Colloidal Quantum Dot Based  Diodes for Cellular Imaging

We report a quantum dot (QD)-based light emitting diode (LED) structure with a solution processed zinc oxide electron transporting layer, with patterning of anode and cathode on the substrate toward cellular imagingapplications. Compared with the use of sputtered thin films, solutionprocessed electron transporting layer improves robustness of the device, as crystalline ZnO shows low CB and VB edge energy levels, efficiently suppressing hole leakage current resulting in LEDs with longer lifetimes. We demonstrate a working lifetime of more than 12 h and a shelf-life of more than 180 days for the devices. Our solution-based process is applicable to microcontact printed and also spin-coated QD films. Further, we demonstrate electroluminescence of QD LEDs at 580and 600-nm emission wavelengths, with typical turn-ON voltage of 12 V.

We compare operating characteristics of an array of individually addressable LEDs fabricated on a single substrate, with average size of each element being 800 μm × 200 μm. The LEDs with spin-coated ETL show a lifetime increase of more than three orders of magnitude compared with devices made using sputtered ETL layer. We find marginal increases in the intensity and uniformity of devices fabricated using a spin-coated ZnO layer. Finally, we demonstrate a compact multicolor light source for cell characterizations by imaging HEMA 3 stained MDA-MB 231 breast cancer cells on-chip. We demonstrate the capability of the devices as a light source by measuring intensity across stained cells with QDLEDs of two different wavelengths and show the correlation as expected with the absorption profile of the fluorescent dye.