Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), CNR-ISTM , Via Elce di Sotto 8, I-06123, Perugia, Italy.
Nano Lett. 2014 Jun 11;14(6):3608-16. doi: 10.1021/nl5012992. Epub 2014 May 8.
Organohalide lead perovskites have revolutionized the scenario of emerging photovoltaic technologies. The prototype MAPbI3 perovskite (MA = CH3NH3(+)) has dominated the field, despite only harvesting photons above 750 nm (∼1.6 eV). Intensive research efforts are being devoted to find new perovskites with red-shifted absorption onset, along with good charge transport properties. Recently, a new perovskite based on the formamidinium cation ((NH2)2CH(+) = FA) has shown potentially superior properties in terms of band gap and charge transport compared to MAPbI3. The results have been interpreted in terms of the cation size, with the larger FA cation expectedly delivering reduced band-gaps in Pb-based perovskites. To provide a full understanding of the interplay among size, structure, and organic/inorganic interactions in determining the properties of APbI3 perovskites, in view of designing new materials and fully exploiting them for solar cells applications, we report a fully first-principles investigation on APbI3 perovskites with A = Cs(+), MA, and FA. Our results evidence that the tetragonal-to-quasi cubic structural evolution observed when moving from MA to FA is due to the interplay of size effects and enhanced hydrogen bonding between the FA cations and the inorganic matrix altering the covalent/ionic character of Pb-I bonds. Most notably, the observed cation-induced structural variability promotes markedly different electronic and optical properties in the MAPbI3 and FAPbI3 perovskites, mediated by the different spin-orbit coupling, leading to improved charge transport and red-shifted absorption in FAPbI3 and in general in pseudocubic structures. Our theoretical model constitutes the basis for the rationale design of new and more efficient organohalide perovskites for solar cells applications.
有机卤化铅钙钛矿彻底改变了新兴光伏技术的格局。尽管原型钙钛矿 MAPbI3（MA=CH3NH3(+)）仅能吸收超过 750nm（约 1.6eV）的光子，但它仍然占据着主导地位。目前，研究人员正致力于寻找具有红移吸收起始、良好电荷输运性能的新型钙钛矿。最近，一种基于甲脒阳离子((NH2)2CH(+) = FA)的新型钙钛矿在带隙和电荷输运方面表现出了比 MAPbI3 更好的性能。研究结果可以从阳离子尺寸的角度来解释，较大的 FA 阳离子有望降低 Pb 基钙钛矿的带隙。为了全面了解尺寸、结构和有机/无机相互作用在确定 APbI3 钙钛矿性质中的相互作用，从而设计新材料并充分利用它们来开发太阳能电池，我们对 A = Cs(+)、MA 和 FA 的 APbI3 钙钛矿进行了全面的第一性原理研究。我们的研究结果表明，从 MA 到 FA 时观察到的四方到准立方结构演变是由于尺寸效应和 FA 阳离子与无机基质之间增强的氢键相互作用的相互作用，改变了 Pb-I 键的共价/离子特性。值得注意的是，观察到的阳离子诱导结构可变性促进了 MAPbI3 和 FAPbI3 钙钛矿中截然不同的电子和光学性质，这是由不同的自旋轨道耦合引起的，导致 FAPbI3 中电荷输运和吸收红移得到改善，并且一般在伪立方结构中也是如此。我们的理论模型为设计用于太阳能电池的新型、更高效的有机卤化铅钙钛矿提供了理论基础。
