The need exists for large area semiconducting devices such as displays, solar panels, and general illumination sources. High performance semiconducting devices, however, typically require wafer based processing. This is due to the need for epitaxial processes, which require single-crystal growth substrates. While amorphous and polycrystalline processes have been developed for the growth of semiconducting materials on glass and metals, in all cases some compromise in device performance is required. Even in the case of organic semiconducting devices, such as organic light emitting diodes (OLEDs), organic solar cells and electronic devices, high performance devices are typically formed via vapor processes within controlled atmospheres. The highest performance devices still benefit from batch based approaches. The need therefore exists for methods in which individual semiconductor elements, which are made using more standard semiconductor processes, can be reassembled into larger devices. In many cases, only a portion of the surface area needs to be covered with the semiconducting elements. This is especially true in the case of displays and general illumination sources. In general lighting applications, brightness levels less than 3000 foot-Lamberts (ftL) are typically preferred and in display applications typical brightness levels of 100 to 200 ftL are used. In many cases, the ability to use the extra surface area for interconnections, addressing elements, conversion materials, energy storage, and a black matrix for enhanced contrast is desirable. In the case of solar applications, the ability to incorporate a concentrator is also advantageous. In many cases the combination of multiple functionalities is desirable (e.g. light emitting diode, solar cell, charge storage, etc.).
While several authors have disclosed methods arraying a variety of semiconducting elements onto a variety of substrate materials, the methods still require a substrate. Carroll in U.S. Pat. No. 4,467,193 and Baker in U.S. Pat. No. 5,771,617 both disclose an array of embedded light emitting diode (LED) chips within a transparent body. Daniels et al in U.S. Pat. Nos. 7,052,924, 7,217,956, 7,259,030, and 7,294,961 disclose the use of a roll-to-roll process and a lamination process to interconnect LEDs and solar cells via transparent conductive sheets. In these cases, some type of supporting substrate is required. The ability to stack, repair, and thermally cool the semiconducting elements is hindered by this approach. In addition, the physical separation and optical properties of the laminating films limit how effectively light can be extracted or impinged on the semiconducting elements. More specifically, high quality displays require contrast enhancement techniques that do not lend themselves to this type of processing. Solar applications involving concentrators are also adversely impacted by this approach.
It would be desirable to create a semiconducting sheet in which a variety of flake-like or chip-like semiconducting elements are arrayed within the sheet such that the thickness of these flake-like or chip-like semiconducting elements is substantially the same as the thickness of the semiconducting sheet. It would also be desirable to create a sheet of semiconducting elements such that interconnections and multiple layers can be formed after the sheet is made. In addition, it would be desirable to incorporate additional functions such as embedded interconnections, a black matrix for contrast enhancement, wavelength conversion materials, charge storage, addressing means, power conversion, and bias control within the surrounding matrix between semiconducting elements and on the semiconducting elements themselves. In U.S. patent application Ser. No. 12/221,304, which is herein incorporated by reference, Zimmerman et al disclose a method of forming a freestanding, substrate-free epitaxial LED chip based on laser liftoff of thick gallium nitride (GaN) from a sapphire wafer. The resulting epitaxial chip ranges from about 10 microns to over 50 microns in thickness, which is substantially thicker than devices grown by metal organic chemical vapor deposition (MOCVD). It would be desirable to use such chips to create a semiconducting sheet. By comparison, the plastic film industry routinely handles films from about 9 microns to 100 s of microns in thickness. Aluminum foils with thicknesses down to a few microns are found in virtually every kitchen.
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