The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad.
This award will support a twenty-four-month research fellowship by Dr. R. Clayton Shallcross to work with Dr. Klaus Meerholz at the University of Cologne in Germany.
The scientific objective of this twenty-four month research fellowship is to develop solution-processable non-volatile organic memory (OMEM), image processing and display devices that may be optically switched via photochromic dithyenylethene (DTE) compounds in the active layer. The project focuses on both light-emitting and non-emitting devices based on organic light-emitting diodes (OLEDs) and ?hole-only? devices, respectively. The overall goal of this research fellowship is to design solution-processable (i.e. crosslinkable) photochromic DTE-based molecules that are specifically sensitive to blue, green, red and IR radiation and incorporate them into working devices, which will ultimately lead to full color image processing and complementary display capabilities that exhibit high contrast between their ON and OFF states (i.e. high ON/OFF ratios), low power consumption, and high fatigue resistance (cyclability).
The broader significance of this project lies in the ability to utilize basic solution processing methods (e.g. spin coating) to easily and inexpensively process thin film non-volatile memory devices. In contrast to conventional device processing conditions utilized for conventional inorganic semiconductors, low-temperature solution processing will be employed to afford high-quality large area multi-layered thin films composed of pure or doped organic semiconductors on a variety of both rigid and plastic substrates. Some of the existing non-volatile memory technologies based predominately on organic compounds in metal-insulator-metal (MIM) and thin film transistor (TFT) formats do show reasonable voltage-induced ON/OFF ratios for current, but are not simultaneously optimized with respect to fatigue resistance (cyclability), power consumption (operating voltage) and ease of processing. Furthermore, light-emitting organic memory (LE-OMEM) devices offer the possibility of not only current read out for conventional memory and image processing applications, but also optical readout due to OLED emission modulation, which may find additional applications in ?smart? display technologies. Rational design of OMEM devices, incorporating a diverse catalog of structurally unique photochromic compounds, offers the prospect of utilizing low-temperature, low-cost and facile solution processing methods for optimizing full color image processing and complementary display devices.
This National Science Foundation (NSF) International Research Fellowship Program (IRFP) award was funded for research into a novel memory technology corresponding to organic light-emitting diodes (OLEDs) that are able to be switched between an emissive ON state and a non-emissive OFF state via an external light source. The photoresponse of these light-emitting organic memory (LE-OMEM) devices is made possible by incorporation of a photoswitchable organic chromaphore (photochromic compound) into the multi-layered stack of what is a fully solution-processable OLED; an expertise of the Meerholz research group at the University of Cologne in Germany, where the fellowship research was conducted. Using specific external light sources with different colors (i.e., wavelengths), the photochromic compound can be reversibly toggled between two stable forms that posses different light absorption properties (i.e., colors) and energetic properties, which are exploited to provide high contrast between the OFF and ON state of the OLED. Such optically programmable multifunctional memory devices have applications in traditional memory devices and signage, where an image can be programmed (WRITE step) into a display and later observed (READ step) via electroluminescence emission. The image or data can be later removed (ERASE step) and re-written. The initial project motivation focused on design and characterization of a range of photochromic compounds that would be sensitive to a range of external light colors; with the key goal of designing colors that would be sensitive to the primary colors of light (red, green and blue). Each device was envisioned to allow for light emission of its complementary color in a working LE-OMEM device. While a number of novel compounds were investigated, the primary research focused on a yellow absorbing (blue appearance) compound that allowed for a blue-emitting OLED that afforded a contrast ratio for light emission between the ON and OFF state of greater than 104, representing a record for such devices in the scientific literature. Additional motivation was focused on realization of multilevel memory characteristics for such devices. Multilevel memory devices are desirable for attaining higher density storage media, where more data can be stored per memory cell compared to traditional binary cells. The LE-OMEM showed the ability for multilevel memory characteristics, whereby incremental conversion of the photochromic compound between the two forms corresponds to predictable and intermediate output characteristics between the totally ON and OFF states. Prototype passively-addressed memory arrays with greater than 100 memory cells showed up to 8 different statistically-discernable levels, corresponding to a 3-fold increase in storage density compared to a traditional binary memory array. This two-year NSF-sponsored research fellowship was able to demonstrate the applicability of photochromic compounds to multifunctional LE-OMEM devices for applications in signage and multilevel memory applications. Furthermore, the fully solution-processable nature of these devices makes them candidates for low-cost and flexible electronic devices, which represents one of the major advantages of organic electronics compared to their inorganic counterparts (e.g. silicon).
