Efficient as-cast semi-transparent organic solar cells with efficiency over 9% and a high average visible transmittance of 27.6%

and a high fill factor of 73.9%. Owing to the complementary absorption spectra in the NIR region, high EQE values between 600 and 830 nm and a favourable transparency window between 400 and 600 nm, while the human eye has the highest sensitivity in the yellow-green wavelength region (500-600 nm), ST-OSCs using an ultra-thin (10-20 nm) Au cathode showed high PCEs of 7.9-9.1% at a high average visible transmittance of 37.3-27.6% in the photopic region. The PCE of 9.1% is one of the highest values reported in the literature for ST-OSCs without any extra treatment and with an AVT of more than 25% in the photopic region so far.