Diacetylene bridged triphenylamines as hole transport materials for solid state dye sensitized solar cells

Article abstract of DOI:10.1039/c3ta11417a

We have synthesized and characterized a series of triphenylamine-based hole-transport materials (HTMs), and studied their function in solid-state dye sensitized solar cells (ss-DSSCs). By increasing the electron-donating strength of functional groups (-H < -Me < -SMe < -OMe) we have systematically shifted the oxidation potential and ensuing photocurrent generation and open-circuit voltage of the solar cells. Correlating the electronic properties of the HTM to the device operation highlights a significant energy offset required between the Dye-HTM highest occupied molecular orbital (HOMO) energy levels. From this study, it is apparent that precise control and tuning of the oxidation potential is a necessity, and usually not achieved with most HTMs developed to date for ss-DSSCs. To significantly increase the efficiency of solid-state DSSCs understanding these properties, and implementing dye-HTM combinations to minimize the required HOMO offset is of central importance.

Full text of DOI:10.1039/c3ta11417a

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Diacetylene bridged triphenylamines as hole transport
materials for solid state dye sensitized solar cells†
Cite this: DOI: 10.1039/c3ta11417a
Miquel Planells,‡a Antonio Abate,‡b Derek J. Hollman,b Samuel D. Stranks,b
c
c
c
c
b
*
Vishal Bharti, Jitender Gaur, Dibyajyoti Mohanty, Suresh Chand, Henry J. Snaith
a
*
and Neil Robertson
We have synthesized and characterized a series of triphenylamine-based hole-transport materials
(HTMs), and studied their function in solid-state dye sensitized solar cells (ss-DSSCs). By increasing the
electron-donating strength of functional groups (–H < –Me < –SMe < –OMe) we have systematically
shifted the oxidation potential and ensuing photocurrent generation and open-circuit voltage of the
solar cells. Correlating the electronic properties of the HTM to the device operation highlights a
significant energy offset required between the Dye – HTM highest occupied molecular orbital (HOMO)
energy levels. From this study, it is apparent that precise control and tuning of the oxidation potential is
a necessity, and usually not achieved with most HTMs developed to date for ss-DSSCs. To significantly
increase the efficiency of solid-state DSSCs understanding these properties, and implementing dye-HTM
combinations to minimize the required HOMO offset is of central importance.
Received 9th April 2013
Accepted 30th April 2013
DOI: 10.1039/c3ta11417a
www.rsc.org/MaterialsA
cell.10 In particular, we have shown that the losses due to series
resistance in the HTM reduces the obtainable power in
Introduction
Signicant improvements in power conversion efficiency
have been reported for solid-state solution-processed solar cells,
which make this cost-effective technology competitive with
established thin-lm solar technologies.1–4 Snaith and co-
workers have recently shown a record efficiencies of over 12%,5,6
using organometallic halide perovskites mesostructured at the
interface with 2,20-7,70-tetrakis(N,N-di-p-methoxyphenylamine)-
9,90-spirobiuorene (Spiro-OMeTAD) as the hole-transport
material (HTM).7,8 In addition, the highest reported efficiencies
(approximately 7%) for solid-state dye sensitized solar cells (ss-
DSSCs) have also been achieved employing Spiro-OMeTAD as
the HTM.9 However, recent studies have demonstrated that the
slow charge transport in Spiro-OMeTAD signicantly limits the
device performance near the maximum power point in the solar
conventional ss-DSSCs by 10 to 20% when generating less
than 10 mA cmꢀ2 photocurrent.11 The recent signicant
enhancements in photocurrent generation with the perovskite
absorbers,5,12,13 and further likely enhancements in ss-DSSCs
will increase the IR losses,10 making enhanced HTMs of para-
mount importance for continued improvements.5,11 Therefore,
together with the other main device components (light
harvesters and electron transporting materials),8,14 the HTM
requires novel solutions to further improve this technology.
Many new materials have been proposed to improve hole
mobility, yet the measured device efficiencies have always
remained lower than Spiro-OMeTAD.15–17 A promising alterna-
tive will be to increase the pore-lling fraction of the meso-
structured photo-electrode by melt inltration,18,19 although
this strongly limits the choice of the materials to those
compatible with the dye thermal stability. The most signicant
progress has been through adding strong oxidants as p-
dopants, thereby increasing the conductivity in Spiro-OMe-
TAD.9,11,20 However, controlled doping can require processing
under inert atmosphere, which is an undesirable method for
producing cost-effective ss-DSSC technologies, and still gives an
upper limit on the conductivity.11
aEastChem – School of Chemistry, University of Edinburgh, Kings Buildings, Edinburgh
EH9 3JJ, UK. E-mail: neil.robertson@ed.ac.uk
bDepartment of Physics, University of Oxford, Oxford OX1 3PU, UK. E-mail: h.
snaith1@physics.ox.ac.uk
cCSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India
† Electronic
supplementary
information
(ESI)
available:
Additional
electrochemical measurements of DATPA derivatives and D102; transmittance
of mesoporous TiO2 lm, absorption and emission spectra of D102 anchored
onto TiO2, metal – hole transport contact resistance, extended crystallographic
table, calculated HOMO energies for DATPA derivatives, detailed description of
mobility measurements and 1H and 13C NMR spectra for compounds 9–12,
uorescence lifetime of D102 on Al2O3 in absence of any of the hole
conductors. CCDC 921284 and 925482. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3ta11417a
Here we present the synthesis and characterization of tri-
phenylamine derivatives with various electron-donating func-
tional groups as HTM in ss-DSSCs (Fig. 1). These molecules are
comprised of two triphenylamine moieties bridged with a
diacetylene group (DATPA). The intramolecular electron trans-
fer and electron valence properties of this type of system have
‡ These authors contributed equally.
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Hexanes/CHCl3 2 : 1) to afford the product as pale solid (2.1 g,
1
90% yield). H NMR (500 MHz, CDCl3) dH: 7.33 (d, J ¼ 8.8 Hz,
2H); 7.26 (m, 4H); 7.10 (d, J ¼ 8.3 Hz, 4H); 7.06 (t, J ¼ 7.3 Hz, 2H);
6.97 (d, J ¼ 8.7 Hz, 2H); 3.02 (s, 1H). 13C NMR (125 MHz, CDCl3)
dC: 148.6; 147.3; 133.3; 129.6; 125.2; 123.8; 122.2; 114.9; 84.1;
76.3. MS EI (m/z): [M]+ calcd for C20H15N: 269.12100; found:
269.12054. Anal. calcd for C20H15N: C, 89.19; H, 5.61; N, 5.20;
found: C, 89.21; H, 5.51; N, 5.17.
Synthesis of 4,40-(buta-1,3-diyne-1,4-diyl)bis(N,N-dipheny-
laniline) (H-DATPA)
Fig. 1 Chemical structure of HTMs (left) and D102 dye23,24 (right) used in this
study. Each of the molecules is comprised of two triphenylamine moieties bridged
with a diacetylene group (DATPA).
2 (269 mg, 1 mmol) was placed in a round bottom ask with 50
mL of CH2Cl2. Tetramethylethylenediamine (TMEDA) (4.5 mL,
30 mmol) and copper(I) chloride (2.9 g, 30 mmol) were added
and the mixture was stirred vigorously for 4 hours under air
with a CaCl2 tube on top. Then, the solvent was removed and the
crude product was puried by column chromatography (SiO2,
Hexanes/CHCl3 2 : 1) to afford the product as yellow solid (251
been previously studied, but have not yet been utilized as a
HTM.21,22 We measure the electrochemical and optical proper-
ties of DATPA derivatives and assess the impact of the electron
donating substituent on ss-DSSC performances. In particular,
we discuss the importance of the offset between the oxidation
potential of the dye and the HTM on the device performances.
1
mg, 95%). H NMR (500 MHz, CDCl3) dH: 7.34 (d, J ¼ 8.8 Hz,
4H); 7.28 (m, 10H); 7.11 (dd, J ¼ 8.3, 1.1 Hz, 8H); 7.08 (t, J ¼ 7.4
Hz, 4H). 13C NMR (125 MHz, CDCl3) dC: 148.7; 147.1; 133.6;
129.7; 125.5; 124.1; 121.8; 114.4; 82.1; 73.7. MS EI (m/z): [M]+
calcd for C40H28N2: 536.22470; found: 536.224675. Anal. calcd
for C40H28N2: C, 89.52; H, 5.26; N, 5.22; found: C, 89.47; H, 5.18;
N, 5.25. m.p. ¼ 246 ꢁC.
Experimental section
Materials
All reagents were purchased from either Sigma-Aldrich or Alfa-
Aesar and they were used as received without further purica-
tion unless otherwise stated. N-Bromosuccinimide (NBS) was
recrystallized from water before use.
Synthesis of 4-bromo-N,N-di-p-tolylaniline (3)
4-Bromoaniline (4.9 g, 29 mmol), 4-iodotoluene (25 g, 115
mmol), potassium tert-butoxide (9.7 g, 87 mmol), copper iodide
(221 mg, 1.16 mmol) and 2,20-bipyridyl (174 mg, 1.16 mmol)
were placed in a round bottom ask and dried under high
vacuum for 30 minutes. Previously degassed dry toluene (120
mL) was added and the mixture was stirred at 120 ꢁC overnight
under N2. Then, a silica plug was run in CH2Cl2 and the solvent
removed. The crude product was puried by column chroma-
tography (SiO2, Hexanes up to Hexanes/CHCl3 4 : 1) to afford
the product as pale yellow solid (3.8 g, 37% yield). 1H NMR (500
MHz, CDCl3) dH: 7.30 (d, J ¼ 8.7 Hz, 2H); 7.09 (d, J ¼ 8.4 Hz, 4H);
Synthesis of N,N-diphenyl-4-((triethylsilyl)ethynyl)aniline (1)
Bis(triphenylphosphine)palladium dichloride (52 mg, 0.075
mmol) triphenylphosphine (39 mg, 0.15 mmol) and copper
iodide (57 mg, 0.3 mmol) were placed in a Schlenk tube and
dried under high vacuum for 30 minutes. Then, a previously
degassed solution of 4-bromotriphenylamine (815 mg, 2.5
mmol), piperidine (2 mL, 20 mmol) and triethylsilylacetylene
(920 mL, 5 mmol) in 15 mL of dry toluene was added via cannula.
The mixture was degassed again by the pump/freeze technique
and stirred at 90 ꢁC overnight under N2. Then, a silica plug was
run in CH2Cl2 and the solvent removed. The crude product was
puried by column chromatography (SiO2, Hexanes/CH2Cl2
4 : 1) to afford the product as yellow oil (933 mg, 97% yield). 1H
NMR (500 MHz, CDCl3) dH: 7.33 (d, J ¼ 8.6 Hz, 2H); 7.26 (m, 4H);
7.09 (d, J ¼ 8.6 Hz, 4H); 7.05 (t, J ¼ 7.4 Hz, 2H); 6.96 (d, J ¼ 8.6
Hz, 2H); 1.05 (t, J ¼ 7.9 Hz, 9H); 0.67 (q J ¼ 7.9 Hz, 6H). 13C NMR
(125 MHz, CDCl3) dC: 148.2; 147.4; 133.2; 129.6; 125.1; 123.7;
122.5; 116.5; 106.8; 90.6; 7.7; 4.7. MS EI (m/z): [M]+ calcd for
6.99 (d, J ¼ 8.4 Hz, 4H); 6.91 (d, J ¼ 8.8 Hz, 2H); 2.34 (s, 6H). 13
C
NMR (125 MHz, CDCl3) dC: 147.6; 145.1; 133.1; 132.1; 130.2;
124.9; 124.1; 113.8; 21.0. MS EI (m/z): [M]+ calcd for C20H18BrN:
351.06171; found: 351.061377. Anal. calcd for C20H18BrN: C,
68.19; H, 5.15; N, 3.98; found: C, 68.33; H, 5.20; N, 4.07.
Synthesis of 4-methyl-N-(4-methylphenyl)-N-(4-((triethyllsilyl)-
ethynyl)phenyl)aniline (4)
C
26H29NSi: 383.20638; found: 383.206001. Anal. calcd
Bis(triphenylphosphine)palladium dichloride (168 mg, 0.24
mmol) triphenylphosphine (125 mg, 0.48 mmol) and copper
iodide (182 mg, 0.96 mmol) were placed in a round bottom ask
and dried under high vacuum for 30 minutes. Then, a previ-
ously degassed solution of 3 (2.8 g, 8 mmol), piperidine (6.3 mL,
for C26H29NSi: C, 81.41; H, 7.62; N, 3.65; found: C, 81.31; H,
7.56; N, 3.69.
Synthesis of 4-ethynyl-N,N-diphenylaniline (2)
1 (3 g, 8.8 mmol) was placed in a round bottom ask with 45 mL 64 mmol) and triethylsilylacetylene (2 mL, 11 mmol) in 40 mL of
of CH2Cl2. Then, tetrabutylammonium uoride 1 M in THF dry toluene was added via cannula. The mixture was degassed
ꢁ
(12 mL, 12 mmol) was added and the mixture was stirred for 1.5 again by the pump/freeze technique and stirred at 90 C over-
hours under N2 at r.t. Aer removing the solvent, the crude night under N2. Then, a silica plug was run in CH2Cl2 and the
product was puried by column chromatography (SiO2, solvent removed. The crude product was puried by column
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chromatography (SiO2, Hexanes/CHCl3 3 : 1 up to 2 : 1) to afford 383.05154; found: 383.051358. Anal. calcd for C20H18BrNO2: C,
the product as yellow oil (1.89 g, 57% yield). 1H NMR (500 MHz, 62.51; H, 4.72; N, 3.65; found: C, 62.52; H, 4.74; N, 3.54.
CDCl3) dH: 7.28 (d, J ¼ 8.8 Hz, 2H); 7.07 (d, J ¼ 8.3 Hz, 4H); 6.98
(d, J ¼ 8.3 Hz, 4H); 6.89 (d, J ¼ 8.8 Hz, 2H); 2.31 (s, 6H); 1.03 (t, Synthesis of 4-methoxy-N-(4-methoxyphenyl)-N-(4-
J ¼ 7.9 Hz, 9H); 0.66 (q, J ¼ 7.9 Hz, 6H). 13C NMR (125 MHz, ((triethyllsilyl)ethynyl)phenyl)aniline (7)
CDCl3) dC: 148.6; 144.9; 133.4; 133.1; 130.2; 125.3; 121.3; 115.4;
Bis(triphenylphosphine)palladium dichloride (65 mg, 0.09
mmol) triphenylphosphine (49 mg, 0.18 mmol) and copper
iodide (71 mg, 0.37 mmol) were placed in a shlenck tube and
dried under high vacuum for 30 minutes. Then, a previously
107.1; 90.2; 21.1; 7.8; 4.8. MS EI (m/z): [M]+ calcd for C28H33NSi:
411.23768; found: 411.237326. Anal. calcd for C28H33NSi: C,
81.69; H, 8.08; N, 3.40; found: C, 81.58; H, 8.15; N, 3.51.
degassed solution of 6 (1.2 g, 3.12 mmol), piperidine (2.5 mL, 25
mmol) and triethylsilylacetylene (780 mL, 4.37 mmol) in 15 mL of
Synthesis of 4-ethynyl-N,N-di-p-tolylaniline (5)
4 (950 mg, 2.3 mmol) was placed in a round bottom ask with dry toluene was added via cannula. The mixture was degassed
ꢁ
12 mL of CH2Cl2. Then, tetrabutylammonium uoride 1 M in again by the pump/freeze technique and stirred at 90 C over-
THF (3.2 mL, 3.2 mmol) was added and the mixture was stirred night under N2. Then, a silica plug was run in CH2Cl2 and the
for 2 hours at r.t. under N2. Aer removing the solvent, the solvent removed. The crude product was puried by column
crude product was puried by column chromatography (SiO2, chromatography (SiO2, Hexanes/CHCl3 3 : 1 up to 2 : 1) to afford
Hexanes/CH2Cl2 4 : 1) to afford the product as pale solid (634 the product as yellow oil (1.36 g, 98% yield). 1H NMR (500 MHz,
1
mg, 93% yield). H NMR (500 MHz, CDCl3) dH: 7.29 (d, J ¼ 8.7 CDCl3) dH: 7.25 (d, J ¼ 8.4 Hz, 2H); 7.04 (d, J ¼ 8.9 Hz, 4H); 6.83 (d,
Hz, 2H); 7.08 (d, J ¼ 8.3 Hz, 4H); 7.00 (d, J ¼ 8.3 Hz, 4H); 6.90 (d, J ¼ 8.9 Hz, 4H); 6.79 (d, J ¼ 8.8 Hz, 2H); 3.79 (s, 6H); 1.03 (t, J ¼ 7.9
J ¼ 8.8 Hz, 2H); 3.00 (s, 1H); 2.32 (s, 6H). 13C NMR (125 MHz, Hz, 9H); 0.65 (q, J ¼ 7.9 Hz, 6H). 13C NMR (125 MHz, CDCl3) dC:
CDCl3) dC: 148.9; 144.8; 133.6; 133.1; 130.2; 125.5; 120.9; 113.8; 156.4; 149.0; 140.5; 133.1; 127.1; 119.4; 115.0; 114.4; 107.2; 89.9;
84.3; 76.0; 21.1. MS EI (m/z): [M]+ calcd for C22H19N: 297.15120; 55.7; 7.7; 4.7. MS EI (m/z): [M]+ calcd for C28H33NO2Si: 443.22751;
found: 297.151344. Anal. calcd for C22H19N: C, 88.85; H, 6.44; N, found: 443.227937. Anal. calcd for C28H33NO2Si: C, 75.80; H,
4.71; found: C, 88.84; H, 6.53; N, 4.83.
7.50; N, 3.16; found: C, 75.87; H, 7.40; N, 2.98.
Synthesis of 4,40-(buta-1,3-diyne-1,4-diyl)bis(N,N-di-p-
tolylaniline) (Me-DATPA)
Synthesis of 4-ethynyl-N,N-bis(4-methoxyphenyl)aniline (8)
7 (1.35 g, 3 mmol) was placed in a round bottom ask with 30
5 (148 mg, 0.5 mmol) was placed in a round bottom ask with 25 mL of CH2Cl2. Then, tetrabutylammonium uoride 1 M in THF
mL of CH2Cl2. TMEDA (2.2 mL, 15 mmol) and copper(I) chloride (4.2 mL, 4.2 mmol) was added and the mixture was stirred for
(1.45 g, 15 mmol) were added and the mixture was stirred 1.5 hours at r.t. under N2. Aer removing the solvent, the crude
vigorously for 4 hours under air with a CaCl2 tube on top. Then, product was puried by column chromatography (SiO2,
the solvent was removed and the crude was puried by column Hexanes/CHCl3 3 : 1) to afford the product as yellow oil (930 mg,
1
chromatography (SiO2, Hexanes/CHCl3 3 : 1) to afford the 93% yield). H NMR (500 MHz, CDCl3) dH: 7.26 (d, J ¼ 8.8 Hz,
product as yellow solid (118 mg, 80% yield). 1H NMR (500 MHz, 2H); 7.06 (d, J ¼ 8.9 Hz, 4H); 6.84 (d, J ¼ 9.0 Hz, 4H); 6.80 (d, J ¼
CDCl3) dH: 7.29 (d, J ¼ 8.7 Hz, 4H); 7.09 (d, J ¼ 8.3 Hz, 8H); 7.01 (d, 8.8 Hz, 2H); 3.80 (s, 6H); 2.98 (s, 1H). 13C NMR (125 MHz, CDCl3)
J ¼ 8.3 Hz, 8H); 6.87 (d, J ¼ 8.8 Hz, 4H); 2.32 (s, 12H). 13C NMR dC: 156.6; 149.4; 140.3; 133.1; 127.4; 119.1; 115.0; 112.8; 84.5;
(125 MHz, CDCl3) dC: 149.1; 144.6; 133.9; 133.5; 130.3; 125.7; 75.8; 55.7. MS EI (m/z): [M]+ calcd for C22H19NO2: 329.14103;
120.5; 113.3; 82.3; 73.5; 21.1. MS EI (m/z): [M]+ calcd for C44H36N2: found: 329.140097. Anal. calcd for C22H19NO2: C, 80.22; H, 5.81;
592.28730; found: 592.28732. Anal. calcd for C44H36N2: C, 89.15; N, 4.25; found: C, 80.10; H, 5.80; N, 4.35.
H, 6.12; N, 4.73; found: C, 89.03; H, 6.06; N, 4.69. m.p. ¼ 206 ꢁC.
Synthesis of 4,40-(buta-1,3-diyne-1,4-diyl)bis(N,N-bis(4-methoxy-
Synthesis of 4-bromo-N,N-bis(4-methoxyphenyl)aniline (6)
phenyl)aniline) (MeO-DATPA)
4-Bromoaniline (2.8 g, 16.5 mmol), 4-iodoanisole (16.5 g, 66 8 (220 mg, 0.67 mmol) was placed in a round bottom ask with
mmol), potassium tert-butoxide (5.5 g, 49.5 mmol), copper 33 mL of CH2Cl2. TMEDA (3 mL, 20 mmol) and copper(I) chlo-
iodide (125 mg, 0.7 mmol) and 2,20-bipyridyl (100 mg, 0.7 mmol) ride were added and the mixture was stirred vigorously for 4
were placed in a round bottom ask and dried under high hours under air with a CaCl2 tube on top. Then, the solvent was
vacuum for 30 minutes. Previously degassed dry toluene (66 mL) removed and the crude product was puried by two column
was added and the mixture was stirred at 120 ꢁC overnight chromatography (SiO2, Hexanes/CHCl3 2 : 1 and then Hexanes/
under N2. Then, a silica plug was run in CH2Cl2 and the solvent Et2O 1 : 1) to afford the product as yellow solid (66 mg, 30%
1
removed. The crude product was puried by column chroma- yield). H NMR (500 MHz, CDCl3) dH: 7.27 (d, J ¼ 8.5 Hz, 4H);
tography (SiO2, Hexanes/CHCl3 4 : 1 up to 2 : 1) to afford the 7.07 (d, J ¼ 9.0 Hz, 8H); 6.84 (d, J ¼ 9.0 Hz, 8H); 6.78 (d, J ¼ 8.5
product as yellow oil (1.87 g, 30% yield). 1H NMR (500 MHz, Hz, 4H); 3.80 (s, 12H). 13C NMR (125 MHz, CDCl3) dC: 56.7;
CDCl3) dH: 7.23 (d, J ¼ 8.8 Hz, 2H); 7.02 (d, J ¼ 9.1 Hz, 4H); 6.82 149.5; 140.0; 133.5; 127.5; 118.8; 115.1; 112.4; 82.3; 73.3; 55.7.
(d, J ¼ 9.1 Hz, 4H); 6.79 (d, J ¼ 8.8 Hz, 2H); 3.79 (s, 6H). 13C NMR MS EI (m/z): [M]+ calcd for C44H36N2O4: 656.26696; found:
(125 MHz, CDCl3) dC: 156.2; 148.1; 140.8; 131.9; 126.7; 122.2; 656.266885. Anal. calcd for C44H36N2O4: C, 80.47; H, 5.52; N,
115.0; 112.5; 55.7. MS EI (m/z): [M]+ calcd for C20H18BrNO2: 4.27; found: C, 80.54; H, 5.43; N, 4.38. m.p. ¼ 186 C.
ꢁ
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133.3; 132.9; 128.5; 125.4; 122.3; 116.6; 106.7; 90.9; 16.8; 7.7; 4.7.
MS EI (m/z): [M]+ calcd for C28H33NS2Si: 475.18182; found:
475.181501. Anal. calcd for C28H33NS2Si: C, 70.68; H, 6.99; N,
2.94; found: C, 67.56; H, 6.73; N, 3.46.
Synthesis of 4-(methylthio)-N-(4-(methylthio)phenyl)-N-
phenylaniline (9)
4-bromo-N-(4-bromophenyl)-N-phenylaniline (4.7 g, 11.6 mmol)
was added to a round bottom ask with 60 mL of anhydrous DMF
and stirred under N2. Sodium thiomethoxide (3.25 g, 46.4 mmol)
was added in small portions and the mixture heated up to 120 ꢁC
and stirred under N2 for 48 hours. Then, the crude product was
poured into 200 mL of aqueous NaHCO3 satured solution and
extracted with 200 mL of Et2O twice. The organic phase was
washed with saturated NaHCO3 and brine, dried over Na2SO4
and the solvent removed. The crude product was puried by
column chromatography (SiO2, Hexanes/CH2Cl2 10 : 1 up to
4 : 1) to afford the product as colorless oil (691 mg, 16% yield).
Note that monoreacted product, 4-bromo-N-(4-(methylthio)
phenyl)-N-phenylaniline, was also obtained with a high yield
(2.35 g, 51% yield). 1H NMR (500 MHz, CDCl3) dH: 7.24 (t, J ¼ 7.9
Synthesis of 4-ethynyl-N,N-bis(4-(methylthio)phenyl)aniline (12)
11 (575 mg, 1.2 mmol) was placed in a round bottom ask with
12 mL of CH2Cl2. Then, tetrabutylammonium uoride 1 M in
THF (1.7 mL, 1.7 mmol) was added and the mixture was stirred
for 2 hours under N2 at r.t. Aer removing the solvent, the crude
product was puried by column chromatography (SiO2,
Hexanes/CHCl3 3 : 1) to afford the product as yellow oil (371 mg,
1
92% yield). H NMR (500 MHz, CDCl3) dH: 7.32 (d, J ¼ 8.7 Hz,
2H); 7.18 (d. J ¼ 8.7 Hz, 4H); 7.02 (d, J ¼ 8.7 Hz, 4H); 6.94 (d, J ¼
8.7 Hz, 2H); 3.02 (s, 1H); 2.74 (s, 6H). 13C NMR (125 MHz, CDCl3)
dC: 148.2; 144.7; 133.3; 133.1; 128.6; 125.6; 122.0; 115.1; 84.0;
76.5; 16.8. MS EI (m/z): [M]+ calcd for C22H19NS2: 361.01534;
found: 361.095338. Anal. calcd for C22H19NS2: C, 73.09; H, 5.30;
N, 3.87; found: C, 70.23; H, 5.32; N, 4.10.
Hz, 2H); 7.18 (d, J ¼ 8.6 Hz, 4H); 7.05 (m, 7H); 2.47 (s, 6H). 13
C
NMR (125 MHz, CDCl3) dC: 147.6; 145.6; 131.8; 129.4; 128.8;
124.8; 124.1; 123.0; 17.1. MS EI (m/z): [M]+ calcd for C20H19NS2:
337.09534; found: 337.095559. Anal. calcd for C20H19NS2: C,
71.17; H, 5.67; N, 4.15; found: C, 71.07; H, 5.62; N, 4.18.
Synthesis of 4,40-(buta-1,3-diyne-1,4-diyl)bis(N,N-bis(4-
(methylthio)phenyl)aniline) (MeS-DATPA)
12 (371 mg, 1 mmol) was placed in a round bottom ask with 50
mL of CH2Cl2. TMEDA (4.5 mL, 30 mmol) and copper(I) chloride
(3 g, 30 mmol) were added and the mixture was stirred vigorously
for 4 hours under air with a CaCl2 tube on top. Then, a silica plug
was run in CH2Cl2 and the solvent removed. The crude product
was puried by column chromatography (SiO2, Hexanes/CHCl3
3 : 2 up to 2 : 3) to afford the product as yellow solid (270 mg, 80%
yield). 1H NMR (500 MHz, CDCl3) dH: 7.33 (d, J ¼ 8.8 Hz, 4H); 7.19
(d, J ¼ 8.6 Hz, 8H); 7.03 (d, J ¼ 8.7 Hz, 8H); 6.92 (d, J ¼ 8.8 Hz, 4H);
2.48 (s, 12H). 13C NMR (125 MHz, CDCl3) dC: 148.4; 144.4; 133.6;
133.6; 128.5; 125.9; 121.5; 114.5; 82.1; 73.8; 16.7. MS EI (m/z): [M]+
calcd for C44H36N2S4: 720.17558; found: 720.175898. Anal. calcd
for C44H36N2S4: C, 73.29; H, 5.03; N, 3.89; found: C, 73.25; H, 5.10;
N, 3.93. m.p. ¼ 218 ꢁC.
Synthesis of 4-bromo-N,N-bis(4-(methylthio)phenyl)aniline (10)
9 (691 mg, 2 mmol) was placed in a round bottom ask with
20 mL of anhydrous DMF and the mixture was cooled down to
0 ꢁC. Then, NBS (365 mg, 2 mmol) was added in small amounts
at 0 ꢁC and the mixture stirred for 3 hours at r.t. The crude
product was dissolved in 200 mL of Et2O and washed with
200 mL of water (ꢂ2) and brine. The organic layer was dried
under Na2SO4 and the solvent removed. The crude product was
puried by column chromatography (SiO2, Hexanes/CH2Cl2
3 : 1) to afford the product as colorless oil (515 mg, 60% yield).
1H NMR (500 MHz, CDCl3) dH: 7.31 (d, J ¼ 0.5 Hz, 2H); 7.17 (d,
J ¼ 8.5 Hz, 4H); 6.99 (d, J ¼ 8.5 Hz, 4H); 6.91 (d, J ¼ 8.5 Hz, 2H);
2.47 (s, 6H). 13C NMR (125 MHz, CDCl3) dC: 146.8; 145.0; 132.6;
132.4; 128.7; 125.0 (ꢂ2); 115.1; 16.9. MS EI (m/z): [M]+ calcd for
C
20H18BrNS2: 415.00586; found: 415.005850.
Methods
Chemical characterization
Synthesis of 4-(methylthio)-N-(4-(methylthio)phenyl)-N-(4-
1H and 13C NMR spectra were recorded on a Bruker Advance 500
spectrometer (500 MHz for 1H and 125 MHz for 13C). The
deuterated solvents are indicated; chemical shis, d, are given
in ppm, using the solvent residual signal as an internal stan-
dard (1H, 13C). MS were recorded on ThermoElectron MAT 900
using electron impact (EI) ionization technique. Elemental
analyses were carried out by Stephen Boyer at London Metro-
politan University using a Carlo Erba CE1108 Elemental
Analyzer. A copy of 1H and 13C NMR spectra have been reported
for samples 9 to 12, where elemental analysis exceeded 0.4% on
carbon (ESI†).
((triethylsilyl)ethynyl)phenyl)aniline (11)
Bis(triphenylphosphine)palladium dichloride (35 mg, 0.05
mmol) triphenylphosphine (26 mg, 0.1 mmol) and copper iodide
(38 mg, 0.2 mmol) were placed in a Schlenk tube and dried under
high vacuum for 30 minutes. Then, a previously degassed solu-
tion of 10 (515 mg, 1.25 mmol), piperidine (1 mL, 10 mmol) and
triethylsilylacetylene (460 mL, 2.5 mmol) in 7.5 mL of dry toluene
was added via cannula. The mixture was degassed again by the
pump/freeze technique and stirred at 90 ꢁC overnight under N2.
Then, a silica plug was run in CH2Cl2 and the solvent removed.
The crude product was puried by column chromatography
(SiO2, Hexanes/CHCl3 4 : 1 up to 2 : 1) to afford the product as
yellow oil (757 mg, 96% yield). 1H NMR (500 MHz, CDCl3) dH: 7.31
Electrochemical characterization
(d, J ¼ 8.6 Hz, 2H); 7.17 (d, J ¼ 8.6 Hz, 4H); 7.00 (d, J ¼ 8.6 Hz, 4H); All cyclic voltammetry measurements were carried out in freshly
6.93 (d, J ¼ 8.6 Hz, 2H); 2.47 (s, 6H); 1.04 (t, J ¼ 7.9 Hz, 9H); 0.66 (q, distilled CH2Cl2 using 0.3 M [TBA][PF6] electrolyte in a three-
J ¼ 7.9 Hz, 6H). 13C NMR (125 MHz, CDCl3) dC: 147.7; 144.7; electrode system, with each solution being purged with N2 prior
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Table 1 Summary of X-ray crystallographic data for H-DATPA and MeO-DATPA
to measurement. The working electrode was a Pt disk. The
reference electrode was Ag/AgCl and the counter electrode was a
Pt rod. All measurements were made at room temperature
using a mAUTOLAB Type III potentiostat, driven by the electro-
chemical soware GPES. Cyclic voltammetry (CV) measure-
ments used scan rates of 100 mV sꢀ1; square wave voltammetry
(SWV) was carried out at a step potential of 4 mV, square wave
amplitude of 25 mV, and a square wave frequency of 15 Hz,
giving a scan rate of 40 mV sꢀ1. Ferrocene was used as the
internal standard in each measurement.
H-DATPA
MeO-DATPA
Chemical formula
Formula weight
Data collection
Structure solution
Structure renement
Color, habit
C40H28N2
536.64
C44H36N2O4
656.78
CrysAlisPro
CrysAlisPro
Superip26
CRYSTALS
Yellow, block
0.30 ꢂ 0.24 ꢂ 0.13
Orthorhombic
Pca21
18.74123(11)
9.94915(7)
18.52083(11)
90
Sir9225
SHELXTL
Yellow, rod
0.39 ꢂ 0.09 ꢂ 0.05
Monoclinic
P21/n
8.05690(10)
9.2642(2)
18.9932(4)
90
90.551(2)
90
1417.60(5)
2
Crystal size (mm)
Crystal system
Space group
˚
a (A)
Optical characterization
˚
b (A)
˚
c (A)
Solution UV-Visible absorption spectra were recorded using a
Jasco V-670 UV/Vis/NIR spectrophotometer controlled with
SpectraManager soware. Photoluminescence (PL) spectra were
recorded with a Fluoromax-3 uorimeter controlled by the
ISAMain soware. All samples were measured in a 1 cm cell at
room temperature with dichloromethane as solvent. Concen-
tration of 2 ꢂ 10ꢀ5 M and 5 ꢂ 10ꢀ6 M were used for solution UV/
Visible and PL, respectively. Time-resolved and time-integrated
PL spectra of lm samples were acquired using a time-corre-
lated single photon counting (TCSPC) setup (FluoTime 300,
PicoQuant GmbH). Samples were photoexcited using a 507 nm
laser head (LDH-P-C-510, PicoQuant GmbH) pulsed at 40 MHz,
with a pulse duration of 117 ps and uence of ꢃ0.1 nJ cmꢀ2. Fits
were carried out using commercial tting soware (FluoFit
v4.5.3, PicoQuant GmbH).
a (ꢁ)
b (ꢁ)
g (ꢁ)
90
90
3
˚
V (A )
3453.38(4)
4
1.263
Z
r(calc.) (g cmꢀ3
)
1.257
0.559
8178/3729
4.7 to 73.7
97.3
m (mmꢀ1
)
0.642
Nꢁ of rcn/unique
42111/28155
4.7 to 76.5
99.8
q range (ꢁ)
Compl. to qmax (%)
R1
0.0427
0.0273
wR2
GooF
0.0941
1.030
0.0592
1.012
indium tin oxide substrate (ITO) and dried at 140 ꢁC for 30 min
in vacuum. The purpose of PEDOT:PSS layer (40 nm) was to
reduce the roughness of ITO as well as to improve the work
function, achieving enhanced hole-only device properties. The
HTMs were spin-coated onto PEDOT:PSS from chloroform
solution (40 mg mLꢀ1) in a nitrogen atmosphere. Finally, Au
contacts (400 nm thick) were applied via thermal evaporation
through a shadow mask in 2 ꢂ 10ꢀ6 Torr vacuum. The work
function of Au and ITO are close to the HOMO energy level of
the HTMs as well as far below the LUMO energy level.30
Therefore, the electron injection barrier is higher than the
corresponding hole injection barrier. As a result, the transport
is dominated by holes. The J–V characteristics of these samples
were measured with a Keithley 2420 Source Meter unit at
room temperature.
Crystallographic details
Crystallographic data were collected using Agilent Technologies
˚
SuperNova with Cu Ka (l ¼ 1.54178 A) radiation at 120(2) K.
Single crystals suitable for X-ray diffraction (XRD) were prepared
by recrystallization in hot 1-azohexane for H-DATPA and slow
evaporation in CH2Cl2 for MeO-DATPA. A summary of data
collection and structure renement is reported in Table 1. The
crystal structures were deposited at CCDC with deposition
number 921284 and 925482 for H-DATPA and MeO-DATPA,
respectively.
Computational details
The molecular structures were optimized rst in vacuum
without any symmetry constrains, followed by the addition of
CH2Cl2 solvation via a conductor-like polarizable continuum
model (C-PCM).27 The presence of local minimum was
conrmed by the absence of imaginary frequencies. All calcu-
lations were carried out using the Gaussian 09 program28 with
the Becke three parameter hybrid exchange, Lee Yang–Parr
correlation functional (B3LYP) level of theory. All atoms were
described by the 6-31G(d) basis set. All structures were input
and processed through the Avogadro soware package.29
Solar cell fabrication
ss-DSSCs were prepared according to a standard procedure,31
and all solvents used for device fabrication were reagent grade
and anhydrous. FTO substrates (15 U sqꢀ1, Pilkington) were
etched with zinc powder and HCl (2 M aqueous solution) to give
the desired electrode patterning. The substrates were cleaned
with Hellmanex (2% by volume in water), de-ionized water,
acetone, and ethanol. The last traces of organic residues were
removed by a 10 minutes oxygen plasma cleaning step. The FTO
sheets were subsequently coated with a compact layer of TiO2
(100 nm) by aerosol spray pyrolysis deposition at 270 ꢁC, using
Charge transport parameters
Charge transport in the HTMs has been investigated according oxygen as the carrier gas. Films of 1.5 mm thick mesoporous
to a published procedure.30 Poly(3,4-ethylendioxythiophene)– TiO2 were then deposited by screen-printing a commercial paste
ꢁ
poly(styrene sulfonate) (PEDOT:PSS) was spin-coated onto (Dyesol 18NR-T). The TiO2 lms were slowly heated to 500 C
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and allowed to sinter for 30 min in air. Once cooled, the samples coating solutions were identical to those used to prepare the
were immersed into a 15 mM TiCl4 aqueous solution for 45 min solar cells. Linear current–voltage curves were obtained for the
at 70 ꢁC and then heated to 500 ꢁC for another sintering step of conductivity measurements by testing in dark condition with a
45 min. Aer cooling to 70 ꢁC, the substrates were immersed in sourcemeter (Keithley 2400, USA) in a two-point contact
a 500 mM dye solution, in 1 : 1 mixture of acetonitrile and tert- setup. All devices were stored in air and in dark for 24 hours
butyl alcohol, for one hour. The dye employed in this study was prior to testing.
D102, previously reported by Horiuchi et al.23 Aer the dyed
lms were rinsed in acetonitrile, the hole conductor matrix was
applied by spin-coating at 1000 rpm for 45 s in air. The solutions
Results and discussion
The HTMs were synthesized according to the procedure
described in Scheme 1. A triphenylamine unit containing para-
substituted methyl and methoxy functional groups was
synthesized by copper-catalyzed Ullmann coupling.33 The thio-
methoxy triphenylamine derivative was synthesized by reacting
dibromotriphenylamine with sodium thiomethoxide,34 followed
by a bromination with NBS. Then, a silyl-protected acetylene
bridge was attached via the Sonogashira cross-coupling reaction
and was subsequently deprotected with uoride.35 Finally,
homocoupling of the terminal alkyl group was achieved via
copper and molecular oxygen.36
Single crystals suitable for XRD analysis were collected to
resolve the crystal packing as shown in Fig. 2. H-DATPA and
MeO-DATPA are each arranged in a herringbone pattern, with
one molecule in the asymmetric unit and a regular stacking
distance between the molecules. The distances between the
planes, dened over the central diacetylene unit and the
for spin coating consisted of 80 mM of hole transporter, 15 mM
of lithium bis(triuoromethylsulfonyl)imide salt and 70 mM of
4-tert-butylpyridine in anhydrous chlorobenzene. Aer drying
overnight, back contacts were applied by thermal evaporation of
200 nm of silver.
Solar cells characterization32
For measuring the device merit parameters, simulated AM 1.5
sunlight was generated with a class AAB ABET solar simulator
calibrated to give simulated AM 1.5, 100 mW cmꢀ2 irradiance,
using an NREL-calibrated KG5 ltered silicon reference cell,
with less than 1% mismatch factor; the current–voltage curves
were recorded with a sourcemeter (Keithley 2400, USA). The
solar cells were masked with a metal aperture dening the
active area (0.12 cm2) of the solar cells. All devices were stored in
air and in dark for 24 hours prior to testing.
˚
directly attached phenyl rings, are 3.61 and 3.44 A respectively.
The shortest intermolecular N–N distances between molecules
Conductivity measurements
˚
Devices for measuring the conductivity of the hole transporter of adjacent stacks were found to be 6.03 A for H-DATPA and
˚
in a dye-sensitized mesoporous TiO2 lm were prepared 5.80 A for MeO-DATPA. These values are indicative of effective
according to a published procedure.31 The preparation (on glass stacking of p-orbitals distributed, as calculated from DFT (see
substrates) was identical to that used for the ss-DSSCs, except next section), on the triphenylamine units and diacetylene
for the absence of the TiO2 compact layer. The electrode pattern bridge. We note that both within the stack and between stacks
was designed for two point contact measurements with a however, shorter interactions were observed for MeO-DATPA.
channel length (direction of current ow) of 200 mm, a channel Close p-stacked molecular arrangements can generate high
width of 6.53 cm, and a lm thickness of 1 mm. The metal-hole charge transport rate in small molecule HTMs.37 Indeed, we
transporter contact resistance was measured to be several will show later that the shorter N–N distance in MeO-DATPA,
orders of magnitude lower than the bulk resistance as estimated thus a closer p–p stacking, corresponds to higher hole
by four point conductivity measurement (see ESI†). The spin- mobility.
Scheme 1 Synthetic procedure for the HTMs preparation. Reaction conditions: (a) CuI, bipy, KOtBu, toluene; (b) NaSMe, DMF; (c) NBS, DMF; (d) triethylsilylacetylene,
(PPh3)2PdCl2, PPh3, CuI, piperidine, toluene; (e) 1 M TBAF, CH2Cl2; (f) CuCl, TMEDA, O2, CH2Cl2.
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Fig. 2 Crystal packing of H-DATPA from a-axis top view together with the p–p stacking (a) and MeO-DATPA (b). Hydrogen atoms have been removed for clarity.
The HTM oxidation potential and energy level alignment
with the HOMO level of the dye are crucial parameters for
constructing high-performance ss-DSSCs. In previous studies,
Durrant and co-workers demonstrated that hole transfer from
the dye to the HTM (dye regeneration) is not limited by kinetic
competition with interface recombination processes.43–45
Rather, dye regeneration depends solely on the oxidation
potential offset that drives the hole from the dye to the
HTM,46–48 which can be estimated by cyclic voltammetry as
described in the experimental section.49 In Fig. 4 we report the
oxidation peaks for the DATPA series and Spiro-OMeTAD
obtained by square-wave voltammetry. As expected, increasing
the substituent electron-donating character on DATPA (H– <
Me– < MeS– < MeO–) we observed a strong shi in the oxidation
peaks to less positive potentials. However, the rst oxidation for
Fig. 3 Normalized UV-Visible absorption (solid line) and photoluminescence
(dashed line) of H-DATPA (black line) Me-DATPA (red line), MeS-DATPA (purple
Spiro-OMeTAD is still another 300 mV less positive than the
MeO-DATPA. All DATPA derivatives show evidence of two
oxidation peaks corresponding to one electron oxidation
process for each triphenylamine unit. For MeO- and MeS-
DATPA, the cyclic voltammetry trace (see ESI†) cannot clearly
elucidate the two-oxidation processes, since the oxidation
potentials for each electron are similar. Therefore, an electro-
chemical pulse technique, such as square-wave voltammetry
line) and MeO-DATPA (green line).
A high level of transparency in the visible to NIR region for
the HTM is a key parameter for ss-DSSC application.38 The
absorption spectra (solid lines, Fig. 3) are similar for each HTM
derivative; the absorption peaks (lmax) are located in the spec-
tral window between 375 and 390 nm, with a half-maximum
peak-width of approximately 70 nm. To study the DATPA
derivatives in ss-DSSCs, we employed an indoline dye (D102),
the absorption spectra of which spans the region 400–650 nm
(ESI†),23,24 which makes this sensitizer ideal to test our new
HTMs. In addition, photoluminescence spectra were also
recorded (dashed line, Fig. 3). The measurements show a clear
red-shi and broadening of the PL spectrum with the series,
which could be explained as an effective S1 energy stabilization
due to the presence of electron-donating groups of various
strengths.39,40 From the intersection of emission and absorption
0
spectra we can obtain the E0–0 transition and, therefore, we can
estimate the optical band gap. In addition, the DATPA emission
peaks are in the visible region where D102 strongly absorbs,
¨
which could enable additional light absorption, via Forster
resonant energy transfer from the HTM to D102.41,42 However,
we can likely neglect this contribution to the photocurrent
since the HTM predominantly absorbs in the UV region, which
is strongly ltered by the TiO2 (see ESI†).
Fig. 4 Square-wave voltammetry of H-DATPA (black line), Me-DATPA (red line),
MeS-DATPA (purple line), MeO-DATPA (green line) and Spiro-OMeTAD (blue
dashed line) plotted vs. ferrocene/ferrocenium.
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Table 2 Summary of the optical and electrochemical properties
the position of the HOMO–LUMO energy levels and their electron
density on the molecules. The HOMO is delocalized through the
p-orbitals of the triphenylamine units and diacetylene bridge for
each derivative. The lowering of the HOMO energy level matches
the electron-donating strength for each group, which is in good
agreement with the oxidation potentials as extracted from square
wave voltammetry. The LUMO energy level, as also reported in
Table 2, seems to be less strongly inuenced by the substituents
on the triphenylamine units. This can be explained by consid-
ering that the LUMO electron density is somewhat localized on
the diacetylene bridge, with a strong p anti-bonding character,
and is therefore not signicantly inuenced by the functional
groups on the para position on the triphenylamine units.
lmax
lem
(nm)a
Egap
EOX
EHOMO
ELUMO
(nm)
(eV)b
(V)c
(eV)d
(eV)e
H-DATPA
Me-DATPA
MeS-DATPA
MeO-DATPA
Spiro-OMeTAD
D102 in solution
D102 onto TiO2
376
382
391
389
385
510
505
434
453
495
490
424
640
670
3.00
2.93
2.85
2.86
3.05
2.19
2.07
+0.52
+0.45
+0.38
+0.30
+0.03
+0.42
+0.58
ꢀ5.33
ꢀ5.23
ꢀ5.14
ꢀ5.02
ꢀ4.64
ꢀ5.18
ꢀ5.41
ꢀ2.33
ꢀ2.33
ꢀ2.29
ꢀ2.16
ꢀ1.59
ꢀ2.99
ꢀ3.34
a
b
Excitation at lmax
. From the intersection of absorption and emission
spectra. c From SWV and CV measurements and referenced to ferrocene.
d
EHOMO (eV) ¼ ꢀ1.4 ECV (V) ꢀ 4.6. e ELUMO ¼ EHOMO + Egap
.
Another signicant parameter to consider for designing new
HTMs for ss-DSSCs is charge transport.9–11 The device prepara-
used here, should be used in order to elucidate both oxidation tion to estimate the charge transport parameters for the
processes. For Spiro-OMeTAD, we observed three oxidation reported HTMs is described in the experimental section. Fig. 6
processes, involving one electron for the rst and second shows the J–V characteristics for the hole-only diodes at room
oxidation and two electrons for the last oxidation process, as temperature. The curves show two regions of conduction, which
reported previously.7,50 We converted the oxidation potentials is Ohmic conduction (with slope ꢃ1) at low voltage and non
(V) to EHOMO (eV) by following the procedure described by Ohmic conduction (with slope >2) at high voltage. The Ohmic
Thompson et al. which employs an empirical linear translation region has been attributed to the background impurity
equal to EHOMO (eV) ¼ ꢀ1.4 ECV (V) ꢀ 4.6.49 The extracted values
are listed in Table 2. It is worth pointing out that we have also
measured the oxidation potential of D102 under the same
conditions used for the HTMs, in order to make a reasonable
estimation of the dye – HTM energy offset (see ESI†). The D102
rst oxidation potential in solution was found at more negative
potential than H-DATPA and Me-DATPA (see Table 2), which
should make these HTMs unable to regenerate the dye.
However, the cyclovoltammetry performed for D102 anchored
on mesoporous TiO2 gives 0.26 V positive shi compared to the
valued collected in solution. Taking in account the new oxida-
tion value, all DATPA derivatives should be able to regenerate
the oxidized dye, as we will discuss aerwards.
To elucidate the electronic properties of DAPTA derivatives,
DFT calculations were performed with Gaussian 09, B3LYP 6-
31G(d) level of theory. Absolute HOMO energy values from DFT
Fig. 6 J–V characteristics of devices experimental (symbols) and calculated (solid
calculations differ slightly from the experimental (ca. 0.4 eV);
however,thetrendisreproducedfaithfully(seeESI†). Fig. 5shows
lines) for H-DATPA (black), Me-DATPA (red), MeS-DATPA (purple), MeO-DATPA
(green) and Spiro-OMeTAD (blue) using the procedure previously reported.30
Fig. 5 Molecular orbital distribution of HOMO (bottom) and LUMO (top) for HTMs at B3LYP/6-31G(d) level of theory (isodensity ¼ 0.04).
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Table
3
Carrier mobility estimated by fitting of current–voltage curves30
Table 4 Device performance parameters for ss-DSSCs fabricated with the
reported in Fig. 6 and conductivity for the lithium-doped HTMs
lithium-doped HTMs
Mobility wihout
Conductivity aer
JSC (mA cmꢀ2
)
Eff (%)
VOC (V)
ff
Li doping (cm2 Vꢀ1 sꢀ1
)
LiTFSI doping (S cmꢀ1
)
H-DATPA
Me-DATPA
MeO-DATPA
Spiro-OMeTAD
0.67
1.13
1.93
6.23
0.15
0.34
1.16
2.96
0.62
0.70
0.89
0.74
0.37
0.43
0.67
0.64
H-DATPA
Me-DATPA
MeS-DATPA
MeO-DATPA
Spiro-OMeTAD
3.0 ꢂ 10ꢀ6
2.8 ꢂ 10ꢀ6
4.0 ꢂ 10ꢀ6
6.0 ꢂ 10ꢀ6
3.6 ꢂ 10ꢀ4
3.9 ꢂ 10ꢀ5
4.0 ꢂ 10ꢀ5
n/a
5.1 ꢂ 10ꢀ5
5.0 ꢂ 10ꢀ5
addition of LiTFSI for the HTM inltrated in a mesoporous TiO2
lm, as described in the Experimental section.31 The measured
values for the LiTFSI-doped HTMs are listed in Table 3. The
conductivity for MeO-DATPA almost identical to Spiro-OMe-
TAD, and the remaining DATPA derivatives measured are all in
the same range. We were unable to measure the conductivity of
devices with MeS-DATPA because the hole transport lms were
inhomogeneous upon inclusion of LiTFSI. It is surprising that
the SCLC mobilities can be so different, yet the conductivities of
the doped lms are comparable. This may be due to a larger
density of low energy charge trap sites in the DATPA HTMs
which are lled upon doping resulting in more comparable
mobility and conductivity to Spiro-OMeTAD. Regardless of
mechanism, the comparable conductivity upon doping is
encouraging for use in the solar cells.
conduction where the injected hole carrier density is smaller
than the intrinsic carrier density in the samples.30
The non Ohmic behavior at high elds has been analyzed in
terms of space charge limited current (SCLC) and tted as
previously reported to extract the charge mobility (Table 3, see
ESI† for more details). DATPA derivatives show charge mobility
about two orders of magnitude lower than Spiro-OMeTAD.
It has been widely demonstrated that lithium bis(tri-
uoromethylsulfonyl)-imide (LiTFSI) needs to be added in the
HTM to efficiently generate photocurrent in ss-DSSCs.51,52
Furthermore, we have recently demonstrated that LiTFSI is
also an strong and stable p-dopant for HTMs.11,53 Therefore, to
compare the charge transport in conditions similar to device
operation, we measured the effective conductivity aer the
Weprepared a setofdevices withDATPAderivatives andSpiro-
OMeTAD as HTMs to measure their respective performance.
Fig. 7a shows the characteristic current–voltage ( J–V ) curves of
Fig. 7 Photocurrent–voltage curves for devices employing DATPA derivatives
and Spiro-OMeTAD as HTM under AM 1.5 simulated sunlight of 100 mW cmꢀ2
equivalent solar irradiance (a) and in the dark (b). The reported J–V curves are
from the device of maximum power conversion efficiency out of a series of four
repeats for each HTM.
Fig. 8 Dependence of the generated photocurrent at short circuit condition
versus Dye – HTM energy offset (a). Schematic diagram of the energy levels at dye-
sensitized TiO2/organic HTM heterojunction (b).54
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the device with maximum power conversion efficiency out of a
seriesof four repeats for each HTM. The characteristics have been
measured under AM 1.5 simulated sun light of 100 mW cmꢀ2
equivalent solar irradiance. Table 4 lists the solar cell perfor-
mance parameters for the J–V curves reported in Fig. 7a.
The short circuit photocurrent density ( JSC) shows a super-
linear increase as HTM oxidation potential decreases. In Fig. 8a
we plot the JSC as a function of the Dye – HTM energy offset,
where we have determined the D102 HOMO level by voltam-
metry of D102-sensitized TiO2 nanoparticles. Conrming what
was previously observed by Kroeze et al.,47 there is an expo-
nential improvement in JSC with this energy offset.48
To investigate the hole transfer between D102 and the
various HTMs (dye regeneration) we have monitored the
quenched emission of the dye adsorbed onto non-injecting
metal oxide, such as Al2O3.5 Mesoporous Al2O3 lms were
prepared in condition similar the TiO2 (see Experimental), then
they were sensitized with D102 and the pores inltrated with the
same HTMs used for the devices reported in Fig. 7. The time-
integrated and time-resolved photoluminescence spectra in
Fig. 9 show an increased quenching of the dye that correlates
directly with the increasing dye – HTM offset and photocurrent
shown in Fig. 8. This quenching is due to a higher hole transfer
yield for the systems with the larger offset, where the hole
distribution lies more strongly on the HTM than on the dye.44,46
We should also note a mismatch for H-DATPA and Me-DATPA,
which is likely to be due to not uniform HTM lm formation, as
we also discuss later for the device reported in Fig. 7.
In contrast to the JSC, there is no consistent trend with the VOC
,
Fig. 9 Monitoring the quenching of the PL from D102 due to hole transfer to
the HTM. (a) Time-integrated PL spectra from film samples excited at 507 nm. (b)
Time-resolved PL decays when monitoring the emission from the samples at 640
nm. Fits to the data were obtained by convoluting biexponential functions with
the instrument response function (IRF). The fits reveal initial dominant ultrafast
components that cannot be accurately resolved by the system and a second
component with time constants of 0.61, 0.45, 0.35 and 0.24 ns for Me-DATPA, H-
DATPA, MeO-DATPA and Spiro-OMeTAD, respectively.
MeO-DAPTA has a higher VOC than Spiro-OMeTAD, but the other
derivatives have a lower VOC. The VOC is generated by the splitting
between the quasi Fermi level for electrons in the TiO2 and for
holes in the HTM. It is inuenced by many factors, including (i)
xed energy level offsets between the materials, i.e. the difference
betweenTiO2 CB energy andthe HTM HOMOenergylevel, (ii) Any
abrupt shi in surface potential at any of the heterojunctions, the
TiO2-Dye-HTM interface for instance, and (iii) the charge density
build up in the photoactive lm (n), which is controlled by the
charge generation (G) and recombination rate (krec), where under
steadystatedn/dt ꢃ G ꢀ nkrec ¼ 0. Inaddition, anyimperfection in
the device can result in dark “leakage” current, which manifests
itself electronically as a low shunt resistance with the impact of
reducing the VOC. The shunt resistance is oen dependent upon
whether the solar cell is illuminated or not, adding further vari-
ability to the open-circuit voltage. The lower photocurrents,
consistent with lower charge generation (hole-transfer) partly
explain the lower than expected VOC. From the dark J–V curves, we
can see that H-DATPA has a shunting issue, and Me-DATPA
exhibits “photo-shunting”. In addition, we can also postulate that
iftheHOMO level ofthe HTM movessignicantly deeper than the
silver electrode work function at this interface, then pinning of
the quasi Fermi level for holes, to the silver work function may
occur, nullifying any potential VOC increase.55 Correctly choosing
the metal/HTM contact may be a requirement to signicantly
enhance the open-circuit voltage of ss-DSSCs.
hole-transfer, or at least operation in the solar cell. Fig. 8b
shows a schematic diagram of the energy levels at the dye-
sensitized TiO2/organic HTM, according to the report of Bis-
quert et al.54 For the devices studied here using Spiro-OMeTAD,
the VOC is 0.74 V. If we assume the determined offset between
the dye and Spiro-OMeTAD HOMO levels of 0.8 V is correct, then
with an optical band gap of 1.9 eV,24 the energy offsets between
the D102 LUMO (or excited state energy) and the maximum
quasi-Fermi level for electrons (E*fn) in the TiO2 under full sun
illumination is 0.4–0.5 eV. This loss-in-potential at the TiO2 side
is reasonably consistent with recent observations with perov-
skite absorbers, where entirely removing the TiO2 and replacing
it with porous alumina resulted in 200 to 300 meV increase in
open-circuit voltage in much thinner devices.5 The most effi-
cient solid-state DSSCs employing organic dyes have a total loss-
in-potential (difference between the optical band gap and open-
circuit voltage) of 0.89 eV,2,8 in comparison to the 1.16 eV for the
system studied here. Clearly minimizing this loss is central to
further enhancements in solar cell performance since it puts a
ceiling on the maximum possible efficiency.56 In principle, the
What is surprising is that we seem to require up to 0.8 eV
offset between the dye and HTM HOMO levels to drive efficient
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dye regeneration process is a single electron transfer, and
should only require a small offset in HOMO levels to occur
efficiently. Indeed, although the measured PL decays in Fig. 9
show signicant speeding up in hole-transfer rate with shiing
of HOMO level, even the slowest quenching system (half life ꢃ1
ns) is still signicantly faster than the nominal lifetime of the
oxidized dye (ms). The hole-transfer from the oxidized dye to the
hole-transporter should thus compete extremely favorably with
recombination of the conduction band electrons with the
oxidized dye. There hence appears to be a signicant missing
bit of information concerning this charge generation mecha-
nism in the ss-DSSC, and understanding rstly why we require
such a large offset, and secondly what dye-HTM properties are
important for minimizing this offset whilst retaining good
operation in the solar cells should be a key focus to signicantly
advance this technology.
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Acknowledgements
We thank the Engineering and Physical Sciences Research
Council (EPSRC) APEX project for nancial support. We thank 20 D. Y. Chen, W. H. Tseng, S. P. Liang, C. I. Wu, C. W. Hsu,
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