Development of a-SiC Thin Film Photoelectrodes and Hybrid PV/a-SiC Devices for Photoelectrochemical Water Splitting
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Development of a-SiC Thin Film Photoelectrodes and Hybrid PV/a-SiC Devices for Photoelectrochemical Water Splitting J. Hu,1) F. Zhu,1) A. Kunrath,1) and N. Gaillard 2) 1
MVSystems, Inc., Golden, Colorado 80401, USA 2
Hawaii Natural Energy Institute, Honolulu, Hawaii 96822, USA
ABSTRACT In this communication, we report our efforts to develop amorphous silicon carbide (a-SiC) thin film photoelectrodes integrated with Si solar cells to form a monolithic, hybrid photovoltaic (PV)/a-SiC device capable of water splitting using sunlight as the only energy source. The main photoelectrochemical (PEC) properties of both the a-SiC photoelectrode and complete hybrid device fabricated by the plasma enhanced chemical vapor deposition (PECVD) technique at low temperature (≤ 200ºC) are discussed. The surface modification with metal nanoparticles, which is critical to PEC performances of the hybrid device, is also described. We show that, with the an a-SiC photoelectrode of p-i-n configuration and a high performance silicon heterojunction solar cell as driver, the photocurrent of the hybrid PV/a-SiC device has reached ~5 mA/cm2. Additionally, the durability of such device has reached ~800 hours in acidic electrolyte. Finally, we describe a roadmap for achieving the solar-to-hydrogen efficiency of >10% by optimizing the device configuration. INTRODUCTION Photoelectrochemical (PEC) dissociation of water into hydrogen and oxygen at a semiconductor-liquid interface, first reported in the early 1970’s [1], offers an environmentally benign approach to hydrogen production. For a semiconductor material to be used for PEC water splitting, its band gap must be of a minimum ~1.7 eV to provide the potential necessary for electrolysis and overcome energy losses in the system. Also, the minority carrier band edge and the Fermi level of the material must straddle the hydrogen and oxygen redox potentials, and the material must be durable in contact with the electrolyte. These criteria set significant restrictions on selection of suitable photoelectrode materials for PEC water splitting. To this day, no single system or material having adequate optical absorption and proper band edge alignment has been identified. Hybrid photovoltaic (PV)/PEC devices address this problem by having a solar cell supplying the additional energy necessary to drive the water splitting reactions [2]. In this approach, a 1.7~2 eV bandgap photocatalyst semiconductor with efficient light absorption can be used in conjunction with the PV cell. We have previously developed a hybrid PV/PEC device consisting of amorphous silicon carbide (a-SiC) photocathode and amorphous silicon (a-Si) tandem solar cell, fabricated by the plasma enhanced chemical vapor deposition (PECVD) technique at low temperature (≤200ºC) [3-6]. Under AM1.5G illumination and in pH2 electrolyte, the Fermi level at the ohmic back contact of this hybrid device is below the H2O/O2 half-reaction potential (by +0.97 V, vs. Ag/AgCl), while the conduction band edge is above the water reduction half-reaction potent
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