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Three-dimensionally ordered macroporous tungsten carbide (3DOM WC) with varying pore size was synthesized using the “inverse-opal” method. Poly{methyl methacrylate} (PMMA) colloidal crystal template with sphere diameters ranging from 180 to 490 nm was used. Two synthesis procedures were performed, the first requiring calcination of tungsten impregnated PMMA colloidal template in air followed by carburization in H2/CH4/N2 atmosphere. The second procedure involved direct carburization of the tungsten impregnated template without prior calcination. The effect of Pt-modification of the template prior to carburization on the resulting material composition was also investigated. The material properties were evaluated using surface and bulk characterization techniques. Finally, cyclic voltammetry was used to investigate the methanol electro-oxidation activity of the 3DOM WC and Pt-modified 3DOM WC in acidic electrochemical environment.

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ABSTRACT:The optical transparency and high electron mobility of graphene make it an attractive material for photovoltaics. We present a field-effect solar cell using graphene to form a tunable junction barrier with an Earth-abundant and low cost zinc phosphide (Zn 3 P 2 ) thin-film light absorber. Adding a semitransparent top electrostatic gate allows for tuning of the graphene Fermi level and hence the energy barrier at the graphene-Zn 3 P 2 junction, going from an ohmic contact at negative gate voltages to a rectifying barrier at positive gate voltages. We perform current and capacitance measurements at different gate voltages in order to demonstrate the control of the energy barrier and depletion width in the zinc phosphide. Our photovoltaic measurements show that the efficiency conversion is increased 2-fold when we increase the gate voltage and the junction barrier to maximize the photovoltaic response. At an optimal gate voltage of +2 V, we obtain an open-circuit voltage of V oc = 0.53 V and an efficiency of 1.9% under AM 1.5 1-sun solar illumination. This work demonstrates that the field effect can be used to modulate and optimize the response of photovoltaic devices incorporating graphene. KEYWORDS: Graphene, zinc phosphide, field effect solar cell, Schottky barrier, earth-abundant materials, photovoltaics T he extraordinary properties of graphene, such as its high carrier mobility, Fermi-level tuning and its high optical transparency, make it an attractive candidate to improve the performance of optoelectronic and photovoltaic devices. 1−3 These properties can be exploited for tackling some of the major challenges in solar energy. One challenge is to exploit Earth-abundant and low-cost synthesis materials with band gaps suitable for solar energy conversion. Besides common PV absorbers like Si, CdTe, and CIGS, there are other materials that have the potential for large scale solar implementation at lower costs, such as zinc phosphide (Zn 3 P 2 ), copper zinc tin sulfide (CZTS), cuprous oxide (Cu 2 O), and iron sulfide (FeS 2 ). 4 However, the lack of doping processes necessary to form homojuntions and the absence of complementary emitter materials to form high quality p−n heterojunction have limited the conversion efficiency of devices incorporating these materials. These limitations can be addressed using graphene, which has previously been used to form graphene−semiconductor junctions 5 and solar cells with CdS and CdSe 6,7 semiconductors as well as with Si,5,[8][9][10][11] demonstrating Schottky barrier behavior. 5 Further advances on graphene−semiconductor junctions were demonstrated by building gate-controlled devices in order to modify the Schottky barrier height and the electrical transport across the junction. This design has been used to build a graphene−Si transistor (barristor) 12 and a graphene−organic thin film junction transistors. 9,13 Herein, we study the barrier tuning of a graphene−Zn 3 P 2 field-effect solar cell with a semitransparent top-gate electrode and demonstrate that this tunin...

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