A study was made on very thin CdTe absorber <1 µm layers to investigate limitations in CdTe collection efficiency. Metal organic chemical vapour deposition (MOCVD) was used to deposit cadmium sulfide (CdS), cadmium zinc sulfide (Cd0.9Zn0.1S) and cadmium telluride (CdTe). Improvements in photon collection in the blue, where the absorption length is shorter, have been achieved using a wider band gap Cd0.9Zn0.1S ternary alloy to replace CdS as the window layer. Solar cell capacitance simulator (SCAPS) modelling software [M. Burgelman, P. Nollet, S. Degrave, Thin Solid Films, 361–362 (2000) 527–532] has been used to calculate device parameters as a function of the absorber layer thickness (controlled by in situ using laser reflectometry). One feature of the MOCVD grown devices is the apparent absence of pin-holes, demonstrated by growth of an ultra-thin absorber (200 nm) with conversion efficiency of nearly 4%.