Tuning the Fermi Level Beyond the Equilibrium Doping Limit Through Quenching: The Case of CdTe

The Fermi level of a material is a fundamental quantity that determines its electronic properties. Thus, the ability to tune Fermi levels is important for developing electronic device materials. However, for most materials, the Fermi level is limited to a certain range in the band gap due to the existence of certain intrinsic compensating defects. Here we demonstrate that quenching can be used as an effective way to overcome this limit, allowing the Fermi levels to be tuned in a much wider range. Taking a photovoltaic material CdTe as a prototype example, we analyzed the physical origin of Fermi level pinning and explained why growing the sample at high temperature followed by rapid quenching to room temperature can overcome the self-compensation limit. We further show that for CdTe, quenching can increase the Fermi level range from about 0.6 to 1.1 eV, which has a great potential in improving CdTe solar cell performance. Our proposed strategy of tuning Fermi level positions beyond the intrinsic equilibrium doping limit is general and can be applied to other semiconductor systems.