The effect of gases on the electrical properties of some organic semiconductors

Effects of gases (primarily NgO^, BF^, Og and NH^) on the electrical properties of molecular single crystals of donors (phthalocyanine and its Mn(ll), Co(ll), Ni(ll), Cu(ll), Zn(ll) and Ph(ll) complexes, anthracene, perylene and the bis(8-hydroxyquinoline) complexes of Cu(ll) and Pd(ll)), electron acceptor (7,7,8,8 tetraeyanoquinodimethane) and an electron donor-acceptor complex (perylene/7,7,8,8 tetracyanoquinodimethane) have been studied. Apparatus was constructed permitting measurement of semiconduction and photoconduction in vacuo and in various gas pressures, over a temperature range of 290-500K.

The conductivity of electron donor and acceptor crystals was only enhanced by electron acceptor and donor gases, respectively, whereas the donor-acceptor complexes were enhanced slightly by both types of gases. Gas effects were confined to the crystal surfaces, except for oxygen on phthalocyanines. The rate and magnitude of conductivity changes on varying the gas pressure were consistent with chemisorption involving electron transfer. Electron transfer between % delocalised orbitals (e.g. NgO^ on phthalocyanines) gave reversible adsorption and gas pressures above 10^Pa resulted in complete surface coverage, each adsorbed molecule producing one surface charge carrier. The more localised c orbitals (e.g. BF-^ on phthalo- cyanineB) resulted in stronger adsorption. Exceptionally, irreversible chemical reaction follows electron transfer (e.g. NgO^ on anthracene). Enhanced semiconduction is accompanied by significant reductions in the activation energy, to values comparable with the photoconduction activation energy. Photoconduction is more sensitive than semiconduction to low gas pressures. Changes in the photoconduction spectral response reflect the influence of adsorbed gases on the exciton dissociation process. Adsorbed NgO^ inhibits the singlet photoconduction in tetracyanoquinodimethane and perylene, but smaller low energy response, with fine structure, possibly triplet bulk photoconduction, persists. Increased surface ionized state density reduces the carrier mobility, resulting in reduced dependence of photocurrent on incident light intensity.

The implications of these findings for the design of organic semiconductor gas detectors are discussed.