Bilayer Dye Protected Aqueous Photocathodes for Tandem Dye

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Bilayer Dye Protected Aqueous Photocathodes for Tandem Dye Sensitized Solar Cells Kevin A. Click, Bradley M. Schockman, Justin T. Dilenschneider, William D. McCulloch, Benjamin R Garrett, Yongze Yu, Mingfu He, Allison E. Curtze, and Yiying Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01911 • Publication Date (Web): 06 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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The Journal of Physical Chemistry

Bilayer Dye Protected Aqueous Photocathodes for Tandem Dye Sensitized Solar Cells Kevin A. Click, Bradley M. Schockman, Justin T. Dilenschneider, William D. McCulloch, Benjamin R. Garrett, Yongze Yu, Mingfu He, Allison E. Curtze, and Yiying Wu* *Email: [email protected] *Phone: (614) 247-7810 Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States

1 ACS Paragon Plus Environment


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ABSTRACT Dye sensitized solar cells (DSSCs) are of broad interest for both solar energy conversion and storage. Furthermore, utilizing water as the electrolyte solvent makes this already cost effective system even more practical and environmentally friendly. Although, using water as the electrolyte solvent introduces new issues such as dye anchor instability and semiconductor degradation. Herein, a bilayer dye design was used which simultaneously improves the aqueous stability by creating a hydrophobic layer over the semiconductor and anchoring group which repels water and ionic/polar species of the electrolyte. We propose the larger recombination in the aqueous based iodide/triiodide redox is due to the low equilibrium constant of triiodide formation in aqueous solvents as opposed to non-aqueous solvents. Consequently, higher concentrations of free iodine exist which has been suggested recently in the literature to be the predominate pathway for recombination in both n-type and p-type non-aqueous dye sensitized systems. Additionally, iodide/iodine has a complex pH dependent equilibrium in water where formation of oxoiodides can now contribute to recombination. Utilizing our bilayer dye (BH2) on NiO, aqueous p-type DSSCs were fabricated using the iodide/triiodide redox and display photocurrents up to 3.0 ± 0.12 mA/cm2 measured under one sun conditions (AM1.5, 100 mW/cm2). The aqueous p-type DSSC photocurrent rivals that of non-aqueous p-type DSSCs while also being stable for months with no sign of photocathode degradation but rather electrolyte decomposition. Furthermore, an aqueous tandem DSSC was fabricated with a completely aqueous electrolyte with all earth abundant materials with no use of any precious metals.


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INTRODUCTION One of the most promising and highly studied solar energy conversion systems are dye sensitized solar cells (DSSCs).1 DSSCs can be integrated into water splitting dye sensitized photo-electrochemical cells (DSPECs) for simultaneous energy conversion and storage.2,3 A tandem DSSC (t-DSSC) combines the highly studied n-type DSSC with the less developed ptype DSSC to produce a t-DSSC which can be more efficient than either n-type or p-type alone.4 This tandem configuration can also be applied to water splitting DSPECs where the overall water splitting reaction is split into two photo-driven half reactions of water oxidation and reduction on the n-type and p-type sides respectively. Furthermore, t-DSSCs can be more cost effective and practical when organic solvents are replaced with water. Water as the solvent for DSSCs have the advantage of being more economical, environmentally friendly, non-flammable and have a lower vapor pressure when compared to organic solvents.5 Moreover, water spitting DSPECs require to some extent aqueous electrolytes. However, dye sensitized systems face new issues when water is used as the electrolyte solvent. The stability of both the semiconductor and dye attachment to the surface is a major concern when low or high pH aqueous conditions are used.6 Furthermore, the use of common and efficient non-aqueous redox mediators such as the iodide/triiodide redox may have entirely different properties when water is used as the electrolyte solvent.7 Recently our group has utilized a bilayer dye design that can protect the semiconductor surface in aqueous conditions.8 Herein, two issues are addressed with our bilayer dye protection strategy when using NiO based aqueous iodide/triiodide DSSCs; (1) instability of dyes when in contact with aqueous electrolytes and (2) new recombination pathways possible when using an aqueous iodide/triiodide redox.



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We previously reported a water splitting DSPEC that was stable when in contact with a pH = 0 (1M HCl) aqueous electrolyte due to the unique protection strategy of our bilayer dye design.8 The complete synthesis and characterization of the bilayer BH series dyes were previously reported by our group.9 We proposed that the bilayer dye design creates a hydrophobic protection layer between polar and/or ionic species of the electrolyte and the semiconductor and anchoring group of the dye (Figure 1). The hexyl groups of the oligothiophenes can aggregate and create a canopy of hydrophobic bulk that repels polar/ionic species in the electrolyte from attacking and de-absorbing the dye attachment from the semiconductor and also can prevent dissolution of the semiconductor when low or high pH is used. This design strategy can improve stability by blocking water’s approach to displace dye binding from the semiconductor surface or dissolution of the semiconductor in low/high pH. Additionally, this protection strategy can also repel ionic species from recombining with holes in NiO. The bilayer protection strategy not only can improve stability but also reduce recombination of these additional pathways of recombination when the iodide/triiodide redox is used in water. The iodide/triiodide equilibrium is highly dependent on solvent interactions. The iodide/triiodide redox when dissolved in water has a much more complex pH dependent equilibria when compared to non-aqueous solvents.7 Numerous oxoiodide complexes can exist in water and their equilibrium and stability are all pH dependent. The possibility for iodide/triiodide/iodine oxidation to oxoiodides presents new pathways for recombination of holes in NiO which has already been shown to be the major issue faced with NiO based p-type non-aqueous DSSCs.10,11 This protection strategy described can be utilized for numerous aqueous dye sensitized applications and in this work we demonstrate the bilayer protection strategy in aqueous p-type DSSCs to address the aqueous concerns of both stability and recombination.


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Only just recently, a p-type aqueous DSSC has been reported using the PMI-6T-TPA dye on NiO with a cobalt based redox couple and achieved inspiring efficiencies of up to 1.6% at 1 sun illuminations; however, the instability of the cobalt based redox couple caused a decrease in efficiency over 60 days.12 Although when a tandem DSSC is implemented, the maximum voltage is now determined by the quasi-fermi level of the electrons and holes in the n-type and p-type semiconductor respectively and is independent on the redox mediator’s reduction potential. The redox couple can play an important role in either n-type or p-type DSSCs, especially in determining the maximum voltage obtainable. The main role of the redox couple in tandem DSSC operation is regeneration of the photo oxidized/reduced dyes. By far, the most prevalent and highly studied redox mediator for DSSCs is the iodide/triiodide redox couple which has been shown to be compatible and efficient for both n-type and p-type DSSCs separately making it a

Figure 1. Chemical structure of the bilayer BH2 dye. good choice for t-DSSCs.13 Here, we utilize an organic dye labeled BH2 to fabricate completely aqueous p-type DSSCs with NiO and an iodide/triiodide redox couple. The bilayer dye protection from BH2 synergistically prevents dye de-absorption improving aqueous stability and slows additional recombination processes identified with aqueous based iodine oxidation to oxoiodides not possible in non-aqueous solvents. The iodide/triiodide redox in aqueous and non-aqueous



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solvents are studied by electrochemical methods such as electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Aqueous p-type DSSCs were fabricated and display photocurrents greater than most non-aqueous p-type DSSCs based on NiO with the iodide/triiodide redox while also being stable for months with no sign of degradation (Table S1). Furthermore, an aqueous t-DSSC was fabricated with a completely aqueous electrolyte with all earth abundant materials with no use of any precious metals. EXPERIMENTAL Chemicals and Materials Chemicals used in this work were purchased from Sigma Aldrich unless otherwise stated. All chemicals were used without further purification. These chemicals include iodine (≥99.99 % trace metals basis), lithium iodide (crystalline powder, 99.9 % trace metal basis), sodium iodide (anhydrous, free-flowing, RediDri™, ACS reagent, ≥99.5 %), nickel (II) chloride (98 %), titanium (IV) chloride tetrahydrofuran complex, guanidine thiocyanate (for molecular biology, ≥99 %), chenodexychloic Acid (GuSCN) (≥97 %), MK-2 Dye (95 %), N-methyl-2-pyrrolidone (MPN). 2-Propanol (certified ACS Plus) was purchased from Fisher Scientific. The tri-block copolymer F-108 (Synperonic©) was purchased from Fluka Analytical. Deionized water was obtained by filtration through a Thermo Scientific™ Barnstead™ E-Pure™ Ultrapure Water Purification System until ≥ 18 MΩ-cm in conductivity. Decon™ Contrex™ AP Powdered Labware Detergent was used for soap water at a 1 % concentration (1.25 oz / gallon). Fluorine doped Tin Oxide (FTO) glass (TEC 7, 1" x 1" x 2.2 mm) was purchased from Hartford Glass Company. Polytetrafluoroethylene (PTFE) (Part#:3/4-5-5498) was purchased from TapeCase Ltd. NiCl2 Solution


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NiCl2 solution was prepared using a similar sol-gel procedure described in the literature.14,15 NiCl2 (1.0 g) and F-108 polymer (0.9 g) were ground into a fine tan powder using a mortar and pestle. The tan powder was added to D.I. H2O (2.5 g) and ethanol (5.0 g). The tan slurry was sonicated and vortexed until clear green. The clear green solution was heated at 30 °C for 3 days in the dark. The white precipitate was centrifuged and the clear green supernatant was collected. NiO Film Fabrication Fluorine doped Tin Oxide (FTO) glass was cleaned by 5 minute sonication cycles of soap H2O, D.I. H2O, and then ethanol. The glass slides were dried at 100°C and cooled to room temperature. Using label paper with the labels removed, scotch tape™ was adhered to the label paper and a hole punch was used to punch 0.36 cm2 holes in the tape. The tape with hole was removed and adhered to the glass slide. A 50 µL drop of the NiCl2 solution was placed above the hole and the solution was doctor bladed across the hole with a microscope glass slide. After 20 minutes sitting at room temperature, the tape was removed and the films were annealed in air at 450 °C for 30 minutes with a ramp rate of 2 °C/min. Film thicknesses were determined using a AlphaStep D-100 profilemeter from KLA-Tencor corporation. Each layer of tape produced around 600 nm thick NiO films. Thicker films were made by repeating the doctor blade and annealing processes. TiO2 Film Fabrication Fluorine doped Tin Oxide (FTO) glass was cleaned by 5 minute sonication cycles of soap H2O, D.I. H2O, and then ethanol. The glass slides were dried at 100 °C and cooled to room temperature. The clean FTO films were immersed in a 40 mM TiCl4 2THF solution in H2O for 60 mins at 65 °C. The FTO films were washed with water and ethanol and dried with a N2 flow. The TiCl4 treated films were then annealed in air at 450 °C for 30 minutes with a ramp rate of 2



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°C/min. Using label paper with the labels removed, scotch tape™ was adhered to the label paper and a hole punch was used to punch 0.36 cm2 holes in the tape. The tape with hole was removed and adhered to the FTO glass slide. Commercial transparent TiO2 paste (Sigma Aldrich) was dropped onto the tape above the hole and was “doctor bladed” across the hole with a microscope glass slide. After 20 minutes sitting at room temperature, the tape was removed and the films were annealed in air at 450°C for 30 minutes with a ramp rate of 2 °C/min. The TiO2 films were TiCl4 treated again using the same procedure described prior. Film thicknesses were determined using a AlphaStep D-100 profilemeter from KLA-Tencor corporation. Platinum Counter Electrodes Holes in the FTO glass were drilled using a 2.0 mm diamond drill from Lasco Diamond Products. The FTO glass with drilled hole were then cleaned by 5 minute sonication cycles of soapy H2O, D.I. H2O, and then ethanol. The FTO glass slides were dried at 100 °C and cooled to room temperature. H2PtCl6 in 2-propanol (64 µL, 10 mM) was evenly dropped across the FTO surface. The films sat at room temperature overnight then annealed in air at 385 °C for 20 minutes at a 2 °C/min ramp rate. Solar Cell Fabrication The NiO films were immersed in N,N-dimethylformamide (DMF) solution of 0.01 mM BH2 overnight (~18 hours). The films were then washed with DMF and dried under N2 flow. A hole the same size of the NiO film (0.36 cm2) was punched into PTFE Tape and applied over the FTO glass slide only exposing the sensitized NiO film. Next, 60 µm thick Surlyn 60 hotmelt foil (Solaronix) was cut into a hallow square with an area of ~ 1 cm2. The platinized counter electrodes and the sensitized NiO electrodes were sealed together face to face using the hotmelt hallow square cut foil by sandwiching the two electrodes together with binder clips and the


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hallow square hotmelt foil between and allowing it to sit in an oven at 100 °C for 3 – 5 minutes. Electrolyte was injected into the cavity between the electrodes by vacuum. This was done by cutting a pipet tip until it fit snugly into the predrilled hole. A capillary tube (closed end down) was placed into the pipet tip. The pipet was filled halfway up with electrolyte and the apparatus was placed in a desiccator and vacuum was applied for 5 – 10 minutes. Upon quenching of the vacuum, electrolyte filled the cavity making sure of no air bubbles. A square piece of hotmelt foil was placed over the hole and a square glass cover slide over the hotmelt. Then a hot iron was pressed onto the glass cover slide to seal the hole. All solar cell fabrication mentioned in the results and discussion section after Figure 5 use either PTFE or Kapton tape to insulate the surrounding FTO of the working electrode from the electrolyte. Tandem Solar Cells Tandem solar cells were fabricated using a similar method as the solar cell fabrication section but rather than using a platinum counter electrode, a MK2 sensitized TiO2 electrode was used. The hole for injecting electrolyte was predrilled in the TiO2 containing electrode. PTFE or Kapton tape was used to cover any exposed FTO for both electrodes. The sensitized films were the same area and shape (0.36 cm2) and were aligned before sealing together with the hotmelt foil. Photovoltaic Methods All J-V curves were recorded using a Reference 600 potentiostat/galvanostat from Gamry Instruments. All solar cells were tested under standard 1 sun conditions (100 mW/cm2, AM 1.5 G) produced by a solar simulator (Small-Area Class B, Solar Simulator, PV Measurements, Inc.). JV curves for p-type DSSCs were recorded at a scan rate of 1 mV/s. JV curves for n-type or tandem DSSCs were recorded at a scan rate of 5 mV/s. The IPCE spectra was measured using a



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QEX7 quantum efficiency measurement system from PV Measurements using silicon diode as the reference cell. Electrochemical Impedance Spectroscopy (EIS) Electrochemical

Impedance

Spectroscopy

was

conducted

with

a

Reference

600

potentiostat/galvanostat from Gamry Instruments between the frequency ranges of 1 MHz to 10 mHz. The working electrode was either the blank FTO film or blank NiO film with both aqueous electrolyte (2M NaI / 20m M I2 / 0.5 M GuSCN / sat. CDCA in D.I. H2O) and a non-aqueous electrolyte (1 M LiI, 100 mM I2 in N-Methyl-2-pyrrolidone (MPN)). The AC amplitude was 10 mV. The cells were all measured under dark conditions. Blank FTO and NiO cells for EIS measurements were fabricated using similar methods described above. For the blank FTO working electrodes, FTO glass was cleaned by 5 minute sonication cycles of soapy H2O, D.I. H2O, and then ethanol. PTFE tape was used to expose a known area of the blank FTO to the electrolytes. Blank NiO films were fabricated identically to the solar cell fabrication but without the sensitization step where the NiO would be immersed in a dye solution. PTFE tape was used here to ensure that only the NiO film was in contact with the electrolytes. Two electrode cells were fabricated identically to how solar cells and platinum counter electrodes were fabricated mentioned above. The EIS data was analyzed using the Gamry Echem Analyst software using a simplex model to fit the data. 3-Electrode Solar Cell Fabrication A three electrode solar cell apparatus was fabricated using a homemade three electrode cell. The working photoelectrode and counter electrodes were constructed identically to the 2-electrode cells described above. An Ag/AgCl (Sat. KCl, 0.198 V vs. NHE) reference electrode was used. The 3-electrode cell was illuminated using a 300 W Xenon lamp with a 1.5 AM filter (Newport)


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and the power density was calibrated to 100 mW/cm2 using a silicon reference cell (accredited by NIST to the ISO-17025 standard) with a KG5 window. The Nernst potential of the electrolyte was calculated using 723 as the Keq of I-/I-3 in H2O.7 RESULTS AND DISCUSSION EIS of Aqueous vs Non-Aqueous Electrolytes Initial attempts to fabricate an aqueous p-type DSSC used previously reported non-aqueous solar cell fabrication techniques.9 Fluorine doped tin oxide (FTO) was used as the transparent conducting oxide substrate, NiO as the p–type semiconductor and BH2 as the dye (Figure 1). Typical non-aqueous concentrations of the iodide/triiodide redox (1 M NaI and 100 mM I2) were used in D.I. H2O. The J-V curve can be seen in Figure S1. Even with no surfactant and a thin NiO film (~ 600 nm), a short circuit current (JSC) density of 1.24 mA/cm2 was obtained but the cell displayed a low open circuit voltage (VOC) of only 21 mV which could be due to the large dark current of the cell. In order to investigate the large dark current, EIS was used to study the differences in charge transfer resistances (Rct) between the FTO-electrolyte interface and the NiO-electrolyte interface of organic and aqueous based solvents for the iodide/triiodide redox. Blank cells were constructed in a two electrode configuration with bare FTO or bare 1 µm thick NiO as the working electrodes and platinum counter electrodes. The blank cells were filled with either a typical p-type non-aqueous electrolyte consisting of 1 M LiI, 100 mM I2 in N-Methyl-2pyrrolidone (MPN) or with an aqueous electrolyte composition of 2M NaI, 20m M I2, 0.5 M Guanidinium thiocyanate (GuSCN), saturated Chenodeoxycholic acid (sat. CDCA) in D.I. H2O (pH = 5)7. The RCT at the working electrodes interfaces were calculated following our previous methods and the Nyquist plots with the fitting parameters are detailed in the supplementary information (Figures S2-S5 and Tables S2-S5).16,17 The RCT results are summarized in Figure 2. 11 ACS Paragon Plus Environment


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Figure 2. Charge transfer resistances (RCT) calculated using EIS of FTO and NiO working electrodes in contact with an aqueous electrolyte (2 M NaI / 20 mM I2 / 0.5 M GuSCN / sat. CDCA / D.I. H2O / pH = 5) and a non-aqueous electrolyte (1 M LiI, 100 mM I2 in N-Methyl-2pyrrolidone (MPN)). The RCT at the FTO-electrolyte interface for the aqueous and non-aqueous electrolytes were calculated to be 2.4 x 106 and 8.9 x 103 Ohms·cm2 respectively at 50 mV. The aqueous electrolyte showed 3 orders of magnitude less RCT at the FTO-electrolyte interface and the NiOElectrolyte at low voltages. For the NiO-Electrolyte interface, the RCT for the aqueous and nonaqueous electrolytes were calculated to be 120 and 12 Ohms·cm2 respectively at 100 mV. This was 2 orders of magnitude less RCT at the NiO-Electrolyte for the aqueous electrolyte when compared to the non-aqueous electrolyte. To understand why there was such a dramatic difference in the EIS results, the differences in equilibrium behavior of iodide and iodine when dissolved in water was considered. Free Iodine Concentration


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Recently, Gerbaldi et at. explored the pH dependent equilibrium when iodide and iodine are dissolved in water in the context of aqueous n-type DSSCs.7 The key difference for water as opposed to non-aqueous solvents is the equilibrium constant (Keq) of triiodide formation seen in equation 1.

The Keq for triiodide formation in MPN and H2O is 4.0 x 106 and 723

respectively.18,19 The significantly smaller Keq for H2O means less triiodide formation and higher free iodine concentration at equilibrium for the aqueous redox. The free iodine concentration ([I2]free) at equilibrium was calculated for our non-aqueous and aqueous system using the Keq for MPN and H2O mentioned prior and following O’Regan et al. methods and equations (2) and (3).20 Where [I-]0 and [I2]0 are the initial iodide and iodine concentrations respectively and [I3-]eq is the triiodide concentration at equilibrium. The [I2]free was calculated to be 28 nM and 14 µM for MPN and H2O respectively. (1)

(2) (3) Having determined the concentration of free iodine in our aqueous electrolyte, we began exploring the possible iodine recombination with NiO. Recombination of holes in NiO can occur by either oxidation of iodide, recombination with the reduced dye anions, or oxidation of free iodine. Our previous studies indicate that recombination between the reduced dye anions is the predominating pathway for non-aqueous NiO based DSSCs.16,17 Similarly, for n-type DSSCs, recombination of electrons in TiO2 can happen by either recombination with the oxidized dye cations, reduction of triiodide, or reduction of free iodine. Recombination has been found to be dominated by the latter two pathways.21 Although, which species contributes to the majority of



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recombination hasn’t been studied until recently by O’Regan et al. who suggests recombination proceeds predominantly by iodine reduction rather than reduction of triiodide for n-type DSSCs.20 Furthermore, a similar trend was recently observed for p-type DSSCs where the predominating pathway for hole recombination was proposed to be oxidation of free iodine over the oxidation of iodide.22 Although in both cases, non-aqueous electrolytes were used meaning the free iodine concentration was only in the nanomolar range. The question then arises of how could such low concentrations support the currents observed. It was suggested that the surface iodine concentration could be higher than the bulk due to iodine interaction with heteroatoms of the dyes.20 Although for the aqueous based iodide/triiodide electrolyte, the bulk iodine concentration is much larger due to the low equilibrium constant when compared to non-aqueous solvents as shown. If iodine is the major recombination pathway for non-aqueous DSSCs, the larger free iodine concentration of aqueous DSSCs could explain such a dramatic increase in dark currents. The effect of iodine concentration on aqueous p-type DSSCs was explored using a 3-electrode solar cell set up. The 3-electrode set up was chosen due to the ability to easily add additions of iodine to the electrolyte and keep the same BH2 sensitized NiO working electrode which minimizes current density deviations caused by minor variations in film thickness if a different film was used for every iodine addition in a 2-electrode setup. Figure 3 shows the 3-electrode setup of a 2 µm thick BH2 sensitized NiO film with a platinized FTO counter electrode and an Ag/AgCl reference electrode. The electrolyte consisted of 2 M NaI, 0.5 M GuSCN in sat. CDCA with various additions of iodine from 20 – 300 mM. The potential axis is referenced to the iodide/triiodide redox potential considering the Nernstian shift in potential as iodine is added to


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the electrolyte and using the Keq mentioned prior. The current density of the cell increased from 20 mM iodine and plateaued once 100 mM iodine was reached and then decreased with increasing concentrations of iodine. Since 2 M NaI was present in the solution, the increase in iodine

will

also

Figure 3. J-V curve of a 3-electrode DSSC with a 2 µm thick BH2 sensitized NiO working electrode, a platinized FTO counter electrode, and a Ag/AgCl (sat. KCl) reference electrode. The electrolyte consisted of 2 M NaI, 0.5 M GuSCN in sat. CDCA with various additions of iodine. increase the triiodide concentration which is responsible for dye regeneration. The potential of the cell reached a maximum after only 50 mM iodine and decreases continually after. Although, the dark current increased continually from the initial 20 mM iodine concentration. The continual increase in dark current can explain the decrease in potential of the cell as iodine concentration was increased. This suggests that the considerable increase of free iodine in aqueous based iodide/triiodide electrolytes is a source of recombination limiting the VOC of the aqueous DSSC. The increase in iodide concentration was also examined (Figure S6) and shows an improvement



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in dark current from 0.5 M – 2.0 M while no additional iodine was added. Although, an increase in iodide concentration after 2.0 M increased the dark current. pH Dependence on Dark Current In order to characterize possible oxidation products of the increased free iodine, cyclic voltammetry (CV) was used to characterize the aqueous electrolyte. Figure 4a shows the CV of 2 mM NaI and 20 µM I2 in 0.1 M citrate or phosphate buffers at both pH = 5 and pH = 7

Figure 4. a) Cyclic voltammetry of a 2mM NaI and 20µM I2 analyte in 0.1M citrate or phosphate buffers at both pH = 5 (red trace) and pH = 7 (blue trace) respectively with a glass carbon working electrode at 50 mV/s scan rate. b) Dark current J-V curves for a 3 electrode solar cell with a 2µm NiO film sensitized with BH2 in contact with a 2M NaI, 20mM I2 electrolyte in 0.1M citric acid buffer at pHs between 3 and 7. respectively at a 50 mV/s scan rate with a glassy carbon working electrode. The first reversible peak at 0.68 V vs NHE is attributed to the oxidation of iodide to iodine and has been well established in the liteature.23 The second and third peaks were quasi-reversible and show pH


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dependence indiciating a proton coupled process. The redox potentials and pH shifts are summeraized in Table 1.

pH

E10ox (V vs NHE) E20ox (V vs NHE) E30ox (V vs NHE)

5

0.68

0.98

1.50

7

0.68

0.88

1.33

Table 1. Summary of electrochemical data shown in Figure 4a.

The latter two peaks could be assigned to oxidation of either iodine, iodide or triiodide to higher oxidation state oxoidodide species such as hypoiodite or periodate. It is non-trival to precisely assign oxidation products due to such a variety of reactions that can occur in the potential and pH region described by the iodide Porbioux diagram.24 The appearance of proton coupled oxidation processes of the iodine containing species is not possible in non-aqueous solvents and can explain why there is such a large dark current and lower charge transport resitance of aqueous based iodide/triiodide DSSCs. As shown in Figure 4a, the potential of the latter two redox processes shift to more negative potentials with increasing pH. The negative shift should increase the thermodyamic driving force for NiO hole recombination with the iodide/triiodide/iodine species in the electrolyte only if the NiO flat-band potential is pinned and not shifted from the pH change as well. In other words, if NiO had a flat-band potential dependence on pH, the potential difference between the latter two redox processes would remain the same as they would both shift. As dicussed prior, we proposed our BH2 dye can create a strong hydrophobic blockin layer on NiO that can repel ionic/polar species. This bilayer protection strategy should prevent adsorption of protons to the surface of NiO and hence pin NiO’s flat-band potential and prevent change with



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pH. With a bilayer dye protection on NiO and it’s flat-band potential pinned, increasing the pH should increase the driving force hence increasing dark currents of the DSSC. In order to probe this pH depedence, a 3-electrode solar cell setup was fabricated and various pHs of a 0.1 M citric acid buffer were introduced. Figure 4b shows the dark current J-V curves with a electrolyte of 2 M NaI and 20 mM I2 in 0.1M citric acid buffers ranging from pH 3 – 6. As the pH increases, the dark current increases as well which is in good agreement with a pinning of the NiO flat-band potential due to our bilayer dye protection. It is favorable then to have a low pH electrolyte for the idodide/triiodide redox to reduce the driving force for oxoiodide formation hence lowering the dark currents of the cell. As shown previously, our dye is ideal for this application since it is stable down to pH = 0 electrolytes. Aqueous p-Type DSSCs As shown above, our initial attempt of an aqueous p-type DSSC had a large area of exposed FTO to the electrolyte (Figure S1 inset). After characterizing the differences of the iodide/triiodide redox in aqueous conditions, additional iodide/iodine/triiodide oxidization at the FTO-electrolyte interface could explain such a dramatic difference in RCT and dark current between the two solvents. To reduce the amount of FTO-electrolyte contact in the aqueous ptype DSSC, polytetrafluoroethylene (PTFE) tape was used to insulate the FTO surrounding the NiO film from the electrolyte (Figure 5 inset). Figure 5 shows two aqueous p-type DSSCs, one fabricated with exposed FTO and the other with no exposed FTO. The cell with no exposed FTO around the NiO film displayed better performance in all photovoltaic parameters with also reduced dark when compared to the cell with exposed FTO to the electrolyte.


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Figure 5. J-V curves of two aqueous p-type DSSCs with a 2 µm NiO film sensitized with BH2 with a 2M NaI, 20 mM I2, 0.5M GuSCN, in sat. CDCA (pH = 5) electrolyte with exposed FTO (red trace) using PTFE tape to insulate the surrounding FTO around the NiO film and the exposed FTO (black trace) with no PTFE tape and using prior fabrication techniques. With the optimized fabrication techniques and reduction of dark currents, we then investigated the long-term stability of the aqueous p-type DSSCs. Figure 6 shows the J-V curves of BH2 on 2 µm thick NiO films with an electrolyte composition of 2 M NaI, 20 mM I2, 0.5 M GuSCN, sat. CDCA in D.I. H2O (pH = 5) fabricated with PTFE tape to minimize FTO-electrolyte contact. The solar cells were tested initially after they were fabricated, then a day later, and finally a week later. The solar cells were stored in the dark until tested again. The JSC was the only photovoltaic parameter

that

decreases

slightly

over

time

while

the

VOC,

FF,

and

ƞ



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Figure 6. J-V curves for 2µm thick NiO films sensitized with BH2 with a 2M NaI, 20m M I2, 0.5 M GuSCN, sat. CDCA in D.I. H2O (pH = 5). Solar cells were stored in the dark between testing at various times. all increase. Table 2 summarizes the average photovoltaic results of 3 solar cells tested at the various times. The dark current of the p-type aqueous solar cells improve with the VOC over time as seen in Figure 6. The photovoltaic parameters of an aqueous p-type DSSC were tested

Jsc (mA/cm2) Voc (mV)

FF

ƞ (%)

Initial

2.9 ± 0.11

79 ± 9

0.24 ± 0.013

0.056 ± 0.013

24 hour

3.0 ± 0.12

103 ± 7

0.26 ± 0.007

0.079 ± 0.005

7 days

2.8 ± 0.05

116 ± 4

0.31 ± 0.049

0.10 ± 0.016

Table 2. Photovoltaic parameters for J-V curves shown in Figure 6. periodically between fabrication and 4 months. The solar cell showed improvement in efficiency even after 4 months. Overall, the JSC is the only photovoltaic parameter that decreases over time while the VOC, FF, and ultimately the ƞ all increase which can be seen in Figure 7.


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Figure 7. J-V curves for 2µm thick NiO films sensitized with BH2 with a 2M NaI, 20m M I2, 0.5 M GuSCN, sat. CDCA in D.I. H2O (pH = 5). Solar cells were stored in the dark between testing at various times. The lower JSC with time was not thought to be dye desorption. The p-type aqueous DSSC was disassembled after 4 months and re-fabricated with fresh electrolyte and the photovoltaic parameters are restored back to almost initial values indicating the change in photovoltaic



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parameters over time was caused by electrolyte interactions and/or decomposition as seen in Figure S7. The change in photovoltaic parameters over time could be due to interactions of the additives within the electrolyte. The exact role the GuSCN additive plays in DSSC operation is still under investigation. It was thought that GuSCN absorbs onto the surface of the semiconductor and slows recombination between the electrolyte.25 More recently, O’Regan et al. proposed a different mechanism in that GuSCN competes with iodine binding to the dye hence reducing the surface concentration of I2 and reducing recombination for n-type DSSCs. For p-type DSSCs, Gibson et al. recently has thoroughly probed the effect of GuSCN on iodine based non-aqueous electrolytes with NiO.22 They found after addition of GuSCN to the electrolyte, there was no NiO band edge dependence indicating no appreciable GuSCN absorption onto NiO. They rather proposed SCN- forms a complex with I2 and inhibits it’s recombination with holes in NiO and reduces surface iodine concentration.22 In this work, the addition of GuSCN immediately reduces dark current while improving the VOC but lowering the JSC simultaneously (Figure S8) which is consistent with aforementioned literature.22 As shown above, the aqueous based iodide/triioide redox has significantly higher concentrations of free iodine meaning GuSCN can have an even larger impact on the photovoltaic performance of aqueous based DSSCs. The GuSCN additive could be playing multiple roles in our aqueous electrolyte. The GuSCN additive may form the I2-SCN complex and inhibit iodine’s oxidation and/or GuSCN could compete with iodine binding to the dye surface and reduce the local concentration of iodine available for recombination at the NiO-Dye interface.


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To investigate if GuSCN can complex the free iodine in aqueous solutions, UV-Vis was used to examine the difference in spectra of iodine solutions with and without GuSCN. Figure 8 shows the UV-Vis spectrum of a saturated solution of iodine dissolved in a 0.1M pH = 5 citric acid buffer to replicate the pH conditions of our electrolyte with and without 1mM GuSCN. Without GuSCN the UV-Vis spectrum shows a mixture of iodine and iodide absorption peaks at 290, 350 and 450 nm. With the addition of 1mM GuSCN, those peaks disappear and new strong peak appears in the 250 nm region which could be attributed to the I2-SCN complex.26 This results shows that GuSCN is able to bind to I2 and possibly inhibit its oxidation that can contribute to the dark current observed. The lower JSC must be due to less available iodine after complexation with SCN- for reaction with iodide to form the triiodide species which is responsible

for

regeneration

of

the

photo-reduced

dye.

Figure 8. UV-Vis of saturated I2 in 0.1M pH = 5 buffer (black trace) then addition of 1 mM GuSCN (red trace). The buffer background (black dotted trace) and 1 mM GuSCN background (blue trace).

Aqueous Tandem DSSCs



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Lastly, since significant photocurrents could be obtained from an aqueous p-type DSSC, an aqueous t-DSSC was explored for which used no platinum counter electrode. For the aqueous ntype DSSC, the MK-2 dye was chosen as the n-type dye due to its stability and performance as an n-type dye for aqueous DSSC applications.27,28 Moreover, MK-2 dye is an organic dye which makes the aqueous tandem DSSC completely precious metal free. The aqueous n-type DSSC was fabricated using commercial TiO2 paste and the same aqueous electrolyte described prior (see

experimental

for

detailed

method).

The

aqueous

n-type

cell

seen

Figure 9. JV curve of the aqueous n-type, p-type and tandem solar cells all using the electrolyte of 2M NaI, 20m M I2, 0.5 M GuSCN, sat. CDCA in D.I H2O (pH = 5.1). The ptype cell (red trace) is a 2 µm NiO cell sensitized with BH2. The n-type cell (blue trace) is a 10 µm TiO2 film sensitized with MK-2 dye. The tandem cell (blue trace) is the combination of the p and n type electrodes shown with illumination through the n-type first. Illumination of the tandem cell first through the p-type side (teal trace) and dark current of the tandem (blue dashed trace). in Figure 9 (black trace) produced an initial Jsc of 0.51 mA/cm2 with a Voc of around 520 mV. The photocurrent of a t-DSSC is limited by the lowest photocurrent producing electrode while 24 ACS Paragon Plus Environment

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the voltages are additive. Considering that, an aged p-type photoelectrode was chosen with photocurrents similar of that of our aqueous n-type DSSC but with maximal VOC since the latter parameter is additive. A newly made TiCl4 treated 10 µm thick TiO2 film was sensitized with MK-2 and used as the n-type electrode for the aqueous t-DSSC. The t-DSSC used the same electrolyte composition of the aqueous n-type and p-type. Figure 9 shows the initial JV curves of the t-DSSC (blue trace) with illumination through the n-type electrode first. The p-type was then illuminated first (teal trace) and the t-DSSC dark current is shown as the dashed blue trace. Illumination through the n-type electrode first produced the best performing t-DSSC. This can be explained by the n-type DSSC’s Jsc alone (0.51 mA/cm2) was lower than the p-type alone (0.83 mA/cm2). The potentials of the p-type and n-type are almost additive for the total Voc of the aqueous tandem DSSC. The photovoltaic parameters of the t-DSSC are summarized in Table 3. Jsc (mA/cm2)

Voc (mV)

FF

ƞ (%)

n-Type

0.51

526

0.72

0.20

p-Type

0.83

142

0.75

0.088

Tandem (n-type)

0.63

621

0.60

0.23

Tandem (p-type)

0.22

604

0.74

0.010

Table 3. Photovoltaic parameters for J-V curves shown in Figure 9. CONCLUSIONS The aqueous iodide/triiodide electrolyte in p-type DSSCs were shown to display larger dark currents and consequently a lower VOC when compared to non-aqueous solvents. We propose the larger recombination in the aqueous based iodide/triiodide redox was due to the low equilibrium constant of triiodide formation in aqueous solvents as opposed to non-aqueous solvents. Consequently, higher concentrations of free iodine exists which has been suggested recently in



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literature to be the predominate pathway for recombination in both n-type and p-type nonaqueous dye sensitized systems. Our calculations show that the free iodine concentration can be 2 – 3 orders of magnitude larger in aqueous solvents when compared to non-aqueous solvents. Lower pH stabilizes the iodide/triiodide redox and can prevent the oxidation of the redox to oxoiodide species such as iodates which can cause electrolyte bleaching. Utilizing the bilayer dye protection of our BH2 dye to pin the valance band of NiO, the dark current increased as a function of increased pH indicating oxoiodide species formation. The addition of GuSCN to the electrolyte lowered the JSC but increased the VOC. The UV-Vis data suggests that the SCN anion binds to iodine hence inhibiting its oxidative recombination but lowers the concentration of triiodide for dye regeneration hence increasing the VOC and decreasing the JSC respectively. Aqueous p-type DSSCs were fabricated with photocurrents up to 3.0 ± 0.12 mA/cm2. The aqueous p-type DSSCs display photocurrents greater than most non-aqueous p-type DSSCs based on NiO and the iodide/triiodide redox while also being stable for up to 4 months with no sign of photocathode degradation but rather electrolyte decomposition. These results indicate that a more stable redox is needed for aqueous DSSC applications. Lastly, an aqueous t-DSSC was fabricated with a completely aqueous electrolyte with all earth abundant materials with no use of any precious metals. ASSOCIATED CONTENT Supporting Information. The supporting information includes a table summarizing the photocurrents of non-aqueous ptype DSSCs that use NiO and the iodide/triiodide redox. A J-V curve of a BH2 p-type DSSC with the iodide/triiodide redox in D.I. H2O. Fitted Nyquist plots with tables of the fitting results


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for the data obtained for Figure 2. J-V curves that show electrolyte composition effects of iodide concentration, CDCA, and GuSCN. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the funding support from the U.S. Department of Energy (Award No. DEFG02−07ER46427). REFERENCES (1)

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TOC Graphic


Bilayer Dye Protected Aqueous Photocathodes for Tandem Dye

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