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PAPER
Cite this: Phys. Chem. Chem. Phys., 2013,
15, 2572
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Lithium salts as ‘‘redox active’’ p-type dopants for
organic semiconductors and their impact in solid-state
dye-sensitized solar cells†
Antonio Abate,za Tomas Leijtens,za Sandeep Pathak,a Joël Teuscher,a
Roberto Avolio,b Maria E. Errico,b James Kirkpatrik,a James M. Ball,a
Pablo Docampo,a Ian McPhersonc and Henry J. Snaith*a
Lithium salts have been shown to dramatically increase the conductivity in a broad range of polymeric and
small molecule organic semiconductors (OSs). Here we demonstrate and identify the mechanism by which Li+
p-dopes OSs in the presence of oxygen. After we established the lithium doping mechanism, we re-evaluate
Received 6th December 2012,
Accepted 11th December 2012
the role of lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI) in 2,2 0 ,7,7 0 -tetrakis(N,N-di-p-methoxyphenylamine)9,9 0 -Spirobifluorene (Spiro-OMeTAD) based solid-state dye-sensitized solar cells (ss-DSSCs). The doping
DOI: 10.1039/c2cp44397j
mechanism consumes Li+ during the device operation, which poses a problem, since the lithium salt is
required at the dye-sensitized heterojunction to enhance charge generation. This compromise highlights
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that new additives are required to maximize the performance and the long-term stability of ss-DSSCs.
Introduction
Organic semiconductors (OSs) are currently considered for a
wide range of applications, including light emitting diodes,1
sensors,2 field effect transistors,3 and solar cells.4–6 Their
solution-processable manufacturing, by means of versatile
printing techniques, provides a large potential for the future
electronic market. One of the original scientific breakthroughs
for organic solids was the ability to modulate their electrical
conductivity using chemical doping, and hence demonstrate
their semiconducting properties.7 Similar to inorganic semiconductors, doping intentionally introduces chemical impurities
into the OSs for the purpose of adjusting material properties to
the specific application; e.g., triphenylamine-based OSs have
a
Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road,
Oxford, OX1 3PU, UK. E-mail: h.snaith1@physics.ox.ac.uk
b
Institute of Polymer Chemistry and Technology (ICTP), National Research Council
of Italy, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy
c
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford,
South Parks Road, Oxford, OX1 3QR, UK
† Electronic supplementary information (ESI) available: Estimation of effective
series resistance due to the hole transporter. Ideal diode fit. Estimation of power
losses through the devices due to series resistance. Estimation of possible
conductivities of hole transporter (Fig. S1). Metal-semiconductor contact resistance (Fig. S2, Table S1). IR and UV-Vis measurements. Solid state 7Li NMR
(Fig. S3). Ground state dye absorption (Fig. S4). Spiro-OMeTAD on SiO2 conductivity in dry O2. See DOI: 10.1039/c2cp44397j
‡ These authors contributed equally.
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been p-doped with Co(III) complexes in solid-state dye-sensitized
solar cells (ss-DSSCs),8 and tetracyano-quinoline derivatives in
organic light emitting diodes (OLEDs).9 Both applications
benefit from the introduction of a p-doped transport layer since
it reduces the charge transport resistance in series with the
heterojunction8,10–12 and helps to achieve Ohmic contacts.13–15
Generally, chemical p-doping agents tend to be comprised of
molecules with relatively high electron affinities, often in the
form of metal–organic complexes or organic resonance structures, which can generate unpaired electrons in the organic
matrix by means of stable charge transfer complexes.16–19
The overall result of such doping is a significant increase in
the concentration of free holes in the OS valence band or
highest occupied molecular orbital (HOMO) levels, with the
associated enhancement in conductivity, especially at high
concentrations.20,21
Recently, lithium salts have been reported to enhance
the conductivity in a broad range of polymeric22,23 and small
molecule24,25 OSs. Yanagida et al. found that lithium salts
enhance the hole transport in polythiophene derivatives.26 They
discussed the effect of different anions without considering the
role of the Li+ ion itself. Very similar properties have been
described for lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI)
in polyphenylenevinylene derivatives.27,28 Li-TFSI has also been
shown to dramatically increase the conductivity and hole
mobility in small molecule OSs, such as 2,2 0 ,7,7 0 -tetrakis(N,Ndi-p-methoxyphenyl-amine)9,90 -Spirobifluorene (Spiro-OMeTAD).29
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To explain the effect of increased conductivity and apparent
increase in mobility, it was previously suggested that the ionic
species in the organic matrix smooth the potential landscape,
which acts to increase the probability of intra-molecular charge
transfer.20,29,30 The resultant 5–10 fold increase in hole mobility
was then used to rationalize the observed 100 fold increase in
conductivity. The Li-TFSI additive was considered redox inactive,
and initial UV-Vis absorption measurements showed no signature
of oxidized hole transporter in solution.29 For this reason, the hole
density was not considered to be significantly affected by addition
of Li-TFSI, leaving at least an order of magnitude increase in
conductivity unaccounted for.
ss-DSSCs incorporating molecular or polymeric holetransporters have also employed lithium salts as additives to
the organic phase, since their incarnation.6,31 The primary role
is to force a negative shift (further from vacuum) in the surface
potential of the TiO2, favouring forward electron-transfer from
the photoexcited tethered dye.32 However, the influence of the
lithium salts in the ss-DSSCs is broad ranging.14,29,32–38 and
despite many studies on the subject a large degree of ‘‘black
magic’’ seems to prevail. Very recently, Cappel et al. reported
that p-doping the hole transporter is fundamental for devices
operation and lithium salts could catalyse the hole transporter
oxidation (p-doping), which takes place at TiO2/dye interface
under illumination.39
The present study discusses the mechanism that drives the
lithium salts to behave as effective p-dopants in a broad range of
polymeric and small molecule OSs. Contrary to previous reports,29
we show that Li+ is involved in redox activity, which leaves free
holes in the organic matrix. This mechanism explains the dramatic
increase in conductivity observed in many lithium salt–hole
transporter systems.22,24–28 After we established the lithium doping
mechanism, we investigate the role of the Li-TFSI in SpiroOMeTAD based ss-DSSCs, specifically looking at its influence on
conductivity of the hole-transporter. We demonstrated the lithium
doping of Spiro-OMeTAD, whereby Li+ is consumed in presence of
oxygen, leading to stabilized p-doping which occurs regardless of
the interface with TiO2 or dye molecules. This scenario helps
explain the variation of device performance as function of the
lithium content, presence of oxygen, and time.
Experimental section
ss-DSSCs were prepared according to a standard procedure,29
and all solvents used in this work were reagent grade and
anhydrous. Briefly, FTO substrates (15 O cm1, Pilkington) were
etched with zinc powder and HCl (2 M) to give the desired
electrode patterning. The substrates were cleaned with Hellmanex
(2% in de-ionized water), de-ionized water, acetone, and ethanol.
The last traces of organic residues were removed by a 10 min
oxygen plasma cleaning step. The FTO sheets were subsequently
coated with a compact layer of TiO2 (100 nm) by aerosol spray
pyrolysis deposition at 300 1C, using air as the gas carrier. 1 mm
thick mesoporous TiO2 films were then deposited by screenprinting a commercial paste (Dyesol 18NR-T). The TiO2 films were
slowly heated to 500 1C and allowed to sinter for 30 min in air.
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The samples were immersed into 0.015 M TiCl4 aqueous
solution for 45 min at 70 1C, and then heated to 500 1C for
another sintering step of 45 min. After cooling to 70 1C, the
substrates were immersed in a 0.5 mM ((5-{1,2,3,3a,4,8bhexahydro-4-[4-(2,2-diphenylvinyl)phenyl]-cyclopenta[b]indole7-ylmethylene}-4-oxo-2-thioxo-thiazolidin-3-yl)acitic acid) (D102),
dye solution (in 1 : 1 mixture of acetonitrile and tert-butyl alcohol)
for one hour. After the dyed films were rinsed in acetonitrile,
the Spiro-OMeTADhole conductor matrix was applied by spin
coating at 1000 rpm for 60 s in air. The solutions for spin
coating consisted of Spiro-OMeTAD dissolved in anhydrous
chlorobenzene (reagent grade) at 15% v/v, assuming a density
of Spiro-OMeTAD of 1 g cm3. This concentration gives 700 nm
capping layer of Spiro-OMeTAD on top of the mesoporous TiO2
layer. tert-Butyl pyridine (tbp) was added to the solution at a
concentration of 33 mol% relative to Spiro-OMeTAD. Lithium
bis(trifluoromethylsulfonyl)imide salt (Li-TFSI) (170 mg ml1 in
acetonitrile) was added to achieve the molar equivalent, relative
to Spiro-OMeTAD listed in the main text. After drying overnight,
back contacts were applied by thermal evaporation of 200 nm of
silver. For measuring the device merit parameters, Solar-simulated
AM 1.5 sunlight was generated with an ABET solar simulator
calibrated to give 100 mW cm2 using an NREL-calibrated KG5
filtered silicon reference cell, and the I/V curves were recorded with
a sourcemeter (Keithley 2400, USA). The solar cells were masked
with a metal aperture defining the active area (0.09 cm2) of the
solar cells.40 The photovoltage and photocurrent decay measurements were performed by a similar method to O’Regan et al. and
as described elsewhere.41–43 All devices were stored in air (dark
condition) for 72 hours before testing.
Devices for measuring the conductivity of the hole conductor
in a dye-sensitized mesoporous titania film were prepared
according to a published procedure.29 The preparation (on glass
substrates) was identical to that used for the ss-DSSCs, the only
differences being that no compact layer of TiO2 was deposited,
the Spiro-OMeTAD solution in chlorobenzene was 10% v/v to
give a thinner capping layer, and the electrode pattern was
designed for two point probe measurements with a channel
length (direction of current flow) of 200 mm, a channel width of
6.53 cm, and a film thickness of 1 mm was used. The spin coating
solutions again consisted of the same concentration of tbp but
varying concentrations of Li-TFSI, as described in the main text.
Linear I/V curves were obtained for the conductivity measurements by testing in ambient light conditions (no light dependence was noted) with a sourcemeter (Keithley 2400, USA) in a
two-point contact setup. The contact resistance was several order
of magnitude lower then bulk resistance in the semiconductor,
as estimated by impedance spectroscopy (see ESI† for details).
The devices for measurements in nitrogen atmosphere were
prepared in a glovebox starting from Spiro-OMeTAD deposition,
left for 48 hours, and tested in a nitrogen atmosphere.
7
Li solid state NMR spectra were recorded at 155.45 MHz on
a Bruker Avance II spectrometer equipped with a 4 mm MAS
probe. A p/2 pulse width of 4.5 ms was used, with a relaxation
delay of 120 s and a spinning speed of 8–10 kHz. Chemical
shifts are referenced to external LiCl (aq) 1 M.
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IR spectra were collected on a Bio-Rad FTS-6000 FTIR
spectrometer fitted with an MCT detector using a DuraSamplIR II
diamond ATR accessory. Each spectrum is the result of 50 co-added
scans at a resolution of 1 cm1 and are displayed as A =
log10(R/R0) where R0 is a blank spectrum of the accessory.
For the UV-Vis spectra, the sample preparation was identical
to that described for the preparation of devices for conductivity
measurements, with the exception that the electrodes were not
sensitized by dye and that no counter-electrodes were deposited. Here the solutions of Spiro-OMeTAD were 10% v/v in
anhydrous chlorobenzene, there was no added tbp, and the
Li TFSI content was varied as described in the text. It should be
noted that the samples were prepared and stored in a clean
room under attenuated UV light. Absorbance measurements of
the films were taken using a commercial spectrophotometer
(Varian Cary 300 UV-Vis, USA) with an internally coupled
integrating sphere (Labsphere, USA). Within the sphere is an
integrated photomultiplier tube that detects the light. Samples
were mounted at the entrance of the sphere, with a diffuse
reflector mounted on an 81 wedge at the exit port. Baseline
measurements were performed on mesoporous TiO2 samples
on glass, the same as the substrates used for the samples.
Results and discussion
The first aim of this study is to understand the effect of lithium
salts addition on the charge transport in OSs. To shed light on
this phenomena, we focused the experiments on a model system
using Li-TFSI, which ensures high electrochemical stability due
to the TFSI anion,44 and Spiro-OMeTAD, which is the state-ofthe-art hole transporting material in ss-DSSCs. Fig. 1 displays the
dependence of the Spiro-OMeTAD conductivity on the Li-TFSI
content. We note that all conductivity values discussed in this
work are ‘‘effective’’ conductivities through a mesoporous film
sensitized with dye. This is the conductivity that is relevant to the
solar cell. The method employed has been previously described
for measuring the effective Spiro-OMeTAD conductivity in conditions similar to device operation.29 A strong trend is evident,
where the conductivity increases from a very low 3 108 up to
3 105 S cm1. Notably, the conductivity reaches the maximum
at the Li-TFSI content usually employed in ss-DSSCs (12–30 mol%
with Spiro-OMeTAD),8 suggesting that this effect on the hole
conduction may be important in the device performance.
So far, this remarkable increase in conductivity has been
exclusively ascribed to the electrostatic charge of the ionic
species added into the organic matrix.26–29 In particular, it
was claimed that the addition of Li-TFSI does not induce charge
transfer complexes, and extra addition of chemical p-dopants is
necessary to oxidize the Spiro-OMeTAD.8,29 However, the UV-Vis
absorption spectra of mesoporous TiO2 films infiltrated with
Spiro-OMeTAD (Fig. 1) show the clear growth of an absorption
band around 520 nm as the concentration of Li-TFSI is
increased, which is indicative of a Spiro-OMeTAD oxidized state
(p-doping).45 The absorption increment with the Li-TFSI content suggests that the salt could be involved in the oxidation of
Spiro-OMeTAD. In a very recent study, Cappel et al. for the first
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Fig. 1 Effective Spiro-OMeTAD conductivity and UV-Vis absorption spectra
(inset) as function of the Li-TFSI content. These films were left for 78 hours in
air before measurement. The solid-line is simply to aid the eye.
time report the observation of oxidized Spiro-OMeTAD (p-doping)
when Li-TFSI is added to the hole transporter in ss-DSSCs.39 They
suggest that the Li+ may catalyse the photo-oxidation of the SpiroOMeTAD, which takes place on dye-sensitized TiO2 surface
under illumination. However, we observe p-doping regardless
the dye-sensitized TiO2 and light exposure (see ESI†), which
suggests a different and more general doping mechanism from
the proposed one.39
We have performed a time series of the conductivity measurements in different experimental conditions (atmosphere
and salt content). In Fig. 2a we show the absorbance of the
oxidized Spiro-OMeTAD probed at 520 nm as a function of time
for a range of Li-TFSI contents. The rate of Spiro-OMeTAD
oxidation seems to increase with Li-TFSI concentration, while
the final degree of oxidation achieved scales approximately
linearly with increasing Li-TFSI concentration. This allows us
to conclude that Li-TFSI does not participate as a catalyst, but
as a reagent being consumed while the reaction progresses.46
It is well known that in the primary processes of photooxidation, molecular oxygen plays the role of p-dopant.47–52 To
probe whether oxygen is involved in the oxidation of SpiroOMeTAD, we prepared devices for measuring conductivity in
oxygen-free conditions with and without the addition of a
standard amount (10 mol% with Spiro-OMeTAD) of Li-TFSI.
In Fig. 2b we show that for devices fabricated with and without
Li-TFSI, but never exposed to air, the conductivity of SpiroOMeTAD is approximately the same. This observation confirms
that the electrostatic effect of the ionic species29 alone is not
sufficient to explain the increase in conductivity, and indeed
raises the question as to whether the ‘‘smoothing of the
potential landscape’’ from the presence of the TFSI counterion has a significant influence at all. Once exposed to air, the
sample with Li-TFSI shows a rapid increase in conductivity of
over two orders of magnitude to reach the same final conductivity value as previously determined for films made in air.
The completion of this process for the same Li-TFSI content
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Fig. 2 Characteristics of Spiro-OMeTAD with different Li-TFSI content in air and
in nitrogen atmosphere. (a) Variation of optical density probed at 520 nm in films
as a function of time in air. The legend gives the mol% of Li-TFSI with respect to
Spiro-OMeTAD. (b) The effective conductivity of Spiro-OMeTAD as a function of
time, before (in nitrogen) and after exposure to air. The legend indicates neat
Spiro-OMeTAD and with 10 mol% Li-TFSI.
moreover seems to occur on a similar timescale as what is
observed in Fig. 2a and is not reversible when the sample is placed
back in oxygen-free conditions, since the conductivity remains at
8 106 S cm1 after 78 hours in a nitrogen environment. On the
other hand, the sample without Li-TFSI (neat Spiro-OMeTAD) has
a generally constant conductivity over time (the slight difference
between nitrogen and air is discussed in ESI†), allowing us to
conclude that both Li-TFSI and oxygen play a central role in the
doping mechanism, and is distinct from the long-know oxygen
doping mechanism.39,49–51,53 A possible contribute of the other air
component (mainly water) was excluded performing a similar
experiment in pure oxygen, as described in ESI.†
In order to identify the reaction products, we have performed
solid-state spectrochemical analyses on materials in conditions
similar to device operation (see Experimental Section for details).
In Fig. 3a we show the 7Li NMR of Li-TFSI and Spiro-OMeTAD
with LiTFSI 10 mol% spectra recorded in the solid-state.54,55 At
the concentration commonly employed in ss-DSSCs,8 Li-TFSI is
perfectly solubilised in the Spiro-OMeTAD film as evidenced by
relatively narrow lines under magic angle spinning, which suggest a weak homonuclear coupling.56 The spectrum of the doped
Spiro-OMeTAD film, detected immediately after the preparation,
contains a single 7Li resonance, slightly shifted from the bulk
Li-TFSI signal due to the different chemical environment. Interestingly, after oxidation occurs (12 hours in air), two different 7Li
resonances are recorded. This indicates a change in the coordination of Li atoms, with the formation of complexes57 which are
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Fig. 3 (a) Solid state 7Li NMR of neat Li-TFSI and Spiro-OMeTAD with the
addition of 10 mol% Li-TFSI collected immediately after the preparation and
after 12 h left in air (the neat Li-TFSI spectrum after 12 h in air is not reported
being very similar to Li-TFSI in (a)). (b) ATR-FTIR spectra of the powder extracted
from the oxidized Spiro-OMeTAD + Li-TFSI sample (which we postulate to contain
LixOy), compared with the Li2O and Li2O2 (top graph) and the spectra of the
starting material (bottom graph).
likely to involve oxygen and the Spiro-OMeTAD molecules.
In particular, the downfield peak (0.02 ppm) is shifted towards
the range of lithium–oxygen species (3.3 ppm for Li2O, 0.3 ppm
for Li2O2 as determined experimentally, see ESI†). The narrow
line width indicates a molecular dispersion of these species
rather than an organization in crystalline structures.58
In order to further investigate the reaction products, we dissolved
the Spiro-OMeTAD Li-TFSI film in anhydrous chlorobenzene/
dimethyl sulfoxide to extract the formed oxides. When the
solution is left in dark and nitrogen for few days, a white powder
precipitates. We washed the filtered solid with anhydrous chlorobenzene and dried it in air. In Fig. 3b we show the ATR-FTIR
spectra of the collected powder. The absorption profile is clearly
different from the starting materials, and a large peak appears at
682 cm1. Similar peaks are observed in the IR spectra of Li2O
and Li2O2 and can be ascribed to Li–O vibrational stretches.59
The data presented above allows us to draw qualitative conclusions concerning the mechanism that drives the lithium salts to
behave as an effective p-dopant in the presence of oxygen, or
otherwise put, enable oxygen to act as a stable p-dopant for organic
semiconductors. The low ionization potential of Spiro-OMeTAD
and the high electron affinity of the molecular oxygen leads to a
weakly bound donor–acceptor complex, which can result in an
effective electron transfer to the oxygen molecule through light or
thermal excitation, as illustrated in eqn (1).60–63 As observed by the
UV-Vis spectra and conductivity measurements, the concentration
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of oxidized Spiro-OMeTAD (Spiro-OMeTAD+O2) is negligible in
the absence of Li-TFSI. However, the addition of the Li+ changes
the equilibrium of eqn (1) in the direction of the oxidized SpiroOMeTAD by consuming O2 as described in eqn (2).59,64–66
Spiro-OMeTAD + O2 2 Spiro-OMeTAD + O2 (1)
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Spiro-OMeTAD + O2 + Li-TFSI - Spiro-OMeTAD + TFSI + LixOy,
(2)
where LixOy stands for lithium oxide complexes, which we
identify as Li2O or Li2O2 from the ATR-FTIR and 7Li NMR data
presented in Fig. 3. The formed Spiro-OMeTAD radical cation
(Spiro-OMeTAD+TFSI) is weakly bound by the highly delocalized charge on the TFSI anion,44 which results in an effective
generation of mobile holes on the organic matrix. We note that
according to the proposed mechanism, it is most likely that the
oxygen radical will form LiO2 upon exposure to Li+ ions, which
readily decomposes to Li2O2.59,64
In the remainder of the manuscript we determine the
impact of the lithium doping on ss-DSSCs performance, by
studying a series of devices with different Li-TFSI content. In
Fig. 4 we show the device figures-of-merit for each Li-TFSI
concentration.40 Confirming what was observed in the literature,33
there is a strong improvement in the short-circuit photocurrent
(Jsc) and fill factor (FF), while the open circuit voltage (Voc)
decreases with the increased Li-TFSI content. Notably, however,
the short-circuit photocurrent begins to decrease as the Li-TFSI
content is increased beyond 10% (molarity with respect to SpiroOMeTAD), although the FF continues to improve giving an optimal
performance around 20 mol% Li-TFSI to Spiro-OMeTAD. This
suggests that Li-TFSI has a broad impact on a number of processes
occurring in the solar cells, which are possibly working in opposite
senses. The ground state absorption in the dye-sensitizer (see ESI†)
and the electron transfer efficiency from the photoexcited dye into
Fig. 4 Devices performance parameters, efficiency (Z) short-circuit current (Jsc), opencircuit voltage (Voc) and fill factor (FF), as function of the Li-TFSI content, extracted from
current voltage curves of solid-state DSCs employing D102 as the sensitizer measured
under AM1.5 simulated sun light of 100 mW cm2 equivalent solar irradiance. Each
point is representative of the four devices prepared per Li-TFSI content.
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Fig. 5 (a) Differential capacitance against voltage measurements for cells containing
a range of Li-TFSI contents. (b) Transport lifetimes (ttrans) at short circuit conditions,
(c) recombination lifetimes (trec) at open circuit conditions and (d) the charge
collection efficiency (Zcol) as a function of charge density for the same range of
Li-TFSI contents as in (a). The lifetime’s reported refer to the time constants obtained
by fitting the current and voltage decays to mono-exponential decay functions.
the TiO2 are strongly influenced by the presence of the Li-TFSI.
This has been observed before for this specific dye, and has been
assigned to a change in the local polarity of the hole-transporter
medium in the vicinity of the dye.31,32 Additionally, the Li-TFSI also
affects the charge dynamics at the TiO2–dye–Spiro-OMeTAD interface as well as electron and hole transport in the TiO2 bulk and
Spiro-OMeTAD respectively.34–36
Disentangling the compounding effects of the lithium salts on
the different interfaces and specifically its influence on transport
is challenging. However, to quantify the influence of Li-TFSI in
more detail, we have performed transient photo-voltage and
photo-current decay measurements on ss-DSSCs with different
Li-TFSI content. From these transient measurements, it is possible to extract estimates for the density of sub band gap states in
the TiO2, recombination lifetime, charge collection lifetime and
charge collection efficiency.67 For the devices studied here, we
find that the density of states in the TiO2 (DOS, Fig. 5a) becomes
deeper and broader as Li-TFSI is added, which is consistent with
Bai et al.,37 and is attributed to a downwards (further from
vacuum) shift in TiO2 conduction band potential. The slight
broadening of the DOS has also been explained as the result of
an increased number of trap states due to Li+ intercalation into
the TiO2.67,68 The trend observed for the DOS fits with the
lowering in Voc observed by increasing the lithium content while
maintaining a constant concentration of tert-butyl pyridine (tBP)
(see Experimental Section for details). In Fig. 5b we show the
charge transport lifetime measured at short-circuit in the devices,
which we observe to decreases with increasing Li-TFSI content, in
contrast to the trend in hole-conductivity in the Spiro-OMeTAD.
This is not surprising, since Fabregat-Santiago et al.38 have
established by impedance modelling that at the low charge
densities observed at short-circuit conditions, the slow multitrapping electron transport processes in TiO2 limits the charge
collection rate from the device.38 Fabregat-Santiago et al. have
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also shown that the addition of Li-TFSI slows down the transport through TiO2 due to the addition of surface traps by
lithium intercalation and hence reduces the detrapping rate
and lowers the electron diffusivity in the TiO2.69 This is consistent with our observations here, where a broader density of
sub band gap states has increased the average trap depth at any
given charge density, and not just induced a shift in surface
potential. At first sight however, this trend of reduced transport
rate with increasing Li-TFSI content would appear to contradict
a beneficial, or indeed significant influence of Increasing
conductivity of Spiro-OMeTAD with increasing Li-TFSI content.
In addition to the slowing transport rate, we also observe the
charge recombination to be similarly slowed by the addition of
Li-TFSI. The decrease in recombination rate has been observed
before,14 and is usually attributed to the shallower DOS as well as a
possible ‘‘screening’’ of the Coulomb attraction between holes in
the Spiro-OMeTAD and electrons in the TiO2 by the high ion
content.70,71 For a solar cell to collect charge efficiently, the absolute
rates for charge collection or recombination are unimportant, but
the relative magnitude of each, and specifically much slower
recombination than transport is important. We show the charge
collection efficiency, estimated by the ratio of the transport rate
over the sum of transport and recombination rates, in Fig. 5d. For
most concentrations of Li-TFSI the efficiency is 98%, indicating
close to perfect compensation of slowing down recombination to
slowing down transport. At very low Li-TFSI content, we observe a
very rapid decrease in charge collection efficiency with charge
density which is mainly due to a significant decrease in the electron
lifetime at short-circuit conditions. This suggests that the electron–
hole attraction at higher charge density cannot be effectively
screened by the low ion content at the TiO2 interface. This, together
with the known influence of lithium ions on electron injection
efficiency72,73 from dye to TiO2, clearly explains the extremely large
jump in measured short-circuit currents upon addition of a small
quantity of Li-TFSI. We note moreover that the photocurrent
generated decreases upon addition of more than 10 mol% Li-TFSI,
although the charge collection efficiency remains high. This suggests that addition of excess Li-TFSI, and resulting high levels of
oxidized Spiro-OMeTAD may actually hinder regeneration of the
hole in the dye after electron injection, though further work is
required to fully understand this photocurrent reduction. Hence, as
far as charge generation and charge collection at short-circuit are
concerned, it appears that a relatively low concentration of Li-TFSI
would be preferable. However, from the device results we observe a
continuous increase in fill factor with increasing Li-TFSI content,
which results in the optimum concentration for efficiency being
much higher that that optimised for photocurrent alone.
The fill factor of ss-DSSCs is determined by many factors, one of
the most important of which can be the series resistance due to
poor charge transport properties of the hole transporter. We note
that although the transport at short-circuit is limited by slow
electron diffusion through the TiO2, under forward bias conditions
near open-circuit the charge density in the film increases considerably, significantly increasing the effective diffusion coefficient
and conductivity in the TiO2 due to the filling of sub-band-gap trap
states. Transport is then likely to be limited by hole migration
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Fig. 6 Series resistances as measured by fitting JV curves to an ideal diode
model (black), calculated series resistances from geometrical estimations using
the conductivity of Spiro-OMeTAD as the only input parameter (grey), and the
associated power losses estimated by a simulation of the JV curves without any
series resistance (open circles, right axis), see ESI† for details.
through the Spiro-OMeTAD which does not have such a strong
charge density dependence upon its hole-mobility.38 To specifically
quantify this effect of lithium doping on the Spiro-OMeTAD on the
current–voltage characteristics, we have fitted the device current–
voltage curves using a one-diode model (see ESI† for additional
details) to extract the series resistance and the power losses at the
maximum power point as function of the Li-TFSI content (Fig. 6).74
We note that the series resistance follows a very similar trend
to the conductivity of Spiro-OMeTAD, confirming that the hole
transport is limiting the charge conduction in the devices at higher
applied bias. This fits well with deduction by Fabregat-Santiago
et al. from impedance spectroscopy on similar devices.38 Indeed,
when we estimate an ‘‘effective’’ charge transport series resistance
due to Spiro-OMeTAD as a function of lithium content from the
conductivity values in Fig. 1 (precise details shown in the ESI†), the
estimated values for series resistance are in extremely good agreement with those estimated from the ideal diode fits to the JV
curves. We estimate the power loss due to series resistance, by
simulating a JV curve will all parameters identical to the one-diode
fits, except the series resistance is reduced to zero. These losses are
shown in Fig. 6, where we observe highly significant loses for all
Li-TFSI contents below 16 mol%, and still 20% losses at 16 mol%
Li-TFSI. It is evident that an effective conductivity above
105 S cm1 is necessary to produce devices that are not significantly limited by a series resistance, especially once the shortcircuit photocurrents are pushed towards 20 mA cm2.75 However,
the detrimental effect of high Li-TFSI concentration on the device
charge transport, DOS, and charge generation results in an overall
compromise for maximum power conversion efficiency. Using a
higher mobility hole-transporter to enable conductivities above
104 S cm1 at lower Li-TFSI contents, would dramatically improve
the solar cell efficiency with all else being equal.
To be more quantitative about the hole-transporter requirements, we can predict the conductivity values possible for holetransporters with different hole mobilities at varying doping levels,
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assuming 100% doping efficiency (see ESI† for details). When a
hole-transporter such as Spiro-OMeTAD with hole mobility on the
order of 2 104 cm2 V1 s1 (an upper-end value based on timeof-flight measurements)76 is used, a conductivity around
104 S cm1 can be attained with just 1–4 mol% doping. The
maximum conductivity attainable for this material, taking
10–20 mol% doping to be a practical limit and assuming a constant
mobility, is then 1–2 103 S cm1. We note that the effective
conductivities expected at these doping levels in a porous TiO2
matrix is expected to be lower due to an increase in physical and
energetic disorder (roughly a factor of three based on the volume
fraction occupied at 80% pore filling), and that the time-of-flight
mobility should be an upper estimate, so that this really is an upper
limit estimate. We can then conclude that a hole transporter with
hole mobility on the order of 103 cm2 V1 s1 would allow for high
conductivity values on the order of 103 S cm1 upon doping below
5 mol%, which should allow for a more optimized system.
Conclusions
In conclusion, we have shown that the lithium salts commonly
added to OSs can be involved in a redox reaction, which leaves an
unpaired electron in the organic matrix. The mechanism we
described demonstrates that the Li+ is an efficient and stable
p-dopant in the presence of oxygen. This lithium doping mechanism clarifies the dramatically increase in conductivity observed in
many lithium salt–hole transporter systems, previously described
in literature.22,24–28 After we established the lithium doping, we
discussed the complex role of Li-TFSI in Spiro-OMeTAD based
ss-DSSCs, specifically looking at its influence on conductivity of the
hole-transporter. We have demonstrated that Li+ is essential in
achieving low series resistances in the solar cells, while it also
facilitates photocurrent generation. The first is accomplished by
oxidation of the Spiro-OMeTAD, during which lithium ions are
consumed, while the latter is dependent on the availability of these
ions to intercalate into the TiO2 and influence the local polarity at
the dye-sensitized heterojunction. It seems then that the current
system employed in making ss-DSSCs is far from optimum, and
that the additive is required for two opposing processes, one which
consumes it and another which is dependent on its presence at the
TiO2 interface. The lithium doping we have deduced could also
explain some of the atmospheric dependencies different research
groups have noted on device performance;39 the atmosphere is
simply reacting with organic components of the device, which can
only be expected to give variable results. To move towards more
stable and reproducible devices, it will be advantageous to
employ different additives, one for enhancing the conductivity
of Spiro-OMeTAD and another to improve charge generation at
the interface.
Acknowledgements
This work was part funded by the European Community’s
Seventh Framework Programme (FP7/2007–2013) under grant
agreement n1 246124 of the SANS project and by the Engineering
and Physical Sciences Research Council (EPSRC) APEX project.
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