Document Actions
Research
Up one levelLight Emitting Devices
Organic light emitting devices (OLEDs) are a highly promising emissive technology for flat panel displays. OLEDs employ soft organic semiconductors to convert injected charge into luminescent excitons, in a process known as electroluminescence. But in order to be widely adopted, OLEDs must not only match the color purity and long-term stability of competing technologies, they must realize their maximum potential efficiency. We have focused on the efficiency of electroluminescence, and particularly the effects of spin on exciton formation in organic semiconductors. We have measured the fundamental efficiency limits in OLEDs and demonstrated a new method for improving performance that we term ‘extrafluorescence’. We have also demonstrated a new method for obtaining high efficiency blue OLEDs and developed a novel technique for analyzing the optical properties of OLEDs. See our work on: An active matrix OLED display prototype built by Samsung.
- Fundamental efficiency limits and extrafluorescence
- Saturated and efficient blue phosphorescent OLEDs
- Optical models of organic light emitting devices
Photovoltaic Devices
Our initial work employed biological materials extracted from plants and photosynthetic bacteria. This was perhaps the first demonstration of the integration of photosynthetic protein-molecular complexes in solid state devices. These first devices focused on the charge generation function of solar cells. Subsequently, we have worked on the light gathering function since this offers the greatest opportunity for cost savings and gains in performance. Our work on light gathering seeks to use a film of organic dyes to absorb light. The energy must then be transferred to the charge generating structure. We have investigated using ‘near field’ energy transfer using surface plasmon polaritons to achieve this. See our work on:
Photovoltaic (PV) cells remain an expensive source of electrical power. Our work addresses the cost of PV energy. We follow the example of photosynthesis by separating the two functions of a solar cell: absorbing light and generating charge. This allows us to improve efficiency and potentially lower costs. For example, it is wasteful to employ large areas of intensively engineered PV cells merely to absorb light. A simple film of organic dyes, for example, is cheaper and more absorptive.
- Integration of Photosynthetic Complexes with Solid-State Photovoltaics
- Surface Plasmon Polariton Mediated Energy Transfer in Organic Photovoltaic Devices
Fundamental studies of disorder
Soft semiconductors are materials that are held together by van der Waals forces rather than the stronger covalent bonds that are more typical of conventional semiconductors such as Si. Examples of soft semiconductors are molecular crystals and thin films of molecules and polymers. Due to weak intermolecular interactions, disorder is found in all soft semiconductors. Even molecular crystals possess thermodynamically-stable molecular vacancies that cannot be removed by annealing.
We examine the influence of disorder on charge injection and charge transport in disordered semiconductors. Charge injection dominates the performance of many organic devices. For example, the operating voltage of OLEDs based on small-molecular weight materials is typically determined by charge injection. We have proposed that interfacial disorder is a critical determinant of charge injection into soft semiconductors.
See our work on:
Understanding the electric field dependence of charge-carrier mobility is central to the rational design of organic semiconductor devices. We present an analytic description of mobility by considering non-equilibrium carrier distributions within a percolation framework. The theory is compared to measurements by Brütting, et al [Organic Electronics 2, 1 (2001)] of the current-voltage and mobility of the archetype small molecule tris(8-hydroxyquinoline) aluminum. The theory accurately reproduces the temperature, carrier density, and electric field dependences of the experimental data.
Finally, we have pursued direct measurements of energetic disorder in soft semiconductors. We measure the density of states as a function of energy in thin films of copper phthalocyanine (CuPC).The density of states is an important factor in understanding charge transport in organic semiconductors. Using Kelvin probe force microscopy we find an exponential density of states with a characteristic energy of 0.11eV over a 0.5eV range of the highest occupied molecular orbital of CuPC. We also find that the technique is limited by charge trapping and hysteresis at low densities of states, and non-uniform potential profiles within the CuPC film at high densities of states.
- Charge injection
- Charge transport
- Measurement of density of states
Nanoelectromechanical Memories and Switches
Carbon Nano-Switches for Low-Leakage Circuit Applications K.M. Milaninia, C.E. Schmitt, A.I. Akinwande, A.P. Chandrakasan, and M.A. Baldo Sponsorship: FCRP IFC, DARPA Nanoelectromechanical switches (NEMS) exhibit minimal leakage current in the off state. Consequently, they may find application in low power electronics. This work focuses on the fabrication of a vertically oriented nano-switch using a carbon fiber or nanotube as the active component. The device schematic is shown in Figure 1, and Figure 2 shows an SEM image of the self-aligned fabrication process used to create the nano-switch [1]. The device consists of a carbon nanotube/fiber grown directly on a highly doped silicon substrate between two contacts which are electrically isolated from the substrate by an insulator. The device is actuated when a voltage is applied between the substrate and one of the contacts. This causes the nanotube to be pulled into and eventually make physical contact with one of the contacts and allows current flow between the substrate and the contact.
More ...
Sensors
Chemical and biological sensors are perhaps the next big opportunity for soft semiconductor materials. The advantage of soft semiconductors in this application is twofold: firstly, soft materials can follow the biological model of shape recognition using conformational signal transduction; secondly, soft semiconductors are an ideal interface between conventional electronic materials and the environment. We are investigating signal transduction devices in both chemical sensors and proteomic sensors.