COMMUNICATION pubs.acs.org/JACS Improved High-Efficiency Organic Solar Cells via Incorporation of a

bS Supporting Information ABSTRACT: The power conversion efficiencies of bulk heterojunction (BHJ) solar cells can be increased from 5 to 6.5 % by incorporating an ultrathin conjugated polyelectro-lyte (CPE) layer between the active layer and the metal cathode. Poly[N-900-heptadecanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were chosen for the photoactive layer. CPEs with cationic poly-thiophenes, in both homopolymer and block copolymer configurations, were used to improve the electronic char-acteristics. The significant improvement in device perfor-mance and the simplicity of fabrication by solution pro-cessing suggest a promising and practical pathway for improving polymer solar cells with high efficiencies. Bulk heterojunction (BHJ) solar cells based on blends com-prising conjugated polymers and fullerene acceptors are the subject of considerable investigation because of their potential to enable the fabrication of low-cost devices that convert sunlight into electricity.1 Significant improvements in power conversion efficiencies (PCEs) due to combined improvements in materials design,2 interface control,3 self-assembly of donor and acceptor phases,4 and device fabrication and engineering5 have recently appeared. The fundamental quantities that determine the PCE include the open-circuit voltage (Voc), the short-circuit current density (Jsc), and the fill factor (FF), as shown in eq 1