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

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.

DFG Programme Research Grants