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(Ag,Cu)(In,Ga)Se₂ solar cells with bandgaps of ≈1.45 eV with a large spread in absorber stoichiometry are characterized with the intention of assessing the effect of composition on the stability of the devices. This material is observed to have a poor diffusion length, leading to very strong dependence upon the depletion region width for charge carrier collection. The depletion width is observed to depend strongly upon the stoichiometry value and shrinks significantly after an initial period of dark storage. It is also seen that the depletion width can be varied strongly through light-soaking and dry-heat treatments, with prolonged annealing leading to detrimental contraction and light soaking leading to expansion which increases current collection. The extent of depletion width variation in response to the treatments is also clearly linked to absorber stoichiometry. Consequently, the device performance, particularly the current output, exhibits a stoichiometry dependence and is considerably affected after each round of treatment. Possible causes of this behavior are discussed.

The stability of thin-film solar cells spanning a wide range of compositions within the (Ag,Cu)(In,Ga)Se-2 material system is evaluated over time, after dry-heat annealing and after light soaking, and the role of Ag and Ga content is explored. Ag-free CuInSe2 is relatively stable to annealing and storage, while Cu(In,Ga)Se-2 suffers a degradation of fill factor and carrier collection. High-Ga (Ag,Cu)(In,Ga)Se-2 suffers degradation of carrier collection after prolonged annealing, reducing the short-circuit current by approximate to 12%. Ga-free (Ag,Cu)InSe2 loses up to a third of open-circuit voltage and a quarter of fill factor after all treatments are applied. All samples suffer voltage losses after light soaking, with the Ga-free devices losing up to 50 mV and those containing Ga losing up to 90 mV. Ag incorporation leads to a significant reduction in doping, and a significant increase in the response of doping to treatments, with the depletion width of (Ag,Cu)(In,Ga)Se-2 samples expanding from approximate to 0.1 mu m as-grown to beyond 1.0 mu m after all treatments, compared to the Cu(In,Ga)Se-2 sample variation of approximate to 0.1-0.3 mu m. Connections between Ag content, doping instability, and performance degradation are discussed.

Wide-gap, high-Ga (Ag,Cu)(In,Ga)Se₂ thin-film solar cells with a wide range of Ag contents are fabricated and characterized before and after dark storage, dark annealing at 85 degrees C, and light soaking. A 1:4 ratio of Ag to Cu enhances initial device performance significantly, with excess Ag enhancing carrier collection at the expense of open-circuit voltage and fill factor for close-stoichiometric devices. For off-stoichiometric devices, increased open-circuit voltages are offset by losses in carrier collection. Efficiency degradation after treatments is typically increased with additional Ag alloying. A second observation is a behavior threshold identified slightly below an Ag to Cu ratio of 1:1. For compositions below the threshold, the doping response to light soaking and dark annealing is similar to that exhibited by low-Ga Cu(In,Ga)Se₂. Above the threshold, light soaking reduces net doping and that dark annealing can even increase net doping. Furthermore, devices above the threshold exhibit a far greater doping responsivity than those below and display a strong dependence of initial performance and stability on group-I/group-III stoichiometry. A third observation is that all devices lose approximate to 1-2% (absolute) in efficiency after a 3 h light soak, indicating that this loss originates from the high-Ga content (1:3 In:Ga), rather than the Ag alloying.