Charge Transport and Recombination in a Nanoscale Interpenetrating Network of n-Type and p-Type Semiconductors:  Transient Photocurrent and Photovoltage Studies of TiO2/Dye/CuSCN Photovoltaic Cells

Solid-state dye-sensitized photovoltaic cells have been fabricated with TiO2 as the electron conductor and CuSCN as the hole conductor. These cells involve the nanoscale mixing of crystalline n-type and p-type semiconductors in films that are more than 100 times thicker than the individual n- and p-type domains. Charge transport and field distribution in this kind of material are as yet unexplored. We have used photocurrent and photovoltage transients, combined with variation in the layer thickness, to examine the limiting factors in charge transport and recombination. Charge transport (t1/2 ≈ 200 μs) is found to be similar to that in dye-sensitized electrolyte cells. Recombination at Voc (t1/2 ≈ 150 μs) is 10 times faster than in electrolyte cells, and recombination at short circuit (t1/2 ≈ 450 μs) is 100 times faster. In the solid-state cells, the similarity of the charge transport and recombination rates results in a low fill factor, and photocurrent losses, both important limiting factors of the efficiency. A simple model is given, and suggestions are made for improvements in efficiency.