## In Uence of Molecular Polarizability on the Active Layer of Organic Photovoltaic Cells: A Case Study of Poly (3-hexylthiophene)

ABSTRACT

The complexity of the microstructure of the active layer in organic photovoltaics (OPVs) poses a unique challenge in improving the efficiency of OPV devices. Molecular dynamics (MD) simulation provides a direct route to determining this microstructure. However, for a donor material like poly (3-hexylthiophene), (P3HT)n, approximations made in all previous force eld for MD simulation has been the neglect of explicit polarization. We looked at the morphology of (P3HT)n using MD simulations at different temperatures where we confirmed the semi-crystalline behavior of P3HT between temperatures of 300 K and 400 K. In line with this, we developed force elds from ab initio data with and without inclusion of explicit molecular polarizability for dimers of (P3HT)1 with monomers optimized at the MP2/cc-pvtz level.

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 Regioregular 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
List of Figures
1.1 IPV and OPV difference . . . . . . . . . . . . . . . . . . . . . 2
1.2 Electronic and Optical process in OPV . . . . . . . . . . . . . 3
1.3 Hole mobility variation with Molecular weight . . . . . . . . . 5
2.1 Chemical structure of P3HT . . . . . . . . . . . . . . . . . . . 9
2.2 Chemical structure of RR-P3HT . . . . . . . . . . . . . . . . . 10
2.3 Chemical structure of RRa-P3HT . . . . . . . . . . . . . . . . 10
2.4 Electro-negativity in water molecule . . . . . . . . . . . . . . . 12
2.5 Induced dipole . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.6 Polarization in a neutral molecule . . . . . . . . . . . . . . . . 15
3.1 Curve for Slater and Gaussian type functions . . . . . . . . . . 23
4.1 Developed molecular structures . . . . . . . . . . . . . . . . . 32
4.2 Starting geometry . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.3 Chemical structure of P3HT monomer . . . . . . . . . . . . . 36
4.4 Molecular polarizability plots for (P3HT)n (n=\0″,1,2,3) . . . 38
4.5 RDF(C1{C1)at different temperature . . . . . . . . . . . . . . 39
4.6 RDF (C2{C2) at different temperature . . . . . . . . . . . . . 40
4.7 RDF (C3{C3) at different temperature . . . . . . . . . . . . . 41
4.8 RDF (C4{C4) at different temperature . . . . . . . . . . . . . 42
4.9 RDF (C5{C5)at different temperature . . . . . . . . . . . . . 43
4.10 RDF (S{S) at different temperature . . . . . . . . . . . . . . . 44

CHAPTER ONE

Introduction

As the world is faced with the issues of ever-decreasing fossil fuel and ever increasing environmental crises resulting from greenhouse gas production, photovoltaic (PV) technology has become a main focus of attention. The success of any PV technology depends on its efficiency, lifetime and cost.

Silicon-based solar cells (a type of IPV) have high efficiencies. However, the high cost to efficiency ratio of silicon-based solar cells and their complex production process has generated interest in developing alternative PV cells such as organic photovoltaics (OPVs). The obvious difference between IPVs and OPVs in terms of their photo-conversion mechanisms (Fig. 1.1) is that light absorption in OPV cells leads to the production of exciton mobile excited states while in IPVs, it leads directly to the creation of free electron-hole pairs.

Figure 1.1: Illustration on the difference between organic and inorganic solar cells when considering the photo-conversion process.

Despite the fact that OPVs have lower efficiencies, developing high performance organic photovoltaic devices as sources of sustainable energy has been an important issue in research conducted worldwide in recent years due to the low manufacturing cost of the devices and their use in the fabrication of flexible devices. In the last decade, the performance of OPVs has improved steadily, reaching a power conversion efficiency (PCE) as high as 7% [1]. It is the rest PV technology capable of generating electricity at a cost on par with conventional fuels, making it a cost-effective renewable energy source without government subsidies. Although, there are other possible applications, the most common and promising application of OPV technology are in organic solar cells (OSCs).

In spite of the tremendous advancement in the power conversion efficiency of organic solar cells over the past decade, major efforts are still needed to understand and optimize all electronic and optical processes (Fig. 1.2) taking place in OPV devices to ensure a continuous increase of their performance.

Figure 1.2 depicts how an OPV device operates the conversion of the incident solar irradiation to electrical current which involves processes such as photon absorption, exciton discussion, charge separation and collection of charges.

Figure 1.2: Illustration of Electronic and Optical process in OPV.

One of the major causes of the low efficiency of OPVs has been the difficulty in improving the morphology of the active layer. Recent studies indicate that the efficiency of organic solar cells is highly correlated to the morphology of the interface between the donor and acceptor materials of the active layer (Fig. 1.2), which in turn depends on the preparation conditions, the crystallization of the particular materials, and the interaction between its donor and acceptor molecules. The problems in designing a high efficiency OPV are complicated by the fact that the texture, or morphology of the donor acceptor blend (which is sensitive to the exact conditions of how the blend was processed into a thin lm) has a dramatic effect on the performance of the OPV [2]. In addition, photo-generation factors such as the exciton discussion length, charge separation and charge collection which affect the performance
are greatly influenced by the material morphology. For example exciton diffusion length has been estimated to range between 10 { 100 nm. This implies that, the active layer interface should have as large an area as possible, and the morphology of the donor and acceptor materials (mostly conjugated polymers and Fullerene derivatives respectively) should be such that the charge carriers have unrestricted conduction pathways to their respective electrodes as suggested by Benanti and Venkataraman (2006). This brings to mind the concept of bulk heterojunction as introduced by Tang in 1986 [8]. By using bulk heterojunction as an alternative active layer structure, Tang recognized that the interface between the donor and acceptor materials, not the electrode contacts is the key to determining the photovoltaic properties of an OPV [3]. In a nutshell, to account for these characteristic properties, the interface morphology of OPV has to be well controlled. This, makes the material morphology essential to improving the efficiency of OPVs.

The question then is, how do we improve the constituents of the active layer in OPVs and what measures need to be taken with regards to the donor or acceptor materials used in OPVs in order to give the charge carriers unrestricted pathways to their respective electrode? These are the questions we wish to answer in this work with a focus on the poly 3-hexylthiophene (P3HT) donor material. P3HT exhibits lower band gap, broader spectra and also better hole mobility compared to other classic conjugated polymers such as poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV), MDMO-PPV, Zn Pc and even other polythiophenes that are broadly used as the donor material in OPV.

A lot of research work has been done both experimentally and theoretically in studying this material. For example, Kline et. al., in their work, showed a clear correlation between the eld-eect mobility of regioregular P3HT (RR-P3HT) and its molecular weight with mobility values increasing from 1:710􀀀6 to 9:410􀀀3 cm2v􀀀1s􀀀1 as the molecular weight is increased from 3:2 to 36:5 kD, where they suggested that optimizing the molecular weight (MW) of conjugated polymers could lead to significant improvements in the device performance [4]. Computationally, Nelson et al., confirmed that via molecular dynamics (MD) simulations, coarse-grained models of the molecular packing can be used to rationalize the MW dependence of hole mobility in regioregular polythiophene (depicted by Fig. 1.3). It can be observed in figure 1.3 that the simulated hole mobility (led stars) increases with increasing MW which is in agreement with the experimental observations(open triangles) [5].

Phase transition kinetics in P3HT has also been studied using differential scanning calorimetry (d.s.c.) where it was revealed that the phase transitions in P3HT are characterized by two separate, i.e. one fast and one slow, crystal.

Figure 1.3: Simulated low-eld limit time-of-light mobility (lled stars) in comparison with measured Field-Elect Transistors mobilities(open triangles). Formation processes, which supports the existence of the ordered nematic
state in the melt of P3HT [6].

Due to the complex nature of the active layer microstructure, experiments to determine the morphology on the nanoscale are difficult. Molecular dynamics (MD) simulations offer a direct route to determining these microstructures. MD simulations however depend on accurate force elds.

Force elds determined from experiments are the most popular for simulations of P3HT. However, these force elds are not accurate. For example, Freisner and coworkers implemented adjustments to the (Optimized Potential for Liquid Simulation) OPLS-2005 force eld in order to improve its ability to model systems such as P3HT where they show the impact of these changes on the dihedral angle distributions, persistence lengths, and conjugation length distributions observed during molecular dynamics simulations [7]. Despite the fact that P3HT has a huge molecular polarizability, one particular approximation in all previous force elds is the neglect of explicit polarization.

In this work, we plan to examine the in uence of the molecular polarizability of P3HT on its morphology at different temperatures and to develop a polarizable force eld based on ab initial data.

Organization of Thesis Work

This thesis is organized as follows:

Chapter Two: The properties, structure of P3HT monomer chains (denoted (P3HT)n) are studied. Polarizability in molecules is also described.

Chapter Three: An overview of computer simulation is given.

Chapter Four: MD simulations of (P3HT)n (n=\0″, 1, 2 and 3) and development of a polarizable ab initial force eld are described.

Chapter Five: Conclusions and suggestions for future studies are given.

References

[1] Xiangjian Wan, Guankui Long, Lu Huang, and Yongsheng Chen, Graphene A Promising Material for Organic Photovoltaic Cells Adv. Mater., 2011, 23, 53425358.

[2] A. Opitz, J. Wagner, W. Brutting, Ingo Salzmann, N. Koch, J. Manara, J. P aum, A. Hinderhofer, and Frank Schreiber, \Interfaces: Correlation Between Morphology and Solar Cell Performance”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 16, No. 6, 2010.

[3] Travis L. Benanti and D. Venkataraman, Organic solar cells: An overview focusing on active layer morphology Photosynthesis Research, 87: 7381, 2006.

[4] Chiatzun Goh, R. Joseph Kline, Michael D. McGehee, Ekaterina N. Kadnikova and Jean M. J. Frchet, \Molecular-weight-dependent mobilities in regioregular poly(3-hexyl-thiophene) diodes”, Appl. Phys. Lett. 86
122110, 2005.

[5] Jenny Nelson, Joe J. Kwiatkowski, James Kirkpatrick and Jarvist M. Frost, \Modeling Charge Transport in Organic Photovoltaic Materials”, Department of Physics, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, United Kingdom, 2009.

[6] Yue Zhao, Guoxiong Yuan, Mario Leclerc and Philippe Roche, \A calorimetric study of the phase transitions in poly(3-hexylthiophene)”, Polymer Vol. 36 No. 11, pp. 2211-2214, 1995.

[7] Kateri H. DuBay, Michelle Lynn Hall, Chuanjie Wu, David R. Reichman, and Richard A. Friesner, \Accurate Force Field Development for Modeling Conjugated Polymers”, Journal of Chemical Theory and Computation.

[8] C. W. Tang, \Two-layer organic photovoltaic cell”, Appl. Phys. Lett. 48 183, 1986.