Pentacene-assisted planarization of photo-active layers for high performance tandem organic photovoltaics

In organic tandem solar cells, the morphology of the photo-active layers is not usually stable when their fabrication processes are made with several thermal treatments. Here, we report pentacene-assisted planar tandem organic photovoltaic device based on the active layers of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61-butyric acid methyl ester (PCBM), fabricated in air-ambient. An additive of pentacene was introduced into the active layers of the bottom sub-cells for planarization on their surfaces. The surface morphology of pentacene-based active layer was maintained to be flat from 80 to 160 °C, and allowed for fabricating the planar active layers and interconnecting layer of tandem devices against thermal treatment. State-of-the-art homo-tandem organic solar cells were achieved with an average power conversion efficiency (PCE) of 3.5%, while their best single junction solar cells of P3HT:PCBM achieved a PCE of 3.2%. We showed the first application of high performance tandem organic photovoltaics with three times of high temperature annealing processes at 160 °C. Our work demonstrates a practical way to design highly efficient tandem organic solar cells with other powerful and well-chosen bandgap energy active layers during thermal treatments.