Multifunctional Nickel Film Protected n‐Type Silicon Photoanode with High Photovoltage for Efficient and Stable Oxygen Evolution Reaction

Abstract

n‐type silicon (n‐Si) is a promising photoanode candidate for photoelectrochemical (PEC) water splitting. However, severe chemical corrosion, sluggish reaction kinetics as well as extremely low photovoltage are critical limitations hampering its PEC performance. This paper describes the introduction of a metallic nickel (Ni) thin film as a multifunctional layer that 1) forms a Schottky junction to extract a high photovoltage, 2) provides an electrocatalytic surface for oxygen evolution reaction (OER), and 3) protects Si against corrosion. Upon introducing a high‐quality Al2O3 tunneling layer to optimize the Si/Ni interface and loading nickel oxide hydroxide to further accelerate the OER, this metal–insulator–semiconductor junction photoanode achieves a record high photovoltage of 640 mV and energy conversion efficiency of 3% in n‐Si based photoanodes without np or np+ buried homojunction, yielding an air mass 1.5G photocurrent density of ≈28 mA cm−2 at 1.23 V (vs reversible hydrogen electrode) for oxygen evolution in alkaline solution with a stability of more than 80 h.

Quantifying Losses and Assessing the Photovoltage Limits in Metal–Insulator–Semiconductor Water Splitting Systems

“…The SiO 2 layer is evident from the ≈0.5 nm white layer between the Si and HfO 2 . An adventitious SiO 2 layer has been observed in other ALD studies on HF‐etched Si,31,48,53 and it was similar for all samples so its effect should be uniform among samples.…”

“…Finally, we note that several MIS water splitting systems have achieved over 600 mV of photovoltage through a combination of using relatively high doped Si and using high‐quality oxides with minimal surface states 30,45,53. As shown by the black region in Figure 6, even an ideal system with optimized insulator thickness may still fall significantly short of the photovoltage limits.…”

“…Many insulators have been utilized in silicon‐based MIS photocatalyst systems. These include SiO 2 ,28–41 TiO 2 ,42–47 HfO 2 ,48,49 Al 2 O 3 ,42,50–53 SrTiO 3 ,54 and ZrO 2. 55 Initially, MIS systems were motivated by their ability to improve the chemical stability of semiconductors, but recently many design strategies have been identified to improve the efficiency of these systems.…”

“…Another strategy to obtain a large barrier height is through the pinch‐off effect, where a high barrier electrolyte or oxide can compensate for low barrier electrocatalytic metal nanoparticles 29,32,34,40,41,57. Previous reports have also demonstrated that the insulator layer can serve as a passivation layer to remove defects and minimize barrier height losses from Fermi level pinning 50,51,53. Furthermore, several studies have shown that annealing is an effective method to further passivate interfacial defects and improve the photovoltage 31,45,47.…”

Dissociating Water at n‐Si Photoanodes Partially Covered with Fe Catalysts

“…In order to prevent the degeneration of Si photoanodes, numerous studies reported on the use of protection layers consisting of insulating or semiconducting materials. These layers are typically applied onto Si by physical deposition methods such as atomic layer deposition (ALD) . In addition to this protective layer, a catalytic coating, generally deposited by evaporation or sputtering, is commonly used to enhance the OER kinetics …”