A hybrid electron donor comprising cyclopentadithiophene and dithiafulvenyl for dye-sensitized solar cells

Summary Two new photosensitizers featured with a cyanoacrylic acid electron acceptor (A) and a hybrid electron donor (D) of cyclopentadithiophene and dithiafulvenyl, either directly linked or separated by a phenyl ring, were synthesized and characterized. Both of them undergo two reversible oxidations and strongly absorb in the visible spectral region due to a photo-induced intramolecular charge-transfer (ICT) transition. To a great extent, the electronic interaction between the D and A units is affected by the presence of a phenyl spacer. Without a phenyl ring, the D unit appears more difficult to oxidize due to a strong electron-withdrawing effect of the A moiety. In sharp contrast, the insertion of the phenyl ring between the D and A units leads to a broken π-conjugation and therefore, the oxidation potentials remain almost unchanged compared to those of an analogue without the A group, suggesting that the electronic coupling between D and A units is relatively weak. As a consequence, the lowest-energy absorption band shows a slight hypsochromic shift upon the addition of the phenyl spacer, indicative of an increased HOMO–LUMO gap. In turn, the direct linkage of D and A units leads to an effective π-conjugation, thus substantially lowering the HOMO–LUMO gap. Moreover, the application in dye-sensitized solar cells was investigated, showing that the power conversion efficiency increases by the insertion of the phenyl unit.


Introduction
Dye-sensitized solar cells (DSSCs) have been intensively investigated as an alternative to silicon-based solar cells [1][2][3][4]. Although devices with the most commonly used dyes based on polypyridyl transition-metal complexes show excellent photo-voltaic performances with high power conversion efficiencies of over 11% [4], metal-free organic dyes have significant advantages in several aspects. These comprise for example large molar extinction coefficients, ease of synthesis, fine- tuning of structural and electronic properties, and low-cost production [1][2][3][4]. Particularly, the hitherto best DSSC based on organic sensitizers shows an efficiency of 10.3% [5]. Among the most efficient organic dyes are those featured with an electron-donor (D) and an electron-acceptor (A) unit linked through a π-bridge, leading to a broad and intense optical absorption band in the visible spectral region due to an effective intramolecular charge transfer (ICT) from D to A units. To develop highefficient DSSCs, a variety of organic donors [2,[6][7][8] have been used in the construction of photosensitizers. Not surprisingly, tetrathiafulvalene (TTF), as a strong π-electron donor, has been incorporated into different D-π-A systems for numerous potential applications [9][10][11][12][13][14]. However, TTF-sensitized solar cells have rarely been explored [15][16][17], mainly due to the high-lying HOMO energy levels leading to a thermodynamically unfavorable dye regeneration. To overcome this problem, we recently applied a Schiff-base reaction to obtain a rigid and planar quinoxaline-fused TTF-based dye that shows an intense optical ICT absorption over a wide spectral range and a substantially stabilized HOMO, leading to a power conversion efficiency of ca. 6.5% [17]. This example represents the currently best performance for TTF-sensitized solar cells. An alternative approach is based on dithiafulvene (DTF), which from a structural point of view, can be treated as half of a TTF unit. DTFbased D-π-A sensitizers have been proven quite promising with high power conversion efficiencies of up to 8.3% [18,19].
Taking into account all these considerations, we set ourselves the synthetic task to prepare two new molecular dyes ( Figure 1) configured with DTF-substituted 4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) as an electron-donor and cyanoacrylic acid as an electron-acceptor moiety. The incorporation of the rigid and coplanar electron-donating moiety CPDT to the DTF core could increase the electrondonating ability of the dyes, beneficial for the electron-injection process. In addition, on the one hand, the presence or absence of a phenyl ring between the D and A units in the organic sensitizers is expected to tailor the frontier orbital energy level, which is an essential aspect for good device performance. On the other hand, the presence of side chains on both DTF and CPDT moieties could prevent dye aggregation and thus retard charge recombination. Herein, we describe the preparation, characterization and the electronic properties of two new organic dyes as well as their application in DSSCs.

Electronic properties
To estimate the energy values of the frontier orbitals (HOMO and LUMO) of dyes 1 and 2 that are essential for the evaluation of two critical processes in DSSC devices, namely the electron injection from the photo-excited sensitizer to the TiO 2 Scheme 1: Synthetic routes to the target dyes 1 (top) and 2 (bottom).
conduction band and the dye regeneration, the corresponding electrochemical and photophysical properties were investigated. As depicted in Figure 2, cyclic voltammograms of dyes 1 and 2 in CH 2 Cl 2 show two reversible oxidation waves. To gain additional insight into the redox processes, the cyclic voltammetry of the reference compound 6 was carried out as well. The latter undergoes two reversible oxidations at 0.44 V and 0.75 V vs Fc + /Fc, hence at values which are lower than the oxidation potentials of the individual components DTF [24] and CPDT [25]. This observation indicates the formation of an extended hybrid donor as the HOMO is delocalized over the DTF and CPDT moieties. Compared to 6, dye 2 is only marginally more difficult to oxidize as evidenced by slightly more positive values for both oxidation potentials (0.02 V and 0.03 V, res-pectively, Table 1). This is due to the electron-withdrawing effect of the cyanoacrylic acid. Obviously, the electronic interaction between D and A units is fairly weak because the π-conjugation is partially broken upon the insertion of the phenyl ring which can adopt a non-coplanar conformation. In contrast to 2, 1 is much more difficult to oxidize as its oxidation potentials (0.63 V and 0.99 V vs Fc + /Fc) are substantially shifted to positive values by 0.17 V and 0.21 V, respectively, suggesting a stronger electronic interaction between the D and A moieties. Clearly, the insertion of the phenyl ring has a significant effect on the redox potentials. Moreover, the energies of the HOMO levels of dyes 1 and 2 are −5.36 eV and −5.17 eV, respectively (Table 1), and thus they are lower than the energy level of the Co 3+ /Co 2+ redox shuttle [26,27]. It can   therefore be deduced that the oxidized dyes can efficiently be regenerated.
The UV-vis absorption measurements of sensitizers 1 and 2 were performed in CH 2 Cl 2 solutions and the spectra are shown in Figure 3. Both of them reveal quite similar absorption patterns with an intense and broad absorption band in the visible region peaking at 577 nm (17330 cm −1 ) and 548 nm (18250 cm −1 ), respectively, with high extinction coefficients on the order of 6 × 10 4 M −1 cm −1 ( Table 1). The lowest-energy absorption band is attributed to ICT transitions originating from electronic excitations from MOs spread over the hybrid donor to the MO localized on the cyanoacrylic acid. Compared to 2, sensitizer 1 shows for the lowest-energy absorption band a higher extinction coefficient and a slight red shift by virtue of the more extended π-conjugation. Based on the onset of the lowest-energy absorption band, the HOMO-LUMO gap is estimated to be 1.82 eV for 1 and 1.86 eV for 2. Therefore, the values of their LUMO energy levels of −3.54 eV and −3.31 eV (Table 1) ensure an efficient electron injection from the photoexcited dyes to TiO 2 . As a consequence, the direct linkage of the D and A units leads to an effective π-conjugation and a planar molecular configuration, thus substantially lowering the HOMO-LUMO gap, while the insertion of the phenyl spacer causes a partially interrupted π-conjugation, hence the electronic interactions between the D and A units are weaker. This result is in good agreement with the aforementioned electrochemical data.

Photovoltaic properties
DSSC devices based on sensitizers 1 and 2 were investigated with both I − /I 3− and Co 2+ /Co 3+ electrolytes and the detailed photovoltaic parameters, such as short-circuit photocurrent density (J sc ), open-circuit voltage (V oc ), fill factor (FF), and power conversion efficiency (η) are listed in Table 2. As depicted in Figure 4, the presence of the phenyl spacer has a pronounced effect on the DSSC device performances. Although dye 1 has a slightly better light-harvesting ability and the HOMO level of 2 is energetically higher than that of 1, both the J sc and V oc values increase on going from 1 to 2, leading to an increase of the η value from 2.18% to 4.12% with cobalt tris(bipyridine)-based redox mediator. With iodide/triiodide   as redox shuttle, both devices based on dye 1 and dye 2 showed higher photocurrent densities of 9.26 mA cm −2 and 12.26 mA cm −2 , respectively. However, they showed lower photovoltages of 485 mV and 493 mV. As a result, in the presence of the iodine-based electrolyte, the PCEs for 1 and 2 were 3.19% and 4.13%, respectively. The higher photocurrent of dye 2 is probably due to a lower charge recombination because of the large distortion between the donor and the anchoring acceptor. On the other hand, the planar structure of 1 with full π-conjugation increases the propensity of dye aggregation on the TiO 2 surface, leading to a decrease of the electron-injection yield, too.
Overall, the dyes show a relatively poor DSSC performance compared to the analogous systems reported in the literature so far [18,19], which is mainly due to substantial charge recombination losses during the electron-injection process, very probably caused by self-aggregation on the TiO 2 surface and fast back-electron transfer upon photoexcitation.

Conclusion
In summary, we have synthesized two new organic photosensitizers featured with a DTF-CPDT hybrid as an electron donor and a cyanoacrylic acid either directly linked or linked through a phenyl spacer. Both of them show two reversible oxidations and strongly absorb in the visible spectral region. As evidenced by electrochemical and optical data, the electronic interaction between the D and A units is greatly affected by the presence of a phenyl spacer. For dye 1, without a phenyl ring, the D unit appears more difficult to oxidize due to a strong the electronwithdrawing effect of the A moiety, and also the lowest-energy absorption band shows a slight bathochromic shift compared to that for dye 2 with a phenyl spacer. As a consequence, the direct linkage of the D and A units leads to an effective π-conjugation, thus substantially lowering the HOMO-LUMO gap and increasing the light-harvesting ability. The application in dyesensitized solar cells was further investigated, showing that the power conversion efficiency increases by insertion of a phenyl ring spacer. To further enhance the device performance by retarding charge recombination upon photoexcitation and increasing the charge injection efficiency, appropriate modifications of the structures of sensitizers based on the DTF-CPDT hybrid donor are currently undertaken.

Experimental General
Air and/or water-sensitive reactions were conducted under nitrogen in dry, freshly distilled solvents. 1

Fabrication of DSSCs
In a similar manner as described in [15], electrodes with a 4 μm