# A star-shaped triphenylamine p-conjugated system with internal charge-transfer as donor material for hetero-junction solar cells

A. Cravino, S. Roquet, P. Leriche, O. Alévêque, P. Frere, J. Roncali

► To cite this version:

A. Cravino, S. Roquet, P. Leriche, O. Alévêque, P. Frere, et al.. A star-shaped triphenylamine p-conjugated system with internal charge-transfer as donor material for hetero-junction solar cells. Chemical Communications, 2006, 1416, pp.1416. 10.1039/b516781g. hal-00084375

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# A star-shaped triphenylamine p-conjugated system with internal charge-transfer as donor material for hetero-junction solar cells

Antonio Cravino, Sophie Roquet, Philippe Leriche, Olivier Alévêque, Pierre Frère and Jean Roncali

Groupe Systemes Conjugues Lineaires, CIMMA, UMR CNRS 6200, Université d'Angers, 2 Bd Lavoisier F-49045 Angers

Introduction of dicyanovinyl groups on a triphenylamine-based conjugated system leads to an intramolecular charge transfer which extends the spectral response and raises the open-circuit voltage of the resulting hetero-junction solar cells.

Organic solar cells are a focus of considerable research effort motivated by the perspective of achieving low-cost, lightweight and flexible power sources.¹⁻⁸ Bulk hetero-junctions based on interpenetrated networks of p-conjugated polymers and soluble C₆₀ derivatives have been intensively investigated in the past decade and power conversion efficiencies in the 4–5% range have been recently reported.⁵,⁶ In parallel, significant progress has been also accomplished in multi-layer hetero-junctions based on small molecules⁷⁻¹¹ and efficiencies in the 3–4% range have been published.⁹,¹⁰

Low dimensional p-conjugated systems such as e.g. oligothiophenes lead to organic semi-conductors with highly anisotropic electrical and optical properties,¹¹⁻¹⁵ which poses specific problem for device fabrication. Thus, whereas a vertical orientation of the conjugated chains on the substrate is known to improve mobility in organic field-effect transistors,¹²⁻¹⁴ such an orientation is detrimental for solar cells as it strongly reduces the absorption of the incident light as well as hole transport to the electrodes.¹¹,¹⁵,¹⁶

Organic glasses derived from triphenylamine (TPA)-based compounds have been widely investigated as active material for hole transport and electroluminescence.¹⁷ Whereas the amorphous character of these materials offers possibilities to develop active materials for solar cells with isotropic optical and charge-transport properties, the use of TPA-based materials for photovoltaic conversion has been scarcely considered. Shirota and co-workers have used TPA-based starburst molecules containing nitro or dimesitylboryl acceptor groups as donor in bilayer heterojunctions. However, the conversion efficiency of these cells was limited to 0.4 and 0.1% respectively under monochromatic irradiation in the absorption band of the donor (y440 nm).¹⁷b

We now report preliminary results on a star-shaped tris[4-(2-thienyl)phenyl]amine core modified by peripheral electron-withdrawing dicyanovinyl groups (1) and its use as donor material for the realization of hetero-junction solar cells. We show that the creation of an intramolecular charge-transfer (ICT) leads simultaneously to an extension of the spectral response and to an increase of the open-circuit voltage of the resulting solar cells.

The synthesis of compound 1 is depicted in Scheme 1. A Stille coupling of the commercially available tribromotriphenylamine 4 with the stannyl derivative of thiophene gave tris[4-(2-thienyl)phe-nyl]amine

3¹⁸ in 85% yield. Vilsmeier formylation of compound 3 gave tricarboxaldehyde 2 in 90% yield. Knoevenagel condensation of compound 2 with malononitrile gave the target molecule tris(4-(5-dicyanomethylidenemethyl-2-thienyl)phenyl]amine 1 in 76% yield.### $$

The cyclic voltammogram of compound 1 shows a reversible anodic wave peaking at 1.09 V vs Ag.AgCl (Fig. 1). This high value shows that the electron-withdrawing dicyanovinyl groups induce a large increase of the oxidation potential compared to compound 3 ($E_{pa}$ 5 0.51 V vs Ag.Ag⁺).¹⁸ Furthermore, the stability of the cation radical indicated by the reversibility of the oxidation process appears as a positive fact for the use of the corresponding material for hole transport.

The UV-Vis spectrum of compound 1 in CH₂Cl₂ shows a first absorption band with a $l_{max}$ at 368 nm and a second more intense band with $l_{max}$ at 510 nm (Fig. 2). Based on the $l_{max}$ of compound 3 (366 nm),¹⁸ the first absorption band can be assigned to a p2p* transition and the main absorption band at 510 nm to an ICT between the donor and acceptor parts of the molecule. The optical spectrum of a thin film of 1 thermally evaporated on glass exhibits a $l_{max}$ at 545 nm and a red shift of the absorption onset to y680 nm. (Fig. 3 and ESI{). These large shifts suggest the occurrence of strong intermolecular interactions in the solid state. The low energy absorption edge of the spectrum leads to a band gap $E_g$ of y1.80 eV. Powder X-ray diffraction spectra of the films did not show any peak, confirming that compound 1 is amorphous similarly to many TPA derivatives.¹⁷

Hetero-junction solar cells of 6 mm diameter have been realized on ITO substrates spin-coated with a 60 nm film of Baytron P¹. Layers of donor 1 and C₆₀ fullerene as acceptor of y25 nm thickness were successively grown by thermal evaporation under vacuum and the devices were completed by deposition of a 60 nm thick aluminium top electrode.

As shown in Fig. 3, the UV-Vis absorption spectrum of the bilayer is more or less the sum of the spectra of the two components. The spectrum shows a first intense band with $l_{max}$ at ca 350 nm corresponding to the absorption of C₆₀ and to the first absorption band of compound 1 and a second broad band with $l_{max}$ at 520 nm. The apparent blue shift of the ICT band of 1 in the spectrum of the bilayer is due to the convolution with the C₆₀ absorption shoulder around 450 nm. The photo-current action spectrum recorded under monochromatic irradiation shows that the incident photon conversion efficiency (IPCE) presents a first shoulder around 350 nm and a broad maximum of ca 28% in the 470–540 nm region.
<figure><img src="image_2.png" /><figcaption>Scheme 1</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 1 Cyclic voltammogram of compound 1 (ca 10<sup>24</sup> M) in 0.10 M Bu<sub>4</sub>NPF<sub>6</sub>/CH<sub>2</sub>Cl<sub>2</sub> scan rate 100 mV s<sup>21</sup>.</figcaption></figure>

The onset of the photo-current at y700 nm shows that the ICT in
compound 1 effectively extends the spectral response of the hetero-
junction.

Fig. 4 shows the current density-voltage (J–V) characteristics of the cell in the dark and under simulated AM 1.5 solar illumination at 100 mW cm<sup>22</sup> light intensity. The curve recorded in the dark shows a rectification behaviour at ca. 1.70 V with a current onset around 1.20 V (see ESI{). Under illumination the cell delivers a short-circuit current density (J<sub>sc</sub>) of 3.65 mA cm<sup>22</sup>. The J–V curves show a persistent slope at negative voltages which can be attributed to various causes such as high series resistance, electron-hole recombination, low shunt resistance or photocon-ductivity. Further work is needed to clarify this point.

<figure><img src="image_4.png" /><figcaption>Fig. 2 UV-Vis absorption spectrum of compound 1 in CH<sub>2</sub>Cl<sub>2</sub>.</figcaption></figure>

The cell presents an open-circuit voltage (V<sub>oc</sub>) of 0.963 V. This
high value can be related to the high oxidation potential of 1.<sup>19,20</sup>
Combined with a rather low filling factor of 0.29, these J<sub>cs</sub> and V<sub>oc</sub>
values lead to a power conversion efficiency of 1.02%.

To summarize we have shown that the introduction of peripheral dicyanovinyl acceptor groups on a tris[4-(2-thienyl) phenyl]amine leads simultaneously to an extension of the absorption towards longer wavelengths and to an increase of the oxidation potential. Preliminary results on prototype bi-layer hetero-junction solar cells confirm that this type of donor material allows extended spectral r

response, high open-circuit voltage and very promising power
conversion efficiency to be combined. Although a detailed
understanding of the consequences of ICT on exciton dissociation,
charge recombination and transport clearly requires further work
and, in particular, detailed photo-physical studies, these first results
clearly provide a strong inducement to synthesize new donors materials
<figure><img src="image_5.png" /><figcaption>Fig. 3 UV-vis absorption spectra of 25 nm films of C<sub>60</sub> (dashed line), compound 1 (dotted line) bilayer (solid line) and EQE under monochromatic irradiation (empty circles).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4 J–V curves of ITO/Baytron/1/C<sub>60</sub>/Al devices recorded in the dark (empty squares) and under 100 mW cm<sup>22</sup> white light illumination (black circles).</figcaption></figure>

designed according to the same concept and to extend the
evaluation of these materials to other type of solar cells such as
bulk hetero-junctions. Work in these directions is now underway.

Notes

$$

M.p. 325 uC. ¹H NMR (CDCl₃) d 7.79 (s, 1H), 7.71 (d, 1H, ³J 5
4.13Hz), 7.64 (d, 2H, ³J 5 8.69Hz), 7.41 (d, 1H, ³J 5 4.09 Hz), 7.20 (d, 2H,
³J 5 8.69 Hz), ¹³C NMR (CDCl₃) (155.6, 150.4, 4147.8, 140.2, 133.9, 128.0,
127.8, 124.8, 124.1, 114.2, 113.4, 76.3. ¹R (KBr) (cm⁻¹) 2220 (CMN),
UV-vis l<sub>max</sub> nm (loge) 368, 509 (5.1). MS MALDI-TOF C₄₂H₂₁N₇S₃ 719 M⁺.

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