1.

Introduction

Since the discovery of photoinduced electron transfer between semiconducting conjugated polymers and fullerenes, several investigations were made.¹ Bulk heterojunction plastic cells (BHJs) based on composites of semiconducting polymers (as electron donors) and fullerene derivatives (as electron acceptors) were studied for their performances. The BHJ devices exhibit a vast interfacial area dispersed throughout the bulk. However, the overall conversion efficiency of these devices is limited by the carrier collection efficiency, which is greatly influenced by the morphology and the phase separation within the active film.^{2, 3, 4} Generally the morphology optimization of the active layers is of primary importance to improve the exciton dissociation efficiency, the transport of free charge carriers, and their extraction from the photoactive layer. To date, there were not many studies made on the functionalization of C₇₀ for the photovoltaic (PV) application besides those already published on the PC₇₀BM and the PC₈₅BM. Using PC₇₀BM into a photoactive layer increases the η% of the cells in both BHJ samples and tandem cases where it was permitted to reach 6%. In 2008, Heeger and co-workers obtained a 5.6% conversion using a photoactive layer based on PC₇₀BM and the low bandgap polymer PCPDTBT.⁵ Improved photovoltaic properties were obtained for solar cells based on copolymer {thieno[3,4-b]thiophene and benzodithiophene polymers (PTBs)} by Luping Yu's team. The broad absorption of this copolymer increased the short circuit current density (J_SC) of cells to 14.5 mA/cm² with power efficiency around 7.4%.⁶ In our previous work, organic solar cells elaborated from the cyclopropano[60]fullerenes,^{7, 8} provided promising PV performances, with power conversion and current density above 1.2% and 7.2 mA/cm², respectively. The effects of molecular structure on morphology and PV characteristics of RR-P3HT {poly (3,5 Hexylthiophene): cyclopropano[60]fullerenes} were studied. Along the same lines, cyclopropano[70]fullerenes were synthesised in order to study PV characteristics, keeping in mind that this family of fullerene C₇₀ derivatives has a broader absorption in the visible region (see Fig. 1). One should note that this Bingel type approach of fullerene C₇₀ functionalization is original, and was never studied before.

Fig. 1

Molecular structure of cyclopropano[70] fullerene derivatives C₇₀-A, C₇₀-B, and C₇₀-C.

2.

Experimental Section

Compounds C₇₀-A, C₇₀-B, and C₇₀-C and C₇₀ were analyzed in 7 × 10⁻⁴ mol l⁻¹ dichloromethane (CH₂Cl₂) (HPLC grade) solutions containing 5 × 10⁻² mol l⁻¹ n-Bu₄NPF₆ as a supporting electrolyte. Electrochemical experiments were performed in a dry, oxygen-free (<0.1 ppm), argon glove box at room temperature with a platinum working electrode. Electrochemical experiments were carried out with an EGG PAR 273A potentiostat-galvanostat. Reduction potential was measured using a silver wire pseudo-reference electrode in reference with internal standard ferrocenium ion/ferrocene (Fc⁺/Fc) redox couple. The scan rate used in this study was 100 mV/s.

Absorption spectra were recorded at room temperature with a Lambda 19 Perkin–Elmer spectrometer. These measurements were done on thin films based on composite P3HT: C₇₀ fullerene derivatives.

The substrates PEDOT:PSS {Poly(3,4 ethylenedioxythiophene): poly(styrenesulfonate)} coated ITO anode from Merck were first dried in an oven at 150°C during 10 min. The blend of RR-P3HT (Rieke): fullerene derivatives was dissolved in chlorobenzene solution. An active layer of RR-P3HT:cyclopropano[70]fullerene was deposited by spin coating over the PEDOT: PSS from Baytron P. Therefore, the LiF(Aldrich)-Al (Goodfellow) cathode was deposited by evaporation under a 4×10⁻⁷ mBar argon pressure, on top of the photoactive layer.⁹ The size of the active area is 0.32 cm². The cells ITO/PEDOT: PSS(40 nm)/RR-P3HT:cyclopropano[70]fullerene(80 nm)/LiF(0.7 nm)/Al(100 nm) were characterized and annealed in the glove box. The PV measurements were carried out before and after heat treatment. The PV properties were measured using a Keithley 236 unit in the dark and under AM 1.5 illumination with 575 Steuernagel solar simulator, through the back side (ITO) of the solar cells. The conversion efficiency η% is calculated using:

The external quantum yield or incident photon-to-electron conversion efficiency (IPCE) was measured using an Acton Spectra Pro 150 apparatus, by illuminating the solar cells with monochromatic light through the ITO. The relation between IPCE, current I_sc, monochromatic intensity I_inc, and wavelength λ is given by:

3.

Results and Discussion

The electrochemical behavior of cyclopropano[70]fullerene derivatives and parent C₇₀ as reference, has been investigated by cyclic voltammetry at room temperature in CH₂Cl₂ solution using TBAPF₆ as the supporting electrolyte. The corresponding data are listed in Table 1. All derivatives show very similar voltammograms. The curves displayed by compound C₇₀-A and C₇₀ have been chosen as an example (see Fig. 2).

Table 1

Redox potentials of compounds C70-A, C70-B, and C70-C.

Fig. 2

Cyclic voltammograms of derivatives C₇₀-A and C₇₀.

The voltammogram clearly shows 3 one-electron fullerene-based reduction waves at potentials E¹_1/2 = –1.07 V, E²_1/2 = –1.44 V, and E³_1/2 = −1.82 V (half-wave potential versus Fc⁺/Fc), which are assigned to the generation of the anion radical C₇₀^•− , the dianion C₇₀^•2−, and the trianion C₇₀^•3− , respectively, from the cyclopropanoC₇₀ moiety. That is characteristic of the electrochemical behavior of cyclopropano[70]fullerene issued from the Bingel process.^{10, 11, 12} Table 1 shows that all fullerene derivatives C₇₀-A, C₇₀-B, and C₇₀-C are characterized by almost identical reduction potentials.

It can thus be inferred that the differences observed in the structures of these derivatives, i.e., the ester chain length and/or the overall symmetry or nonsymmetry of the molecule, brings no net influence on the reduction ability of the compounds. On the other hand the redox properties of all cyclopropano[70]fullerenes appear to be close to those of parent C₇₀. Table 2 shows a 70 mV negative shift for the first reduction step of the derivatives of C₇₀-A, C₇₀-B, and C₇₀-C compared to the parent C₇₀. This shift can be reasonably attributed to a partial loss of conjugation and alteration of the electron-accepting ability of C₇₀ core further to LUMO energy elevation. These results can be explained by the presence of two –CO₂R groups connected to the cyclopropano ring in C₇₀-A, C₇₀-B, and C₇₀-C compounds. These ester groups are electron acceptors, which obviously increase the reduction potential of the C₇₀ core in both series. The fourth oxidation wave at 0.59 V in the case of C₇₀-A was attributed to the Cyclopronano moiety on C_70.¹⁰

Table 2

Relative quantities in mass of derivatives C70-A, C70-B, and C70-C used in photoactive layer.

The UV-visible absorption spectra of the mixtures deposited as thin films are presented in Fig. 3. We notice that the absorption spectra have the same shape, with the same bands characteristic of the P3HT and C₇₀ derivatives, before and after annealing. Heat treatment improves absorption coefficient of the samples. It involves a 30-nm shift of the spectra toward the highest wavelengths and an increase in absorption intensity with the appearance of shoulders at 565, 610, and 620 nm. This is usually attributed to the crystallization of RR-P3HT, further to phase separation.

Fig. 3

Absorption spectra of various composites RR-P3HT: C₇₀-A, RR-P3HT: C₇₀-B, RR-P3HT: C₇₀- C.

Although the molar concentrations of the fullerene derivatives are identical to those in the PCBM {[6,6]-phenyl-C₆₁-butyric acid methyl ester} cell, PV characteristics delivered by the solar devices are not the same. That confirms the dependence of PV performances on the exact molecular structure and more precisely on the difference or similarity between ester side chains. The electrochemical characteristics showed that these derivatives have very comparable electronic properties. The PV characteristics strongly depend on the nature of the interaction between the donor and the acceptor, and on their behavior inside the active layer. However, molecular structure influences the PV parameters as well as η%. Indeed, the best performances are obtained from cells based on the composite P3HT: C₇₀-B, with J_sc 3.56 mA/cm², V_oc 0.607 V, and ff 0.332 (Table 3). These parameters are improved after annealing at 100°C during 10 min (see Fig. 4). These parameters are smaller than in the P3HT:PC₇₀BM system already studied by Yamanari;¹³ their cells provided a conversion efficiency of 3.8%.

Table 3

PV parameters of solar cells based on composites RR-P3HT: cyclopropano[70]fullerenes.

Fig. 4

Current density/voltage (I–V) characteristic of cells based on composites RR-P3HT: cyclopropano[70]fullerene derivatives.

The heating effect causes phase separation and crystallization of the polymer RR-P3HT which allows the improvement of the absorption of the organic layers. Crystallization of the RR-P3HT increases the mobility of the charge carriers and thus we get J_sc = 9.29 mA/cm² and ff = 0.342. Figure 5 shows a new peak around 430 nm with 71% IPCE which justifies the large J_sc. On the other hand, V_oc slightly decreases. It can be caused by interface modification created during annealing. η% reaches 1.5% in that kind of solar cell.

Fig. 5

IPCE spectra of cell: ITO/PEDO:PSS/P3HT: C₇₀-B /LiF/Al, after annealing 100°C for 10 min.

4.

Conclusions

In the development of BHJ solar cells containing new fullerenes derivatives, it is clear that optimization alone of electronic and optical properties is insufficient.^{14, 15, 16} It is essential to take into account the molecular structure of the materials used, which can influence the interaction between donor and acceptor. These compounds C₇₀-A, C₇₀-B, and C₇₀-C put in evidence the importance of the acceptor chemical structure on PV characteristics. They confirm also that synthesis strategies more direct than for the PCBM molecules are promising.⁷ The higher η% = 1.5% is obtained for devices based on C₇₀-B after heat treatment at 100°C during 10 min.