Dongxin Ma sought to deepen understanding of why perovskites have failed to progress in LEDs at the needed rate. Reduced-dimensional perovskite films form with a mixture of distribution and bandgap. The presence of unsaturated sites at the perovskite grain boundaries, and of thin, strongly-confined perovskites, in an otherwise energetically monodispersed distribution, are both what limits device performance. Carriers injected into the traps or into the strongly-confined perovskites are unable to transfer to lower bandgap domains and they then recombine nonradiatively. These observations motivated her to pursue a route to reduce the trap density and to delete the strongly-confined perovskite population from active layers.
Ma discovered that overly rapid quantum well formation during film synthesis results in undesired traps and strongly-confined reduced-dimensional perovskitess. She then designed a series of new molecular additives with N=O, S=O, P=O, or As=O, to passivate the perovskite grain boundaries by forming coordination bonds with the Pb(2+) cations.
Among all, triphenylphosphine oxide (TPPO) worked best. TPPO-treated reduced-dimensional perovskites showed a high photoluminescence quantum yield (PLQY) of 97% and an external quantum efficiency (EQE) of 14.0% in devices, with a half-life of 33 hours at an initial luminance of 100 cd m(-2).
Ma further designed a bifunctional additive, tris(4-fluorophenyl)phosphine oxide (TFPPO) – that would simultaneously achieve two goals: at one end it would provide the perovskite-passivating moiety, P=O and at the other, the functional group, fluorine atom would bind with the organic cations used to reduce perovskite dimensionality, thereby regulating their release into the reaction during film formation and preventing the formation of strongly-confined wells.
She achieved, as a result, perovskite active layers with monodispersed distribution and near-unity PLQY, enabling LEDs with an EQE of 25.6% and an operating half-life of two hours at an initial luminance of 7,200 cd m(-2), indicating tenfold-enhanced operating stability relative to the reported perovskite LEDs with an EQE exceeding 20%.
Since the current device performance achieved with blue perovskite LEDs is considerably lower than that of green and red devices, Ma also demonstrated a chelating-agent-assisted control strategy and a chloride insertion-immobilization strategy, therefore achieving blue LEDs with improved EQEs and operating stability.