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

Abstract

Cu2SrSnS4 (CSTS) is a promising alternative candidate to Cu2ZnSnS4 (CZTS) for single- or multi-junction photovoltaics (PVs) owing to its efficient light-absorbing capability, earth-abundant, nontoxic constituents, and suitable defect properties. However, as a novel absorber material, several fundamental properties need to be characterized before further progress can be made in CSTS photovoltaics. In this letter, hybrid density functional theory (DFT) calculations have been used to comprehensively characterize for the first time, the electronic structure, band alignment, and optical properties of CSTS. It is demonstrated that CSTS possesses the ideal electronic structure (direct band gap of 1.98 eV and small photocarrier effective masses) and optical properties (high extinction coefficient and wide absorption) suitable for photovoltaic applications. Simulated X-ray photoelectron spectroscopy (XPS) valence band spectra using variable excitation energies show that Cu-3d electronic state dominates the valence band maximum of CSTS. Furthermore, the vacuum-aligned band diagram between CSTS and other common absorbers (CZTS, CIGS, CdTe) and the common n-type partner materials (CdS, ZnO) was constructed, which indicate staggered type-II band alignment at the CSTS/CdS and CSTS/ZnO interfaces. Based on these results, interface band offset engineering and alternative device architectures are suggested to improve charge carrier separation and power conversion efficiencies of CSTS.

Density Functional Theory Insights into the Structural, Electronic, Optical, Surface, and Band Alignment Properties of BaZrS3 Chalcogenide Perovskite for Photovoltaics

“…The effective mass vary inversely with the charge-carrier mobility μ ( μ = e τ m e ( h ) * , where e and τ are the electric charge and mean free path of an electron, respectively). In turn, μ correlates directly with the diffusion coefficient D c ( D normalc = k normalB T e μ , where k B and T are the Boltzmann constant and absolute temperature, respectively) . Based on these relationships, materials with lower effective masses have higher charge-carrier mobility and diffusion.…”

“…The real part is related to the electronic band structure and describes the absorptive behavior and the degree to which a material can be polarized, whereas the imaginary part determines the dielectric losses . A high dielectric constant promotes effective charge screening and decreases charge-carrier recombination. , The static dielectric constant of BaZrS 3 is ∼7.5 (Figure a), which is higher than that of CdTe (6.95) but lower than that of silicon (11.7) and GaAs (12.88). Compared to other perovskite materials, the obtained dielectric function of BaZrS 3 is lower than those of α-SrZrS 3 (∼9.36) and β-SrZrS 3 (15.75) but higher than those of CsPbI 3 (5.8), NH 4 PbI 3 (5.78), , CH 3 NH 3 PbI 3 (6.5), and CH­(NH 2 ) 2 PbI 3 (6.6) .…”

“…where k B and T are the Boltzmann constant and absolute temperature, respectively). 33 Based on these relationships, materials with lower effective masses have higher chargecarrier mobility and diffusion. Therefore, there is a need to determine the m e * and m h * for BaZrS 3 .…”

Rear Interface Modification by the ZnTe Layer Enables High-Efficient Cu2(Zn,Cd)SnS4Thin-Film Solar Cells

“…To resolve the abovementioned problem, physical separation by a rear interface passivation layer between CZTS and Mo is demonstrated to be an effective measure. − Such a layer can not only suppress the detrimental rear interface reaction but also passivate the rear interface if suitable. An excellent passivation layer, namely ZnTe, has already been demonstrated for the back passivation of CdTe thin-film solar cells previously but has not been applied to CZTS thin-film solar cells yet. , As we know, the CZTS owns a similar band structure as CdTe . Furthermore, the conduction band of ZnTe is higher than that of pure sulfide CZTS and Cd-alloyed CZTS (CCZTS), , indicating that a reflection field can be built if this layer is applied between the absorber layer and Mo.…”

“…An excellent passivation layer, namely ZnTe, has already been demonstrated for the back passivation of CdTe thin-film solar cells previously but has not been applied to CZTS thin-film solar cells yet. , As we know, the CZTS owns a similar band structure as CdTe . Furthermore, the conduction band of ZnTe is higher than that of pure sulfide CZTS and Cd-alloyed CZTS (CCZTS), , indicating that a reflection field can be built if this layer is applied between the absorber layer and Mo. Such a reflection field for electrons would benefit the carrier collection in solar devices, inhibiting the rear recombination rate and thus reducing the V OC deficit.…”