(388e) Synthesis and Ligand Engineering of Impurity Free Cu(In,Ga)S2 Nanoparticles for Slot-Die Coated Photovoltaics

Solution processing of colloidal semiconducting nanoparticles has emerged as a promising method to fabricate low-cost, high-throughput, and roll-to-roll compatible thin films for applications such as photovoltaics. Colloidal nanoparticle based photovoltaics have demonstrated the highest hydrazine-free efficiencies for stable thin film materials, reaching 15% with a Cu(In,Ga)(S,Se)₂ absorber¹. However, colloidal nanoparticle routes to photovoltaics generally suffer from contamination originating from various processing steps such as metal salt anion inclusion², low-volatility carbonaceous ligands³, and coordinating coating solvents. In this work, a complete pathway from nanoparticle synthesis to device fabrication is developed, which avoids the all of the aforementioned shortcomings typically present in nanoparticle based photovoltaics.

Making use of the versatile amine-thiol solvent/reactant system, heat up, hot injection, and microwave-assisted solvothermal syntheses of colloidal Cu(In,Ga)X₂ (X=S,Se) nanoparticles were developed using only elemental Cu, In, Ga, Se or their metal sulfides/selenides as precursors. This approach completely avoids anionic impurities that metal chloride, iodide, acetate, nitrate, and acetylacetonate salts can introduce. The resultant nanoparticles, their formation pathway, and their ligand chemistry was characterized using XRD, Raman, XRF, TEM, STEM-EDS, FTIR, ¹H-NMR, and GC-MS. To reduce carbon contamination from native ligands, a wide variety of ligand exchange pathways were explored. While ligand exchange is well established in the nanoparticle literature, it has largely focused on binary nanoparticle systems with well-defined surface chemistry⁴. Little work exists for more complex ternary and quaternary nanoparticle systems such as Cu(In,Ga)S₂. Ligand exchange was screened using microwave assisted solvothermal methods with volatile organics such as pyridines, amines, and thiols, as well as room temperature exchanges with inorganic ligands. Greater than 97% removal of native ligands was achieved using optimized ligand exchange pathways. Judicious selection of volatile solvents enabled cleanly drying colloidal inks with stable mass concentrations exceeding 100 mg/mL.

In order to optimize the coating of the colloidal inks, we bypassed the typically used laboratory scale coating techniques such as spin coating, which is not roll-to-roll compatible and has poor materials utilization. Instead a custom laboratory scale slot die coater based on a modified 3D-printer architecture was used to rapidly proof coating conditions using small amounts of ink under 1 mL⁵. Slot die coating affords nearly 100% materials utilization, roll-to-roll compatibility, exceptional film uniformity, and thickness control. Coating uniformity was assessed using absorption scanning along with profilometry to generate three dimensional reconstructions of films. Optimized slot die coatings were used for full device fabrication via selenization of the nanoparticle film.

The colloidal synthesis methods, ligand exchanges, slot die coating, and device fabrication methods developed in this work hold wider applicability to a variety of semiconducting chalcogenide materials, enabling a pathway to scalable, contamination free, solution processed electronic devices.

References:

(1) McLeod, S. M.; Hages, C. J.; Carter, N. J.; Agrawal, R. Synthesis and Characterization of 15% Efficient CIGSSe Solar Cells from Nanoparticle Inks. Prog. Photovoltaics Res. Appl. 2015, 23 (11), 1550–1556.

(2) Kar, M.; Agrawal, R.; Hillhouse, H. W. Formation Pathway of CuInSe₂ Nanocrystals for Solar Cells. J. Am. Chem. Soc. 2011, 133 (43), 17239–17247.

(3) McLeod, S.; Alruqobah, E.; Agrawal, R. Liquid Assisted Grain Growth in Solution Processed Cu(In,Ga)(S,Se)2. Sol. Energy Mater. Sol. Cells 2019, 195, 12–23.

(4) Boles, M. A.; Ling, D.; Hyeon, T.; Talapin, D. V. The Surface Science of Nanocrystals. Nat. Mater. 2016, 15 (2), 141–153.

(5) Vak, D.; Hwang, K.; Faulks, A.; Jung, Y. S.; Clark, N.; Kim, D. Y.; Wilson, G. J.; Watkins, S. E. 3D Printer Based Slot-Die Coater as a Lab-to-Fab Translation Tool for Solution-Processed Solar Cells. Adv. Energy Mater. 2015, 5 (4).