Application of Fluorescent Molecules as Noninvasive Sensors for Optoelectronic Characterization on Nanometer Length Scales

Determining noninvasively the microscopic optoelectronic parameters of molecular assemblies would constitute an important achievement in material as well as in life sciences. In this contribution, we cope with this challenge by utilizing fluorescent tetraphenyldibenzoperiflanthene (DBP) molecules as optically addressable sensors deterministically positioned in N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine/tris(8-hydroxyquinolato)aluminum (NPB/Alq3) organic light-emitting diodes (OLEDs) as a model system. Measuring in operando the variation of the fluorescence intensity allows for direct correlation with the respective charge carrier distribution and transport processes at spatial resolutions below 15 nm as confirmed by complementary drift-diffusion simulations. Moreover, the molecular sensing technique renders sensitive enough to detect the presence of mobile charge carriers already below the built-in voltage. Operating the OLEDs under reverse bias, the sensing molecules provide information about the internal interface polarization caused by the alignment of the dipolar Alq3 entities during sample preparation. Benchmarking the macroscopic device behavior by means of electrical impedance measurements confirms the accuracy and reliability of the microscopic data obtained by the molecular sensing approach. Furthermore, the comparison demonstrates the dynamic range of the optical sensors being susceptible to variations in the local charge carrier distribution over several orders of magnitude and to interfacial polarization occurring at molecular heterojunctions within the OLED matrix. By its universality the presented sensing concept is applicable to a variety of molecular systems and, thus, being of relevance not only for OLEDs but also for other organic thin film devices like transistors.