Article pubs.acs.org/cm
Naphthalenediimide-Benzothiadiazole Copolymer Semiconductors: Rational Molecular Design for Air-Stable Ambipolar Charge Transport Chunling Gu, Wenping Hu, Jiannian Yao,* and Hongbing Fu* Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Bei-jing, 100190, P. R. China S Supporting Information *
ABSTRACT: Rational design of air-stable ambipolar polymeric semiconductors was achieved by covalently connecting naphthalenediimide (NDI) units with benzothiadiazole (BZ) through thiophene (T) linkers, namely, PNDI-mT(BZ)mT (m = 1, 2), in which well-coplanar mT(BZ)mT moieties as a whole act as donors rather than acceptors reported in previous studies. Decreasing the number of thiophene linkers from m = 2 to 1 lowers both LUMO and HOMO energy levels. As a result, the carriers in organic thin film transistors (OTFTs) could be switched from unipolar p-channel only to ambipolar transport. In ambient conditions, PNDI-2T(BZ)2T presents an average hole mobility of 0.07 ± 0.02 cm2 V−1 s−1, while PNDI-T(BZ)T exhibits balanced ambipolar charge transport in a bottom-gate/top-contact device architecture, the average electron and hole mobilities was 0.05 ± 0.02 (μe) and 0.1 ± 0.03 (μh) cm2 V−1 s−1, respectively. Moreover, OTFTs based on both polymer show good air-stability with negligible changes after stored in ambient over 3 months. KEYWORDS: naphthalene diimide copolymer, air-stable ambipolar polymer semiconductor, balanced ambipolar charge transport, donor−acceptor conjugated copolymer
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INTRODUCTION Ambipolar OTFTs based on polymeric semiconductors with high and balanced electron and hole mobilities are currently of great interest,1 due to their applications in complementary-like circuits2 and in light-emitting transistors.3 Effective methods developed thus far for fabricating ambipolar OTFTs include blending of unipolar p- and n-channel materials3b,4 or using a single-component layer but with asymmetric electrodes of different work functions.5 These methods, however, significantly increase the device complexity and therefore the manufacturing cost. Covalently linked donor−acceptor (D-A) copolymers provide an effective strategy to develop singlecomponent ambipolar semiconductors,6 because fine-tuning of energy levels through appropriate combination of D−A building blocks makes it possible for efficient charge injection of both holes and electrons from the same-type electrodes. Proper selection of acceptor units is a key step for ambipolar D−A polymers, including diketopyrrolopyrrole (DPP), 7 naphthalenediimide (NDI),8 and benzothiadiazole (BZ).9 Recently, improved mobilities of both holes and electrons (over 1 cm2 V−1 s−1) have been obtained for DPP-based copolymers.10 However, most of them were achieved under nitrogen, and few are known to operate in ambient conditions. Development of air-stable ambipolar polymeric semiconductors is of crucial importance for practical applications, especially, for utilizing the superior rheological properties of polymer materials in the printing process. © 2013 American Chemical Society
For achieving high performance air-stable ambipolar polymers, three key criteria need to be met: (i) low-lying lowest unoccupied molecular orbital (LUMO < −4.0 eV) for air-stable electron injection and conduction; (ii) appropriate highest occupied molecular orbital (HOMO) ensuring lowenergy bandgap (
