Photocurrent Enhancement by Spontaneous Formation of a p–n Junction in Calcium‐Doped Bismuth Vanadate Photoelectrodes

Abstract

The application of bismuth vanadate (BiVO4) photoelectrodes for solar water splitting is hindered by the poor carrier transport. To overcome this, multiple donor‐doping strategies (e.g. dual doping, gradient doping) have been explored. Here, we show for the first time the successful introduction of calcium (Ca) as an acceptor‐type dopant into BiVO4 photoelectrodes. Interestingly, instead of generating cathodic photocurrents, the Ca‐doped BiVO4 photoelectrodes show anodic photocurrents with an enhanced carrier separation efficiency. Hard X‐ray photoelectron spectroscopy (HAXPES) shows that this enhancement is caused by out‐diffusion of Ca during the deposition process, which spontaneously creates a p–n junction within the BiVO4 layer. Overall, a significant two‐fold improvement of the AM1.5 photocurrent is obtained upon Ca‐doping. This study highlights the importance of controlled doping beyond simply modifying carrier concentration and may enable new device architectures in photoelectrode materials.

Charge Transport and Gradient Doping in Nanostructured Polypyrrole Films for Applications in Photocurrent Generation

“…For the thicker samples, however, where the doping level is 27.6%, an adequate built-in potential to provide a photoresponse is available. The use of such events has already been reported for photoelectrodes, solar cells, and water reduction applications. − …”

Demonstration of a 50 cm2 BiVO4 tandem photoelectrochemical-photovoltaic water splitting device

“…2,3 A particularly interesting feature of BiVO 4 is its relatively low onset potential, which is typically $0.2-0.4 V RHE (potential with respect to reversible hydrogen electrode, RHE) with appropriate surface modications. 4,5 We, and others, have reported various strategies to modify and improve the performance of BiVO 4 photoanodes, including homogeneous and gradient doping, 5,6 surface modication with co-catalysts, [5][6][7] formation of hetero-/ homo-junctions, 8,9 and nano-structuring. 4 Photocurrent densities as high as 6.7 mA cm À2 at 1.23 V RHE in nanostructured heterojunction samples have been reported using tungsten oxide nanorods as scaffolds for the BiVO 4 top layer with cobalt phosphate (CoP i ) surface modication.…”

Tunnel Junction Engineered Photocarrier Dynamics in Epitaxial Semiconductor Nanowires for Efficient and Ultrafast Photoelectrochemical Photodetectors

“…Under the light illumination, electron–hole pairs are generated, and due to the downward band bending at the p-InGaN nanowire and electrolyte interface, the photogenerated electrons will drift to the nanowire surface (solid arrow in Figure c) and are able to participate in the HER process at the surface; however, due to the built-in electric field at the p–n junction, the electrons may also migrate toward the substrate (dashed arrow in Figure c), making the electron transfer to the nanowire surface not highly efficient. Moreover, the built-in electric field may also block the hole transport toward the substrate, limiting the photoresponse. , As such, a low photoresponse might be expected.…”