Influence of the relative humidity on the performance of polymer/TiO2 photovoltaic cells

Hydrolysis of titanium(IV) isopropoxide (TTIP) is a well-known method for the fabrication of TiO2. Normally it is made via a sol-gel reaction in the presence of water. In this paper we report on the preparation of flat TiO2 films for conjugated polymer/TiO2 photovoltaic cells, from a TTIP/isopropanol solution. It is shown that the morphological structure of the TiO2 film is strongly dependent on the relative humidity during spin-coating of the TTIP/isopropanol solution. In bilayer devices consisting of TiO2/poly[2-methoxy-5-(3 ,7 -dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV), a low relative humidity (<25 %, room temperature) is needed in order to form smooth, transparent TiO2 films. Increasing the relative humidity results in porous TiO2 films with a high surface roughness, which leads to shunted devices. Apart from bilayer devices, bulk-heterojunction (BHJ) hybrid TiO2:MDMO-PPV photovoltaic cells have been made, by spin-coating a mixture of TTIP and MDMO-PPV in toluene. Again a strong relation was found between the relative humidity during spin-coating and the current-voltage characteristics of the devices. However, in contrast to the bilayer devices, the best BHJ devices were made at higher relative humidity. The observed performance dependence on relative humidity is discussed in relation to the TiO2 morphology