  Organic solar cells (OSCs) are uniquely suited for semitransparent
applications due to their adjustable absorption spectrum. However, most
high-performance semitransparent cells reported to date are based on materials
that have shown high power conversion efficiency for opaque devices. We
therefore present a model to assess the optimum efficiency and transparency for
a specific donor and acceptor band gap. The absorption characteristics of both
donor and acceptor are modeled with spectral data of typical absorber materials
from the literature which are adjusted to achieve the desired band gap value.
The results show three distinct regions of high light utilization efficiency if
the photopic curve is employed as a weighting function (corresponding to window
applications), and a broad maximum for the plant action spectrum as a weighting
function (corresponding to greenhouse applications). When comparing these
findings to reported experimental values, it is evident that the band gaps of
the materials used for the experimental studies do not correspond to the maxima
identified by our simulation model. The analysis of the energy levels of
molecules recorded in the literature confirms that all band gaps and therefore
all LUE maxima are chemically feasible so that the performance of
semitransparent OSCs can be further improved by designing materials with
optimized absorption spectra.
