Solvent-vapor induced morphology reconstruction for efficient PCDTBT based polymer solar cells

This paper reports polymer solar cells with a 7% power conversion efficiency (PCE) based on bulk heterojunction (BHJ) composites of the alternating co-polymer, poly[N-9 ''-hepta-decanyl- 2,7-carbazole-alt-5,5-(4',(,)7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT), and the fullerene derivative [6,6]-phenyl C-71-butyric acid methyl ester (PC71BM). As confirmed by transmission electron microscopy, solvent-vapor annealing (SVA) of the thin (70 nm) BHJ photoactive layer by exposure to chloroform vapor, for a short period of time (30 s) after deposition, leads to reconstructed nanoscale morphology of donor/acceptor domains, well-dispersed fullerene phase and effective photo-absorption of BHJ. Consequently, SVA-reconstructed devices with a PCDTBT: PC71BM blend ratio of 1:5 (wt%) exhibit similar to 50% improvement in PCE, with short-circuit current J(sc) = 15.65 mA/cm(2), open-circuit voltage V-oc = 0.87 V, and PCE = 7.03%, in comparison to those of the 1:4 (wt%) blends with SVA treatment. (C) 2013 Elsevier B.V. All rights reserved.