Photoconduction in the archetype organic hole transporting material TPD

Electric field, light wavelength and excitation intensity characteristics of photoconduction have been measured in a diamine derivative (TPD) serving as a common hole-transporting material in organic light emitting diodes. Single layer TPD structures, Quartz/ITO/TPD/Al and Quartz/Al/TPD/Al, have been excited trough a quartz substrate. At wavelengths longer than ≈370 nm and at low excitation levels, the photocurrent is due to singlet exciton-induced injection of electrons at the cathodes. Good agreement with experiment is provided by solving the diffusion equation for the excitons and applying the 1D-Onsager model of geminate recombination of electrons with their image countercharge in the electrode. A bulk generated (hole) photocurrent is shown to dominate with excitation energies (hν) well in excess of 3.2 eV, i.e. λ ⩽ 350 nm, and in the whole spectral range applied for moderate and high excitation intensities.