Decoupling bulk- and surface-limited lifetimes in thin kerfless silicon wafers using spectrally resolved transient absorption pump-probe spectroscopy and computer simulations

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 84-86).One of the key technological objectives to further decrease the cost of silicon (Si) PV and enable manufacturing of crystalline silicon is to improve the quality of thin, kerfless Si wafers to monocrystalline equivalent. To aid wafer manufacturers to develop high-quality thin Si wafer substrates, performance-limiting defects in the bulk of thin kerfless Si wafers must be identified, and a means to accurately measure the bulk lifetime is necessary. With decreasing wafer thickness, however, the impact of surface recombination increases and dominates the effective lifetime measured by conventional methods. Therefore, the ability to decouple bulk-limited lifetime from surface-limited lifetime is desirable, ideally without the need for surface passivation. Herein, spectrally resolved transient absorption pump-probe spectroscopy and extensive Technology Computer Aided Design simulations are used to decouple the bulk- and surface-limited lifetimes of thin kerfless silicon wafers in a single measurement. A range of sample conditions are studied. It is observed that the technique can successfully provide reasonable upper and lower limits to the bulk and surface recombination parameters for thin kerfless silicon wafers.by Sin Cheng Siah.S.M

X-ray absorption spectroscopy elucidates the impact of structural disorder on electron mobility in amorphous zinc-tin-oxide thin films

We investigate the correlation between the atomic structures of amorphous zinc-tin-oxide (a-ZTO) thin films grown by atomic layer deposition (ALD) and their electronic transport properties. We perform synchrotron-based X-ray absorption spectroscopy at the K-edges of Zn and Sn with varying [Zn]/[Sn] compositions in a-ZTO thin films. In extended X-ray absorption fine structure (EXAFS) measurements, signal attenuation from higher-order shells confirms the amorphous structure of a-ZTO thin films. Both quantitative EXAFS modeling and X-ray absorption near edge spectroscopy (XANES) reveal that structural disorder around Zn atoms increases with increasing [Sn]. Field- and Hall-effect mobilities are observed to decrease with increasing structural disorder around Zn atoms, suggesting that the degradation in electron mobility may be correlated with structural changes.United States. Office of Naval Research (ONR N00014-10-1-0937)National Science Foundation (U.S.) (Award CBET-1032955)National Science Foundation (U.S.) (CAREER Award ECCS-1150878

Improved Cu 2 O-Based Solar Cells Using Atomic Layer Deposition to Control the Cu Oxidation State at the p-n Junction

Chemistry and Chemical Biolog

Defect Engineering in Cuprous Oxide (Cu₂O) Solar Cells

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 105-109).This thesis is focused on the development of a cuprous oxide (Cu₂O) thin-film (TF) solar cell that is fabricated by manufacturing-friendly methods such as electro-deposition, sputtering and atomic layer deposition. Due to its bandgap of close to 2 eV, it has the potential of being applied as top cell in a tandem configuration. Firstly, I perform bottom-up cost and price analysis to investigate the economic feasibility of TF and c-Si based tandem photovoltaic modules. Next, I investigate the formation of good ohmic back contacts on Cu₂O absorber layer and demonstrate that low contact resistivity can be achieved with a variety of metals on heavily doped Cu₂O films by forming a tunnel junction. Then, I apply synchrotron-based X-ray absorption spectroscopy (XAS) to characterize two front contact buffer materials: amorphous Zn-Sn-O (a-ZTO) and Sndoped Ga₂O₃. I elucidate a fundamental loss mechanism in the amorphous Zn-Sn-O (a-ZTO) electron-blocking layer that has origin in local structural disorder and establish the structure-process- property relationship of a-ZTO so that the front buffer layer can be optimized for photovoltaics. Then, I investigate the doping mechanism of Sn dopant atoms in TFs and single crystalline Ga₂O₃:Sn by revealing the doping mechanism so that Ga₂O₃:Sn can be optimized for photovoltaics. Lastly. I apply bulk defect engineering to manipulate the intrinsic point defect structure of Cu₂O towards improved device performance. The key results will inform the processing conditions for improving mobility and minority carrier lifetime in Cu₂O. Keywords - Earth-abundant, thin-film solar cells, tandem, defect engineering, cost modeling, synchrotron.by Sin Cheng Siah.Ph. D

Low contact resistivity of metals on nitrogen-doped cuprous oxide (Cu 2O) thin-films

Forming low-resistivity contacts on cuprous oxide (Cu[subscript 2]O) is an essential step toward demonstrating its suitability as a candidate solar cell material. We measure the contact resistivity of three noble metals (Au, Ag, and Pd) on sputtered Cu[subscript 2]O thin-films with a range of nitrogen doping levels. Using the circular transmission line model, specific contact resistivity as low as 1.1 × 10[superscript −4] Ω · cm[superscript 2] is measured for Pd contacts on heavily doped Cu[subscript 2]O films. Temperature-dependent current-voltage measurements and X-ray photoemission spectroscopy are used to determine the barrier heights formed at metal/Cu[subscript 2] Ointerfaces. Thermionic emission is observed to dominate for undoped films, whilst field emission dominates for heavily doped films, highlighting the importance of carrier concentration on contact resistivity. Finally, we demonstrate that low contact resistivity can be achieved on heavily doped Cu[subscript 2] films using Earth-abundant metals, such as Cu and Ni.National Science Foundation (U.S.) (Award DMR-0819762)National Science Foundation (U.S.) (Award ECS-0335765)National Science Foundation (U.S.). CAREER (Award ECCS-1150878

Hall mobility of cuprous oxide thin films deposited by reactive direct-current magnetron sputtering

Cuprous oxide (Cu[subscript 2]O) is a promising earth-abundant semiconductor for photovoltaic applications. We report Hall mobilities of polycrystalline Cu[subscript 2]O thin films deposited by reactive dc magnetron sputtering. High substrate growth temperature enhances film grain structure and Hall mobility. Temperature-dependent Hall mobilities measured on these films are comparable to monocrystalline Cu[subscript 2]O at temperatures above 250 K, reaching 62 cm[superscript 2]/V s at room temperature. At lower temperatures, the Hall mobility appears limited by carrier scattering from ionized centers. These observations indicate that sputtered Cu[subscript 2]O films at high substrate growth temperature may be suitable for thin-film photovoltaic applications.National Science Foundation (U.S.) (Award DMR-0819762)National Science Foundation (U.S.) (Award ECS-0335765

Economically Sustainable Growth of Perovskite Photovoltaics Manufacturing

The significant capital expense of photovoltaics manufacturing has made it difficult for new cell and module technologies to enter the market. We present two technoeconomic models that analyze the sustainable growth of perovskite manufacturing for an R2R single-junction technology and a perovskite-silicon tandem module, focusing on the impacts of economies of scale and average selling price on profitability. We establish a cost range of $3.30/W to$0.53/W for flexible modules manufactured in factory sizes ranging from 0.3 MW/year to 1 GW/year. In addition, we model the cost to manufacture a tandem module consisting of a single-junction perovskite cell stacked in 4-terminal configuration onto a silicon cell and show how an existing manufacturer can grow at a faster rate by co-investing in tandems. Our analyses highlight potential routes to market for perovskite photovoltaics and the possibility to sustainably grow a photovoltaics manufacturing company even in markets with higher labor rates

Tuning Electrical, Optical, and Thermal Properties through Cation Disorder in Cu2ZnSnS4

Chemical disorder in semiconductors is important to characterize reliably because it affects materials performance, for instance by introducing potential fluctuations and recombination sites. It also represents a means to control material properties, to far exceed the limits of equilibrium thermodynamics. We present a study of highly disordered Cu-Zn-Sn-S (d-CZTS) films along the Cu2SnS3-Cu2ZnSnS4-ZnS binary line, deposited by physical vapor deposition. Deposition at low temperature kinetically stabilizes compositions that are well outside of the narrow, equilibrium solid solution of kesterite (Cu2ZnSnS4). Here we study d-CZTS and its thermal treatment using complementary characterization techniques: X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). We find that cations in d-CZTS are highly disordered while the sulfur anions remain in a well-defined, cubic close-packed lattice. On the atomic scale, composition fluctuations are accommodated preferentially by stacking faults. Kinetically-stabilized cation disorder can produce nonequilibrium semiconductor alloys with a wide range of band gap, electronic conductivity, and thermal conductivity. d-CZTS therefore represents a processing route to optimizing materials for optoelectronic device elements such as light absorbers, window layers, and thermal barriers.Army Research Office (Grant W911NF-16-1-0406

The effect of sub-oxide phases on the transparency of tin-doped gallium oxide

There have been a number of studies on the fabrication of Sn-doped gallium oxide (Ga2O3:Sn) films with both conductive and transparent properties using a variety of deposition methods. However, often, synthesis results in films that are not transparent. In this paper, we examine the mechanisms underlying these results in Ga2O3:Sn thin films prepared at various growth temperatures, Sn concentrations, and oxygen partial pressures. With X-ray absorption spectroscopy, transmission electron microscopy and energy dispersive spectroscopy, we find that when films are grown under the oxygen deficient conditions there are Ga sub-oxide and SnOxphases in the Ga2O3:Sn thin film. These Ga sub-oxide phases are only found in non-transparent films, and so we infer that the Ga sub-oxide is responsible for the non-transparency. These observations suggest that to obtain transparent Ga2O3:Sn, films deposition or subsequent annealing must be carefully controlled in both temperature and oxygen partial pressure to avoid the formation of Ga sub-oxide phases