Measurement of minority-carrier diffusion lengths using wedge-shaped semiconductor photoelectrodes

Abstract

Measurement of the photocurrent as a function of the thickness of a light absorber has been shown herein both theoretically and experimentally to provide a method for determination of the minority-carrier diffusion length of a sample. To perform the measurement, an illuminated spot of photons with an energy well above the band gap of the material was scanned along the thickness gradient of a wedgeshaped, rear-illuminated semiconducting light absorber. Photogenerated majority carriers were collected through a back-side transparent ohmic contact, and a front-side liquid or Schottky junction collected the photogenerated minority carriers. Calculations showed that the diffusion length could be evaluated from the exponential variation in photocurrent as a function of the thickness of the sample. Good agreement was observed between experiment and theory for a solid-state silicon Schottky junction measured using this method. As an example for the application of the technique to semiconductor/ liquid-junction photoelectrodes, the minority-carrier diffusion length was determined for graded thickness, sputtered tungsten trioxide and polished bismuth vanadate films under back-illumination in contact with an aqueous electrolyte. This wedge technique does not require knowledge of the spectral absorption coefficient, doping, or surface recombination velocity of the sample. Broader contextThe development of solar fuel systems that convert clean and abundant solar energy to efficient fuels can provide a carbon-neutral energy resource for the future. An efficient solar-fuel system requires light absorbing materials with high quantum efficiency and a band gap that matches the free energy of the water-splitting reaction. However there are few Earth-abundant semiconducting light absorbers that meet these requirements and have a sufficiently long minority-carrier diffusion length to allow charge-carrier collection. To identify and develop new materials, rapid characterization techniques are required to evaluate their electrical and optical properties. Here we theoretically and experimentally demonstrate a simple technique for minority-carrier diffusion length measurement in semiconductors. Our technique utilizes the measurement of photocurrent decay along the thickness gradient of a wedge-shaped semiconductor lm and does not require knowledge of the surface recombination velocity or spectral absorption coefficient of the sample. The proposed technique is quite general and applicable for wide range of semiconductor systems.

Quantification of the loss mechanisms in emerging water splitting photoanodes through empirical extraction of the spatial charge collection efficiency

“…† The sharp decay in f(z) agrees well with expectations for diffusion-limited photocurrent, in which the concentration of minority carriers decays with distance from the space charge region. 24,[35][36][37][38][39][40] From Fig. 2(b), it can be seen that the vast majority of the collected carriers are those generated within the first 10 nm of the surface of the electrode, which is in good agreement with our prior work, in which the collection length was determined to be approximately 20-40 nm by analysis of film thickness-dependent photocurrents.…”

“…[20][21][22][23], requires fabrication of full device stacks that are measured under vacuum, which prohibits use of this method for characterization of photoelectrochemical systems under real operating conditions. To overcome this characterization gap and extract the diffusion length, Pala et al 24 suggested measuring photocurrent changes in wedge-shaped thin films. However, such geometries are difficult to fabricate and their behavior may deviate significantly from the performance of planar structures.…”

Heterogeneous photocatalysts: an overview of classic and modern approaches for optical, electronic, and charge dynamics evaluation

“…Another somewhat innovative approach was recently reported by Pala et al using wedge-shaped semiconductor photoelectrodes. 25 They showed that for a sample with a thickness (w) much larger than w d , the photocurrent (J) is approximated as a constant factor multiplied by the reciprocal exponential of w/L D (i.e., J p exp(Àw/L D )). Using a wedge-shaped semiconductor photoelectrode deposited on a transparent substrate ( Fig.…”

Current progress in developing metal oxide nanoarrays-based photoanodes for photoelectrochemical water splitting

“…Compared with the state-of-art nanoporous photoelectrodes, the PEC performance obtained by the BiVO 4 NW arrays is significantly lower. Some studies indicated that reducing BiVO 4 nanosize to be below its hole diffusion length would significantly enhance electron-hole separation [368,369]. Therefore, reducing the diameter of BiVO 4 NWs is important to achieve high PEC performance.…”