Correlation of the Photoelectrochemistry of the Amorphous Hydrogenated Silicon/Methanol Interface with Bulk Semiconductor Properties

Abstract

Semiconductor/liquid junctions derived from 0.5 μm thick films of amorphous hydrogenated silicon, a‐Si:H, have been investigated in CH₃OH solvent. The a‐Si:H films consist of a weakly doped n‐type layer over a 200Å n+‐a‐Si:H layer on a stainless‐steel substrate. The low series resistance and high ratio of minority carrier collection length to film thickness in this arrangement allows a study of the properties of semiconductor/liquid interfaces with minimal interference from bulk resistance losses. We find that a‐Si:H anodes in 0.02M ferrocene, FeCp₂/0.5 mM FeCp₂⁺/1.5 M LiClO₄/CH₃OH solutions exhibit poor short‐circuit quantum yields and low fill factors with 632.8 nm irradiation, but that these junctions display internal quantum yields of close to unity and high fill factors with short wavelength (λ < 450 nm) irradiation. Photons absorbed within a distance comparable to the minority carrier collection length are efficiently collected, and the fill factors and quantum yields under such conditions are insensitive to increases in photocurrent density over a range of 0.1–2 mA/cm². Solar‐simulated irradiation (88 mW/cm²) from a ELH‐type tungsten‐halogen lamp in the a-Si:H/0.02 M FeCp₂/0.5 mM FeCp₂⁺/1.5 M LiClO₄/CH₃OH system yields open‐circuit photovoltages of 0.75–0.85V, short‐circuit photocurrents of 6–7 mA/cm², and photoelectrode efficiencies for conversion of light to electricity of 2.7%–3.3%. Photovoltages with the acetylferrocene^(+/0) redox system are among the highest reported for any a‐Si:H surface barrier system, and can exceed 0.85V under AM1 illumination conditions. Variation in the redox potential of the solution leads to changes in open‐circuit photovoltage in accord with theory, and does not yield evidence for pinning of the a‐Si:H Fermi level by interface states or by surface oxides over the potential range investigated. The observed photoelectrochemical behavior of these a‐Si:H films is consistent with documented bulk transport and carrier collection properties of a‐Si:H, and does not show evidence for series resistance losses, unusual spectral response characteristics, or recombination sites at the semiconductor/liquid junction. Manipulation of the electronic properties of the a‐Si:H films can thus lead to improved energy conversion parameters and a clearer picture of the chemistry at the a‐Si:H/liquid interface.

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