Localization of charge carriers in crystalline solids has been the subject of numerous investigations over more than half a century. Materials that show a metal-insulator transition without a structural change are therefore of interest. Mechanisms leading to metal-insulator transition include electron correlation (Mott transition) or disorder (Anderson localization), but a clear distinction is difficult. Here we report on a metal-insulator transition on increasing annealing temperature for a group of crystalline phase-change materials, where the metal-insulator transition is due to strong disorder usually associated only with amorphous solids. With pronounced disorder but weak electron correlation, these phase-change materials form an unparalleled quantum state of matter. Their universal electronic behaviour seems to be at the origin of the remarkable reproducibility of the resistance switching that is crucial to their applications in non-volatile-memory devices. Controlling the degree of disorder in crystalline phase-change materials might enable multilevel resistance states in upcoming storage devices.

Disorder-induced localization in crystalline phase-change materials

The growing technological importance of conducting polymers makes the fundamental understanding of their charge transport extremely important for materials and process design. Various hopping and mobility edge transport mechanisms have been proposed, but their experimental verification is limited to poor conductors. Now that advanced organic and polymer semiconductors have shown high conductivity approaching that of metals, the transport mechanism should be discernible by modelling the transport like a semiconductor with a transport edge and a transport parameter s. Here we analyse the electrical conductivity and Seebeck coefficient together and determine that most polymers (except possibly PEDOT:tosylate) have s = 3 and thermally activated conductivity, whereas s = 1 and itinerant conductivity is typically found in crystalline semiconductors and metals. The different transport in polymers may result from the percolation of charge carriers from conducting ordered regions through poorly conducting disordered regions, consistent with what has been expected from structural studies.

Charge-transport model for conducting polymers

This chapter covers the current use and challenges of thin-film silicon solar cells, including conductivities and doping, the properties of microcrystalline silicon (the role of the internal electric field, shunts, series resistance problems, light trapping), tandem and multijunction solar cells (a-Si:H/a-Si:H tandems, triple-junction amorphous cells, microcrystalline–amorphous or “micromorph” tandems), module production (deposition of thin-film layers, substrate materials, laser scribing, cell interconnection, and module encapsulation), and module performance.

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Thin-Film Silicon Solar Cells

This book presents the current level of understanding of the structural, electronic and optical properties of amorphous semiconductors. As amorphous materials depart significantly from the crystalline counterparts, some of the basic problems associated with the validity of the effective mass approximation, whether K is a good quantum number, and concepts of phonons and excitons will be addressed in detail. A significant part of the book is devoted to present recent progress made in the understanding of light-induced degradations in amorphous semiconductors, which is regarded as the most limiting problem in device applications. The monograph presents a comprehensive review of both experimental and theoretical studies on amorphous semiconductors.

Advances in Amorphous Semiconductors

This review concerns the concept of multi-excitation entropy (MEE) and its consequences When a fluctuation involving a large number of excitations occurs, for example, when a large activation barrier is overcome, there must be a large entropy associated with this fluctuation First, the concepts of free energy and entropy, of activated processes and the Arrhenius and Eyring equations are reviewed The tendency to neglect entropy, whose value is difficult to determine, in modelling kinetic processes, is briefly discussed We then present a review of the experimental observations of the phenomenon which is variously known as the Compensation Law, the Isokinetic Rule and the Meyer–Neldel Rule (MNR) These observations include examples from chemistry, condensed matter physics, biology and geology Arguments are then presented for the importance of entropy and particularly of MEE in both kinetics and thermodynamics, when activation energies are large After a discussion of non-entropic models of compensation, we present results which support the MEE model as an explanation of MNR The behaviour of systems with low activation energies, or at high temperatures, to which the MEE model does not apply, is then discussedSeveral consequences of MEE, including applications to the interpretation of experimental data, particularly the unification of models of dc and ac electrical properties of materials are considered The high temperature behaviour of systems which obey the MNR at low temperature is then explained, and the idea of a total entropy, of which the MEE is a part, is introduced, as is the correlation between the two empirical parameters encountered in MNR Finally, these ideas lead to verified predictions of reasonable values of attempt frequencies and cross sections in kinetic processes, which initially appear unreasonable

https://iopscience.iop.org/article/10.1088/0034-4885/69/4/R04/pdf

Multi-excitation entropy: its role in thermodynamics and kinetics

Elementary treatment of transport in amorphous semiconductors.- Review of transport experiments.- A transport theory for a homogenous model.- Detailed presentation of the theory.- The long-ranged random potential.- Temperature dependent reference energies.- The transport properties of hydrogenated amorphous semiconductors.- Concluding remarks.

Peter Thomas

Papers

Electronic Transport in Hydrogenated Amorphous Semiconductors

The Statistical Shift Model for the Meyer-Neldel Rule

Electronic Transport in Disordered Semiconductors

Temperature dependent reference energies

The transport properties of hydrogenated amorphous semiconductors