Elsevier

Chemical Physics Letters

Volume 612, 18 September 2014, Pages 234-239
Chemical Physics Letters

Structural and electronic properties of the P3HT–PCBM dimer: A theoretical Study

https://doi.org/10.1016/j.cplett.2014.08.030Get rights and content

Highlights

  • We obtained two stable isomers of the supramolecular P3HT–PCBM dimer via DFT.

  • Non-covalent bonds are formed between the P3HT and the PCBM in the P3HT–PCBM dimer.

  • The energy gap of the dimer is reduced by about 70% with respect to isolated P3HT.

  • There is a blue-shift on the maximum absorption peaks in the P3HT–PCBM dimer.

Abstract

A density functional theory (DFT) study of the supramolecular dimer formed by a 8-unit oligomer of the poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is presented. A dispersion-corrected exchange-correlation potential was used to afford for the long-range van der Waals interactions. Our calculations predict two stable isomers in which the P3HT 8-mer forms a U-shaped structure surrounding the PCBM. From time-dependent DFT calculations it was determined that the maximum absorption intensity of the P3HT–PCBM dimer undergoes a blue-shift of about 80 nm with respect to the isolated P3HT oligomer in qualitative agreement with the experimental facts.

Introduction

In addition of being a renewable and non-polluting energy source, organic or plastic solar cells have attracted much attention as being an unexpensive, flexible and lightweight device [1], [2]. The organic “bulk heterojunction” (BHJ) solar cells made of blends of an electron donor (usually a π-conjugate polymer like P3HT) and an electron acceptor (usually a fullerene derivative like PCBM) are among the most efficient (η  5%) organic cells reported to date [3], [4], [5], [6], [7], [8]. It has been determined that the BHJ solar cell efficiency depends on important processes like exciton formation and the charge transfer mainly at the interface P3HT/PCBM [1], [2], [3], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Yet there is not a complete understanding of how the P3HT and PCBM are interacting at the molecular level and on what kind of structure they form.

In order to cast some light on how the PCBM and P3HT interact at the molecular level we have performed Kohn–Sham density functional theory (DFT) calculations on the supramolecular dimer formed by a 8-unit oligomer (8-mer) of P3HT and the PCBM in the gas phase. A partial search of the dimer potential energy surface was performed to determine stable structures. Time dependent DFT (TDDFT) calculations have been performed to obtain the absorption spectra of the P3HT–PCBM dimer and the isolated systems. The remainder of the Letter is organized as follows: The computational methodology is introduced in Section 2, which describes the details of computational calculations performed in our study. Our results and conclusions are shared in Sections 3 Results and discussion, 4 Conclusions, respectively.

Section snippets

Computational methodology

In order to “partially” explore the potential energy surface (PES) of the supramolecular P3HT–PCBM dimer in the gas phase, and using information from previous calculations on similar supramolecular systems [29], [30], [31], a family of at least 10 different nuclear geometries of the dimer P3HT–PCBM were constructed to be used as initial structures in the geometry optimizations. The optimized structures of the isolated molecules (P3HT-8mer and PCBM) were used to construct the starting geometries

Results and discussion

Figure 1 shows two stable isomer structures (relative energy 3.126 eV) of the P3HT–PCBM dimer, along with the two initial geometries used in the geometry optimization. All other 8 initial geometries used in geometry optimizations either converged to one of the isomers shown in Figure 1 or diverged (the P3HT moved away from PCBM). As can be seen from Figure 1, in both isomers the P3HT tends to “embrace” the PCBM. In order to better understand these isomer structures, Figure 1(c) shows the same

Conclusions

We have performed Kohn–Sham DFT calculations for the supramolecular dimer formed by the 8-unit oligomer of P3HT and PCBM. The calculations were performed in the gas phase using the Grimme's dispersion-corrected version [32], [33] of the (generalized-gradient-approximation) Perdew–Burke–Ernzerhof exchange-correlation (xc) potential [34]. Considering up to 10 initial geometries of the P3HT–PCBM dimer in a geometry optimization procedure, it was possible to obtain two stable isomers of the

Acknowledgments

J.I.R. thanks SIP-IPN (project 20144466) for financial support and IPN for the “Plaza de Excelencia”. J.I.R. also gratefully acknowledge Prof. Paul Ayers’ group at McMaster University for sharing the SHARCNET computer facilities. B.M.B. is thankful for the computer facilities of the “Dirección General de Cómputo y Tecnologías de Información y Comunicación” (DGTIC, UNAM) and the support by PAPIIT-UNAM (IN119811). A.W.G. acknowledges support by a TRO grant of the University of California San

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