Cross-reference to Related Application

This application claims the benefit of United States provisional patent application serial number 61/660,019 filed June 15, 2012, the entire contents of which are herein incorporated by reference

Field of the Invention

The present invention relates to fullerene derivatives modified with indene-based compounds for application as electron acceptors in organic electronic devices, especially organic solar cells.

Background of the Invention

Bulk heterojunction (BHJ) organic solar cells based on conjugated p-type polymers and n-type fullerene derivatives have been intensively investigated in both academia and industry over the past decade due to their potential of being fabricated on flexible and light-weight substrates using high throughput printing techniques and providing low-cost solar electricity (Gunes et al.,(2007) Chem. Rev. 107, 1324-1338; Krebs et al., (2009) Sol. Energy Mater. Sol. Cells. 93, 394-412; Dennler et al. (2009) Adv. Mater. 21 , 1323-1338; Brabec et al., (2010) Adv. Mater. 22, 3839-3856). Significant progress has been made in this field and the power conversion efficiencies (PCE) of solution-processed polymer solar cells have reached 7-8% primarily due to the development of new low-bandgap p-type materials (Liang et al. (2010) Adv. Mater. 22, E135-E138; Chen et al., (2009) Nature Photon. 3, 649-653; Chu et al., (2011 ) J. Am. Chem. Soc. 133, 4250-4253; Dou et al., (2012) Nature Photon. Feb. 12, 2012, DOI: 10.1038/NPHOTON.2011.356) and better control of the nano-scale morphology of the interpenetrating electron donor/acceptor networks (Peet et al., (2007) Nature Mater. 6, 497-500; Lee et al., (2008) J. Am. Chem. Soc. 130, 3619-3623; Hoven et al., (2010) Adv. Mater. 22, E63-E66). In contrast to the intensive study on p-type materials, the design and synthesis of new fullerene derivatives for applications in solar cells as electron acceptors are relatively unexplored with most work focusing on the modification of PC ₆ iBM ([6,6]-phenyl C ₆ i butyric acid methyl ester) or PC ₇₁ BM ([6,6]-phenyl C ₇ i butyric acid methyl ester) derivatives (Lenes et al., (2008) Adv. Mater. 20, 21 16-21 19; Yang et al., J. Am. Chem. Soc. 2008, 130, 6444-6450; Zhang et al., (2009) Chem. Mater. 21 , 2598-2600; Giacalone et al., (2010) Adv. Mater. 22, 4220-4248). Although a number of chemical reactions have been developed to attach organic functional groups to the fullerene skeletons, including cycloaddition and the addition of nucleophiles and free radicals, it is still challenging to modify the LUMO and HOMO energy levels of fullerene derivatives (Hirsch et al., (2005) Fullerenes: Chemistry and Reactions. Wiley-VCH: Weiheim, Germany; Diederich et al., (1999) 4cc. Chem. Res. 32, 537-545). A recent study showed that that the energy levels of the fullerene derivatives mainly depend on the π electron system of the fullerene cage, and the substituents have little impact on the fullerene energy levels (Lu et al., (2011 ) Mater. Chem. 21 , 4953-4960). However, the organic substituents have huge impact on the other physical properties of the resulting fullerene derivatives, such as solubility, crystallinity, and electron mobility.

The cycloaddition reaction between indene and C ₆ o fullerene in refluxing 1 ,2- dichlorobenzene has been reported in the literature (Puplovskis et al., (1997) Tetrahedron Lett. 38, 285-288). Recently, several groups independently reported that at higher reaction temperatures and longer reaction time, two indene molecules can be attached to one C ₆ o cage. This indene-fullerene bisadduct (ICBA) has a higher LUMO energy level than widely used PC ₆ iBM because the number of the π electrons was reduced to 56 in the ICBA molecule. (He et al., (2010) J. Am. Chem. Soc. 132, 1377-1382; Laird United States patent publication US 2009/0176994 published July 9, 2009 (Laird 2009); United States patent publication US 2010/0132782 published June 3, 2010 (Laird 2010); International patent publication WO 2010/057087 published May 20, 2010 (Williams 2010)). As a result, the BHJ solar cell based on the Poly(3-hexylthiophene) (P3HT)/ICBA blend offered a large open-circuit voltage (Voc) of 0.84 V, compared with 0.58 V for the P3HT/PC ₆ iBM system. In addition, the overall power conversion efficiency was increased from 3.9% to 5.4%. This approach opens a new route to the chemical modification of fullerenes. Although the ICBA/P3HT blend works well, ICBA does not match with the low- bandgap p-type polymers that have low-lying LUMO energy levels because the energy offset between the LUMO energy levels of the polymer and ICBA are too small to provide enough driving force for efficient charge separation in bulk heterojunction solar cells. As a result, the photocurrent is low. As to the indene-fullerene monoadduct (ICMA), its LUMO energy level is suitable for most low-bandgap p-type polymers. However, its solubility is not high enough for solution processing. Especially, the indene-C ₇₀ monoadduct has a very low solubility in common organic solvents.

Therefore, there remains a need for new fullerene derivatives having improved solubility and fine-tuned energy levels for use in organic solar cells Summary of the Invention

In one aspect of the present invention, there is provided a compound of Formula

(I):

F-(lnd-Y) (I) wherein F comprises a fullerene having a surface which comprises six-membered and five-membered rings, and Ind-Y is

wherein Y is an electron donating group.

Y is preferably -XRT or -NR2R3, wherein X is O or S, is substituted or unsubstituted alkyl or aryl, and R ₂ and R ₃ are the same or different and each is independently substituted or unsubstituted alkyl or aryl. R is preferably (-VC^-alkyl, fluorinated C C ₂ o-alkyl, C ₂ -C ₂ o-alkenyl, C ₂ -C ₂ o-alkynyl, C ₆ -C ₂ o-aryl, C ₇ -C ₂ 4-alkaryl, Ci-C ₂₀ -carbonyl or C ₃ -C ₂₀ -alkoxycarbonylalkyl. Preferably, R ₂ and R ₃ are independently Ci-C ₂₀ -alkyl, fluorinated CVC^-alkyl, C ₂ -C ₂₀ -alkenyl, C ₂ -C ₂₀ -alkynyl, C ₆ -C ₂₀ -aryl or C ₇ -C ₂₄ -alkaryl. More preferably, R ₁ is CrCe-alkyl, R ₂ is C C ₆ -alkyl, and R ₃ is C C ₆ -alkyl.

In preferred embodiments of compounds of Formula (I), F is preferably a C ₆ o or C ₇ o fullerene.

In another aspect of the invention, there is provided a film or membrane comprising a compound of the present invention. In yet another aspect of the present invention, there is provided a use of a compound, film or membrane of the present invention in an organic electronic device.

In yet another aspect of the present invention, there is provided a process of producing a compound of Formula (I), the process comprising reacting F with Ind-Y in a 1 ,3-cycloaddition reaction, wherein F and Ind-Y are as defined above. Further features of the invention will be described or will become apparent in the course of the following detailed description.

Brief Description of the Drawings

In order that the invention may be more clearly understood, embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:

Fig. 1 depicts an HPLC plot of purified 5-methoxyindene-C ₇₀ monoadduct where the lack of an appreciable peak at 19.5 min shows the absence of unreacted C ₇₀ .

Fig. 2 depicts an HPLC plot of purified 5-butoxyindene-C ₇₀ monoadduct where the lack of an appreciable peak at 19.5 min shows the absence of unreacted C ₇₀ .

Fig. 3 depicts photo current-voltage characteristics of solar cells based on substituted indene-C ₇₀ monoadducts of the present invention with PCDTBT compared to solar cells based on prior art fullerenes with PCDTBT.

Fig. 4 depicts external quantum efficiency of solar cells based on substituted indene-C ₀ monoadducts of the present invention with PCDTBT compared to solar cells based on prior art fullerenes with PCDTBT.

Description of Preferred Embodiments

Fullerenes (F) are molecules comprising carbon atoms in the form of hollow spheres, ellipsoids or tubes. Preferably, F is in the form of hollow spheres. F is preferably a C ₆₀ or C ₇₀ fullerene. Besides having the Ind-Y moiety covalently bonded to the surface thereof, the fullerene may be unsubstituted or substituted with other substituents bonded to its surface or may contain one or more atoms within its hollow (e.g. metal atoms). The fullerene is preferably unsubstituted except for being substituted by the Ind-Y moiety.

Y is preferably -XPv _! or -NR ₂ R ₃ , wherein X is O or S, R ₁ is substituted or unsubstituted alkyl or aryl, and R ₂ and R ₃ are the same or different and each is independently substituted or unsubstituted alkyl or aryl. is preferably Ci-C ₂₀ -alkyl, fluorinated C C ₂ o-alkyl, C ₂ -C ₂₀ -alkenyl, C ₂ -C ₂₀ -alkynyl, C ₆ -C ₂ o-aryl, C ₇ -C ₂₄ -alkaryl, C C ₂₀ -carbonyl or C ₃ -C ₂₀ -alkoxycarbonylalkyl. Preferably, R ₂ and R ₃ are independently C C ₂ o-alkyl, fluorinated C C ₂₀ -alkyl, C ₂ -C ₂₀ -alkenyl, C ₂ -C ₂₀ -alkynyl, C ₆ -C ₂₀ -aryl or C ₇ -C ₂₄ -alkaryl. More preferably, is Ci-C ₈ -alkyl, R ₂ is C^Ce-alkyl, and R ₃ is C C ₆ -alkyl. Indene-fullerene compounds of Formula (I) may be prepared by a controlled 1 ,3-cycloaddition reaction between an indene-based compound containing an electron donating group and a fullerene in accordance with Scheme 1 :

F + Ind-Y→ F-(lnd-Y) (I)

Scheme 1 - Synthesis of F-( Ind-Y) where F and Ind-Y are as defined above. The reaction may be accomplished in any suitable solvent at an elevated temperature. The solvent is preferably an aromatic solvent, more preferably a halogenated aromatic solvent (e.g. trichlorobenzene). The temperature is preferably in a range of about 150-250°C or 150-210°C, in particular 175- 210°C, for example about 185°C. The reaction is preferably conducted for a time in a range of 5-8 hours. The desired indene-fullerene can be readily separated from unreacted fullerene and other byproducts by chromatography, particularly chromatography on a polar medium, for example silica gel. A non-polar eluent is preferably employed in the chromatography, for example toluene, hexanes or a mixture thereof. Ind-Y may be obtained by any suitable reaction from known starting materials. For example, a 6-substituted-indan-1-one may be reduced to the corresponding 6-substituted-indan-1-ol, and then dehydrated to form a 5-substitued-indene. The substituent on the 5-substituted-indene can then be converted to the desired Y group if needed. Scheme 2 illustrates the formation of Ind-Y.

Y is as defined above and L may be the same as Y or a leaving group that can be replaced by Y by suitable reaction. For the reduction step, any suitable reducing agent may be used, for example sodium borohydride (NaBH ₄ ) and lithium aluminum hydride (UAIH ₄ ), and the reduction performed in a suitable solvent, for example an alcohol (e.g. methanol, etc.). For the dehydration step, any strong acid can be used as a catalyst, for example toluene-p-sulphonic acid (TSA), sulfuric acid (H ₂ S0 ₄ ) or phosphoric acid (H ₃ P0 ₄ ), and the dehydration performed in a suitable solvent, for example an aromatic solvent (e.g. benzene, toluene, etc.). If L is to be replaced with a Y group, L is preferably a leaving group, for example a halogen (e.g. CI, Br or I) and a nucleophilic aromatic substitution reaction may be performed. The nucleophilic aromatic substitution may be accomplished by reacting (II) with MY, where M is a leaving group, for example H or a metal ion (e.g. Na ⁺ , K ⁺ ). In some cases, the nucleophilic aromatic substitution may require a catalyst, for example a cuprous halide, e.g. Cul.

In preferred embodiments of compounds of Formula (I), F is preferably a Ceo or C ₇₀ fullerene and Y is preferably -XF^ or -NR ₂ R ₃ , where X, R ₂ and R ₃ are as defined above. Schemes 3 and 4 illustrate preferred syntheses for these preferred embodiments.

TSA

Ceo or C70, 185°C

Scheme 3 - Synthesis of F^lnd-OR

In Scheme 3, 6-hydroxyindan-1-one is used as a starting material, which can be converted to 6-alkoxyindan-1-one through treatment first with sodium methoxide and then with alkyl bromide or iodide in anhydrous methanol. The product can be easily separated from the unreacted 6-hydroxyindan-1-one by column chromatography on silica gel. 6-Alkoxyindan-1-one is reduced with sodium borohydride in methanol at room temperature to give the corresponding 1-hydroxy-indane derivatives, which can be converted to indene derivatives through dehydration reaction in refluxing benzene using toluene-p-sulphonic acid as the catalyst. Then, the obtained 5-alkoxyindene reacts with fullerenes (C ₆₀ or C ₇₀ ) through a cycloaddition reaction at 185°C in 1 ,2,4-trichlorobenzene. The desired indene-fullerene monoadduct can be easily separated from unreacted fullerene and other byproducts by silica gel chromatography using a mixture of toluene/hexanes as the eluent.

R2R3NH

RiSNa Cul catalyst

1 ,2,4-trichlorobenzene 1 ,2,4-trichiorobenzene

Ceo or Oo, 185°C Ceo or C o, 185°C

Scheme 4 - Syntheses of F- nd-SF ,) and F-(lnd-NR ₂ R ₃ )

In Scheme 4, 6-bromoindan-1-one is used as a starting material. It can be converted to 5-bromoindene via the same reaction sequence as in Scheme 3. Then 5-bromoindene is converted to 5-alkylthoioindene or 5-dialkylaminoindene via nucleophilic substitution reaction. Finally, these indene derivatives comprising electron- donating groups react with fullerenes in the same way as in Scheme 3 to generate the targeted indene-fullerene monoadducts.

Compounds of the present invention have improved electronic properties coupled with improved solubility. Improved solubility facilitates processing the compounds into films and membranes resulting in more homogeneous membranes and better performance characteristics. The electron-donating groups not only improve solubility of the resulting fullerene derivatives, but also permit fine-tuning of the LUMO energy levels. In comparison to unsubstituted indene-fullerenes of the prior art, compounds of the present invention show significant improvements in device fill factor and overall EQE- calibrated power conversion efficiency. Moreover, the electron-donating groups are also polar functional groups that facilitate product purification by chromatography on a polar medium (e.g. silica gel) with a non-polar eluent. In contrast, it is difficult to separate unsubstituted indene-fullerene monoadduct from unreacted fullerene and other byproducts because of their similar polarity.

The compounds may be used as electron acceptors in active layers for organic electronic devices, for example in photovoltaic devices (e.g. solar cells). Use of the compounds in solar cells, especially bulk heterojunction (BHJ) solar cells, is of particular note.

Examples Example 1: Synthesis of 5-methoxyindene-C ₇₀ monoadduct (MOIC ₇₀ MA)

Step 1. Synthesis of 6-methoxyindan-1-ol

Commercially available 6-methoxyindan-1-one was dispersed in 20 mL of methanol with stirring in a 100-mL flask. Then, the mixture was cooled to 0°C in an ice- water bath. NaBH ₄ (290 mg) was added. A lot of bubbles formed, and the solid dissolved to give a clear solution. After stirring at 0°C for 5 min, the ice-water bath was removed. The solution was further stirred at room temperature for 2 h. Then, water (15 mL) was added, and acidified with 2 M HCI solution. The resulting mixture was extracted with dichloromethane. The organic phases were combined and dried over MgS0 ₄ . After removal of the solvent under reduced pressure, 1.00 g of light yellow liquid was obtained. The reaction yield is almost quantative. ¹ H NMR (400 MHz, CD ₃ OCD ₃ + D ₂ 0, δ) 7.08 (d, J = 8.4 Hz, 1 H), 6.93 (d, J = 2.4 Hz, 1 H), 6.75 (dd, J-, = 8.4 Hz, J ₂ = 2.4 Hz, H), 5.10 (t, J = 6.8 Hz, 1 H), 3.76 (s, 3H), 2.90-2.80 (m, 1 H), 2.70-2.60 (m, 1 H), 2.42-2.35 (m, 1 H), 1.90- 1.80 (m, 1 H).

Step 2. Synthesis of 5-methoxyindene 6-methoxyindan-1-ol (0.5 g) was dissolved in 20 mL of anhydrous benzene), and then 10 mg of toluene-p-sulphonic acid was added. The resulting solution was refluxed for 1.5 h, and the water generated during the reaction was azeotropically removed with a Dean-Stark apparatus. After cooling down, hexanes (20 mL) were added, and the solution was washed with NaHC0 ₃ aqueous solution. The organic phase was concentrated to obtained yellow liquid. The crude product was purified by column chromatography using ethyl acetate/hexanes (1 :10) as an eluent. Colorless liquid (0.32 g) was otained in 72% yield. ¹ H NMR (400 MHz, CD ₃ OCD ₃ , δ) 7.34 (d, J = 8.4 Hz, 1 H), 6.99 (d, J = 2.4 Hz, 1 H), 6.90-6.84 (m, 1 H), 6.74 (dd, J ₁ = 8.4 Hz, J ₂ = 2.4 Hz, 1 H), 6.62-6.56 (m, 1 H), 3.79 (s, 3H), 3.31 (m, 2H).

Step 3. Synthesis of 5-methoxyindene-C ₇₀ monoadduct

To a 100 ml_ three-necked flask fitted with a magnetic stirrer, a condenser, and a nitrogen inlet were added 5-methoxyindene (0.18 g), C ₇₀ (0.17 g), and 1 ,2,4- trichlorobenzene (12 ml_). After the system was flushed with N ₂ three times, the reaction mixture was heated to 185°C under N ₂ with stirring. The reaction was followed by High performance liquid chromatography (HPLC) analysis. It was found that after 3 h, the monoadduct accounted for 55% in the reaction mixture. The reaction was stop in 5 h, and the content of the desired monoadduct slightly increased to 58%. However, the amount of indene-C ₇₀ bisadduct reached 14%. After cooling down, methanol (60 mL) was added, and the black precipitate was collected by filtration. The product was purified by column chromatography on silica gel using toluene/hexanes (4:6) as the eluent. The reaction yield was 40%. The purity of the purified product was tested on HPLC equipped with a Cosmosil™ Buckyprep column (10.0 mm x 250 mm, Nacalai USA), and found to be 99.8%, as shown in Fig. 1.

Example 2: Synthesis of 5-butoxyindene-C ₇₀ monoadduct (BOIC ₇₀ MA)

Step 1. Synthesis of 6-butoxyindan-1-one

A 100-mL flask was charged with 6-hydroxyindan-1-one (1.00 g) and anhydrous methanol (20 mL). Then a solution of NaOCH ₃ (25 wt% in methanol, 1.7 mL) was added. After stirring for 5 min, iodobutane (0.85 mL was added. The resulting solution was refluxed for 48 h under N ₂ . After cooling down, the solvent was removed under reduced pressure. The residue was extracted with ethyl acetate, and the extract was concentrated to give brown solids, which were further purified by chromatography. The product (1.01 g) was obtained as colorless solids in 72.6% yield. ¹ H NM (400 MHz, CD ₃ OCD ₃ , δ) 7.44 (d, J = 8.4 Hz, 1 H), 7.22 (dd, J ₁ = 8.4 Hz, J ₂ = 2.4 Hz, 1 H), 7.09 (d, J = 2.4 Hz, 1 H), 4.05 (t, J = 6.4 Hz, 2H), 3.05 (t, J = 6.0 Hz, 2H), 2.65-2.60 (m, 2H), 1.80-1.70 (m, 2H), 1.55-1.45 (m, 2H), 0.97 (t, J = 7.2 Hz, 3H).

Step 2. Synthesis of 6-butoxyindan-1 -ol 6-Butoxyindan-1-ol was prepared from using the same procedure as described for the synthesis of 6-methoxyindan-1 -ol except that 6-butoxyindan-1-one was used as the starting material. The reaction yield was almost quantative. H NMR (400 MHz, CD ₃ OCD ₃ + D ₂ 0, δ) 7.07 (d, J = 8.4 Hz, 1 H), 6.92 (d, J = 2.4 Hz, 1 H), 6.75 (dd, J ₁ = 8.4 Hz, J ₂ = 2.4 Hz, 1 H), 5.09 (t, J = 6.4 Hz, 1 H), 3.96 (t, J = 6.4 Hz, 2H), 2.90-2.80 (m, 1 H), 2.70-2.60 (m, 1 H), 2.42-2.35 (m, 1 H), 1.90-1.80 (m, 1 H), 1.75-1.70 (m, 2H), 1.55-1.45 (m, 2H), 0.95 (t, J = 7.2 Hz, 3H). Step 3. Synthesis of 5-butoxyindene

6-Butoxyindan-1-ol was dehydrated according to the procedure for the preparation of 5-methoxyindene. The crude product was purified by column chromatography to afford colorless liquid in 80% yield. ¹ H NMR (400 MHz, CD ₃ OCD ₃ , δ) 7.33 (d, J = 8.0 Hz, 1 H), 6.99 (d, J = 2.4 Hz, 1 H), 6.88-6.84 (m, 1 H), 6.74 (dd, = 8.0 Hz, J ₂ = 2.4 Hz, 1 H), 6.60- 6.56 (m, 1 H), 3.99 (t, J = 6.8 Hz, 2H), 3.31 (m, 2H), 1.80-1.70 (m, 2H), 1.55-1.45 (m, 2H), 0.97 (t, J = 7.2 Hz, 3H).

Step 4. Synthesis of 5-butoxyindene-C70 monoadduct

C ₇₀ (158 mg) and 5-butoxyindene (0.213 g) were heated at 185°C in 1 ,2,4- trichlorobenzene (12 mL) for 5 h. After cooling down, methanol (60 mL) was added, and the black precipitate was collected by filtration. The 5-butoxyindene-C ₇₀ monoadduct was separated from unreacted C ₇₀ and byproducts by column chromatography on silica gel using toluene/hexanes (3:7) as the eluent. The reaction yield was 50%, and the purity was 99.5%, as indicated by HPLC (see Fig. 2).

Example 3. Application of substituted indene~C ₇₀ derivatives in organic solar cells as electron acceptors

The photovoltaic (PV) performance of 5-methoxyindene-C ₇₀ monoadduct (MOIC ₇₀ MA) and 5-butoxyindene-C ₀ monoadduct {BOIC ₇₀ MA) were tested in BHJ solar cells using poly[N-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thieny l-2', 1 ',3'- benzothiadiazole)] (PCDTBT) as a p-type material. For comparison purpose, unsubstituted indene-C ₇₀ monoadduct (IC ₇₀ MA) and widely used PC iBM and were also blended with PCDTBT under the same condition. The BHJ solar cells were prepared on commercial glass slides coated with patterned indium tin oxide (ITO). The thickness and sheet resistance of the ITO are 80 nm and 18 Ω/square, respectively. The active area of each solar cell device was 1.0 cm ² with a length:width ratio of 4:1. The substrates were sonicated sequentially in detergent, deionized water, acetone, and isopropanol. Immediately prior to device fabrication, the substrates were treated in a UV-ozone oven for 15 min. First, a poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT- PSS) thin film (30 nm) was spin-coated and then baked at 140°C for 15 min. Secondly, an active layer was spin-coated on top of the PEDOT-PSS from the solutions of the blends of PCDTBT and C ₇₀ derivatives (1 : 3.5 by weight). BCP (5 nm) and aluminum (Al) (80 nm) were deposited on the top of the active layer in vacuum to complete the PV device fabrication. The solar cells (with no protective encapsulation) were then tested in air under AM 1.5G illumination of 100 mW/cm ² (ScienceTech Inc., SS 500W solar simulator), which was calibrated with a KG5 filter covered silicon photovoltaic solar cell traceable to the National Renewable Energy Laboratory (NREL). Current-voltage {l-V) characteristics were recorded using a computer-controlled Keithley 2400 source meter. The external quantum efficiency (EQE) was performed using a Jobin-Yvon Triax spectrometer, a Jobin-Yvon xenon light source, a Merlin lock-in amplifier, a calibrated Si UV detector, and an SR570 low noise current amplifier.

Fig. 3 and Fig. 4 show the current-voltage characteristics and external quantum efficiency (EQE) curves of the fabricated solar cells, respectively. Detailed analysis of the dark l-V curves revealed that the device series resistance decreased from 10 Ω cm ² for the PC ₇₁ BM based device to 4 and 5 Ω cm ² for the MOIC ₇₀ MA and BOIC ₇₀ MA based devices, respectively. As a result, the device fill factor increased from 0.60 to 0.64 for BOICJOMA and even surprisingly to 0.69 for MOIC ₇₀ MA, and the overall EQE-calibrated power conversion efficiency was enhanced from 5.6% to 6.2%. For the IC ₇₀ MA-based device, an ideal interpenetrating electron donor/acceptor network could not be achieved because of its poor solubility. As a result, the device fill factor (0.5) was low, the photocurrent only reached 8.8 mA/cm ² , and the overall power conversion efficiency was only 4.0%, which was significantly lower than the SO/C ₀ M/A-based device. It is worth pointing out that the open-circuit voltage (Voc) of the BO/C ₇₀ M/4-based device is slightly higher than that of the PC ₇₁ BM device. This means that the indene derivatives not only improved the solubility of the resulting fullerene derivatives, but also slightly raised their LUMO energy levels.

These results show that the synthesized 5-alkoxyindene-C ₇₀ derivatives have better performance than both PC ₇ iBM and IC ₇ oMA. The device fill factor increased from 0.50 to 0.64 and even to 0.69, and the overall EQE-calibrated power conversion efficiency was enhanced from 4.0% to 6.2%, a more than 50% improvement.

Other advantages that are inherent to the structure are obvious to one skilled in the art. The embodiments are described herein illustratively and are not meant to limit the scope of the invention as claimed. Variations of the foregoing embodiments will be evident to a person of ordinary skill and are intended by the inventor to be encompassed by the following claims.