This Broadening Participation Research Initiation Grant in Engineering (BRIGE) provides funding for the development of a novel approach to the fabrication of copper-zinc-tin-sulfide alloy (CZTS) photovoltaic cells. The project will investigate critical aspects of the fabrication of solar cells, starting from the selection of viable precursors and ending with device characterization. For the first time, CZTS nanoparticles will be synthesized using a gas-phase, continuous flow non-thermal plasma reactor starting from solutions of metal salts. This process generates a highly reactive environment which is expected to enable high-rate production of high-quality nanoparticles. Particle morphology and composition will be carefully characterized with the goal of tight control of particle stoichiometry which is expected to have a crucial influence on the performance of the photovoltaic device. The particles will be functionalized using in-flight plasma-initiated surface chemistry and the resulting ink is expected to be compatible with roll-to-roll coating techniques. A densified semiconducting layer will be realized by annealing using various techniques. The prototype solar cell will be tested with the goal of improving the power conversion efficiency (currently 10%) as compared to devices based on the same material system. And the feasibility of the plasma-based fabrication approach for the production of efficient CZTS solar cells will be assessed.

If successful, the application of a scalable process for the CZTS system would provide a new, commercially viable approach to renewable energy production. This material system is attractive because it is based on cheap, earth-abundant, non-toxic elements. This project will also advance the field of plasma processing of materials by investigating the gas-phase chemistry and the dynamic of particle formation for a material system that has not been widely investigated.

Project Report

The ultimate goal of this project is to enable the low-cost fabrication of efficient solar cells based abundant materials, specifically CZTS (copper-zinc-tin-sulfide). We aim at developing novel approaches for the production of CZTS nanoparticles, which we will then use to develop printable inks for the deposition of photovoltaic devices based on them. This project focuses more specifically on the synthesis of CZTS nanoparticles. Current state-of-the-art synthesis techniques are batch-type of reactions that often use hazardous solvents [1, 2]. Consistently with the need for a scalable, high-yield continuous approach, we propose to develop a gas-phase technique capable of producing high-quality CZTS particles. Intellectual merit Our choice is to develop a spray-pyrolysis system in which the appropriate chemical precursors to CZTS are dissolved in a volatile solvent. The solvent + precursor mixture is then nebulized and fed into a high-temperature reactor, such as a tube-furnace, to produce nanocrystals. We have found that metal carbamates are good precursors for this process. Following simple wet-phase recipes [3] we are starting from metal salts (copper chloride, tin chloride, zinc chloride) to produce copper diethyldithiocarbamate [Cu(dedc)2], tin tetraethyldithiocarbamate [Sn(dedc)4] and zinc diethyldithiocarbamate [Zn(dedc)2]. Using this process-precursors combination it is possible to produce crystalline CuS, SnS and ZnS nanoparticles, CZTS nanoparticles and nanostructured CZTS thin films. Overall, we have demonstrated that it is easy to produce CZTS nanoparticles using a continuous-flow, spray pyrolysis technique, which to our knowledge none has demonstrated yet. Future work will be on annealing the particle layer to give a dense, uniform coating, and on building a 1 cm2 photovoltaic cell based on our particles. This will answer the fundamental question whether this approach is suitable for the large-scale production of efficient, low-cost solar cells. There has been significant interest in the community about developing a spray pyrolysis technique for the direct growth of CZTS thin films onto a flat substrate (such as molybdenum-coated soda-lime glass). Given the promising results obtained with respect of the synthesis of CZTS nanopowder, we have also investigated the direct growth of thin films with very promising results [4]. Over its duration, this grant resulted in the publication of three peer-reviewed publications: a) Mangolini, L., Synthesis, properties, and applications of silicon nanocrystals. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2013. 31(2): p. 020801 b) Davis, P. and L. Mangolini, Single precursor synthesis of copper sulfide nanocrystals using aerosol spray pyrolysis. MRS Communications, 2013. 3(01): p. 57-60 c) Exarhos, S., K.N. Bozhilov, and L. Mangolini, Spray pyrolysis of CZTS nanoplatelets. Chemical Communications, 2014. 50(77): p. 11366-11369 The results of this project have also been disseminated via several poster and oral presentation at international conferences (in particular at the 2013 MRS Spring meeting and at the 2014 AVS International meeting). Broader impacts This grant has supported the following: Four graduate students: Patrick Davis and Ebrahim Farshihaghro have successfully defended their M.S. thesis in September and October 2013; Stephen Exarhos is scheduled to complete his M.S. by the end of the 2014-2015 academic year; Ozgul Yasar is scheduled to complete her Ph.D. during the 2015-2016 academic year. Five undergraduate researchers: Alexis Manuel Penaloza, Michael Rieke, Andi Xie, Jesus Hernandez, Luis Carrasquero. Several high school students who have joined the PI’s research group over the summer as interns. These students are Michelle Wang and Rafael Leon (summer 2012); Tyler Walton and Cameron Schaefer (summer 2013); Kolby Nottingham, Ryan Rasmussen and Hao-chien Cho (summer 2014). In addition, one high school teacher, Barry Reno, also joined the group as an intern during the summer of 2012. As part of outreach activities, the PI has also mentored several high school students who take the ‘Engineering Design and Development’ course (which is part of the Project Lead the Way curriculum), taught by Michael Martin at Martin Luther King High School in Riverside. These students are: Kimberli Graham and Nicole Monarrez (in 2012); Cameron Schaefer, Kyle McCarey, Bradley Kuwahara (in 2013); Holly Everson, Lindsey Irwin and George Alfata (in 2014). Particularly rewarding was the experience with the 2013 group, who designed and built from scratch an external attachment to a wheelchair designed to prevent leg muscle atrophy in paraplegic patients (see figure). 1. Todorov, T.K., K.B. Reuter, and D.B. Mitzi, High-Efficiency Solar Cell with Earth-Abundant Liquid-Processed Absorber. Advanced Materials, 2010. 22(20): p. E156-E159. 2. Todorov, T. and D.B. Mitzi, Direct Liquid Coating of Chalcopyrite Light-Absorbing Layers for Photovoltaic Devices. European Journal of Inorganic Chemistry, 2010(1): p. 17-28. 3. Khare, A., et al., Size control and quantum confinement in Cu2ZnSnS4 nanocrystals. Chemical Communications, 2011. 47(42). 4. Exarhos, S., K.N. Bozhilov, and L. Mangolini, Spray pyrolysis of CZTS nanoplatelets. Chemical Communications, 2014. 50(77): p. 11366-11369.

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University of California Riverside
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