Heat degradation of sputter-deposited Cu(In,Ga)Se2 solar cells and modules: Impact of processing conditions and bias

Highlights: • Pre-selenization affects efficiency and reliability of Cu(In,Ga)Se2 modules. • Low pre-selenization increases efficiency but leads to poor reliability performance. • Degradation of short circuit current originates from CuSe divacancy complexes. • Loss of fill factor is attributed to an increase of an in-stack energy barrier. • Bias during heat exposure affects efficiency degradation. -- Abstract: We report accelerated heat degradation studies on fully encapsulated Cu(In,Ga)Se₂ modules as a function of film growth parameters, in particular back contact selenization (preSe), as well as the impact of bias (light/voltage) during heat degradation. We show that pre-Se conditions have a profound effect on the heat stability of the device, whereby reduced preSe, while increasing initial efficiency, results in strong heat degradation, driven by a combination of reduced space-charge region and reduced minority carrier lifetime (as evident from external quantum efficiency measurements) in the light-soaked state and resulting in strong degradation of short-circuit current. This is also accompanied by a stronger increase in the shallow acceptor concentration (as measured by capacitance-voltage profiling) in the degraded state, suggesting that the SeCu divacancy complex (V_Se-V_Cu) is likely responsible. In this case, appearance of a high concentration of deep acceptor states accompanies increased shallow doping upon light-soaking, with the former reducing bulk lifetime and the latter further affecting electron collection due to narrow depletion width. This result suggests that bulk structural properties of the absorber film are strongly impacted by the back contact selenization conditions, making the film more susceptible to heat degradation. In the second part of this paper we show that electrical or light bias during heat exposure reduces degradation, in particular almost fully eliminating the above short-circuit current loss. This is a surprising result as usually the positive effects of bias are attributed to interfacial changes, while our results demonstrate that bulk properties can be improved as well.