In2S3 Atomic Layer Deposition and Its Application as a Sensitizer on TiO2 Nanotube Arrays for Solar Energy Conversion

In₂S₃ atomic layer deposition (ALD) with indium acetylacetonate (In(acac)₃) and H₂S was studied with quartz crystal microbalance (QCM), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy techniques. Subsequent In ₂S₃ ALD on TiO₂ nanotube arrays defined a model semiconductor sensitized solar cell. For In₂S₃ ALD on initial Al₂O₃ ALD surfaces, the In₂S ₃ ALD displayed a nucleation period of ∼60-70 cycles followed by a linear growth region. These results were obtained under ALD conditions that were not completely self-limiting for the In(acac)₃ exposure because of the low In(acac)₃ vapor pressure. The growth per cycle decreased at higher temperature over the temperature range from 130 to 170 °C at these same reactant conditions. The growth per cycle was 0.30-0.35 A per cycle at 150 °C as determined by QCM and XRR measurements at higher In(acac)₃ exposures where the surface reactions were self-limiting chemistry versus In(acac)₃ and H₂S exposures. The FTIR examinations revealed that the nucleation period on Al₂O₃ ALD surfaces may be related to the formation of Al(acac)* species that act to poison the initial Al₂O₃ ALD surface. X-ray diffraction investigations revealed β-In₂S₃ ALD films and X-ray photoelectron measurements were consistent with In₂S₃ films. The In₂S₃ ALD was employed as a semiconductor sensitizer on TiO₂ nanotube arrays for solar conversion. Scanning electron microscopy and energy dispersive X-ray analysis imaging revealed In₂S₃ over the full length of the TiO₂ nanotube array after 175 cycles of In₂S₃ ALD at 150 °C at reactant exposure conditions that were self-limiting on flat substrates. The photoelectrochemical properties of these In₂S₃ ALD-sensitized TiO₂ nanotube arrays with a Co²⁺/Co ³⁺ electrolyte were then characterized by measuring the photocurrent density versus voltage and the external quantum efficiency versus photon energy. A small quantum efficiency of ∼10% was observed that can be attributed to charge recombination losses and charge injection/collection processes.