Dynamics of chemical bath deposition of inorganic thin films for photovoltaic applications

Chemical bath deposition (CBD) is a simple, low cost and scalable technique to deposit inorganic films, but lack of fundamental understanding and control of the underlying chemistry has limited its versatility. We combined the use of a continuous-flow microreactor (CFµR), speciation modeling, and novel time-resolved bath characterization techniques to better understand the evolution of the baths and its impact on film formation and properties. The model system for this study was ZnS, a wide band gap semiconductor whose many applications, such as photovoltaic buffer layers, require uniform and continuous films down to several nanometers thick. In CBD of ZnS, oxygen is often incorporated into the film as oxide and hydroxide to form Zn(S,O,OH). Oxide tunes electronic band levels while hydroxide detrimentally affects electronic device stability. Efforts to understand the film formation and 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 used a CFµR to investigate relationships between bath composition and film properties deposited from a high-performance alkaline recipe. For decreased film oxygen content, the near-neutral deposition regime was explored. Hexamethylenetetramine (HMTA) was successfully used as an additive to maintain near-neutral pH for rapid deposition of continuous film with reduced oxygen impurities. Complexing agents, or ligands, limit the free metal ion concentration and control the film deposition rate, but their dynamic properties are poorly understood. We demonstrated that dynamic speciation modeling is a powerful tool in predicting the ligands’ throttling behavior. Time-dependent deposition rate was captured through spatially-dependent film thickness by CFµR, and confirmed the prediction of the model. Understanding bath dynamics is crucial to advancing CBD from the traditional “recipe-based” perception to a level in which film properties can be fully controlled by bath engineering.Ph.D., Chemical engineering -- Drexel University, 201

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

The dynamic behavior of complexing agents often governs the kinetics of chemical bath deposition (CBD), but dynamics are frequently overlooked in favor of a more simplified description of only initial conditions. Here we demonstrate the importance of complexing agent dynamics using a combination of equilibrium speciation modeling and experiments for the case of ZnS thin films grown with three common complexing agents: ethylenediaminetetraacetate (EDTA), nitrilotriacetate (NTA), and citrate, using a reference recipe that ensured fair comparison. Complexing agents control the deposition rate by limiting the availability of <i>free</i> cations. Speciation modeling was used to simulate the <i>free</i> Zn²⁺ concentration as a function of <i>total</i> Zn²⁺ for baths with the individual agents. On the basis of the primary stability constants, deposition rate was predicted to decrease abruptly with reaction time for the EDTA bath, whereas baths with NTA or citrate should provide deposition rates that decrease only slightly over time. The predicted deposition profiles of the three baths were observed experimentally using a continuous-flow microreactor for CBD and were supported by the measured <i>total</i> Zn²⁺ concentration profiles. Understanding and predicting the dynamic behavior of complexing agents by simulation enables strategic design of CBD processes for many material systems

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

Microreactor Chemical Bath Deposition of Laterally Graded Cd_1–<i>x</i>Zn_<i>x</i>S Thin Films: A Route to High-Throughput Optimization for Photovoltaic Buffer Layers

Cd_1–<i>x</i>Zn_<i>x</i>S (CdZnS) is a promising replacement for the CdS buffer layers in copper indium gallium (di)­selenide (CIGS) solar cells because the wider band gap of CdZnS offers improved optical transmittance of blue light. Chemical bath deposition (CBD) is the state-of-the-art deposition method for CdS and CdZnS. However, CBD of CdZnS is poorly understood, and relationships between bath composition and stoichiometry, microstructure, and optoelectronic properties of the deposited film are lacking. We introduce CBD using a continuous flow microreactor as a new technique to rapidly explore a wide variety of deposition conditions on a single substrate using spatially dependent characterization. X-ray diffraction and X-ray absorption spectroscopy indicate that the film is a mixture of nanocrystalline CdZnS and amorphous Zn­(O,OH,S). Over the length of a single substrate, films showed increasing Zn:Cd ratio in the nanocrystalline phase, increasing amorphous content, and increasing quantum confinement, and resultant monotonic increase in band gap from 2.42 to 2.75 eV. Microreactor CBD (μR-CBD) enables rapid identification of CdZnS compositions that are ideal candidates for thin film photovoltaics, as well as determination of the CBD conditions required to deposit them