Guideline for Efficiency Enhancement in Semi‐Transparent Thin‐Film Organic Photovoltaics with Dielectric Mirrors

Semi-transparent organic solar cells (ST-OSCs) show a unique potential to be integrated in windows due to their outstanding characteristics such as high transparency and color-adjustability. In order to achieve both, high transparency and high efficiency, the use of dielectric mirrors is an excellent concept. However, such a mirror will not only improve the photocurrent generated by a solar cell but also cause losses in transparency. In this work, a theoretical model is developed that predicts the effect of the dielectric mirror on the balance between photocurrent enhancement and transparency loss depending on the spectral shape of the ST-OSC absorption. Experimental investigations with three fully printed ST-OSCs showing different absorption characteristics underline the validity of these studies. It is concluded that ST-OSCs with broad absorption spectra ranging from the short wavelength region over the visible to at least 950 nm are most suitable for the implementation of a dielectric mirror. A narrower absorption spectrum or a shift of the spectrum toward longer wavelengths makes an increase in photoactive layer thickness more beneficial than the attachment of a dielectric mirror. Moreover, the dielectric mirror approach is an excellent strategy to obtain high photocurrents for materials which cannot be processed at high active layer thicknesses.