Control of charge recombination dynamics in dye sensitized solar cells by the use of conformally deposited metal oxide blocking layers

We report herein a methodology for conformally coating nanocrystalline TiO2 films with a thin overlayer of a second metal oxide. SiO2, Al2O3, and ZrO2 overlayers were fabricated by dipping mesoporous, nanocrystalline TiO2 films in organic solutions of their respective alkoxides, followed by sintering at 435 degreesC. These three metal oxide overlayers are shown in all cases to act as barrier layers for interfacial electron transfer processes. However, experimental measurements of film electron density and interfacial charge recombination dynamics under applied negative bias were vary significantly for the overlayers. A good correlation was observed between these observations and the point of zero charge of the different metal oxides. On this basis, it is found that the most basic overlayer coating, Al2O3 (pzc = 9.2), is optimal for retarding interfacial recombination losses under negative applied bias. These observations show good correlation with current/voltage analyses of dye sensitized solar cell fabricated from these films, with the Al2O3 resulting in an increase in V-oc of up to 50 mV and a 35% improvement in overall device efficiency. These observations are discussed and compared with an alternative TiCl4 posttreatment of nanocrystalline TiO2 films with regard to optimizing device efficiency.