Paul N. Stavrinou

Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends

Control of blend morphology at the microscopic scale is critical for optimizing the power conversion efficiency of plastic solar cells based on blends of conjugated polymer with fullerene derivatives. In the case of bulk heterojunctions of regioregular poly(3-hexylthiophene) (P3HT) and a soluble fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester, PCBM), both blend morphology and photovoltaic device performance are influenced by various treatments, including choice of solvent, rate of drying, thermal annealing and vapour annealing. Although the protocols differ significantly, the maximum power conversion efficiency values reported for the various techniques are comparable (4-5%). In this paper, we demonstrate that these techniques all lead to a common arrangement of the components, which consists of a vertically and laterally phase-separated blend of crystalline P3HT and PCBM. We propose a morphology evolution that consists of an initial crystallization of P3HT chains, followed by diffusion of PCBM molecules to nucleation sites, at which aggregates of PCBM then grow.

Simultaneous optimization of charge-carrier mobility and optical gain in semiconducting polymer films.

The combination of efficient light emission and high charge-carrier mobility has thus far proved elusive for polymer semiconductors, with high mobility typically achieved by cofacial pi-electron system to pi-electron system interactions that quench exciton luminescence. We report a new strategy, comprising the introduction of a limited number of more effective hopping sites between otherwise relatively isolated, and thus highly luminescent, polyfluorene chains. Our approach results in polymer films with large mobility (mu approximately 3-6 x 10(-2) cm2 V-1 s-1) and simultaneously excellent light-emission characteristics. These materials are expected to be of interest for light-emitting transistors, light-emitting diode sources for optical communications and may offer renewed hope for electrically pumped laser action. In the last context, optically pumped distributed feedback lasers comprising one-dimensional etched silica grating structures coated with polymer have state-of-the-art excitation thresholds (as low as 30 W cm(-2) (0.1 nJ per pulse or 0.3 microJ cm-2) for 10 Hz, 12 ns, 390 nm excitation) and slope efficiencies (up to 11%).

Indoor optical wireless systems : a review

This paper reviews the research effort to date into optical wireless communication systems, both in academic and industrial contexts. The idea of using the optical medium for wireless communications is not new, having been proposed as a means for indoor communications almost two decades ago. However, the last few years have seen an explosive interest in the potential for free space optical systems to provide portable data communications. One of the prime motivators for reconsidering the use of an optical carrier in the wireless context is the demand for greater transmission bandwidths. The radio frequency spectrum is already exceedingly congested and frequency allocations of sufficient bandwidths are extremely hard to obtain. Further, for the high bandwidth services envisaged, the use of microwave or mm-wave systems will be required, where device technology is currently either expensive or immature. Proponents of optical wireless systems argue that the optical medium is the only cost-effective way to provide high bit-rate mobile services to volume markets. This paper identifies the technical obstacles and limitations of indoor optical wireless delivery in addition to techniques for mitigating these effects and shows that infrared is a viable alternative to radio for certain applications.

1.3 µm Room Temperature Emission from InAs/GaAs Self-Assembled Quantum Dots

We have investigated the growth conditions necessary to achieve strong room temperature emission at 1.3 µm for InAs/GaAs self-assembled quantum dots (QDs) using conventional solid source molecular beam epitaxy (MBE). A relatively high substrate temperature and very low growth rate (LGR) result in long wavelength emission with a small linewidth of only 24 meV. Atomic Force Micrographs obtained from uncapped samples reveal several differences between the LGRQDs and those grown at higher growth rates. The former are larger, more uniform in size and their density is lower by a factor of about 4. LGRQDs have been incorporated in p-i-n structures and strong room temperature electroluminescence detected. The light output of the QD p-i-n diodes is found to be significantly higher than a quantum well (QW) sample at least for current densities up to 0.5 kAcm-2.

Integrated transceivers for optical wireless communications

Line-of-sight free-space optical links can provide extremely high bandwidth communications, but this usually requires that transmitter and receiver are precisely aligned. In order to allow terminals to be mobile, links must be able to track users within their field of view so that the link is maintained. There are various means to do this, but all require complex subsystems with a number of different optical, optoelectronic, and electrical components. A solid-state tracking architecture is introduced and a seven-channel tracking system demonstration described. The system is designed to operate at 155 Mb/s and is, to the best of our knowledge, the first that uses an integrated approach. Arrays of novel resonant cavity LED (RCLED) emitters that operate at 980 nm are used as sources. These are flip-chip bonded to arrays of CMOS driver circuits and integrated with the necessary transmitter optics. The receiver uses a back-illuminated detector array flip-chip bonded to arrays of custom CMOS receivers. All these components are custom and have performance substantially better than nonoptimized commercially available components. In the paper, the design and fabrication of the optics, optoelectronics, and electronics required for this is described. Successful operation of all the subsystems is detailed, together with results from an initial link demonstration.

High‐Efficiency, Solution‐Processed, Multilayer Phosphorescent Organic Light‐Emitting Diodes with a Copper Thiocyanate Hole‐Injection/Hole‐Transport Layer

Copper thiocyanate (CuSCN) is introduced as a hole-injection/hole-transport layer (HIL/HTL) for solution-processed organic light-emitting diodes (OLEDs). The OLED devices reported here with CuSCN as HIL/HTL perform significantly better than equivalent devices fabricated with a PEDOT:PSS HIL/HTL, and solution-processed, phosphorescent, small-molecule, green OLEDs with maximum luminance ≥10 000 cd m(-2) , maximum luminous efficiency ≤50 cd A(-1) , and maximum luminous power efficiency ≤55 lm W(-1) are demonstrated.

Polyfluorene distributed feedback lasers operating in the green-yellow spectral region

We report solid-state, optically pumped poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), distributed feedback lasers that operate in the green-yellow spectral region, previously unaddressed with conjugated polymer gain media. The lasers were fabricated by spin coating F8BT (the gain medium) onto one-dimensional gratings patterned in silica substrates. The emission wavelength could be selected to lie within the range from 558 to 591 nm by controlling the F8BT film thickness and grating periodicity. The minimum lasing threshold of 6.5 nJ per pump pulse (10 ns, 10 Hz, 450 nm) was achieved for a 350 nm spatial period grating with a 180 nm thickness F8BT film. The corresponding emission wavelength was 573 nm and the laser slope efficiency was a relatively high 3%.

High-speed integrated transceivers for optical wireless

Optical wireless LANs have the potential to provide bandwidths far in excess of those available with current or planned RF networks. There are several approaches to implementing optical wireless systems, but these usually involve the integration of optical, optoelectronic, and electrical components in order to create transceivers. Such systems are necessarily complex, and the widespread use of optical wireless is likely to be dependent on the ability to fabricate the required transceiver components at low cost. A number of UK universities are currently involved in a project to demonstrate integrated optical wireless subsystems that can provide line-of-sight in-building communications at 155 Mb/s and above. The system uses two-dimensional arrays of novel microcavity LED emitters and arrays of detectors integrated with custom CMOS integrated circuits to implement tracking transceiver components. In this article we set out the basic approaches that can be used for in-building optical wireless communication and argue the need for an integrated and scalable approach to the fabrication of transceivers. Our work aimed at implementing these components, including experimental results and potential future directions, is then discussed.

Spectral conversion of InGaN ultraviolet microarray light-emitting diodes using fluorene-based red-, green-, blue-, and white-light-emitting polymer overlayer films

We report the fabrication of hybrid organic/inorganic semiconductor light-emitting devices that operate across the entire visible spectrum. The devices are based on a series of blue-, green-, and red-light-emitting polyfluorene materials that convert the emission from an array of micron-sized ultraviolet InGaN light-emitting diodes. We also demonstrate white-light-emitting versions of these hybrid devices by employing single films of carefully adjusted polyfluorene blends in which cascade energy transfer occurs between the constituent materials. The spectral and operating characteristics of the devices are described in detail. Such organic emission layer/inorganic light-emitting diode (LED) array based devices may provide a promising route to the fabrication of low-cost full-color microdisplays and other instrumentation devices.

New light from hybrid inorganic-organic emitters

We present the highlights of a research programme on hybrid inorganic?organic light emitters. These devices combine recent developments in III?V nitride technology (including UV emitting micro-arrays and specifically tailored quantum wells) with conjugated polymers to access the entire visible spectrum. Two types of devices are studied, those based on down conversion of the quantum well emission by radiative transfer and those based on non-radiative resonant energy transfer. The spectral and operating characteristics of the devices are described in detail. Selectable colour micro-arrays and bar emitters are demonstrated. The nature of the non-radiative energy transfer process has also been studied and we find transfer efficiencies of up to 43% at 15?K, with a 1/R2 dependence on the distance between quantum well and polymer layer, suggesting a plane?plane interaction. The relative importance of the non-radiative resonant energy transfer process increases with temperature to be up to 20 times more efficient, at 300?K, than the radiative transfer process.