Effect of Al3+ doping concentration and film thickness of ZnO nanoparticles over the TiO2 photoelectrode in CdS quantum dots sensitized solar cells

•Al3+ doped ZnO nanoparticles increase Jsc and Voc in QDSSCs.•The increase in Voc is due to a reduction in recombination processes.•The increase in Jsc is due to an enlarging of the photogeneration range and a reduction of the transport resistance. In this work was systematically studied the Al3+ doped ZnO nanoparticles effect on the performance of CdS quantum dot sensitized Titania solar cells. Varying the spin coating speed, ZnO nanoparticles with different Al3+ conentrations (00, 0.1. 0.2, 0.4 M %) were deposited over TiO2/CdS. The introduction of undoped ZnO nanoparticles results in a small increase of the Voc (from 524 to 547 mV), impedance analysis indicate that this increase is due to a recombination process reduction. On the other side, low Al3+ doped ZnO increase at the same time the short circuit current (from 7.15 to 8.46 mA/cm2) and the open circuit voltage (from 524 to 576 mV) resulting a 39% of relative enhancement of photoconversion efficiency (from 1.9 to 2.65%). Light absorption and External Quantum Efficiency (QE) measurements indicate that small concentrations of Al3+ promote the formation of new energy levels where extractible charge carriers are photogenerated resulting in an increase in the Jsc meanwhile, impedance analysis show that Al3+ as a dopant in ZnO reduce the recombination process and at the same time increase the transport process, resulting in the observed increase of Voc and contributing to the increase of Jsc.