Elsevier

Polymer

Volume 109, 27 January 2017, Pages 85-92
Polymer

Evaluating the influence of heteroatoms on the electronic properties of aryl[3,4-c]pyrroledione based copolymers

https://doi.org/10.1016/j.polymer.2016.12.013Get rights and content

Highlights

  • Pyrrolopyrroledione (PPD) and thienopyrroledione were polymerized with benzodithiophene to obtain π-conjugated polymers.

  • Optical and electrochemical properties of π-conjugated polymers were investigated to elucidate structure property-relationships.

  • The first instance of a PPD based polymer used in a solar cell is reported.

Abstract

A donor-acceptor-type conjugated copolymer (PBDT-PPD) composed of benzodithiophene (BDT) and pyrrolopyrroledione (PPD) was synthesized using the Stille cross-coupling reaction. Using both experimental and theoretical data, the optical, electrochemical, and photovoltaic properties of PBDT-PPD were compared with those of its sulfur analog, PBDT-TPD, which is composed of BDT and thienopyrroledione (TPD). The optical bandgaps of the polymers were determined to be 1.86 and 2.20 eV, respectively. While both materials possessed similar highest occupied molecular orbital (HOMO) levels, the lowest unoccupied molecular orbital (LUMO) level for PBDT-PPD was raised relative to that of PBDT-TPD. Devices incorporating PBDT-PPD had a higher open-circuit voltage and fill factor, yet drastically lower short-circuit current density (Jsc) than PBDT-TPD leading to a lower power conversion efficiency (PCE). The lack of significant intramolecular charge transfer (ICT) combined with the high LUMO of PBDT-PPD resulted in poor spectral overlap with the solar spectrum, lowering Jsc. Additionally, there was poor electron injection into PCBM, which also reduced the PCE.

Introduction

Since the discovery of the first semiconducting polymer nearly 40 years ago, research involving conjugated polymers has been on the rise. In particular, studies involving bulk hetero-junction (BHJ) organic photovoltaic solar cells (OPVs), has increased exponentially due to their potential low cost production, and use in lightweight, flexible devices [1], [2], [3], [4], [5], [6], [7], [8], [9]. Although improvements in materials and fabrication techniques have led to dramatic increases in OPV performance, as determined by the power conversion efficiency (PCE), a better understanding of structure-property relationships is still desired [10], [11]. The synthesis of conjugated polymers, comprised of alternating π-electron rich and π-electron deficient arylene units, allows for the selective tuning of optical and electronic properties of the material [12], [13], [14]. This “donor-acceptor” strategy has given rise to a variety of materials with desirable properties, such as broad optical absorption bands, deep HOMO energy levels, high charge carrier mobilities, and LUMO levels with appropriate alignment to [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) [8], [15]. In addition, these materials can be further tuned by varying the heteroatoms within the arenes. Indeed, a dramatic impact on the physical, optical, and electronic properties of a material can be achieved through heteroatom substitution [8], [16]. For example, large changes in optical absorption and solubility of many materials have been observed upon replacing of thiophene with the iso-electronic furan or selenophene [16], [17], [18], [19]. While substitution within a group (e.g. the group 16 chalcogens) can at often times have predictable effects, the impact of substitution between groups is often less straightforward [16], [20], [21].

The thiophene containing 5-octylthieno[3,4-c]pyrrole-4,6-dione (TPD) unit has been used as an π-electron deficient moiety in a variety of high efficiency donor-acceptor copolymers [22], [23], [24]. When polymerized with the π-electron rich benzodithiophene (BDT), BHJ OPV performance as high as 5.5% for 1.0 cm2 devices (PBDT-TPD) has been reported [22]. When the thiophene was replaced with furan (FPD), a widening of the optical bandgap was observed, whereas switching with selenophene (SePD) resulted in a reduction of the optical bandgap, relative to TPD [25]. OPVs fabricated from the SePD based polymer showed a greatly reduced short-circuit current density (Jsc), which resulted in a very low PCE of 0.26% [26]. Although an improvement in performance has not been seen with FPD or SePD, exploration outside of the group 16 elements has yet to be researched extensively.

A seldom studied alternative to TPD is the nitrogen analog pyrrolo[3,4-c]pyrroledione (PPD), first reported in 1996 [27]. While, the additional alkyl chain on the nitrogen atom of PPD can potentially increase solubility, the impact of replacing sulfur with nitrogen is not well understood. Recently, PPD was used in a series of donor-acceptor copolymers with varying results, and there was no direct structural comparison to known high performing TPD based materials [28]. Here, a PPD based copolymer was synthesized, characterized and compared to the structurally analogous PBDT-TPD. In addition to the physical methods, we also evaluated both polymers through density functional theory.

Section snippets

Synthesis and characterization of monomer and polymers

The PPD monomer 6 was prepared according the synthetic route as illustrated in Scheme 1 [27]. Diethyl pyrrole-3,4-dicarboxlyate was formed by condensation of diethyl fumarate and p-toluenesulfonylmethyl isocyanide followed by saponification to the dicarboxylic acid, 2. Compound 2 was then converted to the corresponding anhydride by treatment with N,N′-dicyclohexylcarbodiimide, which was ring opened with n-octylamine, and closed with thionyl chloride to give 4. The unalkylated 4 was then

Conclusions

A novel conjugated polymer, PBDT-PPD was synthesized and compared to the well-known sulfur analog PBDT-TPD. Both polymers were used in OPVs and it was found that PBDT-PPD performed worse than PBDT-TPD. Experimental and theoretical studies on the optoelectronic properties of these polymers demonstrated that PBDT-PPD had a lower electron affinity and wider optical bandgap than PBDT-TPD. Furthermore, the ICT was weaker in PBDT-PPD than in PBDT-TPD, and neither material was a particularly good

Materials

Air- and moisture-sensitive reactions were performed using standard Schlenk techniques. Solvents used for palladium-catalyzed reactions were deoxygenated prior to use by sparging with argon for 30 min. The preparation of compounds 6 and 8 are described in the Supporting Information. (4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimethylstannane) (BDT) [38] was prepared according to literature procedures. Thiophene-3,4-dicarboxylic acid was purchased from Oakwood

Synthesis of PBDT-TPD

BDT (193.0 mg, 0.25 mmol) and compound 8 (105.7 mg, 0.25 mmol) were dissolved in toluene (9 mL) and sparged with argon for 30 min. Tris(dibenzylideneacetone)dipalladium(0) (4.9 mg, 2 mol%) and tri(o-tolyl)phosphine (7.1 mg, 9 mol%) were added and the reaction refluxed for 48 h. The polymer was end-capped by refluxing with trimethyl(phenyl)tin (50 mg) for 4 h, followed by refluxing with iodobenzene (0.1 mL) overnight. After cooling to ambient temperature, the mixture was precipitated into

Funding

This work was supported by the 3M Foundation and Iowa State University (ISU) and partially supported by DMR-1410088.

Acknowledgements

We wish to thank Steve Veysey and the ISU Chemical Instrumentation Facility for training and assistance with the thermal analysis. We also thank Dr. Kamel Harrata and the ISU Mass Spectroscopy Laboratory for analysis. The OPVs were fabricated at the ISU Microelectronics Research Center.

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