A novel copolymer from benzodithiophene and alkylsulfanyl-bithiophene: Synthesis, characterization and application in polymer solar cells

https://doi.org/10.1016/j.solmat.2012.04.044Get rights and content

Abstract

We present the synthesis and characterization of a novel copolymer from benzodithiophene and alkylsulfanyl-bithiophene units, poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-4,4′-bis(octylsulfanyl)-2,2′-bithiophene] (PolyS). The structural, electrochemical and photophysical properties of PolyS were investigated by gel permeation chromatography (GPC), thermogravimetric analysis (TGA), NMR, UV–vis, photoluminescence (PL) spectroscopy and cyclic voltammetry (CV). PolyS was used to fabricate bulk-heterojunction polymer solar cells (PSCs) in combination with the [60]PCBM and [70]PCBM (electron acceptors). The device realized using [70]PCBM shows a better current output due to the increased absorption of the blend film compared to the PolyS:[60]PCBM one. The power conversion efficiency of the best PSC realized is 2.3% under 100 mW/cm2 AM 1.5 G illumination.

Highlights

► We synthesized a novel copolymer of benzodithiophene and alkylsulfanyl-bithiophene. ► The structural, electrochemical and photophysical properties were investigated. ► We tested the new material as electron donor in an organic solar cell. ► We obtained a 2.3% efficiency on using [70]PCBM as electron acceptor.

Introduction

In the last decade, polymers solar cells (PSCs) have attracted much attention in the field of renewable energy technologies [1], [2], [3], [4] and they have been intensely studied due to their unique features such as flexibility, light-weight, low fabrication cost and large area processing with high speed [5], [6], [7], [8]. Actually the most successful device structure is based on the bulk-heterojunction (BHJ) concept consisting of an interpenetrating network of electron donor (e.g. p-type conjugated polymers) and acceptor materials (n-type, such as fullerene derivatives) [9], [10], [11]. The blend material is typically sandwiched between two electrodes having different work functions. Conjugated polymers can be a low cost alternative to conventional inorganic semiconductors but, for applications in PSCs, they should possess good filming and absorption properties (absorption coefficients >105 cm−1 and UV–vis spectrum ideally matching the solar spectrum) [12], high hole mobility, and HOMO–LUMO energy levels suitable to be coupled with the acceptor species [13], [14]. The most exploited active material for PSCs is the regioregular poly(3-hexylthiophene) (P3HT). Actually power conversion efficiencies (PCE) up to 7% have been demonstrated using this polymer as donor material and functionalized fullerenes as acceptors [15], [16].

Recently, benzodithiophene (BDT) based materials exhibited very promising properties in polymer solar cells due to their relatively large and planar conjugated structure, which can promote facial π–π stacking of the molecules increasing the charge transport and the red shift of the absorption spectra [17], [18], [19], [20], [21], [22]. Many copolymers of BDT with different conjugated units have been synthesized and the application in PSCs showed promising photovoltaic properties. High PCE values up to 7% have been reached using copolymers of BDT and thieno-thiophene unit [23], [24]. Since BDT unit plays a very important role in these highly efficient photovoltaic polymers, it is interesting to investigate how different units affect the properties (mainly energy gap and position of frontier orbitals) of BDT based copolymers. Among these, bithienyl units as π-bridges are reported to enhance photovoltaic performances of conjugated polymers [25] and 4,4′-bis(alkylsulfanyl)bithiophene appear to be particularly appealing, for it generally enhances polymer solubility in organic solvents, filmability and in poly[4,4′-bis(alkylsulfanyl)bithiophenes] the backbone adopt planar conformation in the solid state, even when the two alkylsulfanyl chains are in a head-to-head junction [26], in contrast to what happens for alkyl chains.

In this paper we present the synthesis and characterization of a novel copolymer from benzodithiophene and alkylsulfanyl-bithiophene units, poly[4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-alt-4,4′-bis(octylsulfanyl)-2,2′-bithiophene] (PolyS). The structural, electrochemical and photophysical properties of PolyS were investigated by gel permeation chromatography (GPC), thermogravimetric analysis (TGA), NMR, UV–vis, photoluminescence (PL) spectroscopy and cyclic voltammetry (CV). PolyS was tested as electron donor in BHJ PSCs with [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM) or [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) as electron acceptors. The PSC devices were characterized by external quantum efficiency (EQE) and current–voltage (IV) measurements under 100 mW/cm2 AM 1.5 G illumination.

Section snippets

General remarks

All air- or moisture-sensitive reactions were performed under argon with dry glassware. All reagents and solvents were purchased from Aldrich and Acros and used as received unless otherwise indicated.

N,N-dimethyl-3-thiophenecarboxamide

In a 1 L round bottom flask, 3-thiophenecarboxylic acid (10 g, 78 mmol) was dissolved in dichloromethane (600 mL) under stirring. Then dicyclohexylcarbodiimide (DCC, 16 g, 78 mmol), 4-(dimethylamino)pyridine (DMAP, 9.52 g, 78 mmol) and dimethylamine hydrochloride (6.36 g, 78 mmol) were added in this order.

Synthesis of PolyS

The synthesis of PolyS was carried out through Stille coupling between 2,6-bis(trimethyltin)-4,8-bis(2-etyl-1-hexyloxy)benzo[1,2-b:4,5-b′]dithiophene (1) [30] and 5,5′-dibromo-4,4′-bis(octylsulfanyl)-2,2′-bithiophene (2) [29] in the presence of a palladium catalyst according to Scheme 1. PolyS is soluble in chloroform, chlorobenzene and THF and possesses a good filmability.

The stannylated comonomer 1 was obtained starting from 3-thiophenecarboxylic acid following the route reported in Scheme 2.

Conclusions

The synthesis, structural, electrochemical and photophysical properties of alternating BDT–bithiophene copolymer PolyS carrying octylsulfanyl side chains have been investigated. PolyS possesses good solubility in common organic solvents, filmability, a proneness to form π-stacks and a moderate solvatochromism. Bis(alkylsulfanyl)bithiophenes are obtained more easily than other comonomers recently employed in the synthesis of organic polymers for photovoltaic applications, as 2-substituted

Acknowledgments

The activity was supported by the Italian Ministry of Economic Development in the framework of the Operating Agreement with ENEA for the Research on the Electric System. The authors thank Dr. M. Lanzi of University of Bologna for GPC measurement.

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