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Efficient All-Printable Solid-State Dye-Sensitised Solar Cell Based on a Low Resistivity Carbon Composite Counter Electrode and Highly Doped Hole Transport Material Timothy William Jones, Noel W. Duffy, and Gregory J Wilson J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 04 May 2015 Downloaded from http://pubs.acs.org on May 4, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Efficient All-Printable Solid-State Dye-Sensitised Solar Cell Based on a Low Resistivity Carbon Composite Counter Electrode and Highly Doped Hole Transport Material Timothy W. Jones,1,* Noel W. Duffy2 and Gregory J. Wilson1 1

CSIRO Energy Flagship, 10 Murray Dwyer Cct, Mayfield West NSW 2304, Australia

2

CSIRO Energy Flagship, Clayton Laboratories, Clayton, Victoria 3169, Australia

* corresponding author; phone: +61 2 4960 6250 Email: [email protected]

KEYWORDS: solid-state dye-sensitized solar cell; chemical doping; carbon counter electrode; printable photovoltaics, series resistance

ACS Paragon Plus Environment

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ABSTRACT Monolithic device architectures provide a route to large-area mesoporous solar cell manufacture through scalable solution processed fabrication. A limiting factor in device scale-up is availability of low-resistivity printable counter electrode materials and reliable doped charge transport materials. Here we report an efficient all-printable monolithic solid-state dye-sensitised solar cell (ss-DSC) based on a high conductivity porous carbon counter electrode and a highly doped 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) hole transport material (HTM). A review of current state-of-the-art printable porous counter electrodes in DSC literature was conducted and identified blends of graphite:carbon black as promising composites for high conductivity electrodes. Direct ex-situ oxidation of spiroOMeTAD produced a stable HTM dopant and its incorporation with one of the lowest resistivity graphite / carbon black composite materials reported to date, drastically decreases device series resistance, particularly that of the porous insulating spacer. Doping improved all performance parameters, and following optimisation we demonstrate scaled-up 1.21 cm2 (1.01 cm2 masked) devices achieving a maximum efficiency = 3.34% (average = 3.05 ± 0.23%).


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1. INTRODUCTION: Inexpensive solar cells based on abundant, non-toxic, solution processable materials have the potential to revolutionise the renewables industry. For the dye-sensitised solar cell,1 significant cost reductions are possible by replacing the counter electrode conducting glass substrate2 with a printable carbon electrode, based on a composite of graphite, carbon black and an inorganic binder.3 In this monolithic all-printable design, porous zirconia spacer acts to prevent direct electrical shunting between the carbon counter electrode and titania semiconductor, improving performance.4 In a liquid junction device, the electrolyte completely penetrates these pores. Due to high ionic mobility coupled with high concentrations, adequate ionic conductivity is attained. However, sealing is troublesome with such high-performance electrolytes, leading to increased chance of cell failure. Thus, a solid-state hole transport material (HTM) is preferred for practical applications.

The

incumbent

molecular

HTM,

2,2',7,7'-Tetrakis-(N,N-di-4-

methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD), has low solubility for solution processing and intrinsically low hole mobility.5 These manifest in decreased pore filling and electronic conductivity in a mesoporous film. This necessitates the need for doping of the HTM to decrease the resistance associated with this device component. The Han group6–8 have been developing solid-state devices based on this particular monolithic all-printable solid-state architecture. A HTM-free perovskite architecture has also been developed,9 however the toxicity of Pb remains an issue. Several approaches have been applied to combat the HTM series resistance problem, including molecular dopants cobalt(III) complex FK102,7 and Lewis acid SnCl4,6 as well as photodoping.8 However, for the spiro-OMeTAD HTM system none of these approaches are ideal. Molecular dopants ionise the spiro-OMeTAD


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when added to the HTM precursor solution. Cobalt complexes have proved effective in increasing HTM conductivity and device performance,10,11 however the generation of spiroOMeTAD+ cation is not 1:1 with the dopant level.10 Moreover, residual cobalt complexes remain in the HTM film, diluting the concentration of the electroactive spiro-OMeTAD. SnCl4 dopant is highly reactive towards hydrolysis, and the reduced SnCl2 form is hygroscopic,12 which would draw moisture into the HTM. The mechanism of photodoping is complex, yet known to involve Li+ and O2.13,14 The process consumes Li+, which has other tasks in a functioning ssDSC, most notably conduction band tuning to increase the driving force for electron injection.15,16 Oxygen photodoping is less readily tuneable than a direct chemical doping process. Furthermore, spectroscopic evidence for lithiated oxides exist upon photodoping.14 Basic oxides are known to attract moisture into the device, and may reduce dye stability and binding strength. A seemingly simpler approach to doping is available, whereby synthesis and isolation of oxidized spiro-OMeTAD is carried out ex-situ. The oxidized compound can then be used to dope the HTM precursor solution in a controllable manner, without residual amounts of the reduced species of oxidant contaminating the film. Cappel et al.13 used this approach to demonstrate functioning ssDSCs, without Li+/O2 doping, however, the reported performance was low. Very recently, Nguyen et al.17 formed and utilised the dication of spiro-OMeTAD as the bis(trifluoromethansulfonimide) (TFSI)† salt to increase the hole conductivity of the HTM in perovskite and traditional ssDSC architectures, and in doing so improved device performance and reproducibility.

†

systematic nomenclature is correctly bis(trifluoromethanesulfonyl)azanide (TFSA) see ‘Resolving ambiguous naming for an ionic liquid anion’ GJ Wilson, AF Hollenkamp, AG Pandolfo, Chemistry International 29 (4), 2007, 16


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In this paper we present the fabrication and optimisation of high performance monolithic allprintable solid-state dye-sensitised solar cells. The performance is optimised via reduction of the resistance associated with the printable carbon counter electrode and the HTM infiltrated within the porous spacer. We report ex-situ synthesis and doping with spiro-OMeTAD(TFSI)2 is a simple, controlled and effective way of increasing the conductivity of a HTM, and ultimately demonstrates more efficient all-printable devices. 2. EXPERIMENTAL Materials Fluorine-doped tin oxide (FTO) substrates were obtained from Pilkington Glass (7 Ω □–1; 2.2 mm). DSL 90-T titania paste (Dyesol) was diluted 10 g paste to 3.9 g alpha terpineol before use. Glacial acetic acid (AcOH), terpineol (mixture of isomers, anhydrous), TiCl4 (99.9%; diluted to 2 M in milli-Q water (>18.3 MΩ cm), chlorobenzene (anhydrous 99.8%), 4-tert-butyl-pyridine (tBP 99%); LiTFSI (99.95%), AgTFSI (97%);

acetonitrile (99.8% anhydrous); titanium

tetraisopropoxide (TTP; 97%), chloroform (anhydrous) diethyl ether (99%) were purchased from Sigma-Aldrich and used as received unless otherwise stated. Spiro-OMeTAD hole conductor (>99%; Lumtec), zirconia nanopowder (30–60 nm; Inframat advanced materials), graphite powder (Riedel-de Haen); carbon black (Printex L6), zirconium tetrabutoxide (ZTP, ~80% technical grade in butanol; Fluka); ethyl cellulose (EC1: 5–15 or EC2: 30–50 mPa s; BioChemika) and D35 dye18 (Dyenamo) were used as received. Paste formulation Porous zirconia paste was prepared by a modification of a literature method.19 In a typical preparation, 6 g ZrO2 powder; 1 g AcOH and 5 g terpineol were placed in a solid yttrium stabilised zirconia (YSZ) mill with 7 × 15 mm ϕ YSZ grinding media in a planetary ball mill


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(Fritsch Pulverisette 6) for 2 h at 250 rpm. A further 15 g terpineol was added and milled for a further 30 min. After this, 2.25 g EC1) was added and milled overnight at 150 rpm to dissolve. Finally, a mixture containing 0.15 mL AcOH, 0.67 mL TTP and 1.1 mL ZTP inorganic binders were added, and milled for a further 45 min. The conducting carbon counter electrode was prepared by ball milling a mixture of graphite and carbon black (6 g total), 2.25 g EC1 and 0.75 g EC2 into a homogeneous powder overnight at 150 rpm. 23 g terpineol was added and milling continued for several hours to dissolve the EC mixture and disperse the carbon blend. Finally, a mixture containing 2 g terpineol, 1.83 mL TTP inorganic binder precursor,20 and 0.2 mL AcOH was added and milled for 1 h until completely homogenised. Synthesis of 2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene dication (Spiro-OMeTAD(TFSI)2) dopant A quantity of Spiro-OMeTAD was dissolved in a minimum volume of chloroform. A solution of AgTFSI oxidant21 dissolved in acetonitrile (2.1:1 mol ratio) was slowly added under stirring. Appearance of a deep red colour, characteristic of oxidized spiro-OMeTAD, was immediate and the solution was allowed to react for a further 2 h. The solution was filtered through 0.2 µm PTFE filter, before being precipitated by addition of diethyl ether. Solution supernatant was decanted, and the resultant dark red solid isolated after drying under vacuum. Fabrication of monolithic dye-sensitised solar cells Counter and working electrode contacts on the FTO substrate were first isolated by ablation of FTO using a 1.6 µm fibre laser. Substrates were then cleaned by successive sonication in detergent (2 % v/v Hellmanex in Milli-Q water) followed by pure Milli-Q water. Substrates were plasma cleaner (Harrick Plasma PDC-002) in air, prior to deposition of a blocking layer applied by spin-casting an acidified 0.5 M TTP solution in 1-BuOH. Residual TTP was removed from contact areas, before thermal annealing at 120 °C and sintering at 500 °C for 30 min. Titania


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films were deposited using a semi-automatic screen printer (ATMA AT60-FA, 90T mesh screen), before being proofed at 70 °C and sintered at 500 °C for 30 min. The films were treated to a secondary 20 mM TiCl4 treatment at 70 °C for 30 min. A porous zirconia overlayer was printed atop the sintered titania film, and proofed at 70 °C. Finally, multiple carbon layers were printed to yield a ca. 24 um carbon layer. The completed substrate was then calcined at 400 °C for 30 min before immersing in dye solution (D35, 0.2 mM in absolute ethanol) overnight. Deposition of HTM was accomplished by drop casting (under an inert N2 atmosphere) a solution containing 150 mM spiro-OMeTAD, 0.1 M tert-butyl pyridine, 12.5 mM LiTFSI and variable Spiro-OMeTAD(TFSI)2 dopant in chlorobenzene. Cyanoacrylic acid anchored-dyes22 and D35 in particular23 are known to be thermally stable, hence the HTM was allowed to penetrate the film for 2 min, before transferring to a hotplate at 70 °C for 30 min to evaporate the solvent. Finally, conductive busbars were applied to the electrodes by ultrasonic soldering. Device measurement and characterisation Cell efficiency and performance parameters were determined via photocurrent-voltage (j–V) curves recorded under a simulated AM1.5 spectrum. A Newport Class A solar simulator (150 W Xe arc lamp) was calibrated to an irradiance of 1.00 ± 0.02 suns using a silicon reference diode fitted with a KG-5 filter (Fraunhoffer ISE, WPVS). Electrical measurements were performed with a Keithley 2400 Sourcemeter, with 4-point terminal connections. Active area was accurately defined with mask apertures (laser cut 0.55 mm stainless steel, 0.25 cm2). Electrochemical impedance spectroscopy was carried out in the dark in a faraday cage between 1 MHz‒100 mHz as a function of applied potential with an Autolab PGSTAT302N fitted with frequency response analyser module (FRA32M). Sheet resistance of the carbon composite films was measured with a Signatone Lucaslabs 4point resistivity probe fitted with a soft tip and small spring constant to prevent excessive


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damage to the film (SP4-40045TBY). Film thickness was measured with a Dektak 1500 profilimeter (Veeco instruments). Carbon composite film morphology was analysed with a Zeiss MA15 scanning electron microscope (SEM).

RESULTS AND DISCUSSION: Carbon composite film characterisation Carbon films of different thickness were prepared by printing several layers laterally offset on borosilicate glass, forming a step-like pattern. The sample was sintered at 400 °C, and the sheet resistance (R⧠ = ρ / d) measured by the 4-point probe method.24 At the location of each measurement, the film thickness (d) was subsequently measured by profilimetry. The specific resistivity (ρ) for the screen printed carbon layer is readily evaluated from a plot of R⧠ vs. d‒1 (Figure 1). 60 50

R□ / Ω □–1

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5:5 Graphite:CB 6:4 G:CB

40 30 20

7:3 G:CB

10 0 0.00

0.05

d–1

0.10

/

0.15

µm–1

Figure 1: Resistivity of the carbon composites evaluated from the slope of the R⧠‒d‒1 plots.

This resistivity will reflect the morphological nature of the counter electrode (packing, porosity, interparticle contact resistance) as well as the bulk resistivity of the individual carbon black and graphite components. Printex L6 carbon black has a bulk compacted resistivity of 360


8

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Ω µm, and a similar graphite material (SFG6) has a value of 115 Ω µm.25 For our graphite, we have evaluated a bulk specific resistivity of 137 Ω µm (see supplementary information). For the three compositions investigated, the resistivity decreases with increasing graphite content. This is simply due to the lower bulk resistivity of graphite compared to the carbon black. For all materials, there is sufficient carbon black to interconnect graphite, and as such we are never in a regime where poor graphite interconnectivity results in an increased composite resistivity. Furthermore, we are not in a regime where the TiO2 inorganic binder present in the composite formed via TTP calcination insulates particles. Following a review of current literature on composite electrodes, the as-prepared carbon composites presented here are compared in detail to those identified in the DSC literature in Table 1. The entries have been prepared in order of decreasing graphite content. For ease of comparison, the composite resistivity has been plotted against the graphite content in Figure 2. We have grouped compositions with similar preparation methods. For fair comparison, we have corrected for the composition of insulating inorganic binder which varies across the samples. The samples prepared here are highly conductive, comparable to the parent materials. For example, the 6:4 graphite:carbon black counter electrodes which contains 55% graphite and 37% carbon black by mass (sintered, including remaining inorganic binder) mass has a specific resistivity of 334 Ω µm. Interestingly, this porous electrode contains 7.6 wt.% TiO2 binder (essentially an insulator) and is attests to the low specific resistivity of our films.

Table 1. Literature review of resistivity of printable graphite/carbon black composites.


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G:CB ratio

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R⧠ / Ω ⧠‒1

d/ µm

ρ /Ω µm

33.55

10

335.5

[10+1a] 7.8 wt.% 400 :2 ZrO2

*

*

160

a

5:1

20 nm ZrO2

400

3.8

50

190

Platinized

Ref. 27

4:1

20 nm ZrO2

400

6.4

50

320

Platinized

Ref. 27

1:0.25

9.1 wt.% 400 ZrO2

52.7

10

527

Ref. 7

3.25:1

TiO2 wt%)

(15 450

5

50

225

Doubles in application due Ref. 3 to electrolyte-induced swelling

75:25

15 wt.% 20 400 nm ZrO2

*

*

940

Control

Ref. 28

75:25

15 wt.% 20 400 nm ZrO2

*

*

813

Platinized

Ref. 28

3:1

None

400

*

*

1010

Control

Ref. 29

3:1

None

400

26

14

364

Decorated electropolymerised PEDOT

75:25

TTP

400

Efficient All-Printable Solid-State Dye-Sensitized ... - ACS Publications

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