Soft contact transplanted nanocrystal quantum dots for light-emitting diodes: Effect of surface energy on device performance.

To realize the full-color displays using colloidal nanocrystal quantum dot (QD)-based light emitting diodes (QLEDs), the emissive QD layer should be patterned to red (R), green (G) and blue (B) sub-pixels in a micrometer scale by solution process. Here, we introduced a soft contact QD-transplanting technique onto the vacuum-deposited small molecules without a pressure to pattern the QD layer without any damage to the prior organic layers. We examined the patternability of QDs by systematically studying the surface properties of various organic layers. As a result, we found that the vacuum-deposited 4,4',4''-tri(N-carbazolyl)triphenylamine (TCTA) layer is suitable for QD-transplanting. A uniform and homogeneous QD patterns down to 2 μm could be formed for all the RGB QDs (CdSe/CdS/ZnS, CdSe@ZnS, and Cd1-xZnxS@ZnS, respectively) with this method. Finally, we demonstrated the R, G, and B QLEDs by transplanting each QD onto the soft TCTA layer, exhibiting higher brightness (2497, 14102, and 265 cd m(-2), respectively) and efficiency (1.83, 8.07, and 0.19 cd A(-1), respectively) than those of the previous QLEDs fabricated by other patterning methods. Because this pressure-free technique is essential for patterning and stacking the QDs onto the soft organic layer, we believe that both fundamental study and engineering approach presented here are meaningful for for the realization of the colloidal QD-based full-color displays and other optoelectronic devices.