Fabrication of double layered hybrid solar cells consisting of low-temperature anatase titanium oxide and conducting polymer

We have prepared a photovoltaic device consisting of poly [2'-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene] (MEHPPV) and an n-type crystalline TiO2 (anatase) thin film by high-temperature process and low-temperature process at a temperature lower than 150 degrees C by sol-gel techniques. The refluxed sol of titanium-tetraisopropoxide (TTI) with water and nitric acid formed anatase phase TiO2 without requiring the high-temperature process, and the wettability of sol is successfully improved by diluting sol with ethanol. The short circuit current J(SC), fill factor, and the power conversion efficiency increase with the heat-treatment temperature of TiO2, Which is attributed to the improvement of series resistance of the TiO2 film. On the other hand, the open circuit voltage remains almost constant (ca. 1,0 V) with the change in heat-treatment temperature between 60 and 120 degrees C, whereas it decreases to 0.76V in the device prepared on the TiO2 film sintered at 500 degrees C, probably owing to the change in crystallinity. The origin of open circuit voltage in indium tin oxide (ITO)/TiO2/MEHPPV/Au is also discussed. The open circuit voltage corresponds well to the energy difference of the conduction band edge of TiO2 and the highest occupied molecular orbital (HOMO) of MEHPPV (ca. 1 eV) in the device consisting of the ITO/low-temperature TiO2/MEHPPV/Au system.