Realizing the stable spectrum in four-chromatic white organic light-emitting diodes by controlling the positions of various emitters in the bipolar interlayer

Maintaining a stable spectrum in white organic light-emitting diodes (WOLEDs) with multi-emissive layers (EMLs) remains a challenge. Herein, four-chromatic (blue-green-red-orange) WOLED with stable spectrum based on ultrathin EMLs and mixed bipolar interlayers was fabricated. We discovered that the locations of various color EMLs in the bipolar interlayers are essential for achieving the stable spectrum. The direct carrier trapping effects which destabilize the spectrum can be suppressed by placing red and green EMLs with both hole and electron traps in the middle of the bipolar interlayer and placing blue and orange EMLs with single-carrier traps on the two sides of the bipolar interlayer respectively. The resulting device exhibited the negligible color coordinate shifts of (0.008, 0.010) during the wide brightness range from 1000 cd m(-2) to 10 000 cd m(-2). Moreover, a high color rendering index of approximately 90 was obtained simultaneously. Our work demonstrated a significant method to achieve stable spectra in multi-EML WOLEDs based on bipolar interlayer.