Solution Synthesis of Metal Sulfide Nanoparticles and Thin Films for Solar Photovoltaics

University of Minnesota Ph.D. dissertation. December 2019. Major: Chemical Engineering. Advisor: Eray Aydil. 1 computer file (PDF); viii, 113 pages.Metal sulfides, such as copper zinc tin sulfide (CZTS), zinc sulfide (ZnS), and tin sulfide (SnS), are sustainable materials suitable for energy production, energy storage, and microelectronics. In thin film solar cells, environmentally benign and earth abundant elements can provide safe alternatives to toxic and scarce materials. CZTS and SnS can replace CdTe and copper indium gallium diselenide (CIGS) as light absorbing materials, while ZnS can replace CdS as the n-type material. SnS also has applications in thermoelectrics, piezoelectrics, lithium ion batteries and valleytronics, many of which take advantage of its layered 2D structure. This thesis focuses on the solution synthesis of metal sulfide nanoparticles and films. CZTS nanoparticles and SnS nanoplates are made using an organic hot-injection synthesis method, while ZnS films are made with chemical bath deposition (CBD). Solution syntheses are potentially more economical than commonly used vapor deposition methods such as evaporation and sputter deposition. Solution based methods are also versatile, scalable, and offer control over nanocrystal size and shape. Furthermore, nanocrystal dispersions can be used to create large area semiconductor films using roll-to-roll coating and processing. We studied CZTS grain growth in coatings comprised of CZTS nanocrystals (NCs). We synthesized CZTS thin films from colloidal nanocrystal dispersions dropcast onto a Mo-coated soda lime glass and annealed in a sulfur environment. Mo is a common electrical contact in thin film solar cells and soda lime glass is used because it contains impurities (Na and K) known to improve grain growth in CIGS and CZTS. Unfortunately, sulfur easily diffuses through the NCs and reacts with Mo to create MoS2. Higher annealing times increases CZTS grain growth but also increase MoS2 growth. To increase CZTS grain growth and restrict MoS2 growth, we incorporated sodium impurities from the vapor phase. CZTS grain sizes improved with increasing sodium concentration, while MoS2 growth was limited. We investigated the synthesis of Zn(S,O) thin films via chemical bath deposition. Zn(S,O) films were deposited on Mo-coated silicon, from aqueous solutions of ZnSO4, SC(NH2)2, and NH4OH. Compositional depth profiles revealed that oxygen incorporation depends on reaction temperature, SC(NH2)2 concentration, and NH4OH concentration. Oxygen percentage increased with increasing reaction temperature and SC(NH2)2 concentration, and it remained constant with increasing ZnSO4 concentration. The NH4OH concentration controls the solubility of ZnS and Zn(OH)2 and, as a result, controls the oxygen percentage in the films. The films with the lowest oxygen contained ~13% oxygen. To reduce oxygen concentration below this level, we used an alternative synthesis based on the thermal decomposition of zinc diethyldithiocarbamate in organic solvents. This resulted in ZnS films with ~7% oxygen, an improvement on the CBD synthesized films, but carbon contamination emerged as a new problem. Finally, we present a facile chemical synthesis of SnS nanoplates with thickness ranging from only a few bilayers (3 - 10 nm) to ~200 nm and lateral sizes of several microns, via thermal decomposition of a single precursor, tin(IV) diethyldithiocarbamate (Sn(dedtc)4) dissolved in oleic acid (OA) and injected into hot (300 - 340 °C) oleylamine (OLA). Using a battery of characterization methods that include tip-enhanced Raman spectroscopy (TERS) and FTIR, we delineate the roles of oleylamine and oleic acid in the synthesis, and rationalize the factors that determine the thickness and lateral sizes of the nanoplates. Initially 3 - 7 nm thick and 10s of nm wide SnS2 nanoplates nucleate and grow but these are subsequently reduced by oleylamine to form SnS nanoplates. The SnS nanoplate morphology depends on the OA/OLA ratio with a narrow window near ~1 yielding few-layer (3 - 10 nm) thick and several micron wide SnS plates. The FTIR and TERS data suggest that few layer SnS nanoplates form near OA/OLA ~1 because deprotonated oleic acid and an oleylamide that forms upon reaction of OA and OLA adsorb and block island nucleation and growth on SnS nanoplates. We demonstrate an intriguing new property of SnS nanoplates whereby nanoplate dispersions respond to an electric field by forming dendritic patterns