N‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

Abstract

Doping in an organic photovoltaic (OPV) device can largely impact its performance. In this work, it is discovered that n‐type electrical doping in the poly(3‐hexylthiophene):[6,6]‐phenyl‐C61‐butyric acid methyl ester active layer can be induced by a certain electron transport layer (ETL), particularly branched polyethylenimine (PEI). Consequently, OPV devices with different ETLs exhibit dramatically different current density–voltage behaviors and external quantum efficiency (EQE) spectra. Using drift‐diffusion modeling, Hall effect, and capacitance–voltage measurements, it is shown that the difference in EQE spectra originates from the different background carrier type and concentration in the OPV active layer, dictated by the specific properties of ETL. Factors that influence electrical doping and device behaviors, such as difference between PEI and another polymeric ETL, polyethylenimine ethoxylated, and the active layer thickness and energy of negative integer charge‐transfer state are elucidated. These findings provide insight into material selection and device design for organic electronic devices.