A production route to high efficiency solar cells made with ultra thin silicon wafers
Reduction in PV system costs can be realised if high efficiency cells can be made with thin silicon wafers. While thin cells of high efficiency have been made using very high quality wafers, no industrially cost effective process for using wafers thinner than 150 µm on solar grade CZ wafers has been demonstrated. In this study a complete industrial process has been developed. Solar cell modelling has been used with the parameters obtainable using commercial grade wafers and industrial processing sequences. Wire sawing has been used in a production unit to cut wafers as thin as 125 µm. Techniques to produce thin silicon foils without wafering have also been investigated. Laboratory processes have verified the experimental models and cell efficiencies over 20% have been demonstrated in 110 µm thick cells using the RP-PERC structure and 18% efficiency has been achieved using lower cost processes. The improvements have been applied to thin cells processed on an industrial cell line and cells (100 cm2) with 17% efficiency have been produced on 140 µm CZ wafers with mechanical yields of 85%. To overcome the stresses experienced in thermal soldering, conducting adhesives that may be applied at ambient temperature, have been evaluated.