Importance of side chains and backbone length in defect modeling of poly(3-alkylthiophenes)

Elucidating the structure of donor-acceptor conjugated polymer aggregates in liquid solution

High-spin donor-acceptor conjugated polymers are extensively studied for their potential applications in magnetic and spintronic devices. Inter-chain charge transfer among these high-spin polymers mainly depends on the nature of the local structure of the thin film and π-stacking between the polymer chains. However, the microscopic structural details of high-spin polymeric materials are rarely studied with an atomistic force field, and the molecular-level local structure in the liquid phase remains ambiguous. Here, we have examined the effects of oligomer chain length, side chain, and processing temperature on the organization of the high-spin cyclopentadithiophene-benzobisthiadiazole donor-acceptor conjugated polymer in chloroform solvent. We find that the oligomers display ordered aggregates whose structure depends on their chain length, with an average π-stacking distance of 3.38 ± 0.03 Å (at T = 298 K) in good agreement with the experiment. Also, the oligomers with longer alkyl side chains show better solvation and a shorter π-stacking distance. Furthermore, the clusters grow faster at higher temperature with more ordered aggregation between the oligomer chains.

Decoupling Planarizing and Steric Energetics to Accurately Model the Rigidity of π-Conjugated Polymers

The π-conjugated backbone of semiconducting polymers gives rise to both their electronic properties and structural rigidity. However, current computational methods for understanding the rigidity of polymer chains fail in one crucial way. Namely, standard torsional scan (TS) methods do not satisfactorily capture the behavior of polymers exhibiting a high degree of steric hindrance. This deficiency in part stems from the method by which torsional scans decouple energy related to electron delocalization from that related to nonbonded interactions. These methods do so by applying classical corrections of the nonbonded energy to the quantum mechanical (QM) torsional profile for polymers that are highly sterically hindered. These large corrections to the energy from nonbonded interactions can substantially skew the calculated QM energies related to torsion, resulting in an inaccurate or imprecise estimation of the rigidity of a polymer. As a consequence, simulations of the morphology of a highly sterically hindered polymer using the TS method can be highly inaccurate. Here, we describe an alternative, generalizable method by which the delocalization energy can be decoupled from the energy associated with nonbonded interactions─the "isolation of delocalization energy" (DE) method. From torsional energy calculations, we find that the relative accuracy of the DE method is similar to the TS method (within 1 kJ/mol) for two model polymers (P3HT, PTB7) when compared to quantum mechanical calculations. However, the DE method significantly increased the relative accuracy for simulations of PNDI-T, a highly sterically hindered polymer (8.16 kJ/mol). Likewise, we show that comparison of the planarization energy (i.e., backbone rigidity) from torsional parameters is significantly more precise for both PTB7 and PNDI-T when using the DE method as opposed to the TS method. These differences affect the simulated morphology, with the DE method predicting a significantly more planar configuration of PNDI-T.

Challenging PM6-like donor polymers for pairing with a Y-type state-of-the-art acceptor in binary blends for bulk heterojunction solar cells

Being fluorine-free and a high performance material as a small organic acceptor molecule, BTP-eC9 has been well mixed with BDT-based PM6 donor polymers for providing satisfactory photovoltaic properties, especially towards future large scale/large area solar cell production. However, as one of the key electrical outputs from such binary active layer materials, the open circuit voltage (V_OC) was limited to ca. 0.84 V, which needs to be further improved for BTP-eC9 to have a bright future. This paper focuses on the molecular design of alkylthio- and alkoxy-phenyl flanked benzo[1,2-b:4,5-b']dithiophene-based conjugated polymers (PBDT-PS-ttTPD or P10 for short and PBDT-PO-ttTPD or P11), which were successfully synthesized and applied as donor materials for pairing with BTP-eC9 in organic photovoltaic (OPV) devices. By fine-tuning the side chains of the benzodithiophene (BDT) moiety, such non-fullerene OPV devices with normal configuration demonstrate an attractively high open circuit voltage (V_OC) of 0.89 and 0.87 V in P10/BTP-eC9 and P11/BTP-eC9 based binary single bulk heterojunction OPV devices, while still maintaining an excellent J_SC of 22.7 and 20.0 mA cm^-2 with a final power conversion efficiency (PCE) of 12.93% and 9.37%, respectively. The alkylthio-phenyl chain substituted BDT polymer exhibits better photovoltaic performance in all aspects than the alternative with alkoxy chains due to the synergistic effect of the alkylthio-phenyl flanked BDT, TPD, and π-bridge (thieno[3,2-b]thiophene).

Local structuring of diketopyrrolopyrrole (DPP)-based oligomers from molecular dynamics simulations

The microscopic structure of high mobility semiconducting polymers is known to be essential for their performance but it cannot be easily deduced from the available experimental data. A series of short oligomers of diketopyrrolopyrrole (DPP)-based materials that display high charge mobility are studied by molecular dynamics simulations to understand their local structuring at an atomic level. Different analyses are proposed to compare the ability of different oligomers to form large aggregates and their driving force. The simulations show that the tendency for this class of materials to form aggregates is driven by the interaction between DPP fragments, but this is modulated by the other conjugated fragments of the materials which affect the rigidity of the polymer and, ultimately, the size of the aggregates that are formed. The main structural features and the electronic structure of the oligomers are fairly similar above the glass transition temperature and at room temperature.

Open-Circuit Voltage of Organic Photovoltaics: A Time-Dependent and Unrestricted DFT Study in a P3HT/PCBM Complex

Solar cells based on blends of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, P3HT/PCMB, constitute one of the most efficient polymer photovoltaic cell types. One of the main factors that determine the efficiency of the solar cells is the open-circuit voltage, V_OC. In this work, we provide an analysis of the parameters affecting the V_OC in a P3HT/PCBM complex. Electronic transitions, excited states, and electron transfer parameters are evaluated under the classical Marcus formalism via the time-dependent and unrestricted CAM-B3LYP/6-31G* methods. The charge-recombination driving force is found to mainly affect the charge-recombination rate constant and, in turn, V_OC. Even though other parameters also determine the value of V_OC like density of states, dimensions of the cell, and microstructure of the donor/acceptor interface, the current work highlights the understanding attained by modeling charge-transfer parameters. The analysis reported here encourage further quantum-chemical investigations in organic photovoltaics with the aim of estimating and improving V_OC, such that more efficient organic solar cells may be predicted.

Torsional Impacts on Quaterthiophene Segments Confined within Peptidic Nanostructures

The co-assembly behavior of peptide-π-peptide and peptide-alkyl-peptide triblock molecules that form one-dimensional (1D) nanostructures under acidic, aqueous environments is dependent on the peptide sequence and the torsional constraints imposed within the nanomaterial volume. Although a hydrophilic tripeptide sequence (Asp-Asp-Asp, DDD-) previously promoted isolation/dilution of minority π-electron components in the matrix of aliphatic peptides, a β-sheet promoting sequence (Asp-Val-Val, DVV-) led to blocks of the two components distributed within larger 1D self-assembled nanostructures. Furthermore, torsional restrictions exerted on the oligoaromatic π-electron unit by the self-assembly process can lead to changes in its conformation (for example, planarity), which has ramifications on its functionality within the peptide matrix. Here, we study this impact on thiophene-based π-electron units with inherently different geometries, viz., relatively planar 2,2':5',2″:5″,2‴-quaterthiophene and 3″,4'-dimethyl-2,2':5',2″:5″,2‴-quaterthiophene, which is twisted at the core bithiophene unit due to the presence of two methyl groups. These peptides were co-assembled at 5 and 20 mol % with peptide- n-decyl-peptide triblock molecules, and the resultant assemblies were studied using UV-vis absorption, photoluminescence, and circular dichroism spectroscopies. We found that torsional restriction in dimethylated quaterthiophene units can impact the stacking behavior of these 1D peptide nanoassemblies and have consequences on their photophysical properties. Additionally, these insights help in the understanding of the dependence of the optoelectronic properties of these materials on both the intrinsic conformation of π-units and the geometric constraints imposed by their immediate local environment under aqueous conditions.

Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes

The backbone conformation of conjugated polymers affects, to a large extent, their optical and electronic properties. The usually flexible substituents provide solubility and influence the packing behavior of conjugated polymers in films or in bad solvents. However, the role of the side chains in determining and potentially controlling the backbone conformation, and thus the optical and electronic properties on the single polymer level, is currently under debate. Here, we investigate directly the impact of the side chains by studying the bulky-substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) and the common poly(3-hexylthiophene) (P3HT), both with a defined molecular weight and high regioregularity, using low-temperature single-chain photoluminescence (PL) spectroscopy and quantum-classical simulations. Surprisingly, the optical transition energy of PDOPT is significantly (∼2,000 cm-1 or 0.25 eV) red-shifted relative to P3HT despite a higher static and dynamic disorder in the former. We ascribe this red shift to a side-chain induced backbone planarization in PDOPT, supported by temperature-dependent ensemble PL spectroscopy. Our atomistic simulations reveal that the bulkier 2,5-dioctylphenyl side chains of PDOPT adopt a clear secondary helical structural motif and thus protect conjugation, i.e., enforce backbone planarity, whereas, for P3HT, this is not the case. These different degrees of planarity in both thiophenes do not result in different conjugation lengths, which we found to be similar. It is rather the stronger electronic coupling between the repeating units in the more planar PDOPT which gives rise to the observed spectral red shift as well as to a reduced calculated electron-hole polarization.

Tuning the electronic and optical properties of NDT-based conjugated polymers by adopting fused heterocycles as acceptor units: a theoretical study

Donor-acceptor conjugated polymers have been successfully applied in bulk heterojunction solar cell devices. Tuning their donor and acceptor units allows the design of new polymers with desired electronic and optical properties. Here, to screen new candidate polymers based on a newly synthesized donor unit, dithieo[2,3-d:2',3'-d']naphtho[1,2-b:3,4-b']dithiophene (NDT), a series of model polymers with different acceptor units were designed and denoted NDT-A₀ to NDT-A₁₂, and the structures and optical properties of those polymers were investigated using DFT and TDDFT calculations. The results of the calculations revealed that the electronic and optical properties of these polymers depend on the acceptor unit present; specifically, their HOMO energies ranged from -4.89 to -5.38 eV, their HOMO-LUMO gaps ranged from 1.30 to 2.80 eV, and their wavelengths of maximum absorption ranged from 538 to 1212 nm. The absorption spectra of NDT-A₁ to NDT-A₆, NDT-A₈, NDT-A₉, and NDT-A₁₂ occur within the visible region (<900 nm), indicating that these polymers are potential candidates for use in solar cells. On the other hand, the absorption spectra of NDT-A₇, NDT-A₁₀, and NDT-A₁₁ extend much further into the near-infrared region, implying that they absorb near-infrared light. These polymers could meet the requirements of donor units for use in tandem and ternary solar cells. Graphical abstract Theoretical calculations by TD-DFT reveal that the optical properties of NDT-based conjugated polymers can be well tuned by adopting different acceptor units, and these ploymers are potential donor materials for tandem and ternary solar cells.

Positional effects of fluorination in conjugated side chains on photovoltaic properties of donor–acceptor copolymers

The position at which conjugated side chains were fluorinated, the meta- or ortho-position in phenyl side chains, was varied to investigate the positional effects of fluorination on the energy levels, crystalline ordering, and photovoltaic properties of the polymers.

Chain conformations and phase behavior of conjugated polymers

Conjugated polymers may play an important role in various emerging optoelectronic applications because they combine the chemical versatility of organic molecules and the flexibility, stretchability and toughness of polymers with semiconducting properties. Nevertheless, in order to achieve the full potential of conjugated polymers, a clear description of how their structure, morphology, and macroscopic properties are interrelated is needed. We propose that the starting point for understanding conjugated polymers includes understanding chain conformations and phase behavior. Efforts to predict and measure the persistence length have significantly refined our intuition of the chain stiffness, and have led to predictions of nematic-to-isotropic transitions. Exploring mixing between conjugated polymers and small molecules or other polymers has demonstrated tremendous advancements in attaining the needed properties for various optoelectronic devices. Current efforts continue to refine our knowledge of chain conformations and phase behavior and the factors that influence these properties, thereby providing opportunities for the development of novel optoelectronic materials based on conjugated polymers.

A Novel Naphtho[1,2-c:5,6-c']Bis([1,2,5]Thiadiazole)-Based Narrow-Bandgap π-Conjugated Polymer with Power Conversion Efficiency Over 10

A novel naphtho[1,2-c:5,6-c']bis([1,2,5]-thiadiazole)-based narrow-bandgap π-conjugated polymer is designed for application in polymer solar cells. Remarkable power conversion efficiencies over 10% can be achieved based on both conventional and inverted device architectures with thick photoactive layers, which are processed by using chlorinated or nonhalogenated solvents, suggesting its great promise toward practical applications based on high-throughput roll-to-roll processing.

Dependence of Excited-State Properties of a Low-Bandgap Photovoltaic Copolymer on Side-Chain Substitution and Solvent

The excited-state properties and chain conformations of a new low-bandgap copolymer based on benzo[1,2-b:4,5-b']dithiophene (BDT) and thieno[3,4-b]thiophene with meta-alkoxyphenyl-substituted side chains in solution were investigated comprehensively. Time-resolved spectroscopy suggested that the excited-state properties were sensitive to the conformations of the copolymer in solution. In addition, excited-state dynamics analyses revealed the photogeneration of triplet excited states by intersystem crossing (ISC) at a rate constant of ∼0.4×10(9)  s(-1) as a result of direct meta-alkoxyphenyl connection to the donor unit BDT irrespective to the macromolecular conformations. According to El-Sayed's rule, the fast ISC herein is correlated with the change of orbital types between singlet and triplet excited states as also shown by quantum chemical calculations. Our studies may shed light on the structure-property relationships of photovoltaic materials.

Naphthodithiophene-Based Conjugated Polymer with Linear, Planar Backbone Conformation and Strong Intermolecular Packing for Efficient Organic Solar Cells

Two donor-acceptor copolymers, PBDT and PNDT, containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b']dithiophene (BDT) and 4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b']dithiophene (NDT), respectively, as an electron-rich unit and 5,6-difluoro-2,1,3-benzothiadiazole (2FBT) as an electron-deficient unit, were synthesized and compared. The introduction of the NDT core into the conjugated backbone was found to effectively improve both light harvesting and the charge carrier mobility by enhancing chain planarity and backbone linearity; the NDT copolymer has stronger noncovalent interactions and smaller bond angles than those of the BDT-based polymer. Moreover, the introduction of the NDT core brings about a drastic change in the molecular orientation into the face-on motif and results in polymer:PCBM blend films with well-mixed interpenetrating nanofibrillar bulk-heterojunction networks with small-scale phase separation, which produce solar cells with higher short-circuit current density and fill factor values. A conventional optimized device structure containing PNDT:PC71BM was found to exhibit a maximum solar efficiency of 6.35%, an open-circuit voltage of 0.84 V, a short-circuit current density of 11.92 mA cm(-2), and a fill factor of 63.5% with thermal annealing, which demonstrates that the NDT and DT2FBT moieties are a promising electron-donor/acceptor combination for high-performance photovoltaics.

Inherent Driving Force for Charge Separation in Curved Stacks of Oligothiophenes

Coexistence of high local charge mobility and an energy gradient can lead to efficient free charge carrier generation from geminate charge transfer states at the donor-acceptor interface in bulk heterojunction organic photovoltaics. It is, however, not clear what polymer microstructures can support such coexistence. Using recent methods from density functional theory, we propose that a stack of similarly curved oligothiophene chains can deliver the requirements for efficient charge separation. Curved stacks are stable because of the polymer's strong π-stacking ability and because backbone torsions are flexible in neutral chains. However, energy of a charge in a polymer chain has remarkably stronger dependence on torsions. The trend of increasing planarity in curved stacks effectively creates an energy gradient that drives charge in one direction. The curvature of these partially ordered stacks is found to beneficially interact with fullerenes for charge separation. The curved stacks, therefore, are identified as possible building blocks for interfacial structures that lead to efficient free carrier generation in high-performing organic photovoltaic systems.

The backbone rigidity and its influence on the morphology and charge mobility of FBT based conjugated polymers

Modulation of the backbone rigidity via proper side chain placement and vinylene group incorporation allowed optimization of solid-state order and OFET performances.

Theoretical study on the torsional potential of alkyl, donor, and acceptor substituted bithiophene: the hidden role of noncovalent interaction and backbone conjugation

The torsional potential of substituted bithiophene is influenced not only by steric repulsion, but also by backbone conjugation and noncovalent interactions.

Effects of conformational symmetry in conjugated side chains on intermolecular packing of conjugated polymers and photovoltaic properties

Introduction of the symmetric conjugated side chain to the conjugated backbone of the polymer was found to improve both the light-harvesting ability of the polymer and its charge carrier mobility, apparently by increasing the packing between the polymer chains.

Fullerene-Based Photoactive Layers for Heterojunction Solar Cells: Structure, Absorption Spectra and Charge Transfer Process

The electronic structure and optical absorption spectra of polymer APFO₃, [70]PCBM/APFO₃ and [60]PCBM/APFO₃, were studied with density functional theory (DFT), and the vertical excitation energies were calculated within the framework of the time-dependent DFT (TD-DFT). Visualized charge difference density analysis can be used to label the charge density redistribution for individual fullerene and fullerene/polymer complexes. The results of current work indicate that there is a difference between [60]PCBM and [70]PCBM, and a new charge transfer process is observed. Meanwhile, for the fullerene/polymer complex, all calculations of the twenty excited states were analyzed to reveal all possible charge transfer processes in depth. We also estimated the electronic coupling matrix, reorganization and Gibbs free energy to further calculate the rates of the charge transfer and the recombination. Our results give a clear picture of the structure, absorption spectra, charge transfer (CT) process and its influencing factors, and provide a theoretical guideline for designing further photoactive layers of solar cells.

Effect of fluorination on electronic properties of polythienothiophene-co-benzodithiophenes and their fullerene complexes

Fluorination of conjugated polymers is a popular way of designing new electron donors for the bulk heterojunction (BHJ) based organic solar cells (OSCs). However, not all fluorinated polymers observed experimentally enhance the power conversion efficiency of OSCs, and the fundamental understanding of the effect of fluorination is not yet fully uncovered. Herein, we report the effect of fluorine substitution on the electronic properties of polythienothiophene-co-benzodithiophenes as well as their complexes with fullerene, using density functional theory (DFT) and time-dependent DFT methods at the molecular level. Systematic computations of energy gaps (E(g)(opt) and E(g)(hl)), ionization potentials (IP), electron affinities (EA), molecular electrostatic potential (MEP) surfaces, and dipole moments (μ) are carried out for these systems. We found that the fluorination of the thienothiophene unit favors lower E(g)(opt), E(g)(hl), IP, and EA as well as stronger μ compared to the fluorination of the benzodithiophene unit, suggesting that efficient exciton dissociation and charge carriers formation may take place efficiently for the former case. These results support recent experimental findings that the performance of polythienothiophene-co-benzodithiophene-based organic solar cells enhances when thienothiophene unit is fluorinated. The present results highlight that more efficient conjugated polymers for OSC can be designed if the gap engineering is carried out by focusing on the low IP, low EA, and high dipole moment.

Side-chain effects on the solution-phase conformations and charge photogeneration dynamics of low-bandgap copolymers

Solution-phase conformations and charge photogeneration dynamics of a pair of low-bandgap copolymers based on benzo[1,2-b:4,5-b(')]dithiophene (BDT) and thieno[3,4-b]thiophene (TT), differed by the respective carbonyl (-C) and ester (-E) substituents at the TT units, were comparatively investigated by using near-infrared time-resolved absorption (TA) spectroscopy at 25 °C and 120 °C. Steady-state and TA spectroscopic results corroborated by quantum chemical analyses prove that both PBDTTT-C and PBDTTT-E in chlorobenzene solutions are self-aggregated; however, the former bears a relatively higher packing order. Specifically, PBDTTT-C aggregates with more π-π stacked domains, whereas PBDTTT-E does with more random coils interacting strongly at the chain intersections. At 25 °C, the copolymers exhibit comparable exciton lifetimes (~1 ns) and fluorescence quantum yields (~2%), but distinctly different charge photogeneration dynamics: PBDTTT-C on photoexcitation gives rise to a branching ratio of charge separated (CS) over charge transfer (CT) states more than 20% higher than PBDTTT-E does, correlating with their photovoltaic performance. Temperature and excitation-wavelength dependent exciton∕charge dynamics suggest that the CT states localize at the chain intersections that are survivable up to 120 °C, and that the excitons and the CS states inhabit the stretched strands and the also thermally robust orderly stacked domains. The stable self-aggregation structures and the associated primary charge dynamics of the PBDTTT copolymers in solutions are suggested to impact intimately on the morphologies and the charge photogeneration efficiency of the solid-state photoactive layers.

Influence of Molecular Conformations and Microstructure on the Optoelectronic Properties of Conjugated Polymers

It is increasingly obvious that the molecular conformations and the long-range arrangement that conjugated polymers can adopt under various experimental conditions in bulk, solutions or thin films, significantly impact their resulting optoelectronic properties. As a consequence, the functionalities and efficiencies of resulting organic devices, such as field-effect transistors, light-emitting diodes, or photovoltaic cells, also dramatically change due to the close structure/property relationship. A range of structure/optoelectronic properties relationships have been investigated over the last few years using various experimental and theoretical methods, and, further, interesting correlations are continuously revealed by the scientific community. In this review, we discuss the latest findings related to the structure/optoelectronic properties interrelationships that exist in organic devices fabricated with conjugated polymers in terms of charge mobility, absorption, photoluminescence, as well as photovoltaic properties.

Computational Study of P3HT/C60-Fullerene Miscibility

Classical molecular dynamics simulations and statistical thermodynamics are used to investigate the miscibility of blends of the conjugated polymer poly(3-hexylthiophene) (P3HT) and fullerene C60 for blend ratios typically used in organic photovoltaic devices over a range of temperatures. Depending on which of two slightly different simulation force fields is used, the calculations suggest that amorphous P3HT/fullerene blends are either miscible or immisicble under typical processing conditions. The former result is consistent with recent experiments and suggests that experimentally observed nano-scale phase separation is driven by polymer or fullerene crystallisation. But the inconsistency between the different force fields indicates that these blends are close to phase coexistence between the separated and homogeneously mixed phases and suggests that care must be taken in interpreting simulation data on P3HT/fullerene blends. These findings have implications for organic photovoltaics, in which the microstructure of conjugated-polymer/fullerene blends plays a crucial role in device performance.

Improved force field for molecular modeling of poly(3-hexylthiophene)

An ab initio-based improved force field is reported for poly(3-hexylthiophene) (P3HT) in the solid state, deriving torsional parameters and partial atomic charges from ab initio molecular structure calculations with explicit treatment of the hexyl side chains. The force field is validated by molecular dynamics (MD) simulations of solid P3HT with different molecular weights including calculation of structural parameters, mass density, melting temperature, glass transition temperature, and surface tension. At 300 K, the P3HT crystalline structure features planar backbones with non-interdigitated all-trans hexyl side chains twisted ~90° from the plane of the backbone. For crystalline P3HT with infinitely long chains, the calculated 300 K mass density (1.05 g cm(-3)), the melting temperature (490 K), and the 300 K surface tension (32 mN/m) are all in agreement with reported experimental values, as is the glass transition temperature (300 K) for amorphous 20-mers.

Theoretical study of torsional disorder in poly(3-alkylthiophene) single chains: intramolecular charge-transfer character and implications for photovoltaic properties

The role of polymer chain morphology on the optoelectronic properties of polythiophenes is an ongoing investigation with the promise of improving organic photovoltaic performance. Chain morphology is predominantly affected by torsional disorder, which causes localization of holes and electrons in the conjugated backbone. Using the model compound oligo(3-methylthiophene), torsionally disordered oligomers were created to compare with a trans-planar oligomer such as found in crystalline poly(3-hexylthiophene). Low lying electronic excitations are calculated using TD-HF and TD-DFT with various long-range corrected functionals. Probability densities of electron and hole were constructed from natural transition orbitals, giving insight into localization and electron-hole overlap. Overlap is found to be substantially higher in disordered oligomers, indicating a stronger Coulombic interaction between electron and hole. Results suggest that improved photovoltaic performance with increased crystallinity is partially explained by stronger light absorption in crystalline polymers and a higher barrier to charge separation in disordered polymers.

Coarse-grained simulations of the solution-phase self-assembly of poly(3-hexylthiophene) nanostructures

Under certain conditions the conjugated polymer poly(3-hexylthiophene) (P3HT) self-assembles into high-aspect-ratio nanostructures (known as nanofibres, nanowires, or nanoribbons) when cooled below its solubility limit in a marginal solvent such as anisole. Such nanostructures are potentially beneficial for organic photovoltaic device performance. In this work, Langevin dynamics simulations of a coarse-grained model of P3HT in implicit anisole solvent are used to study the self-assembly of P3HT nanostructures for polymer chain lengths and concentrations used experimentally to prepare P3HT nanofibres. The coarse-grained model is parametrised to match the local structure and dynamics of an atomistic model with explicit solvent. Nanofibres are also prepared experimentally and characterised by atomic force microscopy and UV-vis spectroscopy. The simulations match the experimental phase behaviour of P3HT in anisole, showing aggregation of P3HT at 293 and 308 K but not at 323 or 353 K. Single-chain simulations at 293 K reveal two distinct nano-scale aggregate morphologies: hairpins and helices. Hairpin aggregates, which are the precursors of nanofibres, are slightly favoured energetically at 293 K for nuclei of the critical size of ≈80 monomers for aggregation. Consequently, chains in multi-chain aggregates adopt the hairpin morphology exclusively in simulations at experimental concentrations at 293 K. The simulated aggregate sizes match experimentally measured nanofibre widths. An estimate of the shift in UV-vis absorption of P3HT due to the change in conjugation length with aggregation in the simulations agrees reasonably well with experiment and shows that most of the spectral red shift that occurs with nanofibre formation is due to increased planarisation of the P3HT chains. In addition to providing insight into the mechanisms of nanofibre formation, the simulations resolve details of the molecular-level organisation of chains in P3HT nanofibres hitherto inaccessible by experiment.