3,8-Dialkoxynaphthodithiophene based copolymers for efficient polymer solar cells

A new donor unit viz. 3,8-bis(2-butyloctyloxy)naphtho[3,2-b:7,6-b']dithiophene (NDT) was developed for a novel donor-acceptor semiconducting polymer. And the new NDT-based polymers (PNDTIT-C and PNDTTT-CF) with thienothiophene units were synthesized. The polymers showed deep HOMO levels of -5.44 eV and -5.51 eV with the optical band gap of similar to 1.6 eV. The electrochemical as well as optical and photovoltaic properties of these polymers were studied. The OPV devices fabricated from the blends of polymer PNDTTT-C/PNDTTT-CF: PC71BM were afforded a high power conversion efficiency of 4.49% and 5.16%, respectively. The optimization of device fabrication included different solvents, different weight ratios and usage of solvent processing additive. To obtain the high PCE a typical processing additive 1,8-diiodooctane was used. The effect of processing additive was studied using AFM analysis. In addition, the field-effect hole mobilities of 3.5 x 10(-4) cm(2)V(-1)s-(1) and 5.7 x 10(-4) cm(2)V(-1)s(-1) were observed for the polymers PNDTIT-C and PNDTTT-CF, respectively. Since we have developed the synthetic process of the 3,8-bis(2-butyloctyloxy)- naphtho [3,2-6:7,6-b']dithiophene(NDT) monomer, the NOT unit will play an important role in future research on conjugated polymer and small molecule design for high-performance organic semiconducting material. (C) 2012 Elsevier B.V. All rights reserved.