Dispositivos poliméricos emissores de luz (Polymer Light Emitting Diodes - PLEDs) têm sido amplamente investigados devido à sua possibilidade de aplicação na fabricação de telas de projeção e displays. As principais vantagens dos materiais poliméricos, nesses casos, são o baixo custo e a possibilidade de processamento em superfícies de grande área, ao contrário do que ocorre para dispositivos contendo cristais líquidos (Liquid Crystal Display - LCD\'s). Apesar de amplamente investigados nos últimos anos, alguns aspectos fundamentais acerca dos mecanismos de injeção de carga nos PLEDs ainda não estão completamente elucidados. Nesta dissertação estudamos as propriedades ópticas, morfológicas e elétricas de dispositivos poliméricos emissores de luz contendo poli(2-metóxi, 5-(2\'-etil-hexilóxi)-1,4-fenileno vinileno) (MEH-PPV) como camada ativa. Inicialmente foi investigada a influência de camadas transportadoras de lacunas (Hole Transport Layer - HTL) e/ou elétrons (Electron Transport Layer - ETL) na eficiência dos dispositivos. As camadas HTL e ETL foram compostas de poli(3,4-etilenodioxithiofeno):poliestireno sulfonado (PEDOT:PSS), e poli(estireno-co-p-estireno sulfonado-co-metaacrilato de metila) (PS-co-SS-co-MMA), respectivamente. Os filmes de PEDOT:PSS foram depositados por centrifugação. Devido ao seu caráter isolante (condutividade elétrica 10-5 S/cm), e por ter nível energético HOMO (Highest Occupied Molecular Orbital) próximo ao nível de Fermi do ITO (Indium Thin Oxide), a utilização do PEDOT:PSS como camada reguladora da injeção de lacunas resultou num aumento do tempo de meia vida do dispositivo em cerca de 10 vezes. No caso de dispositivos contendo a camada de ETL, foi identificada a formação de estados localizados gerados pela sulfonação do poliestireno. Estes estados auxiliam no processo de tunelamento através da camada polimérica. Na segunda parte do trabalho, apresentada no capítulo 4, desenvolvemos um modelo teórico para descrever as regiões das curvas da densidade de corrente elétrica (J) vs. campo elétrico aplicado (F) (dependentes e independentes da temperatura). Este modelo é uma extensão do modelo de Arkhipov, onde inserimos um termo de injeção de carga via tunelamento Fowler-Nordhein através de uma distribuição gaussiana de barreiras de potencial de interface, além do termo de injeção via hopping, já tratado por Arkhipov. O modelo proposto ajustou satisfatoriamente as curvas de J vs. F tanto nos modo de polarização direta, quanto reversa. / Polymer light emitting diodes (PLEDs) have been widely investigated as candidate materials for display fabrication. The main advantages exhibited by PLEDs are the low-cost processing and possibility of large-area display fabrication, in comparison to the conventional liquid crystal displays (LCD\'s). Although the engineering aspects concerning device fabrication and characterization are well understood, some specific points regarding the electrical transport in the bulk and at the interfaces of the devices are not fully explained. In this study, we present a morphological, optical and electrical characterization of PLEDs containing poly(2-methoxi, 5-(2\'-etyl-hexiloxy)-1,4-phenilene vinilene) (MEH-PPV) as the emissive layer. We investigated the influence of hole transport layers (HTL) and/or electron transport layers (ETL) on the efficiency of the devices. The HTL and ETL comprised thin polymeric films of poly(3,4-etylenedioxythiphene):sulfonated polystyrene (PEDOT:PSS) and poly(estyrene-co-p-sulfonated styrene-co-metyl metacrylate) (PS-co-SS-co-MMA), respectively. Devices containing the PEDOT:PSS exhibited a life-time 10 times higher than the devices not containing the HTL material, which is probably due to the controlled hole injection that may be achieved in former devices. In the second set of devices, in which an ETL was incorporated, we observed the formation of localized states in the polymeric ETL layer, which was responsible for improving the tunneling process of charges injected from cathode. A theoretical model concerning the charge injection mechanisms in the PLEDs containing MEH-PPV is presented in chapter 4. The final device architecture was ITO/MEH-PPV/Al, and the J vs. F measurements were taken at temperatures between 120 K and 270 K. The model proposed here is a combination of the Arkhipov´s and Fowler-Nordhein tunneling models, considering a Gaussian distribution of triangular potential barriers. The model takes into account the charge carrier/image charge recombination probability at the interface of the electrode, being very appropriate to explain the dependence of the electric current on the temperature and applied electric field.

Dispositivos poliméricos emissores de luz (Polymer Light Emitting Diodes - PLEDs) têm sido amplamente investigados devido à sua possibilidade de aplicação na fabricação de telas de projeção e displays. As principais vantagens dos materiais poliméricos, nesses casos, são o baixo custo e a possibilidade de processamento em superfícies de grande área, ao contrário do que ocorre para dispositivos contendo cristais líquidos (Liquid Crystal Display - LCD\'s). Apesar de amplamente investigados nos últimos anos, alguns aspectos fundamentais acerca dos mecanismos de injeção de carga nos PLEDs ainda não estão completamente elucidados. Nesta dissertação estudamos as propriedades ópticas, morfológicas e elétricas de dispositivos poliméricos emissores de luz contendo poli(2-metóxi, 5-(2\'-etil-hexilóxi)-1,4-fenileno vinileno) (MEH-PPV) como camada ativa. Inicialmente foi investigada a influência de camadas transportadoras de lacunas (Hole Transport Layer - HTL) e/ou elétrons (Electron Transport Layer - ETL) na eficiência dos dispositivos. As camadas HTL e ETL foram compostas de poli(3,4-etilenodioxithiofeno):poliestireno sulfonado (PEDOT:PSS), e poli(estireno-co-p-estireno sulfonado-co-metaacrilato de metila) (PS-co-SS-co-MMA), respectivamente. Os filmes de PEDOT:PSS foram depositados por centrifugação. Devido ao seu caráter isolante (condutividade elétrica 10-5 S/cm), e por ter nível energético HOMO (Highest Occupied Molecular Orbital) próximo ao nível de Fermi do ITO (Indium Thin Oxide), a utilização do PEDOT:PSS como camada reguladora da injeção de lacunas resultou num aumento do tempo de meia vida do dispositivo em cerca de 10 vezes. No caso de dispositivos contendo a camada de ETL, foi identificada a formação de estados localizados gerados pela sulfonação do poliestireno. Estes estados auxiliam no processo de tunelamento através da camada polimérica. Na segunda parte do trabalho, apresentada no capítulo 4, desenvolvemos um modelo teórico para descrever as regiões das curvas da densidade de corrente elétrica (J) vs. campo elétrico aplicado (F) (dependentes e independentes da temperatura). Este modelo é uma extensão do modelo de Arkhipov, onde inserimos um termo de injeção de carga via tunelamento Fowler-Nordhein através de uma distribuição gaussiana de barreiras de potencial de interface, além do termo de injeção via hopping, já tratado por Arkhipov. O modelo proposto ajustou satisfatoriamente as curvas de J vs. F tanto nos modo de polarização direta, quanto reversa. / Polymer light emitting diodes (PLEDs) have been widely investigated as candidate materials for display fabrication. The main advantages exhibited by PLEDs are the low-cost processing and possibility of large-area display fabrication, in comparison to the conventional liquid crystal displays (LCD\'s). Although the engineering aspects concerning device fabrication and characterization are well understood, some specific points regarding the electrical transport in the bulk and at the interfaces of the devices are not fully explained. In this study, we present a morphological, optical and electrical characterization of PLEDs containing poly(2-methoxi, 5-(2\'-etyl-hexiloxy)-1,4-phenilene vinilene) (MEH-PPV) as the emissive layer. We investigated the influence of hole transport layers (HTL) and/or electron transport layers (ETL) on the efficiency of the devices. The HTL and ETL comprised thin polymeric films of poly(3,4-etylenedioxythiphene):sulfonated polystyrene (PEDOT:PSS) and poly(estyrene-co-p-sulfonated styrene-co-metyl metacrylate) (PS-co-SS-co-MMA), respectively. Devices containing the PEDOT:PSS exhibited a life-time 10 times higher than the devices not containing the HTL material, which is probably due to the controlled hole injection that may be achieved in former devices. In the second set of devices, in which an ETL was incorporated, we observed the formation of localized states in the polymeric ETL layer, which was responsible for improving the tunneling process of charges injected from cathode. A theoretical model concerning the charge injection mechanisms in the PLEDs containing MEH-PPV is presented in chapter 4. The final device architecture was ITO/MEH-PPV/Al, and the J vs. F measurements were taken at temperatures between 120 K and 270 K. The model proposed here is a combination of the Arkhipov´s and Fowler-Nordhein tunneling models, considering a Gaussian distribution of triangular potential barriers. The model takes into account the charge carrier/image charge recombination probability at the interface of the electrode, being very appropriate to explain the dependence of the electric current on the temperature and applied electric field.

Polymer solar cells form a promising technology for converting sunlight into electricity, and have reached record efficiencies over 10% and lifetimes of several years. The performance of polymer solar cells depends strongly on the distribution of electron donor and acceptor materials in the active layer. To achieve longer lifetimes, degradation processes in the materials have to be understood. In this thesis, a set of complementary spectroscopy and microscopy techniques, among which soft X-ray techniques have been used to determine the morphology of polymer:fullerene based active layers. We have found that the morphology of TQ1:PC70BM films is strongly influenced by the processing solvent and the use of solvent additives. We have also found, by using soft X-ray techniques, that not only the light-absorbing polymer TQ1, but also the fullerene is susceptible to photo-degradation in air. Moreover, the fullerene degradation is accelerated in the presence of the polymer. Additionally, this thesis addresses the role of the interfacial layers for device performance and stability. The commonly used hole transport material PEDOT:PSS has the advantage of being solution processable at room temperature, but this layer is also known to contribute to the device degradation. We have found that low-temperature processed NiOx is a promising alternative to PEDOT:PSS, leading to improved device performance. Even for encapsulated polymer solar cells, some photo-induced degradation of the electrical performance is observed and is found to depend on the nature of the hole transport material. We found a better initial stability for solar cells with MoO3 hole transport layers than with PEDOT:PSS. In the pursuit of understanding the initial decrease in electrical performance of PEDOT:PSS-based devices, simulations were performed, from which a number of degradation sources could be excluded. / With the increasing global demand for energy, solar cells provide a clean method for converting the abundant sunlight to electricity. Polymer solar cells can be made from a large variety of light-harvesting and electrically conducting molecules and are inexpensive to produce. They have additional advantages, like their mechanical flexibility and low weight, which opens opportunities for novel applications. In order for polymer solar cells to be more competitive, however, both the power conversion efficiencies and lifetimes need to further improve. One way to achieve this is to optimize the morphology of the active layer. The active layer of a polymer solar cell consists of electron donating and electron accepting molecules whose distribution in the bulk of the film is a major factor that determines the solar cell performance. This thesis presents the use of complementary spectroscopy and microscopy methods to probe the local composition in the active layer of polymer solar cells. The stability of the active layer is studied and the interplay between the photo-degradation of the donor and acceptor molecules is investigated. Additionally, this thesis addresses how the interfacial layers between the active layer and the electrodes can influence device performance and stability. / <p>I publikationen felaktigt ISBN 978-91-7063-739-1</p>

No presente trabalho foi realizado o estudo das propriedades ópticas e elétricas de dispositivos eletroluminescentes poliméricos, conhecidos como diodos emissores de luz poliméricos (PLEDs), e o desenvolvimento de camadas transportadoras de carga (HTL), que visam promover um aumento da eficiência elétrica dos dispositivos. Para o estudo das propriedades ópticas e elétricas dos PLEDs, foram fabricados dispositivos com estruturas do tipo Ânodo/HTL/Polímero Eletroluminescente/Cátodo. Foram apresentadas todas as etapas de fabricação dos dispositivos, assim como seus processos de caracterização. Para o ânodo, foi utilizado um óxido transparente condutor, óxido de índio-estanho - ITO, com tratamento superficial em plasma de oxigênio. Foram estudados três materiais diferentes para as HTLs. Filmes de PAni:PVS ou PAni:Ni-TS-Pc foram depositados pela técnica de automontagem (Layer-by-Layer) e os filmes de PEDOT:PSS foram depositados pelo método de spin-coating. O polímero eletroluminescente utilizado neste trabalho foi o MEH-PPV, também depositado pelo método de spin-coating. Para o cátodo foi utilizado o alumínio, evaporado termicamente. O encapsulamento dos dispositivos foi realizado em atmosfera inerte de argônio para diminuir os efeitos de degradação através do oxigênio e da luz. O emprego de camadas transportadoras de buracos (HTLs) resultou numa sensível diminuição no valor da tensão de operação dos dispositivos, quando empregados filmes de PAni:PVS e PAni:Ni-TS-Pc. Os valores das tensões de operação baixaram de 12 V para cerca de 3 V em relação aos dispositivos fabricados sem a utilização de HTLs. Através da microscopia de força atômica, foi possível determinar a espessura das bicamadas e a rugosidade superficial dos filmes de PAni:PVS para correlacionar estes resultados com a resposta elétrica dos dispositivos. Espessuras de 4nm (para 1 bicamada) resultaram em tensões de operação de 3 V. Foi possível verificar também, por espectroscopia no UV-VIS, que este tipo de filme absorve luz em freqüência diferente daquela emitida pelo MEH-PPV. Medidas elétricas em regime de corrente contínua, curvas de Corrente vs. Tensão e, em regime de corrente alternada, espectroscopia de impedância, foram realizadas em dispositivos para determinar o valor da tensão de operação e estudar os efeitos de interface nas diferentes camadas que compõe um dispositivo. Através das curvas obtidas pela espectroscopia de impedância, foi possível determinar os valores dos componentes dos circuitos equivalentes (capacitores e resistores). Com isso, é possível simular o comportamento destes dispositivos através de circuitos elétricos antes mesmo de serem fabricados. Pelos resultados obtidos, todas as HTLs estudadas contribuíram para uma sensível diminuição no valor da tensão de operação dos dispositivos, apontando-os como excelentes materiais a serem utilizados com o objetivo de alcançar uma maior eficiência e um melhor desempenho destes dispositivos. / In the present work, the study of the optical and electrical properties of polymeric electroluminescent devices known as Polymer Light-Emitting Diodes (PLEDs) and the development of Hole Transport Layers (HTLs) to promote an increase of the electrical efficiency of the devices was performed. PLEDs were constructed with structures like Anode/HTL/Electroluminescent Polymer/Cathode in order to study the optical and electrical properties of these devices. All the stages of the devices production were presented, as well as its characterization processes. For the anode a conductive transparent oxide (Indium Tin Oxide - ITO) with a superficial oxygen plasma treatment was used. Three different materials for the HTLs were used. Films of PAni:PVS or PAni:Ni-TS-Pc were deposited by the self-assembly technique (Layer-by-Layer) and the films of PEDOT:PSS were deposited by the spin-coating method. The electroluminescent polymer used in this work was MEH-PPV, also deposited by the spin-coating method. Aluminum was deposited by thermal evaporation for the cathode. The devices encapsulation was performed in Argon inert atmosphere to reduce the degradation effects through oxygen and light. The use of Hole Transport Layers (HTLs) resulted in a sensitive decrease in the devices operating voltage value when films of PAni:PVS and PAni:Ni-TS-Pc were used. The operating voltage values have decreased from 12 V to 3 V in relation to the devices assembled without the usage of HTLs. By the use of Atomic Force Microscopy measurements the thickness of the bilayers and the surface roughness of the PAni:PVS films was obtained to correlate these results with the devices electric characteristics. Thicknesses of 3 to 4 nm (for one bilayer) resulted in operating voltage of 3 V. It was possible to verify also, by UVVIS Spectroscopy, that this type of PAni:PVS films absorbs light in a different frequency than that emitted by MEH-PPV. Electric measurements in the direct current, Current vs. Voltage curves and, in alternating current, Impedance Spectroscopy, were performed in devices to determine the operating voltage value and to study the interface effects in the different layers used in the devices. Analyzing the curves obtained by the impedance spectroscopy, it was possible to determine the values of the equivalent circuit components (capacitors and resistors) and, with that, to simulate the behavior of these devices through electric circuits even before they were manufactured. By the experimental results, all the HTLs studied have contributed to a sensitive decrease in the devices operating voltage, indicating them as excellent materials to be used to reach a higher efficiency and a better performance of these devices.

In this work a p-i-n type heterojunction architecture for organic solar cells is shown, where the active region is sandwiched between two doped wide-gap layers. The term p-i-n means here a layer sequence in the form p-doped layer, intrinsic layer and n-doped layer. The doping is realized by controlled coevaporation using organic dopants and leads to conductivities of 10-4 to 10-5 S/cm in the p- and n-doped wide gap layers, respectively. The conductivity and field effect mobility of single doped layers can be described quantitatively in a self-consistent way by a percolation model. For the solar cells the photoactive layer is formed by a mixture of phthalocyanine zinc (ZnPc) and the fullerene C60 and shows mainly amorphous morphology. The solar cells exhibit a maximum external quantum efficiency of 40% between 630nm and 700nm wavelength. With the help of an optical multilayer model, the optical properties of the solar cells are optimized by placing the active region at the maximum of the optical field distribution. The results of the model are largely confirmed by the experimental findings. The optically optimized device shows an internal quantum efficiency of around 85% at short-circuit conditions and a power-conversion efficiency of 1.7%.

A numerical model and results for the quantitative simulation of multilayer organic light emitting diode (OLED) and organic solar cell (OSC) are presented. In the model, effects like bipolar charge carrier drift and diffusion with field-dependent mobilities, trapping, dopants, indirect and direct bimolecular recombination, singlet Frenkel exciton diffusion, normal decay and quenching effects are taken into account. For an adequate description of multilayer devices with energetic barriers at interfaces between two adjacent organic layers, thermally assisted charge carrier hopping through the interface, interface recombination, and formation of interface charge transfer (CT) states have been introduced in the model. For the simulation of OSC, the generation of carrier pairs in the mixed layer or at the interface is additionally implemented. The light absorption profile is calculated from optical simulations and used as an input for the electrical simulation. The model is based on three elements: the Poisson equation, the rate equations for charge carriers and the rate equations for singlet Frenkel excitons. These equations are simultaeously solved by spatial and temporal discretisation using the appropriate boundary conditions and electrical parameters. The solution is found when a steady state is reached, as indicated by a constant value of current density. The simulation provides a detailed look into the distribution of electric field and concentration of free and trapped carriers at a particular applied voltage. For organic light emitting diodes, the numerical model helps to analyze the problems of different structures and provides deeper insight into the relevant physical mechanisms involved in device operation. Moreover, it is possible to identify technological problems for certain sets of devices. For instance, we could show that ? in contrast to literature reports - the contact between Alq3 and LiF/Al did not show ohmic behaviour for the series of devices. The role of an additional organic blocking layer between HTL and EML was presented. The explanation for the higher creation efficiency for singlet excitons in the three-layer structure is found in the separation of free holes and electrons accumulating close to the internal interface 1-Naphdata/Alq3. The numerical calculation has demonstrated the importance of controlled doping of the organic materials, which is a way to obtain efficient light emitting diodes with low operating voltage. The experimental results has been reproduced by numerical simulation for a series of OLEDs with different thicknesses of the hole transport layer and emitting layer and for doped emitting layers. The advantages and drawbacks of solar cells based on flat heterojunctions and bulk heterojunctions are analyzed. From the simulations, it can be understood why bulk-heterojunctions typically yield higher photocurrents while flat heterojunctions typically feature higher fill factors. In p-i-n ?structures, p and n are doped wide gap materials and i is a photoactive donor-acceptor blend layer using, e.g,. zinc phthalocyanine as a donor and C60 as an acceptor component. It is found that by introducing trap states, the simulation is able to reproduce the linear dependence of short circuit currents on the light intensity. The apparent light-induced shunt resistance often observed in organic solar cells can also be explained by losses due to trapping and indirect recombination of photogenerated carriers, which we consider a crucial point of our work. However, these two effects, the linear scaling of the photocurrent with light intensity and the apparent photoshunt, could also be reproduced when field-dependent geminate recombination is assumed to play a dominant role. First results that show a temperature independent short circuit photocurrent favour the model based on trap-mediated indirect recombination.

No presente trabalho foi realizado o estudo das propriedades ópticas e elétricas de dispositivos eletroluminescentes poliméricos, conhecidos como diodos emissores de luz poliméricos (PLEDs), e o desenvolvimento de camadas transportadoras de carga (HTL), que visam promover um aumento da eficiência elétrica dos dispositivos. Para o estudo das propriedades ópticas e elétricas dos PLEDs, foram fabricados dispositivos com estruturas do tipo Ânodo/HTL/Polímero Eletroluminescente/Cátodo. Foram apresentadas todas as etapas de fabricação dos dispositivos, assim como seus processos de caracterização. Para o ânodo, foi utilizado um óxido transparente condutor, óxido de índio-estanho - ITO, com tratamento superficial em plasma de oxigênio. Foram estudados três materiais diferentes para as HTLs. Filmes de PAni:PVS ou PAni:Ni-TS-Pc foram depositados pela técnica de automontagem (Layer-by-Layer) e os filmes de PEDOT:PSS foram depositados pelo método de spin-coating. O polímero eletroluminescente utilizado neste trabalho foi o MEH-PPV, também depositado pelo método de spin-coating. Para o cátodo foi utilizado o alumínio, evaporado termicamente. O encapsulamento dos dispositivos foi realizado em atmosfera inerte de argônio para diminuir os efeitos de degradação através do oxigênio e da luz. O emprego de camadas transportadoras de buracos (HTLs) resultou numa sensível diminuição no valor da tensão de operação dos dispositivos, quando empregados filmes de PAni:PVS e PAni:Ni-TS-Pc. Os valores das tensões de operação baixaram de 12 V para cerca de 3 V em relação aos dispositivos fabricados sem a utilização de HTLs. Através da microscopia de força atômica, foi possível determinar a espessura das bicamadas e a rugosidade superficial dos filmes de PAni:PVS para correlacionar estes resultados com a resposta elétrica dos dispositivos. Espessuras de 4nm (para 1 bicamada) resultaram em tensões de operação de 3 V. Foi possível verificar também, por espectroscopia no UV-VIS, que este tipo de filme absorve luz em freqüência diferente daquela emitida pelo MEH-PPV. Medidas elétricas em regime de corrente contínua, curvas de Corrente vs. Tensão e, em regime de corrente alternada, espectroscopia de impedância, foram realizadas em dispositivos para determinar o valor da tensão de operação e estudar os efeitos de interface nas diferentes camadas que compõe um dispositivo. Através das curvas obtidas pela espectroscopia de impedância, foi possível determinar os valores dos componentes dos circuitos equivalentes (capacitores e resistores). Com isso, é possível simular o comportamento destes dispositivos através de circuitos elétricos antes mesmo de serem fabricados. Pelos resultados obtidos, todas as HTLs estudadas contribuíram para uma sensível diminuição no valor da tensão de operação dos dispositivos, apontando-os como excelentes materiais a serem utilizados com o objetivo de alcançar uma maior eficiência e um melhor desempenho destes dispositivos. / In the present work, the study of the optical and electrical properties of polymeric electroluminescent devices known as Polymer Light-Emitting Diodes (PLEDs) and the development of Hole Transport Layers (HTLs) to promote an increase of the electrical efficiency of the devices was performed. PLEDs were constructed with structures like Anode/HTL/Electroluminescent Polymer/Cathode in order to study the optical and electrical properties of these devices. All the stages of the devices production were presented, as well as its characterization processes. For the anode a conductive transparent oxide (Indium Tin Oxide - ITO) with a superficial oxygen plasma treatment was used. Three different materials for the HTLs were used. Films of PAni:PVS or PAni:Ni-TS-Pc were deposited by the self-assembly technique (Layer-by-Layer) and the films of PEDOT:PSS were deposited by the spin-coating method. The electroluminescent polymer used in this work was MEH-PPV, also deposited by the spin-coating method. Aluminum was deposited by thermal evaporation for the cathode. The devices encapsulation was performed in Argon inert atmosphere to reduce the degradation effects through oxygen and light. The use of Hole Transport Layers (HTLs) resulted in a sensitive decrease in the devices operating voltage value when films of PAni:PVS and PAni:Ni-TS-Pc were used. The operating voltage values have decreased from 12 V to 3 V in relation to the devices assembled without the usage of HTLs. By the use of Atomic Force Microscopy measurements the thickness of the bilayers and the surface roughness of the PAni:PVS films was obtained to correlate these results with the devices electric characteristics. Thicknesses of 3 to 4 nm (for one bilayer) resulted in operating voltage of 3 V. It was possible to verify also, by UVVIS Spectroscopy, that this type of PAni:PVS films absorbs light in a different frequency than that emitted by MEH-PPV. Electric measurements in the direct current, Current vs. Voltage curves and, in alternating current, Impedance Spectroscopy, were performed in devices to determine the operating voltage value and to study the interface effects in the different layers used in the devices. Analyzing the curves obtained by the impedance spectroscopy, it was possible to determine the values of the equivalent circuit components (capacitors and resistors) and, with that, to simulate the behavior of these devices through electric circuits even before they were manufactured. By the experimental results, all the HTLs studied have contributed to a sensitive decrease in the devices operating voltage, indicating them as excellent materials to be used to reach a higher efficiency and a better performance of these devices.

A numerical model and results for the quantitative simulation of multilayer organic light emitting diode (OLED) and organic solar cell (OSC) are presented. In the model, effects like bipolar charge carrier drift and diffusion with field-dependent mobilities, trapping, dopants, indirect and direct bimolecular recombination, singlet Frenkel exciton diffusion, normal decay and quenching effects are taken into account. For an adequate description of multilayer devices with energetic barriers at interfaces between two adjacent organic layers, thermally assisted charge carrier hopping through the interface, interface recombination, and formation of interface charge transfer (CT) states have been introduced in the model. For the simulation of OSC, the generation of carrier pairs in the mixed layer or at the interface is additionally implemented. The light absorption profile is calculated from optical simulations and used as an input for the electrical simulation. The model is based on three elements: the Poisson equation, the rate equations for charge carriers and the rate equations for singlet Frenkel excitons. These equations are simultaeously solved by spatial and temporal discretisation using the appropriate boundary conditions and electrical parameters. The solution is found when a steady state is reached, as indicated by a constant value of current density. The simulation provides a detailed look into the distribution of electric field and concentration of free and trapped carriers at a particular applied voltage. For organic light emitting diodes, the numerical model helps to analyze the problems of different structures and provides deeper insight into the relevant physical mechanisms involved in device operation. Moreover, it is possible to identify technological problems for certain sets of devices. For instance, we could show that ? in contrast to literature reports - the contact between Alq3 and LiF/Al did not show ohmic behaviour for the series of devices. The role of an additional organic blocking layer between HTL and EML was presented. The explanation for the higher creation efficiency for singlet excitons in the three-layer structure is found in the separation of free holes and electrons accumulating close to the internal interface 1-Naphdata/Alq3. The numerical calculation has demonstrated the importance of controlled doping of the organic materials, which is a way to obtain efficient light emitting diodes with low operating voltage. The experimental results has been reproduced by numerical simulation for a series of OLEDs with different thicknesses of the hole transport layer and emitting layer and for doped emitting layers. The advantages and drawbacks of solar cells based on flat heterojunctions and bulk heterojunctions are analyzed. From the simulations, it can be understood why bulk-heterojunctions typically yield higher photocurrents while flat heterojunctions typically feature higher fill factors. In p-i-n ?structures, p and n are doped wide gap materials and i is a photoactive donor-acceptor blend layer using, e.g,. zinc phthalocyanine as a donor and C60 as an acceptor component. It is found that by introducing trap states, the simulation is able to reproduce the linear dependence of short circuit currents on the light intensity. The apparent light-induced shunt resistance often observed in organic solar cells can also be explained by losses due to trapping and indirect recombination of photogenerated carriers, which we consider a crucial point of our work. However, these two effects, the linear scaling of the photocurrent with light intensity and the apparent photoshunt, could also be reproduced when field-dependent geminate recombination is assumed to play a dominant role. First results that show a temperature independent short circuit photocurrent favour the model based on trap-mediated indirect recombination.

Two thienoacene dimers based on the thieno[3,2-b]thiophene moiety were efficiently synthesized, characterized and evaluated as active hole-transporting layers in organic thin-film field-effect transistors. Both compounds behaved as active p-channel organic semi-conductors showing averaged hole mobility of up to 1.33 cm² V⁻¹ s⁻¹.

In this work a p-i-n type heterojunction architecture for organic solar cells is shown, where the active region is sandwiched between two doped wide-gap layers. The term p-i-n means here a layer sequence in the form p-doped layer, intrinsic layer and n-doped layer. The doping is realized by controlled coevaporation using organic dopants and leads to conductivities of 10-4 to 10-5 S/cm in the p- and n-doped wide gap layers, respectively. The conductivity and field effect mobility of single doped layers can be described quantitatively in a self-consistent way by a percolation model. For the solar cells the photoactive layer is formed by a mixture of phthalocyanine zinc (ZnPc) and the fullerene C60 and shows mainly amorphous morphology. The solar cells exhibit a maximum external quantum efficiency of 40% between 630nm and 700nm wavelength. With the help of an optical multilayer model, the optical properties of the solar cells are optimized by placing the active region at the maximum of the optical field distribution. The results of the model are largely confirmed by the experimental findings. The optically optimized device shows an internal quantum efficiency of around 85% at short-circuit conditions and a power-conversion efficiency of 1.7%.