Simplified dopant-free color-tunable organic light-emitting diodes

Organic light-emitting diodes (OLEDs) manifest a high performance that has enabled the industrial applications of this technology; nevertheless, achieving high efficiency in these devices via introducing dopants makes the fabrication process quite complicated. In this Letter, we demonstrate color-tunable organic light-emitting diodes (CT-OLEDs) with a simplified structure that incorporates a dopant-free PN heterojunction as the emitting layer. By depositing a hole transport layer, 1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC) as the P-region and co-depositing DMIC-TRZ (LGH001) and 1,3,5-tri[(3-pyridyl)-phen-3-yl] benzene (TmPyPB), the electron transport layer, as the N-region to construct a PN heterojunction, our optimal CT-OLED exhibited a low turn-on voltage of 2.58 V with the maximum current and power efficiencies of 17.8 cd/A and 21.5 lm/W, respectively. We found that enhancing the hole-blocking property of the N-junction for improved charge-carrier balance and higher exciton formation is beneficial to boost the device performance. CT-OLEDs based on TAPC/TmPyPB:LGH001 as PN heterojunction achieved sunlight white emission with correlated color temperature and color rendering indexes of 7765 K and 74, respectively. Importantly, the emission of these devices can be well-tuned by simply regulating the bias voltage. Furthermore, the underlying physics of our developed CT-OLEDs is thoroughly investigated.