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

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.


Introduction
2][3][4] Snaith and coworkers have recently shown a record efficiencies of over 12%, 5,6 using organometallic halide perovskites mesostructured at the interface with 2,2 0 -7,7 0 -tetrakis(N,N-di-p-methoxyphenylamine)-9,9 0 -spirobiuorene (Spiro-OMeTAD) as the hole-transport material (HTM). 7,8In 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 cell. 10In particular, we have shown that the losses due to series resistance in the HTM reduces the obtainable power in conventional ss-DSSCs by 10 to 20% when generating less than 10 mA cm À2 photocurrent. 11The 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 paramount importance for continued improvements. 5,11Therefore, 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.6][17] A promising alternative will be to increase the pore-lling fraction of the mesostructured 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 pdopants, thereby increasing the conductivity in Spiro-OMe-TAD. 9,11,20However, 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. 11ere we present the synthesis and characterization of triphenylamine derivatives with various electron-donating functional 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 transfer and electron valence properties of this type of system have been previously studied, but have not yet been utilized as a HTM. 21,22We measure the electrochemical and optical properties 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.

Materials
All reagents were purchased from either Sigma-Aldrich or Alfa-Aesar and they were used as received without further purication unless otherwise stated.N-Bromosuccinimide (NBS) was recrystallized from water before use.

Chemical characterization
1 H and 13 C NMR spectra were recorded on a Bruker Advance 500 spectrometer (500 MHz for 1 H and 125 MHz for 13 C).The deuterated solvents are indicated; chemical shis, d, are given in ppm, using the solvent residual signal as an internal standard ( 1 H, 13 C).MS were recorded on ThermoElectron MAT 900 using electron impact (EI) ionization technique.Elemental analyses were carried out by Stephen Boyer at London Metropolitan University using a Carlo Erba CE1108 Elemental Analyzer.A copy of 1 H and 13 C NMR spectra have been reported for samples 9 to 12, where elemental analysis exceeded 0.4% on carbon (ESI †).

Electrochemical characterization
All cyclic voltammetry measurements were carried out in freshly distilled CH 2 Cl 2 using 0.3 M [TBA][PF 6 ] electrolyte in a threeelectrode system, with each solution being purged with N 2 prior 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 electrochemical soware GPES.Cyclic voltammetry (CV) measurements used scan rates of 100 mV s À1 ; 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 s À1 .Ferrocene was used as the internal standard in each measurement.

Optical characterization
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.Concentration of 2 Â 10 À5 M and 5 Â 10 À6 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-correlated 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 $0.1 nJ cm À2 .Fits were carried out using commercial tting soware (FluoFit v4.5.3,PicoQuant GmbH).

Crystallographic details
Crystallographic data were collected using Agilent Technologies SuperNova with Cu Ka (l ¼ 1.54178 Å) 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 CH 2 Cl 2 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 CH 2 Cl 2 solvation via a conductor-like polarizable continuum model (C-PCM). 27The presence of local minimum was conrmed by the absence of imaginary frequencies.All calculations were carried out using the Gaussian 09 program 28 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

Charge transport parameters
Charge transport in the HTMs has been investigated according to a published procedure. 30Poly(3,4-ethylendioxythiophene)poly(styrene sulfonate) (PEDOT:PSS) was spin-coated onto 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 mL À1 ) in a nitrogen atmosphere.Finally, Au contacts (400 nm thick) were applied via thermal evaporation through a shadow mask in 2 Â 10 À6 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. 30herefore, 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.

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 sq À1 , 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 TiO 2 (100 nm) by aerosol spray pyrolysis deposition at 270 C, using oxygen as the carrier gas.Films of 1.5 mm thick mesoporous TiO 2 were then deposited by screen-printing a commercial paste (Dyesol 18NR-T).The TiO 2 lms were slowly heated to 500 C

Solar cells characterization 32
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 cm À2 irradiance, using an NREL-calibrated KG5 ltered silicon reference 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 cm 2 ) of the solar cells.All devices were stored in air and in dark for 24 hours prior to testing.

Conductivity measurements
Devices for measuring the conductivity of the hole transporter in a dye-sensitized mesoporous TiO 2 lm were prepared according to a published procedure. 31The preparation (on glass substrates) was identical to that used for the ss-DSSCs, except for the absence of the TiO 2 compact layer.The electrode pattern was designed for two point contact measurements with a channel length (direction of current ow) of 200 mm, a channel width of 6.53 cm, and a lm thickness of 1 mm.The metal-hole transporter contact resistance was measured to be several orders of magnitude lower than the bulk resistance as estimated by four point conductivity measurement (see ESI †).The spin-coating solutions were identical to those used to prepare the solar cells.Linear current-voltage curves were obtained for the conductivity measurements by testing in dark condition with a sourcemeter (Keithley 2400, USA) in a two-point contact setup.All devices were stored in air and in dark for 24 hours prior to testing.

Results and discussion
The HTMs were synthesized according to the procedure described in Scheme 1.A triphenylamine unit containing parasubstituted methyl and methoxy functional groups was synthesized by copper-catalyzed Ullmann coupling. 33The thiomethoxy 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. 35Finally, homocoupling of the terminal alkyl group was achieved via copper and molecular oxygen. 36ingle 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 directly attached phenyl rings, are 3.61 and 3.44 Å respectively.The shortest intermolecular N-N distances between molecules of adjacent stacks were found to be 6.03 Å for H-DATPA and 5.80 Å for MeO-DATPA.These values are indicative of effective stacking of p-orbitals distributed, as calculated from DFT (see next section), on the triphenylamine units and diacetylene bridge.We note that both within the stack and between stacks however, shorter interactions were observed for MeO-DATPA.Close p-stacked molecular arrangements can generate high charge transport rate in small molecule HTMs. 37Indeed, we will show later that the shorter N-N distance in MeO-DATPA, thus a closer p-p stacking, corresponds to higher hole mobility.A high level of transparency in the visible to NIR region for the HTM is a key parameter for ss-DSSC application. 38The absorption spectra (solid lines, Fig. 3) are similar for each HTM derivative; the absorption peaks (l max ) are located in the spectral 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 S 1 energy stabilization due to the presence of electron-donating groups of various strengths. 39,40From the intersection of emission and absorption spectra we can obtain the E 0-0 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 Förster resonant energy transfer from the HTM to D102. 41,42However, we can likely neglect this contribution to the photocurrent since the HTM predominantly absorbs in the UV region, which is strongly ltered by the TiO 2 (see ESI †).
The HTM oxidation potential and energy level alignment with the HOMO level of the dye are crucial parameters for constructing high-performance ss-DSSCs.][45] Rather, dye regeneration depends solely on the oxidation potential offset that drives the hole from the dye to the HTM, [46][47][48] which can be estimated by cyclic voltammetry as described in the experimental section. 49In 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 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 electrochemical pulse technique, such as square-wave voltammetry   used here, should be used in order to elucidate both oxidation processes.For Spiro-OMeTAD, we observed three oxidation processes, involving one electron for the rst and second oxidation and two electrons for the last oxidation process, as reported previously. 7,50We converted the oxidation potentials (V) to E HOMO (eV) by following the procedure described by Thompson et al. which employs an empirical linear translation equal to E HOMO (eV) ¼ À1.4 E CV (V) À 4.6. 49The 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 TiO 2 gives 0.26 V positive shi compared to the valued collected in solution.Taking in account the new oxidation 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 calculations differ slightly from the experimental (ca.0.4 eV); however, the trend is reproduced faithfully (see ESI †).Fig. 5 shows 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 considering 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.
0][11] The device preparation to estimate the charge transport parameters for the reported HTMs is described in the experimental section.Fig. 6 shows the J-V characteristics for the hole-only diodes at room temperature.The curves show two regions of conduction, which is Ohmic conduction (with slope $1) at low voltage and non Ohmic conduction (with slope >2) at high voltage.The Ohmic region has been attributed to the background impurity  Fig. 6 J-V characteristics of devices experimental (symbols) and calculated (solid lines) for H-DATPA (black), Me-DATPA (red), MeS-DATPA (purple), MeO-DATPA (green) and Spiro-OMeTAD (blue) using the procedure previously reported. 30onduction where the injected hole carrier density is smaller than the intrinsic carrier density in the samples. 30he 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. 51,52urthermore, we have recently demonstrated that LiTFSI is also an strong and stable p-dopant for HTMs. 11,53Therefore, to compare the charge transport in conditions similar to device operation, we measured the effective conductivity aer the addition of LiTFSI for the HTM inltrated in a mesoporous TiO 2 lm, as described in the Experimental section. 31The 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.
We prepared a set of devices with DATPA derivatives and Spiro-OMeTAD as HTMs to measure their respective performance.Fig. 7a shows the characteristic current-voltage ( J-V ) curves of    the device with maximum power conversion efficiency out of a series of four repeats for each HTM.The characteristics have been measured under AM 1.5 simulated sun light of 100 mW cm À2 equivalent solar irradiance.Table 4 lists the solar cell performance parameters for the J-V curves reported in Fig. 7a.The short circuit photocurrent density ( J SC ) shows a superlinear increase as HTM oxidation potential decreases.In Fig. 8a we plot the J SC as a function of the Dye -HTM energy offset, where we have determined the D102 HOMO level by voltammetry of D102-sensitized TiO 2 nanoparticles.Conrming what was previously observed by Kroeze et al., 47 there is an exponential improvement in J SC with this energy offset. 48o 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 Al 2 O 3 . 5Mesoporous Al 2 O 3 lms were prepared in condition similar the TiO 2 (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 timeintegrated 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. 44,46e 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 J SC , there is no consistent trend with the V OC , MeO-DAPTA has a higher V OC than Spiro-OMeTAD, but the other derivatives have a lower V OC .The V OC is generated by the splitting between the quasi Fermi level for electrons in the TiO 2 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 between TiO 2 CB energy and the HTM HOMO energy level, (ii) Any abrupt shi in surface potential at any of the heterojunctions, the TiO 2 -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 (k rec ), where under steady state dn/dt $ G À nk rec ¼ 0. In addition, any imperfection in the device can result in dark "leakage" current, which manifests itself electronically as a low shunt resistance with the impact of reducing the V OC .The shunt resistance is oen dependent upon whether the solar cell is illuminated or not, adding further variability to the open-circuit voltage.The lower photocurrents, consistent with lower charge generation (hole-transfer) partly explain the lower than expected V OC .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 if the HOMO level of the HTM moves signi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 V OC increase. 55Correctly choosing the metal/HTM contact may be a requirement to signicantly enhance the open-circuit voltage of ss-DSSCs.
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 hole-transfer, or at least operation in the solar cell.Fig. 8b shows a schematic diagram of the energy levels at the dyesensitized TiO 2 /organic HTM, according to the report of Bisquert et al. 54 For the devices studied here using Spiro-OMeTAD, the V OC 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 TiO 2 under full sun illumination is 0.4-0.5 eV.This loss-in-potential at the TiO 2 side is reasonably consistent with recent observations with perovskite absorbers, where entirely removing the TiO 2 and replacing it with porous alumina resulted in 200 to 300 meV increase in open-circuit voltage in much thinner devices. 5The most efficient solid-state DSSCs employing organic dyes have a total lossin-potential (difference between the optical band gap and opencircuit 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. 56In principle, the 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 $1 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 mechanism 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.

Conclusions
We have presented the synthesis and characterization of triphenylamine derivatives with different electron donating groups to use as hole transport materials in ss-DSSCs.Each of these molecules is comprised of two triphenylamine moieties bridged with a diacetylene group.We systematically changed the energy and the distribution of molecular orbitals by increasing the electron-donating ability of peripheral substituents (-H < -Me < -SMe < -OMe).Our key ndings are that the photocurrent generation appears to be exponentially dependent upon dye-HTM HOMO-HOMO energy offset, but the opencircuit voltage is a complicated convolution of many inuences, requiring more effort to optimize for maximizing the performance of high-voltage ss-DSSCs.A clear requirement to make these simple triphenylamine HTMs as efficient as, or even more efficient than Spiro-OMeTAD, is to shi the oxidation potential to less positive values, and possibly optimized for each individual dye.However, this will be at the cost of open-circuit voltage.More generally, this study clearly identies a signicant energy loss due to dye regeneration in the current state of the art solid-state DSSCs.Understanding the requirement for and minimizing this loss is critical for the future advancement of solid-state DSSCs.

Paper
Journal of Materials Chemistry A

Fig. 1
Fig. 1 Chemical structure of HTMs (left) and D102 dye 23,24 (right) used in this study.Each of the molecules is comprised of two triphenylamine moieties bridged with a diacetylene group (DATPA).

Fig. 2
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.

Fig. 7
Fig.7Photocurrent-voltage curves for devices employing DATPA derivatives and Spiro-OMeTAD as HTM under AM 1.5 simulated sunlight of 100 mW cm À2 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
Fig. 8 Dependence of the generated photocurrent at short circuit condition versus Dye -HTM energy offset (a).Schematic diagram of the energy levels at dyesensitized TiO 2 /organic HTM heterojunction (b).54

Fig. 9
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.

Table 1
23mmary of X-ray crystallographic data for H-DATPA and MeO-DATPA and allowed to sinter for 30 min in air.Once cooled, the samples were immersed into a 15 mM TiCl 4 aqueous solution for 45 min at 70 C and then heated to 500 C for another sintering step of 45 min.Aer cooling to 70 C, the substrates were immersed in a 500 mM dye solution, in 1 : 1 mixture of acetonitrile and tertbutyl alcohol, for one hour.The dye employed in this study was D102, previously reported by Horiuchi et al.23Aer the dyed

Table 2
Summary of the optical and electrochemical properties Excitation at l max .b From the intersection of absorption and emission spectra.c From SWV and CV measurements and referenced to ferrocene.d E HOMO (eV) ¼ À1.4 E CV (V) À 4.6.e E LUMO ¼ E HOMO + E gap . a

Table 3
Carrier mobility estimated by fitting of current-voltage curves 30 reported in Fig.6and conductivity for the lithium-doped HTMs

Table 4
Device performance parameters for ss-DSSCs fabricated with the lithium-doped HTMs