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This thesis summarises a series of works investigating the effect of Ag alloying on the long-term stability and meta-stability of wide-gap (Ag,Cu)(In,Ga)Se₂ (ACIGS) thin-film solar cells. External quantum efficiency, current-voltage, and capacitance-based measurements have been the main characterisation techniques used to investigate these behaviours. Although Ag alloying facilitates high efficiencies and open-circuit voltages (V_OC) in the wide-gap devices (despite the high Ga contents), it is revealed that for a [Ag]/([Ag]+[Cu]) ratio (AAC) in excess of 0.5, the behaviour of ACIGS devices changes significantly. Above the threshold, a strong dependence of net doping concentration (N_net) on group-I/group-III stoichiometry (I/III) is observed, which in turn dictates performance due to low diffusion lengths in high-Ga absorbers. Close-stoichiometric (I/III>0.92) absorbers are almost fully depleted (depletion widths up to 2μm), whilst off-stoichiometric (I/III<0.92) absorbers have high N_net (10¹⁶-10¹⁷cm⁻³). Lightsoaking treatments induce significant reductions in N_net (up to two orders of magnitude), whilst dark storage and annealing lead to increases. The corresponding changes in the depletion widths are large, leading to significant variations in device performance. Independent of Ag contents, high Ga contents lead to V_OC losses after lightsoaking. These meta-stable effects lead to a poor long-term stability in the devices, with one month’s dark storage resulting in 1-2% efficiency losses (absolute) and the persistent V_OC losses contributing a further 1-2% loss (absolute). Additionally, a combination of high Ga and high Ag contents is seen to cause large voltage hysteresis in the devices, which is a further concern for the long-term stability and reliability of modules. It is suggested that defects in ordered vacancy compounds, which segregate to the front surface of the high-Ag ACIGS absorber layers, may explain the dependence of N_net on I/III, whilst the meta-stable variations in N_net are attributed to a currently unknown bulk defect. V_OC losses after lightsoaking are also attributed to a meta-stable defect, with one candidate being the Ga_I antisite. Considering the full range of absorber compositions evaluated, an AAC of 0.2 is seen to provide the best balance between performance and stability for our high-Ga devices.