Ultrathin CdTe photovoltaic solar cells were produced by metal organic chemical vapour deposition in a single horizontally configured growth chamber. Solar cell activation was investigated by varying the duration of the CdCl2 layer deposition and 420°C thermal anneal to promote Cl diffusion into the CdTe. Thicker CdCl2 layers used in activation treatment resulted in a greater degree of sulphur interdiffusion, up to 2 at.-%, into the CdTe layer. The thicker CdCl2 activation layer was necessary to lower the reverse saturation current density for obtaining optimum experimental photovoltaic (PV) device performances. Modelling of the PV performances with equivalent solar cell structure for optimised devices using solar cell capacitance simulation software resulted in an overestimated open circuit voltage (Voc). The simulations showed that reduced acceptor states at the CdTe interface with the intermixed region resulted in the largest decrease in Voc when considering large back surface recombination velocities.