Abstract
The ability to precisely control interfaces of atomic layer deposited (ALD) zinc oxysulfide (Zn(O,S)) buffer layers to other layers allows precise tuning of solar cell performance. The O K- and S K-edge X-ray absorption near edge structure (XANES) of ∼2–4 nm thin Zn(O,S) films reveals the chemical and structural influences of their interface with ZnO, a common electrode material and diffusion barrier in solar cells. We observe that sulfate formation at oxide/sulfide interfaces is independent of film composition, a result of sulfur diffusion toward interfaces. Leveraging sulfur’s diffusivity, we propose an alternative ALD process in which the zinc precursor pulse is bypassed during H2S exposure. Such a process yields similar results to the nanolaminate deposition method and highlights mechanistic differences between ALD sulfides and oxides. By identifying chemical species and structural evolution at sulfide/oxide interfaces, this work provides insights into increasing thin film solar cell efficiencies.
Highlights
A tomic layer deposited (ALD) zinc oxysulfide (Zn(O,S)) films have garnered increasing attention as promising buffer layers in solar cells stemming from the ease with which their composition and thickness can be tuned, a flexibility that allows for adjusting the films’ band gap, conduction band offset and conductivity, among many other properties.[1−3] The versatility of Zn(O,S) to be paired up with several absorber layers (SnS, CIGS, CIS, CZTS) and its large bandgap range (Eg ≈ 2.6−3.8 eV)[4] may allow it to replace the more commonplace but highly toxic CdS buffer layers
Adjusting sulfur content to obtain a slightly positive conduction band offset (CBO ≤ 0.5 eV)[5] and decreasing buffer layer thickness[6,7] reduces interfacial and bulk carrier recombination and allows for straightforward optimization of cell performance.[8−10] to date, there is still controversy on the degree of Zn(O,S) bandgap bowing with sulfur content and thickness.[5,10,11]
It becomes problematic in metal-sulfide ALD, where low growth rates necessitate the films’ long exposure to elevated temperatures, favoring reconfiguration.[12]
Summary
A tomic layer deposited (ALD) zinc oxysulfide (Zn(O,S)) films have garnered increasing attention as promising buffer layers in solar cells stemming from the ease with which their composition and thickness can be tuned, a flexibility that allows for adjusting the films’ band gap, conduction band offset and conductivity, among many other properties.[1−3] The versatility of Zn(O,S) to be paired up with several absorber layers (SnS, CIGS, CIS, CZTS) and its large bandgap range (Eg ≈ 2.6−3.8 eV)[4] may allow it to replace the more commonplace but highly toxic CdS buffer layers. When growing thin Zn(O,S) films on flat SiO2 substrates (Figure 1b), clear thin film and interface information is obtained from the O K-edge TEY XANES spectra (Figure 2a).
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