Understanding Photovoltage in Insulator-Protected Water-Splitting Half-Cells

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis.  ALD-TiO₂ remains 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-TiO₂films 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 in MOS photoanodes due to the range of factors which influence the conductivity and stability of the protection layer as well as the presence of electrically active defect states at the semiconductor/oxide interface. 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.

Figure 1 | Charge transfer in three cell types for water splitting applied to silicon: 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.