Vacancy behavior in Cu(In_1−xGa_x)Se₂ layers grown by a three-stage coevaporation process probed by monoenergetic positron beams

Vacancy-type defects in Cu(In_1−xGa_x)Se₂ (x ≅ 0.45 and 1) grown by a three-stage coevaporation process were probed using monoenergetic positron beams. Measurements of Doppler broadening spectra of the annihilation radiation and positron lifetime spectra showed that two different defect species coexist in the Cu(In_1−xGa_x)Se₂ layers, and these were identified as mono/divacancy-type defects and vacancy clusters, respectively. The vacancy clusters were mainly introduced during the third growth stage, and were located in the subsurface region. The concentration of the defects affected the short-circuit current density and the conversion efficiency of the solar cells. The defect concentration and their depth distributions varied depending on Se beam equivalent pressure, growth time, and post-growth annealing time. The behavior of the vacancy-type defects is discussed also with respect to results obtained using an electron probe micro-analyzer. - Highlights: • We applied positron annihilation to characterize Cu(In_1−xGa_x)Se₂. • Defect species were identified as mono/divacancy-type defects and vacancy clusters. • The conversion efficiency increased with a decreasing vacancy concentration.