The objective of this work is to understand how the thin film characteristics of p-type organic and polymer semiconductors affect their electronic properties in microelectronic applications. To achieve this goal, three main objectives were drawn out: (1) to create single-crystal organic field-effect transistors and measure the intrinsic charge carrier mobility, (2) to develop a platform for measuring and depositing polymer thin films for organic field-effect transistors, and (3) to deposit polythiophene thin films for inorganic-organic hybrid solar cells and determine how thin film properties effect device performance. Pentacene single-crystal field-effect transistors (OFETs) were successfully manufactured on crystals grown via horizontal vapor-phase reactors designed for simultaneous ultrapurification and crystal growth. These OFETs led to calculated pentacene field-effect mobility of 2.2 cm2/Vs. During the sublimation of pentacene at atmospheric pressure, a pentacene disporportionation reaction was observed whereby pentacene reacted with itself to form a peripentacene, a 2:1 cocrystal of pentacene:6,13-dihydropentacene and 6,13-dihydropentacene. This has led to the proposal of a possible mechanism for the observed disproportionation reaction similar to other polyaromatic hydrocarbons, which may be a precursor for explaining the formation of graphite. Several silicon-based and PET-based field-effect transistor platforms were developed for the measurement of mobility of materials in the thin film state. These platforms were critically examined against one another and the single-crystal devices in order to determine the optimal device design for highest possible mobility data, both theoretically based on silicon technology and commercially based on individual devices on flexible substrates. Novel FET device designs were constructed with a single gate per device on silicon and PET as well as the commonly used common-gate device. It was found that the deplanarization effects and poor gate insulator quality of the individual gate devices led to lower overall performance when compared to the common gate approach; however, good transistor behavior was observed with field modulation. Additionally, these thin films were implemented into inorganic-organic hybrid and purely organic solid-state photovoltaic cells. A correlation was drawn between the thin film properties of the device materials and the overall performance of the device. It was determined that each subsequent layer deposited on the device led to a planarization effect, and that the more pristine the individual layer, the better device performance. The hybrid cells performed at VOC = 0.8V and JSC = 55A/cm2.

Photovoltaics (PV) offers a unique opportunity to solve energy and environmental problems simultaneously since the solar energy is essentially free, unlimited, and not localized any part of the world. Currently, more than 90% of PV modules are produced from crystalline Si. However, wafer preparation of cast multicrystalline Si materials account for more than 40% of the PV module manufacturing cost, which can be significantly reduced by introducing the ribbon-type Si materials. Edge-defined film-fed grown (EFG) and String Ribbon Si materials are among the promising candidates for the cost-effective PV because they are grown directly from the Si melt, which eliminates the need for ingot slicing and chemical etch for surface preparation. However, the growth of these ribbon Si materials leads to relatively high concentration of metallic impurities and structural defects, resulting in very low as-grown carrier lifetime of less than 5 µs. Therefore, the challenge is to produce high-efficiency cells on EFG and String Ribbon Si by enhancing the carrier lifetime during the cell processing and to understand the effect of electrically active defects on cell performance through in-depth device characterization and modeling. The research tasks of this thesis focus on the understanding, development, and implementation of defect passivation to enhance the bulk carrier lifetime in ribbon Si materials for achieving high-efficiency cells. It is shown in this thesis that the release of hydrogen from SiNx layer is initially rapid and then slows down with time. However, the dissociation of hydrogen from defects continues at the same pace. Therefore, a short firing provides an effective defect passivation. An optimized hydrogenation process produces a record high-efficiency ribbon Si cells (4.0 cm2) with photolithography (18.3%) and screen-printed (16.8%) contacts. However, active defects are still present even after the optimized hydrogenation process. An analytical model is developed to assess the impact of inhomogeneously distributed active defects on cell performance, and the model is applied to establish the roadmap for achieving high-efficiency ribbon Si cells in the presence of defects. Finally, PC1D simulations reveal that the successful implementation of the surface texturing can raise the cell efficiency to 18%.

The pure organic dye sensitized solar cells (DSSCs) were studied and a new organic dye of donor-π-2acceptors (D-π-2A configuration) was fabricated. This dye, denoted as B2, was investigated and applied in DSSCs. Density functional theory (DFT) was used to examine the electronic distribution of the frontier orbitals of the B2 dye. It was found that intramolecular charge transfer (ICT) between the donor moieties and acceptor moieties of the B2 dye may take place under photo irradiation. The LUMO, LUMO+1 and LUMO+2 of B2 are all distributed on the acceptor moieties and this is very helpful to enhance the intramolecular electron transfer from the donor moieties to the acceptor moieties, which will consequently promote the chance of electron injection into the semiconductor. DSSCs based on B2 demonstrated an power conversion efficiency of 3.62 %. This efficiency value is approximately half of the power conversion efficiency of DSSCs based on N719 (7.69 %) under the same conditions. Femtosecond transient absorption and nanosecond transient absorption (TA), and time-correlated single photon counting (TCSPC) technique were applied to examine the electron transfer processes occurring on the surface of B2/TiO2. B2 dye has life time of the excited states three orders in magnitude shorter than that of N719. The electron injection time from excited B2 to TiO2 is also three orders in magnitude shorter than that from excited N719 to TiO2. It was revealed that the delocalized electrons of π → π* transition for both the B2 dye and the N719 dye could be further guided into the semiconductor, while such injection processes may not happen for the localized electrons in π → π* transition of these dyes. The nanosecond transient absorption and transient emission spectroscopy of the ruthenium bipyridyl sensitizer N719 in different solvents were studied. Three kinds of ZnO nanoparticles were utilized to study the electron transfer process taking place on the interface of N719/ZnO with and without electrolyte by Time-Correlated Single Photon Counting (TCSPC) technique, TA and transient emission spectroscopy. Isopropanol was found to stabilize the singlet excited state of N719 and a related emission band centered at 460 nm was observed in nanosecond time scale. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics on the N719/ZnO interface. In the absence of electrolyte, the electron transfer process on the N719/ZnO interface is dependent upon the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, the impact of ZnO defects upon the electron transfer process is eliminated and the effective electron injection happens from the excited states of N719 to ZnO, in spite of the ZnO particle sizes. The polymer based solar cells were studied and a polymer incorporated with a pyrenylcarbazole pendant was synthesized and applied in the functionalization of multi-wall carbon nanotubes (MWCNT) by noncovalent π-π interaction. The polymer/MWCNT hybrids were isolated and examined. The strong interaction between the polymer and MWCNT in a 1,1,2,2-tetrachloroethane (TCE) solution was investigated. The emission spectra demonstrated an effective quenching of emission from the polymer by the MWCNT. DFT calculations showed an electron delocalization phenomenon between the pyrene and carbazole moieties. The LUMO of the polymer is mainly located on the pyrene moiety while the LUMO+1 of the polymer is predominantly positioned on the carbazole moiety. The electronic transition of LUMO+1→LUMO results in intramolecular charge transfer (ICT) from the carbazole moieties to the pyrene moieties. Femtosecond TA determined the characteristic TA feature of the excited states, which are contributed from both the pyrene and carbazole moieties. The excited state lifetime of the polymer was calculated to be 659 ps and the photo excited electrons can inject into the MWCNT very fast on a time scale of 420 fs. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Donor-acceptor and thiophene based π-conjugated molecules and polymers, along with fullerene derivatives, are extensively used active components in the photoactive layer of organic photovoltaic devices. In this dissertation, we make use of several computational methodologies to investigate structure-property relationships of these organic systems in their molecular forms. We begin with an overview of the field of organic photovoltaics and some of the important problems in organic solar cells that are currently being investigated. This is then followed by a brief review of the electronic-structure methods (e.g. Hartree-Fock theory, Density Functional Theory, and Time-dependent Density Functional Theory) that are employed. We then present the main results of the dissertation. Chapter 3 provides a broad overview on how changes to the donor-acceptor copolymer chemical structure impacts its intrinsic geometric, electronic, and optical properties. Chapter 4 focuses on the characterization of the lowest excited-states and optical absorption spectra in donor-acceptor copolymers. In Chapter 5, we investigate the effects of alkyl side-chain placements in the π-conjugated backbone of oligothiophenes and how that impacts their intramolecular properties as well as the oligomer:fullerene interfacial interactions. Chapter 6 presents our investigation on the role of oligomer:fullerene configuration and reorganization energy on exciton-dissociation and charge-recombination processes. Finally, a synopsis of the work and further considerations are presented in Chapter 7.

The investigation carried out in this project allowed for the development of eleven regioregular π-conjugated alternating copolymers and their implementation in organic solar cells. The eleven synthesized polymers, poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(4,7-dithien-2-yl-2,1,3-benzothiadiazole)] (CB), poly[(2,7-(9,9-dioctyl-9H-fluorene-2,7-diyl))-alt-(1,6-pyrene)] (LP), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(5,5’’’-(3,3’’’-dihexyl-2,2':5',2'':5'',2'''-quarterthiophene))] (CT), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(2,7-9H-fluoren-9-one)] (CF), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(1,6-pyrene)] (CP), poly[(2,7-(9,9-dioctyl-9H-fluorene-2,7-diyl))-alt-(4,7-dithien-2-yl-2,1,3-benzothiadiazole)] (LB), poly[(2,7-(9,9-dioctyl-9H-fluorene-2,7-diyl))-alt-(2,7-9H-fluoren-9-one)] (LF), poly[(5,5’’’-(3,3’’’-dihexyl-2,2':5',2'':5'',2'''-quarterthiophene))-alt-(2,7-9H- fluoren-9-one)] (TF), poly[(2,7-(9,9-dioctyl-9H-fluorene-2,7-diyl))-alt-(4,4'-dioctyl-2,2'-bithiophene)] (oTLT), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(4,4'-dioctyl-2,2'-bithiophene)] (oTCT), poly[(2,7-(9-(heptadecan-9-yl)-9H-carbazole))-alt-(4,4'-dihexyl-2,2'-bithiophene)] (TCT), were investigated using theoretical methods that included semi-empirical geometry optimizations, density functional theory (DFT) energy calculations, and time-dependent density functional theory (TD-DFT) optical absorption predictions. The absorption predictions gave credence to our experimental results in which the absorption of the longer polymer chains underwent a redshift from the monomer absorption. With several of the prepared polymers, bulk-heterojunction photovoltaic cells were fabricated and their photovoltaic activity was investigated. Several of the fabricated cells exhibited photovoltaic efficiencies including polymer/PCBM composites with an aluminum back electrode (CF, CT, P3HT, and MEH-PPV), and also inverted cells with a silver back electrode (CT, P3HT, and MEH-PPV). Several polymers (CF, CT, TCT, LP, oTCT, oTLT, P3HT, and MEH-PPV) were used to solubilize single-walled carbon nanotubes (SWNTs). The solubility of the nanotubes occurred by the polymers’ ability to wrap the tubes, disrupt the bundles (ropes of tubes), and allow for the creation of a homogeneous mixture. Polymer:PCBM:SWNT mixtures were prepared and utilized as the active layer in BHJ solar cells. Some of the inverted cells (with a silver back electrode) that incorporated the nanotube composites (CT, oTCT, oTLT, P3HT, and MEH-PPV) displayed photovoltaic activity. These preliminary results illuminate the photovoltaic behavior of the polymer and provide evidence for their future use in polymer solar cells. / Thesis (Master, Chemistry) -- Queen's University, 2011-02-13 22:09:00.464

Nano structured solar cells provide an opportunity for increased open circuit voltages and and short circuit currents in solar cells due to quantum confinement of the window and absorber materials and an increase in the optical path length for the incident light. In this study, both bulk and nano heterojunctions of CIS/CdS and Cu2S/CdS devices have been fabricated and studied on plain glass substrates and inside porous alumina templates to compare their performance. The devices have also been characterized SEM, XRD and JV measurements. The J-V curves have also been analyzed for series resistance, diode ideality factor and reverse saturation current density.

Thin-film solar cells are widely recognised to have the potential to compete with fossil fuels in the electricity market due to their low cost per peak Watt. The Thin-Film Group at the University of New South Wales (UNSW) is engaged in developing polycrystalline silicon (poly-Si) thin-film solar cells on glass using e-beam evaporation technology. We believe our solar cells have the potential of significantly lowering the manufacturing cost compared to conventional, PECVD-fabricated thin-film solar cells. After years of materials research, the focus of the Group??s work is now moving to the metallisation of evaporated solar cells. Minimising various kinds of losses is the main challenge of the cell metallisation procedure, within which the contact resistance is always a big issue. In this thesis, the contact resistance of aluminium contacts on poly-Si thin-film solar cells on glass is investigated. To the best of the author??s knowledge, this is the first ever contact resistance investigation of Al contacts on evaporated poly-Si material for photovoltaic applications. Various transmission line models (TLM) are employed to measure the contact resistance. An improved TLM model is developed to increase the measurement precision and, simultaneously, to simplify the TLM pattern fabrication process. In order to accommodate the particular requirements of poly-Si coated glass substrates, a TLM pattern fabrication process using photolithography is established. Furthermore, a Kelvin sense tester is set up to ensure an accurate measurement of the contact resistance. After establishment of the TLM technique at UNSW, it is successfully tested on singlecrystalline silicon wafer samples. The thermal annealing process of the contacts is also optimised. Then, the general behaviour of Al contacts on uniformly doped poly-Si films (i.e., no p-n junction) is investigated using the verified TLM technique. The long-term stability of the contacts is also studied. This is followed by an investigation of the contact resistance of the back surface field and emitter layers of different types of poly-Si thin-film solar cells. Finally, a novel contact resistance measurement model is proposed that is believed to be able to overcome the measurement bottleneck of the transmission line models.

Thesis (Ph. D.)--Rutgers University, 2008. / "Graduate Program in Electrical and Computer Engineering." Includes bibliographical references (p. 118-122).

This thesis presents a study of hybrid solar cells, specifically looking at various methods which can be employed in order to increase the power conversion efficiency of these devices. The experiments and results contained herein also present a very accurate picture of how rapidly the field of hybrid solar cells has progressed within the past three years. Chapters 1 and 2 present the background and motivation for the investigations undertaken, as well as the relevant theory underpinning solar cell operation. Chapter 2 also gives a brief review of the literature pertinent to the main types of devices investigated in this thesis; dye-sensitised solar cells, semiconductor sensitized solar cells and perovskite solar cells. Descriptions of the synthetic procedures, as well as the details of device fabrication and any measurement techniques used are outlined in Chapter 3. The first set of experimental results is presented in Chapter 4. This chapter outlines the synthesis of mesoporous single crystals (MSCs) of anatase TiO₂ as well as an investigation of its electronic properties. Having shown that this material has superior electronic properties to the conventionally used nanoparticle films, they were then integrated into low temperature processed dye-sensitised solar cells and achieved power conversion efficiencies of > 3%, exhibiting electron transport rates which were orders of magnitude higher than those obtained for the high temperature processed control films. Chapter 5 further investigates the use of MSCs in photovoltaic devices, this time utilising a more strongly absorbing inorganic sensitiser, Sb₂S₃. Utilising the readily tunable pore size of MSCs, these Sb₂S₃ devices showed an increase in voltage and fill factor which can be attributed to a decrease in recombination within these devices. This chapter also presents the use of Sb₂S₃ in the meso-superstructured configuration. This device architecture showed consistently higher voltages suggesting that in this architecture, charge transport occurs through the absorber and not the mesoporous scaffold. Chapters 6 and 7 focus on the use of hybrid organic-inorganic perovskites in photovoltaic devices. In Chapter 6 the mixed halide, lead-based perovskite, CH₃NH₃PbI_3-xCl_x is employed in a planar heterojunction device architecture. The effects of Lewis base passivation on this material are investigated by determining the photoluminescence (PL) lifetimes and quantum efficiencies of treated and untreated films. It is found that passivating films of this material using Lewis bases causes an increase in the PLQE at low fluences as well as increasing the PL lifetime. By globally fitting these results to a model the trap densities are extracted and it is found that using these surface treatments decreases the trap density of the perovskite films. Finally, these treatments are used in complete solar cells resulting in increased power conversion efficiencies and an improvement in the stabilised power output of the devices. Chapter 7 describes the materials synthesis and characterisation of the tin-based perovskite CH₃NH₃SnI₃ and presents the first operational, lead-free perovskite solar cell. The work presented in this thesis describes significant advances in the field of hybrid solar cells, specifically with regards to improvements made to the nanostructured electrode, and the development and implementation of more highly absorbing sensitizers. The improvements discussed here will prove to be quite important in the drive towards exploiting solar power as a clean, affordable source of energy.

abstract: III-V multijunction solar cells have demonstrated record efficiencies with the best device currently at 46 % under concentration. Dilute nitride materials such as GaInNAsSb have been identified as a prime choice for the development of high efficiency, monolithic and lattice-matched multijunction solar cells as they can be lattice-matched to both GaAs and Ge substrates. These types of cells have demonstrated efficiencies of 44% for terrestrial concentrators, and with their upright configuration, they are a direct drop-in product for today’s space and concentrator solar panels. The work presented in this dissertation has focused on the development of relatively novel dilute nitride antimonide (GaNAsSb) materials and solar cells using plasma-assisted molecular beam epitaxy, along with the modeling and characterization of single- and multijunction solar cells. Nitrogen-free ternary compounds such as GaInAs and GaAsSb were investigated first in order to understand their structural and optical properties prior to introducing nitrogen. The formation of extended defects and the resulting strain relaxation in these lattice-mismatched materials is investigated through extensive structural characterization. Temperature- and power-dependent photoluminescence revealed an inhomogeneous distribution of Sb in GaAsSb films, leading to carrier localization effects at low temperatures. Tuning of the growth parameters was shown to suppress these Sb-induced localized states. The introduction of nitrogen was then considered and the growth process was optimized to obtain high quality GaNAsSb films lattice-matched to GaAs. Near 1-eV single-junction GaNAsSb solar cells were produced. The best devices used a p-n heterojunction configuration and demonstrated a current density of 20.8 mA/cm2, a fill factor of 64 % and an open-circuit voltage of 0.39 V, corresponding to a bandgap-voltage offset of 0.57 V, comparable with the state-of-the-art for this type of solar cells. Post-growth annealing was found to be essential to improve Voc but was also found to degrade the material quality of the top layers. Alternatives are discussed to improve this process. Unintentional high background doping was identified as the main factor limiting the device performance. The use of Bi-surfactant mediated growth is proposed for the first time for this material system to reduce this background doping and preliminary results are presented. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017