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Organic semiconductors typically possess low charge carrier mobilities and Langevin-type recombination dynamics, which both negatively impact the performance of organic solar cells and photodetectors. Charge transport in organic solar cells is usually characterized by the mobility-lifetime product. Using newly developed transient and steady state photocurrent measurement techniques we show that the onset of efficiency limiting photocarrier recombination is determined by the charge that can be stored on the electrodes of the device. It is shown that significant photocarrier recombination can be avoided when the total charge inside the device, defined by the trapped, doping-induced and mobile charge carriers, is less than the electrode charge. Based upon this physics we propose the mobility-recombination coefficient product as an alternative and more convenient figure of merit to minimize the recombination losses. We validate the results in 3 different organic semiconductor-based light harvesting systems with very different charge transport properties. The findings allow the determination of the charge collection efficiency in fully operational devices. In turn, knowing the conditions under which non-geminate recombination is eliminated enables one to quantify the generation efficiency of free charge carriers. The results are relevant to a wide range of light harvesting systems, particularly those based upon disordered semiconductors, and require a rethink of the critical parameters for charge transport.

The transient dynamics of transition-metal dichalcogenides is of significant interest for clarifying fundamental manyparticle interactions at the nanoscale as well as for novel applications. We report an ultrafast terahertz study up to 7 THz of the lamellar semiconductor MoS 2 to access the non-equilibrium conductivity of photo-excited indirect e-h pairs in this multi-layered parent compound. While the equilibrium transport is Drude-like, near-IR optical excitation results in a complex photo-induced conductivity that consists of two components. Mobile charge carriers dominate the low frequency response below 2 THz, while at low temperatures an additional excess conductivity is observed that is enhanced around 4 THz. Two time scales appear in the dynamics: a slow ns relaxation due to non-radiative recombination and a faster sub-100 ps decay connected to the high-frequency THz feature. We discuss the broad THz peak within a model of intra-excitonic transitions in MoS 2 . It agrees well with the expected binding energy and oscillator strength, yet results in an anomalous temperature dependence of the exciton fraction requiring an electronically inhomogeneous phase.

The efficiency droop in nitride LEDs is currently attributed to either carrier-density-dependent nonradiative recombination or to carrier leakage, both being discussed in terms of a single-particle picture. Our time-resolved photoluminescence results show that the radiative lifetime is independent of carrier density, while the nonradiative lifetime scales with the inverse of the carrier density. This can not be understood in a single-particle model. By means of a many-particle theory approach we obtain a consistent picture with both radiative and Auger recombination enhanced by excitonic electron-hole correlation. In the high carrier density limit single-particle radiative and Auger recombination are recovered.

The effects of carrier escape from quantum well ( QW ) and the interaction of hot electrons with crystal lattice are of importance to the physical understanding of QW hot carrier solar cells where the cooling dynamics in photo-excited structures affect the cell efficiency. The absorption of high-energy photons produces electron hole pairs with excess kinetic energy, which are dissipated to the lattice thru phonon scattering. These hot electrons alter the conversion efficiency in photovoltaic solar cells. We have studied the hot electron effect in an Al x Ga 1-x As / GaAs structure with quantum wells placed in the intrinsic region similar to the device described in reference . Our results show that hot electrons lead to an increase in short circuit current. The increase in short circuit current is due to carriers escaping from the well without any significant recombination, which may also lead to a higher cell efficiency. These results support the experimental data recently published by others.

We present a review of nitride laser diodes lasing at short ultraviolet wavelengths. The room temperature operations of the AlGaN based laser diodes under pulsed current mode are presented. The optical and temperature characteristics as well as the carrier recombination of the devices are also investigated.

This paper reviews the development of the theory and measurement of recombination noise in semiconductor junction devices. It traces this development from van der Ziel's corpuscular models of noise in junction diodes and transistors through to recent models of shot-noise suppression and non-classical light emission from laser and light-emitting diodes due to Yamamoto and co-workers.

Recently there has been an increasing demand to perform real-time in-situ chemical detection of hazardous materials, contraband chemicals, and explosive chemicals. Currently, real-time chemical detection requires rather large analytical instrumentation that are expensive and complicated to use. The advent of inexpensive mass produced MEMS devices opened-up new possibilities for chemical detection. For example, microcantilevers were found to respond to chemical stimuli by undergoing changes in their bending and resonance frequency even when a small number of molecules adsorb on their surface. In our present studies we extended this concept by studying changes in both the adsorption-induced stress and photo-induced stress as target chemicals adsorb on the surface of microcantilevers. For example, microcantilevers that have adsorbed molecules will undergo photo-induced bending that depends on the number of adsorbed molecules on the surface. However, microcantilevers that have undergone photo-induced bending will adsorb differently molecules on their surfaces. Depending on the photon wavelength and microcantilever material, the microcantilever can be made to bend by expanding or contracting. This is important in cases where the photo- induced bending of microcantilevers for chemical detection.

The model of current carrier dissociation in the films of amorphous molecular semiconductors which are in strong electric fields has been proposed taking into account the possible hole motion within a Coulomb well by the tunnelling and diffusion drift motion. The probability of this process has also been defined. This study is an attempt to explain the experimentally stated dependencies of the quantum yield of photogeneration on temperature T according to which the extrapolation of the dependencies are crossed at one point To1.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Two of the basic types of diffused junction silicon photodiodes are the n/p diode (which can be operated in either a partially-depleted or fully-depleted mode) and the p/n diode (which is usually operated in the unbiased mode). These two types of diodes are illustrated in Figure 1. While both diodes convert incident photons to electrical current by the same process they exhibit entirely different operating and performance characteristics which determine their suitability for particular applications. Several of these characteristics are reviewed below.