Organic semiconductor heterojunctions as charge generation layers and their application in tandem organic light-emitting diodes for high power efficiency

Organic light-emitting diodes (OLEDs) can convert injected charge into photons. The upper conversion limit of single-unit OLEDs is at most one photon per injected electron. Alternatively, tandem OLEDs that vertically stack several single- unit OLEDs via charge generation layers (CGLs) can convert one injected electron into multiple photons, thus achieving greater brightness and current efficiency with a lower current. However, since the driving voltage consumed by conventional tandem devices scales linearly with the number of electroluminescent units, the resulting power consumption would be the same for both the single- unit and tandem OLEDs to obtain the same luminescence; this means that the power efficiency cannot be greatly increased for such tandem devices. Recently we found that using buffer-modified intrinsic organic semiconductor heterojunctions (OHJs) as the CGLs significantly enhanced the power efficiency of the fabricated tandem OLEDs, which was previously suggested to be difficult for tandem devices. In this feature article, we review the recent advances of tandem OLEDs based on OHJs as the CGLs, including the design concept and basic requirements of the energy levels and mobility of involved organic semiconductors. Our results show that the use of OHJs as CGLs is a universal concept for the fabrication of tandem OLEDs with high power efficiency. We believe that the concept of OHJs opens new perspectives for the rational design of CGLs to realize tandem devices with unprecedented improvement in power efficiency. In particular, it has great potential for use in the fabrication of tandem white OLEDs (WOLEDs) for solid-state-lighting.