Organic semiconductor heterojunction as charge generation layer in tandem organic light-emitting diodes for high power efficiency

High-performance tandem organic light-emitting diodes (OLEDs) employing a buffer-modified C₆₀/pentacene organic semiconductor heterojunction (OHJ) as a charge generation layer (CGL) are demonstrated. The unique cooperation of charge generation, transport, and extraction processes occurred in the OHJ-based CGL remarkably reduces the operational voltage. As a result, an approximately twofold enhancement in power efficiency (21.9 lm W^-1 VS 10.1 lm W^-1) can be achieved that has previously been suggested to be difficult for tandem OLEDs. When the pentacene is replaced by zinc phthalocyanine (ZnPc), copper phthalocyanine (CuPc), or phthalocyanine (H ₂Pc), a similar power efficiency improvement can be also achieved. The novel design concept of the buffer-modified OHJ-based CGL is superior to that of the conventional CGLs. The investigations on the operational mechanism are performed, from which it is found that the mobile charge carriers firstly are needed to be accumulated at both sides of the heterojunction interface and then transport along the two organic semiconductors in terms of their good carrier transport characteristics under an external electrical field, and finally inject into the corresponding electroluminescent (EL) units by the interfacial layers.