Charge Mobility in Organic Semiconductors: Linking Theory and Experiments
Lead Research Organisation:
University of Warwick
Department Name: Chemistry
Abstract
Computing the electron transport properties of a material is one of the most difficult tasks in quantum mechanics. In contrast to molecular properties at equilibrium (e.g. ionization potential, equilibrium bond distance) for which quantum chemistry provides a general protocol and a hierarchy of approximations, there is neither a unique model to describe electron transport nor a well established link between computations and electric measurements. The problem has a distinct theoretical and computational aspect: 1. THEORY. A typical theory of charge transport assumes that the electronic and vibrational structure of the material at equilibrium are computable, and develops a simplified 'model Hamiltonian' that, incorporating a few material dependent parameters, should capture the essence of charge transport in the real system. Advanced approximation methods from quantum dynamics are used to study the model Hamiltonian, obtaining an analytical expression that relates the materials parameter and the observable quantities (e.g. the charge mobility). The model Hamiltonian appropriate for organic semiconductors is different from that used for their inorganic counterpart. The charge carriers of organic conjugated compounds are localized by a combination of two effects: (i) the reorganization energy (i.e. a displacement of the nuclei that stabilizes the charge carriers) and (ii) the disorder (electrons are completely delocalized in a perfectly ordered lattice, while they are localized by the presence of structural disorder). The cooperation of these two effects is what makes organic semiconductors unique among electronics materials. In this project, the appropriate model Hamiltonian for polymeric semiconductors will be built. The role of the different material parameters (reorganization energy, static and dynamic disorder, electronic coupling between monomers, etc...) on the charge mobility will be determined studying numerically the time evolution of the wavefunction that describes the charge carrier. An analytical formulation that relates the charge mobility with the material parameter will be also proposed. 2. COMPUTATION. A charge transport theory links microscopic quantities to experimental observables. Computational chemistry methods will be used in this project to evaluate all the microscopic quantities needed by the theory to make quantitative predictions. The reorganization energy and the electronic coupling between monomers will be computed using quantum chemical methods as already described in the literature. The effect of static and dynamic disorder will be evaluated using a combination of classical molecular dynamics (MD) and quantum chemistry. A classical MD simulation of few polymer chains will be used to model the conformational space explored by the polymer at a given temperature. For a large number of geometries explored by the classical trajectory, the electronic Hamiltonian of the system will be evaluated via quantum chemical calculations. The resulting time dependent electronic Hamiltonian will describe the static and dynamic disorder of the polymeric system. Together with the other material dependent parameters, it will be used to evaluate the charge mobility of the material. An extensive comparison of the proposed theoretical and computational analysis with the available experimental data will provide a much clearer picture of the transport mechanism in organic polymers.
Organisations
Publications
Troisi A
(2007)
Prediction of the Absolute Charge Mobility of Molecular Semiconductors: the Case of Rubrene
in Advanced Materials
Cheung DL
(2008)
Effect of substrate geometry on liquid-crystal-mediated nanocylinder-substrate interactions.
in The Journal of chemical physics
Cheung DL
(2009)
A realistic description of the charge carrier wave function in microcrystalline polymer semiconductors.
in Journal of the American Chemical Society
Cheung DL
(2009)
Computational study of the structure and charge-transfer parameters in low-molecular-mass P3HT.
in The journal of physical chemistry. B
Troisi A
(2009)
Transition from dynamic to static disorder in one-dimensional organic semiconductors.
in The Journal of chemical physics
Troisi A
(2009)
Charge transport in semiconductors with multiscale conformational dynamics.
in Physical review letters
Vehoff T
(2010)
Charge transport in organic crystals: role of disorder and topological connectivity.
in Journal of the American Chemical Society
Liu T
(2011)
Structural variability and dynamics of the P3HT/PCBM interface and its effects on the electronic structure and the charge-transfer rates in solar cells.
in Physical chemistry chemical physics : PCCP
Carvalho A
(2012)
Charge Injection Rates in Hybrid Nanosilicon-Polythiophene Bulk Heterojunction Solar Cells
in The Journal of Physical Chemistry C
Maggio E
(2012)
Evaluating Charge Recombination Rate in Dye-Sensitized Solar Cells from Electronic Structure Calculations
in The Journal of Physical Chemistry C
Description | Computing the electron transport properties of a material is one of the most difficult tasks in quantum mechanics. In contrast to molecular properties at equilibrium (e.g. ionization potential, equilibrium bond distance) for which quantum chemistry provides a general protocol and a hierarchy of approximations, there is neither a unique model to describe electron transport nor a well established link between computations and electric measurements. The problem has a distinct theoretical and computational aspect: 1. THEORY. A typical theory of charge transport assumes that the electronic and vibrational structure of the material at equilibrium are computable, and develops a simplified 'model Hamiltonian' that, incorporating a few material dependent parameters, should capture the essence of charge transport in the real system. The model Hamiltonian appropriate for organic semiconductors is different from that used for their inorganic counterpart. The charge carriers of organic conjugated compounds are localized by a combination of two effects: (i) the reorganization energy (i.e. a displacement of the nuclei that stabilizes the charge carriers) and (ii) the disorder (electrons are completely delocalized in a perfectly ordered lattice, while they are localized by the presence of structural disorder). The cooperation of these two effects is what makes organic semiconductors unique among electronics materials. In this project, we built a generalized model Hamiltonian for all types of organic semiconductors (polymers, liquid crystals, molecular) and studied how the wavefunction evolves in each of these systems. 2. COMPUTATION. A charge transport theory links microscopic quantities to experimental observables. Computational chemistry methods have been used in this project to evaluate all the microscopic quantities needed by the theory to make quantitative predictions. The effect of static and dynamic disorder was evaluated using a combination of classical molecular dynamics (MD) and quantum chemistry. A classical MD simulation of the material was used to model the conformational space explored by the system at a given temperature. For a large number of geometries explored by the classical trajectory, the electronic Hamiltonian of the system was evaluated via quantum chemical calculations. We developed a new approximated quantum chemical method to perform rapidly the large scale calculations required in this project. Moreover, we developed some specialized methods to compute material parameters that were difficult to evaluate like the external reorganization energy. The results of this research, published on the top journals of chemistry (JACS), physics (PRL) and material science (Adv. Mat.) show that it is possible to connect formal theories with the study of realistic materials opening an all new approach to describe charge transport in organic semiconductors. The key findings can be summarized as follows: (i) We demonstrated that quantities that where considered inaccessible to the computation like the density of electronic states of polymeric material can be indeed computed and compared with the experimental results. (ii) We were able to compute the detailed electronic structure of localized charge carriers in polymeric material and determine to what structural deformation of the polymer these states are related. (iii) We proposed a very general model for the charge carrier dynamics in a material where this dynamics occurs in the same timescale of nuclear motions. While our findings are of immediate interest to the experimental community (particularly strong in the UK), the methodology we developed can be easily adapted for the study of many electron and exciton transfer problem in soft materials and we expect that it will be soon adopted by many other theoretical scientists. |
Exploitation Route | Design new polymeric materials for semiconducting industry. |
Sectors | Chemicals |
Description | Microscopic Modelling of Excitonic Solar Cell Interfaces |
Amount | £750,000 (GBP) |
Funding ID | 239988 |
Organisation | European Research Council (ERC) |
Sector | Public |
Country | Belgium |
Start | 10/2009 |
End | 09/2013 |
Description | Microscopic Modelling of Excitonic Solar Cell Interfaces |
Amount | £750,000 (GBP) |
Funding ID | 239988 |
Organisation | European Research Council (ERC) |
Sector | Public |
Country | Belgium |
Start | 10/2009 |
End | 09/2013 |
Description | Theory of interfaces in organic electronics |
Amount | £30,844 (GBP) |
Funding ID | RFG/2010/0270 |
Organisation | The Leverhulme Trust |
Sector | Charity/Non Profit |
Country | United Kingdom |
Start | 10/2010 |
End | 09/2012 |
Description | Theory of interfaces in organic electronics |
Amount | £30,844 (GBP) |
Funding ID | RFG/2010/0270 |
Organisation | The Leverhulme Trust |
Sector | Charity/Non Profit |
Country | United Kingdom |
Start | 04/2010 |
End | 05/2012 |