Thin film solar cells have the potential to be an important contributor to the global energy supply by the mid-21 st-century. CdTe based solar cells, achieving laboratory efficiencies of over 16 %, are highly attractive due to their near optimal band gap for conversion of the AMl.5 spectrum to electricity.
This work establishes a baseline process for CdTe solar cells grown by metal organic chemical vapour deposition, doubling device conversion efficiency to over 10 %. Work is reported towards lowering the amount of material and process steps used, particularly absorber thickness. In order to accurately control the final absorber thickness, in situ laser monitoring has been employed. This has allowed precise thicknesses to be grown, and an accurate investigation of the effect of absorber thickness with material and device performance. Several problems come about as the absorber thickness is decreased, including
junction proximities (pn junction, Back contact, Conduction glass (CG)/ window layer front contact), and obtaining high layer quality and uniform thickness, which are not limited by the specific deposition method utilized (i.e. process independent). The introduction of a novel highly doped back contact layer (BCL), has been shown to lower the contact series resistance R_s of devices (from 10 Ω·cm² to 2 Ω·cm²). This has allowed an all-dry process to be achieved, removing the need for any wet chemical etching.

The CdS/CdTe interface has been shown to be heavily absorbing in the blue region (λ< 500 nm) of the visible spectrum, limiting short circuit current Use) response. To obtain maximum photocurrent, the optical band gap (Eg) of the window layer has been extended, through the use of alloying the CdS with Zn (DEZn), creating a cadmium zinc sulphide (Cd₁__xZn_xS) ternary alloy. This has increased Eg from 2.4 eV to 2.7 eV, allowing higher transmission of
(λ< 500 nm) higher energy photons to the pn junction, increasing efficiency to over 13 %.
Results are supported by spectral response studies, and is the first reported beneficial use of Cd₁__xZn_xS layers in CdTe based solar cells.
It was shown that a good electron affinity (Χ) match to CdTe, could be achieved for low Zn concentrations, but at high Zn concentrations (x over 0.1 ), junction performance deteriorated due to a positive conduction band-offset, and demonstrated an optimum Zn alloy concentration for the particular growth conditions used. Ultra thin absorber cells ( < 1 µm) do not show the classical "roll-over" behaviour at the back contact region, and has been attributed to an increased incorporated doping level. A relationship between the ZnO buffer layer and the shunt resistance is reported, improving shunt resistance for standard (2 µm) thick baseline CdTe devices, but decreases shunt resistance (lower value) for ultra thin absorber devices, decreasing performance. The closer proximity of the back contact and the pn junction for ultra thin devices, appear not to severely
impede device performance, with only small decreases observed in the back region of the spectral response (800-900 nm).