Tuning the Fermi-level of MoOx by plasma-enhanced ALD for carrier selective contact solar cell application

Molybdenum oxide (MoOx, x ≤ 3) is a wide bandgap dopant-free material that is used as a hole transport layer in carrier selective contact solar cells. Carrier transport at the interface of these layers with crystalline silicon is driven by tunneling, which demands the layers to be ultra-thin. Atomic layer deposition (ALD) is an excellent tool to deposit such thin films with layer-by-layer accuracy on a large area at low temperatures. However, conventional thermal ALD suffers from very low deposition rates. Plasma Enhanced (PE) ALD can be a solution to make the deposition more industrially viable. For films deposited at 200 °C, the growth per cycle calculated by spectroscopic ellipsometry is obtained as 1.194 Å/Cycle, and transmittance of the films is more than 90 % in the visible region. Activation energy for electrical conductance is measured for the film deposited with various ALD parameters, showing a significant change in the Fermi level position from the conduction band from 0.276 eV to 0.367 eV. By varying deposition parameters, PEALD allows for precise control over film thickness and composition, resulting in a sub-stoichiometric MoOx film due to increased oxygen vacancy concentration.