Significantly enhanced photocurrent for water oxidation in monolithic Mo:BiVO sub(4)/SnO sub(2)/Si by thermally increasing the minority carrier diffusion length

• Published in: Energy & environmental science 2016-06, Vol.9 (6), p.2044-2052
• Main Authors: Zhang, Liming, Ye, Xiaofei, Boloor, Madhur, Poletayev, Andrey, Melosh, Nicholas A, Chueh, William C
• Format: Article
• Language: English
• Subjects: Minority carriers; Photocurrent; Photoelectric effect; Saturation; Semiconductors; Tin dioxide; Titanium dioxide; Tungsten oxides
• ISSN: 1754-5692; 1754-5706
• DOI: 10.1039/c6ee00036c

Transition-metal-oxide semiconductors are promising photoanodes for solar water splitting due to their excellent chemical stability and appropriate bandgaps. However, in absorbers such as BiVO sub(4), TiO sub(2), alpha -Fe sub(2)O sub(3), and WO sub(3), charge carriers localize as small polarons or become trapped, leading to low minority carrier mobilities. This limits the minority carrier collection efficiency in the quasi-neutral region of the light absorber, and lowers the overall photoactivity. In this work, we demonstrate that modestly elevating the temperature activates minority carrier hopping in monoclinic BiVO sub(4), significantly enhancing the saturation photocurrent without a substantial anodic shift of the onset potential, and is an attractive alternative to employing complex passivation layers and nanostructured templates towards achieving the theoretical photocurrent density. Specifically, using a Mo:BiVO sub(4)/SnO sub(2)/Si tandem photoanode/photovoltaic, increasing the absolute temperature by 11% from 10 to 42 degree C elevates the saturation photocurrent from 1.8 to 4.0 mA cm super(-2). This strong temperature enhancement, 3.8% K super(-1), is 5 times greater than that in alpha -Fe sub(2)O sub(3). Concurrently, the onset potential shifts slightly from 0.02 V to 0.08 V versusthe reversible hydrogen electrode (or equivalently, from -1.22 V to -1.13 V versusthe equilibrium potential of oxygen evolution). Our observation contrasts with the prevailing understanding that the energy conversion efficiency generally decreases with temperature as a result of reduced photovoltage. Thermally-activating minority carrier transport represents a general pathway towards enhancing the photoactivity of light absorbers where hopping conduction limits the minority carrier collection in the quasi-neutral region.