Adherent and Conformal Zn(S,O,OH) Thin Films by Rapid Chemical Bath Deposition with Hexamethylenetetramine Additive

Abstract

ZnS is a wide band gap semiconductor whose many applications, such as photovoltaic buffer layers, require uniform and continuous films down to several nanometers thick. Chemical bath deposition (CBD) is a simple, low-cost, and scalable technique to deposit such inorganic films. However, previous attempts at CBD of ZnS have often resulted in nodular noncontinuous films, slow growth rates at low pH, and high ratio of oxygen impurities at high pH. In this work, ZnS thin films were grown by adding hexamethylenetetramine (HMTA) to a conventional recipe that uses zinc sulfate, nitrilotriacetic acid trisodium salt, and thioacetamide. Dynamic bath characterization showed that HMTA helps the bath to maintain near-neutral pH and also acts as a catalyst, which leads to fast nucleation and deposition rates, continuous films, and less oxygen impurities in the films. Films deposited on glass from HMTA-containing bath were uniform, continuous, and 90 nm thick after 1 h, as opposed to films grown without HMTA that were ∼3 times thinner and more nodular. On Cu2(Zn,Sn)Se4, films grown with HMTA were continuous within 10 min. The films have comparatively few oxygen impurities, with S/(S+O) atomic ratio of 88%, and high optical transmission of 98% at 360 nm. The Zn(S,O,OH) films exhibit excellent adhesion to glass and high resistivity, which make them ideal nucleation layers for other metal sulfides. Their promise as a nucleation layer was demonstrated with the deposition of thin, continuous Sb2S3 overlayers. This novel HMTA chemistry enables rapid deposition of Zn(S,O,OH) thin films to serve as a nucleation layer, a photovoltaic buffer layer, or an extremely thin continuous coating for thin film applications. HMTA may also be applied in a similar manner for solution deposition of other metal chalcogenide and oxide thin films with superior properties.

Dynamic Speciation Modeling to Guide Selection of Complexing Agents for Chemical Bath Deposition: Case Study for ZnS Thin Films

“…The three films were nanocrystalline ZnS of mixed cubic and hexagonal phases with small inclusion of oxygen that shifted the peak positions slightly toward higher Bragg angles, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 similar to the analysis of the film deposited from the reference recipe. 20 Annealing for one hour at 200 °C did not significantly change the diffractograms. Overall, the composition and quality of the films were similar, but thicknesses were different due to different deposition rates that stemmed from the throttling behavior of the complexing agents.…”

“…In our previous works, we experimentally measured the time-resolved concentration of S 2-ion in the bath for a more accurate determination of deposition driving forces by this pathway. [18][19][20] In this work, we compare the behavior of complexing agents by using a reference recipe that we recently developed for rapid deposition of low-oxygen ZnS films for photovoltaic buffer layer and nucleation layer applications. 20 The near-neutral chemistry has a different dominant deposition pathway than the typical ion reaction as described above, but the conclusions can be extended to any system that ions, 23 and thus does not complicate the modeling of speciation.…”

“…[18][19][20] In this work, we compare the behavior of complexing agents by using a reference recipe that we recently developed for rapid deposition of low-oxygen ZnS films for photovoltaic buffer layer and nucleation layer applications. 20 The near-neutral chemistry has a different dominant deposition pathway than the typical ion reaction as described above, but the conclusions can be extended to any system that ions, 23 and thus does not complicate the modeling of speciation. Hence, the important concentrations in our comparison of complexing agents on the deposition rate are the total and free Zn 2+ ion concentrations, TAA concentration, and pH.…”

“…The silicon wafers were similarly cleaned but with the omission of the HCl step. Silicon wafers provided a smooth surface and a clean cross-sectional fracture for reliable film thickness measurements, while the Cr/Pdcoating provided excellent film adhesion.In the batch deposition of the films, the stock solution preparation and mixing sequence were performed following the method reported earlier 20. The deposition solution after mixing contained 0.04 M ZnSO4, 0.12 M thioacetamide (TAA), 0.30 M hexamethylenetetramine (HMTA), and the complexing agent.…”

An ammonia-free chemical-bath-deposited ZnS(O,OH) buffer layer for flexible Cu(In,Ga)Se2 solar cell application: an eco-friendly approach to achieving improved stability

“…8 Moreover, devices with CBD-ZnS(O,OH) buffer layers typically show the effects of metastability. [13][14][15][16][17] However, these studies focus only on the fundamental properties of ZnS(O,OH) films. LS is believed to originate from the change of barrier height in the vicinity of the CIGS/ buffer.…”