Understanding and optimizing triplet exciton transfer at organic-inorganic interfaces: Microscopic calculations

光伏在提供清洁和可再生能源方面发挥着重要作用。目前，硅太阳能电池在市场上占据主导地位。然而，它们有一个严重的效率限制：超过硅带隙的光子能量会转化为不需要的热量。单态激子裂变，即由一个单态激子产生两个三态激子，是一种非常有前途的减少热化损失和提高光敏性的方法。然而，要真正利用单线态裂变，仍然需要应对许多挑战。典型的有机单态裂变材料和半导体太阳能电池之间的界面设计起着特别重要的作用：理想情况下，界面有助于有效的三重态激发和/或电荷转移，同时最大限度地减少电荷复合和能量消耗。虽然精心设计和设计的界面在这方面的重要性最近已经被证明，但三重态-激子转移的机制，以及如何加速它，目前还不是真正了解。这促使了本理论的提出。我们的目的是为单重态裂变材料和半导体之间的界面推导出合理的设计原则。在这里，我们主要关注四苯敏化硅。界面传输特性的分析将依赖于能级排列，分子有序的影响，包括缺陷的薄中间层和钝化层的影响，以及界面结合的影响。此外，还将研究热振动和电场效应钝化等动力学效应。计算基于(约束)密度泛函理论和激发态势能面上结构和电子自由度随时间的演化。采用格林函数法(GW+BSE)进行比较和验证。我们研究了由四苯与氢、氯和硼钝化的硅组合而产生的定义明确的原型界面。这些体系具有不同的界面偶极子、能带排列和键位。此外，我们还分析了氮化镓钝化层对电池性能的影响，证明了该钝化层可以使光伏电池具有较高的外量子效率。对上述模型系统的能量和电荷转移特性的比较和微观分析将允许建立和合理化有助于单线态裂变敏化太阳能电池接口设计的明确趋势。

Photovoltaics play an important role for the provision of clean and renewable energy. Presently, silicon solar cells dominate the market. However, they have a serious efficiency limitation: The photon energy in excess of the silicon band gap is transformed into unwanted heat. Singlet exciton fission, in which two triplet excitons are generated from one singlet exciton, is a very promising approach to reduce thermalization losses and to enable better sensitivity to light. However, many challenges still need to be met to really utilize singlet fission. The design of the interface between the – typically organic – singlet fission material and the semiconductor solar cell, where the excitons are harvested, plays a particularly important role: Ideally, the interface facilitates an efficient triplet excitation and/or charge transfer and minimizes at the same time charge recombination and energy dissipation. While the importance of carefully designed and engineered interfaces in this context has recently been demonstrated, the mechanism of the triplet-exciton transfer, and how it can be expedited, is currently not really understood. This motivates the present theory proposal. We aim at deriving rational design principles for the interface between the singlet fission material and the semiconductor. Here we focus primarily on tetracene sensitized silicon. The interface transfer properties will be analyzed in dependence on the energy level alignment, the influence of molecular order, the influence of thin interlayer films and passivation layers including defects, and the influence of the interface bonding. Also, dynamical effects like thermal vibrations and electric-field-effect passivation will be investigated. The calculations are based on a combination of (constrained) density-functional theory and the time-evolution of the structural and electronic degrees of freedom on excited-state potential energy surfaces. Green's function methods (GW+BSE) are used for comparison and verification. We investigate well-defined prototype interfaces resulting from the combination of tetracene with silicon passivated with hydrogen, chlorine, and boron. These systems are characterized by different interface dipoles, energy band alignments and bonding sites. Additionally, we analyze the influence of hafnium oxynitride passivation layers, demonstrated to lead to photovoltaic cells with high external quantum efficiency. The comparative and microscopic analysis of the energy and charge transfer characteristics of the model systems described above will allow for establishing and rationalizing clear trends that help in the interface design of singlet fission sensitized solar cells.