PHOTOVOLTAIC EFFECTS IN PORPHYRIN POLYMER-FILMS AND HETEROJUNCTIONS

Electropolymerized porphyrin films on indium-tin-oxide substrates have been characterized using Rutherford backscattering spectrometry, abso rption spectroscopy, electrical characterization methods and with step profiling. With these methods the density of the films (rho=1.35 g/cm (3)) and the absorption coefficients alpha(lambda) have been determine d. For film thicknesses exceeding 40 nm, silver electrical contacts wi thout shunts are achieved by evaporation. The dark conductivity of the films amounts to 10(-13)-10(-12) Omega(-1) cm(-1). When applying a ba nd model for the conduction in the films, the dark space charge limite d current and the exponent in the relation between photoconductivity a nd illumination intensity (sigma similar to I-gamma, gamma=0.65+/-0.05 ) indicate an exponential trap distribution in the band gap of the fil ms. From the action spectra, filter effects of the photoconductance an d low mobilities are inferred. Spin coating-of acceptor layers on top of the polymer films results in the formation of heterojunctions showi ng photovoltaic behavior, with an open-circuit voltage 0.4-0.6 V. The short-circuit current is controlled by electron transfer at the donor/ acceptor interface only and is limited by filter effects in the bulk a nd by the low conductivity of the materials. The optoelectrical proper ties of the layers are different if analyzed using a mercury contact ( higher dark conductivity, no photoconductivity) which is attributed to the introduction of dopants from ambient air in this case. (C) 1996 A merican Institute of Physics.