Effect of introducing Al2O3 as a tunnelling layer into p-CBTS/n-CdS heterojunction solar cells

We have studied the effect of introducing an insulating ultrathin Al2O3 with a very wide band gap (similar to 7 eV) into a CBTS/CdS heterojunction to analyze the tunnelling effect in a Mo/MoS2/CBTS/Al2O3/CdS/ZnO/AZO/Al solar cell using experimentally calibrated numerical simulation. We first investigated the intra-band tunnelling of electrons from the p-CBTS absorber to the n-CdS emitter through the insulator layer (Al2O3). In the second analysis set, we found that the thickness of Al2O3 must be sufficient (similar to 3 nm) to allow the minority carrier penetration. It is shown that the CBTS/Al2O3/CdS structure enhances the collection efficiency in the short- and long-wavelength regions, resulting in higher performance. Indeed, with an Al2O3 layer between CBTS and CdS, the device exhibits efficiency of 11.89% with V-OC, J(SC), and FF of 1.08 V, 15.45 mA/cm(2) and 71.41%, respectively, compared to the device without Al2O3, which presents an efficiency of 6.75%, V-OC = 0.69 V, J(SC) = 15.09 mA/cm(2) and FF = 64.84%. This study provides a guide to further optimise the performance of kesterite solar cells.