Efficient Perovskite Light Emitting Diodes Enabled\nby Mixed Ruddlesden–Popper (RP) Perovskite–Perovskite\nPlanar Heterojunction between Hole Transport and Emission Layers

Herein,\nwe report mixed Ruddlesden–Popper (RP) (quasi-2D)\nperovskite–perovskite planar heterojunctions at the hole transport\nlayer (HTL) and perovskite emission layer (EML) interface, facilitating\nefficient perovskite light emitting diode (Pe-LED) devices. A chloride-based\nquasi-2D layer of formamidinium lead chloride (FA<sub><i>x</i></sub>PbCl<sub>3</sub>) acts as an HTL, and a bromide-based quasi-2D\nlayer of PEA<sub>2</sub>(FAPbBr<sub>3</sub>)<sub><i>n</i>−1</sub>PbBr<sub>4</sub> with potassium bromide (KBr) additive\nacts as a light emission layer (EML). The solution-processed quasi-2D\nperovskite–perovskite planar heterojunction between the HTL\nand EML interface has an ultrathin interlayer of poly(9,9-bis(3′-(<i>N</i>,<i>N</i>-dimethyl)-propyl-2,7-fluorene)-<i>alt</i>-2,7-(9,9-dioctylfluorene)) (PFN-P1), which prevents\neither halide intermixing at the perovskite–perovskite interface\nor any dissolution of the underneath perovskite HTL by polar solvents\nused in processing the EML, resulting in a conformal quasi-2D perovskite–perovskite\nheterojunction. More importantly, the mixed dimensional perovskite\nHTL transmittance spectral range can be extended, which corresponds\nto low absorption especially in the deep blue and UV region of the\nelectromagnetic spectrum, by introducing an excess amount of formamidinium\nchloride (FACl). When translated to a device, the optimized Pe-LED\ndevices having a quasi-2D perovskite–perovskite planar heterojunction\nbetween the HTL and EML have superior hole transport and hole injection\ndue to a higher charge carrier mobility and favorable energetic alignment,\noutperforming their counterparts based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)\n(PEDOT:PSS) HTL. The optimized Pe-LED devices have current and power\nefficiencies of 38 cd A<sup>–1</sup> and 31 lm W<sup>–1</sup>, an EQE of 9.3%, a turn-on voltage of 3 V, and a high brightness\nof 11 500 cd m<sup>–2</sup>, together with high reproducibility.\nThey emit a saturated green electroluminescence (EL) with an fwhm\nof 24 nm, having CIE coordinates (0.17, 0.75). Significantly, the\nPe-LED devices with a quasi-2D perovskite HTL/EML heterojunction show\nremarkable EL stability of about ∼30 min at 100 cd m<sup>–2</sup>, which is 4 times higher than that of Pe-LEDs with a PEDOT:PSS HTL/perovskite\nEML heterojunction.