Aging and Characterization of High-Bandgap Perovskite Solar Cells

Long-term stability is an essential requirement for the industrialization of perovskite solar cells. Consequently, publications featuring aging experiments have surged in recent years (Figure 1). 

Solar cell degradation is inevitable. Commercial silicon solar cells typically degrade between 0.5% and 0.8% per year. Their guarantee lasts 25 years, but their performance decreases continuously during this period. [1] Stability studies aim to delay or slow down the degradation process as much as possible. Identifying the degradation mechanisms is a difficult task. It requires a holistic approach as all the solar cell parts (electrodes, interfaces, encapsulation) interact with each other, contributing to degradation, which evolves over time.  

The right approach to understanding degradation requires combining long-term performance measurement with advanced material and optoelectrical characterization techniques. The latter allows us to isolate and investigate the decaying part. Without them, stability data is open to interpretation. 

This blog post presents a temperature-dependent degradation study of perovskite solar cells optimized for a bifacial two-terminal tandem device. We observe that higher temperatures damage the perovskite layer rather than accelerating the degradation path. Meanwhile, devices aged at room temperature show an increased mobile ion density compared to fresh devices. 

These conclusions were possible thanks to the combination of material (XRD, SEM) and electrical characterization (JV, capacitance, and transients analyses). 

This study was in collaboration with the Laboratory for Thin Films and Photovoltaics at EMPA. 

Figure 1: percentage of studies that include stability studies of organic/hybrid material-based solar cells. [2] 

Methodology 

In this study, we tested perovskite solar cells made of a perovskite semiconductor (CsFaPbI3) with a bandgap of 1.57eV. The solar cell has a p-i-n configuration. This bandgap is ideal for use as perovskite top cells in bifacial two-terminal tandem devices. In bifacial tandem devices, the rear irradiance increases the current of the bottom cell. A perovskite with a narrower bandgap has a larger current that matches well with the larger current of the bottom cell. 

Figure 2 represents the device structure. The electron transporting layer consists of C60 and SnO2, with MeO-2PACz and ITO used as the hole-extracting layer and contact, respectively.

Figure 2: representation of the device stack for CsFaPbI3 perovskite with a bandgap of 1.57eV. 

The stability tests were conducted using the stress-test platform Litos from Fluxim AG. The instrument has four isolated chambers, allowing independent temperature and light intensity control. To ensure consistent results, parallel aging tests were conducted on four nominally identical samples, each with four lab-scale solar cell devices (pixels). The samples were subjected to different holding temperatures: 25°C, 45°C, 65°C, and 85°C.  

The light bias was a white LED calibrated to one-sun equivalent intensity. Air or humidity in any chamber was not controlled during the tests. The stability assessment was carried out through continuous Maximum Power Point (MPP) tracking. 

In addition to MPP tracking, JV characterizations were performed every 60 minutes to monitor the evolution of the photovoltaic (PV) parameters. For further analysis, impedance spectroscopy (IS), transient photocurrent (TPC), and open-circuit voltage decay (OCVD) measurements were conducted on fresh devices and after the stability tests. These measurements were performed using the all-in-one Paios platform from Fluxim.  

Ageing Results

For an initial evaluation of the performance decay with temperature, we compared the normalized MPP decay of the different perovskite solar cells we tested (Figure 3). 

The decay profiles have a comparable trend among the temperatures, i.e. there is an initial plateau with a rapid performance decrease reaching T80 (the point in time when the device reaches 80% of peak performance). The plateau lasts tens of hours at low temperatures and gets shorter with the increasing temperature.

Figure 3: normalized MPP decay of perovskite solar cells aged at 25°C. 45°C, 65°C, and 85°C. The black cross indicates the T80 lifetime.  

Plotting the T80 lifetime against time expressed in hours (Figure 4) shows a downward trend in the T80 lifetime as the temperature increases. Solar cells on identical substrates and aged at equivalent temperatures exhibit similar T80 values, a crucial aspect for statistical significance. With Litos Lite, you can replicate these stressing conditions across 56 devices simultaneously, making it an optimal tool for robust statistical validation

Figure 4: T80 lifetime versus stressing temperature. Multiple devices are kept at the indicated temperatures to obtain a significant statistical analysis.  

By performing intermittent JV characterizations during the MPP tracking, we recorded the evolution of the working parameters (Figure 5) of our solar cells. There is a strong agreement between the current decay at MPP conditions and the Jsc. Also, Voc and VMPP have a similar trend, decreasing over time at temperatures of 65°C and 85°C. At all temperatures, the Fill Factor decreases with time. 

Figure 5: circled lines are JV parameters from JV scans, continuous lines obtained from MPP tracking. Temperature increases with colors from blue to red.

A decrease in FF indicates either an increase in recombination, a decrease in extraction efficiency, or both.[3] A premature conclusion would be that considering the similarities of decay at all temperatures and the correlation between temperature and T80, temperature accelerates the ageing process.

We need additional characterization techniques to prove the point. 

Material Characterization 

A smaller T80 indicates faster ageing, but a higher temperature might introduce a new ageing mechanism. The devices' material characterization allows us to understand whether they had comparable degradation from the compositional and morphological perspectives.  

We first considered XRD measurements (Figure 6) to verify the presence of new secondary phases. The photoinactive delta phase is present in all aged devices compared to the fresh (unaged) one (black curve). Interestingly, the ratio of PbI2 to perovskite does not exhibit any correlation with temperature (not shown here). This finding suggests that a higher ageing temperature does not correlate with an increased perovskite decomposition.  

Figure 6: XRD measurement of fresh and aged perovskite aged at increasing temperature. 

The SEM images (Figure 7) of devices degraded at 25°C, 45°C, and 65°C reveal a noticeable effect of temperature on the perovskite layer morphology. Specifically, the formation of voids. As the temperature increases, the size and quantity of voids in the perovskite layer increase. This phenomenon indicates faster decay at higher temperatures correlates with reduced contact between the photoactive and transporting layers. Such morphological change in the perovskite layer impacts the charge transport and the formation of electrical shunts. Without proper electrical contact, the electrical characterization of devices aged at higher temperatures is unreliable. Therefore, we cannot conclude that temperature accelerates the same degradation path obtained at 25°C. 

To avoid potentially inconclusive and erroneous interpretations, we performed a detailed electrical characterization of the solar cell aged at 25°C by using PAIOS. In this device, we are sure that the contact with transporting layers is intact.  

Figure 7: SEM cross-sections of perovskite aged at 25°C, 45°C, and 65°C. 

Advanced electrical characterization 

To verify whether charge extraction or the recombination rate changes with ageing, we compared the Transient photocurrent (TPC) and Impedance Spectroscopy (IS) analyses of a fresh and an aged perovskite device (plots not shown here). Neither of the two measurements indicated the formation of an extraction barrier or increased traps.  

From the normalized IS (Figure 8), we observed that the aged device has a higher capacitance at frequencies below 100Hz. This means the aged device has a different mobile ions signature.  

Figure 8: dark impedance spectroscopy (IS) measurements of aged device (red line) fresh device (blue device). 

An increased ion density due to aging would explain the loss in current and the resulting fill factor decrease. We performed Open Circuit Voltage Decay (OCVD) measurements of fresh and aged devices to assess ion density on a device before and after ageing. OCVD keeps the device at open-circuit voltage conditions while under continuous illumination until it reaches steady-state conditions. The voltage decay is recorded right after the illumination is turned off.

Figure 9 is a representative OCVD curve for a perovskite solar cell. The profile entails two main voltage drops. The earliest, around 15 µs, is due to the recombination of excess free charge carriers. It is followed by a stretched shoulder between 1E^3 µs and 1E^4 µs, which is related to the response of the ions. The sharper decay at longer times can be associated with a loss of charges due to the shunt resistance or to the parasitic effects of the measurement setup. Implementing the analysis method developed by Fischer et al. [5], we determined the ion density from OCVD and intensity-dependent JV curves.  

Figure 9: representative response of perovskite solar cells to OCVD measurements.  

The analysis of light intensity-dependent OCVD measurements revealed different ion concentrations in fresh and aged perovskite solar cells. Figure 10 shows that an aged device has a higher ion density than a fresh device of almost two orders of magnitude. We attribute the increased ion density to the decomposed perovskite or ion migration from the transporting layers.[6]  

Figure 10: Ion density of fresh and aged perovskite, as obtained from OCVD measurement at increasing light intensities.  

Thiesbrummel et colleagues [7] reached similar conclusions with other characterization techniques, such as bias-assisted charge extraction (BACE) and charge extraction by linearly increasing voltage (CELIV). They supported their conclusions with drift-diffusion simulations with the software Setfos from Fluxim AG. They could only fit the experimental results obtained from aged perovskite devices by increasing the mobile ion density.  

Conclusions

This blog post presented our temperature-dependent degradation study of perovskite solar cells with composition CsFaPbI3 and a bandgap of 1.57 eV performed with Litos. The material characterization (XRD and SEM) revealed that temperature damages the morphology of the perovskite layer instead of accelerating ageing.

With the all-in-one platform Paios, we combined stability testing with advanced electrical characterization (TPC, IS, and OCVD) on our perovskite solar cells. Compared to the fresh solar cell, we observed a reduction in performance, which was correlated with an increase in mobile ion density in the aged devices.

This study demonstrates that additional characterizations are essential to investigate the degradation mechanisms. Stability measurements based on MPP tracking alone are insufficient and lead to misinterpretations.

We are confident of our results because, with Litos and Paios, we tested multiple solar cells in parallel, an essential aspect to ensure the statistical significance of our analyses.

Acknowledgements

This study was under the Supertandem project. Device fabrication and material characterization were performed by Kothandaraman Radha from EMPA.

References 

[1] D. C. Jordan and S. R. Kurtz, “Photovoltaic Degradation Rates—an Analytical Review,” Progress in Photovoltaics, vol. 21, no. 1, pp. 12–29, Jan. 2013, doi: 10.1002/pip.1182. 

[2] O. Almora et al., “A precise method for the spectral adjustment of LED and multi-light source solar simulators,” Advanced Energy Materials, p. 2303173, Dec. 2023, doi: 10.1002/pip.3776. 

[3] D. Bartesaghi et al., “Competition between recombination and extraction of free charges determines the fill factor of organic solar cells,” Nat Commun, vol. 6, no. 1, p. 7083, Nov. 2015, doi: 10.1038/ncomms8083. 

[4] M. Neukom, S. Züfle, S. Jenatsch, and B. Ruhstaller, “Opto-electronic characterization of third-generation solar cells,” Science and Technology of Advanced Materials, vol. 19, no. 1, pp. 291–316, Dec. 2018, doi: 10.1080/14686996.2018.1442091. 

[5] M. Fischer, D. Kiermasch, L. Gil-Escrig, H. J. Bolink, V. Dyakonov, and K. Tvingstedt, “Assigning ionic properties in perovskite solar cells; a unifying transient simulation/experimental study,” Sustainable Energy Fuels, vol. 5, no. 14, pp. 3578–3587, 2021, doi: 10.1039/D1SE00369K. 

[6] S. Kundu and T. L. Kelly, “In situ studies of the degradation mechanisms of perovskite solar cells,” EcoMat, vol. 2, no. 2, Jun. 2020, doi: 10.1002/eom2.12025. 

[7] J. Thiesbrummel et al., “Ion-induced field screening as a dominant factor in perovskite solar cell operational stability,” Nat Energy, Mar. 2024, doi: 10.1038/s41560-024-01487-w.