Identifying the Nature and Location of Defects in n–i–p Perovskite Cells with Highly Sensitive Sub‐Bandgap Photocurrent Spectroscopy

Defects that exist in perovskite semiconductors and at their interfaces with charge transport layers limit the performance of perovskite solar cells (PSCs). Highly sensitive photocurrent measurements reveal at least two sub‐bandgap defect states in n–i–p PSCs that use tin oxide covered with [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) as the electron transport layer and tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA) as the hole transport layer. Semitransparent PSCs with an optical spacer‐mirror bilayer on top are used to modulate the interference of light. By varying the thickness of the optical spacer and analyzing the changes in the photocurrent spectra using optical simulations, the defect states that produce photocurrent with sub‐bandgap excitation are found to be located near the PCBM‐perovskite interface. This conclusion is supported by quasi‐Fermi level splitting measurements on perovskite n–i–p half stacks. The observations are explained by an enhanced extraction of trapped electrons from the perovskite at the interface with PCBM.