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:
- Fundamental efficiency limits and extrafluorescence
- Saturated and efficient blue phosphorescent OLEDs
- Optical models of organic light emitting devices

An active matrix OLED display prototype built by Samsung.

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.

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.

The light gathering function in photosynthesis is performed by structures known as ‘antennas’. We have worked on antennas for conventional and organic solar cells. The aim is to use a film of organic dyes to absorb light. The energy must then be transferred to the charge generating structure. We have investigated two approaches to this problem:

Antennas for organic solar cells: We have demonstrated the use of surface plasmon polaritons to transfer energy across the contact of an organic solar cell. This approach has demonstrated enhancements in the quantum efficiency of solar cells at selected regions of the spectrum.

Antennas for conventional solar cells: We have built solar concentrators that collect light using organic dyes and concentrate it on solar cells. The architecture is known as a luminescent solar concentrator (LSC). It enables high optical concentration without excess heating in a stationary system. LSCs consist of a dye dispersed in a transparent waveguide. Incident light is absorbed by the dye and then re-emitted into a waveguide mode. The energy difference between absorption and emission prevents re-absorption of light by the dye, isolating the concentrated photon population in the waveguide. In this way, LSCs can achieve high optical concentrations without solar tracking. Unfortunately, the performance of LSCs has been limited by self-absorption losses that restrict the maximum possible concentration factor. Our work has focused on demonstrating an efficient variant of an LSC that exhibits optical concentrations suitable for practical applications. We call this an ‘organic solar concentrator’ (OSC) because it draws upon the organic semiconductor technology. We have developed solar concentrators using both synthetic dyes and photosynthetic materials known as phycobilisomes.

See our work on:
- Integration of Photosynthetic Complexes with Solid-State Photovoltaics
- Surface Plasmon Polariton Mediated Energy Transfer in Organic Photovoltaic Devices
- Organic Solar Concentrators

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.

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.

See our work on:
- Charge injection
- Charge transport
- Measurement of density of states