Electrochromic device and bulk heterojunction solar cell applications of poly 4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-dodecyl-2H-benzo[1,2,3]triazole (PBEBT)

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Abstract

4,7-Bis(2,3-dihydrothieno [3,4-b] [1,4] dioxin-5-yl)-2-dodecyl-2H-benzo [1,2,3] triazole (BEBT) was polymerized both electrochemically (ePBEBT) and chemically (cPBEBT). Since chemical polymerization enabled a soluble polymer in common organic solvents, a single layer electrochromic device of ePBEBT was constructed. The polymer cPBEBT was also used in bulk heterojunction (BHJ) solar cells as the active layer in combination with a soluble fullerene derivative, 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C61 PCBM.

Introduction

The ability to sense or respond to a controllable external stimulus classifies conjugated polymers as candidates for smart materials [1]. Applications of these materials to low cost optoelectronic devices will be possible using solution processable conjugated polymers with promising electrical and physical properties. Recent worldwide research interest is focused on organic solar cells (OSCs) [2], [3], [4], [5] and electrochromic devices (ECDs) [6]. Polymers that are applicable to many fields are regarded as multi-purpose smart materials for organic electronics [7].

Conjugated polymers allow fine tuning in optical properties via structural modifications and, in principle, highly efficient devices can be obtained by design of low band gap polymers. However, the performance of polymer based solar cells is quite low compared with those of their inorganic counterparts due to restricted charge transport and limited exciton diffusion lengths. [8].

Among conjugated polymers, poly-3,4-ethylenedioxythiophene is proven to be an excellent candidate for optoelectronic applications [9] and known as the blue component of ECD-based display devices [10], [11]. Our previous study on the integration of benzotriazole moiety to PEDOT chains resulted in enhancement of optical properties of the resultant polymer [11]. The contrast was improved (53%) compared with those of PEDOT (44%) with a much shorter switching time (1.1 s compared with to 2.2 s for PEDOT) and higher coloration efficiency (211 cm2/C vs 183 cm2/C). Although the pendant alkyl chain on benzotriazole moiety provided high solubility to the monomer (4,7-bis(2,3-dihydrothieno [3,4-b] [1,4] dioxin-5-yl)-2-dodecyl-2H-benzo [1,2,3] triazole (BEBT)), high electroactivity of the oligomers and the polymer resulted in the electrodeposition of insoluble chains on ITO. However, its chemical polymerization succeeded in the production of a soluble polymer. Here we highlight the photovoltaic and electrochromic device applications of PBEBT as the active layer. The ability to play with the structure of the active component will enable one to improve the performance of polymer/organic based devices compared with their inorganic counterparts.

Section snippets

General

Propylene carbonate (PC), tetrabutlyammonium hexafluorophosphate (TBAPF6), poly(methylmethacrylate) (PMMA) and acetonitrile (AN; Merck) were purchased from Aldrich and used without further purification. BEBT was synthesized according to the reported procedure[11]. All electrochemical studies were performed under ambient conditions using a Voltalab 50 potentiostat. Average molecular weights were determined by gel permeation chromatography (GPC) using a Polymer Laboratories GPC 220. All

Synthesis

Synthesis of 4,7-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2-dodecyl-2H-benzo[1,2,3] triazole (BEBT) was performed according to the published procedure [11] Monomer, BEBT, was polymerized either electochemically (ePBEBT) or chemically (cPBEBT; Scheme 1). However, the polymer was soluble only in the case of chemical polymerization. For electrochemical polymerization indium tin oxide coated glass slide (ITO) was used as the working electrode. The monomer was electrochemically deposited on ITO

Conclusion

A donor acceptor donor type monomer, BEBT, was polymerized chemically and electrochemically. The resulting polymers, cPBEBT and ePBEBT, were exploited for solar cell and electrochromic device applications, respectively. Although the preliminary results regarding power conversion efficiency are not high, optimization of the conditions may lead to further research on such benzotriazole bearing materials in the near future. Although the photovoltaic results are low, they can be improved by

Acknowledgments

The authors thank European Science Foundation (ESF, ORGANISOLAR Project), TUBA and GÜNAM for financial supports.

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