Browsing by Subject "Organic photovoltaics"
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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 Morphological effects of organic and inorganic semiconducting materials by scanning probe microscopy(2012-12) Glaz, Micah Sivan; Vanden Bout, David A.; Webb, Lauren J.; Zhu, Xiaoyang; Holliday, Bradley J.; Korgel, Brian A.Solution deposition of thin film photovoltaic materials leads to large variations in the morphological and chemical compositions of the film. In order to improve device functionality, it is important to understand how morphology and chemical composition affects charge generation, separation, and collection. This PhD work will first study bulk methods in order to characterize materials in solution and films. The results are then correlated with microscopy studies examining morphology. Other methods used in this PhD work will directly couple spectra and microscopy. Microscopic regions of such films and devices can be illuminated using scanning confocal microscopy or near-field scanning optical microscopy (NSOM), which allows for one to directly probe regions of the film at or below the optical diffraction limit. By scanning the sample over a fixed laser spot we can simultaneously create image maps of the topographical, electrical and optical properties. This technique, known as laser beam induced current (LBIC) allows one to directly probe a local area of a device with 100-300nm resolution. Along with bulk device efficiency studies, near field and confocal data of inorganic and organic materials are investigated. These include devices fabricated with a blend of P3HT (poly[3-hexylthiophene]) and perylene diimide derivatives, and Cu(InxGa1-x)Se2 [CIGS] nanoparticle devices. Finally, we use a new device architecture, a lateral organic photovoltaic (LOPV) in order to spatially resolve transport in functional organic devices.Item Optimization of material composition and processing parameters for hybrid organic-inorganic solar cells(2010-12) Salpeter, Garrett Morgan; Manthiram, Arumugam; Ferreira, PauloThe widespread adoption of hybrid organic-inorganic solar cells has been delayed by low performance. Improving performance requires a firm understanding of how to optimize both material composition and processing parameters. In this thesis, we examine processing parameters that include solution composition, annealing temperature, and the rates of spin casting and evaporative coating. We also find that the optimal weight ratio for the active layer of a ZnO:P3HT solar cell is 40 wt. % ZnO.Item Probing the chemistry and morphology of material interfaces in emerging photovoltaics(2018-06-15) Griffin, Monroe Patrick; Vanden Bout, David A.; Dodabalapur, Ananth; Henkelman, Graeme A.; Mullins, Charles Buddie; Roberts, Sean T.Emerging photovoltaics hold great promise to add to the existing solar cell market by either becoming cost competitive or accessing new, niche markets unavailable to current technologies. Unfortunately, wide adoption of these systems, in particular organic photovoltaics (OPVs) and perovskite photovoltaics, is hampered by overall poor performance in either efficiency (in OPVs) or in long-term stability (in perovskite photovoltaics). Designing better materials and optimizing device morphology/architectures, with the latter changing intrinsic material interfaces within the devices, has made progress to overcome some of these issues. Since these interfaces affect overall efficiency and stability, understanding interface chemistry (composition) and morphology is important in understanding overall device performance. Characterization of these interfaces requires techniques capable of probing the chemistry-morphology relationship at the nanoscale. Here, we use Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Atomic Force Microscopy (AFM) to characterize these material systems and their device interfaces. In P3HT:PCBM, a classic OPV system, we directly imaged the active layer buried morphology and determined the mixing length of the D/A interface, which is crucial for device performance and had yet to be measured. We determined, contrary to the general understanding, that thermal annealing in this system actually leaves the bulk heterojunction film mostly mixed. Our methodology provides a route to determining this D/A interface length for other OPV systems providing a new metric and insight for better device design principles. For MAPbI₃ perovskites, we studied interfacial behavior to understand light-induced degradation in functioning devices and electrical-induced degradation in films based on contact selection as well as visualized bias-induced ion migration. We determined that the PDI-EH electron transport layer hinders degradation; whereas, a PCBM electron transport layer allows and facilitates it. For electrical degradation, we determined that an ITO electrode induces more degradation than a platinum contact by measuring interface MAPbI₃/ITO mixing. And, by using lateral MAPbI₃ devices, we visualized ion migration due to an applied electric field. For perovskite devices, interfacial behavior dictates long-term performance. Overall, using a combination of ToF-SIMS and AFM to interrogate interfacial chemistry and morphology, we are working towards better devices through understanding limits on efficiency and stability.Item Scanning photocurrent microscopy to investigate materials for photovoltaics(2015-12-03) Gutierrez, Marlene; Vanden Bout, David A.; Webb, Lauren; Roberts , Sean T; Bard, Allen; Holliday, Bradley; Dodabalapur, AnanthAs the world becomes even more dependent on energy there is a dire need to find a clean and renewable energy source. Solar energy has the possibility to provide more then enough clean energy the world needs, yet, it is still not an option available to many due to production cost. New cheaper materials are investigated in hopes of building a low cost and efficient photovoltaic (PV). One method that can dramatically lower the cost of production is to use materials that can be deposited from solutions. Although many candidates fit the requirements needed to be solution processable, this technique consistently makes PVs with much lower efficiencies then their crystalline counter parts. Information about charge transfer and extraction processes of the charge carriers are necessary to optimize them. Scanning photocurrent microscopy (SPCM) is a technique that scans a sample across a focused laser beam and collects photocurrent as a function of position. Photoluminescence and reflectance are collected simultaneously providing information about the morphology and recombination. Information gathered from SPCM gives insight to intrinsic recombination and transport properties of the material. This dissertation will look at multiple systems used in PVs. First the space charge regions of a Langevin polymer with and without an additive are compared to see the effects of morphology on collection. A Langevin polymer is then compared to a non-Langevin to find intrinsic differences between them. A perovskite solar is scanned using different polarized light. Finally, A new thin film and new device architecture was probed using SPCM.Item Synthesis of conjugated polymers and block copolymers via catalyst transfer polycondensation(2013-08) Ono, Robert Jun; Bielawski, Christopher W.; Sessler, Jonathan L.Conjugated polymers hold tremendous potential as low-cost, solution processable materials for electronic applications such organic light-emitting diodes and photovoltaics. While the concerted efforts of many research groups have improved the performance of organic electronic devices to near-relevant levels for commercial exploitation over the last decade, the overall performance of organic light-emitting diode and organic photovoltaic devices still lags behind that of their traditional, inorganic counterparts. Realizing the full potential of organic electronics will require a comprehensive, molecular-level understanding of conjugated polymer photophysics. Studying pure, well-defined, and reproducible conjugated polymer materials should enable these efforts; unfortunately, conjugated polymers are typically synthesized by metal-catalyzed step-growth polycondensation reactions that do not allow for rigorous control over polymer molecular weight or molecular weight distribution (i.e., dispersity). Chain-growth syntheses of conjugated polymers would not only allow for precise control over the aforementioned polymer metrics such as molecular weight and dispersity, but could also potentially create new applications by enabling the preparation of more advanced macromolecular structures such as block copolymers and surface grafted polymers. Our efforts toward realizing these goals as well as toward exploiting chain-growth methodologies to better understand fundamental conjugated polymer photophysics and self-assembly will be presented.