Interface Structure and Band Alignment of CZTS/CdS Heterojunction: An Experimental and First-Principles DFT Investigation

Abstract

We report a phase-pure kesterite Cu2ZnSnS4 (CZTS) thin films, synthesized using radio frequency (RF) sputtering followed by low-temperature H2S annealing and confirmed by XRD, Raman spectroscopy and XPS measurements. Subsequently, the band offsets at the interface of the CZTS/CdS heterojunction were systematically investigated by combining experiments and first-principles density functional theory (DFT) calculations, which provide atomic-level insights into the nature of atomic ordering and stability of the CZTS/CdS interface. A staggered type II band alignment between the valence and conduction bands at the CZTS/CdS interface was determined from Cyclic Voltammetry (CV) measurements and the DFT calculations. The conduction and valence band offsets were estimated at 0.10 and 1.21 eV, respectively, from CV measurements and 0.28 and 1.15 from DFT prediction. Based on the small conduction band offset and the predicted higher positions of the VBmax and CBmin for CZTS than CdS, it is suggested photogenerated charge carriers will be efficient separated across the interface, where electrons will flow from CZTS to the CdS and and vice versa for photo-generated valence holes. Our results help to explain the separation of photo-excited charge carriers across the CZTS/CdS interface and it should open new avenues for developing more efficient CZTS-based solar cells.

First-principles insights into the electronic structure, optical and band alignment properties of earth-abundant Cu2SrSnS4 solar absorber

“…These results suggest that CdS would be a more appropriate choice of partner material for CSTS from an electronic point of view, because of the smaller CBO between CSTS and CdS, which is also often the first successful choice as the n-type partner material for CIGS 66 , CdTe 67 , and CZTS 68 , 69 absorbers. The slightly larger CBO at the CSTS/CdS interface compared to that of the CZTS/CdS interface (0.21 eV) 68 , 69 , indicates that interface and band offset engineering is required to lower the barrier height at the CSTS/CdS interface and promote efficient charge carrier separation. As was demonstrated to improve the performance of SnS solar cells 70 , 71 , a contact metal with lower work function may be needed at the opposite sides of the junction for CSTS.…”

Solution-processed Cd-substituted CZTS nanocrystals for sensitized liquid junction solar cells

“…and their accompanying shake-up satellite peaks illustrate the existence of Cu 2+ ions and an open 3d 9 shell of Cu 2+ . [62,63] The energy separation of 19.85 (± 0.05) eV observed between Cu 2+ (2p3/2) and Cu 2+ (2p1/2), and their shake-up satellite peaks, is assigned to the formation of Cu 2+ , and not of Co 0 or Cu 1+ . [63] The deconvolution of the XPS spectrum of Cu(2p) in Fig.…”

Modelling Interfaces in Thin-Film Photovoltaic Devices

“…The sign of γ interface signifies whether the interface is more (negative value) or less (positive value) stable than the respective bulk materials in isolation. This method has been used to provide significant insights into the stability of the interfaces of PV materials, for example CZTS/CdS (Rondiya et al, 2019;Rondiya et al, 2020), CCTS/CdS (Rondiya et al, 2020) and TiO 2 /hybrid perovskites. (Mosconi et al, 2014).…”