Improved photovoltaic performance of un-doped FAPbI3 thin film based solar cells via de-trapping excitons

Abstract

Stable and photoactive un-doped alpha-phase formamidinium lead iodide (alpha-FAPbI3) thin film can be formed on top of the P3CT/ITO/glass sample via using a dimethylformamide:N-Methyl-2pyrrolidone:dimethyl sulfoxide (DMF:NMP:DMSO) mixture as the precursor solvent. The absorbance spectra, photoluminescence spectra, X-ray diffraction patterns, and Raman scattering spectra show that the molecular packing structure of the fullerene electron transport layer largely influences the photovoltaic performance of the resultant inverted-type FAPbI3 solar cells due to the formation of induced delta-phase FAPbI3 from the amorphous region. In the optimized FAPbI3 solar cells, the power conversion efficiency (PCE) largely increases from 7.64 % to 12.31 % after 15 days due to the formation of photoactive alpha-FAPbI3:delta-FAPbI3:PbI2 composite thin film. Our findings show that it is possible to increase the PCE of un-doped alpha-FAPbI3 solar cells via increasing the phase transition from amorphous FAPbI3 to alpha-FAPbI3.