Effect of side chain arrangement of conjugated polyelectrolytes buffer layer on the photovoltaic properties

•Effect of side chain arrangement on the photovoltaic properties.•The work function the cathode depends on the number of ionic groups of CPE.•Arrangement of side chains affect the electron collection ability.

Conjugated polymer electrolytes (CPEs) are well known materials for reducing the electron injection/collection barrier at the cathode interface by the formation of favorable interface dipole. Regardless of their potential applications as a cathode buffer layer (CBL), the effect of the arrangement of side chains with ionic groups of CPEs on photovoltaic properties is not yet fully investigated. In this research, a series of cationic CPEs based on phenothiazine and fluorene, poly[9,9-bis(6-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene] bromide (PHF), poly[9,9-dihexyl-9H-fluorne-alt-9,9-bis(6-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene] bromide (PHcoF), poly[10-(4-(N,N,N-trimetylammonium)-butyl)-10H-phenothiazine-alt-9,9-bis(6-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene] bromide (PTF), and poly[10-butyl-10H-phenothiazine-alt-9,9-bis(6-(N,N,N-trimethylammonium)-hexyl)-9H-fluorene] bromide (PTcoF) have been applied as CBL materials to demonstrate how the arrangement of side chains with quaternary ammonium bromide (QAB) influences photovoltaic properties. PHF and PTF have QABs on every side chain; whereas, PHcoF and PTcoF have QABs on the side chains alternatively. According to the DFT calculation, the conformation of backbone based on fluorene (PHF and PHcoF) is expected to be not exactly in a zigzag conformation, while the zigzag conformation is expected for the alternated copolymers based on phenothiazine and fluorene (PTF and PTcoF). Thus, a number of accumulated QABs by the spontaneous rearrangement of side chains of PHF and PTcoF at the surface of Al electrode will be bigger than that of PHcoF and PTF. A larger number of accumulated QABs at the interface between the thin film of CPEs and the Al electrode leads to a bigger net induced dipole moment by the accumulated QABs, indicating a larger reduction of the effective WF of the Al cathode and reduction of a Schottky barrier at the cathode interface. As a result, the performances of PSCs with PHF and PTcoF as a CBL are better than those of the device with PHcoF and PTF.

Graphical abstractTo investigate how the arrangement and the number of ionic groups on the side chains by the conformation change of the polymer backbone influence photovoltaic properties, a series of conjugated polymers are synthesized. The performances of polymer solar cells are dependent on the arrangement and the number of ionic groups of the polymer backbone.Figure optionsDownload full-size imageDownload as PowerPoint slide