Browsing by Subject "Solar Cells"

Now showing 1 - 5 of 5
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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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    Green Chemistry Approach for The Synthesis of Transition Metal Sulfides based on Cu2ZnSnS4 (CZTS) and Etching of Their Impurities
    (2017-07) Pinto, Alexandre
    Green Chemistry comprises a set of good practices leading to more sustainable and environmentally friendly chemical processes. In the first chapter, this dissertation introduces the Green Chemistry Principles and shows how these Principles can be applied towards the synthesis of transition metal chalcogenides. The next chapter presents results focused on the synthesis of the multinary sulfide Cu2ZnSnS4 (CZTS) using microwave as heating source. The control over the crystalline phase of CZTS was studied as a function of variations in synthetic conditions, such as source of sulfur excess, initial oxidation states of Cu and Sn sources, temperature, and time. A model explaining two different behaviors according to the sulfur excess source is proposed. The third chapter uses the concepts learned from the previous one to develop the synthesis of solid solutions between Cu2ZnSnS4 and Cu2CoSnS4, generating compounds with the formula Cu2(Zn1-xCox)SnS4. Thin films were prepared from aqueous dispersions of these Cu2(Zn1-xCox)SnS4 compounds, and the stability of the films upon annealing in sulfur atmosphere was analyzed. The fourth chapter describes the development of a milder etching solution based on a mixture of ethylenediamine and 2-mercaptoethanol to eliminate undesired copper sulfide (Cu2-XS) or copper selenide (Cu2-XS) phases from CZTS thin films. The development of this etching solution represents a viable alternative to the widely used etching methods based on potassium cyanide (KCN) use. The fifth chapter extends the application of the etching solution to etch other common undesired phases such as ZnS, SnS2, and CuxZnySnz, and a possible mechanism is proposed for the etching process based in a Lewis acid-base reaction between ethylenediamine and 2-mercaptoethanol
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    Hybrid solar cells from polymers and silicon nanocrystals.
    (2009-12) Liu, Chin-Yi
    This thesis is concerned with the application of silicon nanocrystals (Si NCs) in photovoltaic devices. Two types of novel solar cells, hybrid solar cells and Si NCs-only thin-film photovoltaic devices, have been developed. Hybrid solar cells are made from polymers and Si NCs. The first hybrid solar cells were fabricated by using poly- 3(hexylthiophene) (P3HT) which has a good hole mobility and matches the energy band alignment of Si NCs. The solar cell performance of Si NCs/P3HT devices was studied as a function of the weight ratio of Si NCs/P3HT and Si NC size. Three groups of Si NCs were used in this study: Si NCs 3-5 nm in diameter, 5-9 nm in diameter, and 10-20 nm in diameter. The open-circuit voltage and short-circuit current increased by using the smallest size NCs due to the high surface-area-to-volume ratio and quantum confinement effect. Those results indicate that Si NCs are a good candidate as an electron acceptor in hybrid solar cell application. To improve the efficiency of Si NCs/P3HT hybrid solar cells, we started to optimize the fabrication conditions by modification of the polymer concentration, usage of postproduction heat treatment, and application of different metal electrodes. After optimization, a hybrid solar cell from 50wt% (weight ratio) Si NCs/P3HT annealed at 150 °C for 2 hours with aluminum (Al) electrodes had a power conversion efficiency of 1.47% with a fill factor of 0.47, short-circuit current of 3.8 mA/cm2, and open-circuit voltage of 0.8 V under air mass 1.5 direct (AM 1.5D) one sun illumination. To understand the hole mobility of P3HT before and after post-production heat treatment, a hole-only device was fabricated by depositing gold (Au) electrodes, which block electron injection from the electrodes to Si NCs. The results suggest that the hole mobility of 50wt% Si NCs/P3HT film increases one order of magnitude after heat treatment, due to improved crystallinity in the P3HT, which can enhance hybrid solar cell efficiency. Literature has reported that the compatibility of polymers and nanocrystals plays an important role in hybrid solar cell efficiency. Although P3HT is a good hole conductor and light absorber in solar cell applications, other polymers should be tested to find the best compatibility for Si NCs. Knowing this, P3HT was replaced by poly [2-methoxy-5- (3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in 3-5 nm Si NCs/MDMO-PPV hybrid solar cells. Although Si NCs/MDMO-PPV devices have a higher open-circuit voltage than Si NCs/P3HT devices, the power conversion efficiency of Si NCs/MDMO-PPV devices is not as high as that of Si NCs/P3HT devices. To understand the reasons for the low efficiency from Si NCs/MDMO-PPV devices, the hole mobility of MDMO-PPV, energy band alignment between MDMO-PPV and Si NCs, and absorption spectrum of MDMO-PPV were studied and compared to those of P3HT. To measure the hole mobility of MDMO-PPV, Au electrodes were again utilized to block electron injection into the Si NCs. The results show that the hole mobility of MDMOPPV is lower than that of P3HT. The absorption spectrum of MDMO-PPV (400-600 nm) is narrower than that of P3HT (400-650 nm) so that exciton generation in P3HT is more efficient than in MDMO-PPV under AM 1.5 conditions. Additionally, MDMO-PPV has a lower highest occupied molecular orbital level than P3HT so the efficiency of hole injection from Si NCs into MDMO-PPV may not be as efficient as for P3HT. These reasons explain why the efficiency of Si NCs/MDMO-PPV devices is not as good as Si NCs/P3HT devices. From Si NC solution processing, we found that 10-20 nm bare Si NCs without any surface modification can form a stable cloudy colloid with 1,2-dichlorobenzene. This colloid can be spin-cast onto an ITO substrate to form a continuous and dense thin film. A Schottky photovoltaic device consisting of a single layer of intrinsic Si NCs was fabricated in a glove box to verify that films can be cast from colloid Si NCs. This photovoltaic device has a sandwich structure with a 250 nm Si NC layer between ITO and Al electrodes. Under AM 1.5D one sun illumination, the Si NC Schottky device showed a significant photovoltaic response with a power conversion efficiency of 0.02%, a fill factor of 0.26, short circuit-current density of 0.148 mA/cm2, and open-circuit voltage of 0.51 V. This result suggests that the solution processing of bare Si NCs can be a new way to manufacture low-cost and high-quality silicon-based thin films.
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    Study of Heat Losses in Crystalline Silicon and Perovskite Solar Cells
    (2023-08) Tisha, Zakia Tamanna
    Energy from the sun is plentiful and sustainable, making it an excellent alternative to fossil fuels. Photovoltaic (PV) solar cells can directly convert this solar energy into electricity. However, PV solar cells face challenges in achieving high efficiency as some of the captured energy is lost as heat or through other means, reducing efficiency and performance. Researchers are constantly trying to improve the efficiency of solar cells. Silicon-based solar cells are widely used and have practical efficiency that keeps improving, reaching close to the theoretical limit of around 30%. One approach to increase the output of solar cells is converting the heat losses back into electricity, consequently boosting the overall efficiency of solar conversion. This heat recycling can be achieved by integrating photovoltaic (PV) devices with thermoelectric materials, which capture and recycle wasted heat. This thesis aims to lay the groundwork required for achieving this objective by studying the heat loss mechanisms and conducting evaluations of some of those mechanisms.This research focuses on understanding and categorizing the losses in solar cells, particularly the below bandgap energy and thermalization losses, which are responsible for more than half of the total losses. Two types of solar cells, crystalline silicon (c-Si) and CH3NH3PbI3 perovskite (C-P), are studied to analyze their loss characteristics.
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    Thin-Film Synthesis of Metal Halide Perovskites for Optoelectronics
    (2020-08) Clark, Catherine
    Metal halide perovskites (MHPs), like the archetypal methylammonium lead iodide (MAPbI3), have emerged in the last decade as promising materials for efficient, low-cost optoelectronics. MHP solar cells have already reached efficiencies >25%, rivaling established technologies like single-crystal Si. Yet several challenges prevent the widespread commercialization of MHPs, including their instability in ambient conditions, their toxicity, and the need for scaleable fabrication techniques. Fundamentally, the origins of important material properties relating to carrier transport and recombination are still not well understood. Thin film deposition techniques that enable detailed study of process-structure-property relationships and are commercially relevant are consequently becoming increasingly essential. This thesis seeks to address these challenges through the design, implementation, and utilization of a carrier-gas assisted vapor deposition (CGAVD) method that can grow MHP films with highly tunable stoichiometries and morphologies. Alongside the design of a CGAVD system with six independently controllable experimental parameters, an analytical model is developed and experimentally validated that allows the determination of robust and repeatable growth regimes and the prediction of material deposition rates. Harnessing this technique, we demonstrate the ability to deposit MASnI3 and MASnBr3 films and to systematically vary their compositions across a wide range, and realize corresponding changes in film microstructures (grain size, coverage) and electronic properties (resistivity, carrier concentration, mobility). Control of grain size and film texturing is also achieved independent of stoichiometry via modulation of chamber pressure and substrate temperature. The benefits of CGAVD are further highlighted by the successful growth of novel all-MHP heterojunctions. Two stable pairings are identified: MAPbBr3/MASnBr3 and CsPbBr3/MASnBr3. Design rules to control the mixing of heterojunctions are developed by exploring the dependence of mixing rate on MHP layer composition and grain size. Finally, through a collaboration with Physical Electronics, we optimize the use of XPS depth-profiling for MHPs and investigate which ions are diffusing in a layered structure that exhibits mixing. Moving forward, the incorporation of CGAVD-grown heterojunctions and Pb MHPs into optoelectronic devices will harness the tunability of this system towards a deeper understanding of process-structure-property relationships in MHP thin films and novel layered structures.