Linearly π -conjugated oligothiophenes as simple metal-free sensitizers for dye-sensitized solar cells

Four linear oligothiophenes containing 4, 5 and 7 thiophene rings substituted by a variable number of octyl chains attached at the beta-position of some of the thiophene rings and possessing a terminal cyanoacrylic acid anchoring group have been synthesized. Results of UV-Vis absorption spectroscopy and cyclic voltammetry show that as expected the extension of the p -conjugated system leads to a decrease of the optical gap with an increase of the HOMO level. The four compounds have been evaluated as sensitizers in dye-sensitized solar cells (DSSCs) using a iodide/triiodide liquid electrolyte and the results are discussed in terms of the structure– property relationship with regard to the extension of the conjugated system and the number and position of the octyl side chains using N719 as the reference system. A power conversion efficiency of B 7.30% corresponding to 90% of the value given by N719 under identical conditions has been obtained with one of the heptamers.


Introduction
][3][4][5][6][7][8][9] Recently power conversion efficiencies (PCE) in the range of 12.0% have been reported, 4,5 while prototypes and pre-commercial applications have been discussed. 6Although ruthenium(II) complexes have represented the archetype of sensitizers for almost two decades, [1][2][3] recent years have witnessed the strong emergence of research on metal-free organic dyes.8][9] Furthermore, metal-free chromophores can present high molar absorption coefficients thus allowing their use in thinner light-absorbing layers.During the past decade, a huge number of metal-free sensitizers have been synthesized and evaluated in DSSC cells and PCE exceeding 10.0% have been reported. 9On the other hand, some examples of cells combining metal-free chromophores and quantum dots have been published. 11,12ost of the metal-free sensitizers possess a D-p-A structure where ''D'' represents an electron-donating moiety such as a diarylamine or carbazole, ''p'' a p-conjugating spacer and ''A'' an electron-accepting moiety.3][14][15][16][17][18][19] Thiophene oligomers (nTs) have been frequently used p-conjugating spacers in D-p-A sensitizers, [7][8][9] but there are very few examples of sensitizers based on pure nTs.Thus, a PCE of 3.36% was obtained with a quinquethiophene derivative with two carboxylic groups. 12DSSCs based on simple oligothiophenes with terminal carboxylic or cyanoacrylic acid anchoring groups have been described, 13,14 while efficiencies of B5.0% have been obtained with sensitizers based on oligothiophenes with a terminal dithienothiophene unit. 15A PCE of 6.52% was reported for a cell based on a spiro-system and quaterthiophene with two anchoring groups. 16Very recently, a sensitizer based on a pentamer of 3-hexylthiophene was described and a PCE of 4.40% was reported for a solid-state DSSC. 18The latter result prompted us to publish the results of the work carried out independently in our laboratories on oligothiophene sensitizers. 19n this work, a series of oligothiophene chromophores involving four (4T), five (5T)a n ds e v e n( 7T) thiophene rings with terminal cyanoacrylic group have been synthesized.All oligomers contain octyl side chains in variable number and positions in order to provide appropriate solubility to the chromophore and contributing to prevent inter-chain aggregation.Two different thiophene heptamers have been synthesized that contain either two internal octyl chains on the terthienyl (3T)b l o c k( 7Ti) or three octyl chains in a regio-regular arrangement (7Tr)( F i g .1 ) .We describe the synthesis of the chromophores and the analysis of their electronic properties by cyclic voltammetry, UV-Vis spectroscopy and theoretical calculations.These new chromophores have been evaluated as sensitizers in DSSCs with iodide/triiodide-based conventional liquid electrolyte formulations.The results are discussed in terms of the structureproperty relationship with regard to the effects of the length of the p-conjugated system and the number and position of the octyl side-chains.

Cyclic voltammetry
Fig. 3 shows the oxidative cyclic voltammograms (CV) of the sensitizers in CH 2 Cl 2 in the presence of tetrabutylammonium hexafluorophosphate as the supporting electrolyte.The CV of 4T shows a first quasi-reversible oxidation wave with an anodic peak potential (E pa ) at 1.04 V corresponding to the formation of the cation-radical (Table 1).
The occurrence of a broad cathodic wave on the reverse scan is indicative of a follow-up chemical reaction of the cationradical presumably associated to the formation of the dimer of 4T by coupling at the free a-position. 22In contrast, the CV of the longer oligomers shows two successive reversible oxidation waves indicative of stable cation-radical and di-cation.
As expected, the extension of the oligothiophene chain produces a negative shift of E pa from 1.04 V for 4T to 0.75 V for 7Tr.In the negative potential region, the CV of all compounds presents an irreversible reduction wave with a cathodic peak potential (E pc ) varying from À0.91 V for dye 7Ti to À1.06 V for dye 5T.

UV-Vis absorption spectroscopy
Fig. 4 shows the UV-Vis absorption spectra of the target compounds in methylene chloride solution.The spectrum of the shortest dyes 4T and 5T presents a first absorption band with a maximum (l max ) in the range 350-400 nm which shifts bathochromically with the extension of the conjugated system from 350 for 4T to ca. 430 nm for 7Ti.These l max values are in good agreement with the absorption maxima reported for oligomers of 3-octylthiophene. 23This first band is followed by a second more intense band attributed to an internal charge transfer (ICT).The l max of this second band which is observed at ca. 480 nm for 4T and 5T presents an hypsochromic shift to 457 and 473 nm for 7Tr and 7Ti.For these two septithiophenes the two transitions merge into a single broad band as confirmed by the large increase of FWHM (full width at half maximum) of the absorption band from B0.60 eV for 4T to B1.10 eV for 7Tr and 7Ti.These results show that whereas the l max of the first transition shifts bathochromically with chain extension, the maximum of the ICT and hence the optical HOMO-LUMO gap (DE) converge towards a limit (Table 1).This phenomenon suggests that, beyond a certain chain length, the lengthening of the conjugated chain does not produce any further increase the electron donor strength of the oligothiophene chain.
It is well known that the effective conjugation length (or energy gap) of all p-conjugated oligomers converges towards a limit for a chain length determined by structural factors such as bond length alternation, aromatic resonance energy and planarity. 24,25In this context, the above results suggest that in the specific case of the p-A oligomers discussed here, the maximum chain length effective for ICT is limited to 4-5 thiophene units due to the rotational disorder inherent to oligothiophenes.Nevertheless, the fact that 7Ti absorbs at a longer wavelength than 7Tr remains intriguing.
In order to gain further insight into this question the structures of different conformations of 7Ti and 7Tr have been investigated using DFT/B3LYP/6-31G(d,p) level of theory.Based on the above discussed limit of the effective chain length for ICT only a chain length of four thiophene rings has been considered for calculations and the absorption spectra of these systems have studied by TD-CAM-B3LYP/6-31G(d,p) level in CH 2 Cl 2 as the solvent (see Fig. S5 and S6, ESI †).Table 2 lists the calculated absorption maxima of the terminal 4T fragment of 7Tr and 7Ti.The results show that for both dyes the anti-anti-anti conformation (a-a-a) leads to the longest l max and hence smallest optical gap whereas the anti-syn-syn (a-s-s) conformation leads to the most blue shifted l max .However, comparison of the two series of data shows that for each conformation the 7Tr fragment absorbs at a shorter wavelength.Furthermore, the difference between the shortest and longest l max is of 37 nm for 7Ti and increases to 51 nm for 7Tr.If one considers that the conformation of oligothiophene molecules in solution is a statistical average of different rotational conformers these results suggest that in any case 7Ti should absorb at longer wavelengths.Fig. 5 shows the UV-Vis absorption spectra of the four dyes adsorbed on nanoporous titanium dioxide films.These spectra agree well with the solution spectra of Fig. 4 and confirm that under these conditions 7Ti still absorbs at longer wavelengths than 7Tr.

Photovoltaic properties
The performances of the four molecules as sensitizers have been investigated in DSSCs with liquid electrolytes (see ESI †).Devices based on the well-known highly efficient ruthenium dye N719 were fabricated under the same conditions for comparison.Fig. 6 shows the current density vs. voltage curves of the various devices under AM 1.5 simulated solar illumination with a power light intensity of 100 mW cm À2 through a mask of ca. 10 mm 2 aperture.Under these experimental conditions, the benchmark sensitizer N719 gives a short-circuit current density ( J sc )o f 15.2 mA cm À2 , an open-circuit voltage (V oc ) of 0.745 V and a fill-factor (FF) of 0.72 leading to a power conversion efficiency (PCE) of 8.10% (Table 2).The results obtained with the four oligothiophene dyes show that the extension of the p-conjugated system from four to seven thiophenes leads to an marked increase of J sc from 9.40 to 14.40 mA cm À2 together with a ca.200 mV increase of V oc values leading to PCE values comprised between 3.80 to 7.30%.
The values of the fill factors are high 70-75% and independent of the structure of the sensitizer.The data in Table 3 show that the lengthening of the conjugated chain leads to an increase of V oc .This result might be related to a decrease of charge recombination due to a better delocalization of the positive charge for the longer oligomers.Fig. 7 shows the external quantum efficiency (EQE)    spectra of the various DSSCs.As expected the curves show that the extension of the conjugation length of the sensitizer produces a progressive bathochromic shift of the onset of photo-current from B650 nm for 4T to B715 nm for 7Ti and 725 nm for the reference dye N719.The spectra also reveal a gradual increase of the photo-response which presents a broad plateau of B70% between 400 and 650 nm for 7Ti.
The integration of the EQE response with respect to the solar irradiation spectrum leads to J sc values corresponding to 85-90% of the values measured under white light illumination.
Comparison of the above results with the literature data on other oligothiophene sensitizers reported so far 13,19 shows that the new chromophores can lead to a significantly higher PCE with values reaching up to 90% of the efficiency of N719 in the case of 7Ti.The increase of efficiency with chain length observed between 4T and 7T can be rationalized on the basis of the extension of the effective conjugation and reduced optical gap leading to better light-harvesting properties.On the other hand, the higher PCE values found for the cells based on 7Ti compared to 7Tr are consistent with UV-Vis absorption and theoretical results.
As a representative example, preliminary evaluation of the stability of the cells based on 7Tr under conditions similar to the ISOS-1 protocol developed for organic solar cells has been carried out. 26To this end the cells were aged at room temperature under indoor light and tested at regular time intervals.Fig. 8 shows the variation of PCE and J sc during a period of two months.A first decrease of PCE of about 20% due to a corresponding decrease of J sc is observed after 400 h.However, no significant variation was observed in the following 800 h.This result demonstrates the excellent stability of this p-A compound, even when combined with a conventional iodidetriiodide based electrolyte.

Conclusion
A series of chromophores based on oligothiophenes substituted by octyl chains with a terminal cyanoacrylic acid anchoring group have been synthesized.Optical and electrochemical results show that as expected, the extension of the p-conjugated system leads to a decrease of the optical gap with an increase of the HOMO level.These compounds have been evaluated under simulated solar illumination as sensitizers in DSSCs with diiode/iodide liquid electrolyte chains using N719 as the reference system.Although a general trend indicating an increase of efficiency with the length of the conjugated system is observed, the    results obtained with the two heptamers reveal interesting differences that suggest that the number and position of the alkyl side chains play a major role in the control of both the conformation of the molecule and intermolecular interactions.These results suggest that in addition to light-harvesting properties, the design of sensitizers with a simple linearly p-conjugated structure should also take into account the structural control of intermolecular interactions in order to minimize chargerecombination and deactivation of the excited state through non-radiative intermolecular processes.Further work in this direction is now underway in our laboratories and will be reported in future publications.

a
based on the long wavelength absorption onset.b using E 0 ox with an offset of À4.99 eV for SCE vs. the vacuum level.c determined by E HOMO À DE. d Shoulder.

Fig. 7
Fig. 7 External quantum efficiencies of the DSSCs based on oligothiophene sensitizers under monochromatic irradiation.

Fig. 8
Fig. 8 Variation of the power conversion efficiency (red) and short-circuit current density (blue) for a DSSC cell based on 7Tr upon indoor storage under ambient conditions.

Table 3
Photovoltaic characteristics of the DSSC based on the oligothiophene sensitizers and N719 under sAM 1.5 simulated solar illumination.Data in bold are the best results obtained for each dye