Fig. 1. （a） Schematic diagram of blue quasi-2D perovskite crystal structure， （b） schematic diagram of energy transfer pathways in quasi-2D perovskite quantum well^［23］， （c） energy band structure schematic diagram of common carrier transport layer and light-emitting layer^［30］

Fig. 2. （a） Schematic diagram representing the strategy adopted to form intermediate n phases for blue emission^［32］， TA spectra of pristine （b） and GA10 perovskite film （c）^［33］， （d） UV-Vis absorption and steady-state PL spectra of PEA₂（Rb _x Cs_1-x ）₂Pb₃Br₁₀^［34］，（e） electronic density of states （DOS） of CsPbBr₃， Cs_0.75EA_0.25PbBr₃， and Cs_0.5EA_0.5PbBr₃^［35］

Fig. 3. （a） PL spectra of perovskite PEA₂Cs_1.5Pb_2.5Br_8.5 with 0%~60% IPABr additive^［36］， （b） TA spectra of PEA₂Cs_1.5Pb_2.5Br_8.5 with 0% and 40% IPABr^［36］， （c） the integrated intensity-q relations of GIWAXS patterns for the CsPbClBr₂ nanocrystal films with different ratios between DPPABr and PEABr^［37］， （d） schematic diagram of low-dimensional components engineering of P-PDABr₂^［38］， UV-Vis spectra （e） and PL spectra （f） of different ligand treated quasi-2D perovskite film^［39］

Fig. 4. GIWAXS images of control （a） and 8%-PPNCl （b） blue quasi-2D perovskite film^［40］， （c） schematic diagram of defect passivation and low-dimensional phase regulation of DFBP in the pristine and target perovskites^［41］， （d） PDOS curves of the pristine perovskite， perovskite containing a chloride vacancy and renovated by C＝O group^［42］， （e） PDOS curves of the pristine perovskite， perovskite containing a lead-chloride defect and renovated by hydroxy group^［42］， （f） schematic and DFT calculated destabilization energy of PbBr₂ coordinated with GABA and PEA binding to the surface^［43］

Fig. 5. （a） UV-Vis absorption and PL spectra of quasi-2D perovskite films without and with 20% NaBr^［44］， TA spectra UV-Vis of quasi-2D perovskite films without （b） and with 20% NaBr （c）^［44］， （d） schematic illustrated the rearrangement of phase distribution by adding Na⁺ in quasi-2D perovskites^［44］， UV-Vis absorption spectra （e） and TRPL curves （f） of quasi-2D perovskite films spin-coated on pristine and alkali-treated PEDOT∶PSS films^［45］， （g） PLQY of LiX incorporated blue， green， and red perovskite films^［46］

Fig. 6. In-situ GIWAXS images for the control （a） and DMSO steam-treated （b） quasi-2D phases of perovskite film^［47］， （c） schematic illustration of the suppressed energy transfer losses for the rearranged phase distribution^［47］， （d） schematic diagrams of the vertical domain distribution in the control and target film^［48］， the absorption and normalized PL spectra of the control （e） and thermal gradient treated （f） quasi-2D perovskite film^［48］

Fig. 7. （a） Schematic diagram of rearrangement of the phase distribution of quasi-2D perovskites after CsCl diffusion^［50］， TA kinetics probed at selected wavelengths in quasi-2D perovskites without （b） and with （c） CsCl incorporation^［50］， GIWAXS diffraction patterns of perovskite films on pristine （d） and TEOS-modified （e） PVK layers at different annealing times^［51］

Fig. 8. GIWAXS images of perovskite films grown on the control （a） and PS-modified （b） PEDOT∶PSS layers^［52］， （c） steady-state PL spectra and （d） PLQYs of control and PS-modified perovskite films^［52］， （e） schematic illustration of the formation of n-phase due to different absorption energy between the PBA⁺/Cs⁺ and ［PbBr₆］^4-［53］， （f） DFT calculation results of the absorption energy between the PBA⁺/Cs⁺ and ［PbBr₆］^4-［53］， GIWAXS images of pristine perovskite （g） and GASCN modified （h） perovskite film^［53］， UV-Vis absorption and TA spectra with different time scales of pristine （i） and modified （j） perovskite films^［54］