Elsevier

Synthetic Metals

Volume 259, January 2020, 116231
Synthetic Metals

Effects of π-spacer and fluorine loading on the optoelectronic and photovoltaic properties of (X-DADAD)n benzodithiophene-based conjugated polymers

https://doi.org/10.1016/j.synthmet.2019.116231Get rights and content

Highlights

  • Novel (-X-DADAD)n type benzodithiophene-based polymers were synthesized.

  • Thiophene spacers strongly influence the optoelectronic properties of polymers.

  • Fluorine loading reduces miscibility of polymers with the fullerene acceptor.

  • π-bridged and fluorinated polymer delivered highest PCE of 7.0 % in solar cells.

Abstract

Four new conjugated polymers based on benzodithiophene, thiophene and 2,1,3-benzothiadiazole or 5,6-difluoro-2,1,3-benzothiadiazole were synthesized and investigated as absorber materials for organic photovoltaics. The effect of (bi)thiophene π-spacers and fluorine substitution on the physicochemical and optoelectronic properties of the polymers was revealed and correlations were drawn with their electrical characteristics in organic solar cells. In particular, introducing either thiophene spacers or fluorine substituents does not affect much the photovoltaic performance of the polymers, while the combination of both routes was found to be a promising strategy for improving the charge carrier mobilities and morphology of the polymer-fullerene blends as well as light power conversion efficiency in solar cells based on these materials. The bulk heterojunction organic solar cells based on the π-bridged and fluorinated polymer P4 showed the highest short-circuit current density and power conversion efficiency of 7 %, which is an inspiring value for fullerene-based organic photovoltaics. Most importantly, our findings provide important insights into rational design of high-performance conjugated polymers while pursuing a combination of two efficient backbone functionalization strategies based on introduction of fluorine substituents and π-spacers to control the geometry and electronic characteristics of the polymer chains.

Introduction

Organic solar cells (OSCs) are actively investigated over the past two decades due to their potential to become lightweight, flexible and low cost source of solar energy for a broad range of applications [1]. The power conversion efficiency (PCE) of OSCs depends mainly on the properties of the materials used in the active layer, e.g. conjugated polymers. Various approaches are currently implemented to design high performance conjugated polymers for photovoltaic applications [[2], [3], [4], [5], [6]]. Incorporation of alternating electron donor (D) and electron acceptor (A) moieties into the polymer backbone represents one of the most widely used strategies to design absorber materials with tailored optical and electronic properties. In particular, this so-called “push-pull” approach allows one to control frontier energy levels, band gap, charge carrier mobility of conjugated polymers as well as nanoscale morphology of their blends with acceptor components.

Within the last years, a particular attention is paid to the introduction of electron-withdrawing fluorine substituents in the polymer backbone while modifying either donor or acceptor units or even both of them [7]. Fluorine substitution was shown to be an efficient approach to optimize the optoelectronic properties of conjugated polymers, e.g. by lowering HOMO energy and thus enhancing the open-circuit voltage (VOC) of organic solar cells [[8], [9], [10], [11], [12]].Moreover, introduction of the fluorine atoms makes polymers less miscible with acceptor counterparts thus suppressing the formation of disordered mixed phases contributing strongly to the recombination of charge carriers [13]. Thus, using fluorine-loaded polymers improves active layer morphology and photovoltaic performance (mainly due to increased current density) owing to the blocked loss channel. Difluorobenzothiadiazole represents one of the most popular fluorine-containing building blocks for conjugated copolymers due to its availability, high photostability and attractive optoelectronic properties [14,15].

The introduction of additional π-spacers between the D and A units is commonly used to modulate and optimize planarity of the conjugated backbone, aggregation behavior of polymeric chains and miscibility of the electron donor polymers with the acceptor counterparts, e.g. fullerene derivatives [[16], [17], [18], [19], [20], [21], [22]]. Conjugated π-spacers crucially affect the geometry of the macromolecules and hence their optical, electrochemical, charge transport and photovoltaic properties [23].

In our previous works, we reported the design of (X-DADAD)n conjugated polymers with the extended DADAD donor-acceptor molecular framework. These polymers demonstrate considerably improved optoelectronic properties as compared to the well-known and easily accessible (X-DAD)n structures [24]. Moreover, (X-DADAD)n polymers usually demonstrate good photostability comparable to that of PCDTBT known as one of the most robust conjugated polymers [25,26]. According to the Scharber theoretical model, this type of donor materials with optimal optoelectronic properties can provide 13–15% efficiency in single-junction OSCs [27].

In this work, we synthesized four novel (X-DADAD)n polymers based on benzodithiophene (X), thiophene (D), and benzothiadiazole (A) units and investigated the effects of the thiophene π-spacer and fluorine loading in the polymer backbone on the optoelectronic and photovoltaic properties of the designed materials. The aim of the study was to evaluate the effects of each of the aforementioned modifications and find a proper balance to enable further rational design of promising materials for efficient and stable organic photovoltaics.

Section snippets

Results and discussion

The synthesis of key monomers is shown in Scheme 1. Briefly, compounds 1a and 1b were prepared using Stille cross-coupling reactions between the monomers M1 or M2 [28,29] and tributyl(thiophen-2-yl)stannane (i). Further bromination of 1a-b with N-bromosuccinimide (ii) in 1,2-dichlorobenzene afforded the monomers M3 and M4.

Conjugated polymers P1-P4 were synthesized via palladium-catalyzed (i) Stille polycondensation reaction using monomers M1-M4 and D1 (Scheme 2). The synthesis of D1 was

Conclusion

We synthesized and investigated four novel conjugated polymers based on benzodithiophene, thiophene and 2,1,3-benzothiadiazole or 5,6-difluoro-2,1,3-benzothiadiazole. The effects of fluorine substitution and introduction of π-spacer on the optoelectronic properties, charge carriers mobility, blend morphology, and photovoltaic performance were revealed. While comparing the photovoltaic characteristics of polymers P1 and P2, one can conclude that incorporation of fluorine substituents into

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was funded by the Russian Science Foundation (grant No. 18-13-00205).

References (54)

  • W. Zhao et al.

    Molecular optimization enables over 13% efficiency in organic solar cells

    J. Am. Chem. Soc.

    (2017)
  • W. Wang et al.

    Nonfullerene polymer solar cells based on a main-chain twisted low-bandgap acceptor with power conversion efficiency of 13.2%

    ACS Energy Lett.

    (2018)
  • W. Gao et al.

    Regulating exciton bonding energy and bulk heterojunction morphology in organic solar cells via methyl-functionalized non-fullerene acceptors

    J. Mater. Chem. A.

    (2019)
  • Y. Cui et al.

    Achieving over 15% efficiency in organic photovoltaic cells via copolymer design

    Adv. Mater.

    (2019)
  • R. Peng et al.

    Synergistic effect of fluorine substitution and thio-alkylation on photovoltaic performances of alternating conjugated polymers based on alkylthio-substituted benzothiadiazole-quaterthiophene

    ACS Appl. Energy Mater.

    (2018)
  • S. Furukawa et al.

    Controlling open-circuit voltage in organic solar cells by terminal fluoro-functionalization of narrow-bandgap π-conjugated molecules

    J. Phys. Chem. C.

    (2016)
  • M.L. Keshtov et al.

    Polymer solar cells based on D–A low bandgap copolymers containing fluorinated side chains of thiadiazoloquinoxaline acceptor and benzodithiophene donor units

    New J. Chem.

    (2018)
  • S. Song et al.

    Synthesis and photovoltaic properties of copolymers with a fluoro quinoxaline unit

    J. Polym. Sci. Part A: Polym. Chem.

    (2018)
  • P. Shen et al.

    Synthesis and optoelectronic properties of new D–A copolymers based on fluorinated benzothiadiazole and benzoselenadiazole

    Polym. Chem.

    (2014)
  • N. Leclerc et al.

    Impact of backbone fluorination on π-conjugated polymers in organic photovoltaic devices: a review

    Polymers

    (2016)
  • H. Zhou et al.

    Development of fluorinated benzothiadiazole as a structural unit for a polymer solar cell of 7 % efficiency

    Angew. Chemie Int. Ed.

    (2011)
  • N. Wang et al.

    Fluorinated benzothiadiazole-based conjugated polymers for high-performance polymer solar cells without any processing additives or post-treatments

    J. Am. Chem. Soc.

    (2013)
  • S. Liu et al.

    Benzo[1,2- b :4,5- b ′]dithiophene and thieno[3,4- c]pyrrole-4,6-dione based Donor-π-acceptor conjugated polymers for high performance solar cells by rational structure modulation

    Macromolecules

    (2015)
  • H. Zheng et al.

    Efficiency enhancement in an indacenodithiophene and thieno[3,4-c]pyrrole-4,6-dione backbone photovoltaic polymer with an extended thieno[3,2-b]thiophene π-bridge

    J. Mater. Chem. C

    (2016)
  • H. Bin et al.

    Effects of donor unit and π-bridge on photovoltaic properties of D-A copolymers based on benzo[1,2-b :4,5-c’]-dithiophene-4,8-dione acceptor unit

    J. Polym. Sci. Part A: Polym. Chem.

    (2014)
  • X. Wang et al.

    Effects of π-Conjugated bridges on photovoltaic properties of donor-π-acceptor conjugated copolymers

    Macromolecules

    (2012)
  • A.V. Akkuratov et al.

    Design of (X-DADAD)n type copolymers for efficient bulk heterojunction organic solar cells

    Macromolecules

    (2015)
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