Zinc ferrite (ZnFe₂O₄, ZFO) has gained attention as a candidate material for photoelectrochemical water oxidation. However, champion devices have achieved photocurrents far below that predicted by its bandgap energy. Herein, strong optical interference is employed in compact ultrathin film (8–14 nm) Ti-doped ZFO films deposited on specular back reflectors to boost photoanode performance through enhanced light trapping, resulting in a roughly fourfold improvement in absorption as compared to films deposited on transparent substrates. The spatial charge carrier collection profile and wavelength-dependent photogeneration yield of mobile charge carriers was then extracted via spatial collection efficiency analysis based on optical and external quantum efficiency measurements. We demonstrate that despite the enhanced performance enabled by the light trapping structure, substantial recombination occurs for thin film ZFO photoanodes even within the space charge region of an ultrathin film photoanode. Furthermore, the excitation-wavelength-dependent yield of mobile charge carriers in ZFO is shown to be less than unity across the visible spectrum, ultimately limiting the attainable photocurrent density. These results explain the underperformance of ZFO as a photoanode material and suggest that reduction of the mobile charge carrier yield due to the existence of ligand field states is a dominant loss mechanism for metal-oxides containing Fe metal centers with open d-shell configuration.

中文翻译：

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超薄膜捕光 ZnFe2O4 光阳极中移动电荷载流子产率和传输长度的定量


锌铁氧体 （ZnFe₂O₄， ZFO） 作为光电化学水氧化的候选材料而受到关注。然而，冠军器件实现的光电流远低于其带隙能量所预测的光电流。在此，沉积在镜面背反射器上的紧凑型超薄膜 （8-14 nm） Ti 掺杂 ZFO 薄膜中采用了强光干涉，通过增强光捕获来提高光阳极性能，与沉积在透明衬底上的薄膜相比，吸收率提高了大约四倍。然后，通过基于光学和外部量子效率测量的空间收集效率分析，提取移动电荷载流子的空间电荷载流子收集曲线和波长依赖性光生产率。我们证明，尽管光捕获结构增强了性能，但即使在超薄膜光阳极的空间电荷区域内，薄膜 ZFO 光阳极也会发生实质性的复合。此外，ZFO 中移动电荷载流子的激发波长依赖性产率在整个可见光谱中小于一，最终限制了可达到的光电流密度。这些结果解释了 ZFO 作为光负极材料的性能不佳，并表明由于配体场态的存在而导致的移动电荷载流子产率降低是具有开放 d 壳构型的含有 Fe 金属中心的金属氧化物的主要损耗机制。