Atomic Layer Deposited TiO2-IrOx Alloys Generate High Photovoltage for Silicon Photoanodes
Authors: Hendricks Olivia L. , Chidsey Christopher E.D. , McIntyre Paul C. 
Published: February 2020

Photoelectrochemical (PEC) water splitting is a promising approach for synthesizing chemical fuels from solar energy. First demonstrated by Fujishima and Honda in 1972,1 PEC cell components and design strategies have proliferated in recent years.2 Regardless of the specific device architecture, however, a PEC device requires three main capabilities: a mechanism for extracting photogenerated carriers, an efficient catalyst for water oxidation, and a corrosion resistant coating to prevent oxidation of the underlying photoabsorber.
Integrating efficient catalysts, protection layers, and high-quality semiconductors remains a challenge. The materials properties associated with these three functions are often incompatible. For example, while TiO2 is highly corrosion resistant, its electronic properties are not suitable for generating a large photovoltage. Many catalysts for water oxidation, such NiOx and FeOx, also fail to generate large photovoltages, and they lack stability across a wide pH range. Buried p+n homojunctions are often employed to set the built-in field for extracting photogenerated carriers, making the photovoltage independent of the electronic properties of the catalyst or protection layer. Though photovoltages as high as 630 mV have been achieved using this strategy, buried homojunctions are costly and require extra fabrication steps. Moreover, many relevant semiconductors for PEC water splitting cannot be doped to form high quality homojunctions.
In this work, we use atomic layer deposition (ALD) to fabricate alloys of TiO2 and IrOx that protect silicon from corrosion without compromising the photovoltage. The photoanode structure is shown in Figure 1, in which TiO2-IrOx alloys function as the “M” of an MIS junction. The 2 nm chemical vendor SiO2 functions as the “I,” enabling interface defect passivation without inhibiting collection by the metal contact. By alloying TiO2 with IrOx, we combine the corrosion resistance of TiO2
3 with the high work function4,5 and catalytic activity of IrOx.6 TiO2-IrOx alloys with 18-35% Ir relative to Ti generated average photovoltages of 551 mV on n-type silicon with a maximum photovoltage of 638 mV. TiO2 imparts stability in acid, and the SiO2/alloy interface remained intact after 12 hours of testing in 1 M H2SO4.

Figure
1
. Structure of silicon MIS photoanode coated with TiO2-IrOx alloys grown by atomic layer deposition. The SiO2 thickness is approximately 2 nm as measured by ellipsometry. The chemical vendor oxide is used for all samples.
Charge transport through the alloy/SiO2/Si junction was found to depend strongly on the composition of the alloy. Electrochemical impedance spectroscopy (EIS) demonstrated that tunneling across the SiO2 was the dominant source of resistance in the photoanode, not the bulk conductivity of the alloy. As the % iridium at the SiO2/alloy interface decreases, the density of states at the interface decreases, as well. This results in a higher tunneling resistance.
Though we used silicon as a model system, our ALD alloying approach is particularly valuable for semiconductors that rely on MIS junctions to generate large photovoltages. Unlike other thin film deposition techniques, ALD enables precise control over both the film thickness and composition. As such, ALD is capable of addressing many of the challenges associated with fabricating carrier selective contacts in phtoelectrochemical devices.

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Figure 1