Effect of Chemical Modifications on the Electronic Structure

of Poly(3-hexylthiophene)

Eli�ezer Fernando de Oliveira,1 Alexandre Camilo-Jr,2 Luiz Carlos da Silva-Filho,1,3

Francisco Carlos Lavarda1,4

1POSMAT—Programa de P�os-Graduacao em Ciencia e Tecnologia de Materiais, UNESP—Universidade Estadual Paulista, Bauru,

SP, Brazil

2Departamento de Fısica, Setor de Ciencias Exatas e Naturais, UEPG - Universidade Estadual de Ponta Grossa,

Av. Carlos Cavalcante 4748, Uvaranas, 84030-900 Ponta Grossa, PR, Brazil

3Departamento de Quımica, Faculdade de Ciencias, UNESP—Universidade Estadual Paulista, Av. Eng. Luiz Edmundo Carrijo

Coube 14-01, 17033-360 Bauru, SP, Brazil

4Departamento de Fısica, Faculdade de Ciencias, UNESP—Universidade Estadual Paulista, Av. Eng. Luiz Edmundo Carrijo Coube

14-01, 17033-360 Bauru, SP, Brazil

Correspondence to: F. C. Lavarda (E-mail: lavarda@fc.unesp.br)

Received 5 December 2012; accepted 30 January 2013; published online 22 February 2013

DOI: 10.1002/polb.23274

ABSTRACT: The widespread use of poly(3-hexylthiophene)

(P3HT) in the active layers of organic solar cells indicates that

it possesses chemical stability and solubility suitable for such

an application. However, it would be desirable to have a mate-

rial that can maintain these properties but with a smaller

bandgap, which would lead to more efficient energy harvesting

of the solar spectrum. Fifteen P3HT derivatives were studied

using the Density Functional Theory. The conclusion is that it is

possible to obtain compounds with significantly smaller bandg-

aps and with solubility and stability similar to that of P3HT,

mostly through the binding of oxygen atoms or conjugated or-

ganic groups to the thiophenic ring. VC 2013 Wiley Periodicals,

Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 842–846

KEYWORDS: conducting polymers; conjugated polymers; molec-

ular modeling

INTRODUCTION Poly(3-hexylthiophene) (P3HT) is a polymerderived from polythiophene, but unlike the latter, it is solu-ble in organic solvents,1–3 which makes it more favorable foruse as an electron donor in the active layers of organic solarcells. In comparison to the other polymers used, such aspoly[2-methoxy-5-(20-ethylhexyloxy)-p-phenylene vinylene]and poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenyle-nevinylene], P3HT has been shown to have greater stability,a longer lifetime, and greater efficiency in heterojunction so-lar cells with [6,6]-phenyl-C61-butyric acid methyl ester,which makes its use more attractive.2,4,5–7

Most polymeric materials used as donors in solar cells havea bandgap greater than 2 eV (around 620 nm), which ena-bles the harvesting of up to 30% of the available solar spec-trum.1–3 One way to increase the harvest of incident solarenergy is via a decrease in the bandgap of the polymer.2

This would provide an increased number of collected pho-tons and electrons promoted to the excited state of the poly-mer, generating a greater electrical current density in the de-vice.1,2 Through polymer chemistry, it is known that smallmodifications of the monomeric units can cause profound

changes in the polymer properties, which in turn affects thebandgap of the polymer.8,9 Thus, if we modify the structureof the material’s monomer, we can find a new material withmore appropriate intrinsic properties.

It is known that substitutions using materials that withdrawor donate electrons to the main chain may not show any cor-relation with the properties of substituents.10 This makesthe computer modeling of materials interesting, as it canprovide some proposals for modifications that might obtainan interesting material to be processed.

Experimental studies on the changes in the properties ofP3HT due to electrophilic substitutions occurring at position4, using chlorine and bromine, emphasize the possibility ofobtaining new derivatives with substitutions at this posi-tion.11,12 In this work, we report the results for 15 substitu-tions at position 4: conjugated and unconjugated alkylgroups, halogens, a hydroxyl, an amine, and a phosphine. Fortwo of the halogens mentioned above (Cl and Br), we haveexperimental validation of our modeling methodology, andwe found two substitutions in which the bandgap, stability,and solubility properties are improved.

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MATERIALS AND METHODS

We conducted a study of chemical substitutions in P3HT, withmodifications in position 4 of the thiophenic ring (Fig. 1). Wechose to observe the trend in the variation of the electronicstructure of P3HT through oligomers with 10 repeating units(decamer). Using the optimized P3HT decamer, we modifiedall atoms and/or groups attached at position 4 of the ringand reoptimized the new material.

Apart from P3HT (R ¼ H), we studied the replacement of thehydrogen atom at position 4 with Cl (P3HT-Cl), Br (P3HT-Br),F (P3HT-F), SH (P3HT-SH), OH (P3HT-OH), NH2 (P3HT-NH2),PH2 (P3HT-PH2), CH3 (P3HT-CH3), C2H5 (P3HT-C2H5), C3H7

(P3HT-C3H7), C4H9 (P3HT-C4H9), cyano (P3HT-cyano), a sixcarbon piece of trans-poliacetylene (P3HT-tpa), a phenyl(P3HT-ph), and a phenylenevinylene (P3HT-pv).

The use of side chains of alkyl groups is due to the fact thatthere is evidence in the literature that such substitutionscould rigidify the polymer backbone, possibly making it moreplanar.10 Substitutions performed with chlorine and bromineonly served to verify whether the results obtained with thismethodology would show the same trends observed in theliterature.11,12

The substitutions made with conjugated alkyl groups are anattempt to increase the system conjugation.13 For example, inrelation to the cyano group, there are reports in the literatureof its use in other polymeric systems, such as PPV,14,15 whichresults in a decrease in the bandgap.

The structure optimization was carried out using the semiem-pirical method PM616 with the MOPAC2009 package,17 withthe structures immersed in solvent, as it was not known howthe new polymers would behave after the thin film process-ing. The simulation of the solvent was accomplished using theCOSMO method.18 The solvent chosen for this study was chlo-

roform, as it is the most-used solvent for dissolvingP3HT.1,19,20 We opted to use the PM6 semiempirical methodsince it has been already satisfactorily employed in the studyof photovoltaic properties of P3HT derivatives.21,22

An oligomer with 10 monomer units of P3HT was obtainedemploying a method developed by us and recently pub-lished.23 We used the configuration Head-to-Tail-Head-to-Tailas this provides the best results for photovoltaic devices.1,24

Our results suggests an approximately anti conformation,which is consistent with experimental and theoretical data atlevel ab initio 6-31G and 6-31G* in vacuum.25–27 The moststable conformation obtained for alkyl chains is of the laddertype, which is the same result obtained via ab initiocalculations.25

After geometry optimization, we obtained the electronicstructure data from the decamers via the density functionaltheory (DFT)28 employing the B3LYP hybrid functional,29

while the COSMO method was used to simulate the solventand the chosen basis functions set was 6-31G(2d)(1p)30

with the GAMESS program.31 Today there is a large numberof available functionals, and our option for the B3LYPhybrid functional is based on the fact that it has alreadybeen successfully used in similar compounds.32–34 In addi-tion, this functional has proved to be reliable, generally pre-senting quality results, which is the reason for its wide-spread use.35

We used a semiempirical method for geometry optimizationdue to the system’s size, as well as the large number of con-formations evaluated, as the application of ab initio qualitymethods would make this task computationally infeasible.The methodology for optimizing the structure via a semiem-pirical method and calculating the electronic structure prop-erties via a DFT calculation provides a reduced computationalcost. Furthermore, the literature suggests that, in the study ofpolymers, this method has satisfactory results when com-pared with experimental results and other theoreticalmethods.33

RESULTS AND DISCUSSION

In the Table 1 below, we present the results for P3HT andthe 15 derivatives studied, obtained via DFT/B3LYP/COSMO/6-31G(2d)(1p) calculations. In addition to the bandgap, wealso have results for the dipole moment (MD) and the ioniza-tion potential (IP) of the decamer.

The MD is related to the solubility of the material,36 an im-portant property for the processing of thin polymeric films.The solubility is always related to one particular type of sol-vent (polar or nonpolar). In the case of P3HT, the mostwidely used type of solvent is chloroform, which is a polarsolvent, so it is desirable that the derivative possesses a DMgreater than or equal to that of P3HT so as to not hamperthe processing.

The IP is calculated according to the Koopmans Theorem orapproximation,37 and it is equal to the negative energy ofthe last occupied molecular orbital. The larger the IP, theharder it is to oxidize the material. Thus, the IP is connectedto the material stability.38

FIGURE 1 Structure of the monomer of P3HT and its

derivatives.

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The experimental value of the bandgap of P3HT for opticalmeasurements is 1.9–2.0 eV for the solid state.1–3,33 More-over, the experimental data indicate a vertical transitionenergy for P3HT in chloroform of about 2.85 eV.39 The theo-retical value we obtained is 4.6 eV. This result was expected,as in the solid state the polymer is generally found in amore planar configuration, which increases the bandgap anddecreases the conjugation. Also, a decamer, having a conjuga-tion length of 10 monomers, has a bandgap greater than thepolymer, for which there is an estimated conjugation lengthof 21–27 monomers.40 In calculations to simulate the P3HTin the solid state, which we did using the same method pre-sented here for the geometric conformation most likely pre-dicted for this situation, which is more planar than for thecase with the solvent, the decamer has a bandgap of 3.0 eV.We are interested in a theoretical tool to estimate the varia-tion of the bandgap for the derivatives, and it is known thatthe B3LYP functional provides good results when consideringthe variation of the bandgap for conjugated polymers.33,41

The same can be said for the MD42 and IP.38

Substitutions with halogens tend to present more solubleand stable materials. Only P3HT-F has a bandgap reduction,of approximately 10%, that can be considered significant,and that results in a material in which all the desired prop-erties have been improved relative to P3HT. It is noted thatthe substitutions performed with chlorine and bromine showthe same trends observed in experiments11,12 with compara-tively larger bandgaps than P3HT.

The hydroxyl and amine groups, P3HT-OH and P3HT-NH2,present the same type of behavior as the halogens, showing asignificant decrease in bandgap. The result for the bandgap ofP3HT-OH merits attention, since it is the compound with the

smallest bandgap of all those studied, with a decrease of 37%over P3HT. By analyzing this result, we realize that it comesfrom an increase in the conjugation of the main chain, sincethe optimized geometry for the decamer in solution is close tothe planar, which does not occur with other compounds. It isapparent that the binding of oxygen atoms in polymersderived from thiophene tends to decrease the bandgap, as wasrecently observed. For measurements performed in solution,the poly [3-(4-octylphenoxy) thiophene], which possesses anoxygen atom directly attached to the thiophene ring, possessesa bandgap about 15% smaller than P3HT and 14% smallerthan the poly [3-(4-octylphenyl) thiophene], which has no oxy-gen connected to the thiophene ring.43 In another study, inwhich P3HT derivatives were created by inserting anotheratom between the thiophene ring and the branching hexyl, itis observed that the insertion of an oxygen atom alsodecreases the bandgap, relative to P3HT, by about 17%.44

Interestingly, in this same work it was observed that the inser-tion of one atom of sulfur also reduces the bandgap in relationto P3HT, but with a lower intensity. These two results are con-sistent with what we found in our work, in which we observethe same relationship in terms of decreasing the bandgap.

Derivatives P3HT-CH3, P3HT-C2H5, P3HT-C3H7, and P3HT-C4H9 (saturated alkyl groups) have almost the same stabilityas P3HT, whereas the solubility and the bandgap are worse.The MD has a direct correlation with the size of the group.An analysis of the electronic population of alkyl chainsreveals that the charge transferred to the thiophenic ring ispractically the same, regardless of the size of the chain. How-ever, the final CH3 group becomes more positive as the chainincreases. The growth of this positive charge that is increas-ingly distant from the center of negative charges in the thio-phenic ring explains the behavior of the MD. Apart fromthese data, we found that the optimized geometric conforma-tion in solution of decamers does not point to greater rigidi-fication due to increased planarity, as suggested,10 since theresult obtained shows a decamer that is less planar thanP3HT. Eventually, the process of film formation can lead togreater rigidification due to the interlacing of two of thealkyl ramifications of each monomer.

In all cases studied, the introduction of conjugated alkylgroups (P3HT-cyano, P3HT-tpa, P3HT-ph, and P3HT-pv) pro-motes a decreased bandgap, which is a very interestingresult in terms of a strategy for decreasing the bandgap ofP3HT. This seems to confirm that this type of substitutiontends to increase the system conjugation,13 which is inagreement with the results obtained for PPV with cyano sub-stitutions.14,15 Although P3HT-tpa shows a wide variation inbandgap, P3HT-cyano presents the best set of results for all15 derivatives studied, with improvements in solubility, sta-bility, and bandgap. Note also that the derivatives thatshowed a smaller MD (likely with weakened solubility) arethose that contain phenyl as a substituent.

Figure 2 shows a plot of the gap as function of the inter-ringaverage dihedral angle for pure P3HT and all the derivativesstudied, which gives some information about the structures.

TABLE 1 Electronic Structure Data For P3HT and Its Derivatives

Structure

Dipole

Moment (D) IP (eV)

Bandgap

(eV)

Percentage

Variation in

Bandgap (%)

P3HT 2.355 5.540 4.591 –

P3HT-Cl 10.111 6.166 4.931 þ7.409

P3HT-Br 10.709 6.209 4.906 þ6.876

P3HT-F 11.361 5.584 4.133 �9.958

P3HT-SH 8.008 5.567 4.479 �2.431

P3HT-OH 8.743 4.231 2.898 �36.876

P3HT-PH2 4.145 5.855 4.786 þ4.267

P3HT-NH2 2.562 4.677 4.076 �11.203

P3HT-CH3 1.341 5.594 4.966 þ8.181

P3HT-C2H5 1.793 5.602 4.976 þ8.417

P3HT-C3H7 1.993 5.586 4.977 þ8.417

P3HT-C4H9 2.194 5.575 4.974 þ8.358

P3HT-cyano 23.578 6.501 4.138 �9.839

P3HT-ph 0.655 5.453 4.531 �1.304

P3HT-tpa 3.618 5.096 3.755 �18.197

P3HT-pv 1.609 5.374 4.313 �6.046

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A planar P3HT chain would have 0� for the syn conformation(all the alkyl arms at the same side) and 180� for the anticonformation (successive alkyl arms on opposite sides of thechain). A chain with neighboring rings lying in perpendicularplanes would have 90�, which is practically the case forP3HT-Br. It is possible to see that increasing planarity leadsto a lower gap. With the exception of hydroxyl substituent,the bulky saturated alkyl groups present the largest distor-tion compared to pure P3HT and, as expected, the largestgap. However, the substituent’s size itself does not define theelectronic properties: the conjugated P3HT-tpa (C6H7),although being the longest substituent, has the second low-est gap. This could be caused by a stronger interaction withthe chains’ pi-system, since the frontier orbitals now extendsover the beginning of the C6H7 chain, which does not happenwith the saturated alkyl groups.

CONCLUSIONS

Our results indicate that it can be very interesting to modifyP3HT to obtain lower bandgap materials for use as donorelements in the active layers of organic solar cells.

The replacement of H with OH at position 4 of the thiophenering reduced the bandgap value by 37% compared to P3HT.Although the IP was reduced by 1.31 eV, implying a probablereduction in the oxidation stability of the material, the sizeof the bandgap reduction was so large that it may be inter-esting its synthesis and characterization.

Substitution with conjugated alkyl groups is perhaps thebest option for yielding P3HT derivatives with lower bandg-aps, with all the groups that we employed resulting in mate-rials with reduced bandgaps compared to P3HT.

The best results obtained for substitutions at position 4 ofP3HT, in which the properties of solubility and stability arelikely to be improved, are for the substituents fluorine andcyano, with bandgap reductions of 10%. Finally, smallerbandgaps were observed for the derivatives P3HT-OH (�37%)and P3HT-tpa (�18%), though with reduction of the IP.

ACKNOWLEDGMENTS

The authors would like to thank the Brazilian agency CNPq forfinancial support. This research was also supported by resour-ces supplied by the Center for Scientific Computing (NCC/Grid-UNESP) at Sao Paulo State University (UNESP).

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