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

In 2 S 3 atomic layer deposition (ALD) with indium acetylacetonate (In(acac) 3 ) and H 2 S was studied with quartz crystal microbalance (QCM), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy techniques. Subsequent In 2 S 3 ALD on TiO 2 nanotube arrays defined a model semiconductor sensitized solar cell. For In 2 S 3 ALD on initial Al 2 O 3 ALD surfaces, the In 2 S 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 Å 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 H 2 S exposures. The FTIR examinations revealed that the nucleation period on Al 2 O 3 ALD surfaces may be related to the formation of Al(acac)* species that act to poison the initial Al 2 O 3 ALD surface. X-ray diffraction investigations revealed β-In 2 S 3 ALD films and X-ray photoelectron measurements were consistent with In 2 S 3 films. The In 2 S 3 ALD was employed as a semiconductor sensitizer on TiO 2 nanotube arrays for solar conversion. Scanning electron microscopy and energy dispersive X-ray analysis imaging revealed In 2 S 3 over the full length of the TiO 2 nanotube array after 175 cycles of In 2 S 3 ALD at 150 °C at reactant exposure conditions that were self-limiting on flat substrates. The photoelectrochemical properties of these In 2 S 3 ALD-sensitized TiO 2 nanotube arrays with a Co 2+ /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.