Dispositivos fotovoltaicos e termoelétricos orgânico baseados em poli (2,2' - Bitiofeno)

Orientador: Prof. Dr. Ivo A. HümmelgenDissertação (mestrado) - Universidade Federal do Paraná, Setor de Ciências Exatas, Programa de Pós-Graduação em Física. Defesa: Curitiba, 23/02/2016Inclui referências : f. 90-99Resumo: O presente trabalho teve como objetivo analisar a aplicação do polímero orgânico semicondutor polibitiofeno (PBT) em dois diferentes dispositivos geradores de energia, as celulas solares e os termogeradores. Alem disso, visavamos codepositar eletroquimicamente óxido de grafeno reduzido (RGO) e analisa-lo enquanto componente do compósito da camada ativa de dispositivos fotovoltaicos. Dispositivos fotovoltaicos foram fabricados e analisados tendo como camada ativa o PBT e tambem o composito PBT:RGO. Foram efetuadas medidas I x V dos dispositivos e analises morfológicas dos filmes. Os resultados para o PBT foram equivalentes aos encontrados na literatura, e o compósito PBT:RGO nao se mostrou eficiente para este fim. No que se refere a aplicacao do PBT em dispositivos termoelóetricos, um sistema de medidas termoelóetricas para filmes finos com gradiente de temperatura perpendicular a superfície do filme foi construído. Foram testados diferentes eletrodos inferiores, o ouro e o oxido de índio-estanho (ITO). Como eletrodo superior, utilizamos alumínio. As medidas do coeficiente de Seebeck desse material foram realizadas medindo as temperaturas e a diferenca de potencial entre os eletrodos (inferior e superior). Dispositivos com eletrodo inferior de ITO apresentaram coeficientes de Seebeck negativos. Os dispositivos com eletrodo inferior de ouro, por outro lado, apresentaram coeficientes de Seebeck positivos, e com valores acima dos conhecidos para polímeros, ate o momento. Dado o comportamento dual do material, foram fabricados tambem termogeradores completos com estrutura analoga à estrutura comercial, no entanto, utilizando apenas uma camada polimerica e um único eletrodo, o que simplifica enormemente o processo de preparo do dispositivo. Analisaram-se as espessuras com melhores resultados nessa arquitetura, e a caracterizacao do coeficiente de Seebeck do dispositivo. Os resultados mostraram uma nova possibilidade no que se refere a fabricacão de termogeradores, permitindo um dispositivo completo a partir de um unico material. Palavras-chave: Polibitiofeno, dispositivos fotovoltaicos, celulas solares, dispositivos ter- moeletricos, termoeletricidade, coeficiente Seebeck.Abstract: The present work is intended to analyze the application of the organic polymeric semicon-ductor, polybithiophene (PBT), in two different energy-generating devices, solar cells and thermogenerators. Moreover, we sought to co-deposit electrochemically reduced graphene oxide (RGO) and analyze it as a component of the active layer in photovoltaic devices. The photovoltaic devices were fabricated and analyzed having PBT or PBT:RGO composite as active layer. I x V measurements were carried out in the devices and the morphological properties were analyzed. The results for PBT were equivalent to those found on the literature, and the PBT:RGO composite showed itself ineffe ctive for this purpose. A thermoelectric measurement system for thin films with temperature gradient perpendicular to the film surface was built. PBT was then applied in the fabrication of thermoelectric devices. Gold and indium tin oxide (ITO) were experimented as bottom electrodes, whereas, aluminum was used as top electrode. The measurements of the Seebeck coefficient of this material were carried out by measuring the temperatures and the difference of potential between the electrodes (bottom and the top electrode). The ITO bottom electrode devices showed negative Seebeck coefficients, whereas the Au bottom electrode devices have showed positive Seebeck coefficients. In view of the dual material behavior, complete thermogenerators were also fabricated with structure analogous to the commercial devices, however, having a single layer, which enormously simplifies device preparation proceeding. The best results for the thickness in this architecture were studied, and the characterization of the Seebeck coefficient of the device were also made. The results showed a new possibility regarding the fabrication of thermogenerators, allowing a complete device from one single material as active layer. Key-words: Polybithiophene, photovoltaic devices, solar cells, thermoelectric devices, thermoelectricity, Seebeck coefficient

Enhancing sub-bandgap external quantum efficiency by photomultiplication for narrowband organic near-infrared photodetectors

Detection of electromagnetic signals for applications such as health, product quality monitoring or astronomy requires highly responsive and wavelength selective devices. Photomultiplication-type organic photodetectors have been shown to achieve high quantum efficiencies mainly in the visible range. Much less research has been focused on realizing near-infrared narrowband devices. Here, we demonstrate fully vacuum-processed narrow- and broadband photomultiplication-type organic photodetectors. Devices are based on enhanced hole injection leading to a maximum external quantum efficiency of almost 2000% at −10 V for the broadband device. The photomultiplicative effect is also observed in the charge-transfer state absorption region. By making use of an optical cavity device architecture, we enhance the charge-transfer response and demonstrate a wavelength tunable narrowband photomultiplication-type organic photodetector with external quantum efficiencies superior to those of pin-devices. The presented concept can further improve the performance of photodetectors based on the absorption of charge-transfer states, which were so far limited by the low external quantum efficiency provided by these devices

Strong light-matter coupling for reduced photon energy losses in organic photovoltaics

Funding: Volkswagen Foundation (no.93404) (MCG), individual fellowship of the DeutscheForschungsgemeinschaft (404587082) (AM).Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. Here, we exploit strong coupling by embedding a fullerene-free organic solar cell (OSC) photo-active layer into an optical microcavity, leading to the formation of polariton peaks and a red-shift of the optical gap. At the same time, the open-circuit voltage of the device remains unaffected. This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. Interestingly, this implies that strong coupling can be exploited in OSCs to reduce the driving force for electron transfer, without chemical or microstructural modifications of the photo-active layer. Our work demonstrates that the processes determining voltage losses in OSCs can now be tuned, and reduced to unprecedented values, simply by manipulating the device architecture.Publisher PDFPeer reviewe

Photomultiplication Enabling High‐Performance Narrowband Near‐Infrared Organic Photodetectors

Abstract Continuous monitoring of food quality, blood oxygen, or industrial processes require high‐throughput near‐infrared photodetectors. Due to excellent properties like low‐cost fabrication, flexibility and narrowband response, organic photodetectors (OPDs) have a huge market potential for such applications. An organic donor–acceptor blend with a low‐energy and broad charge transfer (CT) feature is utilized, circumventing the difficulties of obtaining organic materials with significant absorption beyond 1000 nm. The increasing recombination of such low‐energy gap materials that is detrimental for the quantum efficiency is overcome by applying two photocurrent multiplication (PM) mechanisms to the donor–acceptor blend. Combined with an optical micro‐cavity, this OPD achieves a spectral response (SR) of 15 A W−1 at 1092 nm. With its spectrally narrow response of only 18 nm, this OPD technology can be used for highly resolved measurements. Contrary to OPDs working in the photovoltaic mode, this detector is optimized for operation under reverse bias. With its high spectral response, low‐cost readout circuitry like CMOS can be used for signal detection

Photomultiplication-Type Organic Photodetectors for Near-Infrared Sensing with High and Bias-Independent Specific Detectivity

Highly responsive organic photodetectors allow a plethora of applications in fields like imaging, health, security monitoring, etc. Photomultiplication-type organic photodetectors (PM-OPDs) are a desirable option due to their internal amplification mechanism. However, for such devices, significant gain and low dark currents are often mutually excluded since large operation voltages often induce high shot noise. Here, a fully vacuum-processed PM-OPD is demonstrated using trap-assisted electron injection in BDP-OMe:C60 material system. By applying only −1 V, compared with the self-powered working condition, the responsivity is increased by one order of magnitude, resulting in an outstanding specific detectivity of ≈1013 Jones. Remarkably, the superior detectivity in the near-infrared region is stable and almost voltage-independent up to −10 V. Compared with two photovoltaic-type photodetectors, these PM-OPDs exhibit the great potential to be easily integrated with state-of-the-art readout electronics in terms of their high responsivity, fast response speed, and bias-independent specific detectivity. The employed vacuum fabrication process and the easy-to-adapt PM-OPD concept enable seamless upscaling of production, paving the way to a commercially relevant photodetector technology

Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers

Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge- transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff