Photonic effect of a central ultrathin metal film on the performance of series and parallel homo-tandem solar cells

Abstract

We compare the performances of single cells, parallel and series homo-tandem solar cells by focusing on the effect of a central ultra thin metal film acting as an interconnecting layer in the series case or as an electrode in the parallel case. Cells with this metal film are found to outperform classic solar cells thanks to better interference management. Moreover, we find that photonic optimization from an absorption point of view can be accompanied by a reduction of the total amount of active material. We show that the best types of cell strongly depends on the maximum allowed active material thickness to ensure good carrier collection. We also compare ITO and a recently devised ITO-free alternative, the Two-Resonance Tapping Cavity, as transparent electrode and we show how this choice influences photonic optimization. We perform our analysis under the assumption of AM1.5 spectral illumination for a wide choice of realistic materials: PTB7-th:PC71BM, P3HT:PC61BM and DBP:C70, as well as a theoretical material with a Shockley-Queiser type of spectral absorption plus an Urbach tail. With this idealized spectral response, one can capture all the fundamental features and draw general conclusions about optimization of homo-tandem cells.