Solution-processed energy harvesting electronic devices using amine-thiol solvent media
Caleb Miskin, Kevin Bock, Robert Boyne, and Rakesh Agrawal
School of Chemical Engineering, Purdue University, West Lafayette, IN
On February 15, 2008 the National Academy of Engineering unveiled their fourteen grand challenges of engineering for the 21st century. At the top of the list and voted by the public as the most important challenge was the thrust to make solar energy economical. My research is dedicated to solving this millennial challenge by developing routes to high-efficiency, solution-processed photovoltaics (PV) for low-cost and low-energy manufacturing. Traditional PV manufacturing has emphasized high-vacuum techniques which are both costly, slow, and energy intensive. Solution-processed techniques performed at ambient temperatures and pressures relieve much of the cost and energy burden of thin-film manufacturing.
My research has primarily advanced two methods for solution processed PV. In one method, semiconducting nanocrystals are synthesized and then suspended in an appropriate solvent to form an ink. The ink is then applied to a substrate by a variety of high-throughput methods such as spray coating or doctor blading and then annealed to form a polycrystalline absorber layer for solar energy. I have applied this method with great success to Cu2ZnSnS4, a promising earth-abundant, non-toxic semiconductor. A challenge with this material is its propensity to form binary and ternary undesired phases. Using advanced nano-characterization techniques, my colleagues and I have been able to determine the spatially resolved composition of these nanoparticles and have found them to be highly non-uniform.1,2 As a result, I have focused on synthesis techniques aimed at controlling the nucleation and growth of this material to improve nanocrystal compositional homogeneity. Though particles produced through my work still show some non-uniformities, they are greatly improved. I have combined these improved synthetic techniques with optimized conditions for grain growth of the absorber layer to achieve micron-sized densely packed grains, while minimizing the unwanted “fine-grain” layer characteristic of most solution-processed CZTSSe devices. As a result, I have been able to advance the published record efficiency for nanocrystal ink based solar cells of CZTS from 7.2 to 9.0 percent.3
Another promising route to solution-processed PV is by directly coating molecular precursor solutions (rather than first forming nanocrystals) and annealing the coating to form the polycrystalline solar absorber layer. While processing in this manner has long been employed by the organic electronic community, ultimately it is desirable to combine the superior performance of inorganic electronic materials with the facile processing of organic solutions. A major challenge is that many salts and metals that would be useful precursors to such films have poor solubility in organic solvents compatible with roll-to-roll manufacturing techniques. Our group and others have overcome this challenge by developing a mixture of commonly available thiols and amines to dissolve a host of materials that are otherwise insoluble in either solvent by itself.4–6 The solvent system has found significant use in processing photovoltaic absorber layers,7–9 luminescent quantum dot films,7 and other thin films10 making it a very general solvent system for processing of inorganic thin films.
In my work, I have focused on CdTe—which has been by far the most successful technology in terms of production cost ($/peak watt) and energy payback time for thin-film solar cells. The solution-processing of this material may provide another step change in cost for this material. In this research thrust I have demonstrated for the first time the fabrication of CdTe thin films via a solution-processed molecular precursor approach by dissolving CdCl2 and Te metal in ethylenediamine and propanethiol. The films are formed by spin-coating ultra-thin layers of the solution and then annealing each layer until a ~1 μm thick film is achieved. A plane view and cross sectional image of an annealed CdTe flim is shown in Figure 1. By optimizing the annealing and deposition conditions I am actively increasing the efficiency of these devices with a goal to reach those currently achieved in vacuum-based production (15-20%).
While amine-thiol mixtures are truly fantastic solvents for molecular precursors, I have found they are highly useful for nanoparticle synthesis as well. The phase of Cu2ZnSnS4 nanoparticles can be changed from tetragonal kesterite to hexagonal wurtzite simply by changing the solvent and sulfur source from dissolved sulfur in oleylamine to a mixture of oleylamine and dodecanethiol.11 The increased solvating power of amine-thiol mixtures also provides a method to tailor the solubility of monomer in solution, thereby altering the temperatures at which species nucleate. I am using this to pursue routes to increasingly monodispersed CZTS nanoparticles both in terms of size and composition by suppressing the formation of certain phases and favoring the nucleation of others. In addition, I have also been successful in exploiting the alkahest capabilities of amine-thiol mixtures for the controlled synthesis of several other useful binary and multinary nanocrystals.
To better understand this solvent system's properties and chemistry so that it might be improved for current uses and tailored for others, I am collaborating with an analytical chemistry group to perform advanced tandem mass spectrometry (ESI and APCI) on the solvent mixtures and solutions to determine the species present. Based on the compositions found, we are employing quantum chemical calculations and modeling to determine the structure of the species. This insight will allow the further development of this already incredibly versatile solvent system for not only PV but other electronic devices as well.
(1) Carter, N. J.; Yang, W. C.; Miskin, C. K.; Hages, C. J.; Stach, E. A.; Agrawal, R. Sol. Energy Mater. Sol. Cells 2014, 123, 189.
(2) Yang, W.-C.; Miskin, C. K.; Carter, N. J.; Agrawal, R.; Stach, E. A. Chem. Mater. 2014, 26 (24), 6955.
(3) Miskin, C. K.; Yang, W. C.; Hages, C. J.; Carter, N. J.; Joglekar, C. S.; Stach, E. A.; Agrawal, R. Prog. Photovoltaics Res. Appl. 2015, 23, 654.
(4) Walker, B. C.; Agrawal, R. Chem. Commun. (Camb). 2014, 50 (61), 8331.
(5) Webber, D. H.; Brutchey, R. L. J. Am. Chem. Soc. 2013, 135 (42), 15722.
(6) Webber, D. H.; Buckley, J. J.; Antunez, P. D.; Brutchey, R. L. Chem. Sci. 2014, 5 (6), 2498.
(7) Tian, Q.; Wang, G.; Zhao, W.; Chen, Y.; Yang, Y.; Huang, L.; Pan, D. Chem. Mater. 2014, 26 (10), 3098.
(8) Yang, Y.; Wang, G.; Zhao, W.; Tian, Q.; Huang, L.; Pan, D. ACS Appl. Mater. Interfaces 2014, 7 (1), 460.
(9) Zhang, R.; Szczepaniak, S. M.; Carter, N. J.; Handwerker, C. A.; Agrawal, R. Chem. Mater. 2015, 27 (6), 2114.
(10) Antunez, P. D.; Torelli, D. A.; Yang, F.; Rabu, F. A.; Lewis, N. S.; Brutchey, R. L. Chem. Mater. 2014, 26 (19), 5444.
(11) Yang, W.-C.; Miskin, C. K.; Hages, C. J.; Hanley, E. C.; Handwerker, C.; Stach, E. A.; Agrawal, R. Chem. Mater. 2014, 26 (11), 3530.