钙钛矿太阳能电池稳定性分析的 ISOS 协议
Perovskite solar cells (PSCs) are one of the most promising innovative photovoltaic cells. PSCs can provide an opportunity to replace or improve traditional solar cells. However, the long-term stability of these devices is still under debate. Long-term stability is often overlooked when grabbing headlines and promoting investment hype for emerging solar cells. It should be clear that addressing the inherent stability issues of new PV technologies can reduce the risk of bad investments and avoid the hype dictated purely by high PCE commitments. Early stability analysis of laboratory-scale solar cells is critical for developing emerging photovoltaic technologies and accelerating the transition from laboratory to fab.
Existing qualification tests from the International Electrotechnical Commission (IEC) are designed to address field performance issues for silicon panels. Emerging photovoltaic technologies, such as PSCs, require tests tailored to their characteristics. In order to standardize the stability analysis of organic solar cells (OSCs), a series of stress protocols have been subsumed into the International Summit on Organic Photovoltaic Stability (ISOS), [Ree11]. Likewise, it has been suggested to address the mechanisms that affect the stability of PSC operation using a protocol suitable for this particular technology.
How do we conduct aging experiments for emerging PVs?
The recent Nature Energy paper [KHEN20] by some of the most prominent PSC researchers provides important guidelines and will be summarized here.
This blog post provides a quick guide to understanding the differences between the different ISOS protocols used to analyze organic and perovskite solar cells.
Standard ISOS Stability Protocol
The ISOS protocol can be defined by a combination of the following four stressors and their changes:
- Light (visible and ultraviolet): dark or 1-sun equivalent;
- Temperature: Ambient temperature, 65°C or 85°C;
- Environmental pollutants: inert, ambient, controlled humidity;
- Electrical Bias: Open Circuit (OC), MPP Tracking, or Fixed Voltage (Positive or Negative).
ISOS testing can be applied to individual solar cells, pure materials, incomplete solar cell stacks or micro-modules. The main goal is to ensure comparability of results from laboratory-scale equipment across laboratories. Unlike the IEC, the ISOS protocol is not intended to be a standard qualification test and cannot fail, but its results will improve understanding of the failure modes of the solar cell device under test. The ISOS protocol can be divided into 5 main test groups, each focusing on a different stressor:
● ISOS-D: dark storage/shelf life
● ISOS-L: light transmission
● ISOS-O: Outdoor test
● ISOS-T: Thermal cycling
● ISOS-LT: light-humidity-heat cycle
With more and more control over the test parameters, each group is further divided into three levels, it is a step in the sophistication and sophistication of the required laboratory infrastructure. For example, samples can be simply stored in the dark without controlling other parameters, or stored in the dark at specific temperature and humidity (Table 1, from ISOS-D1 to ISOS-D3). The different layers complement each other, and lower-level protocols are not more important than more complex ones. For example, the combination of ISOS-D3 with ISOS-D2 allows to study the effect of humidity on degradation in the dark.
Table 1: Dark storage protocols of increasing complexity (ISOS-D) (from ISOS-D1 to ISOS-D3).
You can find the combination of parameters that determine each protocol and its three levels in the Nature Energy Paper.
What is the purpose of each ISOS protocol implementation?
- ISOS-D (dark storage) tests resistance to oxygen, moisture and atmospheric components (eg: CO2, NOx, H2S). The ambient atmosphere promotes trap formation, perovskite decomposition, and surface charging.
- Using ISOS-L (light immersion) , defect and ion migration and phase segregation are accelerated by exposure. The type of light source used to perform the experiment must be mentioned, as it also affects the degradation dynamics.
- The purpose of ISOS-O (Outdoor) is to provide a realistic assessment of equipment life in a real environment. Although climate is geographically specific and cannot be reproduced, field tests provide information on failure modes and correlations between real weather and accelerated stress conditions.
- The ISOS-T and ISOS-LT tests (thermal and light-humidity-thermal cycling) are designed to study the effects of cyclic weather conditions (solar radiation, temperature and humidity), which are more detrimental than constant stress conditions due to ion migration contacts.
In general, temperature stress accelerates degradation caused by other factors, but some perovskite materials undergo phase transitions, so the effect of temperature is less pronounced and not isolated. For MAPbI3, it is recommended to use temperatures above the tetragonal to cubic phase transition.
Additional ISOS protocol for PSC
Perovskite solar cells present unique degradation mechanisms worthy of dedicated protocols. The community has added these protocols to existing protocols proposed for organic solar cells. The specific agreement of PSC is:
● ISOS-LC : light and dark cycle
● ISOS-V : Electric Bias in the Dark
● ISOS-I : Intrinsic Stability Test
ISOS-LC (light-dark cycle) was used to reveal information about "fatigue" (the effect of preconditioning and cell history on current performance) and perovskite metastability. The latter relies on phenomena such as ion mobility and reversible chemical reactions, which are only effective under cyclic stress conditions.
Using ISOS-V (electrical bias in the dark) , negative or positive bias stimulates ion migration, charge accumulation, and moisture-induced degradation. Both positive and negative stress voltages simulate real working conditions. In light, the solar cell operates at MPP or open circuit voltage when disconnected from the load, while negative bias simulates a shaded solar cell. Since the charge redistribution is (partially) reversible, the recovery conditions in dark storage should be considered after bias stress.
Schematic illustration of affecting perovskite PV stability. Image taken from reference [CHE21]
Ultimately, each protocol can be performed in an inert atmosphere or encapsulated in an ambient atmosphere. This allows a distinction between intrinsic and extrinsic stressors. The latter depends on cell-environment interactions, while the former includes light, temperature, and electrical bias. Degradation of the encapsulant can mask the degradation of the PSC device, so testing to avoid this problem is necessary.
Any of the above protocols fall under the ISOS-I (Intrinsic Stability Test) protocol if performed in an inert atmosphere such as argon or nitrogen and without encapsulation . In this way, it directly addresses the inherent stability problem of solar cells.
Schematic illustration of affecting perovskite PV stability. Image taken from reference [CHE21]
Considerations when evaluating perovskite solar cell stability
To ensure reproducibility of aging experiments, it is good practice to thoroughly document measurement conditions and sample preparation. Especially for perovskite solar cells, it is recommended to:
- Parameters that were not recorded during the aging process were also monitored.
- Perform experiments on multiple cells for statistical purposes and to compensate for the low reproducibility of PSCs.
- Do not refer to the performance of equipment different from the equipment used in the burn-in experiment.
- Don't just measure fast JV scans.
- Be sure to mention the preprocessing of the sample's previous stress history.
- Please declare details about the light source (irradiance, spectrum and type of light).
- JV was measured periodically during the aging process. Preferably in a quasi-steady state. Better MPP Tracking (MPPT). Optimal MPPT combined with regular JV and non-destructive characterization.
FLUXiM developed 3 different instruments to assess the stability of Perovskite Solar Cells
How to compare analytics obtained on different devices?
As for measurements, the analysis of the results also needs to be reliable. The figure of merit helps to compare different devices, and a typical metric used to assess the stability of a solar cell device is the time it takes to reach a certain initial percentage of efficiency.
The most common is T80 , which is the time it takes to reach 80% of the initial efficiency during aging experiments. Despite the simplicity of the concept, determining the T80 of perovskite solar cells is not straightforward. PSC devices under continuous stress exhibit several trends (Figure 1):
- Rapid drop in initial efficiency (so-called aging effect);
- Efficiency recovery after stress removal;
- Non-monotonic behavior with maximum efficiency after hundreds of hours.
Figure 1: (ad) Possible PCE curves when aged under 1-day light.(e) Evolution of PCE over time in the presence of "aging" effects. (f) Evolution of PCE over time in the case of non-monotonic behavior.
Therefore, it is difficult to determine the effective initial efficiency. Variations in the figure of merit T80 can accommodate these uncertainties. TS80 is the stable T80 time, which is 80% of any "stable" efficiency after aging or the maximum absolute efficiency for non-monotonic evolution. If the efficiency recovers after stress relief or cyclic stress, T80 needs to be corrected accordingly.
State-of-the-art PSC devices require more than 1000 hours to reach T80, but longer burn-in times are a challenging task for some laboratories. In this case, the efficiency after 1000 hours (η1000), expressed as a percentage of the initial efficiency, can be used as a stability figure of merit.
Ultimately, once the PSC device becomes more stable, the figure of merit T95 should be considered, which is also in compliance with IEC procedures.
Device lifetime obtained under specific aging conditions in a single protocol is an important figure of merit for understanding sample improvement. On the other hand, the combination of ISOS protocols that differ in only one parameter allows you to understand the effect of stressors on the device. Figure 2 is a graphical representation of how the ISOS protocol is chosen to study the effects of individual stress factors: by fixing light, electrical bias, and temperature (blue arrows), you can study the effects of various atmospheric conditions; by fixing temperature, atmospheric conditions and light (green arrow), you can study the effect of electrical bias.
Figure 2: Parameter combinations (left) to understand the effect of individual stress factors (right).
in conclusion
Emerging thin-film photovoltaics, such as perovskite solar cells, have achieved expected performance, but low reproducibility hinders the possibility of moving from laboratory to industrial scale. Since thin-film photovoltaic materials exhibit different properties than silicon modules, customized aging experiments are required.
Originally created for organic photovoltaics, the ISOS protocol can be adapted to the unique properties of perovskites and used to ensure comparability of aging tests performed in different laboratories.
FLUXiM has been supporting the development of innovative solar cells for over a decade. Released in early 2020, our stability assessment tools, LITOSand LITOS LITE, are designed to address solar cell stability issues while adhering to the standards set by ISOS. These instruments are designed to perform parallel JV and stability measurements on organic and perovskite solar cells of small areas (mm2 to decimeters) using custom-made sample holders. Leading research laboratories such as Fraunhofer ISE, EPFL and Georgia Institute of Technology are already using these instruments.
LITOS and LITOS LITE can acquire the JV characteristics of up to 56 solar cells and keep them at different operating points (MPP, Voc, Jsc) to increase the experimental output. Using LITOS, accelerated degradation analysis can be performed at a maximum light intensity equivalent to 10 suns and strong UV radiation. LITOS LITE works with AAA rated or higher LED solar simulators to help you with indoor and outdoor light experiments.
We designed these instruments with a fundamental idea in mind: solar cells must be characterized and emphasized at the same time. On perovskite solar cells, JV properties are typically collected at low scan rates, and testing multiple samples in tandem is time-consuming. Furthermore, the different devices on the substates may be too unstable to measure them in series under continuous illumination. If we want to have enough statistics to draw reliable experimental conclusions, we need to control the temperature, environment and lighting of the sample, and then collect the curves at the same time.
LITOS can also be coupled with the all-in-one semiconductor parameter analyzer PAIOS. The combined platform automates the continuous pressurization and testing of solar cells. Stress routines can be paused to perform advanced electro-optic characterization. Different experiments, such as transient measurements (CELIV, TPV, TPC, TEL), impedance spectroscopy, CV, IMPS/IMVS, can be performed on one platform without touching the sample. The post-analysis stress routine is restarted and can be paused again to perform the same high-level characterization at a different degradation point. If we then compare cell parameters (e.g. mobility, trap density, charge density) for different aging periods, we can highlight the mechanisms that lead to device degradation. This characterization strategy can also help standardize aging experiments for perovskite solar cells by reducing sources of random error.
