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Energy transfer from an inorganic substrate to a cyanine molecule thin film has been investigated as an excitation method for organic luminescent devices. Cyanine molecule thin films were fabricated on a Si substrate using layer-by-layer assembly and were excited from the backside of the substrate to observe the luminescence. The luminescence intensity depends on the excitation power and excitation energy. Moreover, the dependence of the luminescence intensity on excitation energy clearly shows a profile similar to the absorption spectrum of Si. These results indicate that luminescence is not due to the direct optical excitation of cyanine by the light transmitted through the substrate but due to the energy transfer from the photoexcited carriers in the substrate. Our results demonstrate that such energy transfer can be used to excite organic molecules on inorganic substrates without energy matching between the electrodes and luminescent materials.

This study investigates electron diffusion based on the observation of luminescence from a cyanine thin film fabricated on a semiconductor substrate. When the backside of the substrate was excited by laser light, enlarged intensity profiles, 20 times larger than the laser spot, were observed. An analysis of the spatial enlargement suggests that the enlargement originated from electron diffusion. This method can be used for analyzing electron transport and contributes to the development of emission devices based on energy transfer.