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

Herein, we report mixed Ruddlesden–Popper (RP) (quasi-2D) perovskite–perovskite planar heterojunctions at the hole transport layer (HTL) and perovskite emission layer (EML) interface, facilitating efficient perovskite light emitting diode (Pe-LED) devices. A chloride-based quasi-2D layer of formamidinium lead chloride (FAxPbCl3) acts as an HTL, and a bromide-based quasi-2D layer of PEA2(FAPbBr3)n−1PbBr4 with potassium bromide (KBr) additive acts as a light emission layer (EML). The solution-processed quasi-2D perovskite–perovskite planar heterojunction between the HTL and EML interface has an ultrathin interlayer of poly(9,9-bis(3′-(N,N-dimethyl)-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) (PFN-P1), which prevents either halide intermixing at the perovskite–perovskite interface or any dissolution of the underneath perovskite HTL by polar solvents used in processing the EML, resulting in a conformal quasi-2D perovskite–perovskite heterojunction. More importantly, the mixed dimensional perovskite HTL transmittance spectral range can be extended, which corresponds to low absorption especially in the deep blue and UV region of the electromagnetic spectrum, by introducing an excess amount of formamidinium chloride (FACl). When translated to a device, the optimized Pe-LED devices having a quasi-2D perovskite–perovskite planar heterojunction between the HTL and EML have superior hole transport and hole injection due to a higher charge carrier mobility and favorable energetic alignment, outperforming their counterparts based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The optimized Pe-LED devices have current and power efficiencies of 38 cd A–1 and 31 lm W–1, an EQE of 9.3%, a turn-on voltage of 3 V, and a high brightness of 11 500 cd m–2, together with high reproducibility. They emit a saturated green electroluminescence (EL) with an fwhm of 24 nm, having CIE coordinates (0.17, 0.75). Significantly, the Pe-LED devices with a quasi-2D perovskite HTL/EML heterojunction show remarkable EL stability of about ∼30 min at 100 cd m–2, which is 4 times higher than that of Pe-LEDs with a PEDOT:PSS HTL/perovskite EML heterojunction.