Perovskite light-emitting diodes (PeLEDs), with their significant advantages such as low material cost, high luminous brightness, and tunable emission colors, have become highly promising candidates for next-generation display and lighting technologies. Since their early development, PeLEDs have achieved remarkable performance breakthroughs. This leap stems not only from innovations in the emissive layer material itself, but more importantly from the synergistic effects of overall device structure optimization, enhanced carrier injection and recombination efficiency, and advancements in interface engineering. Advances in interface engineering have effectively reduced energy loss and passivated defects. In this context, the hole transport layer (HTL) located between the emissive layer and the anode plays a crucial role. It directly determines the hole injection efficiency, non-radiative recombination loss at the interface, and the overall operational stability of the device. Therefore, in-depth research and optimization of the HTL are essential for further improving the efficiency and lifetime of PeLEDs, a key step in accelerating the transition of this technology from laboratory research to practical applications in displays, lighting, and bioimaging.
In the p-i-n structure of blue PeLEDs, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate is widely used as a hole transport material due to its high hole mobility, good optical transparency, and solution processability. However, PEDOT:PSS exhibits significant limitations in blue PeLEDs: its energy level mismatch with the perovskite active layer leads to a high hole injection barrier and severe nonradiative recombination; its inherent hygroscopicity introduces environmental moisture, accelerating the degradation and phase separation of the perovskite material; simultaneously, its conductivity is susceptible to processing conditions and environmental factors, resulting in unstable device performance and significant efficiency variations.
To address these bottlenecks, introducing a polymer interlayer between the HTL and perovskite interface to construct a functional bridging layer has become an effective systematic solution. This interlayer structure allows for precise bandgap modulation to achieve efficient hole injection, utilizes molecular-level interface passivation to suppress nonradiative recombination, and establishes a chemically inert barrier to mitigate destructive reactions, thereby synergistically enhancing photoelectric conversion efficiency and device lifetime. Among various options, poly(N-vinylcarbazole) (PVK) often outperforms other polymer hole transport materials due to its excellent film-forming ability, which endows it with superior interface quality and stability. Nevertheless, the inherently low carrier mobility of PVK remains a key bottleneck. Despite attempts to improve charge transport capabilities through doping or additive engineering, overcoming the limitations imposed by the electronic structure of the polymer backbone remains challenging. Therefore, while retaining the existing interface modulation advantages of PVK, there is an urgent need to develop novel polymer structures with high mobility through innovative molecular design.
Previous work reported the undoped polymer HTM, a “polyvinylcarbazole-based polymer,” constructed by combining a non-conjugated polyethylene backbone with carbazole-based “A-type” side chains. When used as a bridging layer between PEDOT:PSS and perovskite, this structural design effectively modulates energy levels, promotes hole transport and its alignment with the perovskite layer, and suppresses nonradiative recombination. Sky-blue PeLEDs (emission wavelength 488 nm) based on this structure exhibited an operating voltage of 3 V and a maximum external quantum efficiency of 3.26%, a 1.27-fold improvement compared to devices without the bridging layer. These performance enhancements strongly validate the superiority of the strategy combining the non-conjugated backbone with A-type nanomesh aromatics. Theoretical studies have shown that introducing strong electron-withdrawing groups (such as cyano, -CN) into the PVK molecular backbone can optimize interfacial charge extraction efficiency by enhancing the molecular dipole moment and improve film stability through intermolecular dipole-dipole interactions.
Therefore, to further explore the potential of the "molecular meshing" strategy and improve device performance, Xie Linghai et al. from Nanjing University of Posts and Telecommunications, while retaining this core strategy, introduced cyano groups to construct a donor-acceptor structure, designing and synthesizing a cyano-functionalized type A nanomesh aromatic polymer, P-CzCN. Experimental characterization shows that P-CzCN exhibits significantly improved hole mobility and excellent defect passivation ability. Combining theoretical calculations and multi-scale characterization, this work systematically elucidates the synergistic regulation mechanism of cyano modification on molecular stacking behavior, carrier transport paths, and interfacial energy level alignment. Blue PeLEDs with P-CzCN bridging layers achieved a maximum luminance of 4040 cd m⁻² and an external quantum efficiency of 5.39% at 488 nm. Under different voltages, the electroluminescence spectrum consistently centers at 488 nm, exhibiting excellent spectral stability. P-CzCN provides an important example for the functionalization of grid-based HTM and is of great significance for advancing the practical application of blue PeLED technology.
