Relating Deposition Conditions to Zn(S,O,OH) Thin Film Properties for Photovoltaic Buffer Layers Using a Continuous Flow Microreactor

Chemical bath deposition (CBD) is commonly used to deposit ZnS thin films as buffer layers in thin film solar cells, but oxygen is often incorporated into the film as oxide and hydroxide to form Zn­(S,O,OH). Efforts to understand the gradation of the film stoichiometry and properties are limited by film thicknesses of ∼50 nm that are smaller than the probe size of many characterization techniques. We use a continuous flow microreactor (CFμR) to investigate relationships between bath composition and film properties by transposing through-plane gradients over ∼50 nm into lateral gradients over centimeters. Zn­(S,O,OH) films were deposited on glass, Cu₂(Zn, Sn)­(S,Se)₄, and CdSe using thiourea (TU) and thioacetamide (TAA) sulfur sources. X-ray photoelectron spectroscopy (XPS) shows increasing S/(S+O) with distance for TU films and the opposite trend for TAA films, spanning a range of 0.42–0.59 on a single substrate. Films on glass comprise highly monodispersed nodules, revealing separate nucleation and growth regimes. Experimental bath sulfide concentration and pH data were incorporated into speciation models, which showed that Zn­(OH)₂ governs nucleation, whereas ZnS promotes growth. The CFμR provides unique insight into CBD of Zn­(S,O,OH) thin film deposition for optimal control of film morphology and stoichiometry for photovoltaic buffer layers

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

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