Novel Terthiophene-Substituted Fullerene Derivatives as Easily Accessible Acceptor Molecules for Bulk-Heterojunction Polymer Solar Cells

Five fulleropyrrolidines and methanofullerenes, bearing one or two terthiophene moieties, have been prepared in a convenient way and well characterized. These novel fullerene derivatives are characterized by good solubility and by better harvesting of the solar radiation with respect to traditional PCBM. In addition, they have a relatively high LUMO level and a low band gap that can be easily tuned by an adequate design of the link between the fullerene and the terthiophene. Preliminary results show that they are potential acceptors for the creation of efficient bulk-heterojunction solar cells based on donor polymers containing thiophene units.


Introduction
The use of renewable energy sources instead of fossil fuel is a necessity for humanity. The Sun is a green and cheap source of energy: 10 minutes of solar irradiation onto the Earth's surface is equal to the total yearly human energy consumption [1,2]. The world energy challenge can be won harnessing the Sun power with photovoltaic technologies. Organic photovoltaic devices (OPVs) based on conjugated polymers and oligomers have received a lot of attention because of their potential for lightweight, flexible, and low cost photovoltaic energy conversion [3][4][5][6][7]. Among them, the most common devices are bulk-heterojunction (BHJ) polymer solar cells made upon blending an electron donor conjugated polymer with an electron acceptor material such as fullerene derivatives [3][4][5][6][7]. Fullerene-based OPV can be fabricated via vapor deposition; however, considering the expected demand for enhancing cost performance by mass production in the near future, application of roll-to-roll processing (i.e., the solvent casting method) appears highly desirable [8]. Therefore, development of stable fullerene derivatives that show both high power conversion efficiency and sufficient solubility in organic solvents is strongly desired [3][4][5][6][7]. Various types of fullerene derivatives for use as OPV acceptor materials have thus been developed. [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) [9,10] is known to be the best blending material among these derivatives as an acceptor with polythiophenes such as regioregular poly(3-hexylthiophene) (P3HT), which is a typical donor partner in polymer solar cells [3][4][5][6][7].
Although PCBM is the most popular acceptor material so far for BHJ polymer solar cells, it is important to explore new easily accessible C60 derivatives as acceptor partners for polymer donor materials with a huge diversity of chemical structures. Many efforts have been devoted to the modification of the PCBM skeleton by introducing substituents on the phenyl ring, exchanging methyl groups with long alkyl chains, an ethyleneoxy moiety, or a perfluoroalkyl chain to tune the miscibility, thermal properties, and energy levels, and the resulting methanofullerene derivatives have been used to control the film morphology, raise the open circuit voltage ( oc ), and improve the device stability [11][12][13][14][15][16][17]. A few years ago, a PCBM analogue containing a thiophene moiety, [6, 6]-thienyl-C61-butyric acid methyl ester (TCBM), has been reported to exhibit device performance close to PCBM with P3HT as the donor [11][12][13][14][15][16][17]. A series of TCBM analogues with different alkyl chains (methyl, hexyl, and ethylhexyl) at the 5 positions of the thiophene ring was then prepared [18]. Like methanofullerenes, fulleropyrrolidines are efficient acceptors for OPV devices and, recently, it was established that the introduction of a thiophene moiety on the pyrrolidine ring has a favorable effect on the power conversion energy (PCE) of a solar cell [19]. It was reported that 1-(2-(2-methoxyethoxy)ethyl)-2-(2-thiophen-2-yl)fulleropyrrolidine and related derivatives with bithiophene or terthiophene are efficient acceptor partners with P3HT, the compound bearing terthiophene being characterized by the highest oc but lower fill factor (FF) and short circuit current ( sc ) due to reduced solubility when compared to the mono-and bithiophene derivatives [19]. In fact the solubility of a fullerene derivative strongly affects the morphology of its composite with P3HT and therefore the efficiency of the cell [20,21]. These interesting results prompted us to design novel soluble terthiophene-substituted fullerene derivatives as easily accessible acceptor molecules for BHJ polymer solar cells (Scheme 1). We prepared both terthiophene-substituted fulleropyrrolidines (F1-F3) and methanofullerenes (F4-F5), with the aim of obtaining soluble acceptor materials with a good affinity for donor polymers based on thiophene units ( Figure 1). As our work was in progress, as expected, it was reported by Saravanan et al. that F2 is a better electron Figure 1: Terthiophene-substituted fullerene derivatives studied in the present work.
acceptor than PCBM for the fabrication of P3HT based bulkheterojunction solar cells [22].

Materials and Methods
General Comments. Solvents were dried by standard procedures: tetrahydrofuran (THF) and toluene were freshly distilled from Na/benzophenone under nitrogen atmosphere; N,N-dimethylformamide (DMF) was dried over activated molecular sieves; triethylamine (Et 3 N) was freshly distilled over KOH. All reagents were purchased from Sigma-Aldrich and were used without further purification (

Synthesis of Compound 5.
The known terthiophene derivative 5 [19] was prepared following a procedure reported for related compounds [26]. To a solution of 2,2 :5 ,2terthiophene (1) (100.7 mg, 0.40 mmol, 1 equiv.) in N,Ndimethylformamide (3 mL), under nitrogen and cooled to 0 ∘ C, was added, in small portions, phosphoryl trichloride (75.1 mg, 0.49 mmol, 1.2 equiv.). The cool bath was then removed and the mixture was stirred for 24 h at 80 ∘ C. After cooling to room temperature, the reaction mixture was neutralized with NaOH (2 mL, 1.25 M) and then diluted with CH 2 Cl 2 and washed with water: the organic layer was dried  F1-F3. Fulleropyrrolidines F1-F3 were prepared by using the Prato cycloaddition procedure [28,29]. A mixture of the suitable aldehyde (1 equiv.), fullerene C-60 (1 equiv.), and sarcosine (8 equiv.) was refluxed for 24 h in anhydrous toluene under a nitrogen atmosphere. After cooling to room temperature, the solvent was evaporated under vacuum and the residue was purified by flash chromatography, as indicated in each case.     6 mL), under nitrogen and cooled to 0 ∘ C, was added, in small portions, tin tetrachloride (255.3 mg, 0.98 mmol, 1 equiv.) and stirred for 2 h. The reaction mixture was diluted with CH 2 Cl 2 and washed with water: the organic layer was dried over Na 2 SO 4 and concentrated. The crude product obtained was purified by flash chromatography, using dichloromethane as eluant, to give 9 as a pure product (347 mg; yield 65%). 1  2.13. Synthesis of Compound 10. The novel compound 10 was prepared as follows. A mixture of 9 (165 mg, 0.3 mmol, 1 equiv.) and -toluenesulfonyl hydrazide (68 mg, 0.36 mmol, 1.2 equiv.) was refluxed in MeOH (0.5 mL) for 18 h. After cooling to room temperature, the organic solvent was removed under reduced pressure and the residue was dissolved in dichloromethane and washed with water. The organic layer was dried over Na 2 SO 4 and concentrated. The residue was purified by flash chromatography, using hexane/ethyl acetate 7 : 3 as eluant, to give 10 as a pure product (148 mg; yield 75%).

Synthesis of F4.
The new methanofullerene F4 was prepared following procedures reported for related compounds [18,30,31]. A mixture of 10 (55.3 mg, 0.084 mmol, 1.2 equiv.), sodium methoxide (4.5 mg, 0.084 mmol, 1.2 equiv.), and dry pyridine (0.84 mL) was stirred at room temperature for 30 min. Then a solution of fullerene C-60 (50.3 mg, 0.07 mmol, 1 equiv.) in o-dichlorobenzene (4.1 mL) was added, and the homogeneous reaction mixture was stirred at 75 ∘ C under nitrogen overnight. Then the mixture was refluxed for 24 h (180 ∘ C); after cooling to room temperature the solvent was evaporated at reduced pressure, and the residue was purified by column chromatography on silica gel with toluene/hexane 6 : 4 as eluent to give F4 as a pure product (yield 55%).

Electrochemical Characterization.
The cyclovoltammetric (CV) characterization was carried out with an Autolab PGSTAT 128N potentiostat, run by a PC with GPES software. The working cell included a Glassy Carbon (GC) disk embedded in Teflon (Amel, surface 0.071 cm 2 ) as the working electrode, a Platinum counter electrode (Metrohm), and an aqueous saturated calomel electrode (SCE, Amel) as the reference electrode. The sample was dissolved in o-dichlorobenzene (≈0.5 mg/mL) and drop coated from a capillary on the GC electrode. The electrolytic solution was acetonitrile (Carlo Erba, HPLC grade) with 0.1 M tetrabutylammonium tetrafluoroborate TBATFB (Fluka, electrochemical grade). The solution was degassed with argon purging. The scan rate was 200 mV s −1 . According to IUPAC recommendations the data have been referred to the Fc + /Fc redox couple (ferrocenium/ferrocene). HOMO and LUMO values were extrapolated from the onset peaks potential.

Preparation and Characterization of Solar Cells.
Solar cells were fabricated on patterned ITO-coated glass substrates previously cleaned with detergent and water and then ultrasonicated in acetone and isopropyl alcohol for 15 min each.
A PEDOT : PSS (Clevios P VP AI 4083) layer was spin-coated at 3000 rpm onto air plasma cleaner ITO-coated substrates to a thickness of around 40 nm and then baked in an oven at 120 ∘ C for 10 min. Fullerenes and P3HT were dissolved separately in chlorobenzene (20 mg/mL) (Carlo Erba, HPLC grade), mixed 1 : 1 w/w obtaining a total concentration of 10 mg/mL, and then stirred overnight at 70 ∘ C. The blend solutions were spin-coated at 600 and 1200 rpm in glove box onto the ITO/PEDOT : PSS substrates. The thickness of the active layers, measured with a Veeco Dektak 150 profilometer, ranged between 50 nm and 130 nm. Then the samples were completed with the thermal evaporation of the Al (80 nm) cathode at a base pressure of 10 −6 mbar. The active device area was 25 mm 2 . The devices were postproduction thermal annealed in glove box (nitrogen filled) at 150 ∘ C for 10 min. The device electrical characterization was carried out at room temperature in glove box. Solar cells were illuminated using a solar simulator (Sun 2000, Abet Technologies) and the light power intensity was calibrated at AM1.5 illumination conditions (100 mW cm −2 ) using a certified silicon solar cell. The current-voltage curves were taken with a Keithley 2602 source measure.

Results and Discussion
Terthiophene is an interesting -conjugated electronreleasing substituent group that can influence both the light absorbing behavior and charge separation process of fullerenes. Interestingly, it was reported that 1-(2-(2-methoxyethoxy)ethyl)-2-(terthiophene)fulleropyrrolidine is an efficient acceptor partner with P3HT, being characterized by a higher oc but lower fill factor and shorter circuit current with respect to related compounds bearing thiophene or bithiophene instead of the terthiophene moiety, attributed to its lower solubility [19]. These interesting results prompted us to design novel soluble terthiophene-substituted fullerene derivatives as easily accessible acceptor molecules for BHJ polymer solar cells (Scheme 1). We prepared both terthiophene-substituted fulleropyrrolidines (F1-F3) and methanofullerenes (F4-F5), with the aim of obtaining soluble acceptor materials with a good affinity for donor polymers based on thiophene units.
The novel terthiophene-substituted fulleropyrrolidines (F1, F3) and the known F2 [22] were prepared following a method similar to that originally developed by Prato and Maggini and coworkers (Schemes 1 and 2) [28,29]. Sarcosine was treated with [C60]-fullerene in the presence of a suitable terthiophene-substituted aldehyde, in toluene, and the mixture was heated under reflux for 24 h under a nitrogen atmosphere.
The novel terthiophene-substituted methanofullerene F4 was prepared following the procedure reported for other methanofullerenes [18,30,31], by reaction of thetosylhydrazone 10 with sodium methoxide and fullerene C-60 (Scheme 3), whereas F5 was synthesized by the Bingel reaction [32,33]  All fulleropyrrolidines and methanofullerenes were purified by silica gel flash chromatography and characterized by elemental analyses, 1 H NMR and UV-visible spectroscopies, mass spectrometry, and cyclovoltammetry (see Section 2). Figures 2 and 3 show the UV-visible absorption spectra of fullerenes F1-F5 along with that of PCBM in chlorobenzene solution.
The fulleropyrrolidine F2 shows a broad absorption band between 350 and 400 nm, as expected for a terthiophene moiety [22]. This band is red shifted in compound F1, in agreement with the presence of a -delocalized bridge between the pyrrolidine and the terthiophene moiety, and blue shifted in compound F3, due to the presence of the hexyl chains, responsible of the tilting of the structure and the final reduced conjugation.
Red shifting is observed in the compounds F4 and F5 in comparison with PCBM. Interestingly also a large enhancement in the of the peak at 330 nm (assigned to the fullerene) is obtained. Theoretically this enhancement can benefit the performance of the solar cells being the absorption in this region complementary to P3HT. In fact, it was reported that fullerenes' derivatives with a better light absorption can lead to a better power conversion efficiency since more photons are available to be converted into electricity [34].
The electrochemical properties of the various fullerenes were examined by cyclic voltammetry (CV). Current potential profiles are shown in Figures 4 and 5.
We use the first oxidation and reduction potentials to estimate the HOMO and LUMO energy levels by means of equations HOMO (eV) = −( OX + 4.8) and LUMO (eV) = −( RED + 4.8), which involve the use of the internal ferrocene standard value of −4.8 eV with respect to the vacuum level [35,36]. The results are summarized in Table 1. Interestingly, all the novel fullerene derivatives have an enhanced LUMO level with respect to PCBM, possibly improving the oc . In fact, Kim and coworkers reported that the oc of an OPV device is determined by the difference between the HOMO level of the p-type semiconductor and the LUMO level of the n-type conductor [37]. Fullerenes F1-F3 and F5 are characterized by a remarkably low band gap due to a relatively high HOMO level.   The use of fullerenes F1, F3, and F4 as acceptor molecules for bulk-heterojunction polymer solar cells was investigated in a preliminary way using P3HT as donor polymer. However, up to now the highest power conversion efficiency, reached with F4, was 0.46% only, although acceptable FF (0.30) and oc (0.48 Volt) were obtained. A wider screening of solvents and thickness in order to optimize morphology and performance is in progress in our laboratories.

Conclusions
In summary, we have prepared five interesting soluble fulleropyrrolidines and methanofullerenes, bearing one or two terthiophene moieties, as potential acceptors for the creation of efficient bulk-heterojunction solar cells based on donor polymers containing thiophene units. These novel fullerene derivatives are characterized by a better harvesting of the solar radiation with respect to traditional PCBM. In addition, they have a relatively high LUMO level and a low band gap that can be easily tuned by an adequate design of the link between the fullerene and the terthiophene.
Besides, it is worth pointing out that the new fullerene derivatives prepared in the present work are also of interest as new molecular building blocks for materials with nonlinear optical (NLO) properties [38][39][40][41][42][43]. In particular, because the methanofullerene F4 is an excellent candidate as secondorder NLO chromophore due to the presence of the highly polarizable electron acceptor C60-fullerene system linked to the donor terthiophene through a cyclopropane group, its quadratic hyperpolarizability is under study in our laboratories.