Optimization and evaluation of variability in the programming window of a flash cell with molecular metal-oxide storage

We report a modeling study of a conceptual nonvolatile memory cell based on inorganic molecular metal-oxide clusters as a storage media embedded in the gate dielectric of a MOSFET. For the purpose of this paper, we developed a multiscale simulation framework that enables the evaluation of variability in the programming window of a flash cell with sub-20-nm gate length. Furthermore, we studied the threshold voltage variability due to random dopant fluctuations and fluctuations in the distribution of the molecular clusters in the cell. The simulation framework and the general conclusions of our work are transferrable to flash cells based on alternative molecules used for a storage media

High Open-Circuit Voltages in Lead-Halide Perovskite Solar Cells - Experiment, Theory and Open Questions

One of the most significant features of lead-halide perovskites are their ability to have comparably slow recombination despite the fact that these materials are mostly processed from solution at room temperature. The slow recombination allows achieving high open-circuit voltages when the lead-halide perovskite layers are used in solar cells. This perspective discusses the state of the art of our understanding and of experimental data with regard to recombination and open-circuit voltages in lead-halide perovskites. A special focus is put onto open questions that the community has to tackle to design future photovoltaic and optoelectronic devices based on lead-halide perovskites and other semiconductors with similar properties

Coherent control of photocurrent in a strongly scattering photoelectrochemical system

A fundamental issue that limits the efficiency of many photoelectrochemical systems is that the photon absorption length is typically much longer than the electron diffusion length. Various photon management schemes have been developed to enhance light absorption; one simple approach is to use randomly scattering media to enable broadband and wide-angle enhancement. However, such systems are often opaque, making it difficult to probe photo-induced processes. Here we use wave interference effects to modify the spatial distribution of light inside a highly-scattering dye-sensitized solar cell to control photon absorption in a space-dependent manner. By shaping the incident wavefront of a laser beam, we enhance or suppress photocurrent by increasing or decreasing light concentration on the front side of the mesoporous photoanode where the collection efficiency of photoelectrons is maximal. Enhanced light absorption is achieved by reducing reflection through the open boundary of the photoanode via destructive interference, leading to a factor of two increase in photocurrent. This approach opens the door to probing and manipulating photoelectrochemical processes in specific regions inside nominally opaque media.Comment: 21 pages, 4 figures, in submission. The first two authors contributed equally to this paper, and should be regarded as co-first author

Modeling and Characterization of Efficient Carrier Multiplication in Highly Co-doped Semiconductors and Disordered Materials

This thesis offers modeling of a newly discovered gain mechanism for various photodetection applications. Conventional avalanche photodetectors have been in use for the past four decades with impact ionization being the underlying carrier multiplication mechanism.However, tradeoff between sensitivity, dynamic range and bandwidth are some of the drawbacks of the present day photodetection technology. The newly discovered cycling excitation process (CEP) can be a potential candidate to address these issues with linear photo response, single photon sensitivity and high gain bandwidth product. The key feature of CEP is introduction of counter dopants in p-n junction silicon diode, with which the efficiency of auger excitation can be enhanced to great extent by facilitating relaxation of k selection rule. Higher uncertainty in k spaces dictates localization of carriers in real space. Hence, an initial hot carrier can excite electron-hole pair between localized states (e.g. from states closer to valence band to states closer to conduction band) at much lower bias. Another essential component of CEP is phonon/field assisted tunneling from localized states to mobile bands. Contrary to other photodetectors, phonons, actually, play a positive role in achieving gain. Experimentally gain of ~4000 at only 4V have been achieved in the CEP test structure along with photo response dependence on input light power, which is helpful for photon number resolving. Temperature dependent measurement also shows the positive role of phonons. Density functional theory calculation shows the change in band structure with doping bulk crystalline silicon with boron (B) and phosphorous (P) simultaneously. Comparison of density of states exhibits existence of states inside band gap. Furthermore, charge density plot clearly demonstrates electron and hole localization centered around P and B atoms respectively. Hence, highly counter doping with BP atoms turns the crystalline silicon into a quasi-disordered material. Since, highly counter doping introduces disorder in silicon, with this notion naturally disordered materials are explored as possible CEP gain media. Amorphous materials have low mobility due to their nature of disorder. Surprisingly, amorphous silicon (a-Si) photodiodes with thin a-Si layer (~40nm) have shown a gain-bandwidth product of over 2 THz with very low excess noise. To unveil the true gain mechanism, the thesis further delves into theoretical modeling and numerical analysis along with experimental data at different frequencies. Evidence of highly effective carrier multiplication process within a-Si as the primary gain mechanism, especially at high frequency is shown. There is also trap-induced junction modulation at much lower frequency. The analysis further suggests that the carrier multiplication process in thin a-Si can be much more efficient than in thick a-Si, even stronger than single crystalline Si in some cases. Although seemingly counter intuitive, this is consistent with the proposed cycling excitation process (CEP) where the localized states in the bandtails of disordered materials such as a-Si relax the k-selection rule and increase the rate of carrier multiplication. A more rigorous quantum mechanical scattering rate calculation also demonstrates the increase of strength of carrier multiplication with the presence of localized states and the increase of ionization coefficient with decreasing thickness of gain medium. A theoretical framework is offered to calculate the carrier multiplication process in a-Si or other disordered materials involving donor acceptor pairs (DAPs) and to answer several key and seemingly counter intuitive questions such as why amorphous silicon can be more efficient carrier multiplication material than single crystal silicon, why low carrier mobility of amorphous material helps rather than hurt carrier multiplication process, and why thin a-Si is more efficient than thick a-Si in carrier multiplication

Chapter Advanced Numerical Simulation of Organic Light-emitting Devices

Materials scienc

Asymptotic solution of a model for bilayer organic diodes and solar cells

The current voltage characteristics of an organic semiconductor diode made by placing together two materials with dissimilar electron affinities and ionisation potentials is analysed using asymptotic methods. An intricate boundary layer structure is examined. We find that there are three regimes for the total current passing through the diode. For reverse bias and moderate forward bias the dependency of the voltage on the current is similar to the behaviour of conventional inorganic semiconductor diodes predicted by the Shockley equation and are governed by recombination at the interface of the materials. There is then a narrow range of currents where the behaviour undergoes a transition. Finally for large forward bias the behaviour is different with the current being linear in voltage and is primarily controlled by drift of charges in the organic layers. The size of the interfacial recombination rate is critical in determining the small range of current where there is rapid transition between the two main regimes. The extension of the theory to organic solar cells is discussed and the analogous current voltage curves derived in the regime of interest

Advanced Numerical Simulation of Organic Light-emitting Devices

Materials scienc