Quantifying and Elucidating Thermally Enhanced Minority Carrier Diffusion Length Using Radius-Controlled Rutile Nanowires

Abstract

The minority carrier diffusion length (L) is a crucial property that determines the performance of light absorbers in photoelectrochemical (PEC) cells. Many transition-metal oxides are stable photoanodes for solar water splitting but exhibit a small to moderate L, ranging from a few nanometers (such as α-FeO and TiO) to a few tens of nanometers (such as BiVO). Under operating conditions, the temperature of PEC cells can deviate substantially from ambient, yet the temperature dependence of L has not been quantified. In this work, we show that measuring the photocurrent as a function of both temperature and absorber dimensions provides a quantitative method for evaluating the temperature-dependent minority carrier transport. By measuring photocurrents of nonstoichiometric rutile TiO nanowires as a function of wire radius (19-75 nm) and temperature (10-70 °C), we extract the minority carrier diffusion length along with its activation energy. The minority carrier diffusion length in TiO increases from 5 nm at 25 °C to 10 nm at 70 °C, implying that enhanced carrier mobility outweighs the increase in the recombination rate with temperature. Additionally, by comparing the temperature-dependent photocurrent in BiVO, TiO, and α-FeO, we conclude that the ratio of the minority carrier diffusion length to the depletion layer width determines the extent of temperature enhancement, and reconcile the widespread temperature coefficients, which ranged from 0.6 to 1.7% K. This insight provides a general design rule to select light absorbers for large thermally activated photocurrents and to predict PEC cell characteristics at a range of temperatures encountered during realistic device operation.

An integrated thermoelectric-assisted photoelectrochemical system to boost water splitting

“…3d). These two features are in agreement with the reports on the influence of temperature on PEC water splitting performance [36][37][38][39] The reduced photogenerated voltage of the photoelectrodes, prolonged diffusion distance of minority carriers and improved surface catalytic activity with the increase of electrolyte temperature have been proposed to be main reasons. These features suggest that the increased V oc of the thermoelectric-PEC cell with the temperature increase is caused by the voltage input from the thermoelectric device.…”

Plasmon‐Enhanced Photoelectrochemical Water Splitting for Efficient Renewable Energy Storage

“…Chueh's group has evaluated the temperature-dependent photocurrent of α-Fe 2 O 3 , [144] BiVO 4 , [145] and reduced TiO 2 [146] and found that the minority carrier diffusion length, L D , increased with heating of the semiconductor material. Figure 11a shows that the diameter-dependent photocurrent generated by TiO 2 nanowires increased monotonically with increasing temperature (from 10°C to 70°C).…”

“…a,b) Reproduced with permission. [146] Copyright 2017, American Chemical Society. c,d) Reproduced with permission.…”

Hematite coated, conductive Y doped ZnO nanorods for high efficiency solar water splitting

“…The reasons for this are twofold, firstly the unfavourable morphology measured by SEM indicates a loss in surface area, restricting the access to electrolyte. Secondly, electronhole recombination stifles the performance of thick Fe 2 O 3 film, due to the short minority carrier path length in α-Fe 2 O 3 [42]. This is evidenced by the largest charge transfer resistance, R CT , indicated by the greatest arc radius in its Nyquist plot, figures S6(A) and (B).…”