Enhanced performance of dye-sensitized solar cells by an Al2O3 charge-recombination barrier formed by low-temperature atomic layer deposition

Al2O3 films are deposited conformally and uniformly by atomic layer deposition (ALD) at 150 degrees C throughout the surface of the nanoporous TiO2 electrode of dye-sensitized solar cells (DSSCs) to serve as charge recombination barriers (CRB). The self-limiting film growth of ALD enables detailed analysis of the thickness dependence of the CRB effects on the DSSCs, revealing the optimal Al2O3 CRB thickness to be that produced by 1 ALD cycle (nominal thickness - 0.1 nm), at which the CRB increases the power conversion efficiency (PCE) of the DSSCs by 14% (to up to 7.8% PCE). Above 1 cycle, the ALD films excessively raise the Fermi level of the TiO2 electrode surface, as determined by ultraviolet photoelectron spectroscopy (UPS), so as to block electron injection at the dye-to-TiO2 heterojunction and to cause significant degradation in the DSSC performance. Instead of its nominal 0.1 nm thickness, the 1-cycle ALD film has an effective CRB thickness of 0.3 nm because of steric hindrance among the ALD precursor molecules and the dye molecules, as analyzed by a graphical model. The optimal thickness of the ALD Al2O3 CRB is considerably thinner than the 0.9-2.5 nm reported for Al2O3 CRB formed by conventional sol-gel processes, but it should better reflect the true value, considering the better infiltrating capability and finer thickness resolution of the ALD films. The low temperature and fine thickness control of the ALD process will broaden the utility of CRB.