Phonon-coupling enhanced absorption of alloyed amorphous silicon for solar photovoltaics

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Using the observed temperature dependence of $a\text{-Si}:\text{H}$ photon absorption spectrum and the weak-phonon interaction second-order transition theory, phonon-coupling enhanced photon absorption is predicted for $a\text{-Si-Ge}$ and $a\text{-Si-Sn}$ alloys. The ab initio calculated electron and phonon properties of the alloyed amorphous phase show minimally altered electronic states and significant redshifted phonon energies. This phonon shift enhances the optical phonon-coupled photon absorption resulting in increased current generation near the optical band edge. We find that this enhancement favors low-energy optical-phonon modes, thus making the soft-bond forming Sn the choice alloying element.