The concept of tandem organic light-emitting devices (OLEDs) provides a pathway for developing highly stable and efficient OLEDs. The connecting structure that bridges adjacent light-emitting units, substantially affects the device performance of tandem OLEDs. In this letter, we introduce an effective connecting structure in which an ultrathin middle metal layer is sandwiched between efficient electron- and hole-injection layers for the tandem OLEDs, which in essence, avoids the use of reactive metals during fabrication. Two-unit tandem OLEDs with such connecting structure exhibit less than double the driving voltage, yet more than double the efficiency, more saturated emission color, and longer operational lifetime compared to those of single-unit devices. A model based on a hypothesis of energy level pinning effect has been proposed as the mechanism of the connecting structure in the tandem devices. This model is also consistent with the results obtained from the photovoltaic effect measurements in tandem OLEDs.
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