Vacuum Deposition, Characterization and Property Engineering of Cu2BaGe1-xSnxSe4 Films and Their PV Applications

Abstract

Kesterite Cu2ZnSnS4-xSex (CZTSSe) has once gained wide attention as a potential alternative to the CdTe and Cu(In,Ga)(S,Se)2 (CIGSSe) photovoltaic (PV) technologies, which are currently facing challenges in terms of scalability due to the use of the scarcity (Te, and In) and toxicity (Cd) of the elements. However, the similarities between the Cu and Zn atoms in terms of cation size and coordination environment result in the formation of a high density of Cu- and Zn-related anti-site defects and defect clusters in the CZTSSe lattice and limit the open-circuit voltage and efficiency of the CZTSSe solar cells. To target suppressing the formation of anti-site defects and related defect clusters, Cu2-II-IV-X4 (II = Sr, Ba; IV = Ge, Sn; X = S, Se) compounds, which have a significantly larger and chemically more differentiated group-2 element (i.e., Ba, and Sr) instead of Zn, have been introduced. Among Cu2-II-IV-X4 compounds, Cu2BaSnS4 (CBTS) and Cu2BaSnS4-xSex (CBTSSe) with trigonal structure (P31 space group) have been the first materials to have gained attention, and their thin-film deposition using both solution- and vacuum-based techniques, as well as their PV devices, have already been demonstrated. On the other hand, there are only a handful of studies on Cu2BaGeSe4 (CBGSe) and Sn-alloyed Cu2BaGe1-xSnxSe4 (CBGTSe) systems, which have the same crystal structure as CBTS and CBTSSe. In this dissertation, we explore film growth, material properties, property engineering methods, and PV application of this relatively unexplored system, CBGTSe, to get a better understanding of this compound as a potential PV material. To achieve this goal, the following studies are conducted and presented throughout this dissertation: (1) development of a deposition process for high-quality CBGTSe films, which yield functioning solar cells, and examination of the associated solar cell properties; (2) characterization of important optoelectronic properties of CBGSe and CBGTSe films and identification of major bottleneck for their solar cell performance; and (3) demonstration of two different film modification strategies (i.e., alloying and doping) for CBGTSe films and investigation of associated changes in the film properties.The first study demonstrates a high-quality CBGTSe film growth via sequential vacuum deposition (i.e., sputtering, and evaporation) of elemental layers (i.e., Cu, Ba, Ge, and Sn) followed by a selenization step to convert the metallic layer of Cu–Ba–Ge–Sn into CBGTSe compound. This work investigates film growth mechanisms via ex-situ analysis and reveals the critical process parameters (i.e., pre-annealing temperature, and Cu-content) for high-quality films. Additionally, functioning solar cell devices based on CBGTSe as light absorber are demonstrated for the first time. Second, the optoelectronic properties of CBGSe (Sn-free) films are also investigated in detail and compared with its isostructural CBTS using various analysis techniques, including temperature-dependent photoluminescence (PL), Hall effect, photoelectron spectroscopies, optical-pumped terahertz probe spectroscopy (OPTP), and open-cell time-resolved microwave conductivity (oc-TRMC), which reveal possible bottlenecks for the solar cell performance and possible directions for the improvement. Next, two different modification approaches—i.e., 1) alloying with Ag, and 2) doping with group-1 (alkali metals) elements—, which have been used for the existing CIGSSe and CZTSSe technologies, are demonstrated to modify overall optoelectronic properties of the CBGTSe films. First, the partial substitution of Cu by Ag is examined as a potential film property modification strategy. The study reveal how much Cu can be substituted with Ag while maintaining its original trigonal crystal structure and how phase purity, morphology, charge carrier properties, band positions, and recombination properties, which are all critical for the PV and optoelectronic applications, change as a function of Ag-content. The intrinsic background carrier densities for CBGTSe films are relatively low (p = ~1012 cm-3) compared to other related chalcogenides (p = 1015–1017 cm-3 for CIGSSe, and CZTSSe), which can limit its applications as photovoltaic, thermoelectronic, and optoelectronic devices. Therefore, as prospective dopants for the CBGTSe films, alkali elements (Li, Na, K, and Rb) are evaluated to address the low hole carrier density and potentially to allow for property tunability. The study demonstrates orders of magnitude enhancement in hole carrier density via alkali-doping. The changes in other film properties (i.e., film morphology, carrier mobility, and minority carrier lifetime) with alkali-element doping are also examined. Additionally, to address inappropriate band alignment within solar cells based on the Cu2-II-IV-X4 family (e.g., Cu2BaGe1-xSnxSe4, Cu2BaSnS4-xSex), which typically has shown noticeably lower electron affinity (EA) than conventional CdS/i-ZnO/ITO buffer/window stacks, we introduce Zn1-xCdxS/Zn1-xMgxO/ZnO:Al as an alternative low-EA buffer/window stack. The low-EA buffer and window layers contribute to improvement in the properties of CBTSSe solar cells and yield a maximum PCE of 6.5% (with MgF2 anti-reflection coating), which represents the current record PCE for CBTSSe-based solar cells. The study reveals that the alternative buffer/window stack improves overall recombination properties for the CBTSSe solar cells from band offset estimation using photoelectron spectroscopy, and recombination property analysis. In addition, device modeling and simulation results provide directions for further improvement of device performance. The works presented in this dissertation provide baseline understanding and knowledge on film synthesis, material properties, property engineering, and associated solar cells for the CBGTSe compound as well as for the relevant Cu2-II-IV-X4 multinary chalcogenide compounds.