Harnessing Photovoltage: Effects of Film Thickness, TiO2 Nanoparticle

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Harnessing Photovoltage: Effects of Film Thickness, TiO Nanoparticle Size, MgO and Surface Capping with DSCs Hammad Cheema, and Jared Heath Delcamp

ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11456 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 31, 2016

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ACS Applied Materials & Interfaces

Harnessing Photovoltage: Effects of Film Thickness, TiO2 Nanoparticle Size, MgO and Surface Capping with DSCs Hammad Cheema and Jared H. Delcamp* *

481 Coulter Hall, Chemistry Department, University of Mississippi, University, MS, 38677, USA.

KEYWORDS: photovoltage, surface-treatment, recombination rates, electron-lifetime, dyesensitized solar cells

ABSTRACT High photovoltage dye-sensitized solar cells (DSCs) offer an exceptional opportunity to power electrocatalysts for the production of hydrogen from water and the reduction of CO2 to usable fuels with a relatively cost-effective, low-toxicity solar cell. Competitive recombination pathways such as electron transfer from TiO2 films to the redox shuttle or oxidized dye must be minimized to achieve the maximum possible photovoltage (Voc) from DSC devices. A high Voc of 882 mV was achieved with the iodide/triiodide redox shuttle and a ruthenium NCS-ligated dye, HD-2-mono, by utilizing a combined approach of: (1) modulating the TiO2 surface area through film thickness and nanoparticle size selection, (2) addition of a MgO insulating layer, and (3) capping available TiO2 film surface sites post film sensitization with an F-SAM (fluorinated self-

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assembled monolayer) treatment. The exceptional Voc of 882 mV observed is the highest achieved for the popular NCS containing ruthenium sensitizers with >5% PCE and compares favorably to the 769 mV value observed under common device preparation conditions.

1. INTRODUCTION Immense research has been carried out to improve and better understand dye-sensitized solar cells (DSCs) since the breakthrough report by O'Regan and Grätzel.1-7 Two possible applications of dye-sensitized solar cells include: (1) direct sunlight-to-electricity production where the overall efficiency for the conversion of sunlight to electricity is most important, and (2) as a power source for isolated solar-to-fuel devices where maximizing photovoltage is important to provide the power needed to drive catalytic reactions. The conversion of solar energy to chemical energy without additional energy sources is gaining research interest.8-10 High photovoltage solar cell devices enable the direct powering of electrocatalysts from sunlight for solar-to-fuel systems.11-16 In this regard, high photovoltage DSC devices have the unique prospect of providing high open-circuit voltage (Voc) photovoltaics from relatively benign materials, which are often stable under practical conditions and have reached >1.4 V.17 Further increasing photovoltages in DSC devices allows for: (1) the use of electrocatalysts at higher overpotentials (the energy difference between the thermodynamically determined reduction potential of a halfreaction and the potential at which catalysis occurs) where rates are often fastest, and (2) the potentials needed to power higher energy catalytic reactions. DSC research has traditionally focused on harvesting photons across a broad absorption range rather than maximizing photovoltages.18-20 This approach is designed to provide high power conversion efficiency (PCE) devices with balanced photocurrents and photovoltages.21 Through this strategy, ruthenium sensitizer based devices have maintained high PCE values, currently


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reaching 11.9% PCE.22 Recently, an impressive 14.3% PCE has been achieved for an organic dye-based DSC device with a blue-shifted device IPCE relative to high-efficiency ruthenium based devices (IPCEonset of ~700 nm versus ~900 nm).23 This high-efficiency device focused on the maximization of photovoltages through a series of careful surface “cappings” post dye sensitization to prohibit the recombination of electrons in TiO2 with the oxidized cobalt redox shuttle.24-25 Several recent studies have focused on seeking DSC devices with maximum photovoltage values through redox shuttle tuning and sensitizer design.17, 26-29 In this study, we sought to develop a generalized, simple strategy for optimizing photovoltages by a TiO2 photoanode preparation protocol applicable to common sensitizers. Ruthenium sensitizers have displayed an efficient generation of electricity at low energy wavelengths (~900 nm), potentially due in part to vibronic states actively injecting electrons prior to intersystem crossing.30 However, relatively slow regeneration from cobalt redox shuttles and rapid transfer of electrons in TiO2 films to cobalt redox shuttles has slowed improvements to PCE and Voc values for ruthenium-based devices relative to organic dyes.6,

30

Retarding

recombination pathways of electrons in the TiO2 film to the oxidized redox shuttle or dye could offer improvements to ruthenium sensitizer based devices. It should be noted that although much of the discussion in this manuscript will focus on recombination of electrons in TiO2 with the redox shuttle, the back electron transfer from electrons in TiO2 to the oxidized dye can be problematic for device performances and Voc optimization with recombination rates capable of surpassing that of typical regeneration rates.31 Reducing the back electron transfer will be an additionally critical component for dyes exhibiting slow regeneration kinetics with redox shuttles designed to give high-voltage devices but is not further addressed in this manuscript.32-35 There has recently been a focused approach to design NCS free sensitizers with lower recombination losses to improve device performances despite increased synthetic challenges.36-39


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An alternative strategy is to modify the film and provide additives to reduce recombination losses while using the already high performing NCS ligated ruthenium sensitizers. These sensitizers are attractive dyes for studying methods of reducing recombination pathways to improve ruthenium-based device performance parameters since many of the most efficient ruthenium sensitizers contain NCS ligands.40-45 Additionally, the fast recombination kinetics of DSC devices based on these types of dyes offers a fundamental challenge in developing a general strategy for reducing transfer rates of electrons in TiO2 with an oxidized redox shuttle. HD-2mono was chosen as a model ruthenium system since optimal devices work with relatively thin TiO2 electrode films (10 µm vs 20 µm for N719, Figure 1).46-47

SCN Ru

SCN N HO2C

O

N

O

N

F F F F F F

F

N CO2H

F F F F F F

(MeO)3Si

HD-2-mono

PFTS

Figure 1. HD-2-mono sensitizer and 1H, 1H, 2H, 2H-perfluorooctyltrimethoxysilane (PFTS) for fluorine self-assembled monolayer (F-SAM).

The choice of TiO2 film composition is also important to photovoltage optimization. Preparation of TiO2 mesoporous photoanodes with faster electron mobility and high photovoltages greatly depends on crystal size, crystallinity, morphology, composition and surface area.48-52 Photoanodes prepared with different TiO2 nanoparticle size and shape such as hollow, microsize spheres, TiO2 nanotubes and hierarchical nanostructures is actively being studied.53-58 In particular, Li et al. found that an increase in pore size and surface area is important in terms of


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increasing PCE and controlling recombination.59 The Ru (II) sensitizer C101 was found to give optimal devices by finely tuning the size, porosity and shape of TiO2 nanocrystals.60 Additionally, Boschloo et al. found surface area, porosity and surface roughness can be controlled by addition of ethylcelluose to commercially available 20 nm size TiO2 paste (18NRT, Dyesol).61 Furthermore, that the impressively high 1.45 V DSC devices was achieved through capping, Mg+ doping and the use of 25 nm TiO2 nanoparticles.17 We sought to analyze the influence of commercially available TiO2 pastes consisting of 20 nm (P20, 18N-RT, Dyesol) and 28-31 nm (P30, DN-GPS-30TS, Dyenamo) particles on photovoltage at varying film thicknesses. The use of an insulating metal oxide layer (MgO) and a fluorinated self-assembled monolayer (F-SAM, PFTS, Figure 1) can both serve to slow recombination rates. Mg(OC2H5)2 (pre-sensitizer TiO2 surface treatment) and 1H,1H,2H,2H-perfluorooctyltrimethoxysilane (PFTS, post-sensitizer TiO2 surface treatment) thin layer deposition are both known protocols; however, the use of these two treatments in a cooperative manner has not been demonstrated for any dye.6263

Surface treatment with metal oxides such as MgO and Al2O3 reduce the rate of recombination

of electrons in TiO2 with oxidized redox shuttles by creating an insulating layer to keep electrons further from the TiO2 film surface.62, 64-65 An additional effect of TiO2 film treatments with MgO is the intercalation of magnesium during sintering which leads to an upward shift in energy of the conduction band of TiO2.17, 66-67 Both effects can lead to an increase in Voc values in DSC devices. PFTS is believed to occupy vacant spaces on the TiO2 surface after staining the electrodes, which blocks the electrolyte from the TiO2 surface to minimize recombination losses. We hypothesized that since the two treatments were operating through different mechanisms of recombination loss mitigation, combining these two approaches could lead to dramatically enhanced Voc values in ruthenium sensitizer based cells. Herein, we study the effect of TiO2 film thickness, variable TiO2 particle sizes, addition of a


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MgO insulation layer, a capping agent and the effect of these strategies simultaneously on opencircuit voltage and the recombination rate of electrons in TiO2 with the oxidized redox shuttle for HD-2-mono based DSC devices.23-25, 55, 59, 62-63, 68-73 It should be noted that prior strategies for reducing the rate of recombination losses and enhancing open-circuit voltage have focused on changing the electrolyte composition to more bulky ionic structures such as tetrabutylammonium iodide,74-75 electrolyte additives such as tert-butylpyridine,76 and synthetic strategies to modify dye structures for better surface coverage such as bulky donor molecule synthesis77-79 and incorporation of tetrabutylammonium salts as counter ions to dye binders.80 While each of these strategies are valuable for the modulation of open circuit voltage, the benefits of a general protocol to minimize the recombination of electrons in TiO2 with the oxidized redox shuttle by film modification is desirable.81

2. EXPERIMENTAL SECTION 2.1 Materials All commercially obtained reagents and solvents were used as received. N719 (“Ruthenizer 535-bisTBA”) was purchased from Solaronix. The HD-2-mono synthetic details have been published previously.46 1H,1H,2H,2H-Perfluorooctyltrimethoxysilane (PFTS) was purchased from BeanTown Chemical company. Magnesium ethoxide was purchased from Sigma Aldrich.

2.2 Photovoltaic Characterization Photovoltaic characteristics were measured using a 150 W Xenon lamp (model SF150B, SCIENCETECH Inc. Class ABA) solar simulator equipped with an AM 1.5 G filter for a less than 2% spectral mismatch. Prior to each measurement, the solar simulator output was calibrated with a KG5 filtered mono-crystalline silicon NREL calibrated reference cell from ABET


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Technologies (model 15150-KG5). The current density-voltage characteristic of each cell was obtained with a Keithley digital source meter (model 2400). The incident photon-to-current conversion efficiency was measured with an IPCE instrument manufactured by Dyenamo comprised of a 175 W xenon lamp (CERMAX, Model LX175F), monochromator (Spectral Products, model CM110, Czerny-Turner dual-grating), filter wheel (Spectral Products, model AB301T, fitted with filter AB3044 [440 nm high pass] and filter AB3051 [510 nm high pass]), a calibrated UV-enhanced silicon photodiode reference and Dyenamo issued software. Measurements are the average of multiple cells (2 or more cells for Table 1; 3 cells for Table 2).

2.3 Device Fabrication Photoanode preparation: TEC 10 FTO glass (10 ΩΩ/sq. sheet resistance: FTO [fluorine doped tin oxide]) was purchased from Hartford Glass. Once cut into 2x2 cm squares the substrate was

submerged in a 0.2% Deconex 21 aqueous solution and sonicated for 15 minutes at room temperature. The electrodes were rinsed with water and sonicated in acetone 10 minutes, followed by sonication in ethanol for 10 minutes. Finally, the electrodes were placed under UV/ozone for 15 minutes (UV-Ozone Cleaning System, Model ProCleaner by UVFAB Systems). A compact TiO2 underlayer is then applied by pre-treatment of the substrate submerged in a 40 mM TiCl4 solution in water (prepared by careful water addition to 99.9% TiCl4 at 0-5 OC). The submerged substrates (conductive side up) were heated for 30 minutes at 70oC. After heating, the substrates were rinsed first with water then with ethanol. A 5-15 μm mesoporous P20 TiO2 layer (particle size: 20 nm, Dyesol, DSL 18NR-T, >99% anatase) or 5-15 μm mesoporous P30 TiO2 layer (particle size: 30 nm, Dyenamo, DN-GPS-30TS, >99% anatase) was screen printed from a Sefar screen (54/137–64W) resulting in 5 μm thickness after each print. Particle sizes were indicated to be typically within ± 2 nm of the average and are >99%


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anatase at the time of paste formulation as indicated by the distributors. A 5 μm TiO2 scattering layer (particle size: >100 nm, Solaronix R/SP) was screen printed onto all studied electrodes. Between each print, the substrate was heated for 7 minutes at 125 oC and the thickness was measured with a profilometer (Alpha-Step D-500 KLA Tencor). The film layers were printed 5 μm at a time in all cases. The substrate was then sintered with progressive heating from 125oC (5 minute ramp from r.t., 5 minute hold) to 325 oC (15 minute ramp from 125oC, 5 minute hold) to 375 oC (5 minute ramp from 325 oC, 5 minute hold) to 450 oC (5 minute ramp from 375 oC, 15 minute hold) to 500 oC (5 minute ramp from 450 oC, 15 minute hold) using a programmable furnace (Vulcan® 3-Series model 3-550). The cooled, sintered photoanode was soaked 30 min at 70 0C in a 40 mM TiCl4 water solution and heated again at 500 oC for 30 minutes prior to sensitization. The complete working electrode was prepared by immersing the TiO2 film into a HD-2-mono solution (0.3 mM, 1:1:1 mixture of acetonitrile, tert-butyl alcohol and DMSO) 20 hours. Chenodeoxycholic acid (CDCA) was added to the dye solution as a co-adsorbent at a concentration of 20 mM. For N719 based electrodes, a 0.5 mM solution was prepared with extra dry ethanol and CDCA (5 mM). The electrodes were immersed in the dye solutions at room temperature for 20 hours. Counter Electrode Preparation: Two holes were drilled through the insulating side to the conductive side of 2 x 2 cm squares of TEC 7 FTO glass (7 ΩΩ/sq. sheet resistance) using a Dremel-4000 with Dremel 7134 Diamond Taper Point Bit submerged in water to reduce glass cracking with Scotch tape on the FTO side to minimize scratching. The electrodes were washed with water and a 0.1 M HCl in EtOH solution. The electrodes were then sonicated in an acetone bath for 10 minutes and dried at 400 °C for 15 minutes. A thin layer of Pt-paste (Solaronix, Platisol T/SP) was slot printed between two Scotch tape pieces on the conductive side. The electrodes were then heated at 450°C for 10 minutes.


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Dye-sensitized Solar Cell assembly: The photoanode and counter electrode were sealed with a 25 μm thick hot melt gasket (Surlyn, Solaronix, “Meltonix 1170-25”) by heating the system at 130 °C under a pressure of 0.2 psi for 1 minute with a sealing machine (Dyenamo, product DNHM01). The electrolyte was added through the pre-drilled holes in the counter electrodes with the rubber sealing tip from a Solaronix “Vac’n’Fill Syringe” until the electrolyte began to emerge from the second counter electrode hole. The holes were sealed with a Surlyn sheet and a thin glass cover by heating at 130 °C under pressure 0.1 psi for 25 seconds. Finally, soldered contacts were added with a MBR Ultrasonic soldering machine (model USS-9210) with solder alloy (Cerasolzer wire diameter 1.6 mm, item # CS186-150). A circular black mask (active area 0.15 cm2) punched from black tape was used in the subsequent photovoltaic studies.

2.4 Photoanode Surface Treatment MgO treatment: A thin layer of MgO was deposited by submerging TiO2 photoanode films (after all TiO2 layers were printed and prior to dye sensitization) in a solution of Mg(OC2H5)2 (0.15 M solution in EtOH) for 20 minutes in the dark. The electrodes were thoroughly rinsed with EtOH and heated to 500 oC at 40oC/minute and held at 500oC for 30 minutes. Upon cooling, the electrode was used as described above for sensitization and cell assembly. PFTS treatment: Sensitized TiO2 films (with an MgO layer if applicable) were submerged in a 0.1 M solution of 97% 1H,1H,2H,2H-perfluorooctyltrimethoxysilane in hexanes for 90 minutes at 30 oC. The electrodes were rinsed with hexanes and assembled as described above.

2.5 Electron Lifetime and Charge Extraction Measurements Electron lifetime measurements, also known as small modulation photovoltage transient measurements, were carried out with a Dyenamo Toolbox (DN-AE01) instrument and software.


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The intensity of the LED light source (Seoul Semiconductors, Natural White, S42182H, 450 nm to 750 nm emission) is varied to modulate the device open-circuit voltage. The base light intensity was modulated by applied voltages of 2.80, 2.85, 2.90, 2.95 and 3.00 V applied to the LED with the 3.0 V bias approaching 1 sun intensity (97%). The direction of illumination was from the photoanode to the counter electrode, and the device was positioned 5 cm from the LED light source. The voltage rise and decay times are fitted with a Levenberg-Marquardt fitting algorithm via LabView, and the electron lifetime was obtained from the averaging of rise and decay times. Charge extraction at open circuit conditions (Qoc) as function of light intensity was carried out with a Dyenamo Toolbox (DN-AE01) instrument and software. Different open-circuit values were achieved by the programmed control of a biased LED (description above) from 2.5 V to 3.2 V. The LED is switched on for 1 second of illumination, then switched off for 10 seconds with a simultaneous switch to short-circuit conditions with a monitoring of current. The total charge is found by integrating the current measured over time.

3. RESULTS AND DISCUSSION 3.1 Effect of Particle Size and Thickness on Photovoltage We hypothesized that larger particle sizes and thinner films would decrease the surface area available for the recombination of electrons in TiO2 with the oxidized redox shuttle and lead to higher photovoltages. HD-2-mono was employed in devices with varying particle sizes (P20 versus P30 at 20 nm versus 30 nm, respectively), film thicknesses ranging from 15 µm to 5 µm, and variable amounts of P20 and P30 nanoparticles within thicker films to evaluate the effects on photovoltage. For direct comparison, the electrolyte composition was held consistent between each cell (E1: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M tert-butylpyridine (TBP), 0.1 M


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guanidinium thiocyanate (GNCS) in acetonitrile and valeronitrile (v/v, 85/15). As a starting reference point this electrolyte gives a power conversion efficiency (PCE) of 10.1% with P20 10 µm thick films. The PCE was measured at AM 1.5G light intensity (1000 W/m2) based on the equation PCE = (Voc x Jsc x FF)/I0 where Voc is the open-circuit voltage, Jsc is the short circuit current density, FF is the fill factor and I0 is the incident sun intensity. This value is in reasonable agreement with the prior literature report of 9.6% PCE (Jsc = 19.4 mA/cm2, Voc = 0.70 V, FF = 0.71).46 Thicker (15 µm total active layer) TiO2 films were first evaluated with varying particle size compositions for films of 15 µm P20 only, 10 µm P20 + 5 µm of P30, 5 µm P20 + 10 µm P30, and 15 µm P30 (Table 1, entries 1-4). In all experiments, an additional 5 µm scattering layer was employed with particles sizes exceeding >100 nm and for mixed films the P20 layer was deposited first. Only a modest effect of particle size on Voc (687-711 mV) is observed for the 15 µm thick films. The highest Voc observed for the pure P30 film is 711 mV. The device Voc values are all lower than the initial Voc (711 mV) obtained for the optimal PCE benchmark device with a 10 µm P20 film. However, decreasing film thicknesses for both P20 and P30 layers, and by default reducing the TiO2 surface area, may lead to a more pronounced effect on voltages where subtle difference in particle size could be important.


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Table 1. HD-2-mono DSC device performances with varying TiO2 film thicknesses and particle sizes. Voc (mV) Jsc (mA/cm2) FF Particle Size Dependence with 15 µm Films and E1 Electrolyte 701 ± 4 13.3 ± 0.4 0.73 ± 0.01 P20 (15 µm) P20 (10 µm) + P30 (5 µm) 699 ± 4 17.7 ± 0.2 0.75 ± 0.01 P20 (5 µm) + P30 (10 µm) 687 ± 8 17.6 ± 0.4 0.72 ± 0.02 711 ± 8 17.2 ± 0.1 0.73 ± 0.01 P30 (15 µm) Particle Size Dependence with 10 µm Films and E1 Electrolyte 711 ± 7 18.7 ± 0.0 0.74 ± 0.01 P20 (10 µm) 715 ± 9 17.3 ± 0.0 0.73 ± 0.01 P20 (5 µm) + P30 (5 µm) 719 ± 8 16.6 ± 0.1 0.73 ± 0.01 P30 (10 µm) Particle Size Dependence with 5 µm Films and E1 Electrolyte 750 ± 5 15.1 ± 0.2 0.75 ± 0.00 P20 (5 µm) 769 ± 4 13.9 ± 0.5 0.71 ± 0.02 P30 (5 µm) Change in E1 Electrolyte (P30; 5 µm films) 750 ± 6 14.4 ± 0.3 0.63 ± 0.02 E2: 1.0 M PMII 802 ± 8 8.9 ± 0.7 0.68 ± 0.04 E3: 0.6 M PMII 12.0 ± 0.4 0.72 ± 0.01 E4: E3 with 0.1 M LiI, 50 mM I2 783 ± 8

Entry Change

PCE %

1 2 3 4

7.1 ± 0.2 9.5 ± 0.2 9.1 ± 0.2 9.4 ± 0.1

5 6 7 8 9 10 11 12

10.1 ± 0.1 9.1 ± 0.1 8.8 ± 0.1 8.9 ± 0.0 7.8 ± 0.1 6.8 ± 0.1 4.8 ± 0.5 6.9 ± 0.2

All films have a P>100 (5 µm) scattering layer. P20, P30 and P>100 refer to commercial TiO2 pastes with a particle size of 20 nm (Dyesol, DSL 18NR-T, 20 nm average particle size), 30 nm (Dyenamo, DN-GPS-30TS, 28-31 nm particle size), and >100 nm (Solaronix, R/SP), respectively. Both P20 and P30 pastes are reported by the distributors to be >99% anatase. Unless noted, devices use electrolyte E1: 1.0 M DMII (1,3-dimethylimidazolium iodide), 0.05 M LiI, 30 mM I2, 0.5 M TBP (4-tert-butylpyridine), 0.1 M GNCS (guanadinium thiocyanate) in acetonitrile and valeronitrile (v/v, 85/15). E2: 1.0 M PMII (1-methyl-3-propylimidazolium iodide), 0.05 M LiI, 30 mM I2, 0.5 M TBP, 0.1 M GNCS in acetonitrile and valeronitrile (v/v, 85/15). E3: 0.6 M PMII, 0.05 M LiI, 30 mM I2, 0.5 M TBP, 0.1 M GNCS in acetonitrile and valeronitrile (v/v, 85/15). E4: 0.6 M PMII, 0.1 M LiI, 50 mM I2, 0.5 M TBP, 0.1 M GNCS in acetonitrile.

The effects of particle size become more apparent at 10 µm film thicknesses. The Voc shows a subtle increase as the amount of P30 increases relative to the benchmark device for 10


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µm of P30 or 5 µm P20 + 5 µm P30 films (711-719 mV, Table 1, entries 5-7, Figure 2). However, a noticeable decrease in Jsc (and PCE) is observed as the amount of P30 increases. A decrease in PCE from 10.1% to 9.1% is observed when 10 µm P20 is compared with a mixed film of 5 µm P20 and 5 µm P30 (Table 1, entries 5 and 6, Figure 2). This drop in PCE is due to a reduction in photocurrent, which is correlated to a lower total dye loading (30% reduction based on UV studies) in the devices with the larger particle paste presumably due to a lower surface area film (Figure S1). A similar reduction in Jsc and PCE was observed as the amount of P30 increases when comparing 5 µm P20 + 5 µm P30 and 10 µm P30 films (Table 1, entries 6 and 7).

Figure 2. IV curve results with varying active layer nanoparticle size and film thicknesses in devices irradiated under AM 1.5G based on HD-2-mono dye (solid lines: 15 µm films; dashed lines: 10 µm films, top). I-V curve results for devices with varying electrolyte compositions


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(bottom). The effects of film thickness on device Voc trends toward higher voltages as thinner films are used by about 10 mV for 15 µm versus 10 µm films when the particle size is held constant. The Jsc values varied between 16.6 to 18.7 mA/cm2 for 15 µm and 10 µm film thicknesses with the exception of the 15 µm P20 films where the Jsc was observed to be reproducibly and significantly lower (13.3 mA/cm2) leading to a substantially lower PCE values (7.1% versus an average of 9.3% for the remaining devices with 15-10 µm thicknesses). Comparison of film thicknesses with electron lifetime, capacitance and photocurrent dynamic measurements versus sun intensity did not reveal an obvious reason for the diminished performance of the 15 µm P20 device (Figure S2 and S3).82-84 Having found thinner TiO2 active layer films and larger TiO2 particles are favorable for higher Voc values, we further tested this trend by preparing 5 µm P20 and 5 µm P30 devices. These devices gave higher Voc values by 39 mV and 50 mV than the corresponding 10 µm films, respectively (Table 1, entries 8 and 9, Figure 2). The decrease in film thickness lowered Jsc values by approximately 3 mA/cm2 which lead to devices with efficiencies of 8.9% and 7.8%. The decrease in Jsc is presumably due to a lower dye loading. The effect of particle size on Voc values was the most dramatic at the 5 µm thickness with a 19 mV higher value for P30 particles when compared with P20. The 5 µm P30 film based devices gave the highest Voc values (769 mV) of the particle size and film thicknesses tested, presumably due to a reduction in available surface area for recombination of electrons in TiO2 with the oxidized redox shuttle. Changes in composition of redox shuttles can play a dramatic role in modulating Voc values in DSC devices. TiO2 surface adsorption of imidazoliums is known, and large sized imidazoliums are believed to slow recombination of electrons in TiO2 with the oxidized


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electrolyte to increases Voc values.85-88 Redox shuttle effects were investigated with the smaller DMII (1,3-dimethylimidazolium iodide) and larger PMII (1-methyl-3-propylimidazolium iodide) while keeping the film thickness and particle size consistent with the highest Voc devices observed above (5 µm P30). Simply changing the electrolyte counter ion directly to PMII (E2) and holding all concentrations constant led to a reduction in Voc of 19 mV. Solutions made with a 1.0 M concentration of PMII become noticeably more viscous than DMII and we rationalized the increase in electrolyte viscosity is a factor leading to the decreased Voc values. Reducing the concentration of PMII from 1.0 M to 0.6 M (E3) gave solutions with more reasonable viscosities and dramatically increased Voc values (52 mV increase from 1.0 M PMII). However, the Jsc dramatically changed from 14.4 to 8.9 mA/cm2 and the device PCE decreased from 6.8% to below 5%. We rationalized this to be the result of a decreased concentration of the I- redox shuttle component. By increasing the amount of LiI and I2 present in the electrolyte (E4), the lowering of Jsc was limited to 12.0 mA/cm2 from 14.4 mA/cm2 with an increase in Voc to 783 mV from 750 mV when compared with E2 based devices for an overall higher PCE of 6.9%. 3.2 Surface Passivation to Block Recombinations After establishing the improved device conditions for high Voc values, we sought to evaluate the effects of surface passivation treatments. Interestingly, treatment of the commonly used P20 TiO2 nanoparticle size and 10 µm TiO2 film thickness with both MgO and PFTS at the prior independently reported concentrations led poor device performance in DSC devices and dramatically lowered fill factors (Table 2, entries 1 and 2; Figure S4). This low performance may in part be due to these treatments reducing the size of already relatively narrow TiO2 film pores and changing the wettability of pore cavities, which may explain the lack of use of these treatments cooperatively in literature despite the individual merits of both treatments being known. However, a dramatic increase in Voc is observed to 827 mV when films with P30 TiO2


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particles were treated with MgO and PFTS. To probe why the P20 10 µm film showed dramatic decrease in PCE and fill factor, the concentration of Mg(OEt)2 and PFTS were reduced to ½ the original concentration in the deposition solution. Surprisingly, reducing the concentration of either component (Mg(OEt)2 or PFTS) led to significant restoration of both the PCE (8.1-8.3%, Table 2, entries 3 and 4, Figure S5) and fill factor (0.72 versus 0.51). The Voc of these devices reached 778-785 mV, which is a substantial increase over the untreated devices at 711 mV. The P30 10 µm film with the standard concentration of Mg(OEt)2 and PFTS devices gave the highest Voc of the surface treated devices examined at this thickness (Table 2, entry 6). Attempts to further enhance the already high Voc through an electrolyte change did not further enhance the Voc (Table 2, entry 7).

Table 2. DSC device results for MgO and PFTS treated and untreated films. Voc (mV) Jsc (mA/cm2) FF PCE % HD-2-mono based devices (10 µm thickness) 711 ± 7 18.7 ± 0 0.74 ± 0.01 10.1 ± 0.1 P20 727 ± 0 14.6 ± 0.5 0.51 ± 0.07 5.7 ± 0.9 P20 + MgO; PFTS 778 ± 8 14.3 ± 0.0 0.72 ± 0.01 8.1 ± 0.3 P20 + ½ MgO; PFTS P20 + MgO; ½ PFTS 785 ± 7 14.5 ± 0.4 0.72 ± 0.02 8.3 ± 0.1 P30 719 ± 8 16.6 ± 0.1 0.73 ± 0.01 8.8 ± 0.1 827 ± 0 11.3 ± 0.6 0.75 ± 0.01 7.3 ± 0.3 P30 + MgO & PFTS 823 ± 5 12.2 ± 0.1 0.65 ± 0.01 6.8 ± 0.1 P30 + MgO & PFTS; E4 HD-2-mono based devices (5 µm thickness) 750 ± 5 15.1 ± 0.2 0.75 ± 0.00 8.9 ± 0.0 P20 755 ± 0 12.6 ± 0.2 0.41 ± 0.08 4.1 ± 0.8 P20 + MgO & PFTS 769 ± 5 13.9 ± 0.5 0.71 ± 0.02 7.8 ± 0.1 P30 824 ± 4 11.8 ± 0.8 0.73 ± 0.03 7.5 ± 0.3 P30 + MgO & PFTS 825 ± 0 9.6 ± 0.5 0.75 ± 0.02 6.0 ± 0.4 P30 + MgO only 838 ± 4 10.2 ± 0.4 0.71 ± 0.02 6.2 ± 0.4 P30 + PFTS only 882 ± 0 6.4 ± 0.9 0.75 ± 0.02 4.4 ± 0.5 P30 + MgO & PFTS; E4 Comparison of N719 Devices at Standard and Optimized Conditions (5 µm thickness) 724 ± 2 13.3 ± 0.6 0.68 ± 0.01 6.6 ± 0.2 P20 814 ± 5 9.8 ± 0.2 0.68 ± 0.02 5.5 ± 0.1 P30 + MgO & PFTS 857 ± 2 7.5 ± 0.2 0.74 ± 0.01 4.9 ± 0.2 P30 + MgO & PFTS; E4

Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Device


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All films have a P >100 (5 µm) scattering layer. Unless noted, devices use electrolyte E1: 1.0 M DMII, 0.05 M LiI, 30 mM I2, 0.5 M TBP, 0.1 M GNCS in acetonitrile and valeronitrile (v/v, 85/15). E4: 0.6 M PMII, 0.1 M LiI, 50 mM I2, 0.5 M TBP, 0.1 M GNCS in acetonitrile.

Surface treatment experiments were performed with 5 µm films with both P20 and P30 particles (Table 2, Figure 3). We rationalized the thinner P20 films may lead to increased performance with the standard concentration of Mg(OEt)2 and PFTS as the thicker film may lead to a decrease in uniform coverage of the treatments throughout the TiO2 film. The P20 5 µm films again displayed a similar reduction in device PCE and FF as was previously shown for the 10 µm films (Table 2, entries 8 and 9). This suggests the surface treatments are detrimentally affecting the outer most surface of the TiO2 film as the deleterious effects occur irrespective of film thickness. Evaluation of 5 µm P30 devices with the MgO & PFTS treatment shows a significant increase in Voc to 824 mV (55 mV increase) compared to untreated films (Table 2, entries 10 and 11).

Figure 3. IV results for devices with and without MgO and/or F-SAM surface treatment both for P20 and P30 TiO2 films. aIndicates E4 electrolyte.


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The potential synergistic effect of each treatment independently was evaluated for the 5 µm P30 film (Table 2, entries 12 and 13). Interestingly, either treatment separately or combined led to Voc values near 830 mV; however, the combined treatment significantly enhanced current values (~15% increase) relative to either treatment alone. The combined treatment is required for overall better device performance and gives a PCE of 7.5% versus 6.2% for a single treatment. An 882 mV Voc was obtained when the optimized film thickness (5 µm) and electrolyte (E4) were used with the MgO & PFTS treatment, which is 99 mV higher than the analogous device without treatment (Table 1, entry 12; Table 2, entry 14). This is a substantially higher Voc than has been previously reported for Ru-NCS dyes with similar electrolyte compositions. A PCE of 5.2% was measured for our champion cell (4.4% on average). The overall objective of defining a general protocol to maximize Voc and minimize recombination of electrons in TiO2 with the oxidized redox shuttle was evaluated through studies on the benchmark sensitizer N719. This dye is often used with thick films (20 µm) for optimal performance. N719 based devices were prepared according to literature parameters which gave an average of 9.6% PCE (Voc = 712 mV, Jsc = 17.4 mA/cm2, FF = 0.74) without an anti-reflective coating (ARKTOP is not used) with standard P20 particle size TiO2 (10.1% PCE reported) as a device and material quality calibration.47 P20 film based devices gave a PCE of 6.6% and a Voc of 724 mV at reduced film thickness of 5 µm (Table 2 entry 15, Figure 4). Combining both surface treatments and the use of P30 nanoparticles gave a Voc of 814 mV (Table 2, entry 16). Through this simple optimization a 90 mV increase in Voc was observed. The use of the E4 electrolyte led to a 43 mV further increase in Voc to 857 mV (Table 2, entry 17). The dramatic enhancement of Voc for N719 under our conditions highlights the generality of this approach.


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Jsc (mA/cm2)

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8

N719_P20 (5µm) N719_P20 (5µm) + MgO/PFTS N719_P20 (5µm) + MgO/PFTSa

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0 0.0

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Voc (V)

Figure 4. IV results for devices based on N719 dye with and without surface treatments. 3.3 Photovoltage Transient and Charge Extraction Measurements Both blocking recombination of electrons in TiO2 with the oxidized redox shuttle and a shifting of the conduction band could result in the higher observed Voc values. To evaluate both of these effects, charge extraction measurements and small modulation photovoltage transient measurements of electron lifetimes were carried out (Figure 5).35,

89

Charge extraction

measurements were plotted as capacitance versus open-circuit voltage and show a ~100 mV shift at the same capacitance to higher energy after MgO & PFTS treatment for 5 µm P30 films. This shift is commonly attributed to a shift in the TiO2 conduction band (CB) and aligns reasonably to the change in Voc observed for these devices (Figure S6).90-91 MgO & PFTS on P20 films only had a modest change in the conduction band energy versus either P20 untreated or P30 untreated films, which again suggests the MgO & PFTS treatment is not uniform throughout the TiO2 film. Interestingly, separately either MgO or PFTS treatment of P30 films leads to a more dramatic upshift of the TiO2 conduction band relative to P30 films with the combined treatment. The conduction band appears to shift in the following order P30 only < P30 MgO & PFTS

Harnessing Photovoltage: Effects of Film Thickness, TiO2 Nanoparticle

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