Determination of exciton diffusion lengths potentially free from electrodes and charge related inaccuracies

Exciton diffusion length is one of the key parameters of an organic semiconductor thin film, and plays an important role in optimizing the performance of organic optoelectronic devices. By the existing photoelectronic methods for measuring the exciton diffusion length, vertical diode structure is used, which may lead to inaccuracies resulting from the electrode and charge related effects, such as enhanced quenching of excitons near the metal/organic interface and annihilation of excitons via charge-exciton interaction. Herein we propose a novel method for measuring the exciton diffusion length by incorporating the testing organic film into an organic phototransistor. In the device sample, the testing organic film is free from electrode contact and electric filed, enabling measurement free from errors related to the effects of metal electrodes. More importantly, in our method the generation and diffusion of excitons is fully isolated from photocarrier transport, eliminating the error associated with charge-exciton interactions. For demonstration, the exciton diffusion length of tin phthalocyanine (SnPc) thin film was determined to be 14.03 ± 2.76 nm, which is in good agreement with that reported in literatures.