Effectiveness of band discontinuities between CIGS absorber and copper-based hole transport layer in limiting recombination at the back contact

Abstract

In this work, different copper-based thin-film materials as a hole transport layer (HTL) are introduced in Cu(In,Ga)Se2 (CIGS) solar cells to limit the recombination rate at the back contact and improve efficiency. To survey the impact of different HTLs on the efficiency of CIGS solar cells, an experimental cell with a record efficiency of 23.35% has been modeled. Then, cuprous oxide (Cu2O), cuprous thiocyanate (CuSCN), and copper sulfide (CuS) thin films as an HTL were added separately to the CIGS cell. It was demonstrated that the open-circuit voltage (Voc) is enhanced owing to the reduction of the back interface recombination, and short-circuit current density (Jsc) is improved thanks to the enhancement of carrier collection when Cu2O or CuSCN HTLs are added to the conventional CIGS solar cell. External quantum efficiency (EQE) has also shown a significant increase in long wavelengths of approximately 1000–1200 nm through this approach. The simulated impedance spectroscopy confirms that the use of Cu2O HTL in the CIGS solar cells significantly reduces back surface recombination, which could improve Voc and fill factor (FF) values. In addition, the utilization of Cu2O HTL in the CIGS solar cells reduces Urbach energy (EU), which is attributed to suitable band alignment at Cu2O/CIGS interface. High efficiency of 27.3% was achieved by adding the Cu2O HTL in CIGS solar cell, which is mainly attributed to the additional carrier extraction routes through HTL.