Charge conduction process and photovoltaic effect in ITO/ArV/CHR/In p–n junction device

Abstract

The present communication deals with the designing of a new p–n junction device made with 4,4′-aryl bipyridine, also known as aryl viologen (ArV), a p-type organic semiconductor and 3-diazophenyl, 4,5 dihydroxynaphthalene, 2,7 disulphonic acid, disodium salt, commonly known as chromotrope 2R (CHR), a n-type organic semiconductor. The electrical and photoelectrical properties of the fabricated p–n junction having configuration ITO/ArV/CHR/In was studied by analysing its current–voltage (J–V) characteristics, capacitance–voltage (C–V) characteristics in the dark and photoaction spectra. The analysis of dark current–voltage (J–V) characteristics at room temperature has been presented in order to elucidate the conduction mechanisms and to evaluate the device parameters. The charge transport conduction mechanism in forward biased condition in the low voltage region is described by the modified Schockley effect. For biases >1.0V, the dark current is a space charge limited current (SCLC) in the presence of exponentially distributed traps. The variation of 1/C2 with voltage also shows the straight line at low frequency indicating the formation of p–n junction between ArV and CHR and the potential barrier height is about 1.18eV at room temperature which decreases with the increase in temperature. The comparison of photoaction spectra with absorption spectra of the ArV–CHR layer reveals that photocurrent in this device is due to the generation of excitons both in ArV and CHR. The excitons generated in ArV dissociated by electron transfer to CHR and those created in CHR are ionised by hole transfer to the ArV. This charge transfer may be driven by the offset between the electron affinities and ionisation potentials of two semiconductors.