Enhancing thermal stability of bulk heterojunction solar cells with fluorenone derivatives

Abstract

The present invention relates to the provision of an organic compound or compounds containing a fluorenone derivative structure or its substituted derivatives to enhance the thermal stability of organic solar cells.

Claims

1. An electron deficient compound used as an additive in an active layer in an organic solar cell wherein said electron deficient compound has a structure: ##STR00026##

2. An organic solar cell having at least one active layer comprising a PCBM acceptor, a donor polymer and a compound having a structure: ##STR00027##

3. The organic solar cell of claim 2, wherein the compound is present in an amount ranging from 0.01 to 5 percent by weight relative to the weight of the donor polymer in the at least one active layer of the organic solar cell.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 shows the schematic diagram of an organic bulk heterojunction solar cell.

(3) FIG. 2 shows the structural configuration of sample BHJ-OSC device.

DETAILED DESCRIPTION OF THE INVENTION

(4) The present invention is not to be limited in scope by any of the specific embodiments described herein. The following embodiments are presented for exemplification only.

(5) The present invention relates to an electron-deficient fluorenone derivative containing a chromophore structure of formula (I) or its substituted derivative, which when it is present in the composition of organic solar cells, it enhances their thermal stability. Specifically, said fluorenone derivative is present in the active layer of organic solar cells whose structure is schematically represented in FIG. 1, in which the active layer is sandwiched between two electrodes (cathode and anode). More specifically, said fluorenone derivative can be utilized in the active layer of a BHJ-OSC, whose structure is for example represented in FIG. 2, in which an active layer comprises a donor polymer, PCBM, optional processing additives, and said fluorenone derivative. Said active layer may be sandwiched in between an ITO cathode and an Al anode with optional hole and electron transport interlayers as represented for example by PEDOT:PSS and LiF, respectively, to promote and facilitate charge carrier injection and transport. Said formula (I) is as follows:

(6) ##STR00017##

(7) The said thermal stability of organic solar cells may be related, but not limited to, facilitation of processing of active layer fabrication and subsequent stabilization of the nanoscale morphology of their active layers by the fluorenone derivative against thermally-induced degradation.

(8) The stabilization presumably arises from the charge transfer interaction between the fluorenone derivative, which is a strong electron acceptor, and the donor moiety of the donor polymer. This in essence results in physically cross-linking the donor polymer molecules by said fluorenone derivative, promoting and facilitating formation of donor polymer nanodomains. The physical cross-linking of the donor polymers also contributes to the isolation and stabilization of PCBM nanodomains, thus ensuring continuous percolated transport pathways for both electron and hole carriers.

(9) Preferably, the fluorenone derivatives of the present invention, that are useful for thermal stabilization of OSCs are represented by the following general formulas:

(10) ##STR00018##
where X is oxygen atom (O) or dicyanomethylene [C(CN).sub.2]; A and B, jointly or separately, are substituents such as alkyl, alkoxy, halogen, cyano, nitro groups and the like; Y is a divalent linkage such as methylene, dimethylene, trimethylene, substituted polymethylene [e.g., CH.sub.2CH(CH.sub.2CH.sub.3)CH.sub.2CH(CH.sub.2CH.sub.3)CH.sub.2], alkylbis(polymethylene)amine [e.g., (CH.sub.2).sub.2NCH.sub.3(CH.sub.2).sub.2], other substituted polymethylene [e.g., COOCH.sub.2OOC; COOCH.sub.2CH.sub.2OOC], and the like; Z is a trivalent linkage such as tris(polymethylene) amine [e.g., [(CH.sub.2).sub.2].sub.3N] and the like; m and n are integers ranging from zero to 3.

(11) More specifically, the fluorenone derivatives that are of particular interest are fluorenylidene malononitrile derivatives and the like represented by the following structures:

(12) ##STR00019##
where A and B, jointly or separately, are substituents such as alkyl, alkoxy, halogen, cyano, nitro groups and the like; R is an alkyl group; R is a divalent linkage such as methylene, dimethylene, trimethylene, substituted polymethylene [e.g., CH.sub.2CH(CH.sub.2CH.sub.3)CH.sub.2CH(CH.sub.2CH.sub.3)CH.sub.2], alkylbis(polymethylene)amine [e.g., (CH.sub.2).sub.2NCH.sub.3(CH.sub.2).sub.2], their substituted forms and the like; R is a trivalent linkage such as tris(polymethylene) amine [e.g., [(CH.sub.2).sub.2].sub.3N] and the like; m and n, jointly or separately, are integers ranging from zero to 3.

(13) The fluorenylidene manolonitriles that are of specific interest to the present invention include but not limited to the following compounds:

(14) ##STR00020##
where R=(CH.sub.2).sub.3CH.sub.3, (CH.sub.2).sub.7CH.sub.3, (CH.sub.2).sub.3CH(CH.sub.3).sub.2, (CH.sub.2).sub.3CH(CH.sub.3)CH.sub.2CH.sub.3;

(15) ##STR00021##
where Y=(CH.sub.2).sub.6; (CH.sub.2).sub.2NCH.sub.3(CH.sub.2).sub.2, CH.sub.2CH(CH.sub.3)CH.sub.2CH(CH.sub.3)CH.sub.2, CH.sub.2CH(CH.sub.2CH.sub.3)CH.sub.2CH(CH.sub.2CH.sub.3)CH.sub.2, (CH.sub.2).sub.2C(CH.sub.3).sub.2CH.sub.2CH.sub.2C(CH.sub.3).sub.2(CH.sub.2).sub.2; and

(16) ##STR00022##
where Z=(CH.sub.2CH.sub.2).sub.3N, (CH.sub.2CH.sub.2).sub.3CCH.sub.2CH.sub.3, (CH.sub.2CH.sub.2CH.sub.2).sub.3CH.

(17) Four illustrative specific examples of fluorenylidene malononitriles useful for the present invention are provided below:

(18) ##STR00023##

(19) In other embodiments of the present invention, the following illustrative fluorenone derivatives are also useful additives of the present invention:

(20) ##STR00024##
where A and B, jointly or separately, are substituents such as alkyl, alkoxy, halogen, cyano, nitro groups and the like; R is an alkyl group; R is a divalent linkage such as methylene, dimethylene, trimethylene, substituted polymethylene [e.g., CH.sub.2CH(CH.sub.2CH.sub.3)CH.sub.2CH(CH.sub.2CH.sub.3)CH.sub.2], alkylbis(polymethylene)amine [e.g., (CH.sub.2).sub.2NCH.sub.3(CH.sub.2).sub.2], their substituted forms and the like; R is a trivalent linkage such as tris(polymethylene) amine [e.g., [(CH.sub.2).sub.2].sub.3N] and the like; m and n, jointly or separately, are integers ranging from zero to 3;

(21) ##STR00025##

(22) The effective amount of fluorenone derivative additive ranges from 0.01% to 5%, and preferably from 0.1% to 2% by weight, relative to the amount of electron donor polymer such as PTB7. The active layer materials (donor polymer, PCBM, and fluorenone derivative) are first dissolved in a suitable solvent such as chlorobenzene (CB) or dichlorobenzene (DCB) together with optional processing additive such as 1,8-diiodooctane (DIO) in the amount of 0.5% to 5% by volume relative to the total volume of active layer coating solution. Illustrative device configuration of the sample BHJ-OSC device is ITO/PEDOT:PSS/active layer/LiF/Al as represented in FIG. 2.

(23) The active layer coating solution (donor polymer/PCBM/fluorenone derivative/DIO/CB) is then spin coated onto PEDOT:PSS-coated ITO glass substrate, and dried in vacuo, followed by heating in an oven at an appropriate temperature from 10 min to 12 hours, which serves to evaluate the thermal stability of the active layer. Subsequently, LiF and Al are thermally deposited under 10.sup.6 Torr. In this device, ITO/PEDOT:PSS serves as the cathode while LiF/Al serves as the anode. Alternatively, after the active layer deposition, the active layer was dried in vacuo overnight. This was followed by thermal deposition of LiF/Al electrode, and the completed devices were then thermally treated at various temperatures, e.g. from room temperature to 90 C., for 10 min. The devices are measured in ambient conditions for PCE determination. A Newport Thermal Oriel 94021 1000 W solar simulator was used to generate a simulated AM 1.5 G solar spectrum irradiation source. The irradiation intensity was 100 mW cm.sup.2 calibrated by a standard silicon solar cell VS 0831.

(24) The following examples are provided to illustrate the invention, which by no means are exhaustive. They are intended to be illustrative only and are not intended to limit the scope of the invention.

Example 1

Synthesis of N,N,N-Tris(9-Dicyanomethylenefluorene-4-Carboxyethyl)Amine (XI)

(25) (a) 4-Carboxy-9-florenylidene malononitrile: A mixture of 9.31 g (0.0415 mole) of fluorenone-4-carboxylic acid and 75 mL of absolute methanol was magnetically stirred and heated to reflux temperature in a round-bottomed flask fitted with a reflux condenser. Subsequently, there was added to the flask 8.23 g (0.125 mole) of malononitrile and 2 drops of piperidine. The mixture was then heated under reflux for 48 hours. The solid product 4-carboxy-9-fluorenylidene malononitrile, was collected by suction filtration, and purified by stirring in 50 mL of boiling methanol for 15 min, followed by filtration and washing successively with 20 mL of methanol. The product was dried under vacuum at 65 C. for 12 hours and weighed 9.01 g.

(26) (b) 4-Chloroformyl-9-fluorenylidene malononitrile: A mixture of 2.74 grams (0.01 mole) of 4-carboxy-9-fluorenylidene malononitrile as obtained in (a) above, and 15 mL of thionyl chloride in a round-bottomed flask equipped with a reflux condenser was magnetically stirred and heated under reflux in a dry nitrogen atmosphere for 6 hours. The solid acid dissolved after 1 hour's heating. As the reaction proceeded, the reaction mixture turned brownish in color, and was a dark brown reaction mixture at the end of the reaction. The reaction mixture was then evaporated at reduced pressure resulting in a solid residue. Thereafter, 30 mL of dichloroethane was added to the mixture to dissolve the crude product. The resulting solution was then evaporated under reduced pressure to remove traces of thionyl chloride. The crude product was recrystallized from methylene chloride/hexane, and the pure 4-chloroformyl-9-fluorenylidene malononitrile obtained weighed 2.79 g after drying under a vacuum at 40 C. for 12 hours.

(27) (c) To a gently stirred solution of 0.58 g (0.002 mol) 4-chloroformyl-9-fluorenylidenemalononitrile, 0.8 mL triethyl amine in 20 mL anhydrous methylene chloride in a round-bottomed flask cooled in an ice bath, a solution of 0.085 g triethanolamine in 1 mL methylene chloride was added slowly. The solution was stirred at room temperature overnight under N.sub.2 atmosphere. At the end of the reaction, a saturated aqueous solution of NaHCO.sub.3 was added. The organic layer was separated and the aqueous phase was extracted three times with dichloromethane. The combined organic layers were washed with brine, dried over MgSO.sub.4, and upon solvent removal under reduced pressure, afforded a solid crude product. Purification by column chromatography through silica gel using dichloromethane as eluant gave 0.35 g of the pure product (XI).

(28) (d) A series of comparative control devices with active layers treated at different temperatures were fabricated as follows: A solution of 5 mg of PTB7, 7.5 mg of PC70BM, and 15.6 L of DIO in 500 L chlorobenzene was spin-coated onto PEDOT:PSS coated ITO substrate. After coating, the active layer was dried or heated at an elevated temperature in an oven overnight (12 hours). Subsequently, LiF and Al are thermally deposited on the active layer under 10.sup.6 Torr. The device was then encapsulated in the glove box to complete the device fabrication. The performance (PCE) of the control devices with their active layers treated at various temperatures, e.g., from room temperature to 90 C., is shown in Table 1:

(29) TABLE-US-00001 TABLE 1 Performance (PCE) of control devices with their active layers treated at various temperatures. Treatment Temp. ( C.) PCE (%) Room Temp 7.8 40 C. 7.1 70 C. 6.2 90 C. 5.3

(30) Another series of sample devices with fluorenylidene malononitrile derivative (XI) additive were prepared in accordance with the procedure of control devices except that 0.04 mg of (XI) was added to the coating solutions. In this embodiment, the effective amount of fluorenylidene malononitrile additive used herein is 0.8% by weight, relative to the amount of electron donor polymer. The performance (PCE) of the sample devices with their active layers treated at various temperatures, e.g., from room temperature to 90 C., is shown at Table 2.

(31) TABLE-US-00002 TABLE 2 Performance (PCE) of sample devices with their active layers treated at various temperatures. Treatment Temperature ( C.) PCE (%) Room Temp 7.8 40 C. 7.7 70 C. 7.2 90 C. 7.0

(32) As can be noted, the PCE values of the sample devices with fluorenylidene malononitrile (XI) additive (Table 2) were superior to those of control devices without the additive (Table 1) at all temperatures. These results clearly show that the sample devices with fluorenylidene malononitrile (IX) additive were far more stable than the control devices.

Example 2

(33) A series of control devices without fluorenylidene malononitrile additive were prepared in accordance with the procedure of EXAMPLE 1 except that after the active layer deposition, the active layer was dried in vacuo overnight. LiF/Al electrode was then deposited on the active layer, and the completed devices were then thermally treated at various temperatures, e.g., from room temperature to 90 C., for 10 min. The performance of control devices at various temperatures is shown in Table 3.

(34) TABLE-US-00003 TABLE 3 Performance of control devices at various temperatures. Treatment Temperature PCE (%) Room Temp 7.8 40 C. 7.4 50 C. 7.3 60 C. 6.8 70 C. 5.8 80 C. 5.1 90 C. 4.8

(35) Another series of sample devices with fluorenylidene malononitrile derivative (XI) were prepared in accordance with the above procedure for the control devices except that 0.04 mg of (XI) was added to the coating solutions. In this embodiment, the effective amount of fluorenylidene malononitrile additive used herein is 0.8% by weight, relative to the amount of electron donor polymer. The performance of sample devices at different temperatures, e.g., from room temperature to 90 C., is shown in Table 4.

(36) TABLE-US-00004 TABLE 4 Performance of sample devices at various temperatures. Treatment Temperature PCE (%) Room Temp 7.9 40 C. 7.6 50 C. 7.4 60 C. 7.2 70 C. 7.1 80 C. 6.3 90 C. 6.1

(37) Again, the PCE values of the sample devices with fluorenylidene malononitrile (XI) additive (Table 4) were superior to those of control devices without fluorenylidene malononitrile additive (Table 3) at all treatment temperatures. These results clearly show that the sample devices with the additive were far more stable than control devices.

Example 3

Synthesis of 3,5-Bis(9-Dicyanomethylenefluorene-4-Carboxymethyl)Heptane (X)

(38) To a gently stirred solution of 0.65 g (0.0022 mol) 4-chloroformyl-9-fluorenylidenemalononitrile, 0.5 mL triethylamine in 20 mL anhydrous methylene chloride in a round-bottomed flask cooled in an ice bath, a solution of 0.016 g 2,4-diethyl-1,5-pentanediol in 1 mL methylene chloride was added slowly. The solution was stirred at room temperature overnight under N.sub.2 atmosphere. At the end of the reaction, a saturated aqueous solution of NaHCO.sub.3 was added. The organic layer was separated and the aqueous phase was extracted three times with dichloromethane. The combined organic layers were washed with brine, dried over MgSO.sub.4, and upon solvent removal under reduced pressure, afforded a solid crude product. Purification by column chromatography through silica gel using dichloromethane:hexane (2:1) as eluent gave 0.26 g of the pure product (X).

(39) A series of sample devices with fluorenylidene malononitrile (X) additive as synthesized above were prepared in accordance with the procedure of EXAMPLE 1 (d) except that 0.04 mg of (X) was added to the coating solutions. In this embodiment, the effective amount of fluorenylidene malononitrile additive used herein is 0.8% by weight, relative to the amount of electron donor polymer. The PCE performance of sample devices with their active layers treated at various temperatures, e.g., from room temperature to 90 C., is shown at Table 5.

(40) TABLE-US-00005 TABLE 5 Performance of sample devices with their active layers treated at various temperatures: Treatment Temperature ( C.) PCE (%) Room Temp 7.9 40 C. 7.8 70 C. 7.6 90 C. 7.3

(41) As can be noted, the PCE values of these sample devices with fluorenylidene malononitrile (X) additive (Table 5) were superior to those of control devices (Table 1) without the additive at all treatment temperatures.

Example 4

Synthesis of [4-Iso-Pentoxycarbonyl-9-Fluorenylidene]Malononitrile (VIII)

(42) A mixture of 9-fluorenone-4-carboxylic acid (1.0 g, 4.46 mmol), isopentyl alcohol (7.863 g, 89.2 mmol), conc. sulfuric acid (0.05 mL) and toluene (20 mL) in a 100-mL round-bottomed flask fitted with a Dean-stark apparatus and a water condenser was magnetically stirred and refluxed for 24 h before cooling to room temperature. After the reaction, the reaction mixture was evaporated under reduced pressure in the presence of NaHCO.sub.3 (0.1 g). Subsequently, methylene chloride (100 mL) was added and the resulting solution was washed with dilute aq. NaHCO.sub.3 solution (2 times) and with water (2 times), and dried with anhydrous MgSO.sub.4. This was followed by filtration and evaporation under reduced pressure to give the crude product which was purified by flash chromatography on silica gel with hexane/ethyl acetate (volume ratio 15/1) as the eluent to afford iso-pentyl 9-fluorenone-4-carboxylate as a yellow oil (1.27 g, 97%).

(43) A solution of isopentyl 9-fluorenone-4-carboxylate (1.2607 g, 4.28 mmol) as obtained above, malononitrile (0.8488 g, 12.85 mmol), and 1 drop of morpholine in 15 mL methanol was magnetically stirred and refluxed in a 50-mL round-bottomed flask for 24 h. After cooling to room temperature, the solid precipitate was filtered, washed twice with methanol, once with water, and dried in vacuo at 50 C. for 10 h. The crude product was recrystallized from acetone and methanol to yield [4-iso-pentoxycarbonyl-9-fluorenylidene]malononitrile (VIII) as an orange solid, m.p., 126 127 C. (1.31 g, 89.3%).

Photovoltaic Characterization

(44) A series of sample devices with fluorenylidene malononitrile derivative (VIII) additive were prepared in accordance with the procedure of control devices of EXAMPLE 1 except that 0.04 mg of (VIII) was added to the coating solutions. In this embodiment, the effective amount of fluorenylidene malononitrile additive used herein is 0.8% by weight, relative to the amount of electron donor polymer. The performance of the sample devices with their active layers treated at various temperatures, e.g., from room temperature to 90 C., is shown at Table 6.

(45) TABLE-US-00006 TABLE 6 Performance of sample devices with their active layers treated at various temperatures: Treatment Temperature PCE (%) Room Temp 8.0 40 C. 7.8 70 C. 7.7 90 C. 6.8

INDUSTRIAL APPLICABILITY

(46) The present invention relates to the provision of an organic compound or compounds containing a fluorenone derivative structure or its substituted derivative to enhance the thermal stability of organic solar cells to prolong its useful life span.