Enhancing π-Delocalization and Suppressing Traps via Doping in Electron Transport Materials for Efficient Semitransparent Organic Photovoltaics

The ultrathin metal electrode in semitransparent organic photovoltaics (STOPVs) usually suffers from limited charge collection capability and conductivity and thus hinders the power conversion efficiency (PCE). Herein, a new strategy of enhancing the π-delocalization of electron transport layer (ETL) via lithium bis(trifluoromethanesulfonyl)imide doping is developed. The enhanced π-delocalization in ETL benefits sizeable intermolecular π-π overlap, prone to harvesting electrons and thereby improving charge collection range. Doping also improves the conductivity of both ETL and ultrathin silver electrode. Furthermore, the trap densities in ETL and STOPV devices are reduced after doping, contributing to suppressed recombination and higher PCE. Consequently, ETL doping maintains an average visible transmittance of ~ 30% while promotes the PCE of STOPVs from 13.0% to 14.3% and light utilization efficiency from 3.74% to 4.15%, which is among the highest values of optical structure-free STOPVs. This work provides a new insight of π-delocalization manipulation in ETL for efficient STOPVs.