Nir Tessler

LASERS BASED ON SEMICONDUCTING ORGANIC MATERIALS

The semiconductor laser is the cornerstone of modern technology and science. It is being incorporated into an increasing number of applications, ranging from element detection through telecommunications to entertainment. This wide spread of applications is due to the spectral range of the semiconductor laser, which extends from the blue to the far IR, and the attainable output power of several tens of watts. Since light-emitting organic materials are also semiconductors, it is an obvious step to try to introduce their inherent advantages into the laser field. While the direct benefits of incorporating organic lasers into applications are very stimulating, the process of making a laser can also teach us much about its constituent material properties. Since organic materials are constantly evolving, the making of lasers incorporates valuable contributions from a variety of disciplines, extending from organic chemistry through physics to device engineering. With this in mind, this review is also intended for those not necessarily familiar with lasers. The rest of the article is organized as follows: A start is made by looking at the definition of a laser and discussing the issues associated with determining its threshold. A historical overview is then given, spanning from the 1960s to the 1990s. Section 5.1 reviews the recent progress in optically pumped lasers and Section 5.2 is devoted to electrical properties of light-emitting diodes (LEDs), which may affect the performance of future electrically pumped lasers. The figures used in this work are largely taken from data acquired by the author and his colleagues. However, the rapid progress in this field is due to a large number of research groups, as the text will show.

Generalized Einstein relation for disordered semiconductors—implications for device performance

The ratio between mobility and diffusion parameters is derived for a Gaussian-like density of states. This steady-state analysis is expected to be applicable to a wide range of organic materials (polymers or small molecules) as it relies on the existence of quasiequilibrium only. Our analysis shows that there is an inherent dependence of the transport in trap-free disordered organic materials on the charge density. The implications for the contact phenomena and exciton generation rate in light emitting diodes as well as channel width in field-effect transistors is discussed.

On carrier injection and gain dynamics in quantum well lasers

A detailed carrier dynamics model for quantum well lasers is presented. The model describes the transport of carriers using full continuity equations and the gain by rate equations for each well separately, and it also takes into account electron-hole interactions which modify the energy band structure. To this end, the model includes Poisson and Schrodinger equations. The model is solved in steady state where it yields nonuniform carrier distributions along the crystal growth axis. Dynamically, the model is solved in the time domain, yielding the evolution of carriers in time and space and highlighting a new effect, photon-assisted carrier transport. The model is also solved in the small-signal regime where the phase lag in gain between wells is determined. >

Hybrid nanocomposite materials with organic and inorganic components for opto-electronic devices

Hybrid structures consisting of semiconducting organic polymers and strongly luminescent semiconductor nanocrystals offer favorable perspectives through highly saturated, tunable emission and tunable absorption, in combination with an easy processability from solution and low materials cost, which is of importance for several high-tech applications such as hybrid organic–inorganic light-emitting diodes and solar cells. This Feature Article describes selected examples of the design and synthesis of polymer compounds and inorganic semiconductor nanocrystals, followed by operation principles and material choices for devices that are based on combinations of polymers with semiconductor nanocrystals.

Tuning energetic levels in nanocrystal quantum dots through surface manipulations.

We demonstrate tuning of the electronic level positions with respect to the vacuum level in colloidal InAs nanocrystals using surface ligand exchange. Electrochemical as well as scanning tunneling spectroscopy measurements reveal that the tuning is largely dependent on the nanocrystal size and the surface linking group, while the polarity of the ligand molecules has a lesser effect. The implications of affecting the electronic system of nanocrystal through its capping are illustrated through prototype devices.

Structures of polymer field-effect transistor: Experimental and numerical analyses

We compare two basic organic field-effect transistor structures both experimentally and theoretically. By using time-resolved analysis, we gain insight into the mechanisms affecting the performance of these structures. Using a two-dimensional numerical model, we focus on the top contact structure and analyze the difference between the two structures.

Structure dependent modulation responses in quantum-well lasers

The authors present a three-level rate equation model, describing structurally dependent modulation bandwidth limitations in quantum-well lasers. They demonstrate an enhanced damping as well as a capacitivelike rolloff in the modulation response due to the transport time and carrier injection bottleneck. They also show that, for relatively short transport times, the dominant effect is that of damping, and they calculate an approximated explicit expression for a structure-dependent nonlinear gain compression coefficient. >

Molecular control of quantum-dot internal electric field and its application to CdSe-based solar cells

Inorganic nanocrystals are attractive materials for solar-cell applications. However, the performance of such devices is often limited by an insufficient alignment of energy levels in the nanocrystals. Here, we report that by attaching two different molecules to a single quantum dot or nanocrystal one can induce electric fields large enough to significantly alter the electronic and optoelectronic properties of the quantum dot. This electric field is created within the nanocrystals owing to a mixture of amine- and thiol-anchor-group ligands. Examining the steady state as well as temporal evolution of the optical properties and the nuclear magnetic resonances of the nanocrystals we found that the first excitonic peak shifts as a function of the capping-layer composition. We also demonstrate that the use of a mixed-ligand-induced electric field markedly enhances the charge generation efficiency in layer-by-layer CdSe-nanocrystal-based solar cells, thus improving the overall cell efficiency.

Photoluminescence of poly(p-phenylenevinylene)–silica nanocomposites: Evidence for dual emission by Franck–Condon analysis

The vibronic mode intensity pattern of the photoluminescence (PL) spectra of poly(p-phenylenevinylene) (PPV) nanocomposites dispersed with 5-nm-diam silica particles shows an apparent redistribution toward the nominal 0–0 mode with increasing silica volume fraction. Franck–Condon analysis of this variation, corrected for refractive index dispersion, reveals the presence of overlapping emission from two excited electronic states separated by 180 meV. The principal emission arises from the molecular exciton while the lower-lying one is assigned to a dipole–dipole coupled two-chain aggregate exciton. The quantum yield of the aggregate emission decreases monotonically with silica loading up to 50 vol %, whereas that of the molecular state exhibits a maximum at 15 vol %. When the samples are photoexcited below the π-π* localization edge, both of these emissions jointly redshift without a change in their relative intensities. When cooled below a transition temperature centered at 120 K, the yield of the aggrega...

Electronic excitations in luminescent conjugated polymers

Abstract We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved and single-layer EL diodes achieve luminous efficiencies above 2 Lumen W−1 and peak brightnesses up to 90 000 cd m−2. We discuss measurements of photoconductivity, photovoltaic response, photoluminescence excitation spectra and stimulated emission in films of PPV. We consider that the photoexcited state in these films of PPV is the intrachain singlet exciton. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments and can be used as the active lasing medium when incorporated in suitable microcavity structures.