Highly Stretchable Phosphorescence Organic Light Emitting Diodes

Stretchable organic light-emitting diodes (<i>s</i>-OLED) have attracted considerable interest as the next generation display technology in various fields, such as stretchable smartphones, 3-dimensional tactile displays, and various bioelectronics devices including optogenetics. In many studies, <i>s</i>-OLEDs have been developed using extrinsic approaches like stretchable interconnects and rigid OLEDs. These extrinsic approaches, however, have critical limitations such as low stretchability, low diode density per unit area, and significant complexity in the device fabrication process. Hence, <i>s</i>-OLEDs consisting of intrinsically deformable materials should be developed for realizing the stretchable displays. Recently, we successfully fabricated an intrinsically <i>s</i>-OLED (<i>is</i>-OLED) by forming the blend of a commercially available fluorescent polymer and a non-ionic surfactant. The mechanical properties of the <i>is</i>-OLED emission layer (<i>is</i>-EML) were significantly enhanced without sacrificing the functional layer’s electronic properties. However, fluorescence polymer EMLs generally have the disadvantage of low internal quantum efficiency (IQE) of 25% at maximum compared to their phosphorescent counterparts, with IQE reaching up to 100%. Interestingly, although phosphorescent EMLs are extensively utilized to prepare exceedingly efficient OLEDs, no work has been explored to employ their potential in the <i>is</i>-OLED application. In this work, we designed a solution-processable and intrinsically stretchable phosphorescent emitting layer (<i>isp</i>-EML) by blending the small molecule emitting dopant Tris(2-phenylpyridine)iridium(III) (Ir(ppy)₃) together with the mixture of a polymer host Poly(9-vinylcarbazole) (PVK), and a plasticizer Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG). The blending of the polymer host and a plasticizer significantly enhanced the stretchability of the EML from 5% to 100% of crack onset strain. The morphology changes of the polymer host according to the ratio of the plasticizer and small molecules were analyzed through transmission electron microscopy (TEM) and atomic force microscopy (AFM), and it was predicted that the change in the conformation of the polymer host improved the mechanical properties. The luminous property of <i>isp</i>-EML was slightly decreased by blending host but, the efficiency of the <i>isp</i>-EML was significantly increased. The efficiency of the PHOLED with <i>isp</i>-EML showed efficient performance (luminance <i>L</i> of 4,950 cd/m², current efficiency <i>ε</i> of 28.2 cd/A, and EQE of 7.25%) than PHOLED without plasticizer at EML (<i>L</i>, <i>ε</i>, and EQE of 6,600 cd/m², 17.5 cd/A, and 4.88%, respectively). Charge mobility and injection property of polymer host were analyzed through the space charge limited current (SCLC) and electrical impedance spectroscopy (EIS). Because morphology modified isp-EML by surfactant blending changed the charge mobility, the charge balance between hole and electron was improved, increasing the efficiency of PHOLED with <i>isp</i>-EML. Furthermore, because PVK has high triplet energy, the blending host with different emitting dopants changed the emitting color of <i>isp</i>-EML with red, green, and blue.