To understand these observations, it should be highlighted that the calculated absorption coefficient with electron-phonon coupling assumes a perfectly crystalline material. In a highly disordered material or near grain boundaries, the local lack of crystalline order renders indirect transitions more probable as they can take place without the intervention of a phonon. The grain size of a CaCuP film with stoichiometric P content is only approximately 50 nm (Figure ) † and both the crystalline fraction and crystallite size degrade for higher P content. In fact, a strong dependence of optical properties on crystal quality is well known for other indirect band gap materials, such as elemental silicon. Going from fully crystalline to microcrystalline, nanocrystalline and finally fully amorphous Si, the absorption coefficient between the indirect and direct band gap of Si rises accordingly by over an order of magnitude. Bulk light scattering effects in the measurement or the approximate treatment of excited states by DFT could also contribute to the experiment-theory discrepancy -the anisotropic nature of CaCuP may result in non-negligible excitonic effects which can increase the absorption coefficient, and are not accounted for in regular DFT. Since experimental absorption exceeds computed absorption over a broad spectral range, it is unlikely that an optically active defect can be responsible for it. Nevertheless, we have calculated the absorption and emission profile of the lowest energy defect, V Cu , which can be found in Figure . † As expected, defect absorption and emission are closely spaced in energy and are not expected to cause broadband absorption.