Helder Barbosa - Academia.edu (original) (raw)
Papers by Helder Barbosa
Springer Proceedings in Physics, 2009
Recently some experimental results have showed that the spatial alignment of conjugated polymer c... more Recently some experimental results have showed that the spatial alignment of conjugated polymer chains on nanometre length scales can influence the behaviour of polymer-based electronic devices, such as light-emitting diodes, field effect transistors, and photovoltaic cells. The effects of chain orientation at electrode-polymer interfaces on the charge injection process and charge mobility through the polymer layer are not well understood. In this work we use a generalized dynamical Monte Carlo method to study the influence of different polymer chain orientation relative to the electrodes surface on the electric behaviour of single-layer polymer diode, namely density current and charge density.
Materials Science Forum, 2008
Polymers have been known for their flexibility and easy processing into coatings and films, which... more Polymers have been known for their flexibility and easy processing into coatings and films, which made them suitable to be applied in a variety of areas and in particular the growing area of organic electronics. The electronic properties of semiconducting polymers made them a serious rival in areas where until now inorganic materials were the most used, such as light emitting diodes or solar cells. Typical polymers can be seen as a network of molecular strands of varied lengths and orientations, with a random distribution of physical and chemical defects which makes them an anisotropic material. To further increase their performance, a better understanding of all aspects related to charge transport and space charge distribution in polymeric materials is required. The process associated with charge transport depends on the properties of the polymer molecules as well as connectivity and texture, and so we adopt a mesoscopic approach to build polymer structures. Changing the potential barrier for charge injection we can introduce holes in the polymer network and, by using a generalised Monte-Carlo method, we can simulate the transport of the injected charge through the polymer layer caused by imposing a voltage between two planar electrodes. Our results show that the way that holes distribute within polymer layer and charge localization in these materials is quite different from the inorganic ones.
Thin Solid Films, 2008
Transparent conducting oxides are widely used as the transparent electrode in polymer light emitt... more Transparent conducting oxides are widely used as the transparent electrode in polymer light emitting diodes (PLEDs). The physical properties of these materials and consequently device performance strongly depend on their processing and surface treatment. The injection of charge from the transparent electrode into the polymer layer occurs by tunnelling through a potential barrier from the electrode to molecules close to it. This barrier is influenced by the difference in the relevant energy levels of electrode material and polymer molecules, the external applied potential, the Coulomb potential of the charges present in the polymer layer and the potential of their image charges on the electrodes, and may also be altered by electrode degradation effects. A better understanding of the effect of varying this potential barrier on the functioning of PLED is necessary to achieve further improvements in these applications. Here we present a theoretical study of the influence of changes in the potential barrier at the transparent electrode, on bipolar charge evolution through thin polymer layers, in the absence of defects and impurity states, while the other electrode functions as an ohmic contact. Results of a mesoscopic model provide insight into bipolar charge injection, charge and recombination distribution throughout the polymer layer, and may suggest new materials and processing methods to optimize these optoelectronic devices.
Materials Science Forum, 2010
Semiconductor polymers are successfully implemented in a broad range of applications such as ligh... more Semiconductor polymers are successfully implemented in a broad range of applications such as light emitting diodes, field effect transistors and photovoltaic devices. Most of the achievements reached in the development of these devices were obtained at experimental level, being difficult to identify individually the influence of each factor that limits and controls these devices efficiency. One of the factors that strongly influence the performance of polymer-based devices is the presence of chemical defects in the polymer strands that change their molecular properties. As a result, these polymer strands can work like traps or deep energetic states for charge transport, leading, for instance, to a decrease on charge mobility. At experimental level it is a difficult task to isolate the influence of each type of chemical defects individually on the molecular properties of the polymer strands. It is in this context that theoretical modelling seems to be the most suitable approach to ge...
Materials Science Forum, 2010
The actual interest on polymer light emitting diodes (PLEDs) is based on the fact that they are e... more The actual interest on polymer light emitting diodes (PLEDs) is based on the fact that they are easy to process, which reduces the cost of fabrication and thus opening a new branch in the electronic market-the low-cost electronics. However, these devices present a limited efficiency compared to their inorganic counterparts mainly due to the unbalanced charge injection, which reduces the fluorescence emission. One of the first strategies to improve PLEDs efficiency was using a bilayer structure composed by two polymers to improve charge injection and transport, and at the same time tunne charge recombination zone to reduce the effect of the electrodes on exciton quenching. Although this is a very ingenious device architecture some of these bilayer devices showed a lower efficiency than it was expected. The reason for that is attributed to the dissolution of the first polymer layer by the solvent used for the deposition of the second polymer layer, which do not allow to create a define polymer/polymer interface. Although cross-linking the first polymer layer can solve this problem, there is not a clear understanding why the presence of a graded interface between both polymer layers can lead to a change on PLED efficiency. In order to clarify the effect of a graded polymer/polymer interface as compared to a sharp one on the functioning of a PLED, we performed computer experiments using a mesoscopic model of a bilayer PLED developed by us that considers the morphology of both polymers at nanoscale and their properties at molecular scale. The results present in this work show clearly a significant change on the charge recombination profile within the polymer device depending on the type of interface formed between the two polymers, which can be a plausible explanation for the loss of efficiency in the bilayer 7-CN-PPV/PPV LED.
International Conference on Applications of Optics and Photonics, 2011
Although semiconducting polymers are very attractive to be used in optoelectronic devices due to ... more Although semiconducting polymers are very attractive to be used in optoelectronic devices due to their molecular structure, they are not pristine semiconductors. After deposition it is possible to find out several structural and chemical defects, with different origins, that strongly influence exciton dynamics since they create deep energetic sites, where excitons can migrate leading to their quenching or reducing exciton diffusion length. By using a self-consistent quantum molecular dynamics method we performed a computational study to understand the influence of well-known polymer defects on excitons dynamics. Our results show that these defects influences mainly intramolecular exciton localization and exciton energy.
International Conference on Applications of Optics and Photonics, 2011
The optimization of polymer-based optoelectronic devices such as light-emitting diodes (LEDs), ph... more The optimization of polymer-based optoelectronic devices such as light-emitting diodes (LEDs), photodetectors and photovoltaic cells requires the understanding how molecular properties and the spatial arrangement of the conjugated strands affect the electronic processes underlying the functioning of these devices. Since some of the important features are determined largely by the individual molecular strands and other features depend strongly on the nanostructure, a multi-scale modelling of materials and device properties is needed. In this work we discuss the atomistic and nanoscale modelling of charge injection, transport and trapping single-carrier diode based on poly(p-phenylene venylene) (PPV), which also applies to other optoelectronic devices.
Thin Solid Films, 2014
To improve the efficiency of organic solar cells is necessary, for instance, to increase the open... more To improve the efficiency of organic solar cells is necessary, for instance, to increase the open-circuit voltage or increase sunlight absorption by covering complementary regions of the solar spectrum. This can be achieved by using a donor-acceptor system composed of two polymers. The versatility of these materials has the advantage of enabling the control of morphology at nanoscale, and thus the design of an adequate interface to improve the device efficiency, using for instance nanoimprint lithography. However, proper control of the molecular organization of both polymers at polymer-polymer interface is difficult and strongly depends on the experimental conditions used.
Materials Science and Engineering: B, 2011
During the last years it has been clear that it is importance to understand and control the nanos... more During the last years it has been clear that it is importance to understand and control the nanostructure of the active polymer layer used in optoelectronic devices, like polymer diodes, solar cells or field effect transistors. Several experimental studies have shown that the nanostructure of polymer thin films used in these optoelectronic devices depends on the conditions used to deposit the polymer layer between the electrodes. As a result, in solid state conjugated polymer chains tend to be stacked and aligned relative to the electrodes creating domains with different sizes that influence the performance of these devices. To understand how the spatial arrangement of polymer chains affects the 2 functioning of optoelectronic devices, we performed computer simulations using our mesoscopic model based on a generalized dynamic Monte Carlo method. We focus our study on the influence of the nanomorphology on the electric properties of polymer light emitting diodes. Our results show that for a pristine polymer layer and in the presence of ohmic contacts between the electrodes and the polymer layer, the electric properties of the device, namely current density, charge density, internal electric field and the number of charges that undergo recombination strongly depends on the polymer morphology at nanoscale.
Materials Science and Engineering: B, 2009
The electric behaviour of polymer diodes has the influence of several factors such as the electro... more The electric behaviour of polymer diodes has the influence of several factors such as the electrodes work function, the experimental conditions used to deposit the active component or the chemistry of the polymer. Although experimentally it is possible to study the effect of some of these factors on the device performance, for instance by changing the chemical structure of the polymer used or the type of electrodes, it is impossible to study individually each one of these effects because changing one of them can influence the others. Quantum mechanical calculations have shown that depending on the chemical structure of the polymer, its intramolecular properties (e.g. ionization potential, electron affinity or intramolecular charge mobility) can be changed. To understand the effect of the intramolecular properties in the performance of polymer diodes we use a generalized dynamical Monte Carlo method that considers the nanostructure of the polymer layer and the main electronic processes involved in diode 2 functioning. Our results show that the influence of the intramolecular properties on the electric behaviour of pristine polymer-diodes with ohmic contacts depends on the morphology of the polymer layer at nanoscale that can alter not only hole and electron current density for the same applied electric field but also charge density and charge distribution inside the polymer layer.
Journal of Non-Crystalline Solids, 2006
We present theoretical investigations of structural and electronic properties of ground-state and... more We present theoretical investigations of structural and electronic properties of ground-state and low-lying excited singlet states in isolated chains of conjugated polymers using a self-consistent quantum molecular dynamics method. With this approach, we have determined the energy of both states as function of the twist angle between two planar segments of the same polymer chain, for polymer chains with variable length. The conjugated polymers investigated here are poly(para-phenylene vinylene) (PPV) and polydiacetylene (PDA). Our results show that the energy of the excited-state increases more than that of the ground-state, as the twist angle increases up to 90º degrees. The change in the twist angle of both polymers leads to a blueshift in luminescence transition energy, the effect being stronger in PPV when the planar segments have similar sizes. The predicted blueshift in both polymers is dependent on the chain length, the effect being more pronounced for shorter-chains.
Journal of Nanoscience and Nanotechnology, 2010
One of the factors that limit the efficiency of polymer-based optoelectronic devices, such as pho... more One of the factors that limit the efficiency of polymer-based optoelectronic devices, such as photovoltaic solar cells and light emitting diodes, is the exciton diffusion within the polymeric network. Due to the amorphous nature the of polymeric materials, the diffusion of excitons is limited by the energetic and spatial disorder in such systems, which is a consequence not only of the chemical structure of the polymer but also from its morphology at nanoscale. To get a deep understanding on how such effects influence exciton dynamics we performed a quantum molecular dynamics simulations to determine the energetic disorder within the polymer system, and Monte Carlo simulations to study exciton diffusion in three-dimensional (3D) polymer networks that present both spatial and energetic disorder at nanometre scale. Our results show clearly that exciton diffusion in poly(pphenylenevenylene) (PPV) occurs preferentially in the direction parallel to the electrodes surface for a polymer-based optoelectronic devices with the orientation of the conjugated strands similar to those obtained by the spin-coating technique and the decay of such excitons occurs preferentially in longer strands which allow us to get insight on exciton behaviour in polymeric systems that are not possible to be obtained directly from the experiments.
Journal of Materials Science: Materials in Electronics, 2008
It is well known that the morphology of polymer-based optoelectronic devices can influence their ... more It is well known that the morphology of polymer-based optoelectronic devices can influence their efficiency, since the ways that polymer chains pack inside the active layer can influence not only the charge transport but also the optic properties of the device. By using a mesoscopic model we carried out computer experiments to study the influence of the polymer morphology on the processes of charge injection, transport, recombination and collection by the electrodes opposite to those where the injection of bipolar charge carriers take place. Our results show that for polymer layers where the conjugated segments have perpendicular and random orientation relative to the electrodes surface, the competition between charge collection and charge recombination is affected when the average conjugation length of the polymer strands increase. This effect is more pronounced with the increase of the potential barrier at polymer/electrode interfaces that limit charge injection and increase charge collection. For these molecular arrangements the intra-molecular charge transport plays a major role in device performance, being this effect negligible when the polymer molecules have their axis parallel to the electrodes. Although the polymer morphology modelled in this work is far from real, we believe that our model can give some insights on the role of the microstructure on the functioning of polymer-based devices.
Journal of Materials Science: Materials in Electronics, 2007
Although optoelectronic devices made of polymers are very attractive ones (low cost, easy to make... more Although optoelectronic devices made of polymers are very attractive ones (low cost, easy to make), problems related to charge transport, exciton quenching, among others, can be an obstacle for their performance. The use of heterojunctions made of two polymers can be a strategy for improving the efficiency of polymer light emitting diodes (PLEDs) at low bias. Here we present a theoretical study of the influence of bilayer structure in a PLED made of PPV and 2-CN-PPV, by adopting a mesoscopic approach. Our results show that the presence of the polymer/polymer interface improves charge injection and leads to a confinement of charges near it, which will increase the number recombination events in the middle of the device compared to the equivalent single-layer PLEDs.
Journal of Materials Chemistry, 2010
One of the strategies to improve the efficiency of organic light emitting diodes (OLEDs) is to do... more One of the strategies to improve the efficiency of organic light emitting diodes (OLEDs) is to dope the active organic semiconducting layer with inorganic salts, leading to the development of a hybrid organic/inorganic hetero-structure. However, it is hard to know from the experiments how each one of the electronic processes underlying the functioning of OLEDs are affected by the accumulation of inorganic ions of different sign at both organic/electrode interfaces. In order to assess these effects, we performed computer simulations by using a multi-scale model that combines quantum molecular dynamics calculations at atomistic scale with Monte Carlo calculations at mesoscopic scale. We focus our attention on the main differences obtained between doped and pristine organic layers, when bipolar charge injection occurs. Our results show a significant drop on the turn-on applied electric field while maintaining rapid response to the applied field as well as a clear increase in recombination rate and recombination efficiency far from the electrodes for the doped situation, which are responsible for the dramatic improvement of doped OLED performance found in the experiments.
The European Physical Journal Applied Physics, 2009
In polymer light emitting diodes (PLEDs) each semiconducting polymer chain consists of a large nu... more In polymer light emitting diodes (PLEDs) each semiconducting polymer chain consists of a large number of conjugated segments linked by kinks or twists and each one of them behaves like a separated straight strand. The length and orientation of the conjugated strands relative to the electrodes surface depend on the deposition conditions used. Atomistic results have shown that the molecular properties of the conjugated strands depend on their length, which can affect the electronic processes involved in PLEDs. The aim of this work is to study the influence of the average conjugation length within the polymer layer on charge injection, trapping and recombination in PLEDs for all polymer strand orientations relative to the electrodes surface obtained experimentally by different techniques. For that purpose we use a mesoscopic model that considers the morphology and the molecular properties of the polymer. Our results show that by increasing the average conjugation length of the active polymer layer the amount of charge injected into the device increases and the recombination probability occurs preferentially in segments longer than the average conjugation length, both effects having implications on the performance of polymer LEDs.
Computational Materials Science, 2013
The study of the average distance that singlet excitons travel during their lifetime in conjugate... more The study of the average distance that singlet excitons travel during their lifetime in conjugated polymers has attracted considerable attention during the past decade, because of its importance in the functioning of many polymer-based optoelectronic devices, like solar cells and photodetectors. Intriguingly, different values of exciton diffusion length have been extracted from experiments on seemingly identical conjugated polymers. Here we use computer simulations to show that the observed discrepancies in the reported values of the exciton diffusion length may arise from differences in the orientation of conjugated polymer strands relative to the substrate surface, a factor which has been mostly overlooked. Our results show that, on pristine polymer nanodomains with conjugated strands perpendicular to the substrate surface, exciton migration length is approximately 30% and 40% lower than on those with parallel and random strand orientation relative to that surface, respectively, resulting from the different contents of physical traps present in nanodomains with different strand orientation. This work underlines the importance of molecular arrangement on exciton migration, and provides a 2 novel theoretical framework for estimating the dependence of the exciton diffusion length with the orientation of conjugated polymers strands within the nanodomains, as well as helping the design of more efficient polymer-based optical and optoelectronic devices, such as optical sensors, photodiodes, photovoltaic cells and white light-emitting diodes.
Plasma Processes and Polymers, 2007
Substantial progress has been made in fabricating optoelectronic devices using polymers as an act... more Substantial progress has been made in fabricating optoelectronic devices using polymers as an active material. In polymer light emitting diodes (PLEDs), a balanced injection of electrons and holes from the electrodes is fundamental to increase their performance. Using a mesoscopic model based on a generalized Monte-Carlo method, we studied the influence of changing zero-field barrier heights at both electrode/polymer interfaces in the performance of a PLED with an active layer of poly(paraphenylenevinylene) (PPV). Our results show that by controlling the electrodes work functions it is possible to tune the region inside the device where charge recombination preferentially takes place.
Springer Proceedings in Physics, 2009
Recently some experimental results have showed that the spatial alignment of conjugated polymer c... more Recently some experimental results have showed that the spatial alignment of conjugated polymer chains on nanometre length scales can influence the behaviour of polymer-based electronic devices, such as light-emitting diodes, field effect transistors, and photovoltaic cells. The effects of chain orientation at electrode-polymer interfaces on the charge injection process and charge mobility through the polymer layer are not well understood. In this work we use a generalized dynamical Monte Carlo method to study the influence of different polymer chain orientation relative to the electrodes surface on the electric behaviour of single-layer polymer diode, namely density current and charge density.
Materials Science Forum, 2008
Polymers have been known for their flexibility and easy processing into coatings and films, which... more Polymers have been known for their flexibility and easy processing into coatings and films, which made them suitable to be applied in a variety of areas and in particular the growing area of organic electronics. The electronic properties of semiconducting polymers made them a serious rival in areas where until now inorganic materials were the most used, such as light emitting diodes or solar cells. Typical polymers can be seen as a network of molecular strands of varied lengths and orientations, with a random distribution of physical and chemical defects which makes them an anisotropic material. To further increase their performance, a better understanding of all aspects related to charge transport and space charge distribution in polymeric materials is required. The process associated with charge transport depends on the properties of the polymer molecules as well as connectivity and texture, and so we adopt a mesoscopic approach to build polymer structures. Changing the potential barrier for charge injection we can introduce holes in the polymer network and, by using a generalised Monte-Carlo method, we can simulate the transport of the injected charge through the polymer layer caused by imposing a voltage between two planar electrodes. Our results show that the way that holes distribute within polymer layer and charge localization in these materials is quite different from the inorganic ones.
Thin Solid Films, 2008
Transparent conducting oxides are widely used as the transparent electrode in polymer light emitt... more Transparent conducting oxides are widely used as the transparent electrode in polymer light emitting diodes (PLEDs). The physical properties of these materials and consequently device performance strongly depend on their processing and surface treatment. The injection of charge from the transparent electrode into the polymer layer occurs by tunnelling through a potential barrier from the electrode to molecules close to it. This barrier is influenced by the difference in the relevant energy levels of electrode material and polymer molecules, the external applied potential, the Coulomb potential of the charges present in the polymer layer and the potential of their image charges on the electrodes, and may also be altered by electrode degradation effects. A better understanding of the effect of varying this potential barrier on the functioning of PLED is necessary to achieve further improvements in these applications. Here we present a theoretical study of the influence of changes in the potential barrier at the transparent electrode, on bipolar charge evolution through thin polymer layers, in the absence of defects and impurity states, while the other electrode functions as an ohmic contact. Results of a mesoscopic model provide insight into bipolar charge injection, charge and recombination distribution throughout the polymer layer, and may suggest new materials and processing methods to optimize these optoelectronic devices.
Materials Science Forum, 2010
Semiconductor polymers are successfully implemented in a broad range of applications such as ligh... more Semiconductor polymers are successfully implemented in a broad range of applications such as light emitting diodes, field effect transistors and photovoltaic devices. Most of the achievements reached in the development of these devices were obtained at experimental level, being difficult to identify individually the influence of each factor that limits and controls these devices efficiency. One of the factors that strongly influence the performance of polymer-based devices is the presence of chemical defects in the polymer strands that change their molecular properties. As a result, these polymer strands can work like traps or deep energetic states for charge transport, leading, for instance, to a decrease on charge mobility. At experimental level it is a difficult task to isolate the influence of each type of chemical defects individually on the molecular properties of the polymer strands. It is in this context that theoretical modelling seems to be the most suitable approach to ge...
Materials Science Forum, 2010
The actual interest on polymer light emitting diodes (PLEDs) is based on the fact that they are e... more The actual interest on polymer light emitting diodes (PLEDs) is based on the fact that they are easy to process, which reduces the cost of fabrication and thus opening a new branch in the electronic market-the low-cost electronics. However, these devices present a limited efficiency compared to their inorganic counterparts mainly due to the unbalanced charge injection, which reduces the fluorescence emission. One of the first strategies to improve PLEDs efficiency was using a bilayer structure composed by two polymers to improve charge injection and transport, and at the same time tunne charge recombination zone to reduce the effect of the electrodes on exciton quenching. Although this is a very ingenious device architecture some of these bilayer devices showed a lower efficiency than it was expected. The reason for that is attributed to the dissolution of the first polymer layer by the solvent used for the deposition of the second polymer layer, which do not allow to create a define polymer/polymer interface. Although cross-linking the first polymer layer can solve this problem, there is not a clear understanding why the presence of a graded interface between both polymer layers can lead to a change on PLED efficiency. In order to clarify the effect of a graded polymer/polymer interface as compared to a sharp one on the functioning of a PLED, we performed computer experiments using a mesoscopic model of a bilayer PLED developed by us that considers the morphology of both polymers at nanoscale and their properties at molecular scale. The results present in this work show clearly a significant change on the charge recombination profile within the polymer device depending on the type of interface formed between the two polymers, which can be a plausible explanation for the loss of efficiency in the bilayer 7-CN-PPV/PPV LED.
International Conference on Applications of Optics and Photonics, 2011
Although semiconducting polymers are very attractive to be used in optoelectronic devices due to ... more Although semiconducting polymers are very attractive to be used in optoelectronic devices due to their molecular structure, they are not pristine semiconductors. After deposition it is possible to find out several structural and chemical defects, with different origins, that strongly influence exciton dynamics since they create deep energetic sites, where excitons can migrate leading to their quenching or reducing exciton diffusion length. By using a self-consistent quantum molecular dynamics method we performed a computational study to understand the influence of well-known polymer defects on excitons dynamics. Our results show that these defects influences mainly intramolecular exciton localization and exciton energy.
International Conference on Applications of Optics and Photonics, 2011
The optimization of polymer-based optoelectronic devices such as light-emitting diodes (LEDs), ph... more The optimization of polymer-based optoelectronic devices such as light-emitting diodes (LEDs), photodetectors and photovoltaic cells requires the understanding how molecular properties and the spatial arrangement of the conjugated strands affect the electronic processes underlying the functioning of these devices. Since some of the important features are determined largely by the individual molecular strands and other features depend strongly on the nanostructure, a multi-scale modelling of materials and device properties is needed. In this work we discuss the atomistic and nanoscale modelling of charge injection, transport and trapping single-carrier diode based on poly(p-phenylene venylene) (PPV), which also applies to other optoelectronic devices.
Thin Solid Films, 2014
To improve the efficiency of organic solar cells is necessary, for instance, to increase the open... more To improve the efficiency of organic solar cells is necessary, for instance, to increase the open-circuit voltage or increase sunlight absorption by covering complementary regions of the solar spectrum. This can be achieved by using a donor-acceptor system composed of two polymers. The versatility of these materials has the advantage of enabling the control of morphology at nanoscale, and thus the design of an adequate interface to improve the device efficiency, using for instance nanoimprint lithography. However, proper control of the molecular organization of both polymers at polymer-polymer interface is difficult and strongly depends on the experimental conditions used.
Materials Science and Engineering: B, 2011
During the last years it has been clear that it is importance to understand and control the nanos... more During the last years it has been clear that it is importance to understand and control the nanostructure of the active polymer layer used in optoelectronic devices, like polymer diodes, solar cells or field effect transistors. Several experimental studies have shown that the nanostructure of polymer thin films used in these optoelectronic devices depends on the conditions used to deposit the polymer layer between the electrodes. As a result, in solid state conjugated polymer chains tend to be stacked and aligned relative to the electrodes creating domains with different sizes that influence the performance of these devices. To understand how the spatial arrangement of polymer chains affects the 2 functioning of optoelectronic devices, we performed computer simulations using our mesoscopic model based on a generalized dynamic Monte Carlo method. We focus our study on the influence of the nanomorphology on the electric properties of polymer light emitting diodes. Our results show that for a pristine polymer layer and in the presence of ohmic contacts between the electrodes and the polymer layer, the electric properties of the device, namely current density, charge density, internal electric field and the number of charges that undergo recombination strongly depends on the polymer morphology at nanoscale.
Materials Science and Engineering: B, 2009
The electric behaviour of polymer diodes has the influence of several factors such as the electro... more The electric behaviour of polymer diodes has the influence of several factors such as the electrodes work function, the experimental conditions used to deposit the active component or the chemistry of the polymer. Although experimentally it is possible to study the effect of some of these factors on the device performance, for instance by changing the chemical structure of the polymer used or the type of electrodes, it is impossible to study individually each one of these effects because changing one of them can influence the others. Quantum mechanical calculations have shown that depending on the chemical structure of the polymer, its intramolecular properties (e.g. ionization potential, electron affinity or intramolecular charge mobility) can be changed. To understand the effect of the intramolecular properties in the performance of polymer diodes we use a generalized dynamical Monte Carlo method that considers the nanostructure of the polymer layer and the main electronic processes involved in diode 2 functioning. Our results show that the influence of the intramolecular properties on the electric behaviour of pristine polymer-diodes with ohmic contacts depends on the morphology of the polymer layer at nanoscale that can alter not only hole and electron current density for the same applied electric field but also charge density and charge distribution inside the polymer layer.
Journal of Non-Crystalline Solids, 2006
We present theoretical investigations of structural and electronic properties of ground-state and... more We present theoretical investigations of structural and electronic properties of ground-state and low-lying excited singlet states in isolated chains of conjugated polymers using a self-consistent quantum molecular dynamics method. With this approach, we have determined the energy of both states as function of the twist angle between two planar segments of the same polymer chain, for polymer chains with variable length. The conjugated polymers investigated here are poly(para-phenylene vinylene) (PPV) and polydiacetylene (PDA). Our results show that the energy of the excited-state increases more than that of the ground-state, as the twist angle increases up to 90º degrees. The change in the twist angle of both polymers leads to a blueshift in luminescence transition energy, the effect being stronger in PPV when the planar segments have similar sizes. The predicted blueshift in both polymers is dependent on the chain length, the effect being more pronounced for shorter-chains.
Journal of Nanoscience and Nanotechnology, 2010
One of the factors that limit the efficiency of polymer-based optoelectronic devices, such as pho... more One of the factors that limit the efficiency of polymer-based optoelectronic devices, such as photovoltaic solar cells and light emitting diodes, is the exciton diffusion within the polymeric network. Due to the amorphous nature the of polymeric materials, the diffusion of excitons is limited by the energetic and spatial disorder in such systems, which is a consequence not only of the chemical structure of the polymer but also from its morphology at nanoscale. To get a deep understanding on how such effects influence exciton dynamics we performed a quantum molecular dynamics simulations to determine the energetic disorder within the polymer system, and Monte Carlo simulations to study exciton diffusion in three-dimensional (3D) polymer networks that present both spatial and energetic disorder at nanometre scale. Our results show clearly that exciton diffusion in poly(pphenylenevenylene) (PPV) occurs preferentially in the direction parallel to the electrodes surface for a polymer-based optoelectronic devices with the orientation of the conjugated strands similar to those obtained by the spin-coating technique and the decay of such excitons occurs preferentially in longer strands which allow us to get insight on exciton behaviour in polymeric systems that are not possible to be obtained directly from the experiments.
Journal of Materials Science: Materials in Electronics, 2008
It is well known that the morphology of polymer-based optoelectronic devices can influence their ... more It is well known that the morphology of polymer-based optoelectronic devices can influence their efficiency, since the ways that polymer chains pack inside the active layer can influence not only the charge transport but also the optic properties of the device. By using a mesoscopic model we carried out computer experiments to study the influence of the polymer morphology on the processes of charge injection, transport, recombination and collection by the electrodes opposite to those where the injection of bipolar charge carriers take place. Our results show that for polymer layers where the conjugated segments have perpendicular and random orientation relative to the electrodes surface, the competition between charge collection and charge recombination is affected when the average conjugation length of the polymer strands increase. This effect is more pronounced with the increase of the potential barrier at polymer/electrode interfaces that limit charge injection and increase charge collection. For these molecular arrangements the intra-molecular charge transport plays a major role in device performance, being this effect negligible when the polymer molecules have their axis parallel to the electrodes. Although the polymer morphology modelled in this work is far from real, we believe that our model can give some insights on the role of the microstructure on the functioning of polymer-based devices.
Journal of Materials Science: Materials in Electronics, 2007
Although optoelectronic devices made of polymers are very attractive ones (low cost, easy to make... more Although optoelectronic devices made of polymers are very attractive ones (low cost, easy to make), problems related to charge transport, exciton quenching, among others, can be an obstacle for their performance. The use of heterojunctions made of two polymers can be a strategy for improving the efficiency of polymer light emitting diodes (PLEDs) at low bias. Here we present a theoretical study of the influence of bilayer structure in a PLED made of PPV and 2-CN-PPV, by adopting a mesoscopic approach. Our results show that the presence of the polymer/polymer interface improves charge injection and leads to a confinement of charges near it, which will increase the number recombination events in the middle of the device compared to the equivalent single-layer PLEDs.
Journal of Materials Chemistry, 2010
One of the strategies to improve the efficiency of organic light emitting diodes (OLEDs) is to do... more One of the strategies to improve the efficiency of organic light emitting diodes (OLEDs) is to dope the active organic semiconducting layer with inorganic salts, leading to the development of a hybrid organic/inorganic hetero-structure. However, it is hard to know from the experiments how each one of the electronic processes underlying the functioning of OLEDs are affected by the accumulation of inorganic ions of different sign at both organic/electrode interfaces. In order to assess these effects, we performed computer simulations by using a multi-scale model that combines quantum molecular dynamics calculations at atomistic scale with Monte Carlo calculations at mesoscopic scale. We focus our attention on the main differences obtained between doped and pristine organic layers, when bipolar charge injection occurs. Our results show a significant drop on the turn-on applied electric field while maintaining rapid response to the applied field as well as a clear increase in recombination rate and recombination efficiency far from the electrodes for the doped situation, which are responsible for the dramatic improvement of doped OLED performance found in the experiments.
The European Physical Journal Applied Physics, 2009
In polymer light emitting diodes (PLEDs) each semiconducting polymer chain consists of a large nu... more In polymer light emitting diodes (PLEDs) each semiconducting polymer chain consists of a large number of conjugated segments linked by kinks or twists and each one of them behaves like a separated straight strand. The length and orientation of the conjugated strands relative to the electrodes surface depend on the deposition conditions used. Atomistic results have shown that the molecular properties of the conjugated strands depend on their length, which can affect the electronic processes involved in PLEDs. The aim of this work is to study the influence of the average conjugation length within the polymer layer on charge injection, trapping and recombination in PLEDs for all polymer strand orientations relative to the electrodes surface obtained experimentally by different techniques. For that purpose we use a mesoscopic model that considers the morphology and the molecular properties of the polymer. Our results show that by increasing the average conjugation length of the active polymer layer the amount of charge injected into the device increases and the recombination probability occurs preferentially in segments longer than the average conjugation length, both effects having implications on the performance of polymer LEDs.
Computational Materials Science, 2013
The study of the average distance that singlet excitons travel during their lifetime in conjugate... more The study of the average distance that singlet excitons travel during their lifetime in conjugated polymers has attracted considerable attention during the past decade, because of its importance in the functioning of many polymer-based optoelectronic devices, like solar cells and photodetectors. Intriguingly, different values of exciton diffusion length have been extracted from experiments on seemingly identical conjugated polymers. Here we use computer simulations to show that the observed discrepancies in the reported values of the exciton diffusion length may arise from differences in the orientation of conjugated polymer strands relative to the substrate surface, a factor which has been mostly overlooked. Our results show that, on pristine polymer nanodomains with conjugated strands perpendicular to the substrate surface, exciton migration length is approximately 30% and 40% lower than on those with parallel and random strand orientation relative to that surface, respectively, resulting from the different contents of physical traps present in nanodomains with different strand orientation. This work underlines the importance of molecular arrangement on exciton migration, and provides a 2 novel theoretical framework for estimating the dependence of the exciton diffusion length with the orientation of conjugated polymers strands within the nanodomains, as well as helping the design of more efficient polymer-based optical and optoelectronic devices, such as optical sensors, photodiodes, photovoltaic cells and white light-emitting diodes.
Plasma Processes and Polymers, 2007
Substantial progress has been made in fabricating optoelectronic devices using polymers as an act... more Substantial progress has been made in fabricating optoelectronic devices using polymers as an active material. In polymer light emitting diodes (PLEDs), a balanced injection of electrons and holes from the electrodes is fundamental to increase their performance. Using a mesoscopic model based on a generalized Monte-Carlo method, we studied the influence of changing zero-field barrier heights at both electrode/polymer interfaces in the performance of a PLED with an active layer of poly(paraphenylenevinylene) (PPV). Our results show that by controlling the electrodes work functions it is possible to tune the region inside the device where charge recombination preferentially takes place.