| AVS 64th International Symposium & Exhibition | |
| Thin Films Division | Wednesday Sessions |
| Session TF-WeM |
| Session: | Thin Film for Photovoltaics |
| Presenter: | Taro Yamada, The University of Tokyo, Japan |
| Authors: | T. Yamada, The University of Tokyo, Japan Y. Sasaki, The University of Tokyo, Japan S. Suzuki, Shinshu University, Japan M. Zhong, The University of Tokyo, Japan K. Teshima, Shinshu University, Japan K. Domen, The University of Tokyo, Japan |
| Correspondent: | Click to Email |
Tritantalum pentanitride (Ta3N5), a visible-light absorbing material (<600 nm), was formed into a thin layer and tested as a photoelectrode in electrolytic solution under simulated solar light. By the aid of cobalt phosphate catalyst (CoPi), the electrode surface evolved O2 at a substantial efficiency for photon energy conversion. By inserting a GaN layer between the catalyst and Ta3N5, the electrode exhibited an improved stability and durability for O2 evolution in aqueous electrolytic solution.
The Ta3N5 film was prepared by high-temperature (1000-1300 K) nitridation of sputtered Ta film on Ta metallic substrate in NH3 flow at the atmospheric pressure. Ta sputtering was performed in a RF magnetron setup with a Ta metal target at 4 Pa of purified Ar introduction pressure. The gas impurities were minimized and a trace amount of O2 was introduced for sputtering. The formed Ta film is amorphous with a thickness approximately 500nm. After nitridation, the formed Ta3N5 is a transparent, orange-colored film with a visible light absorption cutoff at xxx nm. The CoPi catalyst was deposited by simulated solar irradiation in Co(NO3) 3 solution in neutral phosphate buffer.
The O2 evolution performance was examined in pH 13 K3PO4 buffer under potentiostatic control with a Pt counter electrode and a Ag/AgCl reference electrode. At 1.23 V vs RHE of the anodic potential, The photocurrent by a solar simulator at AM1.5G (1 kWm-2) reached 7 mAcm-2 or higher, however rapidly decay in 1 hour. This was due to Ta3N5 oxidation, and we introduced a GaN overlayer (thickness ~ 50 nm) by NH3-nitridation of evaporated Ga2O3 layer (electron-beam heating, largely metallized). The same photoelectrochemical test exhibited a photocurrent higher than 8 mAcm-2, lasting for more than 10 hours, with slow deactivation afterwards. Microscopic observation of the GaN layer showed fine grains (<50 nm in diameter) of GaN, blocking Ta3N5 oxidation. The faradaic O2 evolution was confirmed by anther setup.
The semiconductive properties of Ta3N5 and the GaN/Ta3N5 junction are currently investigated. Both Ta3N5 and GaN in the present case are polycrystalline and contain influential amounts of impurities, reflecting in the resistivity and other transportation properties.
The photoelectrochemical performance of the present GaN/Ta3N5 electrode matches nearly 5 % of solar-to-H2 energetic conversion as a free-run water photo-splitting device by assembling a with H2 evolving photocathode. By improving the quality of the layers of Ta3N5 and the GaN in terms of crystallinity and purity, we can gain a good control for the performance of the photoelectrodes towards efficient solar energy recovery.