| AVS 62nd International Symposium & Exhibition | |
| Thin Film | Wednesday Sessions |
| Session TF+EN-WeM |
| Session: | ALD for Energy |
| Presenter: | Andrew Scheuermann, Stanford University |
| Authors: | A.G. Scheuermann, Stanford University J.P. Lawrence, Stanford University K.W. Kemp, Stanford University O.L. Hendricks, Stanford University A. Walsh, Tyndall National Institute I. Povey, Tyndall National Institute M.E. Pemble, Tyndall National Institute P.K. Hurley, Tyndall National Institute C.E.D. Chidsey, Stanford University P.C. McIntyre, Stanford University |
| Correspondent: | Click to Email |
Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and chemical synthesis. ALD-TiO2 is the most widely used material because of its excellent stability under water oxidation conditions and potential for high electrical conductivity both as an ultrathin film and with thicknesses exceeding 100 nm [1-3]. However, the most conductive ALD-TiO2 films exhibit poor photovoltages of ~ 400 mV and less [3]. Even assuming near-ideal fill factors, these voltages fall far short of the values needed for viable water splitting devices. Photovoltage optimization is especially difficult to achieve in MOS photoanodes because of the presence of a defective metal oxide protection layer and a defective semiconductor/oxide interface in the device structure. Therefore, understanding how to optimize photovoltage and stability is of utmost concern for the advancement of the field.
Here we report a novel observation of photovoltage loss associated with charge transfer in these metal-oxide protected devices, and by eliminating it, achieve photovoltages as high as 630 mV, the maximum reported to date for single-junction water-splitting silicon cells. The loss mechanism is systematically probed in MOS Schottky junction cells compared to buried junction p+n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A capacitor model that predicts this loss is developed, and is related to the dielectric properties of the protective oxide, achieving excellent agreement with the data. From these findings, we extract design principles for simultaneous optimization of charge transfer resistance and interface quality to maximize the photovoltage of metal-oxide protected MOS water splitting devices.
[1] Y.W. Chen, et al. Nature Mat. 2011, 10, 539-544.
[2] A. G. Scheuermann, et al. Energy Environ. Sci. 2013, 6, 2487–2496.
[3] S. Hu, et al. Science 2014, 344, 1005−1009.
Supplemental Figure 1 | Charge transfer in three cell types for water splitting applied to silicon: Shows the Type 0 Semiconductor-Liquid (SL), Type 1 Metal-Insulator-Semicondcutor (MIS), and Type 2 p+n junction. The density of states on either side of the oxides and the excition splitting position with respect to these layers play a crucial role in mediating efficient charge transfer. These effects are so strong that Type 0 protected silicon cells exhibit essentially no photovoltage, Type 1 nSi cells show a linear photovoltage loss with oxide thickness, and Type 2 cells--where the hole concentration on the Si/SiO2 interface is always high--exhibit record photovoltages at all oxide thicknesses and pH values studied.