Impact of Pseudohalide-Based Cesium Formate Salt as an Additive or Substituent on Perovskite Solar Cell Performance

Abstract

The dual roles of pseudohalide formate anions (HCOO⁻), acting as both additives and substituents in perovskite solar cells (PSCs), are investigated and compared. An excess amount of CsHCOO added to the FA0.9Cs0.1PbI3 perovskite, serving as an additive, enhances the photovoltaic performance of PSCs. We observed a significant enlargement of perovskite grain size, improved perovskite crystallinity, preferred facet alignment parallel to the substrate, and reduced PbI2 formation after incorporating CsHCOO additive into the perovskite film. This was achieved by slowing the perovskite growth rate through formate-iodide anion exchange. Moreover, the strong coordination between the formate anion and Pb2+ inhibits the formation of metallic Pb0 on the perovskite film surface, passivating surface defects. Benefiting from these improvements, reduced trap density and surface defect passivation lead to suppressed carrier recombination and longer carrier lifetimes. Incorporating 3 mol% CsHCOO additive in n-i-p structured PSCs improves the power conversion efficiency (PCE) from 17.12% to 18.57%, compared to PSCs without CsHCOO. Additionally, CsHCOO-added PSCs exhibited superior ambient stability, retaining 90% of their original PCE after 1000 hours in a 25±5% relative humidity environment without encapsulation. For the p-i-n structured PSCs, 3 mol% CsHCOO additive enhanced the device PCE to 20.04%, outperforming the device withot CsHCOO additive (PCE = 18.01%). In contrast, substituting CsI with CsHCOO in the perovskite active layer, with a nominal formula of FA0.9Cs0.1PbI3-x(HCOO)x (x=0.03 and 0.1), induced significant PbI2 formation and non-uniform perovskite films, resulting in reduced PSC performance. Our findings highlight the pivotal role of CsHCOO as an additive in enhancing PSC performance by addressing various challenges related to perovskite film quality and device stability.