Stoichiometry-controllable optical defects in Cu_xIn_2-xS_yquantum dots for energy harvesting

The large Stokes shift for CuxIn2-xSy (CIS) quantum dots (QDs) reduces reabsorption losses in luminescent solar concentrators (LSCs). However, reabsorption still occurs due to their broad absorption spectra, which, along with below unity quantum yields, hamper device performance. The origin of these optical properties is heavily debated, and makes it difficult to optimize CIS for LSCs and other energy harvesting devices such as solid-state and sensitized solar cells. Here, we show with density functional theory calculations that anti-site defects form in near-stoichiometric CIS QDs, while copper vacancies charge-compensated by the oxidation of a second Cu atom form in Cu-deficient structures. Both defects lead to large Stokes shifts, but defects only localize holes in the excited-state leading to strong intragap absorption, which is suppressed for paramagnetic defects that localize holes in the ground-state. The relative concentration of each defect and competing defect phases that lead to non-emissive carrier trapping is controllable by stoichiometry and Fermi-level, and optimal chemical processing conditions for energy harvesting applications are discussed.