Effects of exciplex on the electroluminescent and photovoltaic properties of organic diodes based on terbium complex

We fabricated two organic diodes, one of which consists of a double layer structure of TPD/Tb(ACA)3phen and in the other one a mixture layer is inserted between the double layer, i.e., TPD/TPD:Tb(ACA)3phen (1:1, 30 nm)/Tb(ACA)3phen, here TPD and Tb(ACA)3phen are (N,N′-diphenyl-N,N′-bis(3-methyl-phenyl)-1,1′-biphenyl-4,4′-diamine) and tris(acetylacetonato)-(mono-phenothroline) terbium, respectively. Both the devices show electroluminescence (EL) properties under forward bias and photovoltaic (PV) effects under illumination of ultraviolet (UV) light. For the device with a mixture layer, the EL performance and PV effects were both significantly improved. A maximum EL brightness of 150 cd/m2 under bias of 17 V and a maximum efficiency of 1.1 cd/A at 7.5 V were obtained. Moreover, the diode shows a short-circuit current (Isc) of 43 μA cm−2, an open-circuit voltage (Voc) of 1.1 V, a fill factor (FF) of 0.32, and an overall power conversion efficiency (ηPV) of 1.0% under illumination of 365 nm UV light with 1.5 mW/cm2. The improvements of PV- and EL-properties were presumably attributed to the increased intermolecular contacts in the mixture of TPD and Tb-complex. In addition, a shift of EL color from UV-blue to green-yellow was also observed when a mixture layer of TPD with Tb-complex was inserted. The operation mechanisms of the EL- and the PV-processes of the diodes with different structures were further discussed.