Photovoltaic properties of iodine-doped magnesium tetraphenylporphyrin sandwich cells. Optimization of doping, and dark electrical properties

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Abstract
By fabricating, doping, and testing sandwich cells of the structure Al/Al2O3 /magnesium tetraphenylporphyrin (MgTPP)/Au under high vacuum, without contact with the atmosphere, the effect of iodine doping on the dark and photovoltaic response has been determined. The properties were found to be extremely sensitive to small variations in iodine concentration, and a method of doping was developed to give highly reproducible devices with optimum photovoltaic efficiency for this system. The doped device behaves as a metal-insulator-semiconductor structure in which a depletion layer of thickness ∼100 nm and carrier concentration ∼1016 cm3 is formed in the porphyrin near the Al-Al2O3/MgTPP interface. From analysis of the time dependence of the electrical properties after doping, a value of 1×10−15 cm2/s is obtained for the diffusion coefficient of iodine in the MgTPP film. Rectification of the dark characteristics is observed only for optimized devices, with a rectification ratio of 240 obtained for a cell having a 200-nm-thick porphyrin layer. Current transport at moderate field strengths appears to be space-charge limited in the presence of a discrete trapping level in the porphyrin, enhanced at higher fields by Poole–Frenkel emission in the oxide layer.