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The effects of post-deposition processes such as CdCl 2 dip and/or annealing in air on the material and device properties of vacuum-evaporated Au-CdTe/CdS-TO heterojunction solar cells have been investigated. The CdCl 2 dip followed by air annealing at 300 • C for 5 min improved the device efficiency significantly, resulting in decreased CdTe resistivity and enhanced grain size. The temperature-dependent current-voltage analysis indicated that above 280 K interface recombination dominates the current transport mechanism for the as-grown samples, while depletion region recombination starts to be dominant after annealing the samples with CdCl 2 . Below 280 K multistep tunnelling is identified to be the dominant transport mechanism. Frequency-dependent capacitance-voltage studies revealed that after annealing with CdCl 2 the density of interface states decreases and the quality of the heterointerface improves. The capacitance of the CdS/CdTe heterojunctions has been analysed using a model based on the existence of a single dominant trap level, identified at 0.40 eV above the valence band with a concentration of 5.1 × 10 15 cm −3 .

Forward bias recombination (current transport) mechanisms have been evaluated for thin film solar cells and correlated to the in-gap trap levels present. Here CdTe/CdS devices were chosen as an archetypal example of a modern thin film solar cell, and a set of devices with a range of design variables was used in order to reveal the full range of behaviours that may operate to limit current transport. Experimental current-voltage-temperature datasets were compared to mathematical models of transport, and the in-gap traps were evaluated by thermal admittance spectroscopy. The current transport mechanisms operating are presented on a temperaturevoltage diagram. Three regimes were identified: at 'intermediate' voltages, the behaviour was temperature dependent. From 300 K down to 240 K, thermally activated Shockley Read Hall recombination mediated by a 0.38 eV trap (V Cd ) dominated the transport. Between 200 and 240 K the transport was thermally activated but below 200 K the mechanism became dominated by tunnel assisted interface recombination. At 'low' voltages (and for all devices at all voltages when measured at T<200 K) band to band recombination is via multi-step tunnelling through in-gap states. At high voltage, the forward current is dominated by the well-known limiting effect of the back Schottky contact to the CdTe which is in reverse bias. The current transport behaviour is also correlated with the n-CdS thickness and CdCl 2 processing conditions, both of which are critical to device performance.