Donor–Acceptor Alternating Copolymer Nanowires for Highly Efficient Organic Solar Cells

Despite intensive research efforts, however, only a few conjugated PNWs have been successfully introduced into solutionprocessed BHJ solar cells – for example, regioregular poly(3alkylthiophene) (RR-P3AT) and fullerene blend systems. There have not been any reports describing successful control over the evolution of highly crystalline nanowire structures based on highly photoconductive D–A alternating copolymers. The bulky side chains that ensure their solubility tend to induce unwanted structural distortions, which also reduce the opto electronic properties of the resulting polymers. [ 25 ] This limitation has hindered further development of polymer-nanowire-based BHJ-PSCs; therefore, there remains a signifi cant need for the development of new molecular designs and simple fi lm fabrication strategies for preparing crystalline 1D nanowire structures from D–A alternating copolymers that could be useful in higheffi ciency organic solar cells. Here, we report the preparation of PSCs based on 1D nanowire structures composed of a novel D–A alternating copoly mer, poly(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]di thiophenealt -4,7-bis(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3benzothiadiazole) (PBDT2FBT-2EHO). To the best of our knowledge, this is the fi rst demonstration of BHJ-PSCs based on PNWs comprising (D–A)-type conjugated polymers. The PNW structures were successfully prepared through a singlestep fi lm-forming process using a simple binary mixture of solvents. The kinetically controlled BHJ fi lm-formation was found to facilitate the evolution of crystalline 1D nanowires composed of conjugated polymers with non-bulky side chains without sacrifi cing solution processability. The composition of the binary solvent mixture comprising chlorobenzene (CB) and 1-chloronaphthalene (CN) was adjusted to enable fi ne control over the spatial density and size of the nanowires in order to optimize the morphology of the photoactive layer. The resulting BHJ-PSCs with nanowire structures, which were composed of a novel D–A alternating copolymer, yielded outstanding improvements in the PCE of up to 8.2% in conventional singlecell structures – 60% larger than the PCE of a device prepared without nanowires. Details of the synthetic procedures and characterization data for the monomers are available in the Supporting Information. The benzo[1,2-b:4,5-b′]dithiophene (BDT), which consists of a less-electron-rich phenyl ring fused to two fl anking thiophene moieties, was employed as the co-monomer to construct the D–A alternating copolymer with the strongly electrondefi cient unit of 5,6-difl uorobenzo[c][1,2,5]thiadiazole (2FBT) Over the past decade, polymer solar cells (PSCs) have attracted signifi cant attention due to their advantages over alternative silicon-based solar cells: PSCs have the feasibility to be fabricated at low cost using convenient roll-to-roll printing techniques and to be light-weight devices. [ 1,2 ] Signifi cant improvements in the power conversion effi ciencies (PCEs) of bulk-heterojunction (BHJ) solar cells have been recently achieved, and the PCEs of single-junction PSCs now exceed 8.0%. [ 3,4 ] Among the various photovoltaic materials that can be used in BHJ-PSCs, donor– acceptor (D–A)-type conjugated polymers have recently been identifi ed as highly promising materials for use in organic photovoltaics (OPVs) due to their broad light absorption spectrum [ 5 ] and strong intermolecular interactions, which facilitate charge generation and transport. [ 6 ]