The C70 Fullerene Adducts with Low Anisotropy of Polarizability are

Article pubs.acs.org/JPCC

The C70 Fullerene Adducts with Low Anisotropy of Polarizability are More Efficient Electron Acceptors for Organic Solar Cells. The Minimum Anisotropy Hypothesis for Efficient Isomer-Free FullereneAdduct Photovoltaics Denis Sh. Sabirov* Institute of Petrochemistry and Catalysis, Russian Academy of Sciences, 450075 Ufa, Russia S Supporting Information *

ABSTRACT: Currently, higher fullerene adducts (bis-, tris-, and multiadducts) come into use as electron acceptor components for organic solar cells (OSCs) with enhanced output parameters. These compounds have numerous positional isomers, and isomerism crucially influences the performances of photovoltaic devices. Indeed, application of the isolated and purified fullerene adducts (isomer-free fullerene-adduct OSCs) allows increasing power conversion efficiency (PCE) compared to the use of the isomeric mixtures. Previously, we have found that OSCs reveal higher PCE if they utilize the C60 bisadducts with the lowest anisotropy of polarizability. To demonstrate a general nature of the found correlation in the present work, we have theoretically studied anisotropy of polarizability of the selected isomeric C70 mono-, bis-, and higher adducts synthesized and separately tested as OSC electron acceptors in recent experimental works. We have found that, as in the case of C60, less-anisotropic C70 derivatives reveal higher PCE of the corresponding photovoltaic devices. Thus, the correlation between anisotropy of polarizability of fullerene acceptors and power conversion efficiency of OSC based on them has a general nature (regardless of the fullerene type and nature of addends), and we can formulate the minimum anisotropy hypothesis: f ullerene adducts with low anisotropy of polarizability are more ef f icient as electron acceptor materials for organic solar cells than their highly anisotropic regioisomers. This conjecture is supported with congruence of relevant experimental and computational data with scarce exceptions and may be recommended as an auxiliary tool for the molecular design and screening of novel fullerene derivatives promising for organic solar cells. The reasons for the importance of polarizability (and its anisotropy) underlying this principle are discussed.

1. INTRODUCTION Fullerene adducts (mainly the derivatives of the most available C60 fullerene) have become one of the widespread electron acceptor compounds for bulk heterojunction OSCs.1−8 These compounds allow achieving power conversion efficiency up to ∼8%,9 and the challenge of enhancing PCE retains a substantial handicap between the fundamental studies and industrial applications of OSCs based on fullerene adducts. Recently, the research interests in this field are gradually shifted to the replacement of the commonly used fullerene monoderivatives by higher fullerene adducts, in which the fullerene cores bear two, three, or more addends (bis-, tris-, or multiadducts, respectively).10−12 This is partially caused by the successful development of synthetic methodologies and chromatographic purification techniques required for producing chemically pure samples of fullerene derivatives with the desired type, number, and location of functionalizing chemical groups (see very recent reviews12−14). Fullerene moieties in the highly functionalized derivatives have a reduced number of readily polarizable π-electrons due to the attachment of extra addends, which, however, usually enhance the miscibility of fullerene derivatives with donor polymers, tune donor− © 2016 American Chemical Society

acceptor molecular orbitals interactions, and, hence, influence the charge photogeneration dynamics.11,12 Fullerene multiadducts have numerous positional isomers (regioisomers) differing in the relative positions of addends (the isomeric diversity was previously analyzed in reviews14−16 and original works17−25). Identification, chromatographic separation, and purification of such isomers are very hard, nontrivial, and expensive tasks. Therefore, in the first works, fullerene bis- and trisadducts were introduced into photovoltaic devices as mixtures of regioisomers.10,26−28 Later, it has been recognized that using the purified regioisomers substantially enhances key output parameters of OSCs. This has been first demonstrated with the solar cells based on the substituted bis(dihyrdonaphtyl)C60 adducts in the foremost work.29 After that advance, the number of studies on such “isomer-free” fullerene-bisadduct organic solar cells started increasing.9,12,30−36 The cited works deal with derivatives of both C60 and C70 fullerenes. Received: September 15, 2016 Revised: October 10, 2016 Published: October 10, 2016 24667

DOI: 10.1021/acs.jpcc.6b09341 J. Phys. Chem. C 2016, 120, 24667−24674

Article

The Journal of Physical Chemistry C

C70ind2 had been used in photovoltaics as a mixture,34,49,50 but their synthesis, separation, and separate testing in OSCs have been performed in a current work.34 Though this is an undoubted breakthrough in the field, the C70ind2 regioisomers have been isolated as fractions containing one major isomer with admixtures of the minor ones. Only six C70ind2 fractions have been tested as acceptor components of OSCs. In their major isomers, only ab bonds located on the opposite poles of the C70 spheroid are functionalized (Figure 1). Their calculated

As known, positional isomers of fullerene derivatives are characterized by almost equal mean polarizabilities and different anisotropies of polarizability (see review,37 experimental,38 and theoretical studies23,24,39−43). Hence, anisotropy may be invoked as an index discriminating the positional isomers. Furthermore, we have found23 that the calculated anisotropies of dihydronaphthyl-C60 bisadducts are correlated with the key output parameters of the corresponding OSCs studied by Kitaura et al.:29 more isotropic positional isomers reveal higher PCE and open-circuit voltages (VOC). This has encouraged us to estimate anisotropy of polarizability of the widespread types of fullerene bisadducts to find out the least anisotropic compounds that could be more effective in the photovoltaic applications.23−25 Unfortunately, we have also found that the correlation has limited use and is applicable only to the sets of isomeric compounds; that is, it does not allow a comparison between fullerene derivatives with different addends and/or fullerene cores.44 In general, the C60 bisadducts were more deeply studied. Comparative studies on OSCs utilizing the purified C70 bisadducts have appeared very recently.34−36 To study a relation between anisotropy and OSC efficacies in the present work, we have calculated anisotropies of polarizability of the C70 derivatives tested in the above-cited works and collated the computational results with the corresponding PCE measurements.

2. CALCULATION DETAILS The PBE/3ζ density functional theory method (Priroda program45,46) were used for calculations. As we have previously shown on a large set of examples, this quantum chemical method accurately describes/predicts mean polarizabilities and anisotropies of fullerenes and their derivatives (see our review37 and key original works18,23,39,40,42,47,48). After standard DFT-optimizations and vibration modes solving (to prove that the stationary points, respective to the molecules under study, are minima of the potential energy surfaces), the components of polarizability tensors α were calculated in terms of the finite field approach as the secondorder derivatives of the total energy E with respect to the homogeneous external electric field F: αij = −

∂ 2E ∂Fi ∂Fj

(1)

Figure 1. Structures of regioisomeric C70−indene bisadducts C70ind2. Their designations within “clock numeration” taken from ref 34 are kept in the present work. Carbon atoms of polar pentagons of C70 are colored in red. Schematics of mutual arrangement of the indene addends relative to polar pentagons of the C70 cage accompany each structure for clarity.

Tensors α were diagonalized, and their eigenvalues αii (i = x, y, and z) were used for calculating mean polarizability α and its anisotropy a2: α=

1 (αxx + αyy + αzz) 3

a2 =

1 ((αyy − αxx)2 + (αzz − αyy)2 + (αzz − αxx)2 ) 2

(2)

values against the measured PCE and VOC of the corresponding solar cells are collated in Table 1. Their mean polarizabilities vary in the narrow range 137.7−138.3 Å3. The difference in anisotropies a2 is more pronounced. The 2 o’clock-C isomer with the highest calculated a2 value reveals the lowest PCE when used in OSC. In contrast, the low-anisotropic 2 o’clock-B C70ind2 demonstrates maximal efficiency (Table 1). Furthermore, we have tried to find a correlation between a2, PCE, and VOC values. The correlations found are inverse (Figure 2). Unfortunately, two isomers (12 o’clock-A and 5 o’clock-C) are out of the correlation. This falling out may be explained by the influence of impurities when C70ind2 were tested. Anyway, this

(3)

The optimized structures of the studied compounds are available as Supporting Information.

3. RESULTS AND DISCUSSION 3.1. OSCs Based on the C70−Indene Bisadducts. The C70−indene bisadducts C70(C9H8)2 (C70ind2; and oftenabbreviated IC70BA) make up one of the presentable series of the regioisomeric C70 derivatives. It should be noted that 24668


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Table 1. Calculated Eigenvalues of Polarizability Tensors (αxx, αyy, αzz), Mean Polarizabilities (α), Anisotropies of Polarizability (a2) of the C70−Indene Bisadducts and Previously Measured Key Output Parameters of Organic Solar Cells Based on These Compounds measured parameters of the fullerene-based OSCs (taken from ref 34)

calculated parameters (this work)

isomer of C70ind2

PCE (%)

VOC (V)

αxx (Å )

αyy (Å )

αzz (Å3)

α (Å3)

a2 (Å6)

12 o’clock-A 2 o’clock-A 2 o’clock-B 2 o’clock-C 5 o’clock-B 5 o’clock-C cc1.1cc2′.1

1.8 3.6 5.2 0.9 3.1 4.4 n/a

0.72 0.78 0.82 0.74 0.76 0.80 n/a

115.74 117.40 115.90 117.24 118.06 117.07 122.50

132.48 126.80 132.51 119.51 126.63 120.05 138.00

164.94 169.73 164.80 178.09 169.22 177.85 142.39

137.72 137.97 137.74 138.28 137.97 138.32 134.30

1877.37 2335.13 1854.64 3569.68 2252.32 3521.43 327.42

3

3

Figure 3. Structure and schematic of addition of the proposed lowanisotropy positional isomer cc1.1cc2′.1-C70ind2. The functionalized cc bonds are shown in red.

Thus, based on the anisotropy approach, we recommend this isomer of C70ind2 for targeted synthesis and further use in solar elements to enhance their PCE and VOC parameters. We should unfortunately note that cc-adducts were not detected in the reactions yielding the indene derivatives of C70 in ref 34. Nevertheless, the cc bonds are known as the reactive sites of addition to the C70 molecule,15,51 and we hope that obtaining the cc-adducts of this type is a solvable question for synthetic methodology and/or isolation techniques. 3.2. OSCs Based on the Other C70 Higher Adducts. In the second part of the study, we pay attention to other types of the C70 derivatives. These are positional isomers of bisPC71BM (C94H28O4)35 and a complex C70−oxazoline adduct with additional functionalization (C112H38N2O2).36 Only two isomers of each compound were tested in OSCs. Nevertheless, these examples provide the arguments in favor of the anisotropy effect on the OSC performances. Two isomers of bisPC71BM synthesized, separated, and tested in OSC by Deng et al.35 are shown in Figure 4 and called cis- and trans-bisPC71BM as in the mentioned experimental work. This indication clearly describes the structures of two positional isomers: the same addition pattern and different relative arrangement of the substituents in the cyclopropane ring (Figure 4). This difference in the structure affects the a2 values: according to our calculations, trans-isomer is less anisotropic. As shown experimentally,35 this isomer is more effective in OSC (PCE = 1.84%) than its more anisotropic ciscounterpart (PCE = 1.72%) (Table 2). Another pair of the C70 regioisomeric derivatives have an untypical addition pattern:36 two oxazoline moieties (oxz) are annulated to aa bonds located on the opposite poles next to each other. These two addends are additionally surrounded by the benzyl groups attached to the nearest b atoms of the C70

Figure 2. Correlation between the output parameters of organic solar cells (PCE and VOC) based on C70−indene bisadducts and their anisotropies of polarizabilities. The PCE and VOC values are taken from previous experimental work.34 The points corresponding to the isomers, which do not fit the correlation, are circumscribed.

correlation covers four isomers. Notably, the similar relation, when the low-a2 regioisomers occurred most efficiently in OSCs, was previously observed for the C60-dihydronaphthyl bisadducts.23 Using the regioisomers as isotropic as possible is the recommendation point within the anisotropic approach to obtain higher PCE/VOC values of the C70ind2 OSCs. The number of possible positional isomers of C70ind2 equals 31 (taking into account numbers of reactive ab and cc bonds and steric effects to locate large indene addends on the C70 skeleton).24,25 The isomers with the reduced anisotropy of polarizability can be uncovered without necessity to calculate all of them. Previously,24 we have shown that the addition of both addends to the cc bonds located in different hemispheres of C70 substantially reduces anisotropy of polarizability of o-quinodimethane-methano[70]fullerenes C70(CH2)C8H8. The least anisotropic C70(CH2)C8H8 has the addition code cc1cc2′.1.24 It is noteworthy that molecular size of the o-quinodimetane moiety (C8H8) is close to that of the indene addend (C9H8). Therefore, we have calculated the isomer of C70ind2 with the same location of the addends (Table 1, last row). Note that the nonplanar indene moieties are arranged “face-to-face” in this compound (Figure 3). Our assumption is correct, and for the isomer, we obtain the anisotropy of polarizability reduced to 327.42 Å6 (the above-mentioned C70ind2 regioisomers with abfunctionalization have the a2 values greater than 1800 Å6). 24669


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Figure 4. Formulas and structures of two regioisomers of bisPC71BM synthesized and tested in ref 35.

Table 2. Calculated Eigenvalues of Polarizability Tensors (αxx, αyy, αzz), Mean Polarizabilities (α), Anisotropies of Polarizability (a2) of the C70 Bis- and Hexakisadducts and Previously Measured Key Output Parameters of Organic Solar Cells Based on These Compounds measured parameters of the fullerene-based OSCs (taken from ref 35 and 36)

calculated parameters (this work)

C70 derivative

PCE (%)

VOC (V)

αxx (Å3)

αyy (Å3)

αzz (Å3)

α (Å3)

a2 (Å6)

cis-bisPC71BMa trans-bisPC71BMa cis-C70oxz2(CH2Ph)4 trans-C70oxz2(CH2Ph)4

1.72 1.84 0.55 0.46

0.69 0.72 0.95 0.95

130.93 134.16 171.24 171.34

142.03 143.27 174.10 173.98

194.94 189.74 234.74 234.83

155.97 155.72 193.36 193.38

3510.08 2666.31 3858.34 3871.18

a Designated as in experimental work.35 In previous works, cis-bisPC71BM and trans-bisPC71BM are also called as ab1.1ab1′.1 and ab1.1ab1′.2 isomers, respectively.24

dissimilarity, these two isomers reveal very close values of a2. Nevertheless, the calculations indicate the cis-C70oxz2(CH2Ph)4 molecule less anisotropic, and the experiments36 show that this isomer reveals higher PCE (0.55%) than the trans-counterpart (0.46%). These two examples show that the conjecture about higher power conversion efficiencies of the organic solar cells based on fullerene derivatives with lower anisotropy of polarizability remains true in the case of other higher adducts of C70. 3.3. OSCs Based on the C70 Monoadducts. Isomerism is also typical to monoadducts of C70 because its molecule contains the inequivalent bonds. Among them, the ab and cc bonds are the most reactive.53 Chemical functionalization of these bonds leads to 1 isomer in the case of symmetric addends. In the case of a nonsymmetrical moiety added to C70, 1 and 2 regioisomeric structures are theoretically possible for ab and cc additions, respectively.14,52 Such C70 monoadducts, PC71BM (1 pure isomer ab and mixture of 2 cc-isomers, C82H14O2) and C 70 dhn (2 isomers, C 88 H 24 O 4 , dhn is a substituted dihydronaphthyl moiety) have been recently obtained and tested in OSCs by Umeyama et al. (Figure 6).52 We have collated the results of the experimental work with our quantum chemical calculations. For the PC71BM isomers, we additionally provide the results of the previous theoretical study43 performed with another method to demonstrate the reproducibility of the computational results. Both PBE/3ζ and B3LYP/ 6-311G(d,p) methods indicate almost equal mean polarizabilities of the PC71BM isomers and the increase in the anisotropy in the series: cc1.1-PC71BM < cc1.2-PC71BM < abPC71BM (Table 3). As the most anisotropic compound abPC71BM revealed lower PCE value (6.20%) as compared to the

cage (Figure 5). The only difference between two regioisomers of C70oxz2(CH2Ph)4 is in the relative position of N and O atoms in two oxazoline rings. We designated these isomers accordingly cis and trans (Figure 5). Their calculated anisotropies of polarizability and the measured PCE and VOC values are presented in Table 2. Due to the minor structural

Figure 5. Formulas and structures of two regioisomers of C70oxz2(CH2Ph)4 synthesized and tested in ref 36. Hydrogen atoms are omitted for clarity. 24670


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higher PCE values. This remains true in most cases and has only two exceptions (two isomers of C70ind2 and C70dhn). Previously, a similar tendency has been found for the C60 bisadducts.23 Thus, low anisotropy of polarizability of the fullerene adducts corresponds to higher power conversion efficiency of organic solar cells based on them. This hypothesis is verified by most of the studied compounds of C60 and C70. As for limitations, it is applicable only to the isomeric fullerene derivatives (we have shown this by extrapolating the rule to a large set of nonisomeric C60 monoadducts44). Nonetheless, the use of this hypothesis may narrow down the list of potential fullerene adducts and concentrate the synthetic and purification efforts on the producing of the most promising compounds. Polarizability is a fundamental quantity important for diverse physicochemical processes, and therefore, its influence obviously has a complex nature, and our suggestions about its relation to OSCs are quite speculative. Additionally, the calculated values correspond to molecular property, whereas the measurements reflect the dynamic behavior of the bulk material. Nevertheless, we can mention four points of importance of polarizability for acting OSCs. The first point deals with the dependence of dielectric permittivity ε on the polarizability according to Clausius−Mossotti equation (ρ is a density of the matter):

Figure 6. Formulas and structures of the C70 monoadducts synthesized and tested in ref 52.

less anisotropic regioisomers with cc-functionalization (PCE = 6.46% for their mixture).52 In the case of two C 70 dhn regioisomers, we have unfortunately obtained the inconsistent result. Here, higher PCE value (4.04%) corresponds to the most-anisotropic regioisomer ab-C70dhn (Table 3) that breaks the regularity “lower a2 − higher PCE”. Thus, there are exceptions from this correspondence, but in most cases with respect to the C70 adducts (and the C60 adducts studied previously23), our hypothesis works. 3.4. Possible Role of Fullerene-Adduct Polarizability in Organic Solar Cells. We have calculated anisotropy of polarizability of diverse C70 adducts tested previously in OSCs. It is very important that assessing the output parameters of the devices utilized isomeric compounds were performed under the same experimental protocols (the same donor polymers, donor−acceptor ratios, ways of fabrication of OSCs and their testing within each isomeric set), so the devices differed only in the fullerene-adduct component. This allows comparing the measured PCE (and other parameters). As the present study shows, organic solar cells containing regioisomers of the C70 fullerene adducts with low anisotropy of polarizability reveal

−1 ⎛ ρα ⎞ ρ2 α 2 ⎟ ε = 1 + ρα⎜1 − + ⎝ 3 ⎠ 3

(4)

Dielectric constant is considered as a central parameter decisive for choosing the most efficient regimes for OSCs54 Traditionally, this quantity is used in such studies in a scalar form. Based on the anisotropy effect found, tensorial nature of dielectric permittivity and/or its anisotropy should be taken into account to improve the existing models for assessing the OSC efficacy. The most recent studies have confirmed the importance of polarizability and dielectric phenomena on the recombination processes on the donor−acceptor interfaces in OSCs.55,56 In addition, low dielectric constant (and, in a context of the present work, possibly its low anisotropy) is considered as one of the requirements for molecular design of nonfullerene OSC acceptors.57 The second point deals with the polarizability of fullerene charge-transfer complexes generated upon OSC processing.58

Table 3. Calculated Eigenvalues of Polarizability Tensors (αxx, αyy, αzz), Mean Polarizabilities (α), Anisotropies of Polarizability (a2) of the C70 Monoadducts and Previously Measured Key Output Parameters of Organic Solar Cells Based on These Compounds calculated parameters (this work) C70 derivative

PCE (%)

VOC (V)

αxx (Å )

αyy (Å )

αzz (Å3)

α (Å3)a

a2 (Å6)a

ab-PC71BM

6.20

0.80

113.29

118.43

155.46

129.06 (116.9)

1588.09 (1193.17)

111.73

124.15

147.58

111.51

122.38

150.17

127.82 (114.1) 128.02 (114.5)

994.10 (671.84) 1192.10 (952.11)

cc1-PC71BM

3

6.46

0.85

ab-C70dhn

4.04

0.81

120.42

132.16

181.78

144.78

3182.74

cc-C70dhn

2.44

0.75

122.44

134.10

170.49

142.34

1884.01

cc2-PC71BM

a

3

The results of the B3LYP//6-311G(d,p) calculations are taken from previous work.43 24671



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Polarizabilities of the excited states of such complexes exceed 2000 Å3.58 Thus, these entities should be strongly influenced by external electric fields, and their anisotropy of polarizability may affect the charge-transfer complexes decay. Unfortunately, no common trends are currently known as experimental studies of the effect of the electric field on the dissociation give conflicting results (see prospective article59). Third, anisotropy of polarizability affects interface processes (e.g., wetting). As is known, the polarizabilities of wetting liquid αL and wetted solid αS can be used to estimate contact angle of wetting θc according to de Gennes equation:60 cos θc =

2αS −1 αL

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09341. Cartesian coordinates of the optimized fullerene derivatives (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +7 (347) 284 27 50. Notes

The author declares no competing financial interest.

■ ■

(5)

ACKNOWLEDGMENTS This work was financially supported by Russian Foundation for Basic Research (project 16-03-00822).

The OSC composite materials may be roughly approximated as the phase of fullerene adduct (αL parameter) wetting the surface of the polymer phase (αS in eq 5). This seems reliable in the context of the experimental work,26 in which wetting and surface tension were efficiently used to explain the observed molecular structure−device function relationship for OSCs based on the o-xylenyl bisadducts of C60 with different side solubilizing chemical groups. At last, anisotropy of polarizability is associated with the shape of the acceptor molecule. This may be decisive for packing and ordering the acceptor and/or donor−acceptor phases (i.e., influences on morphology of the OSC materials). Effect of disorder (primarily structural or steric) on OSC efficacy is currently under meticulous investigation,61,62 and some studies even stress that PCE depends on morphologydominated properties rather than energetic parameters.63

REFERENCES

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4. CONCLUSIONS In the present work, we have computationally studied polarizability of several isomeric sets of the C70 adducts (C 70 ind 2 , bisPC 71 BM, C 70 oxz 2 (CH 2 Ph) 4 , PC 71 BM, and C70dhn2) recently tested as electron acceptors in organic solar cells. We have found that, in general, the devices utilizing the C70 adducts with low anisotropy of polarizability reveal higher power conversion efficiencies as compared to their highly anisotropic regioisomers. The cases deviating from this trend are C70 monoadducts with substituted dihydronaphthyl moieties and two isomers of C70−indene bisadduct. We do not discuss here the reasons of these mismatches as their scrutinizing requires additional experimental and theoretical studies. In the case of the C70−indene bisadducts, the deviation from the common trend may be associated with the impurity of the samples. Uniting the results of the present study with the previous similar relationship for C60 bisadducts (correspondence “low a2 − high PCE”), we formulate the minimum anisotropy hypothesis: fullerene adducts with low anisotropy of polarizability are more efficient as electron-acceptor materials for organic solar cells than their highly anisotropic regioisomers. We stress that this proposition is suitable for comparison for only regioisomeric fullerene adducts. Taking into account high numbers of isomeric structures corresponding to the C60 and C70 higher adducts and difficulties of their targeted synthesis and/or separation, the application of this rule may facilitate screening and producing novel fullerene derivatives promising for organic photovoltaics. 24672


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The C70 Fullerene Adducts with Low Anisotropy of Polarizability are

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