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Improved Stability and Performance of Visible Photoelectrochemical Water Splitting on Solution-Processed Organic Semiconductor Thin Films by Ultrathin Metal Oxide Passivation Lei Wang, Danhua Yan, David W. Shaffer, Xinyi Ye, Bobby H Layne, Javier J. Concepcion, Mingzhao Liu, and Chang-Yong Nam Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02889 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017
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Chemistry of Materials
Improved Stability and Performance of Visible Photoelectrochemical Water Splitting on Solution-Processed Organic Semiconductor Thin Films by Ultrathin Metal Oxide Passivation Lei Wang†, Danhua Yan†,‡, David W. Shaffer§, Xinyi Ye†, Bobby H. Layne§, Javier J. Concepcion§, Mingzhao Liu†,‡, and Chang-Yong Nam*,†,‡ †
Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, United States ‡
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States
§
Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973, United States
ABSTRACT: Solution-processible organic semiconductors have potentials as visible photoelectrochemical (PEC) water splitting photoelectrodes due to their tunable small band gap and electronic energy levels, but they are typically limited by poor stability and photocatalytic activity. Here, we demonstrate the direct visible PEC water oxidation on solutionprocessed organic semiconductor thin films with improved stability and performance by ultrathin metal oxide passivation layers. N-type fullerene-derivative thin films passivated by sub-2 nm ZnO via atomic layer deposition enabled the visible PEC water oxidation at wavelengths longer than 600 nm in harsh alkaline electrolyte environments with up to 30 µA/cm2 photocurrents at the thermodynamic water-oxidation equilibrium potential and the photoanode half-lifetime extended to ~1000 s. The systematic investigation reveals the enhanced water oxidation catalytic activity afforded by ZnO passivation and the charge tunneling governing the hole transfer through passivation layers. Further enhanced PEC performances were realized by improving the bottom ohmic contact to the organic semiconductor, achieving ~60 µA/cm2 water oxidation photocurrent at the equilibrium potential, the highest values reported for organic semiconductor thin films to our knowledge. The improved stability and performance of passivated organic photoelectrodes and discovered design rationales provide useful guidelines for realizing the stable visible solar PEC water splitting based on organic semiconductor thin films.
1. INTRODUCTION The ever-growing global energy demand has stimulated extensive research efforts focused on developing clean alternative energy sources, and the solar water splitting has been one of them.1-5 Since the first electrochemical water photolysis using TiO2 electrodes by Honda and Fujishima in 1972,6 the direct photoelectrochemical (PEC) water splitting at the semiconductor-electrolyte interface has been extensively investigated.7-9 The ideal semiconductors for PEC water splitting must strongly absorb light in a wide solar spectral range and be efficient in separating photo-generated charge carriers and collecting/transporting them toward the final electrochemical processes.10-12 While large band-gap (Eg) inorganic semiconductors have been widely studied for the PEC water splitting in the past decades,13-15 recent studies are increasingly centered on the semiconductors with narrow Eg because of their wider solar spectral coverages extending to the visible region.4,16 For instance, the PEC water splitting in the wavelength (λ) range over 600 nm was demonstrated from the hematite (Fe2O3, Eg = 2.2 eV) with the incident-photon-to-current conversion efficiency (IPCE)
reaching 46% (at λ = 400 nm).17 The nanoporous bismuth vanadate (BiVO4, Eg = ~2.4 eV) doped with oxygen evolution catalysts also achieved one of the highest photocurrent outputs (~2.7 mA/cm2) at a small applied potential with a spectral coverage up to ~510 nm.18 Despite these advances, it generally remains challenging to tune the inherent Eg of given inorganic semiconductors for further improved PEC water splitting performances. Moreover, the typical fabrication methods for inorganic-based PEC photoelectrodes, such as vacuum deposition, are not costeffective for the large area scalability in general. Meanwhile, organic semiconductors have emerged as useful materials for solar energy conversion, particularly in the photovoltaic device application: Being solutionprocessible at low temperature, they potentially offer significant advantages over the inorganic counterparts because of the cost-effective large-area scaling as well as flexible, light-weight device form factors. Furthermore, their optical and electrical properties (such as Eg and energy levels) can be tuned by controlling their molecular designs to cover a wide solar spectrum.19-23 Such tunable small Eg and electronic properties of organic semiconduc-
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tors are material characteristics in principle utilizable for solar PEC water splitting applications. Indeed, there have been several reports recently, primarily based on the dyesensitized PEC cell device architecture, demonstrating visible-light solar PEC water splitting by using organic dyes as photosensitizers.24 Specifically, organic dyes anchored to mesoporous TiO2 anodes could participate in the water oxidation reaction by transferring photo-excited electrons to TiO2 and holes to the water oxidation catalysts.25-28 Also reported was the direct PEC water oxidation on the solution-processed organic semiconductor thin film. For instance, Kirner et al. used functionalized perylene diimide (PDI) thin films coated with water oxidation catalysts as photoanodes, realizing visible-light water oxidation with photocurrent density (J) over 150 μA/cm2 at 1.6 V applied potential (E) with respect to the reversible hydrogen electrode (E = 1.6 VRHE).24,29 Most recently, the spray-coated poly[benzimidazobenzophenanthroline] (BBL) conjugated polymer thin film combined with Ni-Co catalysts was shown to produce 20 – 30 µA/cm2 oxygen evolution photocurrents by the Sivula group.30 In fact, it is worthy to note that other than these two studies, such a use of solution-processed organic semiconductor thin films for the direct PEC water splitting has been rarely reported, as most of the related prior works are based on vacuumdeposited organic thin films.31-33 Overall, these progresses exemplify the potential of organic semiconductors in visible-light solar water splitting, but all the above demonstrated organic photoelectrodes share one critical issue—the poor stability that, in certain cases, could sustain the PEC water splitting only on the timescale of seconds.24 One potential solution to the problem is the ultrathin metal oxide protective passivation layer applied onto the semiconductor photoelectrodes; we previously demonstrated that the ultrathin (~1 nm) TiO2 passivation applied by atomic layer deposition (ALD) on vertical ZnO nanowire arrays could improve not only their PEC water oxidation performance by removing the surface trap states but also the stability under the strong alkaline electrolyte environment during the PEC water splitting.34 The Sivula group reported one of the first implementations of the ultrathin metal oxide surface layer (0.1 – 2 nm Al2O3) on hematite PEC photoanodes to passivate the surface trap states and reduce the threshold potential required for water oxidation.35 It is the McIntyre group that demonstrated one of the first active uses of ultrathin metal oxide passivation (2 nm TiO2) for improving the stability of PEC photoelectrodes (Si photoanode),36 which was later followed by the application of ultrathin metal oxide (CoOx and TiO2) conformal coatings for improving the stability and activity of BiVO4 photoanodes as reported by Lewis group.37,38 For organic-semiconductor-based photoelectrodes, the application of ultrathin metal-oxide passivation has been relatively less explored, and only recently the Sivula group functionalized a BBL conjugated polymer photoanode by 1 nm thick ultrathin TiO2 tunnel-junction layers to support the Ni-Co water oxidation catalyst to realize the oxygen
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evolution reaction.30 These prior works hint that the ultrathin metal oxide passivation layer may provide a viable solution to the stability issue in the organicsemiconductor-based PEC solar water splitting. Despite these potentials, there have been few systematic studies that actively attempt the application of ultrathin metal oxide passivation to improve the stability of solutionprocessed organic semiconductor PEC photoelectrodes and investigate the critical design parameters of the ultrathin passivation layer for the optimal PEC water splitting performance. In this report, we demonstrate the improved stability and performance of visible-light PEC water oxidation based on solution-processed organic semiconductor thin film photoanodes by applying ultrathin metal oxide passivation layers with controlled thicknesses. Specifically, the organic photoanode is made of the n-type organic fullerene derivative, PC71BM ([6,6]-phenyl C71 butyric acid methyl ester), a solution-processible small-Eg (~2.0 eV) organic semiconductor widely utilized in organic solar cells, with well-established electronic and optical properties.39 The ultrathin (
