Low temperature aqueous electrodeposited TiOx thin films as electron extraction layer for efficient inverted organic solar cells

Abstract

Organic solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl-C 61-butyric acid methyl ester were fabricated with electrodeposited TiOx electron extraction layers 5-180 nm thick. Electrodeposition under ambient conditions is an attractive, facile and viable approach to prepare metal oxide interfacial layers. The TiOx films obtained displayed a linear relationship between thickness and deposition time when fabricated under ambient conditions using an aqueous air stable peroxotitanium precursor. The precursor solution was prepared from titanium isopropoxide using a chelate process, which allowed water to be used as solvent due to considerably decreased sensitivity of the precursor solution towards hydrolysis. Highly conformal TiOx films, typically observed with vacuum deposition techniques, were obtained on the indium tin oxide substrate upon electrogeneration of OH- ions using H2O2 additive. Conversely, significantly rougher films with spherical growths were obtained using NO 3 - additives. Low temperature annealing at 200 C in air was found to greatly improve purity and O stoichiometry of the TiOx films, enabling efficient devices incorporating the electrodeposited TiO x to be made. Using MoOx as the hole extraction layer, the maximum power conversion efficiency obtained was 3.8% (Voc = 610 mV; Jsc = 10.6 mA/cm2; FF = 59%) under simulated 100 mW/cm2 (AM1.5G) solar irradiation, whereas an efficiency of 3.4% was achieved with fully solution processed interfacial layers comprising the electrodeposited TiOx films and a surfactant-modified PEDOT:PSS hole extraction layer. © 2014 American Chemical Society.