A New Class of Solar Cells: Isomeric Boron Carbide Semiconductors with Fourth Quadrant Conductivity
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A New Class of Solar Cells: Isomeric Boron Carbide Semiconductors with Fourth Quadrant Conductivity Ravi B. Billa,1 A. N. Caruso2, and J. I. Brand3,* 1 Department of Mechanical Engineering 2 Department of Physics 3 College of Engineering and Center for Materials Research and Analysis, University of Nebraska-Lincoln 245N WSEC, Lincoln, NE. 68588-0511 USA * Author to whom correspondence should be addresed ABSTRACT Previously, we have made diodes[1,2] and transistors [3] as well as very effective realtime solid state neutron detectors [4] out of semiconducting boron carbide deposited on silicon or silicon carbide.. In this work the recent fabrication of a new class of highly photosensitive boron carbide diodes is discussed. These diodes exploit the electronic behavior differences of the isomers of film precursors, the closo-dicarbododecaboranes. These differences were observed in photoemission and inverse photoemission studies where the HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap variations upon deposition varied strongly with the isomeric configuration. Based on these results, p-n junctions were formed by plasma enhanced chemical vapor of ortho and meta carborane, respectively, on both nickel and aluminum substrates. These diodes exhibit fourth-quadrant conductivity, making them exciting new photovoltaic conversion devices. INTRODUCTION Semiconducting boron carbide was first proposed in 1958 and realized in 1991. Homojunctions, heterojunctions, transistors, high-temperature diodes, and real time solid state neutron detectors have been fabricated from this in the ensuing years[1-5]. Recently, a pure boron carbide (heteroisomeric device) photovoltaic device was formed from this material in a unique way, by exploiting the bandgap differences. Semiconducting boron carbide is not a single identified compound, but belongs to the complex molecular family sometimes referred to as the “borane, carborance, carbocation continuum” [7]. Composed of varying numbers of boron and carbon atoms (with a few terminal and bridging hydrogens) and arranged in arachno and nido open structures and the closed cage (closo) forms, compounds sharing the same stoichiometry can exhibit different chemical reactivity and local electronic distributions [7,8]. Theoretical calculations from semiempirical modified neglect of differential overlap (MNDO) calculations of the isolated molecules [9,10].indicate that the isoelectronic carboranes should be very similar in HOMO-LUMO gap (highest occupied molecular orbital - lowest unoccupied MO). These calculations predicted 10.97 eV fror the orthocarborane and 10.87 eV for the metacarborane. Since the two carboranes differ only in the positions of the two included carbon atoms and theory predicted the electronic
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states to be so close, great similarity in chemical and physical behavior would be expected. Surprisingly, photoemission experiments shoe that molecular films from these two isomers have significantly different band offsets! These photoemiss
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