Optimization of the properties of the molybdenum back contact deposited by radiofrequency sputtering for Cu(In1−xGax) Se2 solar cells

- Optimum parameters for Mo back contacts by RF sputtering are established.
- Apparent discrepancies in results published are solved.
- 17% efficiencies are obtained in unoptimized CIGS cells grown on optimized Mo films.
- A single equipment is used for sputtering of Mo and ZnO without cross-contamination.

In this work, we report on using RF sputtering technique for the deposition of the molybdenum back contact for CIGS solar cells. This allows to realize the back and front contact (Aluminum doped ZnO is used for the front contact) with the same deposition system. We demonstrate that deposition pressure controls the morphology and resistivity of the Mo back contact. This latter point has been the subject of apparently contradicting reports in the literature, but an analysis of the deposition parameters versus sputtering mechanisms allow to understand this spread of results. Full cells have been grown onto the various sets of Mo films, and their electrical properties (efficiencies, Voc, Jcc and fill factors) are analyzed and modeled. From this, we obtain the ideality factors (reflecting the amount of carrier recombination) and with the shunt resistances we find these to be the main parameters which explains the different cells performances versus Mo back contact growth conditions. Low temperature photoluminescence is used as a tool to get a deeper insight on the recombination paths in the samples and support the conclusions drawn from electrical measurements: the molybdenum films grown at 1 Pa yields the best cells results, owing to minimized non radiative recombination. Finally, using the optimum Mo films and improving the quality of the top contact allow us to obtain CIGS solar cells with efficiencies of 17%, although the CIGS, CdS and ZnO layers are not still fully optimized.