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/MoO 3 /EL unit/Al, 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 :CuPc 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.