Highly porous and fully crystalline ZnO films suitable for (flexible) plastic solar cells can be electrodeposited using structure-directing additives at low temperature (< 100 °C). This kind of low-temperature film recently achieved efficiencies of up to 7.2 % at 1 sun, higher than low-temperature TiO₂ films. However, efficiencies are still lower compared to high-temperature TiO₂ films, even if sensitizers with good adsorption properties on ZnO such as the indoline dye D149 are employed, due to lower open-circuit voltages and fill factors.
In order to determine the reasons for these findings and further increase the efficiency, we investigated different ZnO-based cells by electrochemical impedance spectroscopy. Measurements were made at different potentials along the whole I-V-curves in order to explain differences in the fill factor. The morphology and pore sizes of the electrodeposited ZnO films as well as the their dye loading and the kind of sensitizer dye were varied. Sintered films as well as films made from ZnO nanoparticles were investigated for comparison. It has been found that a finer porous structure with smaller ZnO domains leads to a lower electron transport resistance and therefore a higher fill factor, while on the other hand a lower back reaction resistance in these structures leads to a lower open-circuit voltage (see example in the figure). Measures therefore have to be taken to suppress back reaction, e.g. by co-adsorbants on the ZnO surface. Electron transport and back reaction resistances were also found to depend strongly on the adsorption and aggregation behaviour of the dye.