Novel design of organic donor – acceptor dyes without carboxylic acid anchoring groups for dye-sensitized solar cells †

b Organic donor – acceptor dyes, formed by a high-yielding [2 + 2] cycloaddition – retroelectrocyclisation process between aniline-substituted alkynes and tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ), were employed as novel photosensitizers without carboxylic acid anchoring groups in dye-sensitized solar cells (DSSCs). The e ﬃ cient adsorption of the donor – acceptor dyes onto TiO 2 was con ﬁ rmed by UV-vis and IR spectroscopies. The photovoltaic performances of the DSSCs suggested that the triphenylamine derivatives 3 and 4 provide higher current densities ( J sc ) as compared to the corresponding dimethylaniline counter molecules 1 and 2 . This was mainly due to the excellent charge-separation e ﬃ ciencies and lower charge-recombination rates of the triphenylamine moieties. It was also found that the devices sensitized by the TCNQ-adducted dyes 2 and 4 display open-circuit voltages ( V oc ) higher than those of the TCNE-adducted counter dyes 1 and 3 . All these results were reasonably explained by the J – V curve ﬁ tting based on the equivalent-circuit model as well as the comparison between the absorption and incident-photon-to-current-conversion e ﬃ ciency (IPCE) spectra.


Introduction
2][3][4][5][6] The main component of DSSCs is the photosensitizer, which absorbs sunlight and generates electric charges.8][9] However, the development of metal-free organic sensitizers has been ongoing because their physical properties, such as molar extinction coefficients and absorption ranges, can be controlled by their molecular design.The chemical structures of most efficient organic sensitizers consist of donor-p-acceptor systems bearing carboxylic acidbased anchoring groups at the acceptor site. 10The conventional tetracyanated donor-acceptor compounds.We now present a new design concept for organic sensitizers and report for the rst time a detailed study of DSSCs based on donor-substituted TCBDs.Careful selection of both the donor and acceptor groups eventually led to the improvement of the photovoltaic performances.

Cell fabrication and characterization
The DSSC devices were fabricated as follows.A main transparent layer (15 mm) with titania particles (about 20 nm) and a scattering layer (10 mm) with titania particles (about 400 nm) were screen-printed onto the uorine-doped tin oxide (FTO)conducting glass substrate.The lms were then sintered at 500 C for 1 h.The thicknesses of the lms were measured by a Surfcorder ET 4000 (Kosaka Laboratory, Ltd).The lms were treated with a 0.1 M aqueous solution of HCl before examination.Coating of the titania lms was carried out by immersion in a 3 Â 10 À4 M solution of the sensitizers in CH 3 CN/tBuOH (1 : 1, v/v) for 24 h under a nitrogen atmosphere.Deoxycholic acid (DCA, 20 mmol) was added to the solution of the dye as a coadsorbent.The dye-covered TiO 2 electrode and the Pt counter electrode were assembled into a sandwich-type cell and sealed with a hot-melt gasket (60 mm thickness) that was made of the ionomer Surlyn1702 (DuPont).Finally, the electrolyte, which consisted of 2 M LiI and 0.025 M I 2 in CH 3 CN, was injected into the cell and sealed with a cover glass.The current-voltage characteristics were measured using an Advantest R6243 current-voltage unit aer a 10 min wait time in order to achieve thermal equilibrium under AM 1.5G simulated solar light of 100 mW cm À2 , irradiated from a WACOM super solar simulator, with a 0.25 cm 2 mask to dene the cell area.

Molecular design
Four different donor-acceptor dyes 1-4 were prepared by the formal [2 + 2] cycloaddition-retroelectrocyclisation reaction (Fig. 1).As previously reported, all compounds were obtained in quantitative yields at room temperature.The chemical structures were conrmed by NMR, IR, and MS spectrometry, which were consistent with the reported data.The longest wavelength absorption maxima (l max ) and redox potentials of 1-4 are summarized in Table S1.† Also, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were estimated from the rst oxidation potentials (E ox,1 ) and the rst reduction potentials (E red,1 ), respectively, under the assumption of Fc/Fc + ¼ À4.80 eV, and the values are shown in Fig. 1.The HOMO levels of the four dyes are in the range from À5.66 to À5.22 eV, which are sufficiently low as compared to the redox potential level of the iodide/triiodide electrolyte couple (À4.9 eV).This fact indicates the facilitated dye regeneration.On the other hand, the LUMO levels (À4.31 to À4.11 eV) are close to the conduction band of TiO 2 (À4.3 eV), ensuring the formation of the surface complexes of the dyes and TiO 2 . 26,27A comparison of the acceptor moieties suggests that the LUMO levels of the TCNQ-adducts 2 and 4 are lower than those of the corresponding TCNE-adducts 1 and 3, respectively.This result reects the strong electron-accepting power of TCNQ as compared to TCNE.The HOMO levels of 1 and 3 are almost the same, while 4 with the triphenylamine moiety shows a HOMO level lower than 2 with the dimethylaniline moiety.The lower HOMO or smaller ionization potential is a superior feature of the triphenylamine to dialkylaniline derivatives.[33][34] Fig. 1 Chemical structures and energy levels of organic donoracceptor dyes 1-4.

Dye adsorption onto TiO 2
Since 1-4 do not possess any carboxylic acid anchoring groups, the adsorption behaviour on the TiO 2 surface was rst examined.The TiO 2 powder does not show any visible absorption, but it became colourful when immersed into the dye solutions in CH 3 CN.It is known that both TCNE and TCNQ form surface complexes with TiO 2 , and this event newly produces a well-dened visible absorption ascribed to the charge-transfer from the TiO 2 surface to TCNE or TCNQ. 26,27In contrast to these surface complexes, the donor-acceptor dyes 1-4 show visible absorption bands due to an intramolecular charge-transfer.Thus, the colours of TiO 2 with organic dyes were similar to the original dye colours.However, the absorption bands became broadened and the end absorption bathochromically shied when the dye adsorption onto TiO 2 occurred (Fig. 2).The bathochromic shis in the end absorption of the TCNQ-adducts were more signicant than those of the corresponding TCNE-adducts, indicating the noticeable charge-transfer interactions due to the stronger electron-accepting characteristics.Moreover, 3 and 4 with the triphenylamine donor displayed a negligible bathochromic shi in l max as compared to the corresponding dimethylaniline analogues 1 and 2, respectively.This might be due to the suppressed dye aggregation in terms of the propeller structure of the triphenylamine moiety.In addition to the absorption spectra, peak broadening was also observed in the IR spectra.For example, the cyano vibrational peak of 1 detected at 2212 cm À1 broadens on TiO 2 (Fig. S1 †).In the case of the surface complexes of highly symmetric and rigid dyes like TCNX (X ¼ E or Q) on TiO 2 , the cyano vibrational peaks simply split due to the formation of coordinated and uncoordinated cyano groups. 26,27owever, since the attached dimethylaniline and triphenylamine moieties break the symmetry, the cyano groups close to and far from the attached moieties give slightly different vibrational peaks and serve as the anchoring positions to provide the different chemical environments.Furthermore, it is possible to rotate the chemical structures around the single bond.Therefore, the dyes on TiO 2 produce multiple peaks overlapping each other, resulting in the observed broad peak in the IR spectra.

Photovoltaic performances of DSSCs
The chemically adsorbed dyes 1-4 on TiO 2 , as conrmed above, produce the photocurrent density-photovoltage (J-V) curves in the DSSCs (Fig. 3).The detailed photovoltaic data are listed in Table 1.The device sensitized by the triphenylaminesubstituted TCBD 3 exhibited a current density (J sc ) of 0.65 mA cm À2 , which is higher than that sensitized by the dimethylaniline-substituted TCBD 1 (0.12 mA cm À2 ).The TCNQ-adducts displayed a similar trend.The J sc value of the device sensitized by the triphenylamine derivative 4 was 1.71 mA cm À2 , while the device of the corresponding dimethylaniline analogue 2 displayed a J sc of 1.41 mA cm À2 .These results highlight the importance of the triphenylamine moiety as a donor unit.On the other hand, a comparison of the acceptor moieties clearly indicates the superiority of the TCNQ-adducts to the corresponding TCNE-adducts.In particular, the greater open-circuit voltages (V oc ) of the TCNQ-adducts originate from the elongated molecular sizes and suitable energy levels. 35,36It is noteworthy that the V oc of the device sensitized by the TCNQ-adduct 2 is twice as high as that of the corresponding TCNE-adduct 1. Accordingly, the best photoconversion efficiency (PCE) of 0.25% was achieved for the device sensitized by the TCNQ-adduct 4 with the triphenylamine donor.
Fig. 4 shows the action spectra of incident photon-to-current conversion efficiency (IPCE) as a function of the incident wavelength for the DSSCs.In good agreement with the absorption spectra shown in Fig. 2, the IPCE action spectra of the devices sensitized by the TCNQ-adducts are broader than those of the TCNE-adducts.This is due to the extended absorption range of the TCNQ-adducts.It should be noted that the IPCE of the TCNQ-adducts covers the entire visible region.In addition, the overall IPCE values of the TCNQ-adducts are signicantly higher than those of the TCNE-adducts.Although the light absorption amount is not so different on the photoelectrode (Fig. 2) and the HOMO levels of all the dyes are low   enough to accept electrons from the electrolyte, the TCNQadducts have more-expanded p electrons on the HOMO orbitals based on a density functional calculation. 26The expanded p orbitals would stabilize the oxidized states for the smooth electron relay to generate the photocurrent.Similarly, the triphenylamine moiety improves J sc and IPCE due to the p-electron expansion on the HOMO orbitals and better redox properties.Note that reversible oxidation waves ascribed to the triphenylamine units of 3 and 4 are reported. 30

I-V parameter analysis
The J-V curves of the DSSCs based on 1-4 were analysed using the theoretical equation based on the equivalent-circuit model (eqn (1)): where I is the output electric current (I ¼ 0.25J), V is the output electric voltage, e is the elementary charge, k is Boltzmann's constant, T is the absolute temperature, I ph is the photocurrent, I 0 is the reverse saturation current of the diode, n is the ideality factor of the diode, R s is the series resistance, and R sh is the shunt resistance. 37By applying a tting process to the J-V curves, the I-V parameters (I ph , I 0 , n, R s , and R sh ) were obtained with a high accuracy.Assisted by some theoretical models or equations, e.g., Shockley equation for the p-n junction, the I-V parameters have been extensively used to analyse the physics in traditional semiconductor solar cells.Note that J sc , V oc , and FF are convenient to characterise the J-V curve shapes and the PCE, but not appropriate to discuss the photovoltaic performances because the I-V parameters have a complex inuence on the value of J sc , V oc , and FF.To better understand the photovoltaic performances, the I-V parameter analysis is important in solar cell research.][40] Fig. 5 summarizes the I-V parameters obtained by tting the J-V curves of the DSSCs based on 1-4.The I ph values increased when the dimethyaniline donor was replaced by the triphenylamine donor in both cases of the TCNE-(1 / 2) and TCNQadducts (3 / 4) (Fig. 5(a)).This increase in the I ph is due to the enhanced charge separation efficiencies at the interface between the dyes and TiO 2 as already discussed.On the other hand, both of the diode parameter values, I 0 and n, of the devices sensitized by the triphenylamine-based dyes were lower than those sensitized by the dimethylaniline-based dyes (Fig. 5(b) and (d)).Generally, it is difficult to determine whether or not the concept of suppressing the recombination current from TiO 2 to the electrolyte functions from the values of V oc because V oc simultaneously correlates with I ph , I 0 , and n as follows: However, the decrease in I 0 clearly suggests that the triphenylamine moiety suppresses the amount of recombination  current to improve V oc as well as the enhanced I ph .The R s values did not show any remarkable differences between the four devices, implying that the transparent electrode, counter electrode, and electrolyte solutions do not affect the photovoltaic parameters in the cell-assembling process (Fig. 5(c)).Although it is well known that the reduction of R s improves FF, 41 the triphenylamine moiety could realise the high FF without decreasing R s .This improvement is explained by the increase in V oc based on the theoretical background of an ideal solar cell with no R s and no R sh ; the relationship between V oc and FF is given as follows: 42 In contrast, a clear increase in the R sh values occurred when the dimethylaniline donor was exchanged by the triphenylamine donor (Fig. 5(d)).This increase clearly reects the decrease in the leak current, e.g., that from TiO 2 to organic dyes.In short, the triphenylamine moiety regulates the current on the photovoltaic interface to enhance the photoinduced current and to suppress the recombination and leak currents.[45][46][47]

Conclusions
New types of organic donor-acceptor dyes for DSSCs were introduced.They can be synthesized by click chemistry reactions, enabling large scale production at a low cost.The tetracyanated acceptor moieties of the dyes were efficiently attached to the TiO 2 surface without special treatments.In particular, the expanded tetracyanated acceptor, originating from TCNQ, showed better photovoltaic performances than the simple TCBD analogue.Moreover, it was demonstrated that the use of the triphenylamine donor offers signicant advantages, such as enhanced charge separation efficiencies, a lower charge recombination, and suppressed dye aggregation.These are important design concepts of the donor and acceptor moieties.Further development of effective p-spacers that control the electronic communication between the donor and acceptor moieties will lead to high performance organic dyes.

Fig. 5
Fig.5Analysis results of the photocurrent density-photovoltage curves of DSSCs based on 1-4 using the equivalent-circuit model.

Table 1
Summary of DSSC performances a