Mechanistic Insights into UV-Induced Electron Transfer from PCBM to Titanium Oxide in Inverted-Type Organic Thin Film Solar Cells Using AC Impedance Spectroscopy

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Abstract
An inverted organic bulk-heterojunction solar cell containing amorphous titanium oxide (TiOx) as an electron collection electrode with the structure ITO/TiOx/[6,6]-phenyl C61 butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid)/Au (TiOx cell) was fabricated. Its complicated photovoltaic properties were investigated by photocurrent−voltage and alternating current impedance spectroscopy measurements. The TiOx cell required a significant amount of time (approximately 60 min) to reach its maximum power conversion efficiency (PCE) of 2.6%. To investigate the reason for this slow photoresponse, we investigated the influences of UV light and water molecules adsorbed on the TiOx layer. Surface treatment of the TiOx cell with water induced a rapid photoresponse and enhanced the performance, giving a PCE of 2.97%. However, the durability of the treated cell was considerably inferior that of the untreated cell because of UV-induced photodegradation. The cause of the rapid photoresponse of the treated cell was attributed to the formation of hydrogen bonds between adsorbed water molecules and carbonyl oxygen atoms in PCBM close to the TiOx surface. When the TiOx surface was positively charged by UV-induced holes, the carbonyl oxygen in PCBM close to the TiOx surface can quickly join to the TiOx surface, rapidly transporting photogenerated electrons from PCBM to TiOx in competition with the photocatalyzed degradation. The experimental results suggested that the slow photoresponse of the untreated TiOx cell was because the morphology of the photoactive organic layer changed gradually upon irradiation to improve the transport of photocarriers at the TiOx/PCBM:P3HT interface.