Direct Observation of Conductive Polymer Induced Inversion Layer in n-Si and Correlation to Solar Cell Performance

Abstract

Heterojunctions formed by ultrathin conductive polymer [poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)—PEDOT:PSS] films and n-type crystalline silicon are investigated by photoelectron spectroscopy. Large shifts of Si 2p core levels upon PEDOT:PSS deposition provide evidence that a dopant-free p–n junction, i.e., an inversion layer, is formed within Si. Among the investigated PEDOT:PSS formulations, the largest induced band bending within Si (0.71 eV) is found for PH1000 (high PEDOT content) combined with a wetting agent and the solvent additive dimethyl sulfoxide (DMSO). Without DMSO, the induced band bending is reduced, as is also the case with a PEDOT:PSS formulation with higher PSS content. The interfacial energy level alignment correlates well with the characteristics of PEDOT:PSS/n-Si solar cells, where high polymer conductivity and sufficient Si-passivation are also required to achieve high power conversion efficiency.