SeriesInformation ; 27th European Photovoltaic Solar Energy Conference and Exhibition; 44-49 ; Abstract ; Current solar cells are fundamentally limited by their inability to harvest photons with energies less than the absorber optical threshold. Thinfilm solar cells, usually having band gaps above the optimum singlethreshold value of 1.34 eV given by the ShockleyQueisser limit, and a much smaller volume to absorb the light, are particularly prone to this loss mechanism and thus suffer from imperfect usage of the solar spectrum. An elegant way towards overcoming this limitation and using a larger fraction of the incident light is the reshaping of the solar spectrum by upconversion (UC) of photons. In the present work we apply photochemical upconversion, as realized by triplettriplet annihilation in organic molecules, to amorphous silicon thin film solar cells, incoherently transforming light from the 600750 nm to the 550600 nm wavelength range. Employing a moderate concentration of 19 suns, we demonstrate a relative gain of up to 3% in quantum efficiency around 700 nm and a relative overall efficiency increase of 0.2%. We further pinpoint a pathway which will allow significantly increasing the gain by UC and which can adapted to various single threshold devices, including organic and dyesensitized solar cells.