Forbidden Optical Transitions—A New Design Metric for Transparent Conductors

Many semiconductor materials have weak or forbidden transitions at their fundamental band gaps, as is the case in many high-performing n-type transparent conductors (TCs) including Sn-doped In₂O₃ (ITO) and which induces a widened region of transparency. Finding a high-performing p-type TC could enable breakthroughs in optoelectronic devices such as photovoltatics and several high-throughput screenings have been performed. However, most of these screenings assume that the direct fundamental band gap is a good proxy for absorption edge energy, and so far the presence of forbidden transitions has been overlooked in searches for new p-type TCs. To address this, we compute absorption spectra across a set of ~18,000 semiconductors from the Materials Project database, demonstrating that over half have forbidden or weak optical transitions at the band gap and nearly 10% have at least 0.5 eV between their direct gap and allowed gap (a descriptor we call "forbidden energy difference"). We show that compounds with highly localized states at the band edges (as determined by the inverse participation ratio) are most likely to have forbidden transitions, and for these compounds high orbital overlaps correlate with large forbidden energy differences. Next, we use this data set to perform a screening for p-type and n-type TCs that may have been previously overlooked due to the forbidden nature of their band gap, and defect calculations yield a set of new promising candidate ambipolar TCs, p-type TCs, and n-type TCs. Notably, the two best known n-type TCs, In₂O₃ and SnO₂, would have been filtered out in previous screenings, yet we show they emerge successfully from our screening. We have shared our data set via the MPContribs platform, and we recommend that future screenings for optical properties use metrics representative of absorption features rather than band gap alone.