Device physics of perovskite solar cells
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Perovskite solar cell has attracted great attention recently because of its great potential. However, detailed information on electronic defects is still unknown. Here, we report Urbach energy of tail states (16 meV) with sub-gap QE measurement and two defect bands (0.24 eV and 0.66 eV) with CFT measurement. Meanwhile, the attempt-to escape frequency (~1011 Hz) of perovskite is calculated with our CFT data. Diffusion length of perovskite is measured by both photo-conductivity and QE vs. bias, and both methods give a diffusion length of 4-6 Ã Âµm for perovskite, which is the reason we can make high efficient solar cells with it.
Being the top cell of a perovskite-silicon tandem solar cell is one of perovskite’s most importance applications. In order to do that, the perovskite layer has to be sandwiched by two transparent layers, which makes the device a bifacial solar cell. In this project, we successfully made a bifacial perovskite solar cell by depositing CdS:In and ZnO:Al on top of perovskite. The CdS:In buffer layer effectively protects perovskite from plasma attacking during ZnO:Al sputtering. By optimizing the thickness of CdS:In, we achieved 14+% efficiency with light coming either from the top or bottom. This was the second best bifacial perovskite solar cell in the field at that time. The ZnO:Al layer also provided a great encapsulation to protect perovskite, which greatly improved the perovskite lifetime in ambient air from less than 20 minutes to more than 3 months.
As the top cell of perovskite-silicon tandem, the photo-stability of perovskite layer is extremely important. Systematic experiment and detailed device data analysis were performed to understand its degradation mechanism. The photon-induced degradation data of perovskite solar cell suggested that its degradation mechanism is much different from that of a-Si and organic solar cells. We found the degradation of perovskite under light exposure is attributed to the generation and migration of ions. We proposed an ion-generation-migration model which explains every device behavior during the degradation and recovery process of perovskite solar cell. Quantitative relationship between performance degradation and ion density was investigated by transient ionic current measurement. We also provided two approaches to mitigate photon-induced degradation. By increasing the grain size or adding excess PbI2, the degradation was mitigated by 50% respectively. The ion density we calculated from transient ionic current also showed significant reduction by applying those approaches.