Enhanced Power-Conversion Efficiency in Inverted Bulk Heterojunction Solar Cells using Liquid-Crystal-Conjugated Polyelectrolyte Interlayer

Abstract

Two novel liquid-crystal-conjugated polyelectrolytes (LCCPEs) poly­[9,9-bis­[6-(4-cyanobiphenyloxy)-hexyl]–fluorene–<i>alt</i>-9,9-bis­(6-(<i>N</i>,<i>N</i>-diethylamino)-hexyl)-fluorene] (PF6Ncbp) and poly­[9,9-bis­[6-(4-cyanobiphenyloxy)-hexyl]–fluorene–<i>alt</i>-9,9-bis­(6-(<i>N</i>-methylimidazole)-hexyl]-fluorene] (PF6lmicbp) are obtained by covalent linkage of the cyanobiphenyl mesogen polar groups onto conjugated polyelectrolytes. After deposition a layer of LCCPEs on ZnO interlayer, the spontaneous orientation of liquid-crystal groups can induce a rearrangement of dipole moments at the interface, subsequently leading to the better energy-level alignment. Moreover, LCCPEs favors intimate interfacial contact between ZnO and the photon harvesting layer and induce active layer to form the nanofibers morphology for the enhancement of charge extraction, transportation and collection. The water/alcohol solubility of the LCCPEs also enables them to be environment-accepted solvent processability. On the basis of these advantages, the poly­(3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₀-butyric acid methyl ester (PC₆₀BM)-based inverted polymer solar cells (PSCs) combined with ZnO/PF6Ncbp and ZnO/PF6lmicbp bilayers boost the power conversion efficiency (PCE) to 3.9% and 4.2%, respectively. Incorporation of the ZnO/PF6lmicbp into the devices based on a blend of a narrow band gap polymer thieno­[3,4-<i>b</i>]­thiophene/benzodithiophene (PTB7) with [6,6]-phenyl C₇₀-butyric acid methyl ester (PC71BM) affords a notable efficiency of 7.6%