Highly efficient HAT-CN/rubrene intermediate connectors for tandem organic light-emitting diodes: probing charge generation and separation via magnetic field effects

Abstract Tandem organic light-emitting devices (OLEDs) are promising for their high efficiency and operational stability. In this study, high-performance tandem OLEDs were fabricated using 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN)/5,6,11,12-tetraphenylnaphthacene (rubrene) as an efficient organic heterojunction intermediate connector. The optimized device achieves a luminance of 8441.90 cd m −2 and a current efficiency of 8.39 cd A −1 at 100 mA cm −2 , which are 2.56 and 2.58 times those of a single-unit device. Furthermore, when the HAT-CN thickness was varied from 10 to 25 nm, the efficiency of the tandem devices consistently remained at least twice that of single-layer devices, demonstrating excellent process tolerance. The device also exhibits suppressed efficiency roll-off, maintaining 8.57 cd A −1 at 10 000 cd m −2 –3.76 times higher than the reference device. Crucially, we report the manipulation of the magnetoconductance (MC) sign in tandem OLEDs via the ground-state charge-transfer (GSCT) interface. Unlike the positive MC in conventional single-unit devices at low magnetic fields, our tandem OLEDs show a distinctive negative MC, originating from the reverse intersystem crossing (RISC) process within weakly bound polaron pairs at the GSCT interface. We demonstrate that the strength of the RISC process can be tuned by varying the n-layer thickness, bias voltage, and temperature, thereby regulating the charge generation efficiency of the connector and the resultant exciton dynamics—including S ₁/ T ₁ concentration, T ₁ lifetime, and the strength of intersystem crossing and triplet–charge annihilation processes. This work provides profound insights into the spin-related exciton processes in tandem OLEDs, highlighting the critical role of the intermediate connector for developing advanced electroluminescent devices.