Promoting charge-separation in p-type dye-sensitized solar cells using bodipy

Article abstract of DOI:10.1039/c3cc46133e

The viability of applying bodipy sensitisers to NiO-based p-type dye-sensitised solar cells (p-DSCs) has been successfully demonstrated. The triphenylamine donor-bodipy acceptor design promotes a long-lived charge-separated state which is difficult to ach

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Promoting Charge-separation in p-Type Dye-sensitized Solar Cells using
Bodipy.
Jean-François Lefebvrea, Xue-Zhong Suna, James A. Calladinea, Michael W. Georgea, Elizabeth A.
Gibsona*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The viability of applying bodipy sensitisers to NiO-based p-
type dye-sensitised solar cells (p-DSCs) has been successfully
demonstrated. The triphenylamine donor-bodipy acceptor
10 design promotes a long-lived charge separated state which is
difficult to achieve with NiO-based devices. The current was
above 3 mA cm–2 and the IPCE was 28%.
N
N
F2B
BF2
S
S
N
N
N
Tandem dye-sensitised solar cells could provide a route to high
efficiency solar energy conversion at low cost.1 Lindquist et al.
15 proposed in 1999 that replacing the platinised counter electrode
in Grätzel cells with a dye-sensitized photocathode (in series)
should greatly improve the device efficiency by collecting high
energy photons at the top electrode and lower energy photons at
the bottom electrode.2 This method increases the light collection
20 and reduces thermal energy losses; the theoretical efficiency may
be as high as 43%. To date, however, there has not been a tandem
cell reported with power conversion efficiency higher than that of
a state-of-the-art TiO2-based DSC.
Since the rapid charge recombination reactions at the
25 photocathode limit the photoelectric conversion efficiency of the
p-DSCs, the main difficulty of enhancing the performance of
tandem DSCs exists in raising the current generated at the
photocathode to match that generated at the TiO2 anode.3 The
precise reason for fast recombination is still unclear but it has
30 been demonstrated that the dye can be engineered to promote
charge separation, thereby improving the photcurrent.1 A myriad
of different dyes have been reported with TiO2, but very few have
been designed for p-type materials such as NiO.4 The most
efficient sensitisers for NiO have both high extinction coefficients
35 for maximum light harvesting on thin electrodes (e.g. dyes with
favourable π-π* electronic transitions5–7) and charge-transfer
character to promote and maintain charge separation (e.g. π-
donor-acceptor (“push-pull”) functionality5, or with pendant NDI
or C60 electron acceptors8,9).
HO
O
Figure 1 Structure of bodipy dye 1
50 4-carboxy-4’,4’’-di(5-formyl-2-thienyl)triphenylamine with 2,4-
dimethyl-3-ethylpyrrole in the presence of trifluoroacetic acid
(see ESI for details). The resulting dipyrrane was oxidised with p-
chloranil before reacting with boron trifluoride etherate to give 1.
The optical and electrochemical properties are summarised in
55 Table S1. The electronic spectrum of 1 contains absorption
maxima at 365 nm (ε = 56 000 L mol–1 cm–1) and 540 nm (ε =
112 000 L mol–1 cm–1, almost double that of the push-pull
sensitiser P1 at λmax = 481 nm, Figure S3 in the ESI).14 The
absorption and emission spectra are characteristic for a bodipy
60 derivative, with narrow absorption and emission bands and a
small Stokes shift when excited at 488 nm. λmax was the same in
CH2Cl2 or CH3CN solution and when 1 was adsorbed onto NiO
(Figures S4 and S5), suggesting that the electronic properties of
the chromophore were unchanged on binding. However, when 1
65 was excited in the higher energy band (406 nm) the emission
spectrum was solvent dependent: emitting from the higher energy
triphenylamine (475 nm) when in CH3CN; emitting from the
lower energy bodipy (560 nm) when in CH2Cl2.
The electrochemical properties of 1 were determined by cyclic
70 and square wave voltammetry and calibrated vs. FeCp2 /FeCp2
(Figure S13 and S14). A reversible reduction process at –1.46 V
and two closely spaced irreversible oxidation processes at 0.65
and 0.75 V were observed. The estimated driving forces‡ for both
injection (ΔGinj) and regeneration (ΔGreg) are both negative,
+
40
Boradiazaindacene (bodipy) dyes are popular in a range of
applications but have been used infrequently and with moderate
success in n-DSCs.10-13 However, their large extinction
coefficients, electrochemical stability and tuneable absorption
properties (i.e. to optically match photoanodes in tandem cells)
–
75 indicating that 1 is a suitable NiO sensitiser. E(D*/D ) = 0.81 V, is
45 prompted us to test them in p-DSCs. Initially, we have chosen to
use a triphenylamine-thiophene motif used in one of the most
successful NiO photosensitisers (“P1”) coupled to a highly
substituted bodipy (Figure 1).6 The dye was prepared by reacting
suitably positive to allow for efficient charge separation between
geometry and electronic properties. The results supported
experimental evidence that the bodipy was decoupled from the
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Figure 2. J-V curve of 1-sensitized NiO solar cell. Inset: Comparison of
IPCE (line + symbol) and absorption spectrum (line) of 1-sensitized NiO
solar cell to confirm the dye is contributing to the photocurrent. An
absorbance of 2 means the light harvesting efficiency is ca. 99%.
5
triphenylamine-thiophene donor and the calculated optimised
geometry placed the two at an angle of 84 ±0.4º. Figure S.16
shows the electronic distribution in the frontier orbitals: the
HOMO level of 1 is located on the triphenylamine; two
10 degenerate HOMO–1(–2) orbitals and two degenerate
LUMO(+1) levels are located on the bodipy. TD-DFT suggested
that the lowest energy electronic transitions were bodipy-
localised π-π* in nature, with some contribution from a charge
shift from the HOMO to the LUMO.
–
15
NiO p-DSCs were assembled with 1 as the sensitiser and a I3
/I– redox shuttle (see ESI for details). The photocurrent
density/photovoltage curve and spectral response of a typical cell
is given in Figure 2. The cell parameters were JSC = 3.15 mA cm–
2, VOC = 79 mV, FF = 0.31, η = 0.08%. The maximum incident
Figure 3. Transient absorption spectra of 1 top: 10 ps after excitation (1.2
50 μJ), bottom: 2 ns after excitation (1.6 μJ); black solid line = 1 in CH2Cl2,
red dashed line = 1/NiO, blue dotted line = 1/NiO and I3–/I–. Inset: kinetic
traces of 1/NiO (red) at 575 nm and 1/NiO and I3–/I– at 600 nm (blue).
Symbols represent data, lines represent fit of single exponential decay.
20 photon-to-current conversion efficiency (IPCE) was 28%. These
values are superior to many related triphenylamine-based dyes
reported previously.16,17. However, the current and IPCE fall
on NiO (Figure 3 red dashed line). On the ps timescale, the broad
55 features between 400-470 nm corresponding to 11* that were
observed in CH2Cl2 were less intense. Instead a peak at 575 nm
formed rapidly (τrise ≈ 1 ps) which was characteristic of 1– by
comparison with our spectroelectro-chemistry experiments
(Figure S26). A broad featureless absorption persisted throughout
60 the ps- and ns-transient spectra. This is attributed to oxidised NiO
and indicates that the charge-separated state (NiO+/1–) had
formed. The dynamics for each band were characteristically
heterogeneous and we have fitted the rise and decay in each
experiment to first order kinetics giving a short (τ1 ps) and long
65 (τ2 ns) time constant (Table S4) to enable us to compare the
general behaviour of the intermediates. The rapid time constant
for the formation of NiO+/1– can be compared to the lifetime of
11* recorded in solution to give an estimate of the injection
efficiency‡. Using this approach gives a very high efficiency of
70 99% but it should be noted that any slower injection processes
were masked by the subsequent decay of this peak. To try and
provide a more realistic value for this process we have compared
the relative amplitudes after excitation of the signals at 425 nm
short of that achieved with the most successful dye, P1, (JSC
=
5.48 mA cm–2, IPCE = 64%).6 The photocurrents obtained with
25 our bodipy-based dyes are extremely encouraging compared to
bodipy-sensitized TiO2 devices.10,18,19 TiO2-DSCs usually display
far superior efficiencies than p-DSCs but to our knowledge the
highest IPCE for bodipy sensitised n-DSC reported so far is less
than 30%.11 This suggests that the bodipy chromophore may be
30 better suited to the NiO system than conventional TiO2-DSCs.
Further improvements in efficiency are likely to come from
broadening the absorption band by increasing the electronic
coupling between the π-linker and the bodipy and extending the
conjugation by functionalising the dye periphery.
35
To probe the charge-transfer dynamics of this new dye/NiO
system, we performed ps- and ns-transient absorption
spectroscopy (TA) both with 1 in CH2Cl2 and adsorbed on NiO.
Excitation of 1 in solution (Figure 3 black line) at 532 nm
generated an excited state which absorbed between 400-500 nm
40 and decayed over ca. 400 ps to form a new intermediate with
narrower absorption bands at 425 and 650 nm which decayed
much more slowly (τ425 ≈ 300 (+/- 10) ns in air, τ425 ≈860 ns, τ650
≈ 730 ns in Ar) to the ground state. We have assigned this long-
lived transient to a triplet bodipy (31*) excited state (see ESI for
45 further details).20,21
1
(which contains both contribution from 1* and 1–) and 575 nm
75 (which is NiO+/1– only) at early (2 ps) and late (2 ns) times. This
analysis suggests that the charge separation efficiency is closer to
50% and this result is more in agreement with the maximum
IPCE for the solar cells. The peak attributed to NiO+/1– at 575 nm
The transient absorption results were different for 1 adsorbed
2
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(τ2 ≈ 180 ns) lived three orders of magnitude longer than the
Wolfson Merit Award.
charge-separated state of the widely studied P1 dye.16 This
substantial increase in the charge separated state lifetime
60 Notes and references
–
prompted us to investigate the effect of substituting the I3 /I– for
aSchool of Chemistry, The University of Nottingham, University Park,
Nottingham, NG7 2RD, UK. Fax: +44 115 951 3563; Tel: +44 115 951
3523 E-mail: Elizabeth.Gibson@nottingham.ac.uk
† Electronic Supplementary Information (ESI) available: experimental
65 details, electronic spectra, electrochemical characterisation, DFT
calculations, J-V curves for 1 vs. P1, transient absorption spectra,
5
the tris(4,4′-di-tert-butyl-2,2′-dipyridyl) cobalt (III/II) redox
shuttle in our DSCs in an attempt to increase the photovoltage.7
Unfortunately the photocurrent was negligible in this case,
suggesting that the lifetime was still not sufficient for re-
oxidation of 1– by the cobalt electrolyte‡‡.
spectroelectrochemistry of 1/NiO. See DOI: 10.1039/b000000x/
–
τ1 for the NiO+/1– signal differed little in the presence of I3 /I–
-
‡ ΔGinj = e[EVB(NiO) – ED*/D–]; ΔGreg = e[E(I3–/I2•–) – ED/D–]; E(D*/D )
=
10
–
(Figure 3 blue dotted line) and absence of I3 /I– (Figure 3 red
E(D/D-) + E0-0; EVB (NiO) ≈ –0.12 V vs. Fe(Cp)2+/0 15; E°’(I3–/I2•–) = –0.82 V
70 vs. Fe(Cp)2+/0 in acetonitrile15; ηinj = 1 – (τ1/NiO/τ1*)
dashed line, shifted to λmax = 600 nm), hence there was no
measurable effect of the electrolyte on a short timescale.
However, on a ns timescale the signal for NiO+/1– decayed more
‡‡ This is consistent with the work of Le Pleux et al. who reported that
the oxidation of the PMI-NDI sensitiser with Co(III/II) occurred on the µs
timescale ( = 3.5 s) and was competitive with charge recombination.
1. F. Odobel, Y. Pellegrin, E. A. Gibson, A. Hagfeldt, A. L. Smeigh,
15 rapidly (τ2 ≈ 23 ns) in the presence of the electrolyte, indicating
that interception of the electron by the redox shuttle efficiently
competes with recombination. This re-oxidation is very fast
compared to the microsecond timescale reported for re-reduction
of dyes on TiO2.22 The mechanism for the dye regeneration
20 pathway in n-DSCs is still under debate and it is currently
believed that two equivalents of I– per dye molecule are required.
75
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22-24
For p-DSCs, re-oxidation of the dye requires only one
–
molecule of I3 per dye and this may be why we observe this
process proceeding more quickly. Reported timescales for the
25 regeneration reaction range from ps to μs.25,9 Reasons for the
differing kinetics could be the different driving force for the
reaction (reported Greg <200 meV to 1 eV; Greg = 640 meV for
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from the reduction of I3– (λ = 420-475 nm).
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In our TA experiments on 1/NiO in the presence of electrolyte
we also observed an additional peak centred at 425 nm which
decayed with τ = 500 ns. At first we were concerned that the
presence of the heavy atoms in the electrolyte had driven the
S1→T1 conversion as observed by Morandeira et al. for
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35 Coumarin343/NiO25 but the band at 650 nm that accompanied the
3
higher energy peak for 1* in CH2Cl2 was absent in this sample.
–
100
105
110
Instead, we have assigned this transient as I2 . The extinction
–
coefficient of I2 should be three times greater than that of the
extinction coefficient for the reduced bodipy. The different
40 amplitudes at 420 nm vs. 600 nm in the TA spectrum 2 ns after
excitation indicates that both reduced dye and reduced bodipy are
present in similar quantities. Therefore we have compelling
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–
evidence that electron transfer from the photoreduced dye to I3
generating I2– contributes to the photocurrent in p-DSCs.
45
In conclusion, the three orders of magnitude increase in
charge-separated state lifetime is a significant breakthrough in
our efforts to improve the efficiency of dye-sensitized
photocathodes. We anticipate that tuning the electronic coupling
by modifying the substituents on the bodipy will increase the
50 charge-separated state yield and lifetime further. This will enable
higher photocurrents to be obtained and alternative electrolytes to
be used which increase the photovoltage. These results have
wider implications to the field of “solar fuels” since
photocatalysis requires long-lived charge-separated intermediates
55 for the desired chemical reactions to take place before the high-
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We thank the University of Nottingham and The Royal Society
for funding and MWG gratefully acknowledges receipt of a
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