Study of degradation mechanism in halide perovskite solar cells using impedance and modulus spectroscopy
Résumé
Organic inorganic hybrid halide perovskites have emerged as an innovative material with excellent optoelectronic properties, in bulk or as single crystals with low defect density with specific morphologies [1]–[3]. Perovskite solar cells (PSCs) have become a trending technology in photovoltaic research due to a rapid increase in efficiency in recent years. [4].
However, they show a degradation of their performance under operational conditions (light, bias, environmental stress, etc.). To increase their long-term stability is one of the biggest challenges for market applications. The presence of strong internal electric fields, the existence of ferroelectric domains, or the diffusion of ions/defects are suggested as possible causes for the degradation processes. Several authors suggest the existence of native vacancy defects in these materials and attribute to those defects a major role in the control of the optoelectronic properties, such as hysteresis in the photo-induced current-voltage curve, as well as in device degradation and lifetime. [5], [6]
Among various degradation mechanism in PSCs, it’s important to understand the mechanism in both bulk perovskite and at the interfaces between perovskite layer and transport layers.
Electrochemical impedance spectroscopy (EIS) is a powerful technique to examine the charge carrier dynamics in perovskite solar cells. It gives insight about internal electrical processes in PSCs and distinguishes between bulk and interfacial processes [7]–[9]. Each physical parameter can be extracted in the form of resistance (R), capacitance (C) and Warburg capacitance (W) using an equivalent electrical circuit model. In addition, dielectric modulus is applied for studying the microscopic mechanism of charge transport, contribution of grains (crystals) and grain boundaries and recombination dynamics [10].
In this study, we focus on the degradation of inverted planar structure perovskite solar cells through impedance and modulus spectroscopy. Two samples with the same structure are prepared and stored in air and under dark for 30 days. EIS is measured periodically, on the first sample, without J-V measurement to prevent EIS results from being affected by electrical field that occurs during J-V measurements, while J-V characteristic is measured periodically on the second sample. From combined impedance and modulus data, we confirmed that grains and grain boundaries can be distinguished. The results of modulus spectroscopy imply that the grain size decreases and grain boundaries increases which accelerates ionic accumulation and electronic polarization at interfaces. The results of J-V measurement confirms this hypothesis. Furthermore, we prepared two types of PSCs with electron transport layer based on wet- and dry- processes in order to investigate the effect of solvent on degradation mechanism.