Liu, B.Altıntaş, YemlihaWang, L.Shendre, S.Sharma, ManojSun, H.Mutlugün, EvrenDemir, Hilmi Volkan2020-02-102020-02-1020200935-9648http://hdl.handle.net/11693/53202Colloidal quantum wells (CQWs) are regarded as a highly promising class of optoelectronic materials, thanks to their unique excitonic characteristics of high extinction coefficients and ultranarrow emission bandwidths. Although the exploration of CQWs in light‐emitting diodes (LEDs) is impressive, the performance of CQW‐LEDs lags far behind other types of soft‐material LEDs (e.g., organic LEDs, colloidal‐quantum‐dot LEDs, and perovskite LEDs). Herein, high‐efficiency CQW‐LEDs reaching close to the theoretical limit are reported. A key factor for this high performance is the exploitation of hot‐injection shell (HIS) growth of CQWs, which enables a near‐unity photoluminescence quantum yield (PLQY), reduces nonradiative channels, ensures smooth films, and enhances the stability. Remarkably, the PLQY remains 95% in solution and 87% in film despite rigorous cleaning. Through systematically understanding their shape‐, composition‐, and device‐engineering, the CQW‐LEDs using CdSe/Cd0.25Zn0.75S core/HIS CQWs exhibit a maximum external quantum efficiency of 19.2%. Additionally, a high luminance of 23 490 cd m−2, extremely saturated red color with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.715, 0.283), and stable emission are obtained. The findings indicate that HIS‐grown CQWs enable high‐performance solution‐processed LEDs, which may pave the path for future CQW‐based display and lighting technologies.EnglishColloidal quantum wellsCore/shell structuresHot injectionLight‐emitting diodesNanoplateletsArticle10.1002/adma.201905824