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

光合作用在提供清洁和可再生能源方面发挥着重要作用。目前，硅太阳能电池主导市场。然而，它们有一个严重的效率限制：超过硅带隙的光子能量被转化为不需要的热量。单重态激子裂变，其中两个三重态激子从一个单重态激子产生，是一种非常有前途的方法，以减少热化损失，并使更好的灵敏度光。然而，要真正利用单线态裂变，仍然需要应对许多挑战。通常有机的单线态裂变材料和半导体太阳能电池之间的界面（激子在其中被收获）的设计起着特别重要的作用：理想地，界面促进有效的三线态激发和/或电荷转移，并且同时最小化电荷复合和能量耗散。虽然最近已经证明了在这种情况下精心设计和工程接口的重要性，三重态激子转移的机制，以及如何可以加快，目前还没有真正理解。这是本理论提出的动因。我们的目标是推导出合理的设计原则之间的单重态裂变材料和半导体的界面。在这里，我们主要集中在并四苯敏化硅。界面转移特性将被分析依赖于能级排列，分子顺序的影响，薄的层间膜和钝化层，包括缺陷的影响，和界面键合的影响。此外，动力学效应，如热振动和电场效应钝化将进行研究。计算是基于（约束）密度泛函理论和激发态势能表面上的结构和电子自由度的时间演化的组合。用绿色函数法（GW+BSE）进行了比较和验证。我们调查定义良好的原型界面，并四苯与硅钝化氢，氯，硼的组合。这些系统的特征在于不同的界面偶极子，能带排列和键合位点。此外，我们分析了氮氧化铪钝化层的影响，证明导致光伏电池具有高的外量子效率。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的界面设计的明确趋势。

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.