Objective  White organic lighting-emitting diodes (WOLEDs) have attracted significant interest in the fields of flexible flat panel displays and large-area solid-state lighting due to their merits of ultrathin, large-scale and low-cost. Phosphorescent OLEDs can achieve 100% exciton utilization. However, the lack of stable blue phosphorescent materials hinders the commercial application of all phosphorescent WOLEDs. Thermally activated delayed fluorescence (TADF) materials, which can harvest triplet excitons through efficient reverse intersystem crossing (RISC) and achieve nearly 100% internal quantum efficiency (IQE) are emerging as next generation emitters for OLEDs. Therefore, hybrid TADF/phosphorescent WOLEDs have become an alternative for preparing high efficiency and stable WOLEDs. Generally, in WOLEDs, unbalanced carrier transport in light-emitting layers (EMLs) usually leads to narrow exciton recombination regions, which reduces the efficiency and color stability at a high current density. Various methods, including inserting interlayers between EMLs have been proposed to improve color stability. However, the organic-organic barriers between the interlayers and EMLs enlarge the driving voltages and exacerbate exciton accumulation. Therefore, developing WOLEDs with balanced carrier transport and broadening the exciton recombination zones are the key to simultaneously achieving high efficiency and stable white emission.

  Methods  High efficiency hybrid TADF/phosphorescent WOLEDs are prepared in this study. An exciplex system TCAT:DPEPO is chosen as the host to improve charge balance and optimize exciton distribution. Moreover, a cascaded exciton energy transfer route is constructed to improve exciton utilization efficiency. The working mechanism of devices is illustrated by examining host effects in EMLs. Moreover, the carrier balance is further enhanced by optimizing the transport layer.

  Results and Discussions   The bipolar exciplex host (TCTA:DPEPO) and traditional host DPEPO are comparably investigated in blue TADF devices (Fig.1). By modulating the thicknesses of light-emitting layers, high-efficiency hybrid TADF/phosphorescent WOLED_S based on exciplex host have been achieved with excellent color stability and a high color rendering index (CRI) of 88 (Fig.3). The comparison experiment shows that the outstanding performance of hybrid TADF/phosphorescent WOLEDs is attributed to the widened exciton recombination region and reasonable exciton utilization routes (Fig.4). In addition, by optimizing the electron transport layer, the power efficiency is further improved, achieving maximum values of 52.6 lm·W⁻¹ and 19.3% for power efficiency and EQE, respectively (Fig.6).

  Conclusions  High efficiency, color stable and low efficiency roll-off TADF/phosphorescent hybrid WOLEDs based on exciplex host are achieved. In the proposed WOLEDs, an exciplex host is utilized in EMLs to broad exciton recombination region and a cascaded exciton energy transfer route is constructed to improve exciton utilization. Hybrid WOLEDs exhibit excellent color stability and low efficiency roll-off. Maximum values of PE and EQE are 36.4 lm·W⁻¹ and 17.5% (maintaining 18.2 lm·W⁻¹ and 12.3% at 1000 cd·m⁻²), respectively. With balanced white emission, the WOLED reaches a CIE of (0.451, 0.428) and a high CRI of 88. By further optimizing the transport layer of WOLEDs, the EQE is further improved to 19.3%, and a maximum power efficiency of 52.6 lm·W⁻¹ and a CRI of 90 are achieved. The design strategy proposed in this study provides a simple but feasible approach for high performance hybrid TADF/phosphorescent WOLEDs.