Charge Transfer Characterization of ALD-Grown TiO₂Protective Layers in Silicon Photocathodes

Abstract

A critical parameter for the implementation of standard high-efficiency photovoltaic absorber materials for photoelectrochemical water splitting is its proper protection from chemical corrosion while remaining transparent and highly conductive. Atomic layer deposited (ALD) TiO layers fulfill material requirements while conformally protecting the underlying photoabsorber. Nanoscale conductivity of ALD TiO protective layers on silicon-based photocathodes has been analyzed, proving that the conduction path is through the columnar crystalline structure of TiO. Deposition temperature has been explored from 100 to 300 °C, and a temperature threshold is found to be mandatory for an efficient charge transfer, as a consequence of layer crystallization between 100 and 200 °C. Completely crystallized TiO is demonstrated to be mandatory for long-term stability, as seen in the 300 h continuous operation test.

Role of Bismuth in the Electrokinetics of Silicon Photocathodes for Solar Rechargeable Vanadium Redox Flow Batteries

“…It has been demonstrated that it can effectively hinder this reaction thanks to the strong proton adsorption on the TiO 2 , which might restrict H 2 formation . However, besides having an effect in this aspect, TiO 2 layers have been used as protective layers of silicon photovoltaics, thanks to its good stability at different pH conditions, especially when it is not polarized at very negative potentials . Interestingly, the obtained results with the photocathode in presence of bismuth evidence that less negative voltages are necessary for performing the electrochemical reaction, so that less harsh conditions are necessary during cycling, assuring better stability for the photoelectrode and the TiO 2 protective layer.…”

“…Here, we report for the first time on the photoassisted negative half‐reaction (V 3+ /V 2+ ) in presence of bismuth and its role in the electrochemistry of the system, by using a photocathode based on a n + ‐p silicon buried junction (see the Supporting Information for details), protected by a 100 nm TiO 2 layer deposited by atomic layer deposition (TiO 2 /n + ‐p‐Si) . Initially, to analyze the effect of bismuth in the direct‐charge injection, n + degenerated silicon cathodes (TiO 2 /n + ‐Si) were also evaluated in different media.…”

“…[10] However,b esides having an effect in this aspect, TiO 2 layers have been used as protective layers of silicon photovoltaics, thanks to its good stability at different pH conditions, especially when it is not polarized at very negative potentials. [14] Interestingly,t he obtainedr esults with the photocathode in presence of bismuth evidence that less negative voltages are necessary for performing the electrochemical reaction, so that lessh arsh conditions are necessary during cycling, assuring better stability for the photoelectrode and the TiO 2 protectivel ayer.After these tests, measurements with the photocathodes were performed under chopped illumination ( Figure S2). Besides the shift in the potentials also previously observed, a sharp increasei nt he cathodic photocurrent at higherp olarization in the absence of bismuth is seen.…”

“…[12] Here, we report for the first time on the photoassisted negative half-reaction (V 3 + /V 2 + )i np resence of bismuth andi ts role in the electrochemistry of the system,b yu sing ap hotocathode based on an + -p silicon buried junction (see the Sup- porting Information for details), protected by a1 00 nm TiO 2 layer deposited by atomicl ayer deposition (TiO 2 /n + -p-Si). [14] Initially,t oa nalyze the effect of bismuth in the direct-chargei njection, n + degenerated silicon cathodes (TiO 2 /n + -Si)w ere also evaluated in different media. These resultsu nder dark conditions are shown in Figure 1A.I nt he first situation, the measurement was performed with V 3 + in H 2 SO 4 electrolyte and withoutB iCl 3 .U nder that situation, the V 3 + /V 2 + reaction (indicated by number 2) presentsa no nset potential (E onset )o f around À0.45 V RHE (see Table S1), whereas the reverse V 2 + /V 3 + process (indicated as 2')i sa lmost negligible, showingas ignificant peak separation.…”

Photoelectrocatalytic Materials for Solar Water Splitting

“…Being like n‐Si photoanodes, the pn‐Si buried junction and p‐Si based heterojunction were commonly used to construct the photocathodes. The photocathodes with pn + ‐Si buried junction and p‐Si/SiO 2 /Ti MIS heterojunction usually show more positive onset potential (0.5–0.64 V, vs RHE) for water reduction due to its large barrier height. Pt is an active electrocatalyst for hydrogen evolution, but direct deposition of Pt on p‐Si presented a smaller onset potential (0.27 V, vs RHE) .…”