Development of a high-pressure CdS sputtering process for improved efficiency in CIGS-based photovoltaic devices

A new sputtered process for cadmium sulfide (CdS) has been developed that boosted efficiency of a PV device baseline (with single-stage, coevaporated CIGS) by ~ 2.7% (absolute) to an average of ~10%. The process is highly scalable to a roll-to-roll environment, with the maximum efficiency effect occurring at 0.1 mbar (~75 mTorr) sputtering pressure. The efficiency improvement is thought to be due to two main factors: composition and defects. The CdS film deposited at high pressure (HP) contains more oxygen (primarily as CdO) than one deposited at typical pressures, with oxygen content higher towards the CdS-CIGS interface. The HP process produces an interface with less CdS-CIGS intermixing, which results in a junction with ~ 5x fewer defects as measured by admittance spectroscopy. The performance improvement due to HP CdS occurs even with a very thin CdS layer (<;15 nm), thus greatly reducing the total amount of cadmium contained in the cells.