Article pubs.acs.org/cm
Ladder-Type Nonacyclic Structure Consisting of Alternate Thiophene and Benzene Units for Efficient Conventional and Inverted Organic Photovoltaics Yen-Ju Cheng,* Chiu-Hsiang Chen, Yu-Shun Lin, Chih-Yu Chang, and Chain-Shu Hsu* Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan S Supporting Information *
ABSTRACT: A ladder-type nonacyclic thienyl-phenylene-thienylene-phenylene-thienyl TPTPT unit, consisting of alternate interfused thiophene and benzene units, is designed and synthesized. This multifused distannyl-TPTPT monomer was polymerized with two electron-deficient acceptors, 4,7-dibromo-2,1,3-benzothiadiazole BT and 5,8-dibromo-2,3diphenylquinoxaline QX monomers, by Stille coupling reaction to afford two alternating donor−acceptor copolymers, PTPTPTBT and PTPTPTQX, respectively. Because of the covalent planarization of the conjugated framework, PTPTPTBT simultaneously possess excellent solubilities for solution-processability, low bandgaps with suitable position of HOMO/LUMO energy levels, and high hole mobilities. The devices based on the PTPTPTBT/PC71BM blend not only showed a promising PCE of 5.3% with conventional configuration but also achieved a high PCE of 5.9% with inverted configuration. This value is among the highest performance from the inverted solar cells incorporating a donor−acceptor low bandgap polymer. KEYWORDS: ladder-type structure, organic photovoltaics, alternating copolymer
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charge mobility. 6 On the basis of the aforementioned consideration, integration of alternate thiophene and benzene units into a coplanar entity with forced rigidification becomes a novel molecular design. Pentacyclic fused indacenodithiophene (IDT) unit exemplifies a successful system in this category. The devices utilizing the IDT-based D−A polymers have shown superior photovoltaic performance.7 The other pentacyclic arrangement system is the fused diindenothiophene (DIDT) with highly planar structure. We recently reported a DIDTbased alternating copolymer poly(DIDT-alt-dithienylbenzothiadiazole) (PDIDTDTBT) and its corresponding device has shown a moderate power conversion efficiency (PCE) of 1.65% (Figure 1).8 If the 3,7-position of the DIDT units in PDIDTDTBT are covalently rigidified with the 3-position of two adjacent thiophene rings by a carbon bridge, an alternate thienyl-phenylene-thienylene-phenylene-thienyl (TPTPT) nonacylic building block with forced coplanarity will be emerged (Figure 1). Compared to DIDT, TPTPT unit may exhibit improved optical and electronic properties due to the higher thiophene content and extended coplanarity of the conjugated backbone. In this research, we report the synthesis of the distannyl-TPTPT monomer which is copolymerized with 4,7-dibromo-2,1,3-benzothiadiazole BT and 5,8-dibromo2,3-diphenylquinoxaline QX acceptor monomers to afford two alternating D−A copolymers poly(TPTPT-alt-benzothiadia-
INTRODUCTION
Research on excitonic solar cells using organic p-type (donor) and n-type (acceptor) semiconductors has attracted tremendous scientific and industrial interest in recent years.1 The most critical challenge at molecular level is to translate excellent microscopic properties of photoactive materials into optimal macroscopic device characteristics.2 Donor−acceptor (D−A) polymers incorporating tricyclic 2,7-fluorene unit have shown to possess deep-lying HOMO energy levels that are an important prerequisite to guarantee greater open-circuit voltages (Voc).3 Nevertheless, because of high aromatic stabilization energy of the benzene rings, fluorene-based polymers exhibit relatively large optical band gaps (>2 eV) that restrict their absorption ability and, thus, result in insufficient photocurrents. 4H-cyclopentadithiophene (CPDT) appears to be another attractive thiophene-based tricyclic analogue. CPDT-based D−A polymers have shown narrower optical bandgaps and higher hole mobilities, yielding very high short-circuit currents (Jsc).4 Unfortunately, the resulting devices deliver only moderate Voc values (
