The outstanding excitonic properties, including photoluminescence quantum yield (η_PL), of individual, quantum-confined semiconductor nanoparticles are often significantly quenched upon aggregation, representing the main obstacle towards scalable photonic devices. Here we report aggregation-induced emission (AIE) phenomena in the lamellar solids containing layer-controlled colloidal quantum wells (CQWs) of hybrid organic-inorganic lead bromide perovskites, resulting in anomalously high solid-state of up to 94%. Upon forming the QW solids, we observe an inverse correlation between exciton lifetime and , clearly distinct from that in typical quantum dot solid systems. Our multiscale theoretical analysis reveals that in a lamellar solid, the collective motion of the surface organic cations are more restricted to orient along the [100] direction, thereby inducing a more direct bandgap that facilitates radiative recombination. Using the QW solids, we demonstrate ultra-pure green emission by completely downconverting a blue GaN light emitting diode (LED) at room temperature, with a luminous efficacy higher than 90 lm/W at 5,000 cd/m², which has never been reached in any nanomaterial assemblies by far [1].

[1] Jagielski et al., Science Advances, accepted manuscript (2017)