Determination of Cu in CdTe/CdS Devices Before and After Accelerated Stress Testing

The distribution of Cu before and after accelerated stress testing of high-efficiency CdTe/CdS solar cells has been studied using high mass resolution secondary ion mass spectrometry (SIMS). Standard high-efficiency CdTe/CdS devices were used in this work. Back contacts were graphite paste with varying amounts of Cu added in the form of HgTe: Cu powder. The contacts were applied to the devices after back-surface treatment (NP etch). In one device, no intentional Cu was added to the graphite paste. The devices were stressed at open-circuit voltage in light and at 100°C for ∼1000 hours. SIMS depth profiles were performed on devices after stressing and also on companion devices that were not stressed. The stressed devices show an accumulation of Cu in the CdS layer. Copper levels in the CdS are correlated with the amount of Cu in the graphite paste contact, with higher Cu in the contact resulting in more Cu in the CdS. The Cu level in the CdTe layer is shown to be relatively constant for all devices. The Cu levels in these devices are quantified to provide absolute concentrations.