Nano-grain SnO2 electrodes for high conversion efficiency SnO2–DSSC

The nano-grain ZnO/SnO2 composite electrode was prepared by adding 5 w% of the 200–250 nm ZnO particles to the 5 nm SnO2 colloid in the presence of hydroxypropylcellulose (M.W.=80,000). The nano-grain SnO2 electrode was obtained by removing the ZnO particles from the composite electrode using acetic acid. The FE-SEM micrographs revealed that both electrodes consisted of interconnected nano-grains that were ca. 800 nm in size, and the large pores between the grains furnished the wide electrolyte diffusion channels within the electrodes. The photovoltaic properties of the nano-grain electrodes were investigated by measuring the I–V behaviors, the IPCE spectra and the ac-impedance spectra. The nano-grain electrodes exhibited remarkably improved conversion efficiencies of 3.96% for the composite and 2.98% for the SnO2 electrode compared to the value of 1.66% for the usual nano-particle SnO2 electrode. The improvement conversion efficiencies were mainly attributed to the formation of nano-grains, which facilitated the electron diffusion within the grains. The improved electrolyte diffusion as well as the light-scattering effects enhanced the photovoltaic performance of the SnO2 electrode.