High-Performance Perovskite Photoanode Enabled by Ni Passivation

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High-Performance Perovskite Photoanode Enabled by Ni Passivation and Catalysis Peimei Da, Mingyang Cha, Lu Sun, Yizheng Wu, Zhong-Sheng Wang, and Gengfeng Zheng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b00788 • Publication Date (Web): 27 Apr 2015 Downloaded from http://pubs.acs.org on April 28, 2015

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High-Performance Perovskite Photoanode Enabled by Ni Passivation and Catalysis ⊥

⊥

Peimei Da†, , Mingyang Cha†, , Lu Sun‡, Yizheng Wu‡, Zhong-Sheng Wang†,*, and Gengfeng Zheng†,*

†

Department of Chemistry, Laboratory of Advanced Materials, Collaborative Innovation Center of

Chemistry for Energy Materials, Fudan University, Shanghai, China. ‡

Department of Physics, State Key Laboratory of Surface Physics, Laboratory of Advanced Materials,

Fudan University, Shanghai, China. ⊥

P.D. and M.C. contributed equally to this work.

* Address correspondence to: [email protected] (G. Zheng), [email protected] (Z.-S.Wang).

Abstract: Lead halide perovskites have achieved phenomenal successes in photovoltaics, due to their suitable bandgaps, long diffusion lengths and balanced charge transport. However, the extreme susceptibility of perovskites to water or air has imposed a seemingly insurmountable barrier for leveraging these unique materials into solar-to-fuel applications such as photoelectrochemical conversion. Here we developed a CH3NH3PbI3-based photoanode with an ultrathin Ni surface layer, which functions as both a physical passivation barrier and a hole-transferring catalyst. Remarkably, a ACS Paragon Plus Environment

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much enhanced photocurrent density, an unassisted photoelectrochemical conversion capability, and a substantially better stability against water have been achieved, which are exceeding most of the previously reported photoanodes as well as a similar CH3NH3PbI3-based device structure but without the Ni surface layer. Our study suggests many exciting opportunities of developing perovskite-based solarto-fuel conversion.

KEYWORDS

perovskite; solar energy; photoanode; Ni; passivation

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Lead halide perovskite (CH3NH3PbX3, X = Cl, Br, I) solar cells have gained phenomenal successes in recent years, attributed to their attractive features including high absorption coefficient, ambipolar charge-carrier mobilities, long exciton lifetimes/diffusion length, and low exciton binding energy.1 A certified power conversion efficiency of 17.9% has been recorded by National Renewable Energy Laboratory (NREL).2 Very recently, it has also been shown that two perovskite solar cells in tandem can serve as a separated and external power source to generate enough voltage for water splitting.3 On the other hand, the direct utilization of perovskite for solar-to-fuel conversion such as photoelectrochemical (PEC) photoanode materials in electrolytes is even intriguing, which is more attractive to cheaper H2 generation.4 However, due to their hygroscopic amine salts and distorted crystal structure, perovskites are extremely susceptible to UV light,5 solution process,6 temperature,7 oxygen and moisture,8 which not only limit the long-term stability of perovskite-based solar cells, but also nullify their direct use as PEC photoanodes. Coatings of thin films of Pd,9 Pt,10,11 Ni12 or TiO213 have been demonstrated as an effective passivation barrier for improving the stability of Si, GaP and GaAs photoanodes in water oxidation. Among these surface layers, Ni has further exhibited exceptional potential as an earth-abundant catalyst for oxygen evolution reaction, thus enabling enhanced photoactivity.12,13 This dual functional Ni layer has thus inspired us to develop a multi-layered perovskite-based photoanode, which is for the first time, to the best of our knowledge, capable of inheriting the advantages of perovskite solar cells and further functioning under PEC conditions. In order to potentially leverage the findings of this work, herein, the perovskite device was designed and fabricated based on standard multi-layered thin-film solar cell architectures (Figure 1a). Briefly, a thin compact amorphous TiO2 layer was first deposited onto a fluorine-doped tin oxide (FTO) glass substrate. Perovskite (CH3NH3PbI3) was then deposited on top of this TiO2 layer using an antisolvent spin-coating method14 (Methods in the Supplementary Information). Afterwards, the CH3NH3PbI3 surface was covered with spiro-MeOTAD as a hole transporting material (HTM) layer, and then a Au layer as the top electrode. Finally, an ultrathin Ni layer was deposited by magnetron sputtering ACS Paragon Plus Environment

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as the most external layer of this photoanode. Each of these layers should be fabricated as flat with minimum pinholes as possible, thus enabling conformal protection by the subsequent layers. The schematic band diagram of each component is sketched (Figure 1b), where the band positions are well aligned for charge separation. This photoanode can then be completely immersed in an electrolyte to construct a photoelectronchemical cell with the reference and counter electrodes (Measurements in Supplementary Information). Under sunlight (from back illumination), the photogenerated electrons in CH3NH3PbI3 are collected by the compact TiO2-coated FTO and subsequently flow through the external circuit to the counter electrode, where water is reduced by accepting electrons to produce hydrogen gas. The holes are transferring through the spiro-MeOTAD layer to the Au/Ni layers for oxidation of sacrificial electrolytes (such as S2-) in the solution. Importantly, the top Ni layer serves as not only a physical barrier for electrolyte penetration to the active perovskite layer, but also an efficient oxidation catalyst with the generated holes. As a proof-of-concept, perovskite photoanodes with an ultrathin Ni surface layer exhibit a remarkably high photocurrent density, an unassisted PEC conversion characteristic, and a functional stability in an alkaline solution for a record-long PEC test of 15-20 min. The cross-sectional structure of the multi-layered CH3NH3PbI3 photoanode is displayed (Figure 2a). The compact TiO2 blocking layer is approximately 50 nm thick, and the CH3NH3PbI3 layer is 220250 nm as the light harvester. The planar heterojunction between TiO2 and CH3NH3PbI3 is responsible for charge separation.15 The spiro-MeOTAD, Au and Ni layers are ca. 250, 80 and 8 nm, respectively. Each layer is compact and smooth. The film quality of CH3NH3PbI3 has been compared by three different fabrication approaches (Figure S1): the one-step method,16 the two-step method17 and the antisolvent method14 (Methods in the Supplementary Information). The best CH3NH3PbI3 film quality and compactness are obtained for the anti-solvent method (Figure 2b inset, S2 and S3), which is used for all the following device fabrication and measurements. The X-ray diffraction (XRD) pattern of CH3NH3PbI3 film shows a set of sharp diffraction peaks (Figure 2b), which are well indexed to the crystal planes of a tetragonal perovskite structure that is consistent with literature.18 No other peaks except for FTO are observed, indicating the sample purity. ACS Paragon Plus Environment

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Compared to the TiO2 underlayer, the CH3NH3PbI3 film shows a high UV-Vis absorption spectrum across the entire UV and visible regions (Figure 2c), in good accord with other works.14 The photoluminescence spectrum of CH3NH3PbI3 film exhibits a single emission peak at ~770 nm (Figure 2c, inset), characteristic of CH3NH3PbI3.19 UV photoelectron spectroscopy (UPS) of the CH3NH3PbI3 layer is further displayed, where the binding energy is calibrated with respect to He I photon energy (21.21 eV). The valence band edge (EVB) is estimated to be -5.43 eV below vacuum, also consistent with previous reports.20 The photovoltaic performances of the multi-layered CH3NH3PbI3 electrodes were first examined under conventional solar cell conditions. Two types of device structures, all identical in the underneath FTO/TiO2/CH3NH3PbI3/spiro-MeOTAD/Au layers except for the top Ni layer (abbreviated as “with Ni” and “without Ni”, respectively), are compared. Both devices show similar current-voltage (J-V) behaviors under 1-sun (AM 1.5G) illumination, with open-circuit voltages (Voc) of ~ 0.95 V, shortcircuit current density (Jsc) of ~ 19.0 mA/cm2, and fill factors (FF) of ~ 0.59 (Figure 3a). The power conversion efficiency (PCE) values are well reproducible above 10%. In addition, the time-dependent measurements show that the Jsc values of both devices are relatively stable and well correlated to the on/off cycles of light illumination (Figure 3b). It should be noted that as the illumination-off time increases, Jsc gradually decreases, as perovskite cells require light-soaking of several minutes to achieve the best performance.21 Almost no difference is observed for the photocurrent densities of both samples, suggesting that the coating of an ultrathin Ni layer does not impact on the photoelectric conversion performance of perovskites in conventional solar cells. The impedance measurements at V ≈ Voc forward bias under 1-sun illumination (Figure S4) were carried out, and for the Nyquist plots of both solar cell devices (i.e., with versus without the Ni top layer), a mid-frequency semicircle is attributed to Ctotal and Rrec. The capacitance Ctotal reflects the overall charge storage in the device, Rrec represents the recombination resistance, and Rs represents series resistance. Both semicircles are slightly distorted at the highest frequencies and have a third low-frequency feature. These impedance measurements are


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consistent with literature.22 The similarity of the two Nyquist plots shows that Ni top layer doesn’t have an impact on impedance of device. The multi-layered CH3NH3PbI3 electrodes, with and without top-surface Ni coating, were used as photoanodes in a standard three-electrode cell for PEC measurement (Measurements in Supplementary Information). A 0.1 M Na2S solution (pH 12.8) was used as both the electrolyte and hole sacrificial reagent.23 Compared to the pure TiO2 compact layer photoanode, the multi-layered CH3NH3PbI3 photoanode without Ni top layer presents a clear photocurrent increase (Figure 3c, blue and black curves), which is expected due to the enhanced photoabsorption by CH3NH3PbI3. Surprisingly, the photoanode with an ultrathin (~ 8 nm) Ni top layer exhibits a much more remarkable PEC photocurrent enhancement in the full voltage range tested, i.e., -0.6 to 0.8 V vs. Ag/AgCl (corresponding to 0.35 V to 1.75 V vs. reversible hydrogen electrode, RHE) (Figure 3c, red curve). The photocurrent density of the Ni-coated photoanode reaches above 10 mA/cm2 at 0 V vs. Ag/AgCl, exceeding most of the previously reported photoanodes even with different sensitization layers.24-29 Other metals, such as Co and Cr, were also selected as the surface layer on top of the Au layer for comparison. The photocurrent density of Nicoated photoanode is also much higher than Co- or Cr-coated photoanode with the same thickness (Figure S5). This substantially enhanced PEC activity of the CH3NH3PbI3 photoanode with an additional Ni top layer is distinct from their similar photoelectric behaviors in the aforementioned photovoltaic measurements, which can be rationalized as follows. In a photovoltaic cell, the photogenerated holes are transferred through the HTM layer to the Au top electrode, and combined with electrons to complete the circuit. While in a PEC cell, the photogenerated holes need to transfer to electrolytes for oxidation, during which a Ni top layer can serve as a catalyst for effectively enhancing this PEC conversion process.12,13 In addition, the CH3NH3PbI3 photoanodes without the top Au or Au/Ni layers will instantaneously be degraded and peeled off from the electrode surface, without generating any PEC activity, further confirming the effect of the top Au or Au/Ni layers for the stability of the CH3NH3PbI3 photoanodes.


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Furthermore, photocurrent measurement upon illumination on/off cycles shows negligible dark current densities, confirming that the photocurrents of both samples are originated from the lightenabled PEC conversion (Figure 3d), instead of possible side electrochemical reactions from the degradation of CH3NH3PbI3. Comparatively, photocurrent densities of the CH3NH3PbI3 photoanodes upon on/off cycles are slightly lower than those under continuous light illumination, which is also consistent with the light-soaking phenomenon observed in others’ perovskite solar cells.21 Moreover, when a CH3NH3PbI3 photoanode is directly connected with a Pt counter electrode under sunlight without any external bias, gas bubble evolution can be clearly observed (Figure 3e, Movie S1 in Supporting Information), indicating its capability of spontaneous (unassisted) hydrogen production. The photocurrent density was also measured and recorded at every second by directly connecting a digital amperemeter to a Pt counter electrode on one end and the multi-layered CH3NH3PbI3 photoanode on the other end, demonstrating an unassisted photocurrent density with a peak value of 0.97 mA/cm2 (Figure S6). More importantly, the PEC stability of the multi-layered CH3NH3PbI3 photoanodes was interrogated at 0 V vs. Ag/AgCl. It is well-known that the hygroscopic amine salts and distorted crystal structure make perovskites decompose instantly when exposed to water.30 For the CH3NH3PbI3 photoanodes with 80-nm Au but without Ni top layer, a fast drop of photocurrent density from 2.5 to < 1 mA/cm2 is observed within 100 sec (Figure 4a), accompanying by slight color change at several scattered positions at the film, indicating the decomposition of CH3NH3PbI3. Increase of the Au layer thickness from 80 to 110 nm can clearly improve the photocurrent from 2 mA/cm2 to over 4 mA/cm2 (Figure 4b), possibly due to the increased light reflection and photoabsorption. Nonetheless, this photocurrent of the 110-nm Au top-layer photoanode quickly drops to a stabilized level that is similar to the device with a thinner (e.g. 80 nm) Au layer, suggesting a marginal effect of the Au layer thickness on the PEC stability improvement. It should be noted that these results have been obtained from electrodes with the optimal conditions of each material layer. In contrast, the CH3NH3PbI3 photoanodes with an 8nm Ni layer can remarkably increase both photocurrent density and stability (Figure 4c). It should also ACS Paragon Plus Environment

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be noted that the illumination time of each on/off cycle has to be long enough for perovskite photoanodes to achieve its maximum photocurrent (Figure S7). The Ni-coated CH3NH3PbI3 photoanode maintains 56% of its original photocurrent after 100 seconds of PEC test under continuous or repeated on/off illumination, and can sustain a photocurrent density of ~2 mA/cm2 after a record-long 15-20 min of continuous PEC measurement in an alkaline solution (Figure 4d). Cross-sectional SEM images after PEC test reveal that some small parts of the CH3NH3PbI3 layer has been collapsed (Figure 4d, inset), suggesting the gradual deterioration of devices is due to pinholes on the top layers that result in the penetration of electrolyte molecules through these layers. Considerable developments have been achieved in lead halide perovskites in the past few years for photovoltaics, while the highly susceptible feature of these materials to air and water has also significantly limited their applications, in particular, in the solution-based solar-to-fuel conversion such as hydrogen production or artificial photosynthesis.24 It is impressive that Ni coating as thin as 8 nm can effectively increase photocurrent and improve stability as well. Addition of the Ni coating thickness to ~ 50 nm can further increase the device stability slightly. Nonetheless, it has also been noticed that the film quality of each layer should be highly compact and conformal, with as few pinholes as possible, in order to achieve good performance and stability. When the Au layer quality is not high, such as not highly mirror-reflective surface, even a satisfying solar cell characteristic is obtained, the photocurrent degrades to zero instantaneously when the CH3NH3PbI3 photoanode is put into solutions. In our work, the anti-solvent method has been demonstrated as the best approach to obtain the most conformal film quality, while the one-step method as the poorest. In addition, the fabrication details, such as the spincoating rate, the time when the second solvent is added, and so on, can significantly change the film quality and the subsequent PEC results. Ni has long been used as electrodes in water electrolysis at industry scale. Coating of an ultrathin Ni film was very recently demonstrated as an effective means for improving the PEC stability of silicon photoanode, an excellent light absorber but prone to photoanodic corrosion.12 In our work, when Ni is applied for the protection of the CH3NH3PbI3 photoelectrode under an electrochemical condition, the ACS Paragon Plus Environment

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surface of Ni will gradually be converted into nickel oxide, which is also an excellent hole-transferring catalyst.13 X-ray photoelectron spectroscopy (XPS) depth profiling of Ni/perovskite cell shows a thin oxidized Ni layer (Niδ+) evident above metallic nickel (Figure S8). Ni 2p spectra for the same sample taken after three different Ar ion milling times into the surface reveal the gradual change from oxidized Ni to metallic nickel, consistent with the literature.12 This leads to a fast transfer of the photogenerated holes from the inner CH3NH3PbI3 layer towards the external Au and Ni surface and subsequently to the electrolyte for oxidation, as confirmed by the significantly increased photocurrent, which in the meantime suppresses the photocorrosion of the electrode. In comparison, additional Au layers with equal or even much larger thickness only yield a negligible effect on both the PEC activity and stability enhancement, indicating the imperative role of the Ni surface layer. Meanwhile, the perovskite solar cell with 8 nm Ni coating maintains much higher power conversion efficiency than bare perovskite solar cell after stored in ambient environment for 12 days (Figure S9), where both types have similar high initial power conversion efficiency, thus indicating Ni top layer can improve the general instability of perovskite solar cells as proper passivation. Further improvement on the Au and Ni layer fabrication quality such as using molecular beam epitaxy (MBE) may lead to higher stability of the perovskite photoelectrodes. Moreover, the designed band alignment and the conduction band edge of the multi-layered CH3NH3PbI3 photoanode allow for efficient solar energy absorption and charge transfer, enabling unassisted hydrogen production. The valence band edge of CH3NH3PbI3 is, however, too high for oxygen production, thus requiring a sacrificial reagent for oxidation. A possible solution to this challenge is to design a tandem photoanode that mimics the tandem solar cells for full water splitting,3 while the device structure and mechanism need further investigation. In conclusion, we have developed for the first time a multi-layered CH3NH3PbI3 photoanode enabled by surface Ni coating. The ultrathin Ni top layer serves as both a physical passivation barrier and an effective hole-transferring catalyst for enhancing photocurrent and improving stability against water. The photocurrent density is measured over 10 mA/cm2 in 0.1 M Na2S at 0 V vs. Ag/AgCl, ACS Paragon Plus Environment

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exceeding most of the previously reported photoanodes. More importantly, this PEC photocurrent density can be maintained over 2 mA/cm2 after 15-20 min of continuous PEC tests, which is strikingly phenomenal for the CH3NH3PbI3 electrode and also substantially higher than the similar device structure but without Ni coating. Our study indicates that the recent research breakthroughs in perovskite can be well inherited into the PEC field for hydrogen production and artificial photosynthesis, and the development on water-resistive perovskite devices may also be leveraged to improve long-term stability of perovskite solar cells under ambient conditions.


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Acknowledgements. We thank the following funding agencies for supporting this work: the 973 Project (2013CB934104, 2011CB933302, 2015CB921401), the National Science Foundation of China (21322311, 21473038), the Science and Technology Commission of Shanghai Municipality (14JC1490500, 12JC1401500), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for funding this research group (RG#1435-010).

Supporting information. Details of methods and supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes. The authors declare no competing financial interest.


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Niu, G.; Guo, X.; Wang, L. J. Mater. Chem. A 2015, DOI:10.1039/c4ta04994b.


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Figures

Figure 1. a) Schematic illustration of photoelectrochemical test of a Ni-coated perovskite photoanode in a PEC cell using standard three-electrode system. The photoanode is back illuminated from the FTO side. b) Schematic energy diagram of TiO2, CH3NH3PbI3, spiro-MeOTAD, Au and Ni. The energy levels of each material are given in eV. The Ni layer allows transferring of holes to electrolyte and protecting the perovskite layer from electrolyte penetration.


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Figure 2. a) Cross-sectional SEM image of a multi-layered CH3NH3PbI3 photoanode. b) XRD pattern and top-view SEM image (inset) of an annealed CH3NH3PbI3 film coated on FTO substrate. c) UV-Vis absorption spectra of CH3NH3PbI3 on TiO2 (red) and pure TiO2 (black). Inset: photoluminescence spectrum of a CH3NH3PbI3 film collected at 532-nm excitation. d) UPS spectrum of a CH3NH3PbI3 film. Inset: the zoom-in valence band edge region.


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Nano Letters

Figure 3. a) Representative current density-voltage curve for the multi-layered CH3NH3PbI3 solar cells. b) Comparison of Jsc of two multi-layered CH3NH3PbI3 solar cells, with and without top-surface Ni coating, upon on/off illuminations at 1-sun illumination. c) Photocurrent densities of the compact TiO2 layer (black curve), CH3NH3PbI3 photoanodes without (blue curve) and with Ni surface layer (red curve). d) Comparison of photocurrent densities of CH3NH3PbI3 photoanodes (with vs. without Ni surface layer) upon on/off illumination cycles at 1-sun illumination. e) Photograph of a CH3NH3PbI3 photoanode (with Ni top layer) directly connected with a Pt wire for unassisted hydrogen production at 1-sun illumination.


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Nano Letters

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Figure 4. a-c) Comparison of stability of multi-layered CH3NH3PbI3 photoanodes, with and without Ni surface layer, under chopped AM 1.5G simulated sunlight at 100 mW/cm2. d) Stability of a Ni-coated multi-layered CH3NH3PbI3 photoanode under continuous back illumination. Inset: cross-sectional SEM image of a degraded device after PEC test.


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SYNOPSIS TOC


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High-Performance Perovskite Photoanode Enabled by Ni Passivation

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