Tailor and functionalize TiO2 compact layer by acid treatment for high performance dye-sensitized solar cell and its enhancement mechanism

Surface tailoring and functionalization of an annealed TiO₂ compact layer by H₂SO₄ acid was performed to improve the dye-sensitized solar cell (DSSC) performance. Compared to untreated counterpart, the acid-treated compact layer possesses a rougher surface and more hydroxyl groups, which result in increased surface area and enhanced adherence of the compact layer with the mesoporous TiO₂ film by Ti–O–Ti bonds formed by a followed heating process. Impedance measurement was further used to investigate the enhancement mechanism, indicating the acid post treatment of the TiO₂ compact layer reduces the ohmic bulk resistivity while effectively suppressing charge recombination at FTO/electrolyte interface. In DSSCs with untreated TiO₂ compact layer, a significantly increased series resistivity is very likely to be the rate determining factor to limit the charge separation process. Thus, an optimal post acid treatment could reduce the resistivity for high charge transport, resulting in larger short-circuit current for further improvement of power conversion efficiency from 6.60% in DSSC with untreated compact layer to 7.21% in DSSC with acid-treated compact layer. This work also provides fundamental insight of the compact layer for DSSC performance improvement.