The research in David Cohen's laboratory is directed at trying to obtain a detailed understanding of the electronic properties of disordered semiconductors.  Most of such materials currently under study are of interest in efforts to develop inexpensive thin film photovoltaic devices (solar cells).  Indeed, all of the research funding comes from sources, including the Department of Energy and the National Renewable Energy Laboratory (NREL), interested in such technologies. 

Specific semiconducting materials currently being studied include amorphous silicon (a‑Si:H), the amorphous silicon-germanium alloys (a‑Si,Ge:H), nanocrystalline silicon, and copper indium diselenide (CuInSe2 or “CIS”).  In the last case our ongoing studies include compounds in which indium is alloyed with gallium or aluminum, and/or where selenium is alloyed with sulfur.

Fundamental studies amorphous, or non-crystalline, materials such as amorphous silicon represent one of the true "frontier" areas of solid state physics. Whereas solids with crystalline order have yielded to a very sophisticated theoretical description, theories of the behavior of amorphous materials are still very primitive. This lack of a suitable detailed description of such a large group of materials (that also includes all types of glasses) has been challenging some of our fundamental understanding of all solids. 

Cohen's laboratory has been studying the detailed behavior of electronic states within the mobility gap of amorphous silicon and related alloys for over 2 decades.  A "mobility gap" is the analog of the more familiar energy gap in crystalline semiconductors. Very detailed pictures of the energy distribution of these mobility gap states have been obtained by studying the transient response of amorphous silicon samples following electrical or laser pulse excitation. This electronic energy distribution can then be correlated with sample growth parameters, impurity content, or with the performance of amorphous silicon devices.  Indeed, in spite of the lack of a detailed theoretical understanding, these materials have already had an enormous technological impact since they are used to fabricate the “active matrix” of transistors that drive nearly all LCD flat panel television sets and computer monitors.

A relatively newer but related material under study for the past several years in the Cohen Lab is “nanocrystalline silicon” (nc‑Si:H).  This is actually a mixed phase form of silicon consisting of nanocrystallites, 20-30nm in size interspersed with regions of amorphous silicon, plus about 10at.% hydrogen.  Due to its mixed phase nature, nc‑Si:H is perhaps an even more complex material to understand than amorphous silicon itself.  However, by employing experimental methods nearly unique in the world, researchers in Cohen’s laboratory have been able to distinguish whether photo-generated carriers originate in the amorphous region or in Si nanocrystallites.  Such information is expected to greatly aid in the optimization of such mixed phase materials in solar cells; particularly in tandem solar cells structures with amorphous silicon.

Since 1999 the Cohen laboratory has also devoted a large fraction of its resources toward the study of copper indium diselenide and related alloys.  “CIS” is a crystalline semiconductor with the chalcopyrite crystal structure and it is a true ternary; that is, each of three types of atoms occupies a unique site in the crystal lattice.  However, there are many other stable crystal phases of Cu, In, and Se of similar atomic ratios as the chalopyrite phase.  This results in a high degree of compositional variation within the crystal structure of these compounds.  Indeed, if this did not occur, it would be very difficult to obtain CIS with good semiconducting properties since, in semiconductors, an atomic imbalance of even a few parts per million alters (“dopes”) the electronic properties to an enormous degree.  Instead, variations in the atom ratios (stochiometries) can exceed 10at.% in these CIS compounds without significantly modifying their conductivities or how they behave in electronic devices.   Thus, whereas the properties of amorphous silicon are difficult to understand because of structural disorder, the challenge in understanding the properties of the chalcopyrite semiconductors is related to their compositional disorder. 

Work in the Cohen laboratory on the CIS materials began with a number of studies to examine the fundamental electronic properties of this class of materials.  This led, for example, to the discovery of a new defect level in the gap having a threshold of 0.8eV for optical excitation from the valence band.  More notably it was found that this energy threshold did not change when the gap of these CIS materials were increased by Ga or Al alloying.  This indicated that this new defect level would likely become a dominant recombination center in the higher band-gap chalcopyrite alloys, possibly explaining why such higher gap alloys did not perform as well as expected.  This has led more recently to detailed studies of the properties of high band-gap alloys produced by other types of compositional variations, such as by substituting a substantial fraction of selenium with sulfur in addition to substituting In with Ga.