Effect of molar mass on electroluminescence of poly(p-phenylene)

Oligo(p-phenylene)s (OPP) with different degree of polymerisations (DP) of 6,8,10,and poly(p-phenylene)(PPP) of DP=27,63 were synthesized. Thin-films were formed for four OPPs (DP=5,6,8,10) by thermal vacuum deposition. Two PPP samples (DP=27,63) are used in the powder form. All the samples were semicrystalline. Infra-red (IR), UV–VIS absorption, photoluminescence (PL) and electroluminescence (EL) spectra were measured and discussed in terms of the molecular-mass. All the optical peaks shifted to lower wavelengths in proportion to the inverse of the DP. The quantum yield of the ITO/OPP/Al electroluminescent device (ELD) increased with the increase of DP up to DP=10, the highest molar mass p-phenylene compound which can be thermally evaporated without loss of original DP.