In-situ tuning of the zinc content of pulsed-laser-deposited CZTS films and its effect on the photoconversion efficiency of p-CZTS/n-Si heterojunction photovoltaic devices

We report on the concomitant laser ablation of a Cu2ZnSnS4 target with zinc strips affixed to its surface in order to control at will the Zn content of the CZTS films, and study its effect on their optoelectronic properties in view of their integration into photovoltaic (PV) devices. EDX measurements showed the progressive increase of the Zn content of these pulsed laser deposited (PLD) CZTS films with increasing the surface ratio of Zn-strips to CZTS-target (R-Zn/CZTS). XRD and Raman analyses confirmed that the PLD-CZTS films crystallize in the kesterite phase regardless of the R-Zn/ CZTS ratio. The increase of the Zn content of the PLD-CZTS films was found to increase their optical bandgap from similar to 1.6 to similar to 1.9 eV, while their p-type carrier concentration drops by more than 3 orders of magnitude. The PLD-deposited p-CZTS films were directly deposited onto n-silicon to form Al/n-Si/p-CZTS/ITO heterojunction PV devices, of which PCE was found to be dependent on the R-Zn/CZTS ratio. Indeed, we have identified an optimal R-Zn/CZTS ratio of 24% that leads to p-CZTS/n-silicon heterojunctions with a maximum PCE of 2.2% (an order of magnitude higher than that of CZTS/Si solar cells without any Zn addition). Interestingly, this highest PCE was obtained with the PLD-CZTS films exhibiting the highest work function of 4.75 eV, as measured by UPS. The reconstruction of the band energy diagram showed that the highest PCE achieved at the optimal RZn/ CZTS ratio of 24% corresponds to the largest built-in voltage of the p-CZTS/n-silicon heterojunction.