Hybrid LEDs Offer Full-Color Emission

Daniel S. Burgess

Using the down-conversion of ultraviolet radiation by polyfluorene materials, a team of scientists at Imperial College London and at the University of Strathclyde in Glasgow, UK, has generated blue, green, red and white light from arrays of micro-LEDs. The hybrid organic/inorganic LEDs may find a place in microdisplays and in a variety of other applications.

Pairing wavelength-converting organic materials and micropixelated InGaN LEDs, researchers have generated blue, green, red and white light. Courtesy of Donal D.C. Bradley, Imperial College London.

Donal D.C. Bradley, head of the experimental solid-state physics group at Imperial College’s Blackett Laboratory, explained that microdisplays are of interest for projection and near-to-eye systems such as head-mounted displays and view-finders, as well as for mobile phones and other personal electronics. Additionally, he said, the hybrid LEDs may be used as structured illumination and excitation sources for microarray assays and in medical imaging.

In a demonstration of the approach, George Heliotis of Imperial College and Erdan Gu of Strathclyde employed 64 × 64-element InGaN micro-LED arrays from Martin D. Dawson’s team at the university. The matrix-addressable arrays incorporate 20-μm-diameter emitters on a 30-μm pitch, each producing 1 μW of narrowband radiation centered at 370 nm. The polyfluorenes F8DP, F8BT and Red F, proprietary materials from Sumitomo Chemical Co. Ltd. of Tokyo, were used to convert the UV output to blue, green and red wavelengths, respectively. In each case, the investigators spin-coated a 200-nm-thick film of the polymer onto a quartz substrate and brought it into contact with the array.

To produce white light, they similarly prepared polymer films blending the three materials in various proportions. The best white-light LED displayed CIE coordinates of (0.30, 0.34), close to the ideal of (0.33, 0.33). Increasing the drive voltage to the micro-LED array from 4.2 to 5.8 V resulted in an exponential increase in the output intensity and only a small change in the output spectra, likely the result of saturation in nonradiative energy transfer from the F8DP to the other organic materials.

Full-color displays

Because the elements of the micro-LED array are individually addressable, it may be possible to fabricate full-color displays in which the appropriate polymers would be directly patterned on the emitters to produce red, green, blue and white pixels. A potential challenge to the development of such a device involves the different absorption spectra of the polyfluorenes — the spectra of F8DP matches well with the 370-nm micro-LEDs, but it would be better to use 460-nm emitters with F8BT and Red F. Different array geometries, such as stacked and offset 370- and 460-nm micro-LEDs, could address this issue, Bradley suggested, but he noted that the absorption of F8BT and Red F are high enough that this may not be necessary.

The researchers are investigating the use of nonradiative coupling such as dipole-dipole or Förster resonance energy transfer to improve the conversion efficiencies of the hybrid LEDs, he said. Other paths of inquiry will involve the control of emission properties such as wavelength and directionality by adjusting the spatial format of directly patterned micro-LEDs, and the feasibility of achieving lasing from the pixelated hybrid elements.

Applied Physics Letters, Sept. 8, 2005, 103505.