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Acid-Compatible Halide Perovskite Photocathodes Utilizing Atomic Layer Deposited TiO2 for Solar-Driven Hydrogen Evolution Downloaded via 95.85.70.44 on January 5, 2019 at 09:24:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
In Soo Kim,§ Michael J. Pellin, and Alex B. F. Martinson* Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Argonne−Northwestern Solar Energy Research (ANSER) Center, Evanston, Illinois 60208, United States S Supporting Information *
ABSTRACT: Although solution-processable halide perovskite semiconductors exhibit optoelectronic performance comparable to the best photoabsorbers for solar fuel production, halide perovskites rapidly decompose in the presence of water or even humid air. We show that a hybrid electron transport layer, a PC61BM + TiO2 film (18−40 nm thickness) grown over the sensitive absorber by atomic layer deposition, enables photoassisted proton reduction without further encapsulation. These semitransparent photocathodes, when paired with a Pt catalyst, display continuous reduction of H+ to H2 for hours under illumination, even while in direct contact with a strongly acidic aqueous electrolyte (0.5 M H2SO4). Under 0.5 Sun illumination, a photocurrent density of >10 mA cm−2 is observed, and a photovoltage of 0.68 V assists proton reduction, consistent with a structurally related photovoltaic (PV) device. Submersible halide perovskite photoelectrodes point the way to more efficient photoassisted overall water splitting and other solar fuel generation using solution-processed semiconductors with tunable band gaps.
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physical passivation of the photoabsorber with a chargeselective barrier. This approach has recently been applied to several high-purity inorganic semiconductors including Si, GaAs, and GaP.3,10 We consider here low-cost and solutionprocessed halide perovskites, which have attracted much attention as photoabsorbers, primarily for application in PVs. Due to their tunable direct band gap,16−18 small exciton binding energy,19 long carrier lifetime,20 and ample charge carrier diffusion length,21 PV efficiencies in excess of 20% have been certified. Despite such benefits, however, halide perovskites have proven notoriously sensitive to environmental exposure including moisture, oxygen, and heat,22−24 hindering long-term PV operation under heat and humidity in the absence of macroscopic encapsulation and ostensibly disqualifying direct photoassisted solar fuel generation directly in aqueous solution. Several strategies have been proposed to improve the semiconductor’s stability against moisture, including compositional tuning17,25 and two-dimensional perovskites.26,27 A few encapsulation schemes28−30 have also been devised to protect against humidity. However, none have come close to providing the level of passivation necessary for
he Sun provides a predictable source of energy for the foreseeable future, which we are increasingly and more affordably converting into electricity with photovoltaics (PVs).1 However, the intermittency of the solar resource relative to our energy demand, combined with the challenge and cost of densely storing electrical energy, motivates the pursuit of solar-driven (or at least solar-assisted) chemical fuel synthesis. Chemical fuels offer high volumetric energy density; however, semiconductors must meet significantly more stringent standards in order to be intimately integrated into solar fuel generation systems. For example, stability in strongly acidic or alkaline aqueous electrolytes is required for the most facile water splitting systems. Several III−V semiconductors (e.g., GaP) exhibit suitable band gap and excellent charge transport characteristics but are unstable in aqueous electrolytes.2,3 On the other extreme are metal oxides, some of which are relatively stable in aqueous alkaline electrolytes (e.g., Fe2O3) but are transparent to most solar photons due to their large band gaps4−6 or exhibit inadequate charge lifetime and mobility for efficient charge extraction.7,8 Thus, empirical9−12 and theoretical13−15 efforts to identify new semiconductors that may be suitable for solar fuel generation remains an important field of research. A second approach to direct incorporation of sensitive semiconductors into solar fuel generation systems is through © XXXX American Chemical Society
Received: September 5, 2018 Accepted: November 13, 2018
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DOI: 10.1021/acsenergylett.8b01661 ACS Energy Lett. 2019, 4, 293−298
Letter
Cite This: ACS Energy Lett. 2019, 4, 293−298
Letter
ACS Energy Letters
Figure 1. Photoassisted proton reduction with halide perovskite photocathodes. (a) Cross-sectional schematic of a photocathode stabilized against corrosion in 0.5 M H2SO4(aq) via a hybrid ETL comprising PC61BM with TiO2 deposited by ALD. Photogenerated electrons are conducted through the TiO2 to the Pt catalysts, where electrons are used to reduce protons to hydrogen. (b) Photoelectrochemical behavior of ITO/PEDOT:PSS/halide perovskite/PC61BM/TiO2/Pt photocathode tested under 0.5 Sun. The dark response of a ITO/TiO2/Pt electrode reveals the shift in the proton reduction onset potential for the photoelectrode (0.68 V). (c) Photoelectrochemical behavior of the photocathode in the dark and under continuous and chopped illumination reveals the time scale (ms) and magnitude (>10 mA cm−2) of the photocurrent response.
Figure 2. Configuration of halide perovskite-based photocathodes. (a) Overview of the halide perovskite photocathode experiment. (b) False color cross-sectional SEM image of the photocathode. The total thickness of the hybrid ETL, which protects the halide perovskite absorber, is ∼150 nm. (c) Schematic illustration of a full photocathode stack in the dark after equilibration, with approximate relative energy levels of the conduction and valence bands.
nature of the hybrid ETL grown by ALD. The organic component of the ETL, PC61BM, serves two important functions. First, it accepts electrons from the halide perovskite and reduces recombination at this interface more effectively than TiO2 deposited by ALD at a temperature within the thermal budget of the halide perovskite.28 Second, the PC61BM serves to protect the moisture-sensitive halide perovskite film from the ALD process conditions, which includes short doses of low partial pressure water vapor. In the solar fuel generating version reported herein, the triple-cation formulation of the halide perovskite photoabsorber was selected as it offers several advantages over the simple methylammonium lead iodide (MAPbI3), including improved thermal stability, superior optoelectronic performance, and greater reproducibility. Moreover, the addition of Cs has previously been shown to induce smooth and uniform halide perovskite films,25 which we hypothesize to be a critical factor in fabricating and maintaining a pinhole-free hybrid ETL with optimal passivation performance. Additionally, the Al + Au top electrode previously described is replaced by a discontinuous Pt film (∼15 nm effective film thickness, Figure S1) that accepts electrons from the TiO2 in order to catalyze proton reduction because the two-electron chemistry does not occur readily at the TiO2 interface without the aid of a catalyst. Photocathodes lacking ALD TiO2, even in the absence of any electrochemical bias, exhibited instantaneous degradation upon contact with all aqueous electrolytes tested, while those utilizing PC61BM + ALD TiO2 (with or without Pt deposition) did not degrade within 5 h. An idealized energy diagram of the photocathode is illustrated in Figure 2c. The energy level diagram reveals a staggered type-II band offset in which
stabilization against an aqueous electrolyte with the exception of devices utilizing thick and opaque metal to bury standard halide perovskite-based PVs.31−35 Unfortunately, even this costly approach does not endow stability in acidic aqueous environments, precluding its use in the most advantageous electrolytes for water splitting. More importantly, the thick top electrode is completely opaque to solar photons, rendering the device unsuitable for many tandem photoelectrode designs including those in which the perovskite photoelectrode is the larger of two gaps (as is often the case given the nearly ideal band gap of perovskite absorbers for the top cell), those that offer greater flexibility for oxygen-evolving catalyst choice, those that seek to avoid the cost of thick Field’s metal, and those that operate in strongly acidic electrolyte. In contrast, we demonstrate a simple strategy for integrating a conventional halide perovskite absorber into a photocathode that is compatible with strongly acidic aqueous electrolytes and is further semitransparent. The photocathode, shown schematically in Figure 1a, consists of a hole transport layer (HTL, PEDOT:PSS) on conductive glass (ITO) onto which a triple-cation halide perovskite photoabsorber Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(I0.83Br0.17)3 is deposited, followed by a robust hybrid electron transport layer (ETL, PC61BM + ALD TiO2) similar to the one that we recently introduced28 for use in more robust PVs. Finally, a semitransparent ultrathin film of Pt serves as a catalyst for the hydrogen evolution reaction (HER). The complete photocathode stacks are related to the “inverted” perovskite PV device structure previously reported by our group,28 which exhibited remarkable resistance to both extreme heat and humidity due to the apparently pinhole-free 294
DOI: 10.1021/acsenergylett.8b01661 ACS Energy Lett. 2019, 4, 293−298
Letter
ACS Energy Letters
Figure 3. Operational stability of halide perovskite-based photocathodes in acidic water. (a,b) Optical images of a photocathode in direct contact with 0.5 M H2SO4 prior to and during hydrogen generation, where (i), (ii), and (iii) are the Ag/AgCl reference electrode, Pt counter electrode, droplet of 0.5 M H2SO4, and halide perovskite-based photocathode, respectively. (c) Photocurrent density versus time of a photocathode with a nominally 15 nm thick Pt catalyst under continuous illumination. The electrode potential was held at 0 V vs RHE during continuous illumination of 0.5 Sun.
tion. Furthermore, 1000 times more current is passed (72 Coulombs or 4.5 × 1020 electrons/cm2) during stability testing than there are atoms feasibly reduced (Pb2+) in the active layer (∼4 × 1017 Pb atoms/cm2), which further sets an upper limit on potential decomposition-derived current to
