Unraveling surface and bulk trap states in lead halide perovskite solar cells using impedance spectroscopy

Organic–inorganic hybrid perovskites (OIHPs) have been widely recognized as an excellent candidate for next-generation photovoltaic materials because of their highly efficient power conversion. Acquiring a complete understanding of trap states and dielectric properties in OIHP-based solar cells at the steady state is highly desirable in order to further explore and improve their optoelectronic functionalities and properties. We report CH₃NH₃PbI_3−xCl_x-based planar solar cells with a power conversion efficiency (PCE) of 15.8%. The illumination intensity dependence of the current density–voltage (J–V) revealed the presence of trap-assisted recombination at low fluences. Non-destructive ac impedance spectroscopy (ac-IS) was applied to characterize the device at the steady state. The capacitance–voltage (C–V) spectra exhibited some distinct variations at a wide range of ac modulation frequencies with and without photo-excitations. Since the frequency-dependent chemical capacitance $\left(C_{\mu }\right)$ is concerned with the surface and bulk related density of states (DOS) in CH₃NH₃PbI_3−xCl_x, we verified this by fitting the corresponding DOS by a Gaussian distribution function. We ascertained that the electronic sub-gap trap states present in the solution processed CH₃NH₃PbI_3−xCl_x and their distribution differs from the surface to the bulk. In fact, we demonstrated that both surfaces that were adjacent to the electron and hole transport layers featured analogous DOS. Despite this, photo- and bias-induced giant dielectric responses (i.e. both real and imaginary parts) were detected. A remarkable reduction of $C_{\mu }$ at higher frequencies (i.e. more than 100 kHz) was ascribed to the effect of dielectric loss in CH₃NH₃PbI_3−xCl_x. (paper)