Experimental and Theoretical Study into Interface Structure and Band Alignment of the Cu₂Zn_1–xCd_xSnS₄ Heterointerface for Photovoltaic Applications

Abstract

To improve the constraints of kesterite Cu 2 ZnSnS 4 (CZTS) solar cell, such as undesirable band alignment at p–n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu 2 Zn 1– x Cd x SnS 4 through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental–theoretical approach was employed to characterize and assess the optoelectronic properties of Cu 2 Zn 1– x Cd x SnS 4 materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu 2 Zn 1– x Cd x SnS 4 nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu 2 Zn 1– x Cd x SnS 4 helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu 2 CdSnS 4 (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p–n junction in the ultrafast time scale and highlight a route to improve device performances.

Revealing the electronic structure, heterojunction band offset and alignment of Cu2ZnGeSe4: a combined experimental and computational study towards photovoltaic applications

“…4 The lower efficiency of CZTSSe compared to the chalcogenide Cu(In,Ga)(S,Se) 2 (CIGS) counterparts (420% efficiency) has been attributed to several factors, including but not limited to, large open-circuit voltage (V oc ) deficits, [5][6][7] high-defect states, 8 co-existence of secondary phases, 9,10 short carrier lifetime, 11 and unfavorable band offset and alignment at the CZTSSe/buffer interface. 12,13 In addition to these issues, the kesterite based photovoltaic devices suffer from severe band tailing, which particularly reduces the open circuit voltage (V oc ) of solar cells. 14 Rey et al reported the origin of band-tails in kesterite materials and showed that the large band tailing causes voltage losses that limit the efficiency of kesterite-based devices.…”

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.…”

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).…”