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

Dora Demeter, Jean Roncali, Siriporn Jungsuttiwong, Fabio Melchiorre, Paolo Biagini, Riccardo Po

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Dora Demeter, Jean Roncali, Siriporn Jungsuttiwong, Fabio Melchiorre, Paolo Biagini, et al.. Linearly π-conjugated oligothiophenes as simple metal-free sensitizers for dye-sensitized solar cells. Journal of Materials Chemistry C, 2015, 3 (29), pp.7756-7761. 10.1039/c5tc01183c. hal-01391715

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

Dora Demeter,ª Jean Roncali,ª Siriporn Jungsuttiwong,ᵇ Fabio Melchiorre,ᶜ Paolo Biagini*ᶜ and Riccardo Po*ᶜ

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 π-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 an 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 ~7.30% corresponding to 90% of the value given by N719 under identical conditions has been obtained with one of the heptamers.

## 1. Introduction

Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO₂ have received considerable attention in the last two decades.¹⁻⁹ 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.⁶ Although ruthenium(II) complexes have represented the archetype of sensitizers for almost two decades,¹⁻³ recent years have witnessed the strong emergence of research on metal-free organic dyes. Interest in these compounds is motivated by simpler syntheses and purifications, lower cost and greater flexibility for the structural control of the energy levels and light-harvesting properties.⁷⁻⁹ 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.⁹ On the other hand, some examples of cells combining metal-free chromophores and quantum dots have been published.¹¹,¹²

Most of the metal-free sensitizers possess a D-π-A structure where “D” represents an electron-donating moiety such as a diarylamine or carbazole, “π” a π-conjugating spacer and “A”

an electron-accepting moiety. In many cases A is a cyanoacrylic acid group to serve at the same time as an electron-withdrawing unit and an anchoring group on the TiO₂ surface.⁷⁻¹⁰

Whereas many push-pull sensitizers based on this model have been synthesized, examples of sensitizers with a simple π-A structure namely devoid of terminal donor group remain scarce.¹²⁻¹⁹ Thiophene oligomers (nTs) have been frequently used π-conjugating spacers in D-π-A sensitizers,⁷⁻⁹ 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.¹² DSSCs based on simple oligothiophenes with terminal carboxylic or cyanoacrylic acid anchoring groups have been described,¹³,¹⁴ while efficiencies of ~5.0% have been obtained with sensitizers based on oligothiophenes with a terminal dithienothiophene unit.¹⁵ A PCE of 6.52% was reported for a cell based on a spiro-system and quaterthiophene with two anchoring groups.¹⁶ Very recently, a sensitizer based on a penta-mer of 3-hexylthiophene was described and a PCE of 4.40% was reported for a solid-state DSSC.¹⁸ The latter result prompted us to publish the results of the work carried out independently in our laboratories on oligothiophene sensitizers.¹⁹

In this work, a series of oligothiophene chromophores involving four (4T), five (5T) and seven (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 hepta-mers have been synthesized that contain either two internal octyl chains on the terthienyl (3T) block (7Ti) or three octyl chains in a regio-regular arrangement (7Tr) (Fig. 1).

<footnote>ª Group Linear Conjugated Systems, CNRS Moltech-Anjou, University of Angers, 2Bd Lavoisier, 49045 Angers, France. E-mail: jeanroncali@gmail.com</footnote>

<footnote>ᵇ Center for Organic Electronics and Alternative Energies, Department of Chemistry, University of Ubon Ratchathani, Ubon Ratchathani, 34190, Thailand</footnote>

<footnote>ᶜ Centro Ricerche per le Energie Non Convenzionali, Istituto ENI Donegani, ENI S.p.A., via G. Fauser 4, 28100 Novara, Italy. E-mail: paolo.biabgini@eni.com</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1 Chemical structures of the oligothiophene sensitizers.</figcaption></figure>

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 structure-property relationship with regard to the effects of the length of the π-conjugated system and the number and position of the octyl side-chains.

## 2. Results and discussion

### Synthesis

3,4"-Dioctyl terthienyl (1a), 3,3',4''',4'''''-tetraoctyl-quinquethienyl (4a) and 3,3',3"4''',4'''''-hexaoctyl-septithienyl (5a) have been synthesized according to already reported procedures.²⁰ Bromination of terthienyl 1a with NBS gave the mono-bromo compound 1b in 63% yield. Lithiation of 1a with n-butyllithium and reaction with tributylstannyl chloride gave the Stille reagent 1c in quantitative yield (Fig. 2). A Stille coupling reaction between 1c and 2-bromo-5-formyl thiophene (2) give the formyl-quaterthiophene 3 in 92% yield. Carboxaldehydes 4b and 5b were synthesized in 33% and 30% yields by Vilsmeier-Hack formylation of quinque- and septi-thienyl 4a and 5a respectively.²⁰,²¹

2,5-Distannyl-thiophene 6 was synthesized in quantitative yield from 2,5 dibromo-thiophene via a classical double lithiation/stannylation. This reagent was then engaged in a double Stille coupling reaction to give septithienyl 7a in 47% yield. Vilsmeier-Hack formylation of 7a gave the carboxaldehyde 7b in 34% yield. The target compounds were finally obtained by Knoevenagel condensation of carboxyaldehydes 3, 4b, 5b and 7b with cyanoacetic acid in the presence of ammonium acetate to give the corresponding cyanoacids 4T, 5T, 7Tr and 7Ti in 53, 59, 57 and 66% yield respectively (see ESI†).

### Cyclic voltammetry

Fig. 3 shows the oxidative cyclic voltammograms (CV) of the sensitizers in CH₂Cl₂ 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 (Eₚₐ) 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 cation-radical presumably associated to the formation of the dimer of 4T by coupling at the free α-position.²² In 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 Eₚₐ 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 (Eₚc) varying from -0.91 V for dye 7Ti to -1.06 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 (λmax) 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 λmax values are in good agreement with the absorption maxima reported for oligomers of 3-octylthiophene.²³ This first band is followed by a second more intense band attributed to an internal charge transfer (ICT). The λmax 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 ~0.60 eV for 4T to ~1.10 eV for 7Tr and 7Ti. These results show that whereas the λmax of the first transition shifts bathochromically with chain extension, the maximum of the ICT and hence the optical HOMO-LUMO gap (ΔE) 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 π-conjugated oligomers converges towards a limit for a chain length determined by structural factors such as bond length alternation, aromatic resonance energy and planarity.²⁴,²⁵ In this context, the above results suggest that in the specific case of the π-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.
<figure><img src="image_3.png" /><figcaption>Fig. 2 Synthesis of the target compounds. (i) NBS, CHCl<sub>3</sub>/acetic acid (1:1); (ii) n-BuLi, Bu<sub>3</sub>SnCl, THF; (iii) Pd(PPh<sub>3</sub>)<sub>4</sub>, toluene; (iv) DMF, POCl<sub>3</sub>, 1,2-dichloroethane; (v) NCCH<sub>2</sub>CO<sub>2</sub>H, ammonium acetate, glacial acetic acid.</figcaption></figure>

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<sub>2</sub>Cl<sub>2</sub> 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 λ<sub>max</sub> and hence smallest optical gap whereas the *anti*-syn-syn (a—s—s) conformation leads to the most blue shifted λ<sub>max</sub>. 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 λ<sub>max</sub> 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<sup>-2</sup> through a mask of ca. 10 mm<sup>2</sup> aperture. Under these experimental conditions, the benchmark sensitizer N719 gives a short-circuit current density (J<sub>sc</sub>) of 15.2 mA cm<sup>-2</sup>, an open-circuit voltage (V<sub>oc</sub>) of 0.745 V and a
<figure><img src="image_4.png" /><figcaption>Fig. 3 Cyclic voltammograms corresponding to the oxidation of oligothiophene sensitizers in 0.10 M Bu₄NPF₆/CH₂Cl₂, Pt electrodes, scan rate 100 mV s⁻¹.</figcaption></figure>

<table><caption>**Table 1** Data of UV-Vis spectroscopy (in CH₂Cl₂) and cyclic voltammetry (in 0.10 M Bu₄NPF₆/CH₂Cl₂, scan rate 100 mV s⁻¹, Pt electrodes, ref. SCE) for the target compounds</caption><thead><tr><th>Dye</th><th>λ<sub>max</sub><br>(nm)</th><th>λ<sub>max,ICT</sub><br>(nm)</th><th>ΔE<sup>a</sup><br>(eV)</th><th>ε<sub>max</sub><br>(M<sup>-1</sup> cm<sup>-1</sup>)</th><th>E<sub>pa</sub><br>(V)</th><th>E<sub>pc</sub><br>(V)</th><th>E<sub>HOMO</sub><sup>b</sup><br>(eV)</th><th>E<sub>LUMO</sub><sup>c</sup><br>(eV)</th></tr></thead><tbody><tr><td>4T</td><td>350</td><td>480</td><td>2.58</td><td>36000</td><td>1.04</td><td>-1.01</td><td>-5.78</td><td>-3.20</td></tr><tr><td>5T</td><td>372</td><td>479</td><td>2.59</td><td>35000</td><td>0.90</td><td>-1.06</td><td>-5.59</td><td>-3.00</td></tr><tr><td>7Tr</td><td>400<sup>d</sup></td><td>457</td><td>2.71</td><td>32000</td><td>0.75</td><td>-0.95</td><td>-5.54</td><td>-2.83</td></tr><tr><td>7Ti</td><td>430<sup>d</sup></td><td>473</td><td>2.62</td><td>35000</td><td>0.81</td><td>-0.91</td><td>-5.55</td><td>-2.93</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>Fig. 4 UV-Vis absorption spectra of the oligothiophene sensitizers ~2 × 10⁻⁵ M in methylene chloride solutions.</figcaption></figure>

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 π-conjugated system from four to seven thiophenes leads to an marked increase of $J_{sc}$ from 9.40 to 14.40 mA cm⁻² together with a

<table><caption>**Table 2** Calculated absorption maxima for various conformations of the end 4T fragment of 7Tr and 7Ti</caption><thead><tr><th>Dye</th><th>Conformation</th><th>λ<sub>max</sub> (nm)</th></tr></thead><tbody><tr><td></td><td>R = n-octyl<br>A = CN-CH<sub>2</sub>-COOH</td><td>486</td></tr><tr><td rowspan="3">7Tr</td><td>a-a-a</td><td rowspan="3">451</td></tr><tr><td>a-a-s</td></tr><tr><td>a-s-s</td></tr><tr><td rowspan="2">7Ti</td><td>a-a-a</td><td rowspan="2">492</td></tr><tr><td>a-a-s</td></tr><tr><td rowspan="2">7Ti</td><td>a-a-s</td><td rowspan="2">477</td></tr><tr><td>a-s-s</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Fig. 5 UV-Vis absorption spectra of the sensitizers adsorbed on TiO₂ films of ca. 8 µm thickness.</figcaption></figure>

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)

<footnote>a based on the long wavelength absorption onset. b using $E_{ox}^0$ with an offset of -4.99 eV for SCE vs. the vacuum level. c determined by $E_{HOMO} - \Delta E$. d Shoulder.</footnote>
<figure><img src="image_7.png" /><figcaption>**Fig. 6** Current-density vs. voltage for DSSCs based on oligothiophene sensitizers and N719 under AM 1.5 simulated solar illumination.</figcaption></figure>

<table><caption>**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</caption><thead><tr><th>Dye</th><th>J<sub>sc</sub> (mA cm<sup>-2</sup>)</th><th>V<sub>oc</sub> (V)</th><th>FF</th><th>PCE (%)</th></tr></thead><tbody><tr><td rowspan="2">4T</td><td><strong>9.40</strong></td><td>550</td><td>0.73</td><td><strong>3.80</strong></td></tr><tr><td>8.80</td><td>553</td><td>0.72</td><td>3.60</td></tr><tr><td rowspan="2">5T</td><td><strong>12.6</strong></td><td>681</td><td>0.74</td><td><strong>6.40</strong></td></tr><tr><td>12.8</td><td>654</td><td>0.75</td><td>6.30</td></tr><tr><td rowspan="2">7Tr</td><td><strong>14.3</strong></td><td>687</td><td>0.69</td><td><strong>6.70</strong></td></tr><tr><td>13.7</td><td>665</td><td>0.72</td><td>6.50</td></tr><tr><td rowspan="2">7Ti</td><td><strong>14.4</strong></td><td>708</td><td>0.71</td><td><strong>7.30</strong></td></tr><tr><td>14.4</td><td>692</td><td>0.69</td><td>6.90</td></tr><tr><td rowspan="2">N719</td><td><strong>15.2</strong></td><td>745</td><td>0.72</td><td><strong>8.10</strong></td></tr><tr><td>15.0</td><td>727</td><td>0.73</td><td>8.00</td></tr></tbody></table>

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 ~650 nm for 4T to ~715 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 ~70% 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 far¹³,¹⁹ 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

<figure><img src="image_8.png" /><figcaption>**Fig. 7** External quantum efficiencies of the DSSCs based on oligothiophene sensitizers under monochromatic irradiation.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**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.</figcaption></figure>

the ISOS-1 protocol developed for organic solar cells has been carried out.²⁶ To 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 π-A compound, even when combined with a conventional iodide-triiodide based electrolyte.

## 3. 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 π-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 diode/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 proper-
ties, the design of sensitizers with a simple linearly π-conjugated
structure should also take into account the structural control
of intermolecular interactions in order to minimize charge-
recombination 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.

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