Effect of macropores in titanium dioxide layer on the enhancement of photovoltaic conversion efficiency of long-persistence-phosphor enhanced dye-sensitized solar cells

Mesoporous TiO₂ (m-TiO₂) have been often employed to improve the photovoltaic conversion efficiencies (PCEs) of the dye-sensitized solar cells (DSSCs). While when incorporated with a long-persistence-phosphor (LPP) layer, only a limited PCE enhancement was obtained for the resultant DSSCs. This is mainly due to that little incident sunlight can transport through the solid m-TiO₂ photoanode films and then enter and excite the LPP layer. Here in this work, tunable amount of macropores were introduced into hierarchically porous TiO₂ (h-TiO₂) using polyvinylpyrrolidone (PVP) as pore-forming agent. As a result, the optimized DSSCs with h-TiO₂/LPP photoanodes show the highest PCE of 8.05%, which is improved by 29.00% and 14.51%, compared to those with m-TiO₂ and m-TiO₂/LPP ones, respectively. Series of analysis indicates that the macropores benefit the transmission of incident sunlight through the h-TiO₂ layer and reduce the light reflection at the interface of h-TiO₂/LPP. Furthermore, the carbon-doping in TiO₂ caused by PVP shifts up the Fermi level of TiO₂ and the interface of h-TiO₂/LPP enhances the separation of photo-generated carriers, both of which increase the open circuit voltage of the DSSCs. In addition, the DSSCs with h-TiO₂-8/LPP photoanodes can well work in dark with a real PCE of 57.37%.