Charge Formation in Pentacene Layers During Solar-Cell Fabrication: Direct Observation by Electron Spin Resonance

Microscopic characterization of charge carriers in solar cells is useful for high-performance cell fabrication because the formation and accumulation of charges in cells greatly affect the device performance. Electron spin resonance (ESR) is suitable for such characterization because it can directly observe charge carriers with spins in these cells. In this work, the ESR method is applied to organic thin-film solar cells to investigate charge formation in such devices. Heterojunction cells of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/pentacene/C60/bathocuproine (BCP)/Al are investigated. Clear ESR signals are observed by inserting a typical PEDOT:PSS hole buffer layer. From analysis of the dependence of the ESR characteristics on the external magnetic field direction, the bias voltage, and the duration of solar-simulated irradiation, the charges (mobile holes) in pentacene layers are successfully identified and it can be deduced that these holes are formed at the PEDOT:PSS/pentacene interface during device fabrication. This ESR analysis provides useful knowledge for understanding device operation and improving device performance at the microscopic level.