Electronic structure of fullerene derivatives in organic photovoltaics

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Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel 5 6

Electronic structure of fullerene derivatives in organic photovoltaics

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a

8 9

b

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Department of Physics, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Research Laboratory for Surface Science, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama-shi, Okayama 700-8530, Japan

a r t i c l e

1 8 4 2 15 16 17 18 19 20 21 22 23 24 25 26 27

Rie Nakanishi a,⇑, Ayumi Nogimura a, Ritsuko Eguchi b, Kaname Kanai a

i n f o

Article history: Received 14 May 2014 Received in revised form 3 August 2014 Accepted 5 August 2014 Available online xxxx

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Keywords: Fullerene derivatives Electronic structure Organic photovoltaic (OPV) Ultraviolet photoelectron spectroscopy (UPS) Inverse photoemission spectroscopy (IPES)

a b s t r a c t The electronic structures of the fullerene derivatives [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), [6,6]-diphenyl C62 bis (butyric acid methyl ester) (bisPCBM), C70, [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM), [6,6]-phenyl-C61-butyric acid butyl ester (PCBB), [6,6]-phenyl-C61-butyric acid octyl ester (PCBO), [6,6]-thienyl-C61-butyric acid methyl ester (TCBM), and indene-C60 bisadduct (ICBA), which are frequently used as n-type materials in organic photovoltaics, were studied by ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy. We also performed molecular orbital calculation based on density functional theory to understand the experimental results. The electronic structures near the energy gap of the compounds were found to be governed predominately by the fullerene backbone. The side chains also affected the electronic structures of the compounds. The ionization energy and electron affinity were strongly affected by the number of carbons and functional groups in the side chain. Ó 2014 Elsevier B.V. All rights reserved.

29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

44 45

1. Introduction

46

Organic photovoltaics (OPV), particularly OPV devices containing a polymer/fullerene-based bulk heterojunction (BHJ), have attracted much interest because of their potential for low-cost, large-area, lightweight, and flexible devices with simple structures [1–3]. The fullerenes C60 and C70 and their derivatives bearing various functional group side chains have been used as n-type semiconductor materials in OPV devices with high-efficiency photoelectric conversion [4–8]. C60 and C70 are incompatible with solution processes because of their low solubility in common organic solvents. Soluble derivatives have been synthesized by adding functional groups to these fullerene backbones [9,10], thus allowing OPV devices to be fabricated using solution processes such

47 48 49 50 51 52 53 54 55 56 57 58 59

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⇑ Corresponding author. E-mail address: [email protected] (R. Nakanishi).

as spin-coating [11]. [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) is a well-known soluble derivative and has been frequently used as an acceptor in OPVs [4,9]. Electronic structure of the fullerene derivatives has been investigated by some groups so far [12–16]. It has been reported that the side chains of PCBM and [6,6]-phenylC71-butyric acid methyl ester (PC70BM) affect the solubility and morphology of the film, and its electronic structure, which may improve device performance [14–16]. OPV performance, particularly optical absorption, carrier injection, and carrier transport, strongly depends on the electronic structure of the donor and acceptor molecules [17,18]. The electronic structure around the Fermi level (EF), such as the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), plays an important role in determining the optical and transport properties [19]. For example, the correlation between the electronic structure of the donor or acceptor molecules and the open-circuit voltage (VOC) of the device, which is an energetic driving force for electron transfer

http://dx.doi.org/10.1016/j.orgel.2014.08.013 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

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from the donor to the acceptor, is still not fully understood. It is thought that VOC is related to the difference between the LUMO energy of the acceptor and the HOMO energy of the donor [20,21]. Furthermore, the excitons, which are created after light absorption and migrate to the donor/acceptor interface, separate into electrons in the LUMO of the acceptor and holes in the HOMO of the donor. Thus, understanding the electronic structures of donor and acceptor molecules is important for elucidating the mechanisms by which OPV devices operate and for optimizing materials for high-performance devices. Akaike et al. [14,15] reported the effects of the side chain on the electronic structure of PCBM and [6,6]-diphenyl C62 bis(butyric acid methyl ester) (bisPCBM). They concluded that a subtle charge transfer from the side chain to the C60 backbone destabilizes the electronic states of the molecule. They also suggested that the effects of the side chain on the electronic structures of PCBM and bisPCBM may improve the performance of the OPV devices compared with devices containing C60 [22,23]. Their work demonstrates that measures of OPV device performance, such as VOC, JSC, and fill-factor, can be discussed in terms of electronic structure. There are few other studies of the electronic structure of fullerene derivatives. The purpose of this study is to systematically investigate the electronic structure of the fullerene derivatives used in OPVs. Fundamental information about the electronic structure of fullerene derivatives can be expected to guide the synthesis of new molecules optimized for high-performance OPVs. The electronic structures of the fullerene derivatives PCBM, bisPCBM, C70, PC70BM, [6,6]-phenyl-C61-butyric acid butyl ester (PCBB), [6,6]-phenyl-C61-butyric acid octyl ester (PCBO),

O O

C60

[6,6]-thienyl-C61-butyric acid methyl ester (TCBM), indene-C60 monoadduct (ICMA), and indene-C60 bisadduct (ICBA) (Fig. 1) were examined by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). To interpret the experimental results, molecular orbital (MO) calculations were performed. The electronic structures of the fullerene derivatives strongly depended on structural features, including the type of backbone, number of side chains, side chain length, and functional groups. We investigated the effect of side chains by comparing C70 with PC70BM using the same method as Akaike et al. [14,15] The dependence of the electronic structure on the fullerene backbone and the side chain length are discussed by comparing PCBM, PC70BM, PCBB, and PCBO. The difference in electronic structure caused by replacing a phenyl group with a thienyl group in the side chain is also investigated by comparing PCBM with TCBM. The effect of introducing a different type of side chain on the electronic structure was examined by investigating ICBA.

113

2. Experimental and theoretical procedures

133

PCBM (>99.9%), bisPCBM (mixture of isomers, 99.5%), C70 (99%), PC70BM (mixture of isomers, 99%), PCBB (>97%), PCBO (>99%), TCBM (>99%), and ICBA (99%) were purchased from Sigma–Aldrich and used as received. Thin films of PCBM, bisPCBM, PC70BM, PCBB, PCBO, TCBM, and ICBA were spin-coated from chlorobenzene solution (0.4 wt%) in a glovebox filled with N2 at room temperature. The films were spin-coated onto indium tin oxide (ITO)-coated glass substrates at 1500 rpm for 30 s and transferred to a vacuum chamber under N2. The

134

O

CH3

H3C

PCBM

CH3

O

O O

bisPCBM O

O

CH3

O

O

PCBB

PCBO

CH3

S

O O

CH3

TCBM

O O

C70

PC 70 BM

CH3

ICMA

ICBA

Fig. 1. Molecular structures of fullerenes and derivatives: C60, PCBM, bisPCBM, PCBB, PCBO, TCBM, C70, PC70BM, ICMA, and ICBA.

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(a) cutoff

(b)

UPS

Evac = 0

IPES

π

3.1

3.2 3.5

3.6

π∗

ICBA

3.4

3.7

π∗

3.8

4.1

EF = (3.89)

(3.87)

(3.93)

(3.90)

1.8

2.2

(3.86) (4.10)

(4.18)

TCBM Eg =

2.1

2.5

5.66

5.65

1.9

2.3

(3.97)

2.0

2.6

5.68 5.71 5.83 6.02 PCBM bisPCBM C70 PC70BM PCBB PCBO TCBM

5.64

PCBO Intensity / arb. units

π PCBB

PC 70 BM C70

bisPCBM

PCBM

19

18

17 16 10

8

6

4

2

Binding energy / eV

0

-2

-4

-6

= EF

Fig. 2. UPS and IPES spectra of the fullerene derivatives, PCBM, bisPCBM, C70, PC70BM, PCBB, PCBO, TCBM, and ICBA thin films on an ITO substrate. The horizontal axis shows binding energy with respect to EF. The cut-offs in the UPS and UPS/IPES spectra near EF are shown in (a) and (b), respectively.

144 145 146 147 148 149 150 151 152 153

samples were not exposed to air prior to UPS and IPES measurements. Vacuum deposition of C70 was carried out in a vacuum chamber. The thickness of the films was determined with a surface profiler (Dektak 150, Veeco) and was more than 8 nm for all the samples. Prior to use, the ITO substrates were cleaned with acetone (3 min  3) and isopropanol (3 min  2) in an ultrasonic bath, followed by UV/ozone treatment for 15 min. UPS spectra were acquired with an electron spectrometer (SES200, Scienta) using the He I resonance line. The

5.55

ICBA

Fig. 3. The energy diagrams of fullerene derivatives derived from the UPS and IPES results shown in reference to Evac. The dotted lines represent EF and the numbers in the bracket give the value of EF.

base pressures of the preparation and analysis chambers in both the UPS and IPES instruments were 1  107 and 8  108 Pa, respectively. We performed IPES measurements in isochromat mode using laboratory-made apparatus [16]. The samples were damaged by the electron beam during IPES measurements; therefore, the position where the beam hit the samples was changed. The sample current at a maximum during the measurements was about 1 lA at a kinetic energy of 20 V. The repeatability of the data was confirmed. The energy resolution of the UPS (IPES) spectrometer deduced from the Fermi edge of the vacuum-deposited Au film was 0.1 eV (0.4 eV). MO calculations were performed for isolated C60, PCBM, bisPCBM, C70, PC70BM, PCBB, PCBO, TCBM, ICMA, and ICBA molecules based on density functional theory (DFT) using the Gaussian03 package with the B3LYP/6-31G(d,p) basis set. Although three PC70BM isomers coexist, we used only the main PC70BM isomer structure for the MO calculations in this paper [10]. For bisPCBM and ICBA, which also have several isomer structures, we used the optimized molecular structures obtained by DFT calculation.

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3. Results and discussion

175

3.1. Electronic structure of fullerene derivatives

176

Fig. 2 shows the UPS/IPES results for the films of the fullerene derivatives, PCBM, bisPCBM, C70, PC70BM, PCBB,

177

Table 1 exp exp Parameter values for fullerenes and derivatives from the UPS and IPES experiments and DFT calculations. Values of Iexp, Aexp, Eexp g , /h , /e , and Evac for C60 are cited from the literature [14].

C60 [14] PCBM bisPCBM C70 PC70BM PCBB PCBO TCBM ICMA ICBA

Iexp (eV)

Aexp (eV)

Eexp (eV) g

/exp (eV) h

/exp (eV) e

Evac (eV)

Ical (eV)

Acal (eV)

Ecal g (eV)

6.45 ± 0.02 5.66 ± 0.04 5.65 ± 0.04 6.02 ± 0.04 5.55 ± 0.04 5.71 ± 0.04 5.68 ± 0.04 5.83 ± 0.04 – 5.64 ± 0.04

4.5 ± 0.1 3.6 ± 0.1 3.2 ± 0.1 4.1 ± 0.1 3.7 ± 0.1 3.5 ± 0.1 3.4 ± 0.1 3.8 ± 0.1 – 3.1 ± 0.1

2.0 ± 0.1 2.1 ± 0.1 2.5 ± 0.1 1.9 ± 0.1 1.8 ± 0.1 2.2 ± 0.1 2.3 ± 0.1 2.0 ± 0.1 – 2.6 ± 0.1

1.75 ± 0.04 1.77 ± 0.04 1.78 ± 0.04 1.84 ± 0.04 1.62 ± 0.04 1.81 ± 0.04 1.82 ± 0.04 1.73 ± 0.04 – 1.67 ± 0.04

0.2 ± 0.1 0.3 ± 0.1 0.7 ± 0.1 0.1 ± 0.1 0.2 ± 0.1 0.4 ± 0.1 0.5 ± 0.1 0.3 ± 0.1 – 0.9 ± 0.1

4.70 3.89 3.87 4.18 3.93 3.90 3.86 4.10 – 3.97

6.3 5.9 5.7 6.1 5.8 5.9 5.9 5.9 5.8 5.6

3.4 3.1 3.0 3.4 3.3 3.2 3.2 3.3 3.2 3.0

2.9 2.8 2.7 2.7 2.5 2.7 2.7 2.6 2.6 2.6

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PCBO, TCBM, and ICBA, deposited on ITO substrates. The films were obtained by spin coating, except for the C70 film, which was obtained by vacuum-deposition on the ITO substrate. The vacuum levels (Evac) were estimated for the films from the energy of the secondary electron cut-offs (cut-offs), indicated by the vertical bars in Fig. 2(a). Fig. 2(b) shows the experimental UPS/IPES spectra around EF of the films of the fullerene derivatives. Vertical bars indicate the onsets of the UPS and IPES spectra that correspond to the HOMO (p) and LUMO (p*) energy. The energy difference between the HOMO and EF is the height of the hole injection barrier, /exp h ; and the energy difference between the LUMO and EF is the height of the electron injection barrier, /exp e , from the electrode. The adiabatic ionization energy Iexp, electron affinity Aexp, and energy gap Eexp can be calculated from the following formulas: g

195 197

I

 Evac

ð1Þ

198 200

Aexp ¼ /exp  Evac e

ð2Þ

exp

¼

/exp h

3.2. PCBM and bisPCBM

240

Fig. 4 shows the experimental UPS/IPES spectra of PC70BM, C70, bisPCBM, and PCBM films, and the MOs and corresponding to the simulated spectra. The spectrum of bisPCBM was similar to that of PCBM. The simulated spectra based on the DFT calculation were in excellent agreement with the observed UPS/IPES spectra. The simulated spectra were obtained by convoluting the calculated MOs by using a Gaussian function with a full width at half-maximum of 0.3 eV, which corresponded to the experimental energy resolution. The energy of the simulated UPS and IPES spectra were individually adjusted to reproduce the experimental spectra. The Iexp value of bisPCBM was slightly smaller than that of PCBM (Table 1). This was reproduced by the DFT calculations. The calculated Ical value of bisPCBM was smaller than that of PCBM by 0.2 eV, although the observed difference in Ical values of

241

UPS PC 70 BM

201

204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224

225 226 227 228 229 230 231 232 233 234 235 236 237 238 239

exp Eexp ¼ Iexp  Aexp ¼ /exp g h  /e

π∗

π

ð3Þ

exp exp The values of /exp calculated from the h , /e , and Eg UPS/IPES spectra in Fig. 2(b) are listed in Table 1. The spectral shapes of the UPS/IPES spectra in Fig. 2(b) depended on the fullerene backbone. The spectra of ICBA, TCBM, PCBO, PCBB, bisPCBM, and PCBM showed similar spectral shapes around EF because they had the same C60 backbone. The spectrum of PC70BM was similar to that of C70. These results show that the electronic structures around EF of the fullerene derivative series arise mainly from the fullerene backbones. The LUMO energies of almost all the compounds in Fig. 2(b) were very close to EF. In particular, the LUMOs of C70 and PC70BM were almost at EF. Fig. 3 shows energy diagrams constructed from the values listed in Table 1. To discuss the electronic structure of the molecules, the origin in Fig. 3 is the vacuum level. The LUMOs of all the compounds were very close to EF, and the low-lying LUMO explains the n-type electric characteristics of the devices in which they are used. The LUMOs of C70 and PC70BM were just above EF indicating very small /eexp . In contrast, the LUMOs of bisPCBM and ICBA were higher than the LUMOs of the other fullerene derivatives, which probably causes the larger VOC of the devices containing bisPCBM or ICBA [23–26]. The values of Iexp, Aexp, and Eexp calculated from Fig. 3 by g using the formulas (1)–(3) are different among all the fullerene derivatives. There was more variation in the energy of the LUMOs than the HOMOs. The Iexp and Aexp of C70 were larger and the Aexp of bisPCBM and ICBA were smaller than those of the other compounds. The Eexp of C70 and g PC70BM were smaller and Eexp of bisPCBM and ICBA were g larger than those of the other compounds. The small Eexp g of PC70BM causes a wide absorption range in the visible region, which leads to the high power conversion efficiency (PCE) of the OPV devices where PC70BM is used as an acceptor [5–8].

C 70

Intensity / arb. units

203

IPES

bisPCBM

PCBM

A DFT

10

8

6

4

2

0

-2

-4

-6

= EF

Binding energy / eV Fig. 4. Experimental (solid line) and simulated (shaded areas) UPS/IPES spectra of PCBM, bisPCBM, C70, and PC70BM. The simulated spectra were obtained by convoluting the calculated MOs with 0.3 eV and they were individually shifted (0.22 eV (UPS) and 0.06 eV (IPES) for PC70BM, 0.05 eV (UPS) and 0.28 eV (IPES) for C70, 0.55 eV (UPS) and 0.25 eV (IPES) for bisPCBM, 0.25 eV(UPS) and 0.01 eV (IPES) for PCBM) to reproduce the experimental spectra. The horizontal axis shows binding energy with respect to EF. Black bars aligned below the simulated spectra show the calculated MO energies. The calculated MOs labeled A has a large contribution from the carbonyl group in the side chain of PCBM, which is shown in Fig. 9.

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LUMO

HOMO

C60

PCBM

PCBB bisPCBM

LUMO

HOMO

PCBO

TCBM

C70

PC 70 BM

Fig. 5. Calculated HOMOs and LUMOs of C60, PCBM, bisPCBM, PCBB, PCBO, TCBM, C70, and PC70BM. Details of the DFT calculations are given in the experimental section.

257 258 259 260 261 262 263 264

PCBM and bisPCBM was much smaller than the calculated one. However, the Aexp value of PCBM was larger than that of bisPCBM by 0.4 eV (Table 1). The calculated affinity, Acal, of bisPCBM was smaller than that of PCBM by 0.1 eV, which was qualitatively consistent with the experimental results. As a result of the difference in the values of Aexp, the Eexp value of bisPCBM was larger than that of PCBM g by 0.4 eV. The calculated Ecal g value of bisPCBM was smaller

than that of PCBM by 0.1 eV, which disagreed with the experimental results. We performed the DFT calculations on isolated molecules, whereas the experimental values were measured on solid films. In addition, DFT calculations generally cannot reproduce the values of Eexp g . This was the reason why the simulated spectra were shifted individually to reproduce the experimental spectra. Nevertheless, DFT calculations are useful for qualitative confirmation of

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experimental results, particularly for assigning the electronic states around EF. Next, the MOs of fullerene derivatives were examined using DFT. Fig. 5 shows the calculated HOMOs and LUMOs of C60, PCBM, bisPCBM, PCBB, PCBO, TCBM, C70 and PC70BM. The p-orbitals of the C60 backbone made a large contribution to the HOMO and LUMO of all the molecules in Fig. 5, and the HOMO and LUMO were distributed over the C60 backbone. The HOMO of PCBM and bisPCBM also contained some contribution from the atomic orbitals of the bridgehead carbon atom of the side chain. The HOMO of TCBM also contained a small contribution from the atomic orbitals of the sulfur atom on the side chain. Akaike et al. [14,15] explained the effects of the side chain on the electronic structure of PCBM and bisPCBM. They concluded that a subtle charge transfer from the side chain to the C60 backbone destabilizes the electronic states of the molecule. Fig. 6(a) shows the calculated Mulliken charges of C60, PCBM, bisPCBM, C70, and PC70BM. In contrast to uncharged atoms in C60, the two carbon atoms of the C60 backbone of PCBM and bisPCBM bound to side chain, referred to as anchor carbons here, are negatively charged. This means that the side chain donates some electron density to the C60 backbone. The amount of charge in PCBM and bisPCBM donated to the backbone was estimated at 0.23 e and 0.46 e from the DFT calculations, respectively. Here, e is the elementary charge. Thus the partial electrical charge donated from the side chain to the C60 backbone of PCBM and bisPCBM destabilizes the HOMO and LUMO of the molecule and lowers both the ionization energy and electron affinity. This indicates that PCBM and bisPCBM are weaker acceptors than C60.

We also examined C60 derivatives with simple functional groups. Fig. 6(b) shows the calculated Mulliken charges of the fullerene derivatives C61H2 and C60F2 to explain the effects of electron-donating and -accepting side chains. The amount of the charge donated from C61H2 and C60F2 to the backbone was estimated at 0.19 e and 0.60 e, respectively. The calculation for C61H2 shows the partial electrical charge is donated from the methylene group to the C60 backbone. In contrast, the calculation for C60F2 shows that the partial electrical charge is pulled from the C60 backbone toward the fluorine atoms because of their large electronegativity. Fig. 7 shows the molecular orbital energies of C60, C61H2, and C60F2. The 3-fold degenerate LUMO of C60 is split into three orbitals in C61H2 and C60F2. The LUMO of C61H2 is shifted to a higher energy by the destabilization of the orbital energy because of the introduction of the methylene group. In C60F2, the LUMO is shifted to a lower energy by the stabilization of the orbitals energy because of the introduction of the fluorine atoms. Consequently, the Acal value of C61H2 is smaller than that of C60, whereas the Acal value of C60F2 is larger than that of C60. This trend in Acal in simple molecules supports our interpretation of the results for PCBM and bisPCBM.

305

3.3. C70 and PC70BM

329

The UPS and IPES spectra of the PC70BM film had broader peaks than the spectra of the C70 film (Fig. 4). In particular, the UPS spectrum around a binding energy of 8 eV had broad features. The spectral shapes obtained by DFT calculations agreed well with the observed UPS/IPES

330

(a)

(b) -CH2

C60

PCBM

C61H2

bisPCBM

-F2

C70

PC70BM

+ C60F2

Fig. 6. (a) Calculated Mulliken charges of C60, PCBM, bisPCBM, C70, and PC70BM molecules. Red and green indicate negative and positive charges, respectively. (b) Calculated Mulliken charges of C61H2 and C60F2 molecules. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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-3

3 (-3.6)

-5

-6

-7

HOMO (-6.3) 5

C60

(-6.0)

C61H2

C60F2

344

spectra. The calculated ionization energy and the affinity are qualitatively consistent with the experimentally observed trend. The values of Ecal g were qualitatively consistent with the experimentally observed trend. The DFT calculations show that the partial electrical charge, that is 0.23 e from the calculation, donated from the side chain of the PC70BM to the C70 backbone reduces both Iexp and Aexp, indicating that PC70BM is a weaker acceptor than C70. This is also consistent with the experimental results for PCBM and bisPCBM.

345

3.4. PCBM and PC70BM

346

The difference in the electronic structure around the energy gap between PCBM and PC70BM can be explained by the differences in the electronic structures of C60 and C70, because the fullerene backbones make the largest contribution to the electronic structure around the energy gap. The calculated values of Ical and Acal shown in Table 1 explained the observed trends in Iexp and Aexp qualitatively. The value of Eexp of the PC70BM film was smaller than that g of PCBM by 0.3 eV, because of the small Eexp of C70. g The effect of the side chain on the electronic structure was the same for both PCBM and PC70BM. From the calculated Mulliken charges shown in Fig. 6(a), the charge donated to the backbone from the side chain was calculated as 0.23 e for PCBM and PC70BM. The small energy gap in PC70BM is desirable for OPVs. Recently, OPVs using PC70BM have achieved higher PCEs than those using other acceptor materials [5–8]. Differences in the fullerene backbone of PC60BM and PC70BM affect the device properties. Yasuda et al. [27] compared the conversion efficiency of a PC70BM/poly(benzothiadiazole–triphenylamine) cell (2.65%) and PCBM/poly(benzothiadiazole–triphenylamine) cell (1.34%) directly. The efficiency of the PC70BM cell was about twice that of the PCBM cell. The values of Voc were

336 337 338 339 340 341 342 343

347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368

3.5. PCBM, PCBB, and PCBO

381

Fig. 8 shows the experimental UPS/IPES spectra of TCBM, PCBO, and PCBB films, and the MOs and corresponding simulated spectra. The spectra of PCBB and PCBO were similar to that of PCBM. The simulated spectra based on the DFT calculations were in excellent agreement with the observed UPS/IPES spectra.

382

370 371 372 373 374 375 376 377 378 379 380

(-6.3)

Fig. 7. Molecular orbital energies of C60, C61H2, and C60F2 from the MO calculations. The numbers ‘‘5’’ and ‘‘3’’ next to the HOMO and LUMO of C60 are the degeneracy of the orbitals. Numbers in parentheses are the calculated HOMO and LUMO energies.

335

369

UPS TCBM

Intensity / arb. units

Orbital Energy / eV

-4

LUMO (-3.4)

(-3.3)

similar, whereas the Jsc of the PC70BM cell was higher than that of the PCBM cell. The experimental Aexp of the PC70BM film is similar to that of PCBM film, which is consistent with the similar Voc values. However, the Eexp value of g PC70BM film is less than that of PCBM film because the MOs are widely distributed around the energy gap of PC70BM. The smaller energy gap of PC70BM enhanced the light absorption and energy conversion efficiency, and thus improved the exciton generation and charge separation. In addition, PC70BM shows significantly high absorption coefficient in the visible region which must have a great impact on the PCE [10].

π

IPES

π∗

PCBO

C

PCBB

DFT

B

10

8

6

4

2

0

-2

-4

-6

= EF Binding energy / eV Fig. 8. Experimental (solid lines) and simulated (shaded areas) UPS/PES spectra of PCBB, PCBO, and TCBM. The horizontal axis shows binding energy with respect to EF. Black bars aligned below the simulated spectra show the calculated MO energies. Calculated MOs labeled B and C have a large contribution from the carbonyl group in the side chain of PCBB and PCBO, respectively, which are shown in Fig. 9. The simulated spectra were obtained by convoluting the calculated MOs with 0.3 eV and they were individually shifted (0.27 eV (UPS) and 0.84 eV (IPES) for TCBM, 0.29 eV (UPS) and 1.33 eV (IPES) for PCBO, 0.28 eV (UPS) and 1.33 eV (IPES) for PCBB) to reproduce the experimental spectra.

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The electronic structure around the energy gap is unlikely to be altered greatly by increasing the carbon chain length in the side chain of PCBM because the contribution from the carbon chain to the electronic structure around the energy gap is smaller. Fig. 9 shows the calculated MOs with a large contribution from the carbonyl in the side chain of PCBM (A, 4.3 eV), PCBB (B, 4.1 eV), and PCBO (C, 4.0 eV). MOs A–C are indicated on the calculated MOs in Figs. 4 and 8. The three compounds had very similar MOs, suggesting that the effect of the PCBM carbon chain length on the electronic structure of the molecules was weak. The calculated energies of the MOs were also almost independent of the carbon chain length. The values of Ical, Acal, and Ecal of PCBB and PCBO g (Table 1) are similar to those of PCBM. The Ecal g values of PCBB and PCBO were 0.1 eV smaller than that of PCBM. PCBB and PCBO possess butyl and octyl esters in their side chains, respectively. The Iexp values of PCBB and PCBO were almost the same as that of PCBM, within experimental error. However, the Aexp values of PCBO and PCBB were slightly smaller than that of PCBM. Consequently, the Eexp g values of PCBO and PCBB were larger than that of PCBM. These experimental results are not consistent with the calculations. Fig. 10(a) shows the calculated Mulliken charges of PCBB, PCBO, and TCBM. The amount of charge donated to the backbone from the side chain is 0.23 e in PCBB and PCBO. Zhao et al. [28] examined the effect of carbon chain length in substituents of PCBM-like molecules by using cyclic voltammetry (CV), UV–Vis absorption spectroscopy (UV–Vis), atomic force microscope (AFM), and the OPV device performance. They observed that the LUMO energy of the PCBM-like fullerene derivatives did not change substantially with the chain length. In contrast, the PCE of the

A (PCBM)

OPV device was distributed among the molecules. They reported that the difference in the performance of the OPV devices containing different derivatives was caused by the variation in the morphology of the surfaces in their P3HT blend films. They calculated the LUMO energy from the CV results for an isolated molecule and measured the OPV performance with solid films. Our results are qualitatively consistent with the conclusion reported by Zhao et al. [28]. The simulation results for isolated molecules only showed a small change in the electronic structures of PCBB, PCBO, and PCBM. Our results show the slight narrowing of the energy gap from PCBM to PCBB or PCBO. We suggest that the difference we observed in the electronic structure of the PCBB and PCBO films compared with the PCBM film originated from the difference in the morphology of the solid films. As discussed in the literature [14–16], variations in the film morphology of fullerene derivatives may alter the polarization energy of the environment of the molecule, which leads to electronic structure changes. This would explain the observed difference in the values of Iexp, Aexp, and Eexp of PCBB and PCBO g compared with those of PCBM, in contrast to the calculated results. The calculations were performed on an isolated molecule and the UPS/IPES results were obtained from solid films. The difference in the film morphology of PCBB, PCBO, and PCBM may cause a difference in polarization energy. In future work, AFM should be used to investigate the morphology of the derivative films.

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3.6. PCBM and TCBM

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The changes in the electronic structure caused by replacing a phenyl group with a thienyl group in the side chain were investigated by comparing PCBM with TCBM. The spectrum of TCBM was similar to that of PCBM. The

452

B (PCBB)

C (PCBO)

Fig. 9. Calculated MOs with a large contribution from side chain carbonyl group of PCBM (A), PCBB (B), and PCBO (C). Corresponding spectral structures in the UPS spectra are labeled A, B, and C in Figs. 4 and 8.

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(b)

(a)

PCBB

PCBO

-

ICMA

+

TCBM

ICBA

Fig. 10. (a) Calculated Mulliken charges of PCBB, PCBOs, and TCBM. Red and green indicate negative and positive charges, respectively. (b) Calculated Mulliken charges of ICMA and ICBA molecules.

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3.7. bisPCBM and ICBA

465

Fig. 11 shows the experimental UPS/IPES spectra of ICBA films, the MOs, and the corresponding simulated spectra. The simulated spectra based on the DFT calculations agreed well with the observed UPS/IPES spectra. Our results are generally consistent with the reported spectra [29], although there are differences in the HOMO and LUMO energies with those reported by Ze-Lei Guan et al. [29], which is probably caused by the differences in the substrate and thickness of the specimens. It has been reported that an OPV device containing a P3HT/ICBA BHJ film showed a high VOC of 0.84 V and a higher PCE of 6.48%, which is better than the values of OPV devices based on P3HT/PCBM BHJ [25,26]. The value of Aexp for ICBA was

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UPS Intensity / arb.units

463

value of Acal of PCBM was smaller than that of TCBM by 0.2 eV, which was qualitatively consistent with the experimental results. The amount of charge donated to the backbone from the TCBM side chain was calculated as 0.18 e from the calculated Mulliken charge. The amount of the electrons donated to the fullerene backbone was smaller than that of PCBM, which explains the larger values of Iexp and Aexp for TCBM.

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ICBA

467 468 469 470 471 472 473 474 475 476 477

π∗

π

DFT

10 466

IPES

8

6

4

2

0

-2

-4

-6

= EF Binding energy / eV Fig. 11. Experimental (solid lines) and calculated (shaded areas) UPS/IPES spectra of ICBA. Simulated spectra of ICBA were obtained by convoluting the calculated MOs with 0.3 eV and were shifted (0.44 eV (UPS) and 1.16 eV (IPES)) to reproduce the experimental spectra. The horizontal axis shows binding energy with respect to EF. Black bars aligned below the simulated spectra show the calculated MO energies.

smaller than the values for the other compounds (Table 1). The value of Acal for ICBA was also smaller than that of other compounds, which was consistent with the experimental results.

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-2

Orbital Energy / eV

-3

3 -4

(-3.1)

(-3.0) (-3.2)

(-3.0)

LUMO (-3.4)

-5

(-5.7) -6

HOMO (-6.3) 5 C60

(-5.9)

(-5.6) (-5.8)

PCBM bisPCBM ICMA

508

From the MO calculations, the p-orbitals of the C60 backbone contributed greatly to the HOMO and LUMO of ICMA and ICBA, similar to the other derivatives. The HOMO of ICMA and ICBA also contained some electron density at the anchor carbons and at the two carbon atoms in the side chain that are connected to the anchor carbons. The calculated Mulliken charges of ICMA and ICBA are shown in Fig. 10(b). The amount of charge donated to the backbone from the ICBA side chain was 0.18 e. In contrast, the charge donated to the backbone from the ICMA side chain was 0.10 e, which was about half of that of ICBA. This is similar to the case of PCBM and bisPCBM (see Section 3.2). Fig. 12 shows the energy diagram of C60, PCBM, ICMA, and ICBA. C60 has 5-fold degenerate HOMOs and 3-fold degenerate LUMOs; the HOMO and LUMO are split into five and three orbitals, respectively, by the side chains in all the derivatives. The value of Acal for ICBA was 3.0 eV, which was the same as that of bisPCBM, although the amount charge donated to the fullerene backbone from the side chain of bisPCBM was more than double the value for ICBA. The electronic structures of bisPCBM and ICBA suggest that new acceptor molecules with a high LUMO energy and a large electron affinity could be obtained by introducing double electron-donating side chains into the fullerene backbone. The high-energy LUMO of the acceptor would increase the VOC of the OPV device.

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4. Conclusion

510

The UPS and IPES results in this work show that the electronic structure around Eg of the fullerene derivatives is determined by the fullerene backbone. The electronic structure of PCBM, bisPCBM, PCBB, PCBO, and TCBM were similar, and the electronic structure around the HOMO– LUMO gap of PC70BM was similar to that of C70. Thus, for

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Acknowledgements

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This work was supported by a Grant-in-Aid for Scientific Research (Grant No. 24350013) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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References

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ICBA

Fig. 12. Calculated MO energies of C60, PCBM, bisPCBM, ICMA, and ICBA. The numbers 5 and 3 next to the HOMO and LUMO of C60 are the degeneracy of the orbitals. Numbers in parentheses are the calculated HOMO and LUMO energies.

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PC70BM the value of Eg was small, producing a wider absorption range in visible region compared with the C60 derivatives. This is consistent with the higher performance of OPV devices that use PC70BM. However, the side chain affected the electronic structures of the fullerene derivatives as well as their solubility. The ionization energy and electron affinity were mainly determined by the properties of the side chain. Electron-donating side chains decreased the electron affinity of all the derivatives. The low electron affinity increased the VOC of the OPV devices containing the derivatives. Our results bridge the gap between the fundamental knowledge of the electronic structure of n-type materials and their application in OPV devices. The information about the electronic structure of the fullerene derivatives in this work will help in the synthesis of new materials for high-performance OPVs in the near future.

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