PCBM and PCBM-like derivatives

As part of an effort to develop soluble fullerene derivatives for applications to physics and biology [20], phenyl-C61-butyric acid methyl ester (PC60BM) has been first synthesized in 1995 by the group of Prof. Wudl at the University of California in Santa Barbara [21]. It was PC60BM which—due to its sufficient solubility in solvents used for donor-type polymers—allowed for the first successful demonstration of a BHJ device [6], followed by steadily improved performance of PC60BM-based devices.

The solubility of PC60BM in toluene and chlorobenzene is 35 and 50 mg/ml, respectively. As compared to C60, the absorption spectrum of C70 is significantly extended in the visible part of the spectrum, its derivatives have the potential of increased contribution to photocurrent and therefore better performance. Therefore, PC70BM was synthesized [22]. The chemical structures of PC60BM and PC70BM are shown in Fig. 3.5. Unlike PC60BM, PC70BM was found to be a mixture of three different isomers (a, p, and y, as shown in Fig. 3.6). At 80 mg/ml, in chlorobenzene, its solubility was found to be better than that of PC60BM. Since the discovery of PC60BM and PC70BM, they have been the most utilized acceptor materials and used with a large range of donors, resulting in encouraging device characteristics in many cases. Although many new acceptors have been synthesized and evaluated in OPV devices, no general replacement for PCBM allowing for higher performance has been identified to date. The commercial availability of PCBM and the resulting huge body of work optimizing devices and materials (including the p-phase) in combination with PCBM has made the quick development of alternatives challenging.

image126

Figure 3.5 Chemical structures of PC60BM and PC70BM.

image127

Figure 3.6 Three different isomers of C70 monoadduct. A (a), B (p) and

C (Y).

PCBM is synthesized by [3 + 2] cycloaddition reaction between fullerene and diazomethane [21,22]. The synthetic route of PC60BM is shown in Fig. 3.7. While a [5,6] fulleroid, i. e., with a C-C bond cleaved in the fullerene cage, is formed in the initial addition, isomerization, accelerated, for example, by reflux in o-dichlorobenzene, leads to the final, thermodynamically stable [6, 6]-methanofullerene.

PC70BM was synthesized by a similar method with the exception of using C70 to replace C60 [22]. As mentioned above, due to the asymmetric shape of C70, three different [6, 6] addition isomers for its monoadduct are formed. The percent of a isomer was found to be 85%, while в and y, combined, is 15% of the compound. Due to its improved absorption characteristics [22], the acceptor material in many high-performance low-bandgap PSCs has been PC70BM.

enzoylbutyrate methyl 4-benzoylbutyrate p-Tosylhydrazone

image128

PC BM

Подпись:Подпись:Подпись:Подпись:Подпись:image134A lot of modifications to PC60BM and PC70BM have been done and many novel fullerene acceptor materials with PCBM-like structures have been designed and synthesized, as shown in Figs. 3.8 and 3.9. Modifications include the change of the ester group, substitutions on the phenyl ring attached to the cyclopropane carbon (the methano – bridge) or the replacement of the phenyl, e. g., by thiophene. Also, PCBM-based endohedral fullerenes or the replacement of the methano-bridge by nitrogen have been investigated. Electrochemical reduction potentials and LUMO levels of some fullerene derivatives are summarized in Table 3.3, while the photovoltaic performances based on these materials are given in Table 3.4. In the following part, we will talk about the personalities and properties of these materials in detail. Zheng et al. [23] synthesized fullerene derivatives F1-F4. As shown in Table 3.3, these substituted PCBM derivatives F1-F4 have almost identical reduction potentials, which indicate that the variation of the alkyl chain length of the ester group does not affect the reduction potential of PCBM. Therefore, their LUMO levels are nearly same. In a subsequent set of experiments, MEH-PPV was used as donor and these materials were used as acceptors to fabricate PSC devices. The devices using acceptor material with longer alkyl group showed lower Jsc, indicating a reduction of the charge-carrier mobility. Seri et al. [24] reported another PCBM derivative F5, bearing a t-butyl-ester group. Having the same LUMO and HOMO levels as PCBM, the purpose of designing F5 was to achieve efficient electron transport, good solubility in common organic solvents, and enhanced molecular hindrance able to influence the intermolecular interactions in the active layer. However, the PCE of P3HT: F5 was

only 3.03% which is lower than the 3.48% observed for P3HT: PCBM under the same conditions (AM 1.5, 100 mW/cm2), which was mainly caused by the lower Jsc in the former system [24]. Mobility results show PCBM and F5 possessing the same electron mobility (7.0 x 10-4 cm2/V • s) in their mixed films with P3HT whereas the mobilities of pristine F5 and PCBM films were found to be 2.1 x 10-3 cm2/V • s and 2.5 x 10-3 cm2/V • s, respectively. The solubility of F5 in chlorobenzene and toluene is not very good resulting in a low-quality film when spin-coated from these solutions and therefore showing a decreased device Jsc. Yang et al. [25] designed and synthesized F6 and F7. These two materials are readily soluble in common organic solvents (toluene, dichlorobenzene, etc.). They both have more negative reduction potentials than that of PCBM (i. e., less negative LUMO levels) which was attributed to the electron-donating (+I) effect of the alkoxy-group. Compared to PCBM, higher LUMO levels of F6 and F7 are consistent with the increased Voc shown in Table 3.4. P3HT:PCBM films appear to have a better morphology than the corresponding active layers consisting of F6 or F7 mixed with P3HT, resulting in a higher Jsc and overall performance of P3HT: PCBM devices. In agreement with this finding, OFET mobilities of PCBM, F6 and F7 are 2.85 x 10-2 cm2/V • s, 1.02 x 10-2 cm2/V • s and 1.59 x 10 3 cm2/V • s, respectively.

image135

Figure 3.8 Chemical structures of PCBM-like monoadducts.

Table 3.3 Reduction potentials of some fullerene derivatives

E1(red) (v)

E2(red) (v)

E3(red) (v)

Reference

PCBM

-0.64

-1.05

-1.56

[23]

F1

-0.63

-1.05

-1.55

[23]

F2

-0.67

-1.16

-1.6Q

[23]

F3

-0.64

-1.0Q

-1.57

[23]

F4

-0.65

-1.0Q

-1.62

[23]

F5

-1.101

-1.40Q

-1.QQ6

[24]

PCBM

-1.076

[24]

F6

-1.171

-1.544

-2.048

[25]

F7

-1.181

-1.565

-2.087

[25]

PCBM

-1.163

-1.538

-2.040

[25]

F14

-0.8Q

-1.28

-1.7Q

[28]

F15

-0.88

-1.2Q

-1.84

[28]

F16

-0.Q0

-1.30

-1.81

[28]

PCBM

-0.88

-1.27

-1.78

[28]

F21

-0.58

-1.07

-1.51

[31]

PCBM

-0.57

-0.Q8

-1.51

[31]

image136
image137

Renz et al. [26] have investigated the fullerene solubility – current density relationship in PSCs. They synthesized fullerene derivatives F8, FQ, F10 and F12, with solubilities (in chlorobenzene) of 5 g/L, 30 g/L, 22 g/L and 106 g/L, respectively. Films of these fullerene derivatives mixed with P3HT were investigated.

Films with low-solubility fullerenes show micrometer-sized fullerene aggregates, and a significant increase in size or number of those aggregates was observed after annealing. With rising fullerene solubility, these microscopic aggregates become smaller and vanish. Solar cells made with lower solubility fullerene acceptors were found to have low Jsc, but which increased with increasing solubility until reaching a solubility of approximately 25 mg/ml where the current density saturates. Troshin et al. [27] systematically investigated the fullerene solubility – device performance relationship in PSCs. They synthesized F11, F13, F22, F23, F24, F25 and F26, tested their solubility and photovoltaic performance mixed with P3HT. Solubilities of these compounds in chlorobenzene have been reported to be 5 mg/ml, 19 mg/ml, 23 mg/ml, 45 mg/ml, 70 mg/ml, 36 mg/ml and 58 mg/ml, respectively. The Jsc of devices with these acceptors mixed with P3HT increased with rising fullerene solubility. They found the solubility of the fullerene acceptor in chlorobenzene should be in the range from 30 mg/ml to 80 mg/ml for achieving high Jsc and FF. Zhao et al. [28] synthesized fullerene derivative F14, F15 and F16. They have nearly the same reduction potentials and LUMO levels as PCBM. The photovoltaic performance of F14 and F16 was found to be inferior to that of PCBM devices, while the PCE when using F15 is similar to that of PCBM when using P3HT as donor material. Moriwaki et al. [30] synthesized fullerene derivative F20, EThCBM, in which the phenyl of PCBM is replaced by 5-ethylthiophene. The first reduction potentials for F20 and PCBM are -1.147 V and -1.153 V, respectively, corresponding to nearly identical LUMO levels and also resulting in very similar power conversion efficiencies in conjunction with P3HT. F21, bearing a 3,4-dimethoxythiophene group instead of phenyl, in a structure otherwise identical to PCBM, was synthesized by Choi et al. [31] and a first reduction potential again nearly identical to that of PCBM was found. However, mixed with P3HT introducing two methoxyl groups, the device performance was greatly reduced. Zhang et al. [33] synthesized two fullerene derivatives F28 and F29. Unlike PCBM, which is crystalline, F28 and F29 are amorphous materials. In order to suppress crystallization, the phenyl group in PCBM was replaced by bulky triphenyl amine (TPA) or 9, 9-dimethylfluorene (MF) resulting in TPAC60 (F28) and MFC60 (F29). OFET mobilities of PCBM, F28 and F29 are 1.6 x 10-2 cm2/V • s, 1.1 x 10-2 cm2/V • s and 5.4 x 10-3 cm2/V • s, respectively. The thermal stability of photovoltaic devices based on F28 and F29 was found to be better than that of PCBM but PCE was slightly lower when used with P3HT. The thermal stability of active layers consisting of amorphous fullerene derivatives and P3HT has been confirmed by Kim et al. [34] using a dihexyl-fluorene-modified PCBM (F30).

Mikroyannidis et al. [35] designed and synthesized a novel PCBM derivative F31 in which the methyl group of the ester was replaced by a larger dye-like unit (Fig. 3.9). F31 has better solubility than PCBM in common organic solvents. Its solubility in THF is ca. 40 mg/ml, compared to the 25 mg/ml for PCBM. The absorption of F31 in the range from 300 to 900 nm is significantly stronger than that of PCBM, due to the contribution of the dye part. The film absorption coefficient ranged from 5.3 x 104 to 2.5 x 104 M-1 cm-1 for F31 and from 2.4 x 104 to 1.2 x 104 M-1 cm-1 for PCBM. F31 has 0.2 eV higher LUMO level than PCBM and, correspondingly, the device Voc increased from 0.68 to 0.81 V using P3HT as donor material in both cases. Park et al. [36] synthesized two PCBM-like derivatives in which the bridging carbon of the cyclopropane group is replaced by nitrogen. The electron mobilities of these two iminofullerene isomers [5,6], APCBM (an open azafulleroid) and [6,6] APCBM (a closed aziridinofullerene), were found to be 4.1 x 10-2 and 2.3 x 10-2 cm2/V • s, respectively. Consistent with this observation, PCEs of devices having in addition P3HT in the active layer were reported to be 2.8% for the [5,6] and 2.3% for [6,6] isomer.

Ross et al. [37] designed an endohedral fullerene derivative Lu3N@ C80-PCBH (phenyl-Lu3N@C81-butyric-acid-hexyl ester) bearing the same functional group as PCBM (but with a hexyl-ester). Compared with conventional, hollow, fullerenes, endohedral fullerenes have higher LUMO levels due to the interaction between the metals and the fullerene cage. The hexyl ester has been chosen because of its similarities in solubility and miscibility to PCBM (a methyl ester) when processing P3HT-based OPV devices. The first reduction potentials of Lu3N@C80-PCBH and PCBM are reported by Ross et al. [37] to be -1.500 and -1.220 V, respectively, indicating that the former material has a significantly higher LUMO level than the latter one. Their mobilities were found to be 4.0 x 10-4 and 1.4 x 10-3 cm2/V • s, respectively.

Table 3.4 Photovoltaic performance of fullerene acceptors

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Reference

F1

0.85

5.22

43.2

2.45

[23]

F2

0.80

3.48

35.8

1.27

[23]

F3

0.80

2.86

35.4

1.04

[23]

F4

0.50

0.48

34.0

0.11

[23]

PCBM

0.85

4.61

39.9

2.00

[23]

F5

0.603

7.64

66.0

3.03

[24]

PCBM

0.588

8.85

67.0

3.48

[24]

F6

0.65

4.43

35.0

1.01

[25]

F7

0.64

5.83

46.0

1.73

[25]

PCBM

0.60

6.98

43.0

1.81

[25]

F8

0.36

0.70

36.0

0.10

[26]

F9

0.62

8.10

43.0

2.20

[26]

F10

0.64

8.40

52.0

2.80

[26]

F11

0.35

0.40

35.0

0.05

[27]

F12

0.62

8.40

48.0

2.50

[26]

F13

0.64

7.90

53.0

2.70

[27]

F14

0.535

8.10

53.2

2.30

[28]

F15

0.596

9.90

61.5

3.60

[28]

F16

0.540

9.30

56.4

2.80

[28]

PCBM

0.571

9.60

64.6

3.50

[28]

F17

0.52

7.82

56.0

2.31

[29]

F18

0.50

7.23

54.0

2.00

[29]

F19

0.51

7.59

49.0

1.88

[29]

F20

0.628

6.10

58.7

2.25

[30]

PCBM

0.619

6.12

58.5

2.22

[30]

F21

0.53

2.13

29.0

0.33

[31]

PCBM

0.62

10.9

62.0

4.18

[31]

F22

0.60

8.40

50.0

2.50

[27]

F23

0.60

10.16

54.0

3.40

[27]

F24

0.60

9.00

53.0

2.90

[27]

F25

0.60

10.6

58.0

3.70

[27]

F26

0.60

7.90

33.0

1.20

[27]

PCBM

0.64

10.60

55.0

3.70

[27]

F27

0.59

9.00

66.0

3.54

[32]

F28

0.65

9.90

62.0

4.00

[33]

F29

0.65

9.80

59.0

3.80

[33]

PCBM

0.63

10.4

64.1

4.20

[33]

F30

0.68

8.90

52.0

3.16

[34]

F31

0.81

10.30

63.0

5.25

[35]

PCBM

0.68

8.0

54.0

2.93

[35]

[5,6]APCBM

0.58

8.10

60.0

2.80

[36]

[6,6]APCBM

0.58

7.14

56.0

2.30

[36]

Lu3N@C80-PCBH

0.81

8.64

61.0

4.20

[37]

PCBM

0.63

8.90

61.0

3.40

[37]

In general, in the above-described investigations, LUMO energy levels of acceptor materials were tuned by changing the electron donating and withdrawing ability of the units attached to the fullerene core while all other parameters, particularly the donor material, remained the same. The Voc of the devices then changed correspondingly. Due to reasons such as limited solubility, electron mobility or miscibility with donor materials, Jsc and FF of most photovoltaic devices based on such new electron-acceptor materials were found to be lower than those of PCBM devices. Among the molecules discussed to this point, as summarized in Tables 3.3 and 3.4, F31 and Lu3N@C80-PCBH appear to be the most interesting. F31, described above, is a PCBM-like molecule in which the methyl- of the ester group has been replaced by an organic dye, increasing the absorption of the material and therefore Jsc. Lu3N@ C80-PCBH is an endohedral fullerene derivative, and its LUMO energy level is significantly higher than that of PCBM, leading to improved performance, driven by a higher Voc when combined with P3HT. However, limited scalability of the synthesis of endohedral fullerenes, currently carried out by electric arc, and therefore rather high prices, are expected to be major hurdles for the commercial use of endohedral fullerenes and their derivatives.

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