Inverting Transient Absorption Data to Determine Transfer Rates in Quantum Dot–TiO2 Heterostructures

Abstract

Transient absorption spectroscopy is a powerful technique for understanding charge carrier dynamics and recombination pathways. Analyzing the results is not trivial due to nonexponential relaxation dynamics away from equilibrium, leading to a disparity in reported charge-transfer rates. An inversion analysis technique is presented that transforms transient signals back into their original rate equation. The technique is demonstrated on two CdSe/TiO2 heterostructures with different surface states. Auger recombination is identified at higher carrier densities and radiative recombination at lower carrier densities. The heterostructure with additional surface traps exhibits increased trap-state Auger recombination at high carrier densities and changes to radiative recombination at low carrier densities due to a Shockley–Read–Hall process. Carrier-dependent electron-transfer rates are determined and compared to common methods that only capture the magnitude of the charge transfer at specific carrier densities. The presented transient absorption analysis provides direct understanding of the recombination mechanisms with minimal additional analysis or with presumption of decay mechanisms.