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

The minority carrier diffusion length (L_D) 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_D, ranging from a few nanometers (such as α-Fe₂O₃ and TiO₂) to a few tens of nanometers (such as BiVO₄). Under operating conditions, the temperature of PEC cells can deviate substantially from ambient, yet the temperature dependence of L_D 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_2-x 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_2-x 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 BiVO₄, TiO₂, and α-Fe₂O₃, 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^-1. 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.

Broadband quantum efficiency enhancement in high index nanowire resonators

Light trapping in subwavelength semiconductor nanowires (NWs) offers a promising approach to simultaneously reducing material consumption and enhancing photovoltaic performance. Nevertheless, the absorption efficiency of a NW, defined by the ratio of optical absorption cross section to the NW diameter, lingers around 1 in existing NW photonic devices, and the absorption enhancement suffers from a narrow spectral width. Here, we show that the absorption efficiency can be significantly improved in NWs with higher refractive indices, by an experimental observation of up to 350% apparent external quantum efficiency in lead sulfide NW resonators, a 3-fold increase compared to Si NWs. Furthermore, broadband absorption enhancement is achieved in single tapered NWs, where light of various wavelengths is absorbed at segments with different diameters. Overall, the single NW Schottky junction solar cells benefit from optical resonance, near bandgap open circuit voltage, and long minority carrier diffusion length, demonstrating power conversion efficiency comparable to Si and III-V single NW coaxial p-n junction cells but with much simpler fabrication processes.

Long minority carrier diffusion lengths in bridged silicon nanowires

Nanowires have large surface areas that create new challenges for their optoelectronic applications. Lithographic processes involved in device fabrication and substrate interfaces can lead to surface defects and substantially reduce charge carrier lifetimes and diffusion lengths. Here, we show that using a bridging method to suspend pristine nanowires allows for circumventing detrimental fabrication steps and interfacial effects associated with planar device architectures. We report electron diffusion lengths up to 2.7 μm in bridged silicon nanowire devices, much longer than previously reported values for silicon nanowires with a diameter of 100 nm. Strikingly, electron diffusion lengths are reduced to only 45 nm in planar devices incorporating nanowires grown under the same conditions. The highly scalable silicon nanobridge devices with the demonstrated long diffusion lengths may find exciting applications in photovoltaics, sensing, and photodetectors.