Nandy Subhajit, Kaur Kulwinder, Gautam Sanjeev, Chae Keun Hwa, Nanda B R K, Sudakar Chandran
Multifunctional Materials Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India.
Condensed Matter Theory and Computational Lab, Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India.
ACS Appl Mater Interfaces. 2020 Mar 25;12(12):14105-14118. doi: 10.1021/acsami.9b18357. Epub 2020 Mar 13.
Designing solid-state perovskite oxide solar cells with large short circuit current () and open circuit voltage () has been a challenging problem. Epitaxial BiFeO (BFO) films are known to exhibit large (>50 V). However, they exhibit low (≪μA/cm) under 1 Sun illumination. In this work, taking polycrystalline BiFeO thin films, we demonstrate that oxygen vacancies (V) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high and simultaneously. While aliovalent substitution (Ca at Bi site) is used to control the lattice V, Ca and Ti cosubstitution is used to bring out only GB-V. Fluorine-doped tin oxide (FTO)/BiCaFeTiO/Au devices are tested for photovoltaic characteristics. Introducing V increases the photocurrent by four orders ( ∼ 3 mA/cm). On the contrary, is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-V. This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in . The cosubstitution, following a charge compensation mechanism, decreases the lattice V concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve (3-8 V) much larger than that of BiFeO (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice V concentration dominates GB-V concentration. Notwithstanding, both lattice and GB-V contribute to the increase in ; the former weakens ferroelectricity, and as a consequence, undesirably, is lowered well below 0.5 V. Using optimum and , we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.
设计具有大短路电流（ ）和开路电压（ ）的固态钙钛矿氧化物太阳能电池一直是一个具有挑战性的问题。已知外延BiFeO（BFO）薄膜表现出大的 （>50 V）。然而，在1个太阳光照下，它们表现出低的 （≪μA/cm）。在这项工作中，以多晶BiFeO薄膜为研究对象，我们证明了晶格内和晶界（GB）处存在的氧空位（V）可以被明确控制，从而同时实现高的 和 。虽然采用异价取代（Bi位的Ca）来控制晶格V，但Ca和Ti共取代仅用于产生晶界V。对氟掺杂氧化锡（FTO）/BiCaFeTiO/Au器件进行了光伏特性测试。引入V使光电流增加了四个数量级（ ∼ 3 mA/cm）。相反，发现 <0.5 V，而原始BiFeO的 为0.5 - 3 V。Ca和Ti共取代促进了较小微晶的形成，这反过来又增加了晶界面积，从而增加了晶界V。从漫反射光谱和能带结构计算推断，这产生了占据体带隙的缺陷带，导致 增加了三个数量级。遵循电荷补偿机制的共取代显著降低了晶格V浓度，以在增强极化的情况下保持铁电性质。这有助于实现比BiFeO（0.5 - 3 V）大得多的 （3 - 8 V）。值得注意的是，由于Ca取代保持了适度的微晶尺寸，晶格V浓度主导了晶界V浓度。尽管如此，晶格V和晶界V都有助于 的增加；前者削弱了铁电性，结果，不理想的是， 降低到远低于0.5 V。使用最佳的 和 ，我们证明了在AM 1.5一个太阳光照下，固态BFO太阳能电池的效率可达 ∼0.22%。
