Life on the Urbach Edge

The Urbach energy is an expression of the static and dynamic disorder in a semiconductor and is directly accessible via optical characterization techniques. The strength of this metric is that it elegantly captures the optoelectronic performance potential of a semiconductor in a single number. For solar cells, the Urbach energy is found to be predictive of a material's minimal open-circuit-voltage deficit. Performance calculations considering the Urbach energy give more realistic power conversion efficiency limits than from classical Shockley-Queisser considerations. The Urbach energy is often also found to correlate well with the Stokes shift and (inversely) with the carrier mobility of a semiconductor. Here, we discuss key features, underlying physics, measurement techniques, and implications for device fabrication, underlining the utility of this metric.

Automated summary

The Urbach energy is a metric used to describe the disorder in a semiconductor and can be measured directly using optical techniques. It provides a single value that represents the optoelectronic performance potential of a semiconductor. In solar cells, the Urbach energy has been found to predict the minimal open-circuit-voltage deficit of a material. Considering the Urbach energy in performance calculations gives more realistic limits for power conversion efficiency than classical theories. It is also often correlated with the Stokes shift and carrier mobility of a semiconductor.