Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion

Single-threshold solar cells are fundamentally limited by their ability to harvest only those photons above a certain energy. Harvesting below-threshold photons and re-radiating this energy at a shorter wavelength would thus boost the efficiency of such devices. We report an increase in light harvesting efficiency of a hydrogenated amorphous silicon (a-Si: H) thin-film solar cell due to a rear upconvertor based on sensitized triplet-triplet-annihilation in organic molecules. Low energy light in the range 600-750 nm is converted to 550-600 nm light due to the incoherent photochemical process. A peak efficiency enhancement of (1.0 +/- 0.2)% at 720 nm is measured under irradiation equivalent to (48 +/- 3) suns (AM1.5). We discuss the pathways to be explored in adapting photochemical UC for application in various single threshold devices.