We present a photon-counting simulation to evaluate the impact of top cell film quality on overall performance when designing stack tandem photovoltaic devices. We apply this model to the case where the bottom cell material is silicon in order to take advantage of the relatively low cost of manufacture already available. For the top cell, we approach material selection with an understanding that thin-film devices usually can’t reach the output voltage ideality that can be achieved with optimized single crystal devices. With limited thickness for the top cell, a range of photons with energies higher than the band gap may still not be absorbed in the top cell due to the normally smaller absorption coefficients just above the bandgap energy. We use the Tauc formalism for parameterizing near-edge optical absorption and investigate the effects of film thickness and bandgap on expected efficiency values of CdTe and MALI family materials, which are already known to be good candidates for top cell thin-film devices.