Tin oxide based dye sensitized solar cells

Abstract

A sieve-like SnO2 nanoporous sheets (ASnO2) obtained through anodization of tin foil in aqueous ammonia solution was employed in photoanode in DSSC. It was found that the well-interconnected SnO2 sieve walls achieved better charge transportation and successfully suppressed the charge recombination. Thus, ASnO2 exhibits higher Voc and short circuit current compared to ordinary SnO2 nanoparticles. Further study shows that this closely packed pore arrangement of the tin oxide sheets provides higher charge injection rate, suppressed charge recombination and reduced charge transfer resistance, thus better photocurrent and enhanced device performance were obtained. To tailor the conduction band edge of tin oxide to increase open circuit voltage of DSSCs, gallium doped SnO2 nano-cuboids (Ga-SnO2-NC) was synthesised by hydrothermal method with the aid of commercially available SnO2 nanoparticles (SnO2-NP) as the growth seeds. Even though Ga-doping wasn’t found any changes in morphology and crystallinity, however it successfully increased the band edge of SnO2, in which it is capable of suppressing photoelectron backflow and achieving a higher open circuit voltage (Voc ~ 0.74 V). To the best of our known, this high Voc is the highest among SnO2 based electrodes without TiO2 knowledge. Besides, common problem of low fill factor (FF) in tin oxide based DSSC has been solved through gallium doping. Optimum gallium doping which moderate up-shift of tin oxide band edge results in better charge injection from dye to electrode and impedes electron from recombination with electrolyte, which in turn to increase Voc and power conversion efficiency. Despite high Voc and FF were achieved through gallium doping, short circuit current (Jsc) remained relatively low compared to TiO2 based DSSC. As a result, we have found a way to increase Jsc while maintaining high Voc and FF of Ga-SnO2-NC, which is through further doping of molybdenum (Mo) into Ga-SnO2-NC. Optimum Mo doping had significantly increased the Jsc by 73% while keeping Voc and FF of Ga-SnO2-NC relatively unchanged. The enhancement mechanism was studied under electrochemical impedance spectroscopy (EIS) and was found mainly due to the increase in conductivity across mesoporous Mo, Ga-SnO2-NC layer where it significantly reduces the series resistance of the DSSC device and thus increases the short circuit current. With all the parameters such as Voc, Jsc and FF were all increased through simple doping of gallium and molybdenum, tin oxide has the utmost potential to be the alternative to TiO2 as mesoporous semiconducting material in photoanode.