Driven by the need for competitive single-junction efficiencies using low-cost solution-processed manufacturing technology, there has been a sustained interest in tandem architectures in the field of organic photovoltaics [1-3]. The continuous effort recently led to the demonstration of organic tandem solar cell efficiencies above 17% [4], which moves the technology into the range of commercial relevance for low-cost thin-film photovoltaic solutions.

The design of multijunction solar cells requires the optimization of a large number of structural and compositional degrees of freedom, such as, band gaps and layer thicknesses of component materials, but also the interlayer design for the series connection in the case of the industrially more relevant monolithic tandem devices. In this situation, numerical device simulation can provide instrumental insight for the identification of the optimum multilayer configuration. In organic tandems, while optical simulation of the thin-film layer stacks is routinely used, full opto-electronic device simulation including the recombination junction formed by the interlayer region is not common.

In our contribution, we discuss the numerical optimization of organic tandem solar cells using an integrated optoelectronic device simulation approach that captures the essential properties of the complex multilayer device structures, such as, resonator effects in coherent layer stacks, and the impact of organic interface properties for the design of efficient recombination junction layers. The versatility of the simulation framework is achieved by the combination of a fast 1D transfer matrix formalism for thin film optics with a comprehensive and dedicated drift-diffusion solver for organic optoelectronic devices that is furnished with a hopping transport formalism for charge transfer across organic hetero-interfaces [5]. Performance optimization via configuration tuning is enabled by a suite of built-in global and local optimization algorithms.

The modelling and optimization capabilities of the numerical device simulation framework are illustrated on the example of an existing high-efficiency all-organic tandem solar cell implementation [6]. The role of the hopping transfer rate at the recombination junction for the electrical performance is investigated, and the successive optimization of a multi-layer stack reveals a significant performance gain resulting from a complete opto-electronic optimization as compared to the optical optimization only.