Atomic layer deposition of Al-incorporated Zn(O,S) thin films with tunable electrical properties

Abstract

Zinc oxysulfide, Zn(O,S), films grown by atomic layer deposition (ALD) were incorporated with aluminum to adjust the carrier concentration. The electron carrier concentration increased up to one order of magnitude from 10 19 to 10 20 cm -3 with aluminum incorporation and sulfur content in the range of 0 ≤ S/(Zn+Al) ≤ 0.16. However, the carrier concentration decreased by five orders of magnitude from 10 19 to 10 14 cm -3 for S/(Zn+Al) = 0.34, and decreased even further when S/(Zn+Al) > 0.34. Such tunable electrical properties are potentially useful for graded buffer layers in thin-film photovoltaic applications. Cu 2 ZnSn(Se,S) 4 (CZTS), [3][4][5] Compared to the conventional toxic CdS buffer material for CIGS and CZTS solar cells, Zn(O,S) is composed of earth-abundant and non-toxic elements. KeywordsThis ternary n-type buffer material also has the advantage of having the ability to adjust the band alignment through fine tuning of the stoichiometry, which is easily achieved by atomic layer deposition (ALD) through varying the precursor pulse ratios. [10][11][12] Increasing the sulfur content in Zn(O,S) raises the conduction band energy, which is critical in adjusting the conduction band offset (CBO) at the buffer/absorber interface to optimize the solar cell device performance, 13 as illustrated for SnS/Zn(O,S) heterojunctions in Fig. S1 (see Ref. 14). If the conduction band energy of the buffer layer is too low compared to that of the absorber layer, the negative CBO will induce recombination at the buffer/absorber interface via defects (Fig. S1a). 15 If the conduction band energy of the buffer layer is too high compared to that of the absorber layer, the positive CBO at the buffer/absorber interface creates a barrier that prevents electrons from flowing across the junction towards the transparent conducting oxide (TCO) layer ( Although it has been demonstrated that low electron carrier concentration of Zn(O,S) can improve SnS-based solar cells, this can increase contact resistance with the TCO layer by adding series resistance to the solar cell, which reduces the short-circuit current density (J SC ). While a low carrier concentration of Zn(O,S) can be beneficial for the portion of the buffer layer closer to the absorber layer to reduce possible recombination occurring at the absorber/buffer interface, a high carrier concentration of Zn(O,S) can be beneficial for the portion of the buffer layer closer to the TCO layer to reduce contact resistance. Aluminum is a well known dopant for increasing the electron carrier concentration of ZnO for TCO applications. 18,19 In this study, we report that the electron carrier concentration of ALD Zn(O,S) can be either increased or decreased by modifying the stoichiometry of the film with aluminum incorporation, which is potentially useful for graded buffer layers in thin-film solar cell applications.A custom-built hot-wall ALD reactor was used to grow Zn(O,S) and Al-incorporated Zn(O,S) films. Films were grown at a deposition temperature of 120°C in closed valve mode...

Effect of annealing atmosphere on microstructure, optical and electronic properties of spray-pyrolysed In-doped Zn(O,S) thin films

“…No unreacted In 2 O 3 or no phase separation into ZnO and ZnS were observed from the spectra, except a peak shift, which was observed towards lower 2θ due to the lattice expansion originated from the substitution of oxygen by sulphur. The peak profile fitting shows that the estimated lattice constants increase from a = b = 3.25 Å and c = 5.21 Å in ZnO to a = b = 3.26 Å and c = 5.22 Å in the Zn(O,S) that agrees well with previous reports [10,15]. A preferred orientation along the (002) direction was evolved for indium concentration up to 3 wt%, giving a grain growth perpendicular to the substrate surface.…”

“…In general, for buffer layer, a carrier concentration in the range of 10 14 -10 16 cm −3 is desirable, as the higher carrier concentration leads to higher carrier recombination in the absorber [13]. However, the Zn(O,S) layer with a higher carrier concentration can be effective for the portion of the buffer layer closer to the TCO to reduce its contact (and hence series) resistance [14,15]. It is known that a graded buffer layer (n/n ++ ) with increasing carrier concentration from the absorber to the TCO layer enhances the carrier collection efficiency in the thin film solar cells.…”

Breathable Carbon‐Free Electrode: Black TiO2 with Hierarchically Ordered Porous Structure for Stable Li–O2 Battery

“…The bulk electrical conductivities were measured by four‐point probe in a pelletized form and also by a method proposed in the work of Dunn and co‐workers (see also Figure S5, Supporting Information). The DC conductivity of black TiO 2 (10 −2 –10 −1 S cm −1 ) is, at least, five orders of magnitude higher than that of the control TiO 2 (10 −8 –10 −7 S cm −1 ) which may originate from the Ti 3+ ions that generate shallow donor levels just below the conduction band minimum, giving rise to the n‐type conductivity …”

“…[53,54] In addition, similar absorption features from partial electron yield and total electron yield (TEY) indicate that such reduction of Ti 4+ not only occurs in the outermost surface of TiO 2 , given that the detection depth in a TEY mode extends to a few nanometers. [33,63] The scanning electron microscopy (SEM) images of Figure 2e,f exhibit that the electrode consists of mesoporous frameworks or layers which provide reaction site for oxygen reduction. [50,51,59,60] Raman spectroscopy (Figure 2d) also confirms the TiO 2−x structure.…”

Inorganic Materials by Atomic Layer Deposition for Perovskite Solar Cells

“…Atomic layer deposition (ALD) is a powerful tool for growing reproducible conformal pinhole-free high-quality thin-films of inorganic materials. ALD has the advantage of precise fine control of the film thickness and materials properties, such as stoichiometry, morphology, and doping [ 11 , 12 , 13 , 14 , 15 ]. While chemical vapor deposition (CVD) and physical vapor deposition (PVD), including thermal evaporation, e-beam evaporation, molecular beam epitaxy, pulsed laser deposition, and sputtering, have been popular deposition methods in industry, ALD has gained much attention with ultra-thin or pinhole-free conformal coating becoming more important.…”