Na‐induced efficiency boost for Se‐deficient Cu(In,Ga)Se2 solar cells

Among different process routes for Cu(In,Ga)Se-2 (CIGS) solar cells, sufficient Se supply is commonly required to obtain high-quality CIGS films. However, supplying extra Se increases the cost and the complexity. In this work, we demonstrate that extra Na incorporation can substantially increase efficiency of Se-deficient CIGS solar cells, fabricated by sputtering from a quaternary CIGS target without extra Se supply, from 1.5% to 11.0%. The Se-deficient CIGS device without extra NaF reveals a roll-over I-V curve at room temperature as well as significantly reduced J(sc) and fill factor at low temperatures. The electrical characteristics of Se-deficient CIGS films are well explained and modeled by the low p-type doping due to high density of compensating donors and the presence of deep defects possibly originating from the anti-bonding levels of Se vacancies. The significant improvement after extra Na incorporation is attributable to the Na-induced passivation of Se vacancies and the increased p-type doping. Our result suggests that extra Na addition can effectively compensate the Se deficiency in CIGS films, which provides a valuable tuning knob for compositional tolerance of absorbers, especially for the Se-deficient CIGS films. We believe that our findings can shine light on the development of novel CIGS processes, distinct from previous ones fabricated in Se-rich atmosphere. Copyright (c) 2015 John Wiley & Sons, Ltd.