Quantification of the loss mechanisms in emerging water splitting photoanodes through empirical extraction of the spatial charge collection efficiency

Abstract

The operando quantification of surface and bulk losses is key to developing strategies for optimizing photoelectrodes and realizing high efficiency photoelectrochemical solar energy conversion systems. This is particularly true for emerging thin film semiconductors, in which photocarrier diffusion lengths, surface and bulk recombination processes, and charge separation and extraction limitations are poorly understood. Insights into mechanisms of efficiency loss can guide strategies for nanostructuring photoelectrodes, engineering interfaces, and incorporating catalysts. However, few experimental methods are available for direct characterization of dominant loss processes under photoelectrochemical operating conditions. In this work, we provide insight into the function and limitations of an emerging semiconductor photoanode, g-Cu 3 V 2 O 8 , by quantifying the spatial collection efficiency (SCE), which is defined as the fraction of photogenerated charge carriers at each point below the surface that contributes to the measured current. Analyzing SCE profiles at different operating potentials shows that increasing the applied potential primarily acts to reduce surface recombination rather than to increase the thickness of the space charge region under the semiconductor/ electrolyte interface. Comparing SCE profiles obtained with and without a sacrificial reagent allows surface losses from electronically active defect states to be distinguished from performance bottlenecks arising from slow reaction kinetics. Combining these insights promotes a complete understanding of the photoanode performance and its potential as a water splitting photoanode. More generally, application of the SCE extraction method can aid in the discovery and evaluation of new materials for solar water splitting devices by providing mechanistic details underlying photocurrent generation and loss. Broader contextThe next generation of solar energy conversion systems require design and integration of new semiconductor materials. Detailed understanding of the optoelectronic properties of these materials and the driving forces and loss mechanisms that limit device performance is essential to the development of high efficiency systems. Insights into mechanisms of efficiency loss can guide strategies for nanostructuring photoelectrodes, engineering interfaces, and incorporating catalysts. However, few experimental methods are available for direct characterization of dominant loss processes under photoelectrochemical operating conditions. In this work, we provide insight into the function and limitations of an emerging semiconductor photoanode, g-Cu 3 V 2 O 8 , by quantifying the spatial collection efficiency (SCE), which is defined as the fraction of photogenerated charge carriers at every point that contribute to extracted current. Analyzing SCE profiles at different operating potentials while performing different chemical reactions allows distinguishing between bulk and surface losses and between different types of surface recombination mechanisms...

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

“…The performance limitation is commonly attributed to charge carrier losses at bulk and surface defect sites. [ 2e,3 ] Although much work has been done to mitigate these loss mechanisms for n‐type metal oxides like BiVO 4, [ 4 ] a well‐defined mitigation approach has not yet been developed for p‐type materials like CBO.…”

In situ characterizations of photoelectrochemical cells for solar fuels and chemicals

“…4(a). 71 A sharp decay is observed in the SCE profile with increasing distance from the surface. This is consistent with diffusion-limited photocurrent, in which the concentration of minority carriers decays with distance from the space charge region.…”

“…4(b). 71 It can immediately be seen that a modest increase in collection length, ∼2 nm, is observed with increasing potential. This suggests that the main effect of increasing potential is the reduction of surface recombination rather than an increase in the thickness of the space charge region.…”

“…69 In thin-film devices, however, the absorption of light is more complex due to resonance and interference effects, and for this reason, regularization methods which can handle arbitrary charge carrier generation profiles were used for copper indium gallium diselenide (CIGS) thin-film solar cells. 70 Recently, empirical SCE analysis has been applied to two photoelectrode materials for the first time, α-Fe 2 O 3 and γ-Cu 35,71 The results of the SCE analysis for γ-Cu 3 V 2 O 8 thin films are shown in Fig. 4(a).…”

“…The inset shows the photocurrent as a function of applied electrochemical potential. Reproduced with permission from Ref [71]…”