Browsing by Subject "Charge transport"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
Item Development of new experimental techniques for studying transport and recombination in organic and inorganic thin film solar cells(2011-05) Lombardo, Christopher Joseph; Dodabalapur, Ananth, 1963-; Banerjee, Sanjay; Bank, Seth; Korgel, Brian; Tutuc, Emanuel; Yu, EdwardFor more than 20 years, scientists have studied solar cells made from organic semiconductors. Throughout this time, device structures have evolved from bilayer devices to bulk heterojunction (BHJ) devices and even though efficiencies are approaching 10%, scientists still know relatively little about the transport of charge carriers and recombination mechanisms in these materials. Novel structures, based on lateral BHJ solar cells, have proven to be versatile tools to study transport and recombination mechanisms. In addition, these structures can easily be employed by researchers and solar cell manufacturers to determine the quality and measure the improvement of their materials. For these studies, poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been employed due to its wide use among researchers as well as potential for commercialization. DC photocurrent measurements as a function of device length have yielded the mobility-lifetime product and the generation rate of free carriers within these BHJ devices. In addition to these parameters, the recombination rate as a function of light intensity provides information about the mechanisms of recombination. For example, by measuring the recombination rate as a function of applied electric field and light intensity we have found that recombination is unimolecular in nature and shifts to bimolecular at increased electric field strengths. Additionally, the mobility-lifetime product, generation rate, and recombination mechanism have been studied as a function of applied electric field, illumination spectrum, illumination intensity, etc. This information has provided much insight on physics of the P3HT:PCBM material system which did not exist before these studies.Item Device physics and charge transport of field-effect transistors based on advanced organic semiconductors and graphene(2012-12) Ha, Tae-Jun; Dodabalapur, Ananth, 1963-; Neikirk, Dean P; Bank, Seth; Akinwande, Deji; Nadkarni, SuvidThis dissertation consists of six chapters: In the first chapter, electrical and material properties and charge transport in organic semiconductors and graphene based field-effect transistors (FETs) are introduced. In the second chapter, device architectures of indenofluorene-phenanthrene copolymer based thin-film transistors (TFTs) are discussed. The combination of recessed source/drain and surface treatments on electrical contact and low-voltage-operated TFTs with solution-processed high-k dielectric are investigated. In the third chapter, device physics and charge transport of diketopyrrolopyrrole-naphthalene copolymer based TFTs are discussed. Top-gate TFTs with the polymer dielectric exhibit mobilities of ~1 cm2/V-s and charge transport measurements in steady-state and under non-quasi-static conditions reveal device physics in dual-gate configuration. In the fourth chapter, device characteristics and charge transport in ambipolar diketopyrrolopyrrole-benzothiadiazole copolymer based TFTs are focused. The ambipolar polymer TFTs possess balanced electron and hole mobilities which are both > 0.5 cm2/V-s. The trap density of states is calculated using two analytical methods developed by Lang et al. and Kalb and Batlogg. In the fifth chapter, charge transport of diketopyrrolopyrrole-thiophene copolymer based TFTs employing 4-point-probe configuration is studied. Such polymer TFTs possess the mobilities of up to 3 cm2/V-s. The activation energy as a function of carrier concentration represents multiple trapping and thermally release model or Monroe-type model of charge transport. In the sixth chapter, transformation of electrical characteristics of graphene FETs with an interacting capping layer of fluoropolymers and pi-conjugated organic semiconductors is investigated. The electrical properties of graphene by wafer-scale chemical vapor deposition can be favorably tuned by fluorocarbon capping methods.Item Investigation of charge transport in organic photovoltaic materials using lateral device structures(2016-08) Slobodyan, Oleksiy Viktor; Dodabalapur, Ananth, 1963-; Bank, Seth; Vanden Bout, David; Yu, Edward; Wang, ZhengUnderstanding of charge carrier transport and recombination in bulk heterojunction (BHJ) materials is important for continued improvement of organic photovoltaics (OPVs). Solar cell efficiencies now approach 12% and answers to lingering questions create a roadmap for increasing this value. OPVs are made as vertical structures and majority of analyses in literature are directed to this structure. In this dissertation, theoretical and experimental analyses of lateral devices are developed to compliment the knowledge base established with vertical devices. Lateral OPVs offer unique insights into transport and recombination physics in BHJs: they decouple charge extraction from charge photogeneration, allow clear formation of space-charge regions and recombination zone, open the BHJ to probing, and allow comparison of ambipolar to unipolar electron & hole currents. Lateral OPVs are simulated to understand their current-voltage behavior and link it to development of space-charge. Modeling focuses on the intermediate 3um channel length. At this transport length effects of space-charge behavior are clearly present and all photogenerated charge can be extracted. Modeling work is used to support analysis of experimental results. BHJs made of electron-transporter [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and hole transporting polymer poly[3-hexylthiophene-2,5-diyl] (P3HT) and co-polymer poly[2-(5-(4,4-dioctyl-4H-silolo[3,2-b:4,5-b’]dithiophen-2-yl)-3-tetradecylthiophen-2-yl)-5-(3-tetradecylthiophen-2-yl)thiazolo[5,4-d]thiazole] (PDTSi-TzTz) are studied. Transport in PDTSi-TzTz:PCBM is analyzed by profiling the channel potential. The channel potential and current-voltage measurements are used to obtain carrier mobilities and recombination rates. High charge collection efficiency is found even at transport lengths greater than 1 micron. Photocurrent and extracted unipolar injection currents in P3HT:PCBM blends are studied. These measurements yield intensity-dependent mobilities of both electrons and holes. Extraction of both mobility values in the same BHJ point to electron mobility as the limiting factor in OPV performance.Item Lateral device techniques for characterizing organic bulk heterojunction photovoltaic materials(2014-08) Danielson, Eric Lewis; Dodabalapur, Ananth, 1963-This work is focused on developing novel techniques for characterizing organic bulk heterojunction (BHJ) materials for organic photovoltaic (OPV) applications. Polymer:fullerene BHJs are a promising class of photovoltaic materials, but an improved understanding of the charge transport processes and materials science of BHJs is required for them to effectively compete with other photovoltaic systems. Key parameters of BHJ systems that need to be evaluated include both electron and hole mobilities, the carrier concentrations, the recombination mechanism and the recombination coefficient. For these studies, poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) have been characterized due to its wide use among researchers. Traditional characterization techniques have focused on transient measurements in a vertical device configuration, but we demonstrate the use of lateral BHJ devices as materials diagnostic platforms. Lateral devices allow for direct access to the active layer for spatially resolved and environmental effect measurements. The devices are also measured under steady state operation, similar to a working OPV cell. Under these conditions, lateral BHJ devices exhibit space charge limited transport behavior. A detailed charge transport model is presented to describe the potential, electric field, and carrier concentration profiles of lateral BHJ devices, as well as the current versus voltage characteristics of different regions of the device. We are able to calculate the slower carrier mobility from photocurrent measurements of lateral devices and the carrier mobility ratio from the device potential profile, even in ambipolar BHJ systems. In situ potentiometry is used to construct detailed potential profiles of the device channel and calculate both carrier mobilities. The carrier concentration and recombination coefficient are calculated from lateral conductivity measurements, and we show that bimolecular recombination is the dominant mechanism in bulk P3HT:PCBM. A simplified in situ potentiometry and photocurrent measurement technique is presented to measure the time evolution of organic BHJ performance. Due to the open geometry of the lateral BHJ device, we are also able to monitor the change in key charge transport parameters, including the recombination mechanism, in response to environmental degradation, analyte exposure, and ambient temperature. We show increased geminate recombination in P3HT:PC₇₁BM after prolonged light exposure. Lateral BHJ device measurements offer a useful complement to measurements on vertical photovoltaic structures and provide a more complete and detailed picture of OPV materials.Item Metal oxide photoelectrodes for solar water splitting(2015-08) Rettie, Alexander John; Mullins, C. B.; Bard, Allen J; Humphrey, Simon M; Hwang, Gyeong S; Korgel, Brian AEfficient solar water splitting – using sunlight to produce hydrogen from water – has been an ambitious goal of the scientific community for over 40 years. At its heart this is a materials problem, with the photoelectrodes used in a photoelectrochemical cell having to satisfy all the constraints of a photovoltaic material (light absorption, charge transport) as well as being stable in water and having appropriately positioned band edges. Of the metal oxide systems studied for this purpose, we identified iron oxide (hematite, α-Fe2O3), tungsten trioxide (WO3) and an emerging (at the time) material, bismuth vanadate (BiVO4) as the most promising. In this dissertation we sought to understand and address the shortcomings of these materials, namely, carrier transport in BiVO4 and α-Fe2O3 and light absorption in WO3. We synthesized high quality single crystals of undoped and Mo and W-doped BiVO4 using the floating zone technique and carried out fundamental transport measurements. Electrons were shown to form small polarons and the Hall effect mobility was low, ~10-1 cm2 V-1 s-1 at 300 K. Critically, the mobility measured by the Hall effect may be vastly different from the drift mobility. Small-polaron hopping was found to be in the adiabatic regime and anisotropic conductivity was related to the structural arrangement of vanadium ions. Electrons are also thought to form small polarons in α-Fe2O3, but a thorough analysis had not been performed. We grew single crystals of Ti:α-Fe2O3 and characterized their electron transport to evaluate this model and probe the large anisotropy thought to occur between the basal planes. These revealed that the adiabatic small-polaron model was appropriate. Interestingly, electron transport in Ti:α-Fe2O3 was shown to be near-isotropic, contradicting the common view in the literature. Finally, we studied the effects of sulfur or iodine incorporation in WO3 with the aim to improve its visible light harvesting ability. Both of these impurities did increase visible light absorption, but performance was degraded in all cases except for very low concentrations of sulfur doping. These impurities likely form inter-gap defect bands which allow the absorption of longer wavelength light, but also degrade transport properties if present in large amounts.Item Polarization and charge transport in polymer and 2-D material based field-effect transistors(2016-12) Kim, Seohee; Dodabalapur, Ananth, 1963-; Lee, Jack; Bank, Seth R.; Akinwande, Deji; Tao, HuThis work investigates new semiconductors that are currently being considered for use in flexible electronics. It is critical to understand charge transport physics in field-effect transistors (FETs) to get better device performance and integrate them into commercial products. The charge transport studies include understanding of effect of wide-range of applying voltage (sub-threshold and above threshold operation), dominant charge transport mechanisms under specific conditions (band, band-like and hopping transport), and of polarization effect (polaronic and non-polaronic transport). Charge transport in high-mobility diketopyrrolopyrrole (DPP) co-polymers FETs is investigated. Sub-threshold regime conduction, including diffusion- and drift-limited regimes, are accurately modeled with above threshold regime. Based on modeling results, the realistic density of states (DOS) curve of polymer FETs in a wide range of gate voltages is calculated. From modeled sub- and above- threshold regime mobility data, dominant charge transport mechanisms under specific conditions are investigated. Shallow states charge transport is well-described by multiple trap and release (MTR) transport, while hopping transport models such as variable range hopping (VRH) or Gaussian disorder based model (GDM) can describe deep states charge transport. The transition between the conduction regimes is a function of temperature and carrier density. In addition, a polarization effect from polar molecules in both atmosphere and dielectric on the electronic properties of polymer FETs is demonstrated. High-k and low-k surface dielectric devices are measured in polar and non-polar atmospheres. Dipoles in both conditions affect conduction in polymer FETs, but have different aspects, such as uniformly shifted DOS or only shallow states shifted DOS. The improved electrical characteristics of graphene monolayer sheet FETs with fluoropolymer capping is explained via measurements under polar vapor flow, which is reversible and non-destructive. It is found that the higher dipole moment of polar molecules corresponds to better improvement of electrical properties of graphene FETs, including the Dirac voltage shift, mobility and residual carrier concentration. In addition, a similar experiment is applied to graphene nano-ribbon (GNR) FETs. GNR FETs, which have high on/off ratio but some degraded electrical characteristics due to a considerable number of edge defects, also show a highly improved performance under polar vapor.