Direct Observation of Conductive Polymer Induced Inversion Layer in n-Si and Correlation to Solar Cell Performance
Rongbin Wang, Yusheng Wang, Chen Wu, Tianshu Zhai, Jiacheng Yang, Baoquan Sun,* Steffen Duhm,* and Norbert Koch*
1Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nanoscience and Technology, Soochow University, 199 Ren-AiRoad, Suzhou 215123, China.
2Joint International Research Laboratory of Carbon-Based Functional Materials and Devices and Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices
3Institut für Physik and IRIS Adlershof Humboldt-Universität zu Berlin Brook-Taylor-Str. 6, 12489 Berlin, Germany
4Helmholtz-Zentrum Berlin für Materialien und Energie GmbH Albert-Einstein Str. 15, 12489 Berlin, Germany
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 for 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 Si-passivation are also required to achieve high power conversion efficiency.