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Suppressing Förster Resonance Energy Transfer in Close‐Packed Quantum‐Dot Thin Film: Toward Efficient Quantum‐Dot Light‐Emitting Diodes with External Quantum Efficiency over 21.6%

Colloidal II–VI quantum dots (QDs) exhibit near‐unity photoluminescence (PL) quantum yield (QY) when they are dissolved in dilute solution. However, when they are assembled into thin film, the PL QY is decreased significantly due to the presence of nonradiative Förster resonance energy transfer (FRET) among the close‐packed QDs. In this work, the FRET is suppressed by developing a binary‐QD light emission layer (EML), where blue‐QDs are introduced as spacers to spatially separate the red‐QDs. Due to the separation of red‐QDs, the FRET among red‐QDs is effectively suppressed and thus the emission properties of red‐QDs are well preserved. As a result, the PL QY of red‐QD thin film is enhanced by 1.5 times. When the binary‐QD EML is integrated into devices, the resultant QD light‐emitting diodes (QLEDs) exhibit a high external quantum efficiency of 21.64%. The work offers a new, simple and effective approach to suppress the FRET and improve the QY of QD thin film, which may open up a new avenue for developing efficient QLEDs with near‐unity internal quantum efficiency.