Article pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
PTB7:PC61BM Bulk Heterojunction-Based Photocathodes for Efficient Hydrogen Production in Aqueous Solution Wenwen Shi,†,‡,⊥ Wei Yu,†,⊥ Deng Li,†,‡ Doudou Zhang,† Wenjun Fan,† Jingying Shi,*,† and Can Li*,†,§ †
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State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § The Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023, China S Supporting Information *
ABSTRACT: Because of excellent light absorption and charge separation properties, a polymer bulk heterojunction (BHJ) composed of electron−donor and electron−acceptor is very promising to be used for photoelectrode preparation. In this work, a PTB7:PC61BM BHJbased photocathode is fabricated for photoelectrochemical (PEC) hydrogen evolution reaction (HER) in aqueous solution. With CuOx and TiOx thin films as interfacial modification layers, the photocathode with full structure (fluorine-doped tin oxide/CuOx/PTB7:PC61BM/ TiOx/Pt) generates a maximum photocurrent density up to −7.27 mA cm−2 and an onset potential as positive as 0.63 V. The half-cell solar-tohydrogen efficiency is up to 1.5%, outperforming all of the previous BHJbased photocathodes for HER. CuOx is identified to play a role of facilitating charge separation, whereas TiOx improves the charge injection as well as blocks the back migration of holes during the PEC process. The factors including permeation of water and oxygen molecules, incident light spectrum, and current flowing that affect the PEC stability are investigated and discussed.
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INTRODUCTION Converting water into hydrogen fuel using solar light as the sole input energy is a long-term goal for mankind.1−3 Photoelectrolysis offers a feasible strategy to produce hydrogen through solar water splitting.4 In order to couple with the water oxidation half reaction for overall water splitting without extra energy input, efficient and durable photocathodes for hydrogen evolution reaction (HER) are highly desirable. Extensive efforts have been made to fabricate photocathodes based on inorganic semiconductors such as III−V group phosphides, silicon, copper (I)-based chalcogenides, and cuprous oxides.5 However, to date, high cost in materials and electrode fabrication, low solar-to-hydrogen conversion efficiencies, insufficient anticorrosion against irradiation in aqueous environment are still great drawbacks for promising applications.5−9 Alternatively, organic semiconductors, especially polymers, with advantages of tunable band gap, broad light absorption spectra, highly optical absorption coefficient, and solutionprocessable fabrication, have emerged as attractive candidates for light harvesting to generate charged carriers for interfacial chemical reactions.10,11 For example, polymeric materials including graphitic carbon nitrides, polyaniline, polyterthiophenes, and poly(3,4-ethylenedioxythiophene) (PEDOT) have been fabricated to be photocathodes for solar-to-chemical conversion.12−17 However, the intrinsic hurdle of tightly bound © XXXX American Chemical Society
Frenkel excitons produced in polymers prevents the photogenerated excitons from effective separation, resulting in low photoelectrochemical (PEC) activity.18 The architecture of a bulk heterojunction (BHJ) composed of electron−donor and electron−acceptor has been recognized as an effective way to overcome the tightly bound energy and facilitate the exciton dissociation for highly efficient energy conversion in organic solar cells.10 In this regard, the BHJ strategy was applied to construct polymer-based photocathodes for efficient PEC processes. Bourgeteau et al. in 2013 first reported a BHJ-based organic photocathode by using poly-3-hexylthiophene (P3HT):phenylC61-butyric acid (PC61BM) as active layer with PEDOT:PSS as hole-transporting layer (HTL), TiOx as electron-transporting layer (ETL), and MoS3 as HER cocatalysts.19 Thereafter, most efforts have been focused on the development of the above P3HT:PC61BM BHJ-based photocathodes through altering the hole-transporting materials and HER cocatalyst to improve PEC activity. Reduced graphene oxide, NiOx, MoOx, MoS2, WO3, and CuI have been used as interfacial modification layers and the maximal half-cell solarto-hydrogen efficiency (HC-STH) of the integrated photoReceived: November 4, 2018 Revised: February 21, 2019
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DOI: 10.1021/acs.chemmater.8b04629 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials cathode is 1.25% with platinum as HER cocatalyst.20−26 However, the understanding of the modification interlayers on the performance and stability of the organic photocathodes is still unclear. Also, beyond the P3HT:PC61BM absorber layer, other polymer-based BHJs have seldom been reported by far in fabrication of HER photoelectrodes. In this work, we report the photocathodes based on a low band gap polymer BHJ of poly[(4,8-bis-(2-ethylhexyloxy)benzo(1,2-b:4,5-b′)dithiophene)-2,6-diyl-alt-(4-(2-ethylhexyl)3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl] (PTB7):PC61BM and their PEC hydrogen production over a platinum catalyst in an acidic electrolyte. The CuOx and TiOx thin films are deposited as modification interlayers. The photocathode with full structure [fluorine-doped tin oxide (FTO)/CuOx/PTB7:PC61BM/TiOx/Pt] is capable of delivering a high photocurrent of −7.27 mA cm−2 at 0 V and an onset potential as positive as 0.63 V. The HC-STH can be achieved up to 1.5%, which is the highest among the currently reported organic photocathodes. In terms of the PEC measurements and photovoltaic characterization, we discussed and identified the roles of CuOx and TiOx in PTB7:PC61BM BHJ-based photocathodes for HER. Besides, the influences of incident irradiation, oxygen or water permeation in aqueous solution, and the applied bias on the PEC stability based on the BHJ photocathode were evaluated and discussed.
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sequential ultrasonic treatment in detergent, DI water, acetone, isopropanol, and ethanol for 30 min, then transferred to the UV− ozone chamber for a 20 min treatment before use. The CuOx solution was spin-coated and treated by UV−ozone for 15 min.27 Then, the sample was transferred to a nitrogen-filled glove-box to prepare the PTB7:PC61BM active layer with a thickness of ∼90 nm by spincoating of the PTB7:PC61BM (BHJ) solution at 1000 rpm for 1 min. The PTB7:PC61BM (1:1.5 by weight) was dissolved in o-DCB/DIO (97:3 vol %) with a concentration of 16 mg mL−1, then stirred at 80 °C overnight. After the preparation of the TiOx film, the Pt was deposited by photoassisted electrodeposition with a home-made reactor using a Pt foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The plating solution was composed of 0.5 mM H2PtCl6 and 0.5 M Na2SO4. The argon gas was constantly bubbling for 15 min before deposition. Potential control was accomplished by an electrochemical workstation. The deposition potential was 0 V versus SCE with the charge density of ∼25 mC cm−2 and the solution was stirred during the deposition process. PEC Measurements. The PEC measurements were conducted in a three-electrode system with a potentiostat (CH Instruments 660D potentiostat) under simulated AM 1.5G solar light irradiation. The fabricated photocathode, a Pt foil, and an SCE were used as working, counter, and reference electrodes, respectively. The electrolyte solution was 0.1 M Na2SO4 with 0.1 M H2SO4 (pH = 2). The measured potentials versus SCE were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation: VRHE = VSCE +0.242 + 0.059 × pH. All potentials are quoted against the RHE unless otherwise noted. The linear sweep voltammetry (LSV) curves were measured from +0.8 to −0.6 versus SCE at a scan rate of 50 mV s−1. The electrolyte of cyclic voltammetry (CV) test consists of 5 mM K3[Fe(CN)6], 5 mM K4[Fe(CN)6], and 0.5 M Na2SO4. The photocathodes were irradiated by a solar simulator (AM 1.5G) from the solid/solution interface in all cases. Mott−Schottky analysis was carried out at a certain dc potential range with an ac potential frequency of 1 kHz under dark conditions. Electrochemical impedance spectra analysis was performed at 0.5 V with 50 mV amplitude under dark conditions. Faradaic efficiency was tested by recording the photocurrent and the generated H2 simultaneously. The photocurrent was recorded by electrochemical workstation at 0 V versus RHE. The produced H2 was detected by an online gas chromatograph with a 5 Å molecular sieve column. H2 evolution faradaic efficiency was calculated based on the amount of H2 evolved and the charge passed through the electrode. Characterization. The work function of CuOx was measured in air by scanning Kelvin probe microscopy (SKPM) with a Bruker Metrology NanoScope III-D atomic force microscope. The atomic force microscopy (AFM) topography images of the BHJ were obtained by using a MultiMode NanoScope III-D atomic force microscope (Bruker) in the tapping mode. The UV−vis absorption spectra of the photocathodes were recorded on a Cary 5000 UV−vis− near-infrared spectrophotometer. The cross-sectional morphology of the photocathode was imaged by a Quanta 200 FEG scanning electron microscope. The Ti and O elements in the photocathode were analyzed by energy-dispersive X-ray spectroscopy (EDXS). The surface chemical composition of CuOx was measured by X-ray photoelectron spectroscopy (XPS), using a spectrometer (VG ESCALAB MK2) with monochromatized Al Kα as the X-ray source.
EXPERIMENTAL SECTION
Materials. The donor polymers, PTB7 and PTB7-Th (poly[4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co3-fluorothieno[3,4-b]thiophene-2-carboxylate]), were purchased from 1-Material, Inc. The acceptor PC61BM was purchased from Solarmer Materials, Inc. The o-dichlorobenzene (o-DCB, 99%) and 1,8diiodoctane (DIO, 98%) were purchased from Sigma-Aldrich Inc. Copper(II) acetylacetonate (98%), 99.5%, and sodium sulfate (Na2SO4) were purchased from Alfa Aesar and Sinopharm Chemical Reagent Co., Ltd, respectively. All materials were used as received. All electrolyte solutions were prepared using deionized (DI) water (Millipore Milli-Q purification system, resistivity >18 MΩ cm). Preparation of the CuOx and TiOx. CuOx was prepared following the reported article.27 Briefly, 3 mg of Copper(II) acetylacetonate powder was dissolved in DI water (3 mL) and then 0.4 vol % acetic acid (3 mL) was added into the solution. The obtained solution was then ultrasonicated for 5 min to make the powder gradually dissolve. After that, 30 vol % H2O2 was added into the solution with the volume ratios of 3:2. To obtain the CuOx thin film, the solution was spin-coated onto the FTO substrate at a speed of 4000 rpm for 40 s in air, then treated by ultraviolet (UV)−ozone for 15 min. The precursor solution of TiOx was received by solving the tetrabutyl titanate (180 μL) in the mixture solvents of ethanol/ isopropanol (5 mL:5 mL), then stirred for 5 min before adding 10 μL of concentrated hydrochloric acid (HCl). Then, the solution was stirred for 72 h at room temperature in a sealed vial.26 The TiOx film was prepared by spin-coating this solution onto the sample at a speed of 1000 rpm for 1 min in air and keeping in the ambient at room temperature for 2 h. The atomic layer deposition (ALD)-TiOx film was obtained by a SYSKEY ALD system. In every ALD cycle, the deposition temperatures and the successive pulse duration for TiCl4 and H2O precursors are 150 °C, 30 ms and 25 °C, 50 ms, respectively, resulting in a growth rate of 0.5 Å per cycle.28 Then, a fixed number of 100 and 300 cycles is used to achieve ALD-TiOx films of 5 and 15 nm thickness, respectively. Preparation of the Photocathodes. The organic photocathodes were fabricated by subsequent deposition of different layers, according to the following architecture: FTO/CuOx/PTB7:PC61BM/TiOx/Pt. The FTO-coated soda-lime glass substrates were cleaned by
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RESULTS AND DISCUSSION The polymer BHJ consisting of PTB7 as electron−donor and PC61BM as electron−acceptor, with their chemical structures shown in Figure S1, is able to harvest the visible light in the range from 500 to 750 nm (Figure S2). In fabrication of the photocathode with full structure, the FTO substrate is deposited in order with a CuOx thin film, BHJ materials, a TiOx thin film, and platinum nanoparticles, designated as FTO/CuOx/PTB7:PC61BM/TiOx/Pt in this work. It should B
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(CuO).31,37 The Cu2+/Cu+ ratio is estimated to be 2.75:1, which is consistent with the previous report.27 Besides, a strong peak at 531.2 eV in Figure S7b corresponds to O 1s binding energy. Moreover, the EDXS mapping images (Figure S8) of the sample FTO/TiOx illustrate the uniform distribution of the elements of Ti and O. Meanwhile, the FTO/TiOx shows the same electrochemical behavior as titanium oxide in acidic solution, as indicated by the cyclic voltammogram curve as in Figure S9.38 These results together with the above work function and conduction band measurements indicate the production of CuOx and TiOx phases by the present synthesis methods. Figure 1b shows the LSV curves of integrated photocathodes with different material structures. All electrodes display almost zero current in the dark (curve a) during the potential range of 0.65 to −0.3 V. Regardless of the presence of platinum cocatalyst, BHJ/Pt produces a negligible photocurrent (curve b). In the presence of the TiOx or CuOx interlayer, the derived BHJ/TiOx/Pt (curve c) and CuOx/BHJ/Pt (curve d) photocathodes show improved PEC activity by generating the cathodic photocurrent densities of −0.40 and −3.06 mA cm−2 at 0 V with corresponding onset potentials at 0.22 and 0.33 V (defined as the potential where a cathodic photocurrent exceeds −0.1 mA cm−2), respectively. After integrating both TiOx and CuOx layers, the photocathode with full structure (curve e) gives the highest photocurrent density and the earliest onset potential. In addition, without the BHJ layer, the photocathode is not photoactive (Figure S10). This result suggests that only the BHJ layer is the active layer and the interlayers of CuOx and TiOx play significant roles in facilitating the collection of charge carriers from the excited BHJ for solid/electrolyte interfacial chemical reaction. Because of variations in molecular weight and polydispersity index, the performance of the organic BHJ polymer-based optoelectronic devices generally varies from batch to batch.39 Likewise, BHJ-based photocathodes derived from different batches often exhibit some inconsistence in output photocurrent. Figure S11 summarizes the obtained photocurrents over the full-structured photocathodes in 25 batches, the statistical distribution of which is ideally normal. The chopped light LSV curve in Figure 2a indicates the optimal PEC activity achieved on the PTB7:PC61BM BHJ-based photocathode with full structure. The onset potential is at around 0.63 V and the photocurrent density reaches −7.27 mA cm−2 at 0 V, which outperforms the previous P3HT:PC61BM BHJ-based photocathodes (Table S1) for HER in aqueous solution. The HCSTH of the photocathode can be calculated from the LSV curve with the following formula (eq 1)40,41 ÅÄÅ ÑÉ ÅÅ |jph (mA/cm 2) × (|Vb − 0|) (V)| ÑÑÑ Å ÑÑ HC‐STH = ÅÅÅ ÑÑ 2 ÅÅ ÑÑ P (mA/cm ) total ÅÅÇ ÑÑÖ (1)
be noted that the PTB7:PC61BM BHJ-based photovoltaic cell achieves the optimal solar-to-electricity conversion efficiency of over 7% with PEDOT:PSS as HTL and Ca as ETL.29 However, the remarkable hygroscopicity of PEDOT:PSS and reactive Ca makes it incapable of withstanding the erosion of the aqueous solution. In view of a suitable work function and high light transmittance together with solution-processable preparation, nontoxicity and earth-abundant source, waterinsoluble CuOx and TiOx are used herein in the BHJ-based photocathode.30,31 The energy-level diagram for the photocathode with full structure is demonstrated in Figure 1a. The stagger band edges
Figure 1. (a) Energy-level diagram of the proposed organic photocathode; (b) current−potential curves of PTB7:PC61BMbased photocathodes with different structures under AM 1.5G simulated sunlight at 100 mW cm−2 in 0.1 M Na2SO4 aqueous solution (pH = 2).
of PTB7 and PC61BM evidently facilitate the photogenerated excitons separated into holes and electrons. CuOx with high work function of 5.39 eV (Figure S3, measured by SKPM) was used to modify the FTO substrate to reduce the energy barrier for hole extraction by pinning the Fermi level to the positive integer charge-transfer state of the donor materials based on the integer charge-transfer model.32−35 The conduction band position of TiOx is found to be −4.12 eV versus vacuum level (Mott−Schottky plot in Figure S4),36 which is close to the lowest unoccupied molecular orbital of PC61BM (−4.10 eV). In this regard, electrons produced in the active layer can transfer to TiOx, then reach the Pt surface for HER. Therefore, the energy-level arrangement ensures the thermodynamic charge separation and interfacial transfer. The PTB7:PC61BM BHJ on the CuOx surface exhibits uniform morphology as shown in the AFM image (Figure S5), which is beneficial to rapid dissociation of the photogenerated excitons and transportation of carriers.29 The as-prepared polymer BHJ film is ∼90 nm in thickness, whereas the CuOx and TiOx films are ultrathin so that it is difficult for them to be discerned in the cross-sectional scanning electron microscopy (SEM) image (Figure S6). To further confirm the generation of the CuOx phase, we carried out XPS analysis with results as indicated in Figure S7. It is seen from the Cu 2p core-level XPS spectra (Figure S7a) that the broad Cu 2p3/2 peak can be deconvoluted into two peaks, peak 1 (Cu2O) and peak 2
where jph is the photocurrent density achieved under an applied bias Vb. Accordingly, the HC-STH of the current photocathode is determined to be 1.5% at 0.3 V (Figure 2b), which is the highest value among the currently reported organic photocathodes. It is worth mentioning that the simultaneous modification with the CuOx and TiOx interlayer is also effective for fabrication of an active HER photocathode based on the poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxyC
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Figure 3. (a) Charge separation efficiency (Φsep) and (b) Charge injection efficiency (Φinj) of PTB7:PC61BM-based photocathodes with different structures.
Figure 2. (a) Optimal current−potential curve of the full-structured photocathode under chopped light illumination (AM 1.5G) in 0.1 M Na2SO4 aqueous solution (pH = 2). (b) HC-STH of the organic photocathode calculated from the current−potential curve.
facilitating charge separation in the PEC process, which agrees well with its function as a conventional hole transfer layer in the photovoltaic process. Charge injection efficiency is the yield of those electrons that are injected into the electrolyte to reduce protons, or, in other words, are those electrons that do not recombine with holes at surface traps.42 Figure 3b shows that the injection efficiency is increased largely by adding the TiOx interlayer into the photocathode. Similar results are also observed when PTB7Th:PC61BM was used as photoactive layer instead (Figure S12b). It is thus suggested that TiOx helps to extract photogenerated electrons so as to increase the number of electrons to participate in the interfacial chemical reaction. The enhanced charge injection efficiency of TiOx in the PEC process is consistent with the general function of the TiOx film as an electron-transfer layer in the photovoltaic cell. On the other hand, the overcoating of TiOx may eliminate the surface defects by passivation where are general charge recombination centers.43 The simultaneous application of CuOx and TiOx is able to further improve charge injection from 80 to 100% at 0 V. However, the highest charge separation efficiency achieved in Figure 3a is less than 40%, which is far behind the maximum charge injection efficiency of 100%. As a result, the charge separation is surely a key issue which should be addressed for enhancement in the energy conversion efficiency of BHJ-based photocathodes. In order to further investigate the mechanism of the improved charge separation with the CuOx layer, we examined the charge transfer abilities of CuOx layers by the space charge limited current (SCLC) method. A hole-only device with the structure of FTO/HTL1/active layer/HTL2/metal is required to carry out the SCLC measurement according to the related theory, in which HTL is the hole transfer layer and the metal acts as the contacting layer.44 MoO3 is the widely used material for HTL and Au metal with high work function as electrical contact to facilitate the hole transfer for the hole-only device. Thus, to study the effect of CuOx on the hole mobility, the hole-only devices of FTO/CuOx/BHJ/MoO3/Au and FTO/ BHJ/MoO3/Au were fabricated for SCLC evaluation. The results are shown and compared in Figure 4a. It is seen that the calculated hole mobility of FTO/CuOx/BHJ/MoO3/Au (1.39
late] (PTB7-Th):PC61BM BHJ. As shown in Figure S12a, the full-structured photocathode of CuOx/PTB7-Th:PC61BM/ TiOx/Pt generates much higher photocurrent density than the other electrodes with either CuOx or TiOx. Furthermore, the onset potential also positively shifts in large scale on adding the CuOx and/or TiOx interlayers. To identify the roles of CuOx and TiOx thin films in photocathodes for efficient hydrogen production reaction, controlled experiments were performed to determine the charge separation efficiency (Φsep) and injection efficiency (Φinj) of the as-investigated photocathodes. We use [Fe(CN)6]3− anions to scavenge the photogenerated electrons that arrive at the electrode/solution interface. JH O = Jabs × Φsep × Φinj
(2)
J[Fe(CN) ]3− = Jabs × Φsep
(3)
2
6
According to eqs 2 and 3, supposing the electrons were 100% consumed in electrolyte with [Fe(CN)6]3− anions, Φsep is obtained by dividing J[Fe(CN)6]3− by Jabs, Φinj is obtained by dividing JH2O by J[Fe(CN)6]3− (see the Supporting Information). The current−potential curves with and without [Fe(CN)6]3− anions in electrolyte are shown in Figure S13. The charge separation efficiency and injection efficiency of photocathodes were determined and are displayed in Figure 3a,b, respectively. The curves in Figure 3a are evidently divided into two groups, with and without CuOx interlayers. In a wide range of bias potential from 0.2 to −0.2 V, over three times enhancement in charge separation efficiency is observed in the CuOx group, clearly indicating that CuOx facilitates the charge separation. Here, the charge separation efficiency can be defined as the fraction of photogenerated electrons that does not recombine with holes in the bulk.42 The reduced energy barrier between FTO and BHJ layers by the inserted CuOx is beneficial for the rapid holes’ extraction and collection, thus reducing the chances of recombination between holes and electrons. In other words, the recombination is significantly inhibited because of timely migration of holes by the modification layer of CuOx. Therefore, CuOx plays a role of D
DOI: 10.1021/acs.chemmater.8b04629 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 4. (a) J1/2−V characteristics of hole-only devices: FTO/BHJ/MoO3/Au and FTO/CuOx/BHJ/MoO3/Au, (b) Dark J−V of devices: FTO/ BHJ/Al and FTO/CuOx/BHJ/Al; (c) OCP of FTO/CuOx/BHJ/Pt and FTO/CuOx/BHJ/TiOx/Pt photocathode in 0.1 M Na2SO4 (pH = 2) solution.
× 10−5 cm2 V−1 s−1) is eight times higher than FTO/BHJ/ MoO3/Au (1.70 × 10−6 cm2 V−1 s−1), implying faster hole transfer in the presence of CuOx. Because of the low mobility of holes, timely migration and consumption of photogenerated holes is a big issue, which should be addressed for efficient charge separation in photoelectrodes.45 The CuOx interlayer remarkably overcomes the detention of holes by reducing the interfacial energy barrier. To further study the role of CuOx, the dark J−V curves for the devices of FTO/CuOx/BHJ/Al and FTO/BHJ/Al were measured and are compared in Figure 4b. It is noted that the aluminum metal is generally used as cathode material for electron collecting because of its appropriated work function of 4.3 eV. The measured dark J− V of devices show that the device with CuOx exhibits lower current density in the reverse direction than that without CuOx, indicating an enhanced rectification ratio of the entire device.46 This result suggests that the charge extraction is improved to produce a higher photocurrent with the CuOx modification layer.47,48 Considering the strong relationship between the output photovoltage and the open circuit potential (OCP), we measured the OCP of the photocathodes with and without the TiOx layer to further assess the effect of the TiOx layer.49 In Figure 4c, the OCP of the FTO/CuOx/BHJ/Pt photocathode almost keeps constant before and after light exposure, whereas a large shift is found for the FTO/CuOx/BHJ/TiOx/ Pt photocathode upon illumination. The increase in OCP can be attributed to the enhanced photovoltage. The onset potential thus displays a positive shift, which is consistent with the above result shown in Figure 1b. Being a fast and reversible redox couple, ferri/ferrocyanide are often used as probe species to investigate the chargetransfer property at the electrode/solution interface.36,50 To further evaluate the effect of CuOx and TiOx modification layers, we conducted a CV test of ferri/ferrocyanide redox couple on the investigated photocathodes and compared their CV curves in Figure 5a. It should be noted that all photocathodes generate insignificant currents in the electrolyte without ferri/ferrocyanide redox species under illumination (Figure S14), excluding the effects of electrolyte and electrode themselves within the measuring potential range. With the presence of the ferri/ferrocyanide redox, FTO/BHJ/Pt only shows capacitive current. In contrast, FTO/CuOx/BHJ/Pt produces nearly reversible redox peaks with good symmetry in the oxidation/reduction waves, whereas FTO/CuOx/BHJ/ TiOx/Pt demonstrates a single irreversible reducing peak instead. In view of these facts, it is concluded that without modification interlayers between the BHJ and FTO substrate or surface catalyst, the photogenerated charge carriers, neither electron nor hole, are incapable of passing through the
Figure 5. (a) Cyclic voltammograms of FTO/BHJ/Pt, FTO/CuOx/ BHJ/Pt, and FTO/CuOx/BHJ/TiOx/Pt photocathodes in 0.5 M Na2SO4 + 5 mM ferri/ferrocyanide solution (pH = 6.08) under AM 1.5G simulated sunlight irradiation; (b) Nyquist plots of different solid-state devices measured in dark conditions.
interface to be trapped by the metallic platinum particles for catalytic reaction. CuOx facilitates the migration of both holes and electrons to the platinum surface for catalytic oxidation or reduction reactions, whereas the TiOx allows for the transfer of electrons but blocks the holes toward the electrode/solution interface. This may be attributed to a much lower valence band of the TiOx compared to the highest occupied molecular orbital of PC61BM, as shown in the energy-level diagram in Figure 1a. A similar result had been reported by Chorkendorff et al. who recognized the TiOx as a conductive protective layer over silicon photoabsorbers.50 Thus, the CV results suggest that the TiOx layer may favor electron transfer and block holes at the solid/solution interface, which accounts for the significant enhancement in charge injection efficiency. Additionally, the interfacial charge-transfer resistance at the photocathode side is detected by electrochemical impedance spectroscopy (EIS) measurement. The photocathodes were fabricated in solid-state devices by depositing metallic aluminum conductive contact for the test. The Nyquist plots of FTO/BHJ/Al, FTO/CuOx/BHJ/Al, and FTO/CuOx/BHJ/ TiOx/Al devices are presented in Figure 5b. The equivalent circuit consists of a series resistance (Rs) and a parallel resistor−capacitor (RC) circuit. Only one semicircle is observed in the Nyquist plots, suggesting that the chargetransfer and accumulation processes in these devices could be modeled by a single RC unit.51 The fitted impedance parameters of different devices in Table S2 show that the charge-transfer resistance (Rct) reduces sharply from 3582 to 1541 Ω after adding the CuOx interlayer, indicating a more efficient separation of the photogenerated charge and smaller resistance for interfacial charge transfer. The Rct is further decreased to 967.6 Ω when a TiOx interlayer is added together, resulting in faster charge transport inside the photocathode. These results confirm that CuOx and TiOx films are able to E
DOI: 10.1021/acs.chemmater.8b04629 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 6. (a) Current−potential curves of photocathode after different post treatments under AM 1.5G simulated sunlight at 100 mW cm−2 in 0.1 M Na2SO4 aqueous solution (pH = 2); (b) current−potential curves of the photocathode after chronoamperometry polarization at 0.36, 0, and −0.2 V for 200 s under AM 1.5G simulated sunlight at 100 mW cm−2 in 0.1 M Na2SO4 aqueous solution (pH = 2); (c) stability test for different photocathodes with an applied potential of 0 V under AM 1.5G simulated sunlight irradiation in 0.1 M Na2SO4 aqueous solution (pH = 2).
about 75% loss in activity during 1 h of testing. In contrast, it rapidly decays to a negligible level in 2−3 min with only the solution-processed TiOx film (Figure 6c). Continuous production of hydrogen was found to occur over the photocathode with ALD-TiOx for 1 h with a faradaic efficiency of nearly 100% (Figure S15).
reduce the interfacial energy barriers and thus beneficial to charge transfer for the BHJ-based photocathode. By far, PEC stability is still a very challenging issue for polymer BHJ-based photocathodes.52−54 To identify the factors that affect the PEC stability, controlled experiments over the photocathode samples with comparable PEC activities were performed under different conditions. The LSV curves before and after post treatments are recorded in Figure 6a and the onset potential and photocurrent density at 0 V are summarized in Table S3 for comparison. Only a half photocurrent remained after immersing the photocathode in the electrolyte (air saturate) for 30 min in the dark. The penetrated oxygen or water molecules may react with the polymers and cause an impairment in light absorption, making the photocurrent decrease.55,56 The light-soaking experiments were done in a glove box to isolate the photocathode from water and oxygen.57 About 72 and 80% of the photocurrent densities remain after the photocathodes are exposed to light with and without UV radiation (
