ETL is used instead.

An interesting occurrence is that the damage from UV exposure can be partially regenerated by the heat from standard sunlight. This effect is related to our earlier discussion of how bulk defects are not responsible for significant perovskite performance decreases. Since a perovskite’s components are fairly mobile within the crystal structure, adding energy in the form of heat allows the crystal to relax from a high-energy disrupted state from the defects back into the lower energy state of the perovskite crystal, pushing the efficiency back up but not quite to the original level of the pristine cell.

Oxygen Reactivity: Problems And Uses For Perovskite PV Technologies

Pure oxygen is a constant concern for almost everything created by humans, and perovskite solar cells are no exception. As oxygen reacts with an organic halide perovskite it has a strong preference for stripping hydrogen atoms from the organic A components. This removal of hydrogen leaves several reactive species behind that proceed to react with everything around them and disrupt the semiconducting properties of the local crystal.

As with most chemistry, this reaction occurs faster and more frequently at higher temperatures which understandably is a problem for solar cells, requiring very thorough encapsulation of the device from the local environment to prevent it.

Oxygen is not entirely detrimental, however. When there are no organic components to react with, oxygen can actually be a useful additive to help “fill in” defects in the perovskite crystal, both bulk and surface, through a process called passivation. 

This helps not only to increase the performance of the cell by reducing nonradiative recombination in certain types of defects but also to prevent some types of degradation due to a decreased number of active chemical sites within the crystal.

Water Solubility/Reactivity: Considerations For The Formation Of Perovskite Structures

Atmospheric moisture is more than simply a vector for delivering reactive oxygen to a perovskite; it can result in detrimental reactions of its own.

Foremost is the fact that lead halide perovskites are slightly soluble in water. Therefore too much exposure can lead to the perovskite layer dissolving out from between the hole and electron transport layers.

However, it isn’t quite as simple as preventing all water contact. In the proper concentrations, moisture exposure of perovskites during the fabrication process can prevent pinholes in the perovskite films, increase crystal density and reduce locations where non-radiative recombination can occur.

Denser crystals with fewer defects not only have better performance but also show a longer lifespan in testing, so there is a definite potential for risking the usage of water in the production process before the final encapsulation of the finished product.

Surface Defect Passivation: A Path Towards Stable Perovskite Solar Cells?

With all these avenues for degrading perovskite devices, how do researchers and producers aim to keep their designs functioning for the 20 to 30-year lifetime the public expects from solar panels?

There was an example we discussed above where oxygen could be used in inorganic perovskites to passivate the crystal. Passivation is the same process that keeps stainless steel or aluminum from rusting, where a reaction between a specific molecule and the device/object occurs that blocks reactive sites on the material, preventing them from acting as sites for reactive chemicals to attack and damage the device/object.

What this molecule is depends on the material you are attempting to passivate but it must be stable enough after reacting with the perovskite surface that it is unable to be dislodged by anything it might encounter in daily use, whether that is temperature, UV radiation, humidity, or chemicals.

Passivation methods and considerations for perovskites
Passivation methods and considerations for perovskites (source: https://pubs.rsc.org/en/content/articlelanding/2020/ee/d0ee01767a)

Additives For Improved Stability Of The Active Layer In Perovskite Pv Cells

While passivation is the concept of creating a barrier on the surface of a material after it is shaped to prevent future degradation, it is also possible to use less reactive chemicals to exert physical stresses upon the structure of the perovskite during formation to achieve the desired crystallinity. This is to promote the growth of larger crystal grains which lower the number of surface and bulk defects within the device while also minimizing the potential for damage from reactive chemistry.

As an example of the application of unusual chemistry to perovskites, it is possible to promote some perovskites to grow larger crystals during formation using the addition of capsaicin in regulated concentrations. As stated earlier, larger crystal grains equal fewer surface defects in the form of grain boundaries for free charges in the solar cell to be trapped and recombined.

Caffeine is another unexpected additive that has beneficial properties for perovskites. The addition of caffeine to the precursor ink during crystallization acts as an energy barrier for the crystallization, slowing down the rate of crystallization but increasing the quality and density.

By increasing the crystal size and reducing bulk defects within it, ion mobility is reduced. This stabilizes the crystal at higher temperatures and allows a perovskite cell to perform at higher efficiencies under intense sunlight even with the decrease in conductivity due to the suppression of ion transport from normal levels.

It is also possible to simply add a different, large physical 2D crystal to a perovskite to act as a stabilizing feature and a method for improving the conduction of charges between the intrinsic perovskite layer and the electron transport layer.

For example, the usage of graphene for this purpose is very effective, but also currently very expensive and unlikely to improve the commercial validity of the device unless there are significant changes in the price of graphene or other 2D materials. So while there are ways to create improvements or offset shortcomings, it is important to weigh these processes against their future economics.

However, even with all the various ways for perovskite solar cells to fail, there are just as many potential paths to fortifying and stabilizing them against the elements.

Recent publications have showcased extremely impressive results in longevity testing such as the 1000h damp heat examination required by the IEC testing standards as mentioned above. And with further improvements in accelerated aging testing, it is possible to iterate through lifetime testing at a much faster pace than before, accelerating the rate of testing new formulations and constructions until a truly competitive version is found.

Energy Efficiency of Perovskite Solar Cells

With all perovskite benefits (such as high absorption coefficient, high relative permittivity, and shiftable band gap) and barriers (such as UV sensitivity, high defect density, and thermal instability) that have been discussed, how do we know whether perovskites can achieve their potential for industry-leading power conversion efficiencies without requiring energy-intensive fabrication? And is there a reputable and comprehensive source for comparison against other solar cell technologies?

Thankfully, such a source does exist. The National Renewable Energy Laboratory provides detailed information on perovskite energy efficiency as well as many other metrics.

National Renewable Energy Laboratory (NREL): Current Records For State-Of-The-Art Perovskite Solar Cells

Enter the National Renewable Energy Laboratory, more commonly known as NREL, operating in conjunction with the Solar Energy Technologies Office (SETO) of the US Department of Energy. Among NREL’s many functions, they offer certified solar cell testing and publish a chart of world record holders for all solar energy technologies.

NREL record research cell efficiencies, emerging solar technologies specific.
NREL record research cell efficiencies, emerging solar technologies specific. (source: https://www.nrel.gov/PV/assets/pdfs/cell-PV-eff-emergingPV-rev220630.pdf)

NREL’s database shows what solar cell technologies are leading the pack, how quickly some are improving their positions, and which are struggling to build upon past successes.

Note the path that perovskites have taken since they were first recognized as a distinct technology by NREL in 2011. With an initial efficiency of 14% to the latest value in 2022 of 25.7%, as seen in the above image, perovskites have shown one of the most consistently rapid rises in the efficiency of any solar technology since NREL began this archive.

Thanks to the NREL database it is easy to look through the recent and current record holders and find their publications about what makes their cells achieve the results they do.

We have collected the current leaders for the three main categories of perovskite solar technologies and the focus of the research that allowed them to achieve those records: pure perovskite solar cells, perovskite/CIGS tandem cells and perovskite/c-Si cells.

If we sort institutions according to the number of times their cells appeared in the top 3 most efficient solar cells of each category, we produce the following ranking:

  1. Helmholtz-Zentrum Berlin (HZB)
  2. Ulsan National Institute for Science and Technology (UNIST)
  3. École polytechnique fédérale de Lausanne (EPFL)
  4. Centre Suisse d’Electronique et de Microtechnique (CSEM)
  5. Korean Research Institute for Chemical Technology (KRICT)
  6. Massachusetts Institute of Technology (MIT)
  7. University of California, Los Angeles (UCLA)
  8. Nanjing University

Arranging these record holders into their respective fields with the topics of their research, some commonalities begin to appear.

Pure Perovskite cells

 

Perovskite/CIGS tandem cells

 

Perovskite/c-Si tandem cells

Institute

UNIST

 

Institute

HZB

 

Institute

EPFL/CSEM

Date

2021-12-03

 

Date

2020-01-01

 

Date

2022-06-17

Efficiency

25.7%

 

Efficiency

24.2%

 

Efficiency

31.3%

Surface and bulk defect mitigation through anion passivation

 

Improved quality of interface layer and alkali metal doping of PSC and CIGS layers

 

Hybrid vapour/solution processing on textured silicon surface

   
   
               

Institute

KRICT/MIT

 

Institute

HZB

 

Institute

HZB

Date

2019-07-01

 

Date

2019-06-01

 

Date

2021-11-16

Efficiency

25.2%

 

Efficiency

23.3%

 

Efficiency

29.8%

Charge carrier management through improved SnO2 ETL fabrication and separated passivation steps for surface and bulk defects

 

Self Assembled Monolayers for electrode junctions to improve conductivity and reduce surface defects

 

Textured interface and dielectric back reflector

   
   
               

Institute

Nanjing University

 

Institute

UCLA

 

Institute

HZB

Date

2019-12-01

 

Date

2018-11-17

 

Date

2020-01-01

Efficiency

24.2%

 

Efficiency

22.4%

 

Efficiency

29.15%

Antioxidant additives for passivating surface defects in all perovskite tandem cells

 

Highly doped hole transport junction layer between subcells

 

Fast hole extraction via self assembled monolayer interface between PSC and Si subcells

   
   

The current record holder in single-layer perovskite cells, UNIST, focused their efforts on improving the electrical connection between the photo-absorber perovskite layer and the charge transport layers. They succeeded via the design of a special interlayer coating of formamidinium tin chloride perovskite to prevent the formation of the normal density of defects between the layers that interrupt charge flow and lower efficiency.

And it’s not just pure perovskite solar cells that are showcasing exciting growth and ability. Related combined multijunction technologies of Perovskite/CIGS (PS/C) cells and Perovskite/Silicon (PS/Si) also show impressive capability and potential, with PS/Si cells showing a confirmed 31.25% power conversion efficiency.

Looking across all the three types of perovskite cells denoted above it is also possible to see common themes of research in these record holders.

The most obvious is the focus of all these projects on improving the quality of charge conduction between layers of the devices themselves. Whether that is a junction between the PSC layer and the Si or CIGS layer or simply the interface between the intrinsic perovskite layer and the CTL, this seems to be the most important variable to control and improve upon.

Once surface defects (such as the interfaces between different materials) have been sufficiently mitigated, there then arises the consideration of how to passivate the other remaining defects. We see the investigation of other defect mitigation in the research from both the KRICT/UCLA teams and UNIST, as they strive to maximize the performance of pure perovskite devices.

The final area of focus for perovskite solar cell record-holding research is the exploration of the benefits of doping the active materials in the cells. The record-holding HZB research in the fields of PSC/CIGS tandem cells used such a technique which could be a chief strategy to consider and monitor in the years to come.

All of these considerations are topics that have been explored above as known barriers to higher-performance perovskite cells, and seeing the work that these institutions have done to address them is a source of excitement for all those who have high hopes for the promises of perovskites as a powerful tool in the decarbonization toolbox.

Combine this showcased progress with some of the recent improvements in other areas of perovskite research such as ferroelectric inorganic perovskites, rapid-aging testing procedures, incorporation of advanced 2D materials, and advanced manufacturing processes such as vapour-phase or electrodeposition, it is exciting to think about what the cell efficiency chart will showcase for perovskites in the coming years.

A natural question after seeing such rapid technology development is, how does a researcher test a perovskite solar cell’s performance, as well as their potential faults and if a new strategy has corrected them? We’ll discuss that in the next section.

How Are Researchers Testing And Validating Perovskite Solar Cells In Their Labs?

As with any scientific endeavour, the faults, benefits and behaviour of a study’s focus are determined through testing. So when studying perovskite solar cells, how do researchers test them to ensure that they are performing as desired? Especially when, due to the fact that perovskites are still barely a decade old as a technology (and yet to enter the commercial solar market in any major way), there are not yet specific testing standards or protocols for perovskite cells.

To determine the relative capabilities of a solar cell compared to available data for any other technology, you have to be able to achieve provably high-quality values for specific figures of merit that transcend technologies.

The most common being arguably the most important one, Power Conversion Efficiency (PCE), since that is the figure of merit that is used to showcase how good any solar technology is at being a solar power collector.

Beyond this comparison, there are other figures of merit that can inform how the device in question operates and can inform the pathways toward improved performance or explanations for likely failures as mentioned in a previous section.

Some of these other figures of merit include the following:

  • Short Circuit Current (ISC): This is the amount of current that can be induced to flow through the PV device by incident sunlight when no load is applied to the completed circuit (voltage = 0, resistance = 0)
  • Open Circuit Voltage (VOC): This is the voltage that a solar cell experiences when illuminated but without a circuit being completed (current = 0, resistance = infinite)
  • The IV Curve: the graphical relationship between the produced voltage and current of a solar cell while operating under illumination at various resistive loads between 0 and infinity
  • The Maximum Power Point (MPP): The point on the IV curve where the product of the current and voltage produces the highest value of power
  • Fill Factor: the ratio of how close to ideality a certain solar cell is given by the ratio of the MPP divided by the product of the ISC and VOC.
Quick reference sheet for Figures of Merit related to Fill Factor
Quick reference sheet for Figures of Merit related to Fill Factor

These figures of merit are affected by both internal and external factors for the device in question and thus in order to ensure that the values collected for them are accurate it is necessary to account for these potential influences at all stages of testing.

As shown earlier, perovskites have several potential sources of defects and degradation. These defects and degradation pathways can have detectable effects on the outcomes of any tests for the standard figures of merit needed to make accurate comparisons for the viability of a new solar PV technology.

While the tests for the above-mentioned figures of merit are common across different PV materials and designs, there needs to be either mitigation of specific perovskite defects or testing to identify the primary barriers to achieving desired properties.

Examples of Controlling Variables During Testing: Reducing Temperature For Stable Perovskite Solar Cells And Comparative Standards

Since no solar technology is perfectly efficient there will always be some energy that is unable to be converted to electricity and ends up as heat within the cell itself.

This heat is not only a mechanism for degrading the long-term performance of a perovskite cell but also a reason for the fill factor to diverge from any tested values as heat tends to increase electrical resistance in materials, which pushes the IV curve further away from its ideal shape.

However, since different materials for different types of solar technologies handle the generation, retention, and rejection of heat in significantly varied ways, it is difficult to make comparisons between them at the elevated temperatures that occur during operation.

Different materials have different heat capacities and thus under the same intensity of illumination will have different stable temperatures. To that end, there needs to be a testing mechanism that allows for accurate data collection while managing heat so standard comparisons can be made. Traditionally this has been done with actively cooled substrates but improvements in electronics, detector sensitivities, and sampling resolutions have opened a new option for this.

New Techniques For Perovskite Solar Technology Testing: Flash/Strobe-Based Testing

With the advent of light sources that could be rapidly and repeatedly brought to fully stable operating status within very short periods, the concept of flash testing has arisen.

By pulsing the light source of a solar simulator for between 1 ms to 30 ms, depending on the device design and experimental requirements, it is possible to record reliable responsivity and IV data for a solar cell without any undue heating occurring.

This allows testing to more easily maintain a standard temperature and thereby make temperature-agnostic comparisons of the electrical performance of different solar cells.

Beyond concerns for heat, short-duration light exposure testing also has the ability to give new information that is not easily achievable with continuous-wave solar simulation.

Short-duration phenomena such as charge transport lifespans/recombination timing can be more easily determined by measuring the time it takes for detected current from the cell to reach a chosen baseline after the flash has ceased through a process known as photocurrent decay measurement. 

Longevity Testing For Perovskite Solar Cell Technology

When testing solar cells, however, heat is not strictly unwanted.

Temperature is a measure of the amount of motion in an arrangement of atoms and since temperature changes are an inevitable experience in a solar cell’s lifetime, it is thus a major driving force for the long-term degradation of crystalline devices and materials. This is due to thermal motion within crystals displacing atoms from the lattice or causing diffusion of dopants within the crystal lattice. Thus the capability to simulate a long-term operation of a solar panel within a short time is a very useful ability.

By increasing the temperature of the device being tested beyond what would be experienced during normal operations (note: not beyond any temperature that would cause a phase change in the device that would alter the operational characteristics!), it is possible to rapidly “age” a perovskite cell to make educated predictions about the potential lifespans of any device being tested. 

This is considered an acceptable method as, barring catastrophic failures, in a fully encapsulated solar cell, heat is the only remaining source of degradation. Therefore, applying more heat (up to certain limits as stated above) is treated as equivalent to a longer time spent at lower temperatures.

Proper longevity testing as shown in the above example is not simply about preventing exposure to sources of defects or degradation, but rather controlling the amount of exposure to highlight the most useful data.

The prescribed figures of merit that are used as comparative standards are useful for judging the viability of a new technology against incumbents and competitors, but varying conditions beyond the standards is how researchers can expose undiscovered flaws or unexpected boons, and ultimately chart paths toward better devices.

The accelerated testing protocols for perovskites are potentially going to require additional experimental design considerations that vary slightly from traditional solar cell technologies in order to collect the same information. This is due to the different material behaviour from one formulation to another.

One of the chief remaining challenges for perovskite researchers is therefore to establish “objective, trusted validation methods that can adequately screen for real-world failure modes.” Some initiatives are working toward that end, such as the Solar Energy Technologies Office (SETO) funding of the Perovskite Photovoltaic Accelerator for Commercializing Technologies (PACT) Validation and Bankability Center.

The hope is that PACT’s field and lab testing will ultimately improve understanding and confidence in perovskite real-world durability.

What Applications Exist For Perovskite Solar Cells In The Near Future?

So now that we’ve discussed the history, construction, capabilities, potential problems, and test practices, where are people thinking of applying perovskite solar cell technology?

One possible application is the replacement of current large-scale panels in residential and commercial energy production. However, what less-obvious options are unlocked due to some of the unique characteristics of perovskites not shared by current commercial solar technologies?

Lightweight Perovskite Films And The Vehicles They Enable (VIPV)

For stationary solar devices, weight and flexibility are not primary considerations beyond potential compliance with standards.

With the reduced weight and higher flexibility brought on by the thin film perovskite construction, opportunities for integrated solar power in vehicular platforms are no longer seeming out of the realm of possibility for the mass market.

For some possible specialty applications, such as climate monitoring or long-duration observation of remote locations for ecological or national-interest purposes, the idea of a solar-powered drone, something not considered viable with traditional silicon solar cells, is very attractive.

NASA prototype solar powered long duration drone Helios. Weight savings are very important for aerial electric vehicles. This means that integration of lightweight flexible perovskite solar technology is of great interest to projects such as this.
NASA prototype solar powered long duration drone Helios. Weight savings are very important for aerial electric vehicles. This means that integration of lightweight flexible perovskite solar technology is of great interest to projects such as this. (source: https://www.nasa.gov/sites/default/files/images/370876main_ED01-0209-5_full.jpg)

Even simpler, the use of perovskites as a method of range extension for less specialized battery electric vehicles (BEVs), called Vehicle Integrated PV (VIPV), ranging in size from e-bikes and cars to private planes, is under consideration. 

With the ability to apply thin films to curved surfaces without the weight of thicker silicon panels, the amount of surface that can be covered by these devices is maximized while minimizing the amount of energy spent moving that weight.

Sadly, a fully solar-powered vehicle with “infinite range” for consumers is likely to be a fairly niche application, barring significant improvements to energy storage technologies. A built-for-purpose vehicle might be able to supply enough power, but given the generally low surface area available for solar cell integration even with flexible thin films, the use of thin-film integrated solar cells on BEVs is looking to be limited to a method of range extension rather than a sole power source.

Lightyear 0 solar powered personal car, rear ¾ view with full solar cell coverage of roof and rear visible.
Lightyear 0 solar powered personal car, rear ¾ view with full solar cell coverage of roof and rear visible. (source: https://www.forbes.com/sites/afdhelaziz/2020/11/19/why-lightyear-is-the-most-game-changing-car-company-on-the-planet/?sh=15763ce271ee)

Applications For Perovskite Integration Into Commercial Infrastructure

If reducing reliance on electrical infrastructure for your personal mobility isn’t enough, there is also potential for perovskites to be integrated more directly into residential and commercial building construction.

Standard roof or shingle mountings are well-known solar cell applications, and the benefits of lighter and more efficient collectors are fairly obvious, but the truly interesting application is how effective perovskites work as transparent solar panels in windows.

The idea of solar windows can seem a little counter-intuitive as windows are meant to let light through, while solar power is about capturing as much light as possible. Nevertheless, there has been research into the topic.

Some efforts focus on harvesting the non-visible portions of the spectrum (UV and IR) to allow for more normal-looking transparent windows.

The other school of thought is looking at more dual-purpose designs where shading and power generation are linked together to provide electricity while helping prevent the interior from overheating.

Reducing the need for air conditioning while also providing energy to be used in the complex itself during the hottest and thus most likely heaviest electrical demand periods can help stabilize energy usage for both large and micro-grids, and helps adapt our cities for the changing circumstances of the world.

Energy On The Go And Wherever You Are: Perovskite Wearables And the Internet of Things

The changing circumstances of the world are not merely environmental but also technological.

The drive for smaller and more integrated computer components has resulted in the dispersion of connected devices and sensors in more and more aspects of our lives. The Internet of Things (IoT), as it is called, is an exciting field full of possibilities such as the integrated examination and automation of our entire world in unprecedented fidelity.

Figuring out how to power all of those devices, especially the ones that are not able to be connected to the grid, is difficult. With high efficiency, low cost, low-light capable perovskite solar cells, the range of possible devices is expanded.

As an even more unexpected possibility, for devices that are either too small to have a battery attached, or too energy-intensive to have one last for long enough to be useful, researchers have begun incorporating perovskite solar technology into clothing.

This would work as a kind of power hub source to supply extra energy to some of the many commonly-used devices such as headphones, smartwatches, or fitness trackers.

Imagine being able to wirelessly recharge your phone just by putting it in your jacket pocket while you are out on a walk!

It could also open up options for new additions to clothing such as integrated light systems for fashion or safety, or supply a larger power budget for something like smart glasses.

Beyond the personal and into the more serious applications, there are a large number of applications where perovskites can save lives.

Disaster sites, refugee camps, and other locations where threats to people’s well-being exist are always in dire need of energy to run devices. This energy is needed both to improve conditions for residents/victims and support emergency workers during whatever operations they might find themselves undertaking in their aid.

Mobile power packs, able to charge themselves without relying solely on infrastructure that might not be in place, could mean the difference between life and death for someone.

Perovskites have incredible potential to be implemented in a wide range of important applications.

The vast scope of possible perovskite solar cell recipes, as well as the complexity of transport mechanisms, requires rapid and accurate testing. As perovskites are known to be sensitive to external factors such as heat, it is important to have methods of testing that are as controllable as possible without any excess exposure to otherwise invisible parts of the solar spectrum such as UV and infrared.

Perovskite Solar Cell Technology Summary

Perovskites are an exciting new field of solar technology that are promising impressive improvements to current solar technologies, but still require overcoming several barriers before mass adoption is truly viable.

  • Perovskite describes a structure, not a specific chemical composition
  • Mixing and matching components gives unprecedented design flexibility for building fit-for-purpose solar devices
  • Altering crystal structures can also give beneficial physical and electronic properties
  • Inexpensive and energy-efficient fabrication process options (spin-coating, roll-to-roll production, vapour deposition, electrodeposition) promise economic production options at large scales
  • Defect tolerance allows perovskites to perform at high efficiencies even without the single crystal structure of other solar cell materials
  • Multijunction construction is possible due to the flexibility of chemical composition allowing the bandgap to be shifted purposefully for different layers
  • High absorption coefficient means that thin films and low light operations are viable designs for perovskite cells
  • Perovskites utilize several different mechanisms for charge transport (lattice conduction, ion conduction, polaron motion, and charge injection) through the intrinsic layer to the CTLs, meaning there are multiple considerations for how to maximize collection due to each having their own optimal conditions.
  • Several barriers remain for perovskites to achieve acceptably long operational lifetimes:
    • Rapid degradation in UV
    • Dissolution by water exposure
    • Degradation through oxygen exposure
    • Degradation through excessive heat and/or cold
  • Testing for perovskites is currently nearly identical to testing other solar cells so equipment is transferable across projects
  • Lightweight, flexible, efficient perovskite solar cell would unlock novel applications for solar power generation
    • Vehicle Integrated Photovoltaics (VIPV)
    • Building Integrated Photovoltaics (BIPV)
    • Fabric Integrated PV (FIPV)

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