Metrology and process optimization for large area monolithically integrated Cu(In,Ga)Se2 modules

Cu(In,Ga)Se 2 (CIGS) thin film photovoltaic (PV) is emerging from niche market status. However, it is still a challenge for manufacturers to realize CIGS's full potential as the most efficient thin film PV material for cost-effective manufacturing of large area monolithically integrated modules. Large area solar modules operate differently from small area cells due to the circuit structure and the complication from non-uniformities, analysis of which benefits from the use of specialized metrologies. We demonstrate that electroluminescence (EL) provides an effective and convenient way to explore device non-uniformities, defects and other features, which can be useful in process optimization for higher efficiency and yield. We have used this technique to diagnose and solve CdS buffer process problems. Changing chemical dispensing sequence has been used to eliminate excessive particle generation due to homogeneous nucleation. Mo scribe-through at P2 or P3 mechanical scribes affecting the device performance can also be easily identified with EL. Lower argon gas sputter pressure in the range of 3 - 13 mTorr has been tied to higher Mo film density and better scribe-through resistance resulting in reduced panel breakage during final lamination. Faster transport speed during Mo sputtering result in higher specular reflectance from the Mo surface and was optimized to eliminate CIGS peeling from Mo. A root cause for the peeling is proposed. Process optimization to address these issues identified by EL has enabled HelioVolt to achieve a recent baseline large area module efficiency of over 13% with yields of over 90%, and tight distributions providing a recent champion module efficiency of 13.7%.