Kevin P. Pipe

Engineered doping of organic semiconductors for enhanced thermoelectric efficiency

Significant improvements to the thermoelectric figure of merit ZT have emerged in recent years, primarily due to the engineering of material composition and nanostructure in inorganic semiconductors (ISCs). However, many present high-ZT materials are based on low-abundance elements that pose challenges for scale-up, as they entail high material costs in addition to brittleness and difficulty in large-area deposition. Here we demonstrate a strategy to improve ZT in conductive polymers and other organic semiconductors (OSCs) for which the base elements are earth-abundant. By minimizing total dopant volume, we show that all three parameters constituting ZT vary in a manner so that ZT increases; this stands in sharp contrast to ISCs, for which these parameters have trade-offs. Reducing dopant volume is found to be as important as optimizing carrier concentration when maximizing ZT in OSCs. Implementing this strategy with the dopant poly(styrenesulphonate) in poly(3,4-ethylenedioxythiophene), we achieve ZT  =  0.42 at room temperature.

Transparent and conductive electrodes based on unpatterned, thin metal films

Transparent electrodes composed of ultrathin, unpatterned metal films are investigated in planar heterojunction (PHJ) and bulk heterojunction organic photovoltaic (OPV) cells. Optimal electrode composition and thickness are deduced from electrical and optical models and experiments, enabling a PHJ-OPV cell to be realized using a silver anode, achieving power conversion efficiency parity with an analogous cell that uses an indium tin oxide anode. Beneficial aspects of smooth, unpatterned metal films as transparent electrodes in OPV cells are also discussed in the text.

Fiber based organic photovoltaic devices

A fiber-shaped organic photovoltaic cell is demonstrated, utilizing concentric thin films of small molecular organic compounds. Illuminated at normal incidence to the fiber axis through a thin metal electrode, the cell exhibits 0.5% power conversion efficiency, compared to 0.76% for a planar control device. The fiber device efficiency is nearly independent of illumination angle, increasing its power generation over the planar counterpart for diffuse illumination. Losses due to partial shading of the fiber surface are minimal, while the coated fiber length is limited only by the experimental deposition chamber geometry—factors favoring scale-up to woven energy harvesting textiles.

High thermal conductivity in amorphous polymer blends by engineered interchain interactions

Thermal conductivity is an important property for polymers, as it often affects product reliability (for example, electronics packaging), functionality (for example, thermal interface materials) and/or manufacturing cost. However, polymer thermal conductivities primarily fall within a relatively narrow range (0.1-0.5 W m(-1) K(-1)) and are largely unexplored. Here, we show that a blend of two polymers with high miscibility and appropriately chosen linker structure can yield a dense and homogeneously distributed thermal network. A sharp increase in cross-plane thermal conductivity is observed under these conditions, reaching over 1.5 W m(-1) K(-1) in typical spin-cast polymer blend films of nanoscale thickness, which is approximately an order of magnitude larger than that of other amorphous polymers.

Computational sprinting

Although transistor density continues to increase, voltage scaling has stalled and thus power density is increasing each technology generation. Particularly in mobile devices, which have limited cooling options, these trends lead to a utilization wall in which sustained chip performance is limited primarily by power rather than area. However, many mobile applications do not demand sustained performance; rather they comprise short bursts of computation in response to sporadic user activity. To improve responsiveness for such applications, this paper explores activating otherwise powered-down cores for sub-second bursts of intense parallel computation. The approach exploits the concept of computational sprinting, in which a chip temporarily exceeds its sustainable thermal power budget to provide instantaneous throughput, after which the chip must return to nominal operation to cool down. To demonstrate the feasibility of this approach, we analyze the thermal and electrical characteristics of a smart-phone-like system that nominally operates a single core (~1W peak), but can sprint with up to 16 cores for hundreds of milliseconds. We describe a thermal design that incorporates phase-change materials to provide thermal capacitance to enable such sprints. We analyze image recognition kernels to show that parallel sprinting has the potential to achieve the task response time of a 16W chip within the thermal constraints of a 1W mobile platform.

CCD-based thermoreflectance microscopy: principles and applications

CCD-based thermoreflectance microscopy has emerged as a high resolution, non-contact imaging technique for thermal profiling and performance and reliability analysis of numerous electronic and optoelectronic devices at the micro-scale. This thermography technique, which is based on measuring the relative change in reflectivity of the device surface as a function of change in temperature, provides high-resolution thermal images that are useful for hot spot detection and failure analysis, mapping of temperature distribution, measurement of thermal transient, optical characterization of photonic devices and measurement of thermal conductivity in thin films. In this paper we review the basic physical principle behind thermoreflectance as a thermography tool, discuss the experimental setup, resolutions achieved, signal processing procedures and calibration techniques, and review the current applications of CCD-based thermoreflectance microscopy in various devices.

Suppressing molecular motions for enhanced room-temperature phosphorescence of metal-free organic materials

Metal-free organic phosphorescent materials are attractive alternatives to the predominantly used organometallic phosphors but are generally dimmer and are relatively rare, as, without heavy-metal atoms, spin–orbit coupling is less efficient and phosphorescence usually cannot compete with radiationless relaxation processes. Here we present a general design rule and a method to effectively reduce radiationless transitions and hence greatly enhance phosphorescence efficiency of metal-free organic materials in a variety of amorphous polymer matrices, based on the restriction of molecular motions in the proximity of embedded phosphors. Covalent cross-linking between phosphors and polymer matrices via Diels–Alder click chemistry is devised as a method. A sharp increase in phosphorescence quantum efficiency is observed in a variety of polymer matrices with this method, which is ca. two to five times higher than that of phosphor-doped polymer systems having no such covalent linkage.

Enhanced optical field intensity distribution in organic photovoltaic devices using external coatings

An external dielectric coating is shown to enhance energy conversion in an organic photovoltaic cell with metal anode and cathode by increasing the optical field intensity in the organic layers. Improved light incoupling in the device is modeled using transfer matrix simulations and is confirmed by in situ measurement of the photocurrent during growth of the coating. The optical field intensity in optimized cell geometries is predicted to exceed that in analogous devices using indium tin oxide, both cell types having equivalent anode sheet resistance, suggesting a broader range of compatible substrates (e.g., metal foils) and device processing techniques.

Comprehensive heat exchange model for a semiconductor laser diode

By measuring the total energy flow from an optical device, we can develop new design strategies for thermal stabilization. Here we present a comprehensive model for heat exchange between a semiconductor laser diode and its environment that includes the mechanisms of conduction, convection, and radiation. We perform quantitative measurements of these processes for several devices, deriving parameters such as a lasers heat transfer coefficient, and then demonstrate the feasibility of thermal probing for the nondestructive wafer-scale characterization of optical devices.

Internal cooling in a semiconductor laser diode

A thermal model of a diode laser structure is developed which includes a bipolar thermoelectric term not included in previous models. It is shown that heterostructure band offsets can be chosen so that there are thermoelectric cooling sources near the active region; this method of cooling is internal to the device itself, as opposed to temperature stabilization schemes which employ an external cooler. A novel laser structure is proposed that is capable of internal cooling in the Ga/sub 1-x/In/sub x/As/sub y/Sb/sub 1-y/-GaSb material system with /spl lambda/ = 2.64 /spl mu/m.