The Effect of Al2O3 Barrier Layers in TiO2/Dye/CuSCN Photovoltaic Cells Explored by Recombination and DOS Characterization Using Transient Photovoltage Measurements

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Abstract
Solid-state dye-sensitized solar cells of the type TiO2/dye/CuSCN have been made with thin Al2O3 barriers between the TiO2 and the dye. The Al2O3-treated cells show improved voltages and fill factors but lower short-circuit currents. Transient photovoltage and photocurrent measurements have been used to find the pseudo-first-order recombination rate constant (kpfo) and capacitance as a function of potential. Results show that kpfo is dependent on Voc with the same form as in TiO2/dye/electrolyte cells. The added Al2O3 layer acts as a “tunnel barrier”, reducing the kpfo and thus increasing Voc. The decrease in kpfo also results in an increased fill factor. Capacitance vs voltage plots show the same curvature (∼150 mV/decade) as found in TiO2/dye/electrolyte cells. The application of one Al2O3 layer does not cause a significant shift in the shape or position of the capacitance curve, indicating that changes in band offset play a lesser role in the observed Voc increase. Cells made with P25 TiO2 have, on average, 2.5 times slower recombination rate constants (longer lifetimes) than those made with colloidal TiO2. The cells with P25 also show 2.3 times higher trap density (DOS), which results in little change in the Voc between the two types of TiO2. It is further noted that the recombination current in these cells cannot be calculated from the total charge times the first order rate constant.