Effects of exciton and charge confinement on the performance of white organic p−i−n electrophosphorescent emissive excimer devices

The quantum efficiency of triplet excimer-based white organic $\mathrm{p-i-n}$ light-emitting devices (WOLEDs) is shown to depend exponentially on the thickness of the emissive layer (EML), while the voltage increases approximately linearly with EML thickness. The EML consists of the square planar Pt excimer emitting complex, platinum(II) [2-(4’,6’-difluorophenyl-N, ${\mathrm{C}}^{2^{\prime }})$ (2,4-pentanedionato)] doped into N,N’-dicarbazolyl-3,5-benzene, and the electron capture length within the EML is found to vary from $\text{90±10}$ to $\text{120±10\hspace{0.05em}Å,}$ depending on whether or not the transport layers are p or n doped. The $\mathrm{p-i-n}$ WOLED exhibits peak external quantum and power efficiencies of $\text{(5.2±0.5)\%}$ and $\text{(11±1)\hspace{0.05em}}\mathrm{lm}/\mathrm{W},$ respectively, and at $\text{500\hspace{0.05em}}\mathrm{cd}/{\mathrm{m}}^2$ these efficiencies decrease to $\text{(4.2±0.4)\%}$ and $\text{(4.3±0.4)\hspace{0.05em}}\mathrm{lm}/\mathrm{W}.$ The device has color coordinates of (0.35, 0.43) and a color rendering index of 75. We also demonstrate the importance of an electron blocking layer that reduces the leakage of excitons and charge out of thin EMLs, thereby improving the quantum efficiency of devices by a factor approaching 3, as compared to devices lacking the blocking layer.