ECS Journal of Solid State Science and Technology, 2 (9) P333-P337 (2013) P333 2162-8769/2013/2(9)/P333/5/$31.00 © The Electrochemical Society Continuous Microreactor-Assisted Solution Deposition for Scalable Production of CdS Films Sudhir Ramprasad, a,b, z Yu-Wei Su, b,c Chih-Hung Chang, b,c Brian K. Paul, b,d, * and Daniel R. Palo a,b, e a Energy Processes and Materials Division, Pacific Northwest National Laboratory, Corvallis, Oregon 97330, USA b Microproducts Breakthrough Institute and Oregon Process Innovation Center, Corvallis, Oregon 97330, USA c School of Chemical, Biological, & Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, USA d School of Mechanical, Industrial, & Manufacturing Engineering, Oregon State University, Corvallis, Oregon 97331, USA Solution deposition offers an attractive, low temperature option in the cost effective production of thin film solar cells. Continuous microreactor-assisted solution deposition (MASD) was used to produce nanocrystalline cadmium sulfide (CdS) films on fluorine doped tin oxide (FTO) coated glass substrates with excellent uniformity. We report a novel liquid coating technique using a ceramic rod to efficiently and uniformly apply reactive solution to large substrates (152 mm × 152 mm). This technique represents an inexpensive approach to utilize the MASD on the substrate for uniform growth of CdS films. Nano-crystalline CdS films have been produced from liquid phase at ∼90 ◦ C, with average thicknesses of 70 nm to 230 nm and with a 5 to 12% thickness variation. The CdS films produced were characterized by UV-Vis spectroscopy, transmission electron microscopy, and X-Ray diffraction to demonstrate their suitability to thin-film solar technology. © 2013 The Electrochemical Society. [DOI: 10.1149/2.003309jss] All rights reserved. Manuscript submitted March 29, 2013; revised manuscript received May 29, 2013. Published June 11, 2013. Cadmium Sulfide (CdS) thin films are commonly used as het- erojunction partners in cadmium telluride (CdTe) and copper indium gallium di-selenide (CIGS) thin-film solar cells. 1–3 Thin CdS films are used to maximize the amount of light absorbed in the active area of the solar cells 4 while still thick enough to minimize shunting. 5 Chopra et al. 1 and Mitzi et al. 6 in their reviews on thin film solar cells have em- phasized the necessity of low-cost manufacturing techniques for CdTe and CIGS thin film solar cells. Solution deposition techniques offer a promising economical pathway for cost effective PV manufacturing. Among the solution based deposition techniques, electrodeposition 7–11 and chemical surface deposition 12,13 ex- hibit substantial potential for facile integration with large scale production. In the case of screen printing, it is challenging to produce films less than 10 μm thick, 14 a full two orders of magnitude too high for solar PV-relevant CdS films. In addition, there is a cost burden because of the substantial heat-treatment required to produce high quality films. 15 Doctor blading can only be used for solution chemistries that do not aggregate or crystallize at high concentration. 16 Spray pyrolysis has been reported in the literature, 17 however it necessitates higher deposition temperature (>400 ◦ C). Chemical bath deposition (CBD) is an extensively investigated solution-based method for generating CdS films. 18–23 Although, CBD has been implemented for large scale manufacturing, 7 lower material utilization and significant waste generation continue to be issues. Continuous solution deposition process offers advantages over several shortcomings of the CBD process. The success of integrat- ing continuous solution deposition into industrial scale production is largely dependent on the choice of coating technique. There is a need in thin film coatings industry for simple and cost effective processes that can be used in non-ideal environments and involving aggressive chemicals, high temperature, and challenging reaction chemistries. The requirement is to create a thin layer of aqueous reactive solu- tion on the substrate without long upstream hold-up time. These long hold-up times will cause the solution to age, leading to undesirable precipitation reactions. 12,24 The reaction mixture for CdS production is time sensitive, with longer times favoring homogenous formation of undesired particles that will aggregate, create poor films, cause equipment fouling, and reduce overall material yield. Groups led by Chang 25–27 and Baxter 28,29 have demonstrated the incorporation of microchannel devices in a continuous process of ∗ Electrochemical Society Active Member. e Present Address: Barr Engineering, Hibbing, Minnesota 55746, USA. z E-mail: sudhir.ramprasad@pnnl.gov solution deposition for semiconductor films. These researchers have shown that using microreactor technology has enabled enhanced con- trol on process parameters for challenging reaction chemistries com- monly involved in generating semiconductor films. Based on a sim- ilar framework, Paul et al. 30 have developed a deflected plate flow cell method for coating CdS films by microreactor-assisted solution deposition (MASD) process. Although, this approach was capable of producing uniform CdS films on 150 mm substrate, it is not suitable for conveyorized reel-to-reel manufacturing. The work described here adapts the MASD for deposition of CdS films and demonstrates a route to process scale-up. MASD facilitates precise control of solution temperature and residence time prior to application. The objective of this paper is to demonstrate a near room temperature continuous-flow deposition process capable of depositing CdS films with high film uniformity operating under atmospheric pressures. The rod coater developed during this study is simple and cost effective to implement, in comparison to other thin film deposition techniques. It requires no hold-up time and facilitates precise control of solution temperature and residence time prior to application. We report a scalable deposition unit that shows a pathway to larger scale manufacturing. Methods and Materials Description of experimental process characteristics.— Positive displacement pumps (Acuflow Series III) were used to pump each stream of reagents at a constant flow rate. All chemical reagents used were of ACS grade (>99% purity). Cadmium chloride provided the cadmium source, and thiourea provided the sulfur source. Stream A consisted of cadmium chloride (0.004 M), ammonium chloride (0.04 M), and ammonium hydroxide (0.04 M) in water. Stream B con- sisted of thiourea (0.08 M) in water. Special care should be taken in the handling of cadmium-containing solutions, including personal pro- tective equipment, adequate ventilation, and proper disposal of waste. The reagent reservoirs were placed on analytical balances (Ohaus) and the changes in the mass were recorded throughout the duration of the test. The reagents from the two streams were mixed in a T-mixer (Idex, Inc.) before entering the heat exchanger. Thermocouples at the inlet and outlet of the heat exchanger recorded the temperature of the fluid. Commercial soda lime glass (Pilkington TEC-15) with a transparent conducting oxide (TCO) layer was employed as the sub- strate for film deposition. The TCO layer consisted of fluorine-doped tin oxide (FTO). A LabView (National Instruments) program was ecsdl.org/site/terms_use address. 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