Characterizing the role of iodine doping in improving photovoltaic performance of dye-sensitized hierarchically structured ZnO solar cells.

We report two novel types of hierarchically structured iodine-doped ZnO (I-ZnO)-based dye-sensitized solar cells (DSCs) using indoline D205 and the ruthenium complex N719 as sensitizers. It was found that iodine doping boosts the efficiencies of D205 I-ZnO and N719 I-ZnO DSCs with an enhancement of 20.3 and 17.9 %, respectively, compared to the undoped versions. Transient absorption spectra demonstrated that iodine doping impels an increase in the decay time of I-ZnO, favoring enhanced exciton life. Mott-Schottky analysis results indicated a negative shift of the flat-band potential (V(fb)) of ZnO, caused by iodine doping, and this shift correlated with the enhancement of the open circuit voltage (V(oc)). To reveal the effect of iodine doping on the effective separation of e(-)-h(+) pairs which is responsible for cell efficiency, direct visualization of light-induced changes in the surface potential between I-ZnO particles and dye molecules were traced by Kelvin probe force microscopy. We found that potential changes of iodine-doped ZnO films by irradiation were above one hundred millivolts and thus significantly greater. In order to correlate enhanced cell performance with iodine doping, electrochemical impedance spectroscopy, incident-photon-current efficiency, and cyclic voltammetry investigations on I-ZnO cells were carried out. The results revealed several favorable features of I-ZnO cells, that is, longer electron lifetime, lower charge-transfer resistance, stronger peak current, and extended visible light harvest, all of which serve to promote cell performance.