The performance of perovskite solar cells are continuously increasing, recently reaching efficiencies of over 20%. However, the stability of these devices is not as impressive as their efficiency: more comprehensive studies on degradation mechanisms and a deeper understanding of the weak spots of the different available stacks are necessary to bring perovskite solar cells to the next level.
We studied the stability of perovskite solar cells, encapsulated and measured in air, for two different structures: N-I-P (glass/ITO/TiO2/Perovskite/Spiro-OMeTAD/Au/barrier) and P-I-N (glass/ITO/NiO:Cu/Perovskite/PCBM/ZnO/Al/barrier). In several parallel experiments, we stressed both structures with light or with temperature, and we compared the results with devices under shelf-life conditions. We found that for both architectures, the main stress factor was light. Looking at the whole set of experiments, we observed different degradation pathways for P-I-N and N-I-P structures. For P-I-N devices we found the Aluminum contact as the main degradation factor, while for the N-I-P devices there are strong indications that the photocatalytic property of the TiO2 layer is the major cause of fast degradation. A second set of devices was stressed in dark, at temperatures increasing from 45 °C to 100°C: for P-I-N and N-I-P devices we observed that the deviation between shelf life and temperature induced degradation happened at different temperatures. The P-I-N devices, compared with the N-I-P devices, shown higher thermal stability.
The influence of encapsulation on device stability was also evaluated, with parallel experiments between encapsulated devices measured in air, and devices without encapsulation measured in inert atmosphere.
These experiments helped us to identify the weak spots in our devices. Moreover, we gained a better comprehension of the degradation factors for our devices: this helps us to choose the best ageing conditions for accelerated lifetime tests, allowing a faster screening of new stacks and helping us to focus our work on the most promising stacks. The understanding of degradation pathways will also help us to define encapsulation requirements and specifications.