Efficient Unbalanced Absorption Organic-Perovskite Nonmonolithic Tandem Solar Cells in Parallel Connection

Recent results on tandem solar cells involving halide perovskites look very promising in obtaining economic solar cells with efficiencies >30%. Current research in the literature is about finding and optimizing materials for tandem solar cells, but only a few works focus on the tandem architecture. In the case of monolithic integration, in series connection of subcells is usually considered. However, such architectures come with a current matching condition, which narrows the choice of absorbers and questions device performances under real-world conditions where spectral variations are observed. In this work, we investigate the possibility of in parallel connection considering two absorbers with close band gap energies (i.e., nonideal configuration for current-matching condition): CH3NH3PbI3 (1.59 eV) and PM6:Y6 (1.36 eV). We show experimentally that if the currents are summed, the tandem open-circuit voltage (Voc) tends to be reduced to the lowest single junction Voc. We perform a simulation based on the single-diode equivalent circuit model to investigate the voltage matching condition and the impact of subcell series resistances on tandem performance. Our findings show that nearly matching open-circuit voltages is preferable to avoid any loss in power efficiency. Finally, we offer a solution that allows the combination of absorbers with different band gap energies: association of cells to control subcells' Voc while maintaining the same active area. With such a strategy, an almost 21% power efficiency nonmonolithic tandem is demonstrated in parallel connection architecture, opening the path toward efficiency in parallel connection monolithic tandem.

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