Origin of Different Photovoltaic Activities in Regioisomeric Small

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The Origin of Different Photovoltaic Activities in Regioisomeric Small Organic Molecule Solar Cells: The Intrinsic Role of Charge Transfer Processes Santu Biswas, Anup Pramanik, and Pranab Sarkar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02821 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 9, 2018

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The Journal of Physical Chemistry

The Origin of Different Photovoltaic Activities in Regioisomeric Small Organic Molecule Solar Cells: The Intrinsic Role of Charge Transfer Processes Santu Biswas, Anup Pramanik, and Pranab Sarkar∗ Department of Chemistry, Visva-Bharati University, Santiniketan- 731235, India E-mail: [email protected]

Abstract Regioisomeric small organic molecules are recently reported to show remarkable difference in their photovoltaic activities when blended with suitable donor/acceptor counterpart. Structural difference as a result of positional diversity of the foreign substituent leads to different intermolecular interaction, dipolar orientation and thus they acquire different morphologies. This severely influence the local transport and carrier recombination leading to different photovoltaic power conversion efficiencies. However, the molecular level understanding relating to the carrier transfer and the associated dynamical properties remains unexplored which could focus on the rational description of such phenomena. Herein, we apply Marcus theory of electron transfer rates on some experimentally realized and model composite systems consisting regioisomeric molecules which provides a general understanding of such intrinsic dynamical behavior, the precise control of which could offer better photovoltaic materials.

∗ To

whom correspondence should be addressed

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INTRODUCTION π-conjugated small organic molecules have recently drawn serious attention in fabricating costeffective photovoltaic devices. 1–3 There are two major advantages which attract them in solar cell industry. Firstly, their structural flexibility is helpful for optimizing random blend morphology in bulk heterojunction (BHJ) technique 4,5 and secondly, the electronic and optical band-gap of the molecules could easily be tuned through chemical functionalization. 1,6–8 In the last couple of years, a significant effort has been devoted for the rational design and synthesis of conjugated small organic molecules and their successful implementation in photovoltaic devices. 3,9–12 In most of the cases, fullerenes and their derivatives are the common acceptors. 13,14 However, performance of the photovoltaic cells composed of organic molecules is very much dependent upon the donor architecture comprising different types of electron-rich and electron-deficient counterparts which modulate intra/inter molecular charge transfer and recombination dynamics. 15–19 Such kind of structure-property relationship is also modulated in different regioisomeric molecules. The electron-rich or electron-deficient substituents in different topological geometries induce different molecular orbital distributions, molecular dipoles, and intermolecular interactions resulting different crystalline geometries in the thin film of the molecules which severely affect the electrical and optoelectronic properties resulting different photovoltaic efficiencies. 20–25 The inter-crystallite ordering between neighboring crystallites may also affect local transport and recombination rates thereby imparting different photovoltaic activities in different regioisomers. 22,23,26 However, correlating such phenomena guided by the regioisomeric effect is very difficult and in fact there are no clear guidelines for the ideal design for modulating the morphological characteristics. Welch et al. 25 first demonstrated how the positions of a heteroatom (pyridyl N-atoms) within a π-conjugated backbone of a molecule can influence the electronic structure as well as the molecular shape. Such changes in the building blocks of the materials are also translated to supramolecular assemblies within crystalline domains. This specific class of materials, when serves as the donor component in bulk heterojunction solar cells, severely affects the power conversion efficiency. It is therefore essential to have a clear understanding on the structure-property relationship for predicting the pho2


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tovoltaic efficiencies of regioisomeric molecules. The basis of such relationship should lie on the electron transfer kinetics guided by the relative energy positions of the individual components in the composite system. We thus provide here the charge transfer and charge recombination dynamics of some recently studied systems composed of regioisomeric molecules as donor. 20 It has been demonstrated that charge recombination rate for a particular regioisomer is very low in comparison to that of the other and such phenomenon has been rationalized with the prediction of a better photovoltaic material where recombination rate is further slow down by additional electron withdrawing group with a number of possible nonbonding interactions at the terminal positions of the molecule. 21 To the best of our knowledge, this is the first theoretical approach for correlating the electronic effect of regioisomeric molecular properties with the charge transfer and recombination rates which may guide photovoltaic performance of the device. First of all, we investigate the ground state electronic properties of the regioisomeric molecules and see how the position of heteroatom can influence their frontier energy levels. We then compute the optical absorption properties of the individual molecules to see their effectiveness as dye. The molecular composites are then made with PC61 BM as a global acceptor. Now, on the basis of the ground and excited state properties of those composites, we compute the exciton binding energy and intermolecular charge transfer properties and finally, we determine the charge transfer and recombination rates on the basis of Marcus theory. 27,28 This gives a clear overview how the geometric parameters of the individual dyes affect the charge transfer properties within the composites which in turn guide the energy conversion efficiency in solar cell devices.

MODEL AND COMPUTATION As a reference, we first choose distal and proximal isomers of fluorine substituted benzothiadiazole and thiophene containg π-conjugated molecules (dis- and prox-FBT, distal and proximal positions of a fluorine atom with respect to central core) which have been synthesized recently by Li et al. 20 (long alkyl chains have been replaced by methyl group for computational advantage) and then

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modify such systems by replacing terminal thiophene rings with methylidene rhodanine moiety (FBTR). We also study the corresponding unfluorinated molecules (BT and BTR) for comparison, all the structures of which are shown in Figure 1. The geometry optimizations of the individual molecules and the respective composites were performed by using density functional theory (DFT) methodologies within G09 29 program packages where all the elements were represented by 6-31G∗∗ basis functions. Different kinds of density functionals such as B3LYP, HSE06, PBE1PBE, etc. were used to calibrate the geometric parameters and electronic properties with the experimentally determined values. It is known that the hybrid functional B3LYP fits well for representing the optimized ground state geometries for the individual and composite systems. 7,17,30 However, as could be found in Table 1, HSE03/HSE06 reproduces the electronic energy gap quite accurately, while PBE1PBE estimates the HOMO energy of the individual molecules with reasonable good accuracy which is a very important parameter for determining the charge transfer rate. 17,30,31 We performed time-dependent DFT (TDDFT) calculations to have the electronic excitation energy along with the absorption spectra of the molecules. The same table (Table 1) indicates that the long-range corrected density functional, CAM-B3LYP compares the computed absorption maxima (λmax ) of the molecules very nicely with that of the experimentally determined values for regioisomeric FBT molecules. 20 Note that the TDDFT calculations have been performed in chloroform medium to see the effect of solute-solvent interactions in the absorption spectra as also done in experimental condition. 20 We used polarizable continuum model (PCM), using the integral equation formalism variant (IEFPCM) 32 as implemented in G09. 29 The Generalized Mulliken–Hush (GMH) model 33 was employed to compute the charge transfer integral values at the donor–acceptor interface. 34,35 During electronic excitation, the dipolar change was evaluated by employing finite field method on the excitation energies. 36 All the analyses relating to the excited states of the materials were performed with the help of ’Multiwfn’, a multifunctional wavefunction based analyzer, 37,38 the details of which could be found in supporting information (SI).

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RESULTS AND DISCUSSION Li et al. 20 have recently shown that between dis- and prox-FBT, the distal isomer with a larger optical band-gap and deeper HOMO level shows superior photovoltaic power conversion efficiency (PCE) with an appreciable value of open-circuit voltage. This study provides a useful guideline for the generation of optoelectronic devices through the regioisomeric control of molecular properties. The small structural change in the two regioisomers leads to a slight difference in their electrochemical and optical properties. It has been demonstrated that the orientation of the fluorine atom leads to differences in the conformational diversity and the pair-wise electrostatic potential of the molecules which influence the overall lattice orientations. Although a detailed mechanism of such crystallization in the thin film remains unexplored, the authors attribute a lower photovoltaic performance for prox-FBT/PC71 BM blend in comparison to that of the dis isomer. However, the authors did not enlighten on the charge transfer or recombination processes for the two regioisomers which may be very crucial for determining the photovoltaic activities. The deeper HOMO position of disFBT (0.23 eV lower than that of prox-FBT as obtained from cyclic voltametric measurement) 20 is in excellent agreement with our quantum chemistry calculation using PBE1PBE/6-31G** level of theory (please see Table 1). It has been demonstrated that the antibonding character of C-F bond in the HOMO of prox-FBT up-shifts its relative position than that of dis-FBT for which the HOMO experiences non-bonding nature of the C-F bond (see Figure S1 in SI). 20 The HOMO of prox-FBT is even higher than the corresponding unfluorinated molecule (BT) also (see Table 2 and Table S1 of SI). In solution, both prox-FBT and dis-FBT exhibit almost identical absorption band (shown in Figure 2) with a maximum absorption at λ = 516 and 517 nm, respectively which are in excellent agreement with the experimental data as obtained by Li et al. 20 The same authors determine a larger molar absorption coefficient for dis-FBT than that of prox-FBT, however, our computed oscillator strengths for dis- and prox-FBT are 2.28 and 2.19, respectively using CAM-B3LYP/6-31G∗∗ level of theory. Note that fluorination to the BT molecule shifts the λmax to the blue region marginally as could be found in Table 2. Upon photoexcitation an organic chromophore produces a bound 5



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electron-hole pair, called exciton. 39 The capability of dissociating such exciton into free polarons and thereafter injecting hot electron to the adjacent electron acceptor determines the photovoltaic performance of the donor chromophore. The energy requirement for breaking the Coulombic attractive force between the electron and hole is termed as exciton binding (Eb ) energy which can be calculated as below Eb =

∑ d∈D,a∈A

ε

qd qa ; lda

(1)

where qd and qa are the partial charges accumulated over donor (D+ ) and acceptor (A− ) parts with a separation lda and ε is the dielectric constant of the medium. Alternatively, Eb can be estimated as the difference between the electronic and optical band-gap energy (HOMO-LUMO energy difference and first excitation energy, respectively). 40 Within this approximation, the computed exciton binding energies for dis- and prox-FBT molecules are 0.25 and 0.33 eV, respectively indicating the possibility of exciton dissociation and the values are in consistence with that of the similar other molecules. 17,30,41 It is worth mentioning that the electronic and optical bandgaps are very sensitive to the density functional used in DFT/TDDFT calculations. As already stated, PBE1PBE determines the HOMO energy (EHOMO ) quite accurately and on the other hand, long-range corrected functional CAM-B3LYP estimates the optical band-gap very nicely. 30,41 The hybrid functional B3LYP, however, is also a good choice for determining ground state properties. 17,20 We here adopted two different strategies for computing ground electronic state and excitation energies relating to dynamical properties; combination of PBE1PBE/CAM-B3LYP and combination of B3LYP/CAM-B3LYP for estimating HOMO energy and first excitation energy (∆E0−0 ) and thereafter determining electronic band-gap from the estimation of corrected LUMO corr = E 31 (ELUMO HOMO + ∆E0−0 ). Both the strategies give similar trend in charge transfer and charge

recombination dynamics. Effectiveness of the organic molecules as donor could be realized by calculating the intermolecular charge transfer rate of the donor-acceptor (D–A) composite which can be viewed as a model for BHJ solar cell. In the optimized geometry, the acceptor PC61 BM is placed vertically over the planer surface of the donor molecule. In an organic solar cells, the electron injection process in6


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volves photoinduced electron transfer from D to A component via an intermediate charge-transfer (CT) state. 42 The lowest CT state is thus a key intermediate electronic state which plays a determining role not only in the exciton-dissociation but also in the charge-recombination dynamics. 43 We analyze the nature of excited states and find that the low-lying excited states both for disand prox-FBT–PC61 BM composites correspond to local excitation (LE) involving either D or A counterpart (please see Figure 3). Even, in some cases the excitation involves both types of local excitations, however, the complete charge transfer state is of prime importance. The first CT state for dis-FBT–PC61 BM appears at 2.89 eV (S13 ), while that is S14 for the prox isomer as shown in Figure 4. Although the absorption intensities of these transitions are relatively low, the underlying states are mainly responsible for the charge transfer dynamics within the composite materials as already stated. Due to intermolecular charge transfer characteristics, these states show a large charge transfer length (lCT ) of 6.98 and 6.07 Å for the dis and the prox isomers, respectively. As shown in Equation Eq. (2), the charge transfer kinetics is mediated by another important parameter λ , the reorganization energy of the D–A composite. The experimentally and theoretically determined λ values (the details of the theoretical calculations for inner and outer reorganization energy values are shown in SI) for these types of small organic molecular composites run in the range of 0.5 eV. 17,30,44 With this, we present the charge transfer and recombination dynamics within the composite materials for two regioisomeric donors dis- and prox-FBT. As already stated, we employ Marcus theory 28 where rates (k) of charge transfer (CT ) and recombination (CR) processes at a particular temperature T are determined on the basis of free energy change (∆G) of the corresponding process s k=

4π 3 (∆G + λ )2 2 |V | exp − DA h2 λ kB T 4λ kB T

! (2)

where VDA is the charge-transfer integral between donor and acceptor states that can be estimated on the basis of Generalized Mulliken–Hush (GMH) model 33 µtr ∆E

VDA = p

(∆µ)2 + 4(µtr )2

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(3)


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where µtr is the transition dipole moment along the major axis, ∆µ is the change in dipole moment between the ground (S0 ) and the excited (Sn ) state and ∆E is the vertical excitation energy. ∆µ can be computed by using finite field method on the excitation energy. 45,46 The calculated ∆µ, µtr and thereafter VDA values of the composite systems are shown in Table 3. We recapitulate that the first intermolecular charge transfer states for dis- and prox-FBT composites are S13 and S14 , respectively, while in case of unsubstituted BT it is S9 . So, those states have been selected for the VDA calculations. Now, the free energy change for charge recombination process (∆GCR ) can be evaluated as the difference in ionization potential of the donor (EIP (D)) and electron affinity of the acceptor (EEA (A))

∆GCR = EIP (D) − EEA (A)

(4)

Within Koopman’s approximation, EIP (D) is the HOMO energy of the donor, while EEA (A) can be evaluated as the sum of HOMO energy of the acceptor and its first excitation energy which have be determined quite accurately as already mentioned. Now, from the estimated value of ∆GCR , one can calculate the free energy change for charge transfer (∆GCT ) by using Rehm–Weller equation 47

∆GCT = −∆GCR − ∆E0−0 − Eb

(5)

where ∆E0−0 and Eb are the energy of the lowest excited state of free-base donor and the exciton binding energy, respectively. The computed values of different free energies and their corresponding rates for dis- and prox-FBT–PC61 BM composites are shown in Table 3 using PBE1PBE/CAMB3LYP level of theory. The similar results with B3LYP/CAM-B3LYP combination are shown in Table S2 in SI. It is revealed that dis isomer experiences faster charge transfer and very slower charge recombination than the prox isomer which is evidenced from the recent experimental observation where dis isomer is reported to show greater PCE. 20 So, we demonstrate that the regioisomeric effect has a severe effect in hot electron transfer and electron-hole recombination dynamics. In particular, the slower recombination rate, in case of dis-FBT, may be ascribed as the more neg8


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ative value of ∆GCR because of its deeper HOMO energy position as already described. This may further be cited due to poorer electron-hole overlap (S) and larger charge transfer length (lCT ) in its first CT state as compared to those of prox-FBT. Now, to see the precise effect of fluorine substitution on the BT molecule, we have performed similar computations on the unsubstituted molecule BT. We see that the charge transfer rate is not affected too much in comparison to that of the dis-FBT, however, in this case also the charge recombination rate is too high. This ensures that, fluorine substitution precisely at the distal position with respect to the central core leads to slower recombination of charge carriers ultimately leading to high PCE. At this point, it should be pointed out that the Marcus theory is validated in weak coupling condition and it adopts the classical low frequency harmonic oscillation without taking into account the vibrational quantum effect. 48 However, photoexcitation leads to population of upper vibrational levels which requires the detailed consideration of electron-phonon coupling. As a matter of fact, vibrational quantum tunneling effect comes into the picture and in the high frequency regime the rate expression is more correctly given by Marcus-Levich-Jortner equation. 48–50 Very recently, Sun and his coworkers 51–54 show that vibrational quantum tunneling effect facilitates the charge recombination process by favoring the Coulombic attraction between photogenerated charges. Their extensive studies reveal that the low frequency approximation in Marcus theory overestimates the charge transfer rates while the charge recombination rates are underestimated. However, for very similar systems (proximal and distal isomers) it is expected to follow the similar trend in calculated rate constants using Marcus theory at least at zero external field which we are particularly interested in. Moreover, our calculated values can nicely explain the experimental observation that the distal isomer of FBT shows better power conversion efficiency than the corresponding proximal isomer. Another point that should be discussed over here is that the density functional has a precise effect on the calculated rate constants for charge transfer and recombination processes. 17 Especially, for noncovalent composite materials where the van der Waal interactions are more important, the dispersion corrected functionals can change the ground state properties. To look into the matter, we have performed some additional calculations using ωB97X-D for the composite materials. It has

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been observed that the intermolecular distance and the orientation are changed a bit which affect the VDA values. But as we have already introduced, the rate constants are much more dependent upon ∆GCT /CR (exponential factor), they are not affected so much, even their qualitative trend is also retained. Now, in organic photovoltaics, the exciton dissociation and thereafter transport of charge carriers occurs under the influence of electric field generated via the difference in work-functions of the source and counter electrodes. 55 So, the carrier transport under the influence of external field is an important issue. 30,41,53 However, in absence of any external bias, the effect of the workfunctions of the electrodes on the charge transport process can be understood by calculating the the open-circuit voltage (VOC ) of the BHJ solar cell. VOC of such cell can simply be estimated D A as VOC = 1/e(|EHOMO | − |ELUMO |) − 0.3 V , 8,56 where 0.3 eV is considered as the minimum en-

ergy difference between LUMO of the donor and the LUMO of the acceptor required for effective charge separation. The computed VOC values of the different composites are given in Table 3. The large value of VOC indicates the deep-lying HOMO states of the donors and this ascertains the suitability of the materials for photovoltaic devices. Our results are in qualitative agreement with the recent experimental results where the FBT-fullerene composites are reported to show high VOC ' 1 V. 20 We also calculate the energy loss (Eloss ) of such systems following the equation: Eloss = ∆E0 − eVOC 57 as shown in Table 3, where ∆E0 is the first excitation energy of the composite system. Let us now delve our attention to the effect of change in molecular architecture which introduces a number of non-bonding interaction in the molecular backbone. In view of this, we have replaced the terminal thiophene donor parts of the D-A-D-A-D system by methylidene rhodanine group. Due to the additional nonbonding interactions within the molecule (N...F, N...S, etc.), the planarity of the system is now changed. 21,58 Additionally, electron withdrawing nature of the methylidene rhodanine moiety and extended delocalization within the molecule 21 suppress the HOMO energy to a great extent as could be found in Table 2. The corresponding HOMO isosurface plot is shown in Figure S1 in SI. Surprisingly, the overall effect reduces the charge

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recombination rate drastically (please see Table 3). Our computation indicates that, both for the unsubstituted and fluorine substituted systems (BTR, dis-FBTR), the recombination rates are lowered by almost 108 times compared to that of thiophene terminated systems. The charge transfer rates are affected, however, not in a great extent. Interestingly, here also the fluorine substituted distal isomer (dis-FBTR) shows relatively slower recombination dynamics (kCR = 2.0×10−8 s−1 ) indicating its high photovoltaic performance. Such an order of low recombination rates for similar methylidene rhodanine terminated composite materials was reported on the basis of Langevin model of bimolecular recombination. 26 As shown in Figure 4, for dis-FBTR, the lowest CT state is S15 with an electron-hole separation of 6.16 Å. Comparing the charge transfer and recombination rates we conclude that the proposed donor molecule with end-terminated methylidene rhodanine will show better photovoltaic efficiency in its distal regioisomeric form in comparison to that of the already synthesized molecule with thiophene end-termination. These systems are also predicted to show higher VOC and lower Eloss as given in Table 3.

CONCLUSION In summary, on the basis of ground and excited state electronic structure calculations employing DFT and TDDFT methodologies we provide a general view how different regioisomeric small organic donor molecules influence the charge transfer and charge recombination dynamics in a composite system with suitable acceptor which necessarily guide the power conversion efficiency of the fabricated devices. Firstly, we observe that the position of the foreign element affects the HOMO energy level of the molecule in a great extent. Secondly, we demonstrate that although the positional effect does not affect the charge transfer rate very much, it severely influences to slow down the charge recombination rate for a particular regioisomer (distal) which has deeper HOMO level. As a matter of fact, that particular regioisomer provides higher open circuit voltage, lower energy loss and thus offers better photovoltaic performance. More electron withdrawing group like methylidene rhodanine at the donor site further lowers the HOMO level which could gener-

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ate better photovoltaic material. The fluorine substituted dis-FBTR is predicted to show superior photovoltaic performances with extremely low recombination rate. We do believe that the present understanding will provide a guideline for inducing regioisomeric effect for improving the power conversion efficiency in small organic molecule-based solar cells.

Supporting Information Available Detailed methodologies for computing excited state properties and reorganization energy; table showing HOMO/LUMO energies and internal reorganization energy using B3LYP/CAM-B3LYP level of theory; table showing charge transfer/recombination rates using B3LYP/CAM-B3LYP level of theory. This material is available free of charge via the Internet at http://pubs.acs. org/.

Acknowledgement The work is supported by the research grants from CSIR (File No. 01(2916)/17/EMR-II) and DST (Ref. No. SR/NM/NS-1005/2016), New Delhi, Government of India. S.B. is thankful to CSIR for providing him Senior Research Fellowship (SRF).

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Table 1: Computed gas-phase HOMO/LUMO energies, their difference and absorption maximum (λmax , in chloroform medium) for dis- and prox-FBT molecules using different level of theory. Values in parentheses are the first excitation energies (in eV) in gas phase. The corresponding experimental data 20 are given for comparison. Method B3LYP HSE03 HSE06 PBE1PBE M06-2X CAM-B3LYP

EHOMO (eV) dis prox -4.96 -4.79 -5.10 -4.92 -4.83 -4.65 -5.20 -5.01 -6.18 -5.98 -6.20 -5.99

ELUMO (eV) dis prox -2.76 -2.69 -3.26 -3.19 -3.02 -2.95 -2.68 -2.61 -1.97 -1.90 -1.67 -1.60

EHOMO−LUMO (eV) dis prox 2.20 2.10 1.84 1.73 1.81 1.70 2.52 2.40 4.21 4.08 4.53 4.39

ωB97X-D Expt.

-6.83 -6.63 -5.39 -5.16

-1.11 -3.52

5.72 1.87

-1.04 -3.39

5.59 1.77

λmax (nm) dis prox 671 691 674 703 677 706 617 643 470 471 517 516 (2.45) (2.42) 499 496 522 520

Table 2: Computed gas-phase HOMO energy, corrected LUMO energy (adding ∆E0−0 to EHOMO ) and absorption maximum (λmax ) in chloroform medium for the studied molecules using PBE1PBE/CAM-B3LYP level of theory. System BT prox-FBT dis-FBT dis-FBTR BTR PC61 BM

EHOMO (eV) -5.06 -5.01 -5.20 -5.61 -5.49 -5.97

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corr ELUMO (eV) -2.62 -2.59 -2.75 -3.33 -3.22 -3.55

λmax (nm) 519 516 517 564 566 –



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Table 3: Comparing charge transfer/recombination rates and associated photovoltaic properties of the composite systems computed in gas phase using PBE1PBE/CAM-B3LYP level of theory (VOC = open-circuit voltage, Eloss = energy loss, ∆µ = dipole moment difference between ground and excited state, µtr = transition dipole between ground and excited state). System BT–PC61 BM

VOC (V) 1.21

Eloss (eV) 1.21

∆µ µtr (a.u.) (a.u.) 8.87 0.065

VDA (eV) 0.020

prox-FBT–PC61 BM

1.16

1.26

8.77

0.041

0.013

dis-FBT–PC61 BM

1.35

1.07

9.48

0.035

0.011

dis-FBTR–PC61 BM

1.76

0.52

8.11

0.047

0.018

BTR–PC61 BM

1.64

0.64

7.91

0.105

0.040

20


∆GCR in eV ∆GCT in eV (kCR in s−1 ) (kCT in s−1 ) -1.51 -0.93 4 (2.2×10 ) (2.6×1011 ) -1.46 -0.96 (6.4×104 ) (6.5×1010 ) -1.65 -0.80 1 (1.9×10 ) (5.0×1011 ) -2.06 -0.22 −8 (2.0×10 ) (1.6×1012 ) -1.94 -0.33 (1.1×10−4 ) (2.2×1013 )

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Figure 1: Geometries of the unsubstituted and fluorine substituted regioisomeric donor and acceptor (PC61 BM) molecules obtained from B3LYP/6-31G** level of computation. The gray, white, blue, red, cyan, and yellow colored tubes indicate the C, H, N, O, F, and S atoms, respectively.

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Figure 2: Absorption spectra of the dis- and prox-FBT molecules computed by using CAMB3LYP/6-31G∗∗ level of theory in chloroform medium.

Figure 3: Charge density difference (CDD) plots for some low-lying excited states of dis- and prox-FBT–PC61 BM composites which correspond to local excitation (LE). Blue and red colors indicate hole and electron densities, respectively.

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Figure 4: Charge density difference (CDD) plots along with some excited state properties for the lowest charge transfer (CT) states of the donor molecule–PC61 BM composites. Blue and red colors indicate hole and electron densities, respectively.

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Graphical TOC Entry

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Origin of Different Photovoltaic Activities in Regioisomeric Small

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