Semiconducting Aromatic Boron Carbide Films for Neutron Detection and Photovoltaic Applications

Semiconducting aromatic-boron carbide composite/alloyed films formed by plasma enhanced chemical vapor deposition from carborane and aromatic precursors have been demonstrated to be excellent detectors for thermal neutrons because of the large 10B cross section. The electronic properties of these films derived from XPS show that the properties of boron carbide can be tuned by co-deposition of aromatic compounds and carborane. Aromatic doping results in narrower indirect band gaps (1.1 - 1.7 eV vs ~3 eV for orthocarborane-derived boron carbide without aromatics) and average charge transport lifetimes (as long as 2.5 ms for benzene-orthocarborane and 1.5 - 2.5 ms for indole-orthocarborane) that are superior to those of boron carbide (35 µs). The films also show enhanced electron-hole separation that is also superior to those of boron carbide where the states at the top of the valence band is made of aromatic components while states at the bottom of the conduction band is a combination of aromatic and carborane moeities. These properties result in greatly enhanced (~850%) charge collection, relative to films without aromatic content, in thermal neutron exposures at zero-bias, and are gamma-blind. Such films are therefore excellent candidates for zero-bias neutron detector applications. These properties also show little variation with increasing aromatic content beyond a critical concentration, indicating that at some point, excess aromatic results in the formation of regions of polymerized aromatic within the film, rather than in additional carborane/aromatic linkages. While previous studies on these aromatic-boron carbide materials indicate the potential for neutron detection due to the narrowed band gap, enhanced electron-hole separation and charge transport lifetimes compared to the boron carbide counterpart, the mechanisms of charge transport and photoconductivity (important for photovoltaic applications) of these materials have remained unexplored. Properties such as narrowed band gap, efficient electron-hole separation and long charge transport lifetimes, are also desirable in photovoltaic applications. This, plus ease of fabrication and environmental robustness makes aromatic-boron carbide films promising candidates for photovoltaic applications. Plasma enhanced chemical vapor deposition (PECVD) has been used to synthesize these aromatic-boron carbide composite films by co-deposition of pyridine, aniline or indole with orthocarborane/metacarborane. Film chemical composition and bonding were characterized by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), variable angle spectroscopic ellipsometry (VASE) and (in collaboration with Dowben Group at UNL) charge transport and photoconductivity measurements. Results show narrowed band gaps (indirect) where the top of the valence band is made up of the aromatic moiety and the conduction band minimum us made of aromatic and carborane moeities, improved charge carrier mobilities that is stoichiometry and frequency dependent (aniline-orthocarborane films). Photoconductivity measurement results obtained from ~2.6:1 indole-orthocarborane film show fourth quadrant conductivity. I(V) curves indicate a photocurrent of 2.36 µA at zero bias, with an appreciable open-circuit voltage of 1V. The ability for these aromatic-boron carbide films to operate at zero bias for both neutron detection and photovoltaic applications is an excellent advantage that indicates low cost of operation of these materials.