This dissertation addresses the following issues. Firstly, to reduce the efficiency roll-off at high current densities or brightness, of green, fluorescent organic light-emitting (OLEDs), we introduced a double-doped structure into the emissive layer. It includes two thin buffer layers and a broad emissive region stacked with two doped layers. This modification better controls charge injection/transport and recombination, boosting EL and PL efficiency. Secondly, aiming for highly efficient phosphorescent OLEDs surpassing the theoretical EQE limit of ~20%, a new class of platinum(II)-based phosphorescent complexes have been designed and synthesized serving as both emitters and electron transporters in straightforward undoped bi-/tri-layered devices. Achieving this without costly doping techniques, these OLEDs boast a relatively low turn-on voltage, extremely high power efficiency, and stable emission color dependent on applied voltages. This design anticipates reduced or no efficiency roll-off even at brightness levels exceeding 20,000 cd/m2, far surpassing DOE technology requirements (only 500-1500 cd/m2). This work sheds light on the influence of molecular design on crystalline packing and optoelectronic device performance and accelerates the development of efficient and stable Pt-based emitters.