RAINBOW solar cells for a more sustainable future

Large-scale implementation of technologies capable of harvesting renewable energy sources is in the agendas of most countries as a key strategy to reduce/mitigate the effects of climate change. Organic photovoltaics (PV) is one of those technologies. Single-junction solar cell efficiencies are limited by thermodynamics to a maximum theoretical efficiency of ≈33%. So far, the most promising strategy to overcome this fundamental limitation consists of building a tandem of multiple semiconducting junctions (Fig. 1a), with different bandgaps, matching the energy of incoming photons to minimize losses. However, in the case of organic PV, the experimental gains obtained so far are still very modest, mainly due to technical difficulties with the fabrication.At ICMAB-CSIC we have found a solution to this problem [1]: We propose an alternative tandem concept based on spectral splitting, in which various semiconducting junctions with cascading bandgaps are laid side by side, thus the name RAINBOW (Fig. 1b). Each lateral sub-cell receives a fraction of the spectrum that closely matches their main absorption band (Fig. 1c). The RAINBOW concept has been experimentally validated (monolithic two-cell device), showing that this geometry can lead to a reduction in the losses and an improvement in efficiency of ca. 50% with respect to the best sub-cell (Fig. 1d). Our results demonstrate the significant potential of RAINBOW solar cells, inviting the community to explore new material combinations and tandems with more than two sub-cells, increasing efficiencies in organic PV to rival those of their inorganic counterparts.