Light Emission Mechanisms of Solid-Solution Host Crystalline Organic Light-Emitting Diodes

Crystalline organic light-emitting diodes (C-OLEDs) have captured progressively extensive attention in the field of optoelectronic devices due to their lower driving voltage, higher current density, and increased brightness in comparison with amorphous OLEDs. In particular, solid solutions, which can integrate multiple components while maintaining the crystal structure of the matrix, making it an advantageous strategy for promoting the development of C-OLEDs. However, the luminescence mechanisms and processes involved are not yet fully understood. In this study, we systematically investigated the luminescence mechanisms of C-OLEDs using a unipolar crystalline host (2FPPICz), a bipolar organic solid-solution (Bp-OSS) host (2FPPICz:2Fn in a 10:2 ratio), and emitting guests with varying energy level distributions. Conductivity measurements of the emitting layer films revealed that the energy levels of the guest materials significantly influence the transport behaviors of electrons and holes. In the Bp-OSS, electrons primarily transport through the 2Fn pathway, while holes migrate via the 2FPPICz channel. Furthermore, transient electroluminescence (TrEL) and electroluminescence (EL) performance tests were conducted on C-OLEDs with different guests. The TrEL spikes observed after applying a reverse bias pulse indicate distinct charge distribution and recombination processes within the emitting layer. The comparison of the EL performance shows that in the single-host (2FPPICz) system, the luminescence process is dominated by energy transfer, whereas in the Bp-OSS host system, charge trapping dominates. For efficient emission in host–guest doping system based on organic solid solutions (OSS), the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels of the guest should fall within the energy range of the host. This work provides a crucial guidance for designing high-performance C-OLED host–guest doping systems.