Browsing by Subject "CZTS"

Now showing 1 - 7 of 7
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    Copper zinc tin sulfide (Cu2ZnSnS4) photovoltaic material development and thin film solar cells
    (2016-03) Zhang, Liyuan
    Copper zinc tin sulfide (Cu2ZnSnS4, or CZTS) is emerging as an alternative light absorbing material to the present thin film solar cell technologies such as Cu(In,Ga)Se2 and CdTe. All the elements in CZTS are abundant, environmentally benign, and inexpensive. In addition, CZTS has a band gap of ~1.5 eV, the ideal value for converting the maximum amount of energy from the solar spectrum into electricity. CZTS has a high absorption coefficient (>104 cm-1 in the visible region of the electromagnetic spectrum) and only a few micron thick layer of CZTS can absorb all the photons with energies above its band gap. A two-stage process of CZTS thin film synthesis is presented, which consists of sequential thermal evaporation of copper, tin and zinc layers followed by a heat treatment in the presence of sulfur vapor (sulfurization) in a sealed quartz ampoule. The metal precursor stacking order, deposition rate and thickness of each metal layer can be adjusted to give uniform metal precursor stacks of controlled morphology and composition. The effects of sulfurization temperature, time, substrate material, metal precursor stacking order, and back contact layer on the morphological and structural properties of the CZTS films are investigated. Observations of grain size changes and compositional modification are made and explained in terms of the likely secondary phases present. CZTS thin film solar cells were fabricated and the effects of chemical composition were studied both on the absorber layer properties and on the final solar cell performance. It is confirmed that CZTS thin film chemical composition affects the carrier concentration profile, which then influences the solar cell properties. Only a small deviation from the optimal chemical composition can drop device performance to a lower level, which confirms that the CZTS solar cells with high conversion efficiency existed in a relatively narrow composition region. Besides CZTS absorber chemical composition study, post deposition rapid thermal annealing (RTA) was conducted and its influence on solar cell performance was studied. It is observed that post deposition RTA would lead to an increase of device performance. Through C-V measurement results, we have shown that post RTA of CZTS solar cell affects the CZTS/CdS interfacial defect concentration and zero bias depletion depth, which means the defect-related charge at CZTS/CdS interface reduces and it improves Voc and the fill factor.
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    Exciton Dynamics in Alternative Solar Cell Materials: Polymers, Nanocrystals, and Small Molecules
    (2014-07) Pundsack, Thomas
    To keep fossil fuel usage in 2040 even with 2010 usage, 50% of global energy will need to come from alternative sources such as solar cells. While the photovoltaic market is currently dominated by crystalline silicon, there are many low-cost solar cell materials such as conjugated polymers, semiconductor nanocrystals, and organic small molecules which could compete with fossil fuels. To create cost-competitive devices, understanding the excited state dynamics of these materials is necessary.The first section of this thesis looks at aggregation in poly(3-hexylthiophene) (P3HT) which is commonly used in organic photovoltaics. The amount of aggregation in P3HT thin films was controlled by using a mixture of regioregular and regiorandom P3HT. Even with few aggregates present, excited states were found to transfer from amorphous to aggregate domains in <50 fs which could indicate efficient long-range energy transfer.To further study P3HT aggregation, a triblock consisting of two P3HT chains with a coil polymer between them was investigated. By changing solvents, aggregation was induced in a stable and reversible manner allowing for spectroscopic studies of P3HT aggregates in solution. The polarity of the solvent was adjusted, and no change in excited state dynamics was observed implying the excited state has little charge-transfer character.Next, the conduction band density of states for copper zinc tin sulfide nanocrystals (CZTS NCs) was measured using pump-probe spectroscopy and found to be in agreement with theoretical results. The density of states shifted and dilated for smaller NCs indicative of quantum confinement. The excited state lifetime was found to be short (<20 ps) and independent of NC size which could limit the efficiency of CZTS photovoltaic devices.Finally, triplet-triplet annihilation (TTA) was studied in platinum octaethylporphyrin (PtOEP) thin films. By analyzing pump-probe spectra, the product of TTA in PtOEP thin films was assigned to a long-lived metal-centered state. To elucidate the mechanism of TTA, the annihilation dynamics were modeled using second order kinetics as well as Förster and Dexter energy transfer. Dexter energy transfer provided the best fits and the most reasonable fitting parameters.
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    Investigating the effect of Sodium and Potassium on the formation of Copper Zinc Tin Sulfide Films
    (2015) Wrasman, Cody
    Copper Zinc Tin Sulfide (CZTS) has the potential to produce inexpensive solar cells from earth abundant materials. The effects of sodium and potassium ions on the formation of Copper Zinc Tin Sulfide crystals from metal precursors was investigated. It was found that the addition of these ions resulted in increases in the CZTS crystal size under identical processing conditions. Written to complete the URS requirement for Cody Wrasman.
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    New materials for chalcogenide based solar cells
    (2013-06) Tosun, Banu Selin
    Thin film solar cells based on copper indium gallium diselenide (CIGS) have achieved efficiencies exceeding 20 %. The p-n junction in these solar cells is formed between a p-type CIGS absorber layer and a composite n-type film that consists of a 50-100 nm thin n-type CdS followed by a 50-200 nm thin n-type ZnO. This dissertation focuses on developing materials for replacing CdS and ZnO films to improve the damp-heat stability of the solar cells and for minimizing the use of Cd. Specifically, I demonstrate a new CIGS solar cell with better damp heat stability wherein the ZnO layer is replaced with SnO2. The efficiency of solar cells made with SnO2 decreased less than 5 % after 120 hours at 85 oC and 85 % relative humidity while the efficiency of solar cells made with ZnO declined by more than 70 %. Moreover, I showed that a SnO2 film deposited on top of completed CIGS solar cells significantly increased the device lifetime by forming a barrier against water diffusion. Semicrystalline SnO2 films deposited at room temperature had nanocrystals embedded in an amorphous matrix, which resulted in films without grain boundaries. These films exhibited better damp-heat stability than ZnO and crystalline SnO2 films deposited at higher temperature and this difference is attributed to the lack of grain boundary water diffusion. In addition, I studied CBD of Zn1-xCdxS from aqueous solutions of thiourea, ethylenediaminetetraacetic acid and zinc and cadmium sulfate. I demonstrated that films with varying composition (x) can be deposited through CBD and studied the structure and composition variation along the films' thickness. However, this traditional chemical bath deposition (CBD) approach heats the entire solution and wastes most of the chemicals by homogenous particle formation. To overcome this problem, I designed and developed a continuous-flow CBD approach to utilize the chemicals efficiently and to eliminate homogenous particle formation. Only the substrate is heated to the deposition temperature while the CBD solution is rapidly circulated between the bath and a chilled reservoir. We have demonstrated Zn1-xCdxS films for a variety of (x) values, with and without varying (x) across film thickness.
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    Solution Synthesis of Metal Sulfide Nanoparticles and Thin Films for Solar Photovoltaics
    (2019-12) Trejo, Nancy
    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.
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    Synthesis and characterization of copper zinc tin sulfide nanoparticles and thin films.
    (2012-07) Khare, Ankur
    Copper zinc tin sulfide (Cu2ZnSnS4, or CZTS) is emerging as an alternative material to the present thin film solar cell technologies such as Cu(In,Ga)Se2 and CdTe. All the elements in CZTS are abundant, environmentally benign, and inexpensive. In addition, CZTS has a band gap of ~1.5 eV, the ideal value for converting the maximum amount of energy from the solar spectrum into electricity. CZTS has a high absorption coefficient (>104 cm-1 in the visible region of the electromagnetic spectrum) and only a few micron thick layer of CZTS can absorb all the photons with energies above its band gap. CZT(S,Se) solar cells have already reached power conversion efficiencies >10%. One of the ways to improve upon the CZTS power conversion efficiency is by using CZTS quantum dots as the photoactive material, which can potentially achieve efficiencies greater than the present thin film technologies at a fraction of the cost. However, two requirements for quantum-dot solar cells have yet to be demonstrated. First, no report has shown quantum confinement in CZTS nanocrystals. Second, the syntheses to date have not provided a range of nanocrystal sizes, which is necessary not only for fundamental studies but also for multijunction photovoltaic architectures. We resolved these two issues by demonstrating a simple synthesis of CZTS, Cu2SnS3, and alloyed (Cu2SnS3)x(ZnS)y nanocrystals with diameters ranging from 2 to 7 nm from diethyldithiocarbamate complexes. As-synthesized nanocrystals were characterized using high resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and energy dispersive spectroscopy to confirm their phase purity. Nanocrystals of diameter less than 5 nm were found to exhibit a shift in their optical absorption spectra towards higher energy consistent with quantum confinement and previous theoretical predictions. Thin films from CZTS nanocrystals deposited on Mo-coated quartz substrates using drop casting were found to be continuous but highly porous. Annealing CZTS nanocrystal films at temperatures as low as 400°C led to an intense grain growth; however, thin films from CZTS nanocrystals cracked on annealing due to their high porosity. Although quantum confinement in CZTS is only accessible in nanocrystals of diameters less than 5 nm, the high volume of the ligands as compared to the volume of the nanocrystals makes it a challenge to deposit continuous compacted thin films from small nanocrystals. Films deposited from thermal decomposition of a stoichiometric mix of metal dithiocarbamate complexes were found to be predominantly CZTS. These films from complexes were found to be continuous but microporous. The diameter of the spheres making up the microporous structure could be changed by changing the anneal temperature. The structural composition of the final film could be altered by changing the heating rate of the complexes. CZTS exists in three different crystal structures: kesterite, stannite, and pre-mixed Cu-Au (PMCA) structures. Due to the similarity in the crystal structures, it is extremely difficult to distinguish them based on X-ray diffraction. We computed the phonon dispersion curves for the three structures using ab-initio calculations, and found characteristic discontinuities at the Γ-point which can potentially be used to distinguish the three. In addition, the Γ-point phonon frequencies, which correspond to the Raman peak positions, for the three structures were found to be shifted from each other by a few wavenumbers. By deconvoluting the experimental Raman spectra for both CZTS and Cu2ZnSnSe4 (CZTSe) using Gaussian peaks, we observed that the most intense Raman scattering peak in both CZTS and CZTSe is a sum of two different peaks which correspond to scattering from their respective kesterite and stannite phases. The electronic, structural, and vibrational properties of a series of CZTS-CZTSe alloys (CZTSSe) were studied using ab-initio calculations. The S-to-Se ratio and the spatial distribution of the anions in the unit cell were found to determine the energy splitting between the electronic states at the top of the valence band and the hole mobility in CZTSSe alloys and solar cells. X-ray diffraction patterns and phonon distribution curves were found to be sensitive to the local anion ordering. The predicted Raman scattering frequencies and their variation with x agree with experimentally determined values and trends.
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    Synthesis, deposition, and microstructure development of thin films formed by sulfidation and selenization of copper zinc tin sulfide nanocrystals
    (2014-08) Chernomordik, Boris David
    Significant reduction in greenhouse gas emission and pollution associated with the global power demand can be accomplished by supplying tens-of-terawatts of power with solar cell technologies. No one solar cell material currently on the market is poised to meet this challenge due to issues such as manufacturing cost, material shortage, or material toxicity. For this reason, there is increasing interest in efficient light-absorbing materials that are comprised of abundant and non-toxic elements for thin film solar cell. Among these materials are copper zinc tin sulfide (Cu2ZnSnS4, or CZTS), copper zinc tin selenide (Cu2ZnSnSe4, or CZTSe), and copper zinc tin sulfoselenide alloys [Cu2ZnSn(SxSe1-x)4, or CZTSSe]. Laboratory power conversion efficiencies of CZTSSe-based solar cells have risen to almost 13% in less than three decades of research. Meeting the terawatt challenge will also require low cost fabrication. CZTSSe thin films from annealed colloidal nanocrystal coatings is an example of solution-based methods that can reduce manufacturing costs through advantages such as high throughput, high material utilization, and low capital expenses. The film microstructure and grain size affects the solar cell performance. To realize low cost commercial production and high efficiencies of CZTSSe-based solar cells, it is necessary to understand the fundamental factors that affect crystal growth and microstructure evolution during CZTSSe annealing. Cu2ZnSnS4 (CZTS) nanocrystals were synthesized via thermolysis of single-source cation and sulfur precursors copper, zinc and tin diethyldithiocarbamates. The average nanocrystal size could be tuned between 2 nm and 40 nm, by varying the synthesis temperature between 150 °C and 340 °C. The synthesis is rapid and is completed in less than 10 minutes. Characterization by X-ray diffraction, Raman spectroscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy confirm that the nanocrystals are nominally stoichiometric kesterite CZTS. The ~2 nm nanocrystals synthesized at 150 °C exhibit quantum confinement, with a band gap of 1.67 eV. Larger nanocrystals have the expected bulk CZTS band gap of 1.5 eV. Several micron thick films deposited by drop casting colloidal dispersions of ~40 nm CZTS nanocrystals were crack-free, while those cast using 5 nm nanocrystals had micron-scale cracks. We showed the applicability of these nanocrystal coatings for thin film solar cells by demonstrating a CZTS thin film solar cell using coatings annealed in a sulfur atmosphere. We conducted a systematic study of the factors controlling crystal growth and microstructure development during sulfidation annealing of films cast from colloidal dispersions of CZTS nanocrystals. The film microstructure is controlled by concurrent normal and abnormal grain growth. At 600 °C to 800 °C and low sulfur pressures (50 Torr), abnormal CZTS grains up to 10 µm in size grow on the surface of the CZTS nanocrystal film via transport of material from the nanocrystals to the abnormal grains. Meanwhile, the nanocrystals coarsen, sinter, and undergo normal grain growth. The driving force for abnormal grain growth is the reduction in total energy associated with the high surface area nanocrystals. The eventual coarsening of the CZTS nanocrystals reduces the driving force for abnormal crystal growth. Increasing the sulfur pressure by an order of magnitude to 500 Torr accelerates both normal and abnormal crystal growth though sufficient acceleration of the former eventually reduces the latter by reducing the driving force for abnormal grain growth. For example, at high temperatures (700-800 oC) and sulfur pressures (500 Torr) normal grains quickly grow to ~500 nm which significantly reduces abnormal grain growth. The use of soda lime glass as the substrate, instead of quartz, accelerates normal grain growth. Normal grains grow to ~500 nm at lower temperatures and sulfur pressures (i.e., 600 °C and 50 Torr) than those required to grow the same size grains on quartz (700 °C and 500 Torr). Moreover, carbon is removed by volatilization from films where normal crystal growth is fast. There are significant differences in the chemistry and in the thermodynamics involved during selenization and sulfidation of CZTS colloidal nanocrystal coatings to form CZTSSe or CZTS thin films, respectively. To understand these differences, the roles of vapor pressure, annealing temperature, and heating rate in the formation of different microstructures of CZTSSe films were investigated. Selenization produced a bi-layer microstructure where a large CZTSSe-crystal layer grew on top of a nanocrystalline carbon-rich bottom layer. Differences in the chemistry of carbon and selenium and that of carbon and sulfur account for this segregation of carbon during selenization. For example, CSe2 and CS2, both volatile species, may form as a result of chalcogen interactions with carbon during annealing. Unlike CS2, however, CSe2 may readily polymerize at room temperature and one atmosphere. Carbon segregation may be occurring only during selenization due to the formation of a Cu-Se polymer [i.e., (CSe2-x)] within the nanocrystal film. The (CSe2-x) inhibits sintering of nanocrystals in the bottom layer. Additionally, a fast heating rate results in temperature variations that lead to transient condensation of selenium on the film. This is observed only during selenization because the equilibrium vapor pressure of selenium is lower than that of sulfur. The presence of liquid selenium during sintering accelerates coarsening and densification of the normal crystal layer (no abnormal crystal layer) by liquid phase sintering. Carbon segregation does not occur where liquid selenium was present.