Physical Origins of the Transient Absorption Spectra and Dynamics in Thin-Film Semiconductors: The Case of BiVO₄

Abstract

Transient absorption (TA) spectroscopy is uniquely suited for understanding kinetic processes initiated by light over vast ranges of time. In combination with white light probes, the recorded differential absorption spectra can contain spectroscopic signatures characteristic of specific charge carrier population densities. However, disentangling the often-complex and convoluted spectra is made challenging without robust analysis methods relating the underlying physical mechanisms to the spectral components. In this work, we address the origin of the transient spectra of a model system of emerging solar energy harvesting materials using a monoclinic BiVO 4 thin film. Using ground-state optical properties of the semiconductor, we find the main derivative-like spectral response to be related to shifting and broadening of oscillators, rather than specific carrier-related transitions. However, by using the Drude optical model of free carriers, we also identify the transient response related to freehole density. Importantly, sample heating from the optical pumping, which begins at ∼10 ps and plateaus by ∼200 ps, dominates the overall spectral response at longer times. On the basis of a physical model of the spectral response, a kinetic model is developed that describes the pump power dependence of the free-hole density, as well as the temporal evolution of the spectral changes associated with shifting and broadening of oscillators. First-principles density functional theory calculations are used to rationalize experimental measurements. This comprehensive approach to analyzing and modeling the TA spectra offers a generalizable basis for understanding the complex pump−probe data, reveals thermal heating artifacts that are frequently erroneously assigned to long-lived photocarriers, and offers a path to eliminating ambiguity in analysis of photocarrier dynamics in solid-state systems.

Impact of Oxygen Vacancy Occupancy on Charge Carrier Dynamics in BiVO4 Photoanodes

“…In the study herein, we focus on a strong optical feature observed in transient spectroscopic studies of BiVO4 around its bandgap (λ = 470 nm, see Figure 2 below). [27][28][29] This feature appears spectroscopically analogous to a similar feature observed in α-Fe2O3 (λ = 580 nm), which we have previously suggested to originate from unoccupied oxygen vacancy states in α-Fe2O3. 30,31 There is however considerable dispute over the assignment of these features in both metal oxides, with other studies relating this spectral feature to photogenerated holes that undergo water oxidation, 27,[32][33][34][35] or suggesting this feature results from structural changes resulting from thermal effects of pulsed laser excitation.…”

“…However in BiVO4, the energetics and the derivative-like nature of the spectral response suggest that the feature may be due to phenomena such as electroabsorbance effects, as suggested elsewhere. 28 Whilst further work is necessary to understand the exact spectroscopic origin of this optical feature, our experimental observations strongly correlate this spectral fingerprint to changes in the concentration of…”

“…30,31 There is however considerable dispute over the assignment of these features in both metal oxides, with other studies relating this spectral feature to photogenerated holes that undergo water oxidation, 27,[32][33][34][35] or suggesting this feature results from structural changes resulting from thermal effects of pulsed laser excitation. 28,36 Such controversies are symptomatic of our limited understanding of the optical signals observed following optical excitation of these metal oxides. Herein, we address these ambiguities by directly modulating the population of V 5+ /V 4+ states in BiVO4 using five complementary electrochemical, thermal and all-optical methods.…”

Fermi Level Engineering of Passivation and Electron Transport Materials for p‐Type CuBi2O4 Employing a High‐Throughput Methodology

“…[ 24 ] Typically, temperature‐induced changes in optical properties due to laser‐induced heating from carrier recombination are of critical importance to deconvolve in order to ascertain information related to excited carrier populations. [ 25 ] In this case, however, because we are comparing only CBO with and without Cu 1.5 TiO z , any changes in dynamics can be attributed to a change in electronic‐related components as the thermal properties of the films are assumed to be effectively identical.…”

Ptychography of Organic Thin Films at Soft X-ray Energies

“…The two detectors were used for simultaneous collection of the differential transmission (dT) and the differential reflection (dR) signal. The differential absorption (dA) signal was calculated using the following equation 33 i k j j j j y…”