The Effects of Device Geometry and TCO/Buffer Layers on Damp Heat Accelerated Lifetime Testing of Cu(In,Ga)Se Solar Cells

In Cu(In,Ga)Se₂ solar cells encapsulated with polyethylene terephthalate (PET) or glass top sheets, the effects of damp heat (D-H) accelerated lifetime testing (ALT) depend on water vapor transmission rate (WVTR) of both transparent conducting oxide (TCO) and the intrinsic zinc oxide (i-ZnO) buffer, as well as device geometry. PET top sheets have a WVTR of ~10 g/m²·day, and glass has a WVTR of 0. Previously, coupons encapsulated with PET degraded to 50% of initial efficiency after 1000 h D-H ALT. We show that PET encapsulated coupons degrade at the same rate as glass encapsulated coupons after 2000 h D-H ALT to 92% of initial efficiency. The only change from previous work is that, here, i-ZnO covers the entire coupon surface, not the just active area. The WVTR of the i-ZnO/TCO stack is 2 × 10^-3 g·H₂O/m²·day. A set of unencapsulated devices went through D-H ALT, one where scribing was used to define the active area of the device and another without scribing; both were protected only by 50-nm i-ZnO. The bare-unscribed device performed as well as the previous glass and PET encapsulated coupons after 1500 h D-H ALT; the bare-scribed device degraded to 78% of initial efficiency, indicating that TCO integrity is a critical ALT parameter.