Perovskite Solar Cell Stability Measurements
Perovskite solar cells have significant stability challenges that must be addressed before they can be considered suitable for large-scale manufacturing. In the early stages of perovskite solar cell production, stability issues were rarely reported or addressed in scientific papers. However, extensive research has been conducted since then, with ongoing efforts to improve the stability of these solar cells.
Starting with a standard I-V curve (or J-V curve) measurement is essential. However, perovskite solar cells are susceptible to an effect known as hysteresis. This means that the device's efficiency, as measured through a forward voltage sweep, may differ from that obtained through a backward voltage sweep. As of 2023, the most credible explanation for this effect is the migration of ions through the device once the voltage is applied. Consequently, relying solely on a J-V curve may sometimes make it challenging to accurately measure operational efficiencies. Therefore, you should aim to take some stability measurements in your PSC research to show how your device will work in real life conditions.
So how should you measure perovskite solar cell to show both stability and efficiency? The current stability standards for silicon PV dictates industrial PV solar panels must last for 20-25 years in real world conditions to be fit for market. Perovskite solar cells, being an emerging technology, are still in the developmental stage and cannot yet meet these industry standards. However, this does not mean that stability issues should be overlooked or disregarded. It remains an active topic of discussion in research communities, where scientists are striving to define accurate assessment methods for both perovskite solar cell efficiency and stability.
In this article, we will outline some of the suggested protocols for measuring perovskite solar cells, as proposed by the International Summit on Organic Photovoltaic Stability (ISOS). Additionally, we will demonstrate how the Ossila IV testing software can be utilized to measure solar cell efficiency.
Guide to Measuring Stability of Perovskite Solar Cells
A 2020 paper published in Nature Energy titled "Consensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS procedures" (Citation: M. V. Khenkin et al., Nature Energy, 2020, 5, 35-49) addresses various challenges faced by perovskite solar cells. The paper suggests a checklist of measurements that researchers should consider to assess their devices more accurately and comprehensively. Implementing these protocols will enable researchers to test perovskite devices uniformly, thereby advancing research at an accelerated pace.
Title: Consensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS procedures
Citation: M. V. Khenkin et al., Nature Energy, 2020, 5 35-49.
From this paper, we have summarized some of the suggested measurements, including some of the details about these experiments that you should report. These are listed in the table below.
| Aim | Measurement | Details that should be reported |
|
When initially characterizing devices |
J-V curve Performance tracking (such as MPP tracking or current stabilization) EQE |
Details about the light source and solar simulator used (type, spectral irradiance, intensity, calibration steps) measurement scan speed and any preconditioning done before. Tracking protocol (including test duration and algorithm used) Calculated JSC compared to measured JSC (for EQE measurements). |
|
For any aging studies |
Periodically recorded J-V curves Extended performance tracking (MPP tracking, stabilized metrics) |
Frequency of measurement Parameters of individual scans (see initial characterizations) Measurement environment (light, temperature, atmosphere, bias condition e.g. held at VOC) |
Taking Stability Measurements
Even if you don't see hysteresis in your PSC I-V curve, we recommend performing some sort of stabilized measurement on your PSC devices to support your initial metrics. The Ossila Solar Cell I-V Test System allows you to take stabilized current measurements quickly and easily.
Additionally, the Lifetime Testing programme in the Ossila Testing Software will measure J-V sweeps intermittently over a set time period at given intervals. This feature is particularly useful for conducting aging studies on perovskite devices, in order to assess their robustness in various situations. You can conduct these measurements inside a glove box to test intrinsic defects only. You can also measure devices in ambient conditions, with appropriate encapsulation. The more stability measurements you take, the better understanding you gain of how well your solar cell will perform in real-world conditions.
Stabilized Current Measurements
Before taking stabilized current measurements, you will need to measure the current-voltage curve for your chosen pixel(s). From this measurement, you can find the maximum power value, and the corresponding maximum power voltage (VMP). This is the voltage where your device will be at its best operational performance.
During the measurement, you will hold your chosen pixel at the maximum power voltage and track the output current for the duration of your measurement. The Ossila Solar Cell Testing Kit comes with free, compatible software which will perform this stabilized measurement for you. All you need to do is input the device area and choose a holding voltage, and the system will run the measurement for the desired time.
In some systems, like the Ossila I-V testing software, the holding voltage may adjust slightly to maximize performance. By knowing the holding voltage and measured current, you can calculate and track power output over time.
It is standard protocol to conduct stabilized current measurements over a minimum time frame of 1 minute. However, if the device does not achieve a stable output within this timeframe, consider extending the measurement duration.
By performing stabilized current measurements, you can further investigate the operational performance of your devices. A stable PSC device will maintain consistent efficiency over time, similar to the initial sweep. In this case, you have shown that you're J-V sweep is accurately measuring the performance of your device.
However, for some devices as you track the efficiency over time, the measured efficiency will start to fall. These devices will likely have shown hysteresis in their J-V sweeps, but sometimes this can reveal another problem in the device. In some cases, the efficiency of the device will keep falling, indicating a serious stability issue that could be missed with a J-V sweep alone.
Often the devices will stabilize at a lower PCE than the initial sweep. This initial drop in performance is often referred to a performance "burn-in". In most cases, the output performance will be slightly lower than the sweep as the system adjusts to constant measurement. However, if this stabilized PCE is significantly lower than the initial PCE from the J-V sweep, this may indicate a more serious device issue.
In all three cases, taking a stabilized current measurement is a quick and easy way to investigate further the true operational performance of your devices.
Solar Simulator
