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

摘要

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 In2S3 ALD on TiO2 nanotube arrays defined a model semiconductor sensitized solar cell. For In2S3 ALD on initial Al2O3 ALD surfaces, the In2S3 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 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.