Solution-processed metal halide perovskites have emerged as highly promising materials for light-emitting devices such as LEDs and lasers. The external quantum efficiency (EQE) of perovskite LEDs (PeLEDs) has increased rapidly since its first demonstration, and optically pumped perovskite lasers under continuous-wave operation at room temperature has been achieved. Although much research has been devoted to improving the EQE of PeLEDs, their efficiency starts to drop significantly under high injection current density (J). Suppressing efficiency roll-off at high J is required for achieving ultra-bright LEDs, and ultimately, electrically driven lasers since the key parameter of J×EQE or luminance needs to be large enough to exceed a threshold for lasing. The detrimental behavior of efficiency roll-off is likely caused by a combination of charge injection imbalance, Auger-induced luminescence quenching and Joule heating. This phenomenon, also known as efficiency droop, can be quantified by the critical current density (J_c) at which the EQE reduces to half of its peak value. In this talk, we discuss our work in improving J_c and maximum luminance of PeLEDs. We first optimize the hole transport layers for balancing charge injection in quasi-2D PeLEDs, and achieved simultaneously high EQE (16.2%) and luminance (~31,000 cd/m²). Nevertheless, we ran into a limit in increasing J_c of quasi-2D PeLEDs, likely due to the poor charge transport with the insulating long-chain PEABr layer and increased non-radiative Auger recombination rate originated from the enhanced local charge carrier density in the multiple quantum well structures. These motivated us to further contrast the performance of quasi-2D and 3D perovskites. 3D perovskites generally can withstand higher J; however, their photoluminescence quantum yield (PLQY) is usually lower due to small exciton binding energy. We explored surface passivation to improve the PLQY of 3D perovskites by introducing excess KBr during CsPbBr₃ synthesis. The resulting luminance improves by about 4-fold to ~120,000 cd/m² and J_c increases by 20-fold to ~800 mA/cm². Subsequently, we investigated thermal-induced luminescence quenching and fabricated current-focusing devices with nanoscale current injection areas. Using pulsed current injection, we increased J_c up to ~60 A/cm² while operating the device at J values up to ~1 KA/cm² without measurable damage to the device, ultimately leading to a luminance of 7.6 Mcd/m². We further explored the effect of substrate heat conductivity by comparing glass and sapphire substrate based PeLEDs, and showed that the device performance can be further improved using sapphire substrates.