Holman Research Group – Arizona State University Photovoltaics Research Case Study

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Exploring The Inner Workings Of Perovskite Silicon Tandem Solar Cells

The Holman Research Group at Arizona State University (ASU) focuses on developing solar energy by researching methods for manufacturing and characterizing solar cells. They have in-house capability to manufacture silicon, perovskite, and perovskite-silicon tandem solar cells. Additionally, their research partnerships extend further to work on CdTe, III-Vs, and more.

Due to the complex nature of tandem cells compared to monolithic materials such as silicon and CdTe, traditional characterization techniques are insufficient. No traditional gas-based solar simulator has the ability to control the spectrum being used to illuminate the device without the aid of external additions such as filters or frequency combs.   

Enter the Pico LED solar simulator from G2V Optics. With its spectral tunability and the ability to rapidly change between spectral presets, thanks to Python scripting, Ph.D. student Mason Mahaffey is able to confidently design experiments that would have been completely impossible with the light sources that were available to him prior to getting a Pico.

ASU Photovoltaic Cell Research Labs

Mason Mahaffey is a 3rd year Ph.D. student in electrical engineering at Arizona State University. He works under Professor Zachary Holman, in collaboration with the Rolston Lab under the oversight of Project TEAM UP.

TEAMUP is a cooperative effort to produce commercializable Perovskite/Silicon tandem cells which is funded by the US Department of Solar Energy Technologies Office (SETO). Both the Holman and Rolston labs are based out of the School of Electrical, Computer, and Energy Engineering of the Ira A. Fulton Schools of Engineering. Additionally, they work closely together at the MacroTechnology Works building at ASU.

“The Rolston Group and the Holman Group both work [together]. Our labs share resources, expertise, and space. ASU is a highly collaborative university and has lots of common lab spaces for researchers to use shared equipment.” – Mason Mahaffey, Ph.D. student at ASU

Mason’s research examines the potential for perovskite silicon tandem solar cells as next-generation improvements to current solar energy technologies. Relying on both traditional testing protocols (such as standard IV testing, and SunsVOC testing), and novel ones that have been produced in-house – these protocols can improve confidence in results by corroboration while simultaneously highlighting sources of error that any single test might have missed or obscured.

This proprietary protocol is called Suns External Radiative Efficiency (SunsERE) testing and was created by 2 years of joint work between Mason and the former Assistant Research Professor of the Holman lab, Dr. Arthur Onno. This innovative method of solar cell testing allows researchers to determine failure modes and reasons for deviation from theoretical expectations in IV analysis of solar energy devices.


The Rolston Research Group, Mason Mahaffey located mid-center in dark blue with white CAS text
The Rolston Research Group

The Challenges of Perovskite Tandem Solar Cell Testing

When exploring and developing new solar cell technology there is no straightforward path, most times you are left solving problems as they arise. Perovskite solar cells research is no different. While there are some challenges all perovskite researchers seem to share such as improving long term stability, Mason’s work is focused more on creating measurement tools and techniques to access the complete picture of a solar cell’s device performance. This means that Mason must be precise in his work in order to validate novel processes that can have unique barriers to overcome.

How Do You Address “Observer Effects” When Collecting Solar Cell Data?

When building test cells with novel constructions in a lab, the characterization has to be exceedingly thorough in order to determine the most likely paths to failure when trying to create larger, potentially commercially viable solar cell devices.

Any physical interaction with a solar cell sample for the purpose of determining electrical properties influences those same measurements. The simple and necessary act of applying electrical contacts to the devices can add layers of losses that make it nearly impossible to determine the properties of the interiors of the optical layers of a solar cell traditionally.

Even if they are necessary for the final operation of the devices, “observer effects” such as taking measurements through these contacts can obfuscate the actual properties of devices, displaying internal resistances significantly higher than the actual values, and hide sources of loss in plain sight. If these discrepancies are not discovered and accounted for then they could lead to incorrect results or interpretations related to the true performance of the solar cell.

Balancing between what is required to keep pushing forward and what becomes an observer effect is a fine balance all perovskite researchers must tackle.

Failure to do so could hamper the outcomes of the research.

Novel Solar Cell Testing Methods Create Unique Hurdles

Not all testing methods fit every type of perovskite solar cell research. While some do share commonalities, such as needing electrical connections between the sample, the sensor, and the measurement device, others require a more creative and flexible approach. Mason, Dr. Onno, and Professor Holman saw the obstacle and overcame it in a novel way.

This approach they dubbed the SunsERE.

Just like IV and SunsVoc, SunsERE allows for the creation of a device operational curve (called an implied JV curve). The implied JV curve reflects the full potential of a solar cell (a JV curve with perfect contacts). The implied JV curve can reveal the quality of the light-absorbing material of the solar cell and provide a check on device quality before the device is fully completed because it is measured optically.

SunsERE requires precise control over not only the wavelength of the incident light but also the exposure time on scales of tens of milliseconds.

The issue arising from this new way of analyzing research cells became obvious to the Holman group when it came to needing to switch on the scale of milliseconds without losing their precise control over the wavelength. A standard lamp or traditional solar simulator could only do one or the other, even when equipped with a physical filter addon, but not both.

How Do You Test A Device That Requires Different Inputs For Different Parts Of The Same Device?

Perovskites are a relative newcomer to the solar photovoltaic world, and there has been significant interest in determining the viability of combining the material in a multijunction system with traditional silicon solar cells. These ideas have impressive results, as the current efficiency record for these tandem cells is over 33%

With any trailblazing construction of a solar test cell comes some growing pains.

These pains arise when attempting to determine the most likely path of failure or trying to create larger and potentially commercially viable devices. Mason found that the original testing setup that had worked for single junction silicon cells could not provide useful data on new perovskite silicon tandem devices.

This issue was because tandem cells’ behaviour is wavelength dependent. Meaning, what can promote desired behaviour in one junction might be ignored entirely by the other junctions or even be detrimental. These detrimental setbacks can be bulk material degradation, surface defects between layers, or the spectrum that reaches the layers not matching the material’s bandgap, as some examples.

Equipped with all this knowledge and his experience, a new challenge for Mason and the Holman lab arose: how do you test a device that requires different inputs for different parts of the same device? 

This left Mason looking for a solution, as he was constrained by his existing broadband white LED light source.

A cutaway schematic of a perovskite silicon tandem cell and how it absorbers different frequencies of light in different subjunctions
A cutaway schematic of a perovskite silicon tandem cell and how it absorbers different frequencies of light in different subjunctions

ASU and G2V: Engineering A Solution For Perovskite Solar Cells Testing

Spectral Tunability: A Critical Criteria Which Allows For Multijunction Solar Cell Testing

Shifting to testing tandem solar cells from a previous focus on pure silicon solar cell devices necessitated some changes to how the Holman and Rolston labs performed their tests. This manifested in needing to collect data from two different PV materials that operate most effectively in different bands of light. A single broadband light source, whether a xenon-arc,  metal halide, or white LED lamp, was no longer sufficient or capable of providing useful light.

With the purchase of a Pico Class AAA solar simulator from G2V, Mason can now shift the wavelengths of light (using software), on his samples to specifically target the preferred wavelengths of each layer of his devices.

The tunability our LED solar simulator provides became key to carrying out their exploratory journey into tandem devices from pure silicon without the need for any physical spectral filters like what would be required with a more traditional broadband point source.

Python Programming for Rapid Perovskite Tandem Solar Cell Testing and Compatibility

Compounding the usefulness of the spectral tunability for Mason, the Holman, and Rolston labs is the ability to build unique programs for timing, intensity and spectrum with the integrated Python API for custom scripting capability.

“By far the advantages of [the Pico] is the ability to measure tandems accurately out to the spectral range available, the ability to tune the illumination, and code [in specific requirements for any researcher’s needs]. …There are definitely ways that I want to push the boundaries of this product to accommodate every little need that I have. It has simplified my research in many ways, [and] given me some ease in giving me great, reliable solar cell measurements.” -Mason Mahaffey – Ph.D. student at ASU

The ability to precisely control the exposure time of samples to specific wavelengths allows Mason to produce results in a significantly shorter time period than if he was having to deal with manually changing filters and operating shutters.

An interesting application the team at Holman labs worked out was the ability of the Pico to rapidly switch the light source not just in intensity but also across the spectrum. This state-of-the-art approach makes it easier for Mason and colleagues to pursue their research into improving methods for how to characterize these cells.

Combining spectral tuneability with Python scripting became the perfect pairing for their perovskite silicon tandem solar cell testing.

There was still one challenge that needed a solution, developing an efficient way to carry out their proprietary SunsERE application.

Multiple Solar Cell Testing Protocols For I-V Curve Confirmation And Illumination

Corroborating behaviours with other experimental procedures can help isolate the cause and magnitudes of confounding sources of error. For example, series resistance. Mason and the rest of the Holman and Rolston research groups are working to illuminate these faults and sources of resistance to produce more efficient perovskite silicon tandem solar cells that better match with theoretical and simulated examples.

When searching for the most accurate information about the performance of a device, it is important to factor in all possible sources of error and design experiments to be able to highlight these sources accurately. Standard IV curves are insufficient for determining performance characteristics for the cells that Mason, Professor Holman, and Rolston lab are working with.

To counter this, he built his testing programs around three different testing methods to evaluate and quantify as many sources of error as possible that can distort a traditional IV curve, and improve the information garnered from a standard IV test.

SunsVOC testing helps to corroborate IV testing for determining not only the current maximum power point of the device but the theoretical MPP without series resistance losses. SunsVOC testing is also simpler to collect information from than IV testing and allows for a more rapid collection of data from multiple experiments using different lighting parameters.

Utilizing the variable output of the Pico, Mason can quickly produce SunsVOC measurements across the entire working spectrum of any device he is working on. This quick measurement generates an accurate picture of where any efficiency losses might be localized in a particular layer or if there is a fault in the light-absorbing layer itself.

In order to be able to accurately reproduce the behaviour of the cell running through an IV sweep with SunsVOC, the intensity of the light is shifted from full one sun equivalent down to below the minimum 0.1 suns that the Pico is capable of usually.

To this end, G2V is continuing to work closely with Mason to provide attenuators to help drive the intensity down by an order of magnitude, expanding the scope of the tests over the entirety of their sample’s IV profiles.

To add a third layer of corroboration, the Holman lab pioneered work into the third measurement system, SunsERE. This process is still proprietary to this lab, so we cannot explain it in its entirety. We can share that it is a completely non-contact method of investigating the electrical properties of solar cell materials.

It uses different lighting conditions, spectral, temporal, and intensity varying, to determine the true IV curve without any series resistance effects via measuring radiative emissions after exposure to very precisely calibrated light sources using the Pico.

This acts as a calibrated bias source that can be tuned to interact preferentially with any specific band gap for subjunctions or different materials.

This capability allows the SunsERE process to be calibrated to work for any device in question, whether monolithic or multijunction.

By using a stand-off method for determining IV characteristics of photovoltaic materials, Mason and the Holman Lab can more accurately investigate the interactions between tandem layers and other multijunction devices due to minimizing the number of potential error sources.

The Future of Tandem Solar Cells and their Testing For ASU

“The Pico is particularly important for our research because we do research on tandem solar cells… These tandem cells absorb light differently at different wavelengths, and because of that, having a monochromatic light source, or even a visible light white light source, is insufficient for providing a full picture of the behavior for our solar cells.”

-Mason Mahaffey – Ph.D. student Arizona State University

Fortified with the right tools they need to validate their modules, The Holman lab, the Rolston lab, and the rest of Project TEAMUP are well-positioned to produce invaluable research on novel methodologies for investigating photovoltaic materials’ behaviour and performance.

This is exciting as it can lead to faster pathways for improving device efficiency, characterizing novel multijunction cells that utilize a wider spectral range than traditional cells, and bolstering attempts by these upcoming technologies to obtain commercial certification.

Together, G2V and research groups like the Holman and Rolston labs, where Mason Mahaffey is pursuing Ph.D. accreditation, are contributing to the design, improvement, and development of the technologies needed for a sustainable future.

Learn how our Pico and Sunbrick products replicate solar light and can advance your work.

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