Synthesis and optoelectronic properties of thermally cross-linkable fluorene derivative containing hole-transporting triphenylamine terminals

This paper describes the synthesis of a solution-processable and thermally cross-linkable 2,7-bis-[4-bis(4-vinylphenyl)aminophenyl]-9,9-dihexylfluorene (VTF) and its application as hole-transporting layer in multilayer polymer light-emitting diodes (PLEDs). The thermal, photophysical, and electrochemical properties of VTF were investigated by differential scanning calorimetry, thermogravimetric analysis, optical spectroscopy, and cyclic voltammetry. The VTF is readily cross-linked via vinyl groups by heating at 180 °C for 30 min to obtain homogeneous film with excellent solvent resistance. Multilayer PLEDs (ITO/PEDOT:PSS/cured-VTF/MEH-PPV/Ca/Al) were readily fabricated by spin-coating process using cross-linked VTF as hole-transporting layer (HTL). The maximum brightness (13,640 cd/m2) and current efficiency (0.69 cd/A) were superior to those without HTL (ITO/PEDOT:PSS/MEH-PPV/Ca/Al: 7810 cd/m2, 0.28 cd/A). In addition, the cured-VTF could replace conventional hole-injection layer (PEDOT:PSS) to reveal comparable performance (8240 cd/m2, 0.44 cd/A). Current results indicate that the VTF with four thermally cross-linkable terminal vinyl groups is a promising optoelectronic material, which is readily processed by wet processes.