Performance Limitations of Wide-Gap (Ag,Cu)(In,Ga)Se2 Thin-Film Solar Cells

The effect of absorber stoichiometry in (Ag,Cu)(In,Ga)Se-2 (ACIGS) solar cells with bandgaps (E-g) > 1.40 eV is studied on a large sample set. It is confirmed that moving away in composition from ternary AgGaSe2 by simultaneous reduction in Ga and Ag content widens the chalcopyrite single-phase region and thereby reduces the amount of ordered vacancy compounds (OVCs). As a consequence, a distortion in current-voltage characteristics, ascribed to OVCs at the back contact, can be successfully avoided. A clear anticorrelation between open-circuit voltage (V-OC) and short-circuit current density (J(SC)) is detected with varying absorber stoichiometry, showing decreasing V-OC and increasing J(SC) values for [I]/[III] > 0.9. Capacitance profiling reveals that the absorber doping gradually decreases toward stoichiometric composition, eventually leading to complete depletion. It is observed that only such fully depleted samples exhibit perfect carrier collection, evidencing a very low diffusion length in wide-gap ACIGS films. The results indicate that OVCs at the surface play a minor or passive role for device performance. Finally, a solar cell with V-OC = 0.916 V at E-g = 1.46 eV is measured, which is, to the best of our knowledge, the highest value reported for this bandgap to date.

Automated summary

The study focused on the impact of absorber stoichiometry in ACIGS solar cells with bandgaps greater than 1.40 eV, showing that moving away from AgGaSe2 composition can reduce ordered vacancy compounds, leading to improved device performance. An inverse correlation between V-OC and J(SC) was observed, with capacitance profiling revealing enhanced carrier collection in fully depleted samples. Additionally, the measurement of a solar cell with V-OC = 0.916 V at E-g = 1.46 eV represents the highest reported value for this bandgap to date.