Generalized Method to Extract Carrier Diffusion Length from Photoconductivity Transients: Cases of BiVO4 , Halide Perovskites, and Amorphous and Crystalline Silicon

Long diffusion lengths of photoexcited charge carriers are crucial for high power conversion efficiencies of photoelectrochemical and photovoltaic devices. Time-resolved photoconductance measurements are often used to determine diffusion lengths in conventional semiconductors. However, effects such as polaron formation or multiple trapping can lead to time-varying mobilities and lifetimes that are not accounted for in the conventional calculation of the diffusion length. Here, a generalized analysis is presented that is valid for time-dependent mobilities and time-dependent lifetimes. The diffusion length is determined directly from the integral of a photoconductivity transient and can be applied regardless of the nature of carrier relaxation. To demonstrate our approach, photoconductivity transients are measured from 100 fs to 1 µs by the combination of time-resolved terahertz and microwave spectroscopy for BiVO4, one of the most studied metal oxide photoanodes for photoelectrochemical water splitting. The temporal evolution of charge carrier displacement is monitored and converges after about 100 ns to a diffusion length of about 15 nm, which rationalizes the photocurrent loss in the corresponding photoelectrochemical device. The presented method is further validated on a−Si:H, c−Si, and halide perovskite, which underlines its potential to determine the diffusion length in a wide range of semiconductors, including disordered materials.Received 14 April 2022Revised 2 August 2022Accepted 12 August 2022DOI:https://doi.org/10.1103/PRXEnergy.1.023008Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCarrier generation & recombinationDiffusionElectrical conductivityOptical conductivityPhotoconductivityPhotocurrentPhysical SystemsAmorphous materialsAmorphous semiconductorsComplex materialsEnergy applicationsEnergy materialsThin filmsTechniquesMicrowave techniquesTerahertz spectroscopyCondensed Matter, Materials & Applied Physics