Temperature-dependent time response characteristic of photovoltaic performance in planar heterojunction perovskite solar cell

Abstract

In recent years, perovskite solar cell (PSC) has achieved power conversion efficiency as high as over 20 %, making it competitive with high-efficiency thin film solar cells such as CuInGaSe and CdTe solar cells. However, the critical issue of reliability and stability for PSC should be addressed since a significant degradation of photovoltaic (PV) performance at low temperature has been found regardless of planar mesoporous PSC. To reveal the degradation of PV performance in PSC, the temperature-dependent PV performance of the planar PSC is investigated in detail. A PSC sample is loaded into a cryostat chamber connected to a compressor and illuminated by a halogen lamp. The operating temperature varies from 200 K to 325 K and the current-voltage (J-V) characteristic of planar PSC is measured at different scan rates from 10 V/s to 0.0017 V/s. At a fast scan rate of 10 V/s, the PSC shows a low PV performance at either low temperature or high temperature. The short-circuit current (JSC), open-circuit voltage (VOC) and maximum power point (PMPP) are found to decline with the temperature decrteasing. Moreover, the J-V curve also shows the S-shape characteristic. This suggests that the inefficient transport of photo-generated carriers occurs in the PSC. Ions such as Pb2+, CH3NH3+ and I-vacancies cause the screening effect of built-in field and the photo-generated carriers cannot be separated nor collected efficiently. As a result, JSC and VOC show small values in J-V curves measured at a fast scan rate. However, the degradation in PV performance is temporary. The PV performance gradually reaches a steady state at different operating temperatures with scan rate going down to 0.0017 V/s. The PMPP and VOC increase with temperature decreasing. These results indicate that a long illumination time is necessary for PSC to reach a steady state. After long-time illumination under biased condition (i.e., J-V curves measured at slow scan rate), the built-in field is compensated for by the external bias and the ions piling in the interface regions have enough time to diffuse towards the opposite direction. Thus, the screening effect of built-in field is reduced and the PV performance of PSC reaches a steady state. According to the result of device simulation, the increasing VOC at low temperature is attributed to the enhanced built-in potential difference and the reduced recombination rate of carriers. The temperature-dependent external quantum efficiency measurements of planar PSC before and after light illuminationis are performed to investigate the mechanism of carrier transport. It reveals that the separation and collection efficiencies of photo-generated carriers can be improved significantly after light illumination due to the fact that the screening effect of built-in field is reduced. These findings help understand the carrier transport mechanism in planar PSC.