Metal halide perovskite semiconductors have demonstrated remarkable potentials in solution‐processed blue light‐emitting diodes (LEDs). However, the unsatisfied efficiency and spectral stability responsible for trap‐mediated non‐radiative losses and halide phase segregation remain the primary unsolved challenges for blue perovskite LEDs. In this study, it is reported that a fluorene‐based π‐conjugated cationic polymer can be blended with the perovskite semiconductor to control film formation and optoelectronic properties. As a result, sky‐blue and true‐blue perovskite LEDs with Commission Internationale de l'Eclairage coordinates of (0.08, 0.22) and (0.12, 0.13) at the record external quantum efficiencies of 11.2% and 8.0% were achieved. In addition, the mixed halide perovskites with the conjugated cationic polymer exhibit excellent spectral stability under external bias. This result illustrates that π‐conjugated cationic polymers have a great potential to realize efficient blue mixed‐halide perovskite LEDs with stable electroluminescence.

Perovskite light-emitting diodes (LEDs) have attracted broad attention due to their rapidly increasing external quantum efficiencies (EQEs) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] . However, most high EQEs of perovskite LEDs are reported at low current densities (< 1 mA cm -2 ) and low brightness. Decrease in efficiency and rapid degradation at high brightness inhibit their practical applications. Here,

Neutral π-radicals have potential for use as light emitters in optoelectronic devices due to the absence of energetically low-lying non-emissive states. Here, we report a defect-free synthetic methodology via mesityl substitution at the para-positions of tris(2,4,6-trichlorophenyl)methyl radical. These materials reveal a number of novel optoelectronic properties. Firstly, mesityl substituted radicals show strongly enhanced photoluminescence arising from symmetry breaking in the excited state. Secondly, photoexcitation of thin films of 8 wt% radical in 4,4’-bis(carbazol-9-yl)-1,1’-biphenyl host matrix produces long lived (in the order of microseconds) intermolecular charge transfer states, following hole transfer to the host, that can show unexpectedly efficient red-shifted emission. Thirdly, covalent attachment of carbazole into the mesitylated radical gives very high photoluminescence yield of 93% in 4,4’-bis(carbazol-9-yl)-1,1’-biphenyl films and light-emitting diodes with maximum external quantum efficiency of 28% at a wavelength of 689 nm. Fourthly, a main-chain copolymer of the mesitylated radical and 9,9-dioctyl-9H-fluorene shows red-shifted emission beyond 800 nm.

Hybrid halide perovskites are promising candidates for next-generation optoelectronic applications. They show heterogeneity at multiple lengthscales in their structural, chemical and optoelectronic properties [1]. Characterization techniques capable of revealing the structure-property relations are key to further understand this family of new materials.