Enhancement of photocurrent by columnar Nb-doped TiO2 compact layer in dye sensitized solar cells with low temperature process of dc sputtering

The compact layer of niobium doped TiO2 (TNO) with columnar structure was prepared by a low temperature process of dc magnetron sputtering with high growth rate, and its effect on the carrier transport properties in dye sensitized solar cells (DSSCs) was examined by comparing with granular TNO compact layer prepared by a sol-gel method followed by a heat treatment at high temperatures. With a reactive dc magnetron sputtering using Nb-Ti alloy target and Ar-O2 mixed gas, columnar structure with preferred orientation along [220] was obtained when non-heated FTO/glass substrate was located closely to the high density plasma covering the target. The short circuit photocurrent density Jsc of columnar layer showed an increase even at thick region of > 200 nm in contrast to that of granular one. The enhancement of Jsc was mainly explained by longer electron life time τe than granular one which may be originated from the dense structure of columnar layer covering FTO, as well as the small series resistance of columnar layer due to the reduction of inter grain resistance and the small electron mass along a-axis. However, the leak current from the surface of columnar grains was found to increase with the strong preferred orientation because of the dense surface trapping sites which might be caused by the low temperature crystal growth activated by adjacent plasma. For thinner region of ≲ 200 nm, the difference between columnar and granular layer was small, and the effect commonly obtained for compact layer was confirmed. Thus, even with the thickness of ∼ 100 nm being commonly used for compact layer, columnar layer made in this study has an advantage from the viewpoint of low temperature process and high growth rate.