Density of Surface States at CdSe Quantum Dots by Fitting of Temperature-Dependent Surface Photovoltage Transients with Random Walk Simulations

Surface photovoltage (SPV) transients were measured for a monolayer of CdSe quantum dots (QDs) on planar indium tin oxide (ITO) substrate at temperatures between 200 and 40 °C. By fitting with a random walk simulation, the energy distribution and density of electronic surface states at the QDs were determined. SPV transients show the relaxation of separated charge carriers. This relaxation was described by a model based on an isolated QD approximation. In this model Miller–Abrahams hopping of holes between surface states and the first excitonic state of CdSe QDs and recombination of holes with electrons separated at the ITO surface were considered as mechanisms leading to charge relaxation. Parameters of the model were obtained by a statistic multiparameter fitting procedure of measured SPV transients. All SPV transients could be excellently fitted by using the same Couchy energy distribution of hole traps at the CdSe QD surface. The number of hole traps at CdSe QD surfaces determined by the simulation was between 11 and 6 per QD and therefore in the range of 10 13 cm –2 .