Influence of Molecular Polarizability on the Active Layer of Organic Photovoltaic Cells.
Abstract
The complexity of the microstructure of the active layer in organic photovoltaics (OPVs) poses a unique challenge in improving the efficiency of OPVdevices.
Molecular dynamics (MD) simulation provides a direct route todetermining this microstructure.
However, for a donor material like poly (3-hexylthiophene), (P3HT)n, approximations made in all previous force fieldfor MD simulation has been the neglect of explicit polarization.
We looked atthe morphology of (P3HT)n using MD simulations at different temperatureswhere we confirmed the semi-crystalline behavior of P3HT between temperatures of 300 K and 400 K.
In line with this, we developedforce fields fromab initio data with and without inclusion of explicit molecular polarizabilityfor dimers of (P3HT)1 with monomers optimized at the MP2/cc-pvtz level.
Contents
Dedication ii
Acknowledgement iii
Abstract iv
Nomenclature ix
1 Introduction 2
2 Atomistic Structure and Dynamics of Poly(3 hexyl-thiophene) 8
2.1 Structure and Properties of P3HT . . . . . . . . . . . . . . . . 8
2.1.1 Regioregularity in P3HT . . . . . . . . . . . . . . . . . 9
2.2 Molecular Interactions . . . . . . . . . . . . . . . . . . . . . . 11
2.2.1 Short Range Repulsive Interaction . . . . . . . . . . . 11
2.2.2 Dipole Interaction . . . . . . . . . . . . . . . . . . . . . 12
2.2.3 Polarization . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.4 Dipole Polarizability . . . . . . . . . . . . . . . . . . . 15
2.2.5 Static Dipole Polarizability . . . . . . . . . . . . . . . . 15
3 Computer Simulation 19
3.1 Theoretical Models . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2 Basis sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.2.1 Types of Basis Set . . . . . . . . . . . . . . . . . . . . 23
3.3 Geometry Optimization . . . . . . . . . . . . . . . . . . . . . 25
3.4 Force Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.4.1 Force Field Development . . . . . . . . . . . . . . . . . 27
4 MD Simulation of Poly(3-hexylthiophene) 31
4.1 Development of P3HT Molecular Structure and Geometry Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.2 Molecular Dynamics Simulation . . . . . . . . . . . . . . . . . 34
4.3 Results and Discussions . . . . . . . . . . . . . . . . . . . . . 36
4.4 Development of Ab Initio Polarizable Force Fields for (P3HT)1 38
4.4.1 Ab Initio Atomic Partial Charges . . . . . . . . . . . . 45
4.4.2 Ab Initio Distributed Polarizabilities . . . . . . . . . . 47
4.4.3 Ab Initio Lennard Jones parameters . . . . . . . . . . . 47
5 Conclusions and Recommendations 51
5.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.2 Recommendation . . . . . . . . . . . . . . . . . . . . . . . . . 51
Appendix 52
Introduction
As the world is faced with the issues of ever-decreasing fossil fuel and everincreasing environmental crises resulting from greenhouse gas production,photovoltaic (PV) technology has become a main focus of attention.
Thesuccess of any PV technology depends on its efficiency, lifetime and cost.Silicon-based solar cells (a type of IPV) have high efficiencies.
However, thehigh cost to efficiency ratio of silicon-based solar cells and their complex production process has generated interest in developingalternative PV cells suchas organic photovoltaics (OPVs).
The obvious difference between IPVs andOPVs in terms of their photo-conversion mechanisms (Fig. 1.1) is that lightabsorption in OPV cells leads to the production of exciton mobile excitedstates while in IPVs, it leads directly to the creation of free electron-holepairs.
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