Enhanced Efficiency of Tandem Organic Light-Emitting Diodes via Manipulating Heterojunction Composition of Charge Generation Unit

Tandem organic light-emitting diodes (OLEDs) with an organic photovoltaic heterojunction (OPV-HJ) as a charge generation unit (CGU), consisting of fullerence (C-60) and copper phthalocyanine (CuPc), are fabricated. The device structures are ITO/EL unit/C-60-CuPc HJ/MoO3/EL unit/AI, where EL unit is composed of NPB/Alq(3)/LiF. The efficiency improvement of tandem OLEDs is achieved by manipulating the structure and composition of C-60-CuPc HJs. The charge generation and transport characteristics of CGUs are strongly dependent on the detailed structures of C-60-CuPc HJs and the optimized CGU is 0.5:1 C-60: CuPc bulk-HJ. The improved efficiency of the tandem device with 0.5:1 C-60:CuPc bulk-HJ as CGU demonstrates to be the result of better charge carrier balance induced by the optimal charge transport and charge generation of C-60:Cu Pc bulk-HJs according to their surface morphology and absorption spectra. These results shed light on that C-60:CuPc OPV-HJ-based CGU is a promising connection unit for the fabrication of high-efficiency tandem OLEDs.