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

As promising candidates for future applications in building-integrated photovoltaics, semitransparent organic solar cells (ST-OSCs) have made tremendous progress. However, power conversion efficiency (PCE) of the ST-OSCs is limited by intrinsic narrow absorption spectra concentrated in the near infrared region (NIR), weak extinction coefficient, and mismatched molecular energy levels of thin active layers. Here, an efficient ST-OSC based on a donor/acceptor electron pair of a trifluorinated polymer donor PBFTT and a tetrachlorinated acceptor IT-4Cl was fabricated. Due to halogenation, photovoltaic materials show stronger extinction coefficient, improved crystallinity and higher charge carrier mobility; PBFTT shows lower electronic energy levels, and IT-4Cl shows a red-shifted absorption spectrum. As a result, the PBFTT:IT-4Cl pair shows matched energy levels, complementary absorption spectra in the NIR region and a good blend morphology. Hence, as-cast OSCs based on PBFTT:IT-4Cl achieved a high PCE of 11.1% with a high short-circuit current density of 19.7 mA cm^-2 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.