Interplay Between Fullerene Surface Coverage and Contact Selectivity of Cathode Interfaces in Organic Solar Cells

Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces have an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques as variable-angle spectroscopic ellipsometry and capacitance-voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in four donor/acceptor systems: PCPDTBT, P3HT, PCDTBT and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene rich interface at the cathode is a prerequisite to enhance contact selectivity, and consequently power conversion efficiency

Nanoscale investigation and control of the interfacial properties of organic solar cells and organic thin-film transistors

Las propiedades de las películas de semiconductores orgánicas y, en particular, de las interfases involucradas, son uno de los aspectos más prominentes en relación con la eficiencia de los dispositivos orgánicos. La interfase formada entre dos materiales orgánicos puede influenciar las propiedades electrónicas y ópticas de los dispositivos de diferentes manteras: por los mecanismos de crecimiento, la morfología, la densidad de defectos y la estructura electrónica. El impacto de la orientación molecular en interfases de materiales orgánicos es una de las cuestiones menos entendidas y menos investigadas en relación con la eficiencia de células solares orgánicas. Mediante el uso de microscopia de sonda cercana (SPM) y fotoluminiscencia, se ha demostrado en esta tesis una correlación clara entre la orientación molecular en la interfase de DIP (donor)/PTCDI-C8 (aceptor) y la formación de un estado de transferencia de carga para aquellas heteroestructuras en las que el solape de los orbitales p en moléculas adyacentes es favorecido. Otro tipo de interfase de materiales orgánicos de gran relevancia se encuentra en los transistores orgánicos de película delgada (TFTs), en el que el dieléctrico es funcionalizado con películas orgánicas autoensambladas (SAMs). El uso de SAMs es una tecnología muy prometedora en la manufacturación de transistores orgánicos para conseguir voltajes de operación deseados dado que el voltaje umbral de operación puede ser modulado mediante la elección de las SAMs. El origen físico de este fenómeno ha sido muy debatido en la literatura y permanece una cuestión abierta. Microscopia de sonda Kelvin ha sido empleada como herramienta para explorar las propiedades electrónicas de la interfase entre DNTT (semiconductor orgánico) y dos SAMs con cadenas alquílica terminadas en grupos metil o metil fluorinados. Dicho estudio en correlación con la operación de los TFTs con DNTT ha revelado que el voltaje umbral depende de la capacitancia del dieléctrico solamente para la SAM fluorinada y se ha determinado que se debe a la interacción electrónica en la interfase entre DNTT y los grupos F de la SAM. En conjunto, los estudios realizados en esta tesis combinan una serie de métodos sistemáticos y técnicas complementarias que han permitido abordar el efecto de procesos electrónicos en interfases de relevancia en células solares y TFTs. Los resultados de esta tesis ponen de manifiesto la importancia del control de las propiedades estructurales y electrónicas de las interfases de materiales orgánicos como paso necesario para mejorar la eficiencia de dispositivos.Thin-film and interface properties of organic semiconductors are among the most prominent aspects with regard to the overall performance of organic electronic devices. The interface formed between two organic materials can influence the electronic and optical properties of organic electronic devices by determining the growth mechanisms, morphology, defect density and the electronic interface structures of organic films. The impact of the relative molecular orientation at the organic/organic interface on the performance of organic solar cells is one of the less understood factors and thus, it represents an outstanding opportunity for research and technologies based on the control of the local molecular ordering of the organic molecules in donor/acceptor organic photovoltaics. Using state-of-the-art scanning probe microscopy techniques and photoluminescence studies a clear link between the relative molecular orientation of the DIP (donor)/PTCDI-C8 (acceptor) heterostructures and an emissive charge transfer state is demonstrated, which is ultimately associated with an efficient π-orbital overlap at the interface. Another extremely interesting organic/organic interface is the one found in organic thinfilm transistors (TFTs), where the gate dielectric contains organic species such as selfassembled monolayers (SAMs). The use of SAMs opens an appealing path of research in manufacturing TFTs with the desired operating voltages, due to the observation that the threshold voltage can be modulated using different SAMs. Revealing the underlying mechanisms of this phenomenon, which is known as threshold-voltage shift, signifies a considerable challenge. Kelvin probe force microscopy (KPFM) was used as a powerful tool to explore at the nanoscale the electronic properties at the interface between DNTT and two different SAMs namely an alkly- and a fluoroalkylphosphonic acid SAM. A systematic series of KPFM investigations combined with the analysis of the transistor parameters reveals gate-oxide capacitance-dependent threshold-voltage shift as a result of interface electronic interactions at the DNTT/fluoroalkyl SAM interface. On the contrary, the DNTT transistors with the alkyl SAMs exhibit a small capacitanceindependent threshold-voltage shift, associated with the intrinsic dipole-induced electrostatic potential of the SAM. Together, the studies carried out in this thesis represent innovative approaches utilizing controlled organic semiconductor processing methods and complementary techniques, which enabled us to achieve a better understanding of different electronic processes at the interfaces involved in organic solar cells and organic thin-film transistors. This thesis emphasizes the relevance of achieving controlled interface architectures with exciting potential for future interface engineering in organic electronic devices

Influence of the relative molecular orientation on interfacial charge-transfer Excitons at donor/acceptor Nanoscale heterojunctions

We address the impact of the relative orientation between donor (D) and acceptor (A) molecules at the D/A heterojunction on the exciton dissociation. For this purpose, two-dimensional heterojunctions of diindenoperylene (DIP) and N,N'-dioctyl-3,4,9,10-perylene tetracarboxylicdiimide (PTCDI-C) deposited onto SiO/Si are grown, which exemplify two model interfaces with the π-staking direction either perpendicular or parallel to the interface. Aspects related to the morphology of the heterojunctions and charge photogeneration are studied by scanning probe force methods and photoluminescence (PL) spectroscopy. Results from PL spectroscopy indicate that the exciton dissociation is influenced by the different relative molecular orientations of A and D. For the configuration with stronger orbital overlap between A and D at the interface, the exciton dissociation is dominated by recombination from an interfacial charge-transfer state. © 2014 American Chemical Society

Nanoscale investigation and control of the interfacial properties of organic solar cells and organic thin-film transistors

Las propiedades de las películas de semiconductores orgánicas y, en particular, de las interfases involucradas, son uno de los aspectos más prominentes en relación con la eficiencia de los dispositivos orgánicos. La interfase formada entre dos materiales orgánicos puede influenciar las propiedades electrónicas y ópticas de los dispositivos de diferentes manteras: por los mecanismos de crecimiento, la morfología, la densidad de defectos y la estructura electrónica. El impacto de la orientación molecular en interfases de materiales orgánicos es una de las cuestiones menos entendidas y menos investigadas en relación con la eficiencia de células solares orgánicas. Mediante el uso de microscopia de sonda cercana (SPM) y fotoluminiscencia, se ha demostrado en esta tesis una correlación clara entre la orientación molecular en la interfase de DIP (donor)/PTCDI-C8 (aceptor) y la formación de un estado de transferencia de carga para aquellas heteroestructuras en las que el solape de los orbitales p en moléculas adyacentes es favorecido. Otro tipo de interfase de materiales orgánicos de gran relevancia se encuentra en los transistores orgánicos de película delgada (TFTs), en el que el dieléctrico es funcionalizado con películas orgánicas autoensambladas (SAMs). El uso de SAMs es una tecnología muy prometedora en la manufacturación de transistores orgánicos para conseguir voltajes de operación deseados dado que el voltaje umbral de operación puede ser modulado mediante la elección de las SAMs. El origen físico de este fenómeno ha sido muy debatido en la literatura y permanece una cuestión abierta. Microscopia de sonda Kelvin ha sido empleada como herramienta para explorar las propiedades electrónicas de la interfase entre DNTT (semiconductor orgánico) y dos SAMs con cadenas alquílica terminadas en grupos metil o metil fluorinados. Dicho estudio en correlación con la operación de los TFTs con DNTT ha revelado que el voltaje umbral depende de la capacitancia del dieléctrico solamente para la SAM fluorinada y se ha determinado que se debe a la interacción electrónica en la interfase entre DNTT y los grupos F de la SAM. En conjunto, los estudios realizados en esta tesis combinan una serie de métodos sistemáticos y técnicas complementarias que han permitido abordar el efecto de procesos electrónicos en interfases de relevancia en células solares y TFTs. Los resultados de esta tesis ponen de manifiesto la importancia del control de las propiedades estructurales y electrónicas de las interfases de materiales orgánicos como paso necesario para mejorar la eficiencia de dispositivos.Thin-film and interface properties of organic semiconductors are among the most prominent aspects with regard to the overall performance of organic electronic devices. The interface formed between two organic materials can influence the electronic and optical properties of organic electronic devices by determining the growth mechanisms, morphology, defect density and the electronic interface structures of organic films. The impact of the relative molecular orientation at the organic/organic interface on the performance of organic solar cells is one of the less understood factors and thus, it represents an outstanding opportunity for research and technologies based on the control of the local molecular ordering of the organic molecules in donor/acceptor organic photovoltaics. Using state-of-the-art scanning probe microscopy techniques and photoluminescence studies a clear link between the relative molecular orientation of the DIP (donor)/PTCDI-C8 (acceptor) heterostructures and an emissive charge transfer state is demonstrated, which is ultimately associated with an efficient π-orbital overlap at the interface. Another extremely interesting organic/organic interface is the one found in organic thinfilm transistors (TFTs), where the gate dielectric contains organic species such as selfassembled monolayers (SAMs). The use of SAMs opens an appealing path of research in manufacturing TFTs with the desired operating voltages, due to the observation that the threshold voltage can be modulated using different SAMs. Revealing the underlying mechanisms of this phenomenon, which is known as threshold-voltage shift, signifies a considerable challenge. Kelvin probe force microscopy (KPFM) was used as a powerful tool to explore at the nanoscale the electronic properties at the interface between DNTT and two different SAMs namely an alkly- and a fluoroalkylphosphonic acid SAM. A systematic series of KPFM investigations combined with the analysis of the transistor parameters reveals gate-oxide capacitance-dependent threshold-voltage shift as a result of interface electronic interactions at the DNTT/fluoroalkyl SAM interface. On the contrary, the DNTT transistors with the alkyl SAMs exhibit a small capacitanceindependent threshold-voltage shift, associated with the intrinsic dipole-induced electrostatic potential of the SAM. Together, the studies carried out in this thesis represent innovative approaches utilizing controlled organic semiconductor processing methods and complementary techniques, which enabled us to achieve a better understanding of different electronic processes at the interfaces involved in organic solar cells and organic thin-film transistors. This thesis emphasizes the relevance of achieving controlled interface architectures with exciting potential for future interface engineering in organic electronic devices

Nanoscale investigation and control of the interfacial properties of organic solar cells and organic thin-film transistors

Las propiedades de las películas de semiconductores orgánicas y, en particular, de las interfases involucradas, son uno de los aspectos más prominentes en relación con la eficiencia de los dispositivos orgánicos. La interfase formada entre dos materiales orgánicos puede influenciar las propiedades electrónicas y ópticas de los dispositivos de diferentes manteras: por los mecanismos de crecimiento, la morfología, la densidad de defectos y la estructura electrónica. El impacto de la orientación molecular en interfases de materiales orgánicos es una de las cuestiones menos entendidas y menos investigadas en relación con la eficiencia de células solares orgánicas. Mediante el uso de microscopia de sonda cercana (SPM) y fotoluminiscencia, se ha demostrado en esta tesis una correlación clara entre la orientación molecular en la interfase de DIP (donor)/PTCDI-C8 (aceptor) y la formación de un estado de transferencia de carga para aquellas heteroestructuras en las que el solape de los orbitales p en moléculas adyacentes es favorecido. Otro tipo de interfase de materiales orgánicos de gran relevancia se encuentra en los transistores orgánicos de película delgada (TFTs), en el que el dieléctrico es funcionalizado con películas orgánicas autoensambladas (SAMs). El uso de SAMs es una tecnología muy prometedora en la manufacturación de transistores orgánicos para conseguir voltajes de operación deseados dado que el voltaje umbral de operación puede ser modulado mediante la elección de las SAMs. El origen físico de este fenómeno ha sido muy debatido en la literatura y permanece una cuestión abierta. Microscopia de sonda Kelvin ha sido empleada como herramienta para explorar las propiedades electrónicas de la interfase entre DNTT (semiconductor orgánico) y dos SAMs con cadenas alquílica terminadas en grupos metil o metil fluorinados. Dicho estudio en correlación con la operación de los TFTs con DNTT ha revelado que el voltaje umbral depende de la capacitancia del dieléctrico solamente para la SAM fluorinada y se ha determinado que se debe a la interacción electrónica en la interfase entre DNTT y los grupos F de la SAM. En conjunto, los estudios realizados en esta tesis combinan una serie de métodos sistemáticos y técnicas complementarias que han permitido abordar el efecto de procesos electrónicos en interfases de relevancia en células solares y TFTs. Los resultados de esta tesis ponen de manifiesto la importancia del control de las propiedades estructurales y electrónicas de las interfases de materiales orgánicos como paso necesario para mejorar la eficiencia de dispositivos.Thin-film and interface properties of organic semiconductors are among the most prominent aspects with regard to the overall performance of organic electronic devices. The interface formed between two organic materials can influence the electronic and optical properties of organic electronic devices by determining the growth mechanisms, morphology, defect density and the electronic interface structures of organic films. The impact of the relative molecular orientation at the organic/organic interface on the performance of organic solar cells is one of the less understood factors and thus, it represents an outstanding opportunity for research and technologies based on the control of the local molecular ordering of the organic molecules in donor/acceptor organic photovoltaics. Using state-of-the-art scanning probe microscopy techniques and photoluminescence studies a clear link between the relative molecular orientation of the DIP (donor)/PTCDI-C8 (acceptor) heterostructures and an emissive charge transfer state is demonstrated, which is ultimately associated with an efficient π-orbital overlap at the interface. Another extremely interesting organic/organic interface is the one found in organic thinfilm transistors (TFTs), where the gate dielectric contains organic species such as selfassembled monolayers (SAMs). The use of SAMs opens an appealing path of research in manufacturing TFTs with the desired operating voltages, due to the observation that the threshold voltage can be modulated using different SAMs. Revealing the underlying mechanisms of this phenomenon, which is known as threshold-voltage shift, signifies a considerable challenge. Kelvin probe force microscopy (KPFM) was used as a powerful tool to explore at the nanoscale the electronic properties at the interface between DNTT and two different SAMs namely an alkly- and a fluoroalkylphosphonic acid SAM. A systematic series of KPFM investigations combined with the analysis of the transistor parameters reveals gate-oxide capacitance-dependent threshold-voltage shift as a result of interface electronic interactions at the DNTT/fluoroalkyl SAM interface. On the contrary, the DNTT transistors with the alkyl SAMs exhibit a small capacitanceindependent threshold-voltage shift, associated with the intrinsic dipole-induced electrostatic potential of the SAM. Together, the studies carried out in this thesis represent innovative approaches utilizing controlled organic semiconductor processing methods and complementary techniques, which enabled us to achieve a better understanding of different electronic processes at the interfaces involved in organic solar cells and organic thin-film transistors. This thesis emphasizes the relevance of achieving controlled interface architectures with exciting potential for future interface engineering in organic electronic devices

Maternal Dietary Patterns and Gestational Diabetes Risk: A Case-Control Study

Background. Maternal dietary patterns play an important role in the progress of gestational diabetes mellitus (GDM). The aim of the present study was to explore this association. Method. A total of 388 pregnant women (122 case and 266 control) were included. Dietary intake were collected using a food frequency questionnaire (FFQ). GDM was diagnosed using a 100-gram, 3-hour oral glucose tolerance test. Dietary pattern was identified by factor analysis. To investigate the relation between each of the independent variables with gestational diabetes, the odds ratio (OR) was calculated. Results. Western dietary pattern was high in sweets, jams, mayonnaise, soft drinks, salty snacks, solid fat, high-fat dairy products, potatoes, organ meat, eggs, red meat, processed foods, tea, and coffee. The prudent dietary pattern was characterized by higher intake of liquid oils, legumes, nuts and seeds, fruits and dried fruits, fish and poultry whole, and refined grains. Western dietary pattern was associated with increased risk of gestational diabetes mellitus before and after adjustment for confounders (OR = 1.97, 95% CI: 1.27–3.04, OR = 1.68, 95% CI: 1.04–2.27). However, no significant association was found for a prudent pattern. Conclusion. These findings suggest that the Western dietary pattern was associated with an increased risk of GDM

Influence of the relative molecular orientation on interfacial charge-transfer excitons at donor/acceptor nanoscale heterojunctions

We address the impact of the relative orientation between donor (D) and acceptor (A) molecules at the D/A heterojunction on the exciton dissociation. For this purpose, two-dimensional heterojunctions of diindenoperylene (DIP) and N,N'-dioctyl-3,4,9,10-perylene tetracarboxylicdiimide (PTCDI-C8) deposited onto SiO2/Si are grown, which exemplify two model interfaces with the π-staking direction either perpendicular or parallel to the interface. Aspects related to the morphology of the heterojunctions and charge photogeneration are studied by scanning probe force methods and photoluminescence (PL) spectroscopy. Results from PL spectroscopy indicate that the exciton dissociation is influenced by the different relative molecular orientations of A and D. For the configuration with stronger orbital overlap between A and D at the interface, the exciton dissociation is dominated by recombination from an interfacial charge-transfer state.Financial support by the Spanish Government (projects MAT2010-20020 and NANOSELECT CSD2007-00041) and by the German Research Foundation (DFG) within the priority program SPP1355 is acknowledged.Peer Reviewe

Interplay between Fullerene Surface Coverage and Contact Selectivity of Cathode Interfaces in Organic Solar Cells

Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces has an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques such as variable-angle spectroscopic ellipsometry and capacitance–voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in three additional donor/acceptor systems: PCPDTBT, P3HT, PCDTBT, and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene-rich interface at the cathode is a prerequisite to enhance contact selectivity and consequently power conversion efficiency

Threshold-Voltage Shifts in Organic Transistors Due to Self-Assembled Monolayers at the Dielectric: Evidence for Electronic Coupling and Dipolar Effects

The mechanisms behind the threshold-voltage shift in organic transistors due to functionalizing of the gate dielectric with self-assembled monolayers (SAMs) are still under debate. We address the mechanisms by which SAMs determine the threshold voltage, by analyzing whether the threshold voltage depends on the gate-dielectric capacitance. We have investigated transistors based on five oxide thicknesses and two SAMs with rather diverse chemical properties, using the benchmark organic semiconductor dinaphtho­[2,3-b:2′,3′-<i>f</i>]­thieno­[3,2-<i>b</i>]­thiophene. Unlike several previous studies, we have found that the dependence of the threshold voltage on the gate-dielectric capacitance is completely different for the two SAMs. In transistors with an alkyl SAM, the threshold voltage does not depend on the gate-dielectric capacitance and is determined mainly by the dipolar character of the SAM, whereas in transistors with a fluoroalkyl SAM the threshold voltages exhibit a linear dependence on the inverse of the gate-dielectric capacitance. Kelvin probe force microscopy measurements indicate this behavior is attributed to an electronic coupling between the fluoroalkyl SAM and the organic semiconductor