Investigation of Charge Accumulation States in Polymer Solar Cells using Light-Induced Electron Spin Resonance Spectroscopy

Charge accumulation states in organic solar cells were studied in detail by a microscopic characterization with light-induced electron spin resonance (ESR) spectroscopy during device operation using a typical polymer solar cell of indium-tin oxide (ITO)/poly (3,4-ethylenedioxythiophene):poly (4-styrenesulfon ate) (PEDOT:PSS)/regioregular poly(3-hexylthiophene) (P3HT):fullerene [6,6]-phenyl C-61-butyric acid methyl ester (PCBM)/LiF/Al. Two light-induced ESR signals with a narrow and broad linewidth were observed where the broad component with a slow accumulation rate clearly correlated with the deterioration of the device performance. From the ESR analysis, the charge accumulation state causing the device deterioration is ascribed to holes at P3HT polymer-chain ends with residual bromines in amorphous regions in the active layer. Preventing the charge accumulation seems indispensable to develop highly durable polymer solar cells with high performance.