Solution-Processed Thin Film Deposition and Characterization of Multinary Chalcogenides: Towards Highly Efficient Cu2BaSn(S,Se)4 Solar Devices
Emerging multinary chalcogenide semiconductors have long been in the spotlight for addressing challenges related to the scarcity and toxicity of commercially available solar energy converting materials, CdTe and Cu(In,Ga)(S,Se)2. Kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) materials have emerged as alternatives to address these issues for environmentally benign, low-cost photovoltaic (PV) and photoelectrochemical (PEC) applications. However, the significant degree of detrimental defect formation associated with near identical coordination and atomic size for all the Cu/Zn/Sn cations involved in the structure prevent the PCE values for these materials from reaching the level comparable to already commercialized thin-film solar energy converting technologies. The I2-II-IV-VI4 (I = Li, Cu, Ag; II = Ba, Sr; IV = Si, Ge, Sn; VI = S, Se) family of materials offer tunable structural, optical, and electronic properties that can potentially enable their pervasive application in the solar energy conversion industry, in addition to promise of Earth-abundance and nontoxicity through their individual components. In addition, this family of materials is expected to be disorder-resistant because the constituent atoms share dissimilar chemistry, ionic size, and coordination. As a first example from this family of compounds, experimental and computational analyses for Cu2BaSn(S,Se)4 (CBTSSe) agree with the assertion that the large size and distinct coordination for Ba relative to Cu/Sn inhibits III and III antisite disordering and related band tailing. However, while the CBTSSe system seems to meet the expectation of antisite defect resistance, the broader implications of atomic size/coordination discrimination as a defect control mechanism and the solar energy converting potential of this material remain to be resolved. In this dissertation, the goals, therefore, are fourfold: to develop low-cost thin-film processing methods that deliver efficient solar absorber device performance, to further study disorder within CBTSSe solution-processed films using different measurement techniques, to examine the validity of the constituent element size/coordination discrepancy strategy to control disorder/band tailing in multinary chalcogenide semiconductors, to reveal factors that limit PV performance within CBTSSe devices for future high-performance Earth-abundant photovoltaic technologies. After giving scientific motivations of this dissertation in Chapter 1 and a general introduction to Earth-abundant defect-resistant chalcogenide materials, their depositions, and device applications in Chapter 2, Chapter 3 confronts the first solar absorber device application challenge, i.e., reducing solar cell cost, by investigating solution-based synthetic pathways for thin-film deposition of CBTSSe material. Efficient PEC devices are constructed using CBTSSe thin films deposited from molecular solution and ball milling approaches to provide a preliminary measure of the solution-processed film quality relative to existing co-sputtered CBTSSe analogs. The molecular solution path involves the dissolution of Ba(NO3)2, Cu(CO2CH3)2, and SnI2 in low-toxicity solvent DMSO, followed by spin coating, sulfurization and selenization steps at high temperatures. The ball milling approach, on the other hand, involves dispersion of ball-milled Cu2S, BaS, Sn, and S particles in low-toxicity ethanol, followed by sulfur and selenium treatments at high temperatures. After addressing the challenge related to the formation of a BaSO4 secondary phase (i.e., modifying pre-bake conditions toward high temperatures—e.g., 540 °C—and supplying sulfur during the pre-baking step), a micron thick, single phase CBTSSe absorber layer with grains as large as 4.5 µm and band gap (E_g) of 1.68 eV was grown from the molecular solution approach. The films prepared from molecular solution and ball milling approaches (micron thick and with band gap E_g = 1.56 eV) were employed in a Pt/TiO2/CdS/CBTSSe photocathode structure, exhibiting ~10 mA/cm2 and 5.54 mA/cm2 photocurrent densities, respectively, at 0 V reversible hydrogen electrode (VRHE). The performance levels of these first solution-deposited CBTSSe PEC devices are parallel to the performance of analogous vacuum-deposited CBTSSe films, which offered ~12 mA/cm2 current density at 0 VRHE. The high quality CBTSSe films deposited from the molecular solution approach mentioned in this chapter enabled us to evaluate CBTSSe material defects, optoelectronic properties and PV performance parameters, and how they connect with processing variables and device geometries in Chapters 4-6. These chapters report power conversion efficiencies (PCE) enhancing from 2.9% to a world record value of ~6.5% for this new absorber. Chapter 4 explores intrinsic properties of compositionally stoichiometric large E_g material CBTS (~2 eV) and alloyed CBTSSe with ~1.6 eV E_g using several spectroscopic techniques, including low-temperature photoluminescence (LT-PL). In addition, for alloyed CBTSSe, we diagnose PV device performance limiting factors for a 2.9% PCE device. LT-PL measurements of CBTSSe show a defect PL emission line at ~1.5 eV and deep defect feature at 1.15 eV. This measurement shows no apparent excitonic luminescence features (unlike CBTS). Tracing the unique photoluminescence signatures of these emission lines under various excitation intensities and temperatures helps conclude that the 1.5 eV emission line belongs to shallow quasi donor-acceptor pair (QDAP) recombination, whereas the 1.15 eV emission belongs to deep QDAP transitions. On the other hand, the LT-PL spectrum for CBTS consists of several weak near-band-edge PL emission lines and strong emissions at 2.08 eV and 2.11 eV, and broad/weak defect bands at 1.95/1.6 eV. Based on findings from excitation and temperature dependent PL analysis, PL lines are attributed to bound exciton (at 2.08 eV), free exciton (at 2.11 eV), donor-acceptor pair (1.95 eV), and free to bound transitions (1.6 eV). LT-PL analyses point out that the potential fluctuations are more substantial in Zn-based CZTSSe than CBTSSe, which in turn has stronger fluctuations than CBTS. LT-PL, time-resolved terahertz (THz) and photo electron spectroscopy as well as capacitance voltage analyses on CBTSSe films point out that the presence of deep defects within the band gap, short bare film surface minority carrier lifetimes (~50 ps), non-ideal band alignment at the CdS/CBTSSe interface (i.e., cliff-like 0.63 eV conduction band offset measured by ultraviolet photoemission spectroscopy) and low density of charge carriers are the primary issues that limit the maximum achievable (1.31 V) VOC to 0.47 V for the 1.59 eV E_g material, resulting in 2.9% PCE devices. The physical measurements provided on the stoichiometric solution-processed CBTSSe absorber bring up critical directions for future performance improvement of the devices based on this Earth-abundant I2-II-IV-VI4 family. Chapter 5 focuses on the first VOC enhancement strategy, i.e., improving intrinsic material quality (e.g., minimizing deep defects and increasing charge carriers) by process optimization through Cu stoichiometry tuning. This chapter targets to increase our understanding of the phase stability and optoelectronic property sensitivity for CBTSSe material. According to XRD and SEM analyses, phase purity is sustained throughout a Cu content range (as measured by [Cu] / [Ba+ Sn]) of nominally 0.94 to 1.01, and grain enlargement correlates with Cu content. THz spectroscopy and Hall effect reveal that the lifetime, carrier mobility, p-type conductivity and hole density (~1013 cm-3) are nominally independent of Cu content. The champion PCE exceeds 4.7% for all copper compositions in the phase pure region, with a record value of 5.1%, representing the highest performance level achieved to this point for a solution-processed device using the indium tin oxide (ITO)/intrinsic zinc oxide (i-ZnO)/CdS/ CBTSSe device architecture. This PCE is equivalent to that for the best vacuum-deposited devices using the same device structure and represents 30% relative change in VOC (0.605 V) compared to the devices with 2.9% PCE. These findings suggest that CBTSSe films and solar cells may be less sensitive to Cu stoichiometry compared to kesterite materials and therefore that they may provide a more stable material platform to prepare high quality thin-film solar cells. Finally, Chapter 6 focuses on the second VOC enhancement strategy, i.e., improving interface quality by developing a device architecture that minimizes non-idealities related to band alignment. Chapter 6 demonstrates a >25% improvement in VOC (from ~0.60 V to ~0.76 V) and corresponding enhancement in PCE (~5.1 % to ~6.2 % without anti-reflection coating; ~6.5 % with MgF2 anti-reflection coating, representing the world record PV PCE for the I2-II-IV-IV4 family of materials) for solution-processed CBTSSe solar cells, by introducing an alternative buffer/window stack with a lower electron affinity relative to the conventional CdS/i-ZnO/ITO stack, more in accord with the low electron affinity of the CBTSSe layer. The front stack used for this study consists of a successive ionic layer adsorption and reaction-deposited Zn1-xCdxS buffer combined with sputtered Zn1-xMgxO/Al-doped ZnO window/top contact layers. We have investigated the impact of the front buffer/window by examining the device properties using a combined experimental and device simulation (SCAPS-1D) approach. We show the importance of considering both the buffer and window layer band positions relative to the absorber in optimizing CBTSSe solar cell performance. These chapters help to examine the general validity of the proposition that atomic size/coordination discrimination can be used to target needed defect resistance within the broader family of complex multinary chalcogenide films by comprehensive screening of semiconductor prospects for CBTSSe films obtained via a solution processing approach and targeted towards light-absorbing applications, particularly in PV and PEC solar. The processing-property understanding gained for solution-processed CBTSSe in these chapters can be translated to other I2-II-IV-VI4 family members towards highly efficient solar devices.
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