Thesis (Ph.D.)--University of Central Florida, 2008. / Adviser: Neelkanth G. Dhere. On t.p. "x" and "y" are subscripts. Includes bibliographical references (p. 130-142).

Dissertation (Ph.D.)--University of Toledo, 2009. / Typescript. "Submitted as partial fulfillment of the requirements for the Doctor of Philosophy in Physics." Bibliography: leaves 132-148.

Thesis (M.S.)--Florida State University, 2006. / Advisor: Peter Kalu, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed Sept. 22, 2006). Document formatted into pages; contains x, 77 pages. Includes bibliographical references.

In today's "silicon age" in which we live, field-effect transistors (FET) are the workhorse of virtually all modern-day electronic gadgets. Although silicon currently dominates most of these electronics, layered 2D transition metal dichalcogenides (TMDCs) have great potential in low power optoelectronic applications due to their indirect-to-direct band gap transition from bulk to few-layer and high on/off switching ratios. TMDC WSe2 is studied here, mechanically exfoliated from CVT-grown bulk WSe2 crystals, to create a few-layered ambipolar FET, which transitions from dominant p-type behavior to n-type behavior dominating as temperature decreases. A high electron mobility μ>150 cm2V-1s-1 was found in the low temperature region near 50 K. Temperature-dependent photoconduction measurements were also taken, revealing that both the application of negative gate bias and decreasing the temperature resulted in an increase of the responsivity of the WSe2 sample. Besides TMDCs, Group III-VI van der Waals structures also show promising anisotropic optical, electronic, and mechanical properties. In particular, mechanically exfoliated few-layered InSe is studied here for its indirect band gap of 1.4 eV, which should offer a broad spectral response. It was found that the steady state photoconduction slightly decreased with the application of positive gate bias, likely due to the desorption of adsorbates on the surface of the sample. A room temperature responsivity near 5 AW-1 and external quantum efficiency of 207% was found for the InSe FET. Both TMDC’s and group III-VI chalcogenides continue to be studied for their remarkably diverse properties that depend on their thickness and composition for their applications as transistors, sensors, and composite materials in photovoltaics and optoelectronics.

xiii, 84 p. ; ill. / This dissertation describes the deposition and characterization of an amorphous thin film with a composition near that of CuInSe 2 (CIS). The creation of an amorphous intermediate leads to a crystalline film at low annealing temperatures. Thin films were deposited from elemental sources in a custom built high vacuum chamber. Copper-selenium and indium-selenium binary layered samples were investigated to identify interfacial reactions that would form undesired binary intermediate compounds resulting in the need for high temperature annealing. Although the indium-selenium system did not form interfacial compounds on deposit, indium crystallized when the indium layer thickness exceeded 15 angstroms, disrupting the continuity of the elemental layers. Copper-selenium elemental layers with a repeat thickness of over 30 angstroms or compositions with less than 63% selenium formed CuSe on deposit. Several deposition schemes were investigated to identify the proper deposition pattern and thicknesses to form the CIS amorphous film. Simple co-deposition resulted in the nucleation of CIS. A simple stacking of the three elements in the older Se-In-Cu at a repeat thickness of 60 angstroms resulted in the nucleation of CuSe and sometimes CIS. The CIS most likely formed due to the disruption of the elemental layers by the growth of the CuSe. Reduction of the repeat thickness to 20 angstroms eliminated the nucleation of CuSe, as predicted by the study of the binary Cu-Se layered samples, but resulted in the nucleation of CIS, similar to the co-deposited samples. To eliminate both the thick Cu-Se region, and prevent the intermixing of all three elements, a more complex deposition pattern was initiated. The copper and selenium repeat thicknesses were reduced into a Se-Cu-Se-Cu-Se pattern followed by deposition of the indium layer at a total repeat thickness of 60 angstroms. At a Se:Cu ratio of 2:1 and the small repeat thickness, no Cu-Se phases nucleated. Additionally, the Cu-In interface was eliminated. For this deposition scheme, films with a selenium rich composition relative to CuInSez were generally amorphous. Those that were Cu-In rich always nucleated CIS on deposit. Annealing of all samples produced crystalline CIS. / Adviser: David C. Johnson

With the advent of the 21st century, one of the serious problems facing mankind is harmful effects of global warming. Add to that the ever increasing cost of fuel and the importance of development of clean energy resources as alternative to fossil fuel has becomes one of the prime and pressing challenges for modern science and technology in the 21st century. Recent studies have shown that energy related sources account for 50% of the total emission of carbon dioxide in the atmosphere. All research activities are focused on developing various technologies that are capable of converting sunlight into electricity with high efficiency and can be produced using a cost-effective process. One of such technologies is the CuIn1-xGaxSe2 (CIGS) and its alloys that can be produced using cost-effective techniques and also exhibit high photo-conversion efficiency. The work presented here discusses some of the fundamental issues related to high volume production of CIGS thin film solar cells. Three principal issues that have been addressed in this work are effect of reduction in absorber thickness on device performance, micrononuniformity involved with amount of sodium and its effect on device performance and lastly the effect of working distance on the properties of molybdenum back contact. An effort has been made to understand the effect of absorber thickness on PV parameters and optimize the process parameters accordingly. Very thin (

The use of size and shape tunable quantum confinement nanocrystals has many potential applications for use in semiconductors, optics and sensors. The synthesis of lead sulfide (PbS) and copper indium diselenide (CuInSe���) nanoparticles are of particular interest for use in semiconductor, optoelectronics and bio-medical applications. The continuous synthesis of lead sulfide (PbS) and copper indium diselenide (CuInSe���) nanocrystals was undertaken in this work. Quality colloidal nanocrystal synthesis requires three components: precursors, organic surfactants and solvents. The synthesis of the nanocrystals can be thought of as a nucleation event, followed by a subsequent growth period. Both the nucleation and growth rates were found to be dependent upon factors such as temperature, growth time, and precursor concentration. For a continuous flow system the residence time (at nucleation and growth conditions) was also found to be important. In order to separate the nucleation and growth events, injection techniques were employed to achieve rapid nucleation of nanocrystals with final size dictated by the growth temperature and/or residence time through the growth zone of the reaction system. Experimental parameters to investigate the size, shape, and composition of synthesized nanocrystals included injection temperature, growth temperature, residence time, and concentration of organic surfactants. Size tunability was accomplished for both PbS and CuInSe��� nanocrystals where particle sizes less than 10 nm were achieved and the resulting nanocrystal compositions were found to be at the approximate stoichiometric ratios for both PbS and CuInSe���. The materials used for the process tubing and pumps were found to be important as chlorinated reaction byproducts were found to react with the stainless steel tubing and pump heads. Post processing was also found to be important in order to remove any possible reaction by-products and residual precursors from the surface of synthesized nanocrystals. When at least one dimension of the nanocrystal approaches the exciton Bohr radius, the bandgap for the nanocrystal increases. UV-VIS spectroscopy was used to optically characterize synthesized PbS nanocrystals from our continuous flow synthesis. The absorption spectra for the particles demonstrated an absorption onset showing a large blueshift compared to that of bulk PbS. The blueshift matches closely with literature reports of the quantum confinement effect that would be desired when synthesizing PbS nanoparticles at diameters that are less than the PbS exciton Bohr radius of 18 nm. / Graduation date: 2013

Σκοπός της παρούσας διπλωματικής εργασίας είναι η μελέτη της επίδρασης της γωνίας πρόσπτωσης της ηλιακής ακτινοβολίας στη λειτουργία φωτοβολταϊκών κυττάρων, σε περιοχές μεσαίου γεωγραφικού πλάτους. Συγκεκριμένα μελετήθηκε η επίδραση της γωνίας αυτής στη λειτουργία δύο ΦΒ πλαισίων ενός μονοκρυσταλλικού πυριτίου (sc-Si) και ενός δισεληνοϊδιούχου χαλκού (CIS). Τα πλαίσια τοποθετήθηκαν σε ταράτσα κτηρίου του Τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας υπολογιστών του Πανεπιστημίου Πατρών. Οι μετρήσεις ξεκίνησαν τον Οκτώβριο του 2009 και ολοκληρώθηκαν το Σεπτέμβριο του 2010. Τα ΦΒ πλαίσια είχαν νότιο προσανατολισμό δεδομένου ότι τοποθετήθηκαν σε τόπο του βόρειου ημισφαιρίου με γεωγραφικό πλάτος 38ο 32’ . Κατά την πειραματική διαδικασία οι μετρήσεις γίνονταν σε πολλαπλές κλίσεις κάθε φορά και για τα δύο πλαίσια . Μετρήθηκαν το ρεύμα βραχυκύκλωσης, η τάση ανοιχτοκυκλώσεως, η θερμοκρασία πλαισίου και η ηλιακή ακτινοβολία. Υπολογίστηκε ο βαθμός απόδοσης , ο συντελεστή ποιότητας και η γωνία πρόσπτωσης, ανά κλίση ΦΒ πλαισίου. Υλοποιήθηκε αλγόριθμος για τον θεωρητικό υπολογισμό της βέλτιστης γωνίας κλίσης έτσι ώστε η γωνία πρόσπτωσης να γίνεται ελάχιστη ανά χρονική στιγμή (μέγιστης παραγόμενης ενέργειας ) και έγινε σύγκριση της με τα πειραματικά αποτελέσματα. Διαπιστώσαμε πειραματικά την εξάρτηση της παραγόμενης ισχύος από την γωνία πρόσπτωσης της ηλιακής ακτινοβολίας . Επιπλέον μέσω της πειραματικής διαδικασίας υπολογίστηκε η βέλτιστη κλίση πλαισίων ανά εποχή για τόπους γεωγραφικού πλάτους ίδιου με τον τόπο των πειραματικών μετρήσεων . / -

Investigations performed in order to understand the electronic and optoelectronic properties of field effect transistors based on few layers of 2D Copper Indium Selenide (CuIn7Se11) are reported. In general, field effect transistors (FETs), electric double layer field effect transistors (EDL-FETs), and photodetectors are crucial part of several electronics based applications such as tele-communication, bio-sensing, and opto-electronic industry. After the discovery of graphene, several 2D semiconductor materials like TMDs (MoS2, WS2, and MoSe2 etc.), group III-VI materials (InSe, GaSe, and SnS2 etc.) are being studied rigorously in order to develop them as components in next generation FETs. Traditionally, thin films of ternary system of Copper Indium Selenide have been extensively studied and used in optoelectronics industry as photoactive component in solar cells. Thus, it is expected that atomically thin 2D layered structure of Copper Indium Selenide can have optical properties that could potentially be more advantageous than its thin film counterpart and could find use for developing next generation nano devices with utility in opto/nano electronics. Field effect transistors were fabricated using few-layers of CuIn7Se11 flakes, which were mechanically exfoliated from bulk crystals grown using chemical vapor transport technique. Our FET transport characterization measurements indicate n-type behavior with electron field effect mobility µFE ≈ 36 cm^2 V^-1 s^-1 at room temperature when Silicon dioxide (SiO2) is used as a back gate. We found that in such back gated field effect transistor an on/off ratio of ~ 10^4 and a subthreshold swing ≈ 1 V/dec can be obtained. Our investigations further indicate that Electronic performance of these materials can be increased significantly when gated from top using an ionic liquid electrolyte [1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)]. We found that electron field effect mobility µFE can be increased from ~ 3 cm^2 V^-1 s^-1 in SiO2 back gated device to ~ 18 cm^2 V^-1 s^-1 in top gated electrolyte devices. Similarly, subthreshold swing can be improved from ~ 30 V/dec to 0.2 V/dec and on/off ratio can be increased from 10^2 to 10^3 by using an electrolyte as a top gate. These FETs were also tested as phototransistors. Our photo-response characterization indicate photo-responsivity ~ 32 A/W with external quantum efficiency exceeding 10^3 % when excited with a 658 nm wavelength laser at room temperature. Our phototransistor also exhibit response times ~ tens of µs with specific detectivity (D*) values reaching ~ 10^12 Jones. The CuIn7Se11 phototransistor properties can be further tuned & enhanced by applying a back gate voltage along with increased source drain bias. For example, photo-responsivity can gain substantial improvement up to ~ 320 A/W upon application of a gate voltage (Vg = 30 V) and/or increased source-drain bias. The photo-responsivity exhibited by these photo detectors are at least an order of magnitude better than commercially available conventional Si based photo detectors coupled with response times that are orders of magnitude better than several other family of layered materials investigated so far. Further photocurrent generation mechanisms, effect of traps is discussed in detail.

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