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

The effect of absorber stoichiometry in (Ag,Cu)(In,Ga)Se2(ACIGS) solar cells withbandgaps (Eg) > 1.40 eV is studied on a large sample set. It is conrmed thatmoving away in composition from ternary AgGaSe2by simultaneous reduction inGa and Ag content widens the chalcopyrite single-phase region and therebyreduces the amount of ordered vacancy compounds (OVCs). As a consequence, adistortion in currentvoltage characteristics, ascribed to OVCs at the back contact,can be successfully avoided. A clear anticorrelation between open-circuit voltage(VOC) and short-circuit current density (JSC) is detected with varying absorberstoichiometry, showing decreasingVOCand increasingJSCvalues for [I]/[III] > 0.9.Capacitance proling reveals that the absorber doping gradually decreases towardstoichiometric composition, eventually leading to complete depletion. It isobserved that only such fully depleted samples exhibit perfect carrier collection,evidencing a very low diffusion length in wide-gap ACIGS lms. The results indicatethat OVCs at the surface play a minor or passive role for device performance.Finally, a solar cell withVOC¼ 0.916 V atEg¼ 1.46 eV is measured, which is, to thebest of our knowledge, the highest value reported for this bandgap to date