Novel electron donating π-conjugated organic materials based on indoloindole unit : synthesis, characterization and optoelectronic application

Abstract

Heteroacene-containing fused aromatic system has been extensively investigated in the area of fundamental study of molecular property and application to organic devices based on unique photophysical and electronic properties. Recently, heteroacenes have been emerged as a promising backbone unit of organic semiconducting materials due to their remarkable charge carrier mobility, and excellent environmental stability. Among various heteroacens, pyrrole-containing heteroacene backbone exhibits strong electron donating nature originating from the low ionization potential, and possesses planar structure, controllable solubility, and easy functionalization. Based on thier unique properties, indolo[3,2-b]indole, which is one of the pyrrole-containing heteroacene, is suitable for the semiconducting material in optoelectronic system. Herein, I designed and characterized indolo[3,2-b]indole (IDID) derivatives. By UV/vis spectroscopy and measurement of energy levels, IDID derivatives are clearly confirmed as strong electron donor. To explore electron donating nature of IDID derivatives, I carried out CT complex formation using IDID derivatives as an electron donor, and 2,7-dinitro-9-fluorenone (DNF), and 2,4,7-trinitro-9-fluorenone (TNF), and 7,7,8,8-tetracyanoquinodimethane (TCNQ) as an electron acceptor. Through analyzing the photophysical properties of CT complexes, IDID derivatives were proven to be excellent donor materials for the formation of ground state CT complex with acceptors. Furthermore, using CT co-crystals of IDIDp-TCNQ as the semiconducting active elements, single-crystal OFETs were prepared by solvent vapor annealing (SVA) process, which showed ambipolar p-/n- type field effect mobility up to 1.27x10-3 cm2V-1s-1and 3.40x10-2 cm2V-1s-1, respectively. On the other hand, IDIDp single-crystal OFETs showed only p-type field effect mobility up to 2.29x10-2 cm2V-1s-1. Secondly, using strong donating nature of IDID core and intramolecular charge transfer (ICT) characteristic of the acceptor-substituted IDID, I designed and synthesized low bandgap A-D-A and D-A-D type triad molecules using IDID as a donor moiety and DPP as an acceptor moiety for high efficiency bulk-heterojunction small molecule organic solar cell (SMOSC). Through comparing optical, electrochemical properties and device performances of A-D-A and D-A-D type IDID-DPP derivatives, A-D-A type triad molecule was found to be an excellent donor molecule in OSCs, where solution processed organic solar cells based on a blend of HD-IDID-EH-DPP as a donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an acceptor exhibited VOC of 0.73 V, JSC of 10.24 mAcm-2, FF of 55.6% and PCEs as high as 4.15%. On the other hand, organic solar cells based on a blend of HD-DPP-EH-IDID as a donor and and PC61BM as an acceptor exhibited VOC of 0.64 V, JSC of 4.23 mAcm-2, FF of 54.0% and PCEs as high as 1.46%.