Dissipation of Charge Accumulation and Suppression of Phase Segregation in Mixed Halide Perovskite Solar Cells via Nanoribbons

Mixed halide hybrid perovskite compounds such as MAPbI1.5Br1.5 are among highly acclaimed absorber materials for solar cells due to their easy tunability of band gap. These compounds are intrinsically unstable and often demix into separate phases upon white light illumination, and this undesired process commonly known as photoinduced phase segregation presently impedes the development of mixed halide perovskite (MHP)-formulated solar devices. Understanding the pathways that lead to such phase segregation and controlling them are the key to solve their device instability problems. The miscibility gap that I/Br possesses at room temperature easily allows the separation of the mixed phase of MAPbI1.5Br1.5 into two phases as MAPbBr3 and MaPbI3 on irradiation, which leads to a continuous decrease in Voc in these devices. The low-lying valence band (VB) of the segregated iodine-rich phase facilitates trapping of holes, leading to migration of I– ions toward grain boundaries. Here, we investigate the effects of TiO2-intercalated graphene nanoribbons (GNRs) to hinder the migration and accumulation of I– at grain boundaries. It is found that the remarkably low charge resistance and low electron impedance of the modified electron transport layer (ETL), along with a reduced polaron-induced strain gradient in MHP due to GNRs facilitate against phase segregation, hole accumulation, and carrier recombination. This enables the device to retain its Voc unlike in devices without GNR wherein Voc decreases to zero in less than 60 min of illumination. Using conductive atomic force microscopy (AFM) studies, we reveal the electrical conductivity of TiO2 and TiO2-GNR films. We conclude by positing that TiO2-GNR with improved charge transport properties reduces electric field-induced halide ion migration that may give rise to a decrease in Voc upon continuous light illumination.