Transformation of solar energy into chemical fuels is an attractive energy conversion transformation to address the intermittency of power generation in photovoltaics, which has been a long-standing challenge for development of solar energy ^[1] . Stored solar fuels may then be used when sunlight does not reach the solar panels, enabling continuous, solar-generated energy availability. The fuel cycle addressed in this work is the splitting of water to molecular hydrogen and oxygen, which can be used as fuels or as high-energy precursors in fuel synthesis.

We utilize a cobalt-containing water oxidation catalyst (Co-Pi) that assists the water to oxygen reaction ^[2] . Co-Pi, can be formed either via electrodeposition from Co²⁺ ions in aqueous solutions containing potassium phosphate (KPi) or by processing supported thin-film (800 nm thick) cobalt metal anodes in KPi at pH 7. Here, we use doped silicon wafers, Figure 1, as substrates for either electrodeposition of Co-Pi or processing of cobalt metal thin films into Co-Pi catalyst. Photoanode structures of ITO/Si/Co/Co-Pi, ITO/Si/ITO/Co-Pi and ITO/Si/ITO are prepared and tested for photo-assisted water oxidation activity.

Differences in water oxidation performance of each photoanode are particularly clear in Figure 2, which shows a comparison of steady-state current versus applied potential under dark and light conditions. The ITO/Si/ITO electrode passes very little current (10 μA/cm²) compared to the Co-Pi loaded electrodes and shows negligible change between light and dark conditions. The ITO/Si/ITO/Co-Pi electrode exhibits increasing dark current densities as voltage is applied and demonstrates additional increase under illumination (current offset between dark and light conditions reaches 200 μA/cm² at 1.35 V). The ITO/Si/Co/Co-Pi electrode achieves current densities higher than the ITO/Si/ITO/Co-Pi electrode in both dark and light conditions. The dark/light current offset over the catalytic regime of the electrode (between 0.85 V to 1.35 V) increases from 100 μA/cm2 (at 0.85 V) to 1 mA/cm² (at 1.35 V) and even higher. These results demonstrate the importance of material selection and device engineering for harnessing the solar spectrum towards catalytic chemical fuel production. The robust and abundant silicon electrode and direct processing of the cobalt metal produces an improved light-assisted water oxidation device.

1. T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets and D. G. Nocera, “Solar energy supply and storage for the legacy and nonlegacy worlds,” Chem. Rev. vol. 110, pp. 6474-6502, 2010.
2. M. W. Kanan and D. G. Nocera, “In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co²⁺,” Science vol. 321, pp. 1072-1075, 2008.

Figure 1: (a) Non-resonant pumping scheme that is impeded by exciton annihilation, preventing polariton build up in the well. (b) Intra-cavity pumping scheme that populates polaritons along the entire dispersion curve. (c) Device structure. (d) Spectra showing the overlap of the emission of the DCM pump with the absorption of the strongly coupled J-aggregate.

The strongly coupled states of light and matter in microcavities known as polaritons offer exciting possibilities for the study of condensation, superfluidity, and other condensed matter phenomena and show promise as a radically new class of optoelectronic devices based on the macroscopic coherence of light and matter ^[1] . In particular, organic materials allow for strong coupling and polariton lasing to be achieved at room temperature and with substantially reduced requirements for cavity quality factor ^{[2] [3]} . In our work, we demonstrate room temperature polariton lasing in a lambda-thick microcavity where a highly absorbing thin film of J-aggregates serves as the strong coupling material (Figure 1c). Typically, polaritons in the cavity are created by excitation with non-resonant laser pulses. Subsequently, the hot excitons couple to the cavity and move along the polariton dispersion towards the bottom of the polariton trap (Figure 1a). However, a loss mechanism known as annihilation  is present in organic materials which prevents a high density of polaritons from being formed in the trap, and hence polariton condensation (lasing) cannot be achieved. We employ a new pumping scheme known as intra-cavity pumping that circumvents annihilation losses inherent to organic materials at high excitation densities (Figure 1b) by pumping the polariton mode from within the cavity using another organic material (the laser dye DCM) (Figure 1c and 1d). Using this flexible cavity architecture, polariton lasing at room temperature has been achieved. Current work focuses on the application of the polariton laser as a low-energy, ultra-fast all-optical switch and on a better understanding of the properties of the polariton condensate.

1. H. Deng and Y. Yamamoto, “Exciton-polariton Bose-Einstein condensation,” Reviews of Modern Physics, vol. 82, pp. 1489-1537, May 2010.
2. S. Kéna-Cohen and S. R. Forrest, “Room-temperature polariton lasing in an organic single-crystal microcavity,” Nature Photonics, vol. 4, pp. 371-375, Apr. 2010.
3. J. R. Tischler, M. S. Bradley, Q. Zhang, T. Atay, A. Nurmikko, and V. Bulovic, “Solid state cavity QED: Strong coupling in organic thin films,” Organic Electronics, vol. 8,  pp. 94-113, 2007.