Understanding the effect of heteroatoms on structural and electronic properties of conjugated polymers

Accepted Manuscript Understanding the effect of heteroatoms on structural and electronic properties of conjugated polymers Ram S. Bhatta , Mesfin Tsige PII:

S0032-3861(14)01087-8

DOI:

10.1016/j.polymer.2014.11.050

Reference:

JPOL 17446

To appear in:

Polymer

Received Date: 5 September 2014 Revised Date:

14 November 2014

Accepted Date: 15 November 2014

Please cite this article as: Bhatta RS, Tsige M, Understanding the effect of heteroatoms on structural and electronic properties of conjugated polymers, Polymer (2014), doi: 10.1016/j.polymer.2014.11.050. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Understanding the effect of heteroatoms on structural and electronic properties of

Ram S. Bhatta* and Mesfin Tsige*

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conjugated polymers

Department of Polymer Science, The University of Akron, Ohio 44325, United States

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Corresponding authors: [email protected]; [email protected]

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*

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Abstract Heteroatom-containing conjugated polymers are promising candidates for designing efficient polymer solar cells. However, fundamental understanding of the role

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of heteroatoms on structure-property relationships of these polymers is not yet fully understood. This work, based on first-principles calculations at the molecular level, uncovers how fluorine and oxygen introduction on poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-

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cyclopenta[2,1-b;3,4-b’]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]

(PCPDT-BT)

affect structural and electronic properties. Systematic computations of torsional defects,

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energy gaps, molecular electrostatic potential surfaces and dipole moments are carrier out for PCPDT-BT and its fluorine and oxygen derivatives. We found that oxygen derivative favors lowest-energy planar conformation, low energy gaps and high ground-to-excited state dipole differences. The present results further suggest that oxygenation might

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increase charge dissociation and reduce charge recombination in the excited state,

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supporting the recent experimental findings.

Keywords: Energy gaps; Molecular electrostatic potential surface; Torsional potential

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1. Introduction Polymer solar cells (PSCs), bulk heterojunction (BHJ) devices containing conjugated polymers as electron donors and fullerene derivatives as electron acceptors,

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are potential candidates to meet the world’s growing demand for clean, renewable and sustainable energy [1-3]. PSCs are environmentally friendly and are well recognized for their flexibility, lightweight, low cost, and ease of processing [4-6]. Despite their

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promise, PSCs are still suffering from their lower performance and stability compared to their inorganic counterparts [7]. The current power conversion efficiency (PCE) of PSCs

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has reached ~10.6% [8], but this would need to be at least doubled [9] for practical realization of PSCs by average citizens. Performance of PSCs depends on various factors such as band gaps of polymers, chemical structures and the bulk heterojuction morphology [10-12]. Among them, chemical structure of the conjugated polymer plays

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an important role because the chemical modification changes frontier molecular orbitals such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as intermolecular and intramolecular interactions

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[13,14]. Consequently, these interactions affect energy offset between the HOMOs of polymer and fullerene derivative changing open circuit voltage (Voc), and energy offset

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between the LUMOs of polymer and fullerene derivative changing the charge transport, which in turn affect the overall performance of PSCs. In recent years, several experimental studies [15-27] have focused on structural

modifications of the conjugated polymers by adding electronegative fluorine atoms and their effect on the performance of PSCs. These experimental studies are in agreement that the performance of PSCs is enhanced upon the partial fluorination of each monomer unit.

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In fact, our recent first-principles studies [28-30] on fluorinated derivatives of polythienothiophene-co-benzodithiophenes, that agreed with the experimental findings, have shown that the fluorination of thienothiophene unit increases the effective

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conjugation, HOMO offset and dipole moment, and decreases exciton binding energies.

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Figure 1. Schematic representation of PCPDT-BT, PCPDT-DFBT and PDTP-DFBT. The interring torsional angle, α, is zero when S-C-C-C (connected to N) angle is in trans conformations shown. In

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the present computations, the methyl group replaces R, R1 and R2.

Recently [8], oxygenated derivates of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-

cyclopenta[2,1-b;3,4-b’]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]

(PCPDT-BT)

have been synthesized and revealed that oxygenated derivative poly[2,7-(5,5-bis-(3,7dimethyl

octyl)-5H-dithieno[3,2-b:2’,3’-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3-

benzothiadiazole)] (PDTP-DFBT) performs better than fluorinated derivative poly[2,6-

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(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]-dithiophene)-alt-4,7-(2,1,3difluorobenzothiadiazole) (PCPDT-DFBT) (Fig. 1). Although the synthesis and the performance of PCPDT-BT, PCPDT-DFBT and PDTP-DFBT are studied experimentally,

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many important aspects from the first-principles study remain unexplored. In particular, effect of oxygenation on conformation, frontier orbital energies and dipole moments have never been addressed so far.

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In this study, we perform systematic first-principles calculations on the structural and electronic properties of PCPDT-BT, PCPDT-DFBT and PDTP-DFBT to investigate

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the effect of fluorine and oxygen additions. We compute torsional potentials, frontier molecular orbital energies, band gaps, molecular electrostatic potentials and dipole moments at the molecular level. The understanding of these fundamental properties is crucial for optimizing the design of efficient conjugated polymers for high performance

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PSCs. 2. Computational details

All calculations on PCPDT-BT, PCPDT-DFBT and PDTP-DFBT oligomers up to

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the hexamer were carried out using density functional theory (DFT) [31] and time dependant DFT (TDDFT) [32] combined with dispersion corrected [33] Becke’s three–

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parameter Lee-Yang-Parr exchange-correlation functional (B3LYP) [34] and 6-31G(d) basis set. Our previous experience on the first-principles studies [28-30] of similar polymers has shown that the dispersion corrected B3LYP functional captures accurate nanostructural and electronic properties. This method has also been implemented to produce reliable structural and electronic properties of similar conjugated polymers in various studies [35-37]. In the context of the present work, some computations were also

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performed at the coulomb attenuated and long-rage corrected functionals, CAM-B3LYP [38] and LC-BLYP [39], respectively combined with the same basis set for the sake of comparison. The large-scale calculations were performed using Kraken and Blacklight

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computational resources that were provided by the extreme science and engineering discovery environment (XSEDE) [40]. NWChem 6.1 [41] and Gaussian 09 [42] ab initio packages were used to perform these computations.

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Two different types of geometry optimizations of PCPDT-BT, PCPDT-DFBT and PDTP-DFBT oligomers were performed using DFT methods. First, oligomers were fully

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optimized without any constraint to determine the minimum energy conformations. Second, nanostructures were partially optimized at the constrained inter-ring torsional angle (α) by rotating two planar halves of each polymer in the range 0˚ ≤ α ≤ 180˚ to investigate the effect of torsional disorder on structural and electronic properties. The

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exited state calculations were performed using TDDFT methods starting from the ground state optimized geometries. 3. Results and discussion

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3.1. Nanostructures and torsional defects

All lowest energy conformations of PCPDT-BT, PCPDT-DFBT and PDTP-DFBT

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oligomers up to the hexamer investigated in the present study were found to contain planar backbones (Fig. 1). Vibrational frequencies of selected oligomers were calculated to ensure that each of the fully optimized conformations represented a local minimum of energy. Some additional fully relaxed geometry optimizations of dimers were also performed starting from non-planar geometries (for example, α ≈ 25˚) as inputs to make sure that the optimized structure represented the lowest energy conformation. Irrespective

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of the starting geometry, the optimized geometry converged to the same conformation with the planar backbone. This shows that the solid-state packing structures of PCPDTBT, PCPDT-DFBT and PDTP-DFBT are expected to have a planar backbone in the

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homogeneous environment.

However, since the BHJ morphology of a PSC contains a blend of polymers and fullerene derivatives, the situation might be different because of the heterogeneous

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environment. For example, different semicrystalline and crystalline domains have been reported in the BHJ morphology of PTB7 and fullerene derivatives [43-45]. These

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domains might be expected to contain substantial conformational disorder, which are usually caused by the inter-ring torsional defects in the polymer chains. A usual approach to probe these conformational defects is to calculate the torsional potential as a function of inter-ring torsional angle as was done in our previous studies on polythiophene (PT)

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and poly(3-alkylthiophenes) (P3ATs) [46-48]. These torsional profiles are very informative to investigate the propensity of conformational disorders in otherwise regular conformations.

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Beginning with the planar PCPDT-BT, PCPDT-DFBT and PDTP-DFBT oligomers, the backbone torsional potentials were computed by rotating two planar halves

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in steps around the central inter-ring bond. Figure 2 shows the one-dimensional torsional poentials for these polymers up to the hexamer. The relative torsional energy increases with the increase in torsional angle, reaches to the maximum and decreases with further increase in the torsional angle. Each of these torsional potentials has two minima: one at

α ≈ 0˚ and the other at α ≈ 180˚. At α ≈ 90˚, there is a maximum that is the highest barrier between the two torsional minima.

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Figure 2. Torsional potentials for (a) PCPDT-BT, (b) PCPDT-DFBT and (c) PDTP-DFBT. Dotted lines are guide to the eyes.

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Figure 3. Images of (a) HOMO (α = 0˚), (b) LUMO (α = 0˚), (c) HOMO (α = 90˚) and (d) LUMO (α = 90˚) for PCPDT-BT. Red and blue colors represent negative and positive phases of the orbitals,

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respectively.

The major factor responsible for the nature of torsional profile depicted in Figure

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2 is the electronic conjugation along the conjugated backbone. At α ≈ 0˚ or 180˚, the planar geometry favors more extended electronic conjugation across the conjugated rings of monomer units (Figure 3). The electronic conjugation is decreased when the nonplanarity of the conjugated backbone increases and at α ≈ 90˚, the electronic conjugation is broken thereby reducing the conjugation length from n to n/2 (Figure 3). Consequently, frontier molecular orbirtals get more separated and the torsional energy of an oligomer is

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increased. Hence, the barrier at α ≈ 90˚ is a result of the breakage of the electronic conjugation. Other factors affecting the torsional potential are steric hindrance and hydrogen bonding. The minimum energy conformation is stabilized by less steric

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hindrance and stronger hydrogen bonding between hydrogen and heteroatoms.

Interestingly, PCPDT-BT, PCPDT-DFBT and PDTP-DFBT oligomers show different convergence behavior of torsional barrier compared to other conjugated

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polymers such as PT and P3AT [47,49]. Although barrier heights of PT and P3AT converge beyond the octamer [47,49], PCPDT-BT, PCPDT-DFBT and PDTP-DFBT do

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no show any noticeable change in barrier height with oligomer length, and resemble closely with PTB7. This behavior of the rapid convergence of torsional barrier with the oligomer length is associated with the enhanced conjugation length, additional details can be found in our previous study [28]. At the B3LYP level, the change in barrier heights

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from dimer to tetramer for these polymers is ≤ 14 meV. The overall change in barrier height from dimer to hexamer is ≤ 17 meV, which is less than kT at room temperature. The fluorination of the BT unit increases the barrier height by ~15 meV and the

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introduction of the oxygen atom to the CPDT unit increases the barrier height by ~ 20 meV. Table 1 summarizes the barrier heights computed using different DFT methods in

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which the relative variations of barrier heights are consistent across the different DFT methods. However, B3LYP performs better in terms of absolute values of barrier heights and resembles closely with sophisticated perturbation theory calculations [28].

Table 1. Torsional barrier heights (eV) for polymers computed using different DFT functionals Polymer B3LYP CAM-B3LYP LC-BLYP PCPDT-BT PCPDT-DFBT PDTP-DFBT

0.311 0.326 0.346

0.195 0.207 0.221

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0.161 0.186 0.199

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3.2. Energy gaps and torsional defects We start with the computed energy gaps of the lowest energy conformations of PCPDT-BT, PCPDT-DFBT and PDTP-DFBT. The computed optical gap ( E gopt ) and

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HOMO-LUMO gap ( E ghl ) of these polymers are plotted in Figure 4. The computed values of E gopt represent the vertical excitation energies from the ground state to the first excited state and are directly comparable to the optical band gap observed from UV-vis

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absorption spectroscopy. The computed values of E ghl represent the valence to conduction

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band energy difference in the ground state and are directly comparable to the band gap measured from cyclic voltametry. The available experimental values of E gopt and E ghl are

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also included in Figure 4 for comparison.

Figure 4. Effect of fluorine and oxygen additions on energy gaps. Experimental values of energy gaps are from Ref. [8].

As shown in Figure 4, both computed and experimental values of E gopt and

E ghl increase with the fluorination of the BT unit and decrease with the introduction of an oxygen atom to the PCPDT unit. For PCPDT-BT, PCPDT-DFBT and PDTP-DFBT, the

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computed values of E gopt are 1.34 eV, 1.40 eV and 1.34 eV, respectively at the B3LYP level and the reported experimental values are 1.48 eV, 1.51 eV and 1.38 eV,

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respectively. Similarly, the computed values of E ghl are 1.58 eV, 1.64 eV and 1.58 eV, and the experimentally observed values of E ghl are 1.62 eV, 1.82 eV and 1.65 eV, respectively for these polymers. Underestimation of computed E gopt and E ghl compared to the corresponding experimental values can be attributed as the lack of intermolecular

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interactions and solvent effects in the present computation that are present in the

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experimental measurements. Importantly, the relative variations of these energy gaps agree fairly well between experimental and computed results. These results are consistent with the observed blue and red shift of spectra for PCPDT-DFBT and PDTP-DFBT, respectively relative to PCPDT-BT [8]. The increase in the band gap with the fluorination of PCPDT-BT is consistent with our previous findings [30] that the addition of fluorine similar

conjugated

polymer,

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atoms on

polythienothiophene-cobenzodithiophene,

increases the band gap. Importantly, the addition of oxygen on PCPDT-DFBT decreases the band gap and hence PDTP-DFBT has the lower band gap compared to other

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conjugated polymers such as PT, P3HT and fluorinated polythienothiophene-

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cobenzodithiophenes [30,47].

Energy gaps are very sensitive to torsional defects. The torsional defect hinders

the effective overlapping of p-orbitals of carbon atoms between the interacting conjugated rings and hence the extended π-conjugation is reduced. Consequently, energy gaps of conjugated polymers are increased. Figure 5 shows the effect of the torsional defect on E ghl for PCPDT-BT, PCPDT-DFBT and PDTP-DFBT. In Figure 5, E ghl represents the difference between HOMO and LUMO energies in the ground state when

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the two planar halves of each polymer are rotated about the central inter-ring torsional angle, α. At α = 0˚ and 180˚ (i.e. planar conformation) E ghl has minimum values for all

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polymers. At α > 0˚ and α < 180˚, E ghl increases and reaches a maximum at α = 90˚. All computed and reported values of E ghl are plotted relative to Eghl of planar PCPDT-BT for

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the purpose of direct comparison.

Figure 5. Torsional dependence of HOMO-LUMO gaps. Energies are plotted relative to HOMO-LUMO gap of PCPDT-BT at α = 0˚. Experimental data (open squares) are from Ref. [8].

As α approaches 90˚, the two planar halves of each polymer are orthogonal to

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each other and the effect of torsional disorder on E ghl is at the maximum. At α =90˚,

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values of E ghl for PCPDT-BT, PCPDT-DFBT and PDTP-DFBT are 0.47 eV, 0.55 eV and 0.43 eV, respectively, higher relative to E ghl (1.944 eV) of PCPDT-BT at α = 0˚. At the minimal torsional defect (i.e. α ≈ 0˚), the fluorination on the BT unit increases E ghl by ~0.04 eV whereas the oxygen addition to the CPDT unit decreases E ghl by ~0.03 eV relative to E ghl (1.944 eV) of PCPDT-BT. These values are consistent with the reported

E ghl for these polymers as depicted in Figure 5. Relative variation of E ghl can be explained

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in terms of electron withdrawing and electron donating effects of fluorine and oxygen atoms, respectively. Strong electron-withdrawing fluorine atoms in BT unit pull electrons from the conjugated ring and increase the band gap. On the other hand, the introduction

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of the oxygen atom into the CPDT unit to form the DTT unit decreases the band gap by increasing the conjugation. These effects are captured consistently by different DFT functionals (Table 2). However, coulomb attenuated and long-range corrected functionals

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overestimate the absolute values of E ghl . Absolute values computed at the B3LYP level are in good agreement with the reported experimental values.

1.944 1.984 1.910

3.985 4.018 3.955

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PCPDT-BT PCPDT-DFBT PDTP-DFBT

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Table 2. HOMO-LUMO gaps (eV) of polymers computed using different DFT functionals Polymer B3LYP CAM-B3LYP LC-BLYP Ref. [8]

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5.973 5.998 5.948

1.918 1.962 1.910

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Figure 6. Torsional dependence of (a) HOMO and (b) LUMO energies for polymers as shown. Energies are

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plotted relative to HOMO and LUMO energies of PCPDT-BT at α = 0˚.

The variation of the band gaps shown in Figure 5 can be better understood in

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terms of HOMO and LUMO energies. Figure 6 shows the dependence of HOMO and LUMO energies of PCPDT-BT, PCPDT-DFBT and PDTP-DFBT on the torsional defect,

α. At α ≈ 0˚, fluorination of the BT unit decreases HOMO energy by ~0.17 eV relative to the HOMO energy (-5.36 eV) of PCPDT-BT. However, the decrease in the HOMO energy is only ~0.14 eV relative to the HOMO energy of PCPDT-BT when oxygen atom is added into the CPDT unit. LUMO energies of PCPDT-DFBT and PDTP-DFBT are decreased by ~0.13 eV and ~0.18 eV, respectively, relative to the LUMO energy (-2.74

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eV) of PCPDT-BT when fluorine and oxygen atoms are added into the BT and CPDT units. Importantly, relative variation of LUMO and HOMO energies does not follow the trend for PCPDT-DFBT and PDTP-DFBT. In other word, PDTP-DFBT has ~ 0.03 eV

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higher HOMO energy and ~ 0.05 eV lower LUMO energy compared to PCPDT-DFBT. These changes in the HOMO and LUMO energies clearly explain the nature of variation of E ghl among PCPDT-BT, PCPDT-DFBT and PDTP-DFBT shown in Figure 5. At the

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maximum torsional defect, LUMO energies of PCPDT-DFBT and PDTP-DFBT reach ~0.04 eV and ~0.02 eV relative to the LUMO energy of PCPDT-BT at α = 0˚.

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Figure 7 shows the variation of E gopt for PCPDT-BT, PCPDT-DFBT and PDTPDFBT with the torsional defect ranging from α = 0˚ to 180˚. Experimentally observed

E gopt are also included in Fig.7 in terms of relative scale. Figures 5 and 7 show qualitatively similar trend. However, they differ quantitatively because E gopt involves the

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excitation of electrons from the ground state to the excited state. Absolute values of E gopt are always less than E ghl because of the coulomb interaction between a hole in the HOMO

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level and an electron in the LUMO level. This holds true for all torsional angles shown in figures 5 and 7. For example, E gopt of PCPDT-BT increases up to ~0.26 eV at α = 90˚

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relative to E gopt of PCPDT-BT at α = 0˚. This variation is ~0.21 eV lower compared to

E ghl of PCPDT-BT. At α ≈ 0˚, PCPDT-DFBT has ~35 meV higher, but PDTP-DFBT has ~31 meV lower E gopt relative to PCPDT-BT. Relative increase in E gopt when fluorine atoms are added to the BT unit is in very good agreement with the reported data. However the decrease in E gopt when an oxygen atom is added into the CPDT unit is slightly overestimated at the B3LYP level. Importantly, the present results are supported

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by the experimental observations that fluorination of the BT unit increases E gopt whereas oxygenation of CPDT unit decreases E gopt . The torsional defect increases E gopt and reaches

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maximum at α = 90˚. At α = 90˚, values of E gopt are ~0.33 eV and ~0.21 eV for PCPDT-

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DFBT and PDTP-DFBT, respectively relative to E gopt (1.72 eV) of PCPDT-BT.

Figure 7. Torsional dependence of Optical gaps as shown. Energies are plotted relative to optical gap of PCPDT-BT at α = 0˚. Experimental values of optical gaps are from Ref. [8].

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3.3. Molecular electrostatic potential surfaces The addition of an oxygen atom to the CPDT unit increases the electron density in

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PDTP-DFBT. In PCPDT-DFBT, fluorine atoms pull electron density towards them by exhibiting inductive effect when bonded to carbon atoms of the BT unit. As a result, electron densities of oxygenated and fluorinated polymers are changed compared to polymers without O and F atoms. The changes in the electron densities can be visualized by constructing molecular electrostatic potential (MEP) surfaces. These surfaces allow three-dimensional visualization of the different charged regions of oligomers.

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Figure 8. MEP surfaces of (a) PCPDT-BT, (b) PCPDT-DFBT and (c) PDTP-DFBT.

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Figure 8 shows the MEP surfaces of PCPDT-BT, PCPDT-DFBT and PDTP-

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DFBT dimers. An area of blue color in the MEP surface corresponds to a lower electron density and has a positive charge. On the other hand, red color on the MEP surfaces corresponds to a high electron density and has a negative charge. PCPDT-DFBT involves fluorinations of benzothiadiazole unit, whereas PDTP-DFBT involves fluorination and oxygenation of benzothiadiazole and cyclopentadithiophene units, respectively. Due to the inductive effect, the MEP surface of PCPDT-BT (Figure 8(a)) is distinct with less extended red color compared to the MEP surface of PCPDT-DFBT (Figure 8(b)).

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Similarly, the MEP surface of PDTP-DFBT (Figure 8(c)) represents the extension of negative

charge

on

both

benzothiadiazole

and

cyclopentadithiophene

units.

Consequently, the extension of red color (negative charge) increases in the order PCPDT-

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BT < PCPDT-DFBT < PDTP-DFBT.

3.4. Dipole moment

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The variation of electron densities in the MEP surfaces (Figure 8) implies the fluctuation of dipole moments across the polymers. The lowest energy conformations of

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PCPDT-BT, PCPDT-DFBT and PDTP-DFBT computed at the B3LYP level have dipole moments (µg) of 2.83 D, 2.56 D and 2.58 D, respectively in the ground state. Interestingly, PCPDT-BT has slightly higher µg compared to PCPDT-DFBT and PDTPDFBT. Stuart et al. [50] have reported similar variation of µg for similar polymers in

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which fluorinated polymers have lower µg than the unfluorinated polymer. Although PCPDT-DFBT and PDTP-DFBT have slightly smaller µg than PCPDTBT in the ground state, addition of heteroatom increases the dipole moment significantly

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in an excited state (Table 3). Excited state dipole moments (µe) for planar PCPDT-BT,

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PCPDT-DFBT and PDTP-DFBT are 4.25 D, 6.21 D and 8.76 D, respectively. This shows that the dipole moment increases substantially with the introduction of fluorine and oxygen atoms. The change in the ground to excited state dipole moment [51] ( ∆µ = [(µ gx − µ ex ) 2 + (µ gy − µ ey ) 2 + (µ gz − µ ez ) 2 ] ) for PCPDT-DFBT and PDTP-DFBT are 1/ 2

2.35 D, 3.85 D and 6.21 D, respectively. The larger value of ∆µ for PDTP-DFBT compared to other polymers suggests that PDTP-DFBT promotes increased charge dissociation in the excited state thereby reducing the charge recombination.

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Table 3. Computed ground and excited state dipole moments (Debye) and their differences for polymers at α = 0˚. Polymer µg µe ∆µ 2.83 2.56 2.58

4. Conclusions

4.25 6.21 8.76

2.35 3.85 6.21

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PCPDT-BT PCPDT-DFBT PDTP-DFBT

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The effect of heteroatoms on structural and electronic properties of PCPDT-BT derivatives was investigated employing first-principles calculations. The lowest energy

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conformations of these polymers computed using ground state DFT methods were found to be planar. The backbone torsional barrier heights for each of these polymers were found to be almost independent of the chain length. The change in the barrier height going from the dimer to the hexamer is less than kT at room temperature, suggesting that

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the torsional potential of the dimer can be applied for the molecular modeling of longer oligomers. Among these polymers, PCPDT-BT dimer has the smallest torsional barrier height (~ 0.31 eV). PCPDT-DFBT and PDTP-DFBT have higher torsional barrier heights

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by ~15 meV and ~35 meV, respectively, compared to the barrier height of PCPDT-BT. PCPDT-DFBT was found to have higher Eghl and Egopt whereas PDTP-BDBT was

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found to have lower Eghl and Egopt compared to PCPDT-BT based on DFT and TDDFT computations. These fluctuations in the energy gaps are caused by the alteration in the HOMO and LUMO energies. In other words, the introduction of an oxygen atom onto the PCPDT unit of PCPDT-DFBT decreases LUMO energy but increases HOMO energy. The torsional defect present in the dimers of these polymers rises Eghl and Egopt up to ~0.5 eV and 0.3 eV, respectively compared to the ideal planar conformations. Consequently,

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the ground to excited state dipole difference was found to be significantly increased when oxygen is added onto the PCPDT unit of PCPDT-DFBT. The relatively smaller energy gaps and larger ground to excited state dipole difference of PDTP-DFBT compared to

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PCPDT-BT and PCPDT-DFBT indicate that PDTP-DFBT favors efficient charge separation in the excited state. Based on the present results, PDTP-DFBT has more potential to be an efficient electron donor for PSC devices, in agreement with the recent

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experimental findings.

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Acknowledgements We are grateful to the National Science Foundation (Grant No. DMR0847580) for financial support. This work used Kraken and Blacklight computational resources that are

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provided by the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation Grant No. ACI-1053575. The

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authors thank additional support from the University of Akron.

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Highlights Structural and electronic properties of PCPDT-BT derivatives are investigated. PDTP-DFBT has lower energy gaps compared to PCPDT-BT and PCPDT-DFBT. PDTP-DFBT has the highest change in the ground to excited state dipole moment. PDTP-DFBT has the highest torsional barrier.

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