Increasing the quantum efficiency of nickel phthalocyanine films by oxygen adsorption

An examination of the surface photovoltage indicates that when oxygen adsorbs on a nickel phthalocyanine polycrystalline film, one form adsorbs irreversibly with a sticking probability of 9 × 10^?3 and a second form adsorbs reversibly with a sticking probability >0.1. The reversibly adsorbed oxygen can be removed by evacuating the ambient oxygen, while the irreversible form can only be removed by heating the sample to 433 K. The irreversibly adsorbed oxygen causes an order of magnitude increase in the photovoltage, even though a comparison of the photovoltaic relaxation times indicates that this oxygen has actually slightly lowered the energy band bending at the surface depletion layer. This increase in the photovoltage is therefore attributed to an increased quantum efficiency of minority carrier injection in a process which is analogous to that observed for oxygen in the bulk.