The surface photovoltage of an increasing series of polyarenes: Anthracene, tetracene, and pentacene Available to Purchase

The surface photovoltage signals and the associated relaxation times generated by a laser pulse at the surface depletion layers of anthracene (0.8 μV, 5.6 msec), tetracene (12. μV, 10.0 msec), and pentacene (17.5 μV, 20.0 msec) appear to vary with the increasing amount of electron delocalization. As expected, the photovoltage of these materials depends logarithmically on light intensity until a saturation value corresponding to the complete energy band flattening at the surface is achieved, and this energy band bending is larger for pentacene than it is for tetracene. The photovoltage signal is observed to decay exponentially following the laser pulse with a relaxation time that is independent both of the wavelength and intensity of the light. It is established that this is in agreement with theoretical predictions based on a simple model involving the recombination of the photoinjected charge. The photovoltage spectral dependence of all three polyarenes have maxima which correspond to maxima in the corresponding optical absorption spectra due to the allowed singlet–singlet transitions. In addition, the photovoltage spectrum of anthracene has maxima that correspond to the ’’forbidden’’ singlet–triplet transitions, which are comparable in size to the photovoltage arising from the allowed singlet–singlet transitions. This observation implies that the dissociation of excitons to form free carriers is independent of the distance of the exciton from the anthracene surface. The corresponding singlet–triplet transitions for tetracene and pentacene are outside the spectral region examined and thus were not observed.