Hidden reaction pathways inform additive engineering for perovskite LEDs.

Perovskite light-emitting diodes (PeLEDs) have emerged as a promising class of solution-processed light-emitting devices, combining high efficiency with facile fabrication1-5. Amino-acid-based molecular additives are widely used to regulate crystallization and suppress defects in perovskite thin films6-8, yet these molecules are chemically reactive in precursor solutions, in which their transformations and products remain poorly understood. Here we resolve and control the hidden solution-phase chemistry of a ubiquitous additive, 5-aminovaleric acid (5AVA), whose solution-phase reactivity has remained unclear6,9-15. We show that 5AVA undergoes competing in situ reactions during precursor preconditioning, yielding two chemically and functionally distinct products: a beneficial organoamidinium zwitterion and a detrimental Lewis-base lactam. By explaining this reaction dichotomy and establishing targeted reaction-pathway control, we selectively use the beneficial species. Direct synthesis and incorporation of the purified zwitterion decouple its intrinsic passivation function from confounding by-products, yielding perovskite films with a near-unity mean peak photoluminescence quantum yield (PLQY) of 97.2% and PeLEDs with a champion external quantum efficiency (EQE) of 37.1%, alongside a mean peak EQE of 32.8%. Finally, we demonstrate that this reaction-pathway control concept can be extended to further additive chemistries and emission colours, highlighting its potential as a chemistry-driven strategy for rational additive design in PeLEDs.