Photovoltaic effect and conduction mechanism in metal-insulator-metal cells containing metal-free phthalocyanine

Abstract

Metal-insulator-metal cells containing metal-free phthalocyanine sandwiched between two metals (six different combinations of gold, lead and aluminium) have been prepared, and their photoelectrical properties studied in ultra-high vacuum. With irradiation incident on the non- substrate electrode the spectral response and sign of the photovoltage are consistent with conduction by injected holes. At low voltages, the photocurrent-voltage curves can be quantitatively explained by a space- charge-free theory of conduction. Energy barriers to hole injection, and hence the built-in field, are not determined by the work function of the metal, but in each case the higher barrier occurs on the non-substrate side of the phthalocyanine. This fact, together with the observed variation of the built-in field with irradiance and the failure of the space-charge-free conduction model at high voltages, is explained by assuming that the metals make ohmic contact to the phthalocyanine and that the effective barriers to injection are determined by space-charge effects of holes trapped near the metal-insulator interface. The trap density is highest at the non-substrate side of the film and is approximately uniform with energy. The dark current exceeds the saturation photocurrent at high voltages, which suggests that the mechanism of photoinjection into the bulk is probably exciton dissociation at defects near the illuminated electrode rather than exciton-induced photoinjection directly from the metal.    

Reasons for the low photovoltaic power-conversion efficiencies in these cells, and theoretical limitations on more-ideal cells of this type, are discussed.