Effect of back contact surface nitriding on the growth of Cu₂ZnSnSe₄ and solar cells performances

Interfacial performance optimization is one of the significant means to improve the efficiency of Cu₂ZnSnSe₄ (CZTSe) solar cells. Here, nitridation treatment was applied to the molybdenum back contact surface by plasma-assisted molecular beam epitaxy (PA-MBE), and provide a feasible method for compound optimization of back contact and a novel understanding of the CZTSe growth mechanism. It is found that the nitridation changes the CZTSe growth process and regulates the elemental distribution. A MoN_x layer forms on the back contact surface, which inhibits the decomposition reaction of the bottom absorber layer with molybdenum and reduces the generation of over thick MoSe₂. This interlayer also improves carrier transport efficiency by forming quasi-ohmic contact and provides a hole-selective transportation to reduce interfacial carrier recombination, due to its high work function. Finally, the efficiency of CZTSe solar cell is improved by 32% from 4.93% to 6.51% accompanied with the increase of V_OC, J_SC, and FF.