Comparison of Dye-Sensitized ZnO and TiO₂ Solar Cells:  Studies of Charge Transport and Carrier Lifetime

Abstract

Nanocrystalline particles of ZnO and TiO 2 of approximately equal size (∼15 nm) were used to prepare mesoporous electrodes for dye-sensitized solar cells. Electron transport in the solar cells was studied using intensity-modulated photocurrent spectroscopy and revealed very similar results for ZnO and TiO 2 . Apparent activation energies for electron transport in nanostructured ZnO of e0.1 eV were calculated from the temperature dependence of transport times under short-circuit conditions. The lifetime of electrons in the nanostructured semiconductors was evaluated from open-circuit voltage decay and intensity-modulated photovoltage spectroscopy. Significantly longer lifetimes were obtained with ZnO. Despite the reduced recombination, ZnO-based solar cells performed worse than TiO 2 cells, which was attributed to a lower electron injection efficiency from excited dye molecules and/or a lower dye regeneration efficiency. The internal voltage in the nanostructured ZnO film under short-circuit conditions was about 0.23 V lower than the opencircuit potential at the same light intensity. Results may be explained using a multiple trapping model, but as electrons are usually only shallowly trapped in ZnO, an alternative view is presented. If there is significant doping of the ZnO, resulting band bending in the nanocrystals will form energy barriers for electron transport and recombination that can explain the observed properties.

Efficient and stable CH3NH3PbI3-sensitized ZnO nanorod array solid-state solar cells

“…In DSCs similar V oc values are indeed found. 32 Origin of the lower V oc seems to be the relatively fast recombination in ZnO NRA/ perovskite solar cells, which is most evident for the devices with relatively long nanorods.…”

ZnO Tetrapods: Synthesis and Applications in Solar Cells

“…The chemical stability of ZnO is, however, less than that of TiO 2 , which was found to be problematic in the dyeadsorption procedure. Low electron injection efficiency from excited dye molecules to ZnO is also an obstacle to obtaining high efficiency [66][67][68].…”

Role of Resonance Energy Transfer in Light Harvesting of Zinc Oxide-Based Dye-Sensitized Solar Cells

“…Although most of the reported works on DSSC are based on TiO 2 porous thin films, various structures of ZnO are also being used for DSSC fabrication. 5,[17][18][19] The advantages of using ZnO over TiO 2 are its direct band gap (3.37 eV), higher exciton binding energy (60 meV) compared to TiO 2 (4 meV), 20 22 However, the efficiency of the DSSC based on ZnO nanostructures is still very low (5%). 23 One of the possibilities to enhance the efficiency of ZnO-based DSSC is the enhancement of the light absorbing ability of the DSSC.…”