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

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....