Design of Substituent in Tertiary Phosphine Oxide for Surface Defect Passivation in Perovskite Solar Cell

Organic-inorganic hybrid halide perovskites receive significant attention since the report of a 9.7%-efficient and 500-h-stable solid-state perovskite solar cell (PSC) in 2012. Owing to their intriguing optoelectronic characteristics and simple solution processability, power conversion efficiency (PCE) of PSCs swiftly increased from 9.7% to 26.1% in a decade. However, solution process and ionic nature of the perovskite might generate defects on the surface and grain boundary, which can affect device efficiency and stability. Interfacial engineering was proposed as one of effective methods to passivate defects. Here, we report effect of substituent in tertiary phosphine oxide (PO) passivating materials on photovoltaic performance and stability of PSCs, where the studied substituents were alkyl group with different chain length (triethylphosphine oxide (TEPO), tributylphosphine oxide (TBPO), trihexylphosphine oxide (THPO)) and cyclohexyl, phenyl, and hexterocyclic groups (tricyclohexylphosphine oxide (TCPO), triphenylphosphine oxide (TPPO) and TMPPO). From the current-voltage measurements together with theoretical calculation, TMPPO was found to be most effective substituent. Density functional theory (DFT) calculation revealed that TMPPO interacts with perovskite surface via binding of both oxygens in P=O and morpholine (oxygen in cyclohexyl group) with undercoordinated Pb on the perovskite surface. X-ray photoelectron spectroscopy showed that the Pb 4f peak shifted to lower binding energy, which confirmed the interaction via electron donating from TMPPO to the undercoordinated Pb²⁺ ions. Photoluminescence, space-charge limited current and electrochemical impedance spectroscopy were studied to understand the defect passivation. The recombination by surface traps was found to be substantially reduced by TMPPO, which eventually improved the power conversion efficiency from 21.78% to 23.72% mainly due to the improved open-circuit voltage. Moreover, operational stability was improved by the TMPPO post-treatment and bare perovskite film maintained its alpha phase over 1000 hours under light at ambient condition.