The bulk photovoltaic (PV) effect exhibited by non-centrosymmetric systems gained research interest due to the observed large open-circuit voltage. Ferroelectric systems exhibiting anomalous photovoltaic effects are mostly crystallized with multi-phase coexistence. Hence, the computational difficulty in building a multi-phase system restricts the detailed photovoltaic studies through phenomenological and shift current theory. In this work, ferroelectric Ba_1−x(Bi_0.5K_0.5)_xTiO₃ (BBKT) oxide is designed to crystallize in single-phase tetragonal symmetry with improved polarization characteristics, and it is found to exhibit large PV response. Both experimental and theoretical studies on BBKT samples reveal ∼18% reduction in bandgap compared to the parent BaTiO₃. Short-circuit current measured as a function of light intensity and light polarization angle reveal linear and sinusoidal response, respectively. The observed features are in accordance with phenomenological theory. Remarkably, the x = 0.125 sample displays ∼8 times higher open-circuit voltage (7.39 V) than the parent compound. The enhanced PV effect is attributed to the large shift current along the z direction as evidenced by the additional charge-center shift of the valence band occupied by the O-2p orbital and the conduction band occupied by the Bi-6p orbital. Notably, the degenerate Bi-p_z state at the conduction band minimum in BBKT favors the large shift current response in the z direction.

中文翻译：

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BaTiO3基铁电氧化物中的体光伏效应：实验和理论研究

由于观察到的大开路电压，非中心对称系统表现出的整体光伏（PV）效应引起了研究兴趣。表现出异常光伏效应的铁电系统大多与多相共存而结晶。因此，建立多相系统的计算困难限制了通过现象学和位移电流理论进行的详细光伏研究。在这项工作中，铁电体Ba _{1- x}（Bi _0.5 K _0.5）_x TiO ₃（BBKT）氧化物被设计为具有改善的偏振特性的单相四方对称结晶，并且发现其表现出大的PV响应。对BBKT样品的实验和理论研究均表明，与母体BaTiO ₃相比，带隙降低了约18％。根据光强度和偏振角测得的短路电流分别显示出线性和正弦响应。观察到的特征符合现象学理论。值得注意的是，x  = 0.125的样品的开路电压（7.39 V）比母体化合物高约8倍。增强的PV效应归因于沿z的大位移电流由O-2 p轨道所占据的价带和Bi-6 p轨道所占据的导带的附加电荷中心位移所证明的方向。值得注意的是，BBKT中导带最小值处的简并Bi- p _z状态有利于z方向上的大位移电流响应。