Kesterite structured semiconductors copper zinc tin sulfide, selenide and sulfoselenide semiconductors (Cu₂ZnSnS₄, Cu₂ZnSnSe₄ and Cu₂ZnSn(S,Se)₄ respectively) have great potential for the next generation of photovoltaic devices. The bandgap of the sulfoselenide can be tuned from 1.5eV to 1.0eV by varying to ratio of S/(S+Se) from pure sulfide to selenide. Contrary to crystalline silicon solar cells that have dominated the market, this family of semiconductors are made from nontoxic earth abundant elements that allow for the potential to scale up to terawatt production levels. These thin film photovoltaics have a drastically lower cost of manufacturing as they require a smaller initial energy input and less raw material. This work explores the chemistry behind the formation of these semiconductor materials. The molecular precursor solutions were made using Cu₂(tu)₆Cl₂ for the copper source, as the copper is already in the 1+ oxidation state in addition to being coordinated by thiourea. This was then dissolved using dimethyl sulfoxide (DMSO) as the solvent. Using this as a basis, a total of six solutions were made by varying both the tin and zinc compounds added including tin (II) and tin (IV) chlorides and acetates, zinc (II) chloride and a zinc thiourea complex, Zn(tu)₂Cl₂. These solutions were then spin coated onto molybdenum coated soda lime glass to form CZTS films under thermal and further annealed in selenium vapor to form CZTSSe. In some cases, a thin cadmium sulfide (CdS) layer was deposited onto the CZTSSe film via chemical bath deposition to protect the film from photoluminescence measurement. In between each step, samples were collected and characterized using x-ray diffraction, Raman spectroscopy and photoluminescence to determine the film’s composition and detect possible binary or ternary impurities. Results from the six solutions are compared and CZTSSe formation mechanisms are discussed.

With the ever growing global population, demand for new abundant sources of energy is on the rise. Solar power is one such source, and despite technology dating back several decades, it has failed to gain major traction due to both the cost of materials and processing of silicon semiconductors, which currently dominate the market. One promising replacement for silicon is copper tin zinc sulfide (CZTS), and its selenated derivatives, CZTSSe, and CZTSe. The bandgap of these materials is adjustable by varying the ratio of sulfur and selenium, S/(S+Se), and lies between 1.0 and 1.5 eV, an ideal range for photovolatics (PV). CZTS solar cells consist of earth abundant materials which are inexpensive to process. One such method, the molecular precursor route, involves making solutions of the precursor materials, including copper (I), zinc (II), tin (IV), thiourea (tu) as a sulfur source, and dimethyl sulfoxide (DMSO). Once the solution forms, thin films are spin coated onto molybdenum (Mo) coated substrate and thermally annealed at 500°C to form the CZTS crystal structure and absorber layer before subsequent processing steps. In this solution, metal-thiourea, metal-DMSO and metal-thiourea-DMSO complexes of various geometries and constitution are formed. Using density functional theory (DFT), the stability of these complexes has been probed to determine their relative stability in order to understand the contents of the solution and to optimize the precursor solution to improve the efficiency of CZTS. By choosing precursor chemicals which form complexes of varying stability, the CZTS molecular ink formulation can be modified and CZTS absorber material quality can be improved. Complexes such as Zn(DMSO)₂Cl₂, Zn(tu)₂Cl₂, and Sn(DMSO)₆ have been investigated using DFT calculation methods within Gaussian. Both the vacuum and solvated energies have been investigated and compared to gain insight into what complexes are present in the molecular precursor solution.

Defect characterization of a completed solar cell is an important and often tricky step of improving the efficiency of photovoltaic devices. There are many well documented and useful techniques that already exist for doing this. Examples of more prominent techniques include photocurrent vs voltage (JV) and External Quantum Efficiency (EQE) measurements. In general however when preforming these measurements the cell is taken to be homogenous over the area of the cell. Cells tend to be more and less efficient in certain regions however. Laser Beam Induced Current (LBIC) is a method where a local photocurrent is generated in the cell with a laser beam. Using this method of local excitation a high density LBIC map can be generated to give insight into the non-homogenous features of the cell by taking a large number of LBIC measurements and recording the XY position that each point was taken at. Depending on the wavelength of laser used to generate the LBIC map different layers of the cell may also be probed. LBIC can be used to figure out what layers, where in fabrication, and possible after-fabrication damages that lead to drops in efficiency in certain regions of the cell. This knowledge can be applied in future fabrications to increase the overall efficiency of the cell.