Effects of Substituted Side Chains on the Optical and Electrical Properties of D-A Conjugated Copolymers
- PDF / 15,576,309 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 67 Downloads / 271 Views
Effects of Substituted Side Chains on the Optical and Electrical Properties of D-A Conjugated Copolymers Shinuk Cho1,*, Sangkyu Lee2, Minghong Tong3, Jung Hwa Seo3, Alan J. Heeger3 1
Department of Physics, University of Ulsan, Ulsan 680-749, South Korea. Energy Materials Research Center, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, South Korea. 3 Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 931065090, U.S.A. 2
ABSTRACT The effects of substitute side chain were investigated using donor-acceptor (D-A) conjugated copolymers consisting of a cyclopentadithiophene (CPDT) derivative and dithienyl-benzothiadiazole (DTBT). The intrinsic properties of the copolymers were significantly altered by perturbations of the intramolecular charge transfer (ICT). The absorption of PCPDT-ttOTBTOT (P2), which assumed a tail-tail configuration, tended to blue-shift relative to the absorption of PCPDT-TBTT (P1). The absorption of PCPDT-hhOTBTOT (P3), which assumed a head-head configuration, was blue-shifted relative to that of P2. The electrical transport properties of field-effect transistors (FETs) were sensitive to the side chain position. The field-effect mobility in P2 (μ2=1.8×10–3 cm2/V·s) was slightly lower that in P1 (μ1=4.9×10–3 cm2/V·s). The mobility of P3, however, was very low (μ3=3.8×10–6 cm2/V·s). Photoexcitation spectroscopy showed that the charge generation efficiency (shown in transient absorption spectra) and polaron pair mobility in P1 and P2 were higher than in P3, yielding P1 and P2 device performances that were better than the performance of devices based on P3.
INTRODUCTION Because of the low band-gap property together with relatively high hole mobilities, conjugated copolymers containing electron donor–acceptor (D-A) repeating units have drawn considerable attention for use as active materials in organic electronic devices, such as bulk heterojunction (BHJ) photovoltaic (PV) cells and polymer field-effect transistors (FETs) [1-5]. Most polymer electronic devices are fabricated by solution processing, which represents an attractive advantage of polymer electronics over their inorganic counterparts. Although the conjugated core units of D-A conjugated copolymers convey solubility, D-A copolymers have relatively low solubility if they have a high molecular weight (Mw). Because the best performances have usually been obtained using high Mw materials [6,7], achieving high molecular weight with good solubility is anticipated to be an important goal for achieving high performance polymer electronic devices. As a simple way to improve the solubility of high Mw D-A copolymers, alkyl or alkoxy side chains may be introduced at the thiophene unit. Substitution of side chains in conventional conjugated polymers, however, often alters the configuration of the conjugated chain, thereby affecting the electronic properties. Consequently, the polymers display properties that differ compared to the original analogs [8,9]. Such side chain effects can critically affe
Data Loading...
