White organic light-emitting diodes with fine chromaticity tuning via ultrathin layer position shifting

Nondoped white organic light-emitting diodes using an ultrathin yellow-emitting layer of rubrene (5,6,11,12-tetraphenylnaphtacene) inserted on either side of the interface between a hole-transporting $4,4′$-bis[$N$-(1-naphtyl)-$N$-phenylamino]biphenyl (⁠$α$-NPB) layer and a blue-emitting $4,4′$-bis(⁠$2,2′$-diphenylvinyl)-$1,1′$-biphenyl (DPVBi) layer are described. Both the thickness and the position of the rubrene layer allow fine chromaticity tuning from deep blue to pure yellow via bright white with CIE coordinates (⁠$x=0.33$⁠, $y=0.32$⁠), an $ηext$ of 1.9%, and a color rendering index of 70. Such a structure also provides an accurate sensing tool to measure the exciton diffusion length in both DPVBi and NPB (8.7 and $4.9nm$⁠, respectively).