Organic Electronics 14 (2013) 3222–3227

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Organic Electronics

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Letter

Electronic structure and surface morphologyof [6,6]-phenyl-C71-butyric acid methyl ester films

1566-1199/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.orgel.2013.09.031

⇑ Corresponding author.E-mail address: j6213620@ed.noda.tus.ac.jp (A. Nogimura).

1 Present address: Emergent Functional Polymers Research Team, RIKENCenter for Emergent Matter Science, Wako-shi, Saitama 351-0198, Japan.

Ayumi Nogimura a,⇑, Kouki Akaike b,1, Rie Nakanishi a, Ritsuko Eguchi c, Kaname Kanai a

a Department of Physics, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japanb Photoelectric Conversion Research Team, Advanced Science Institute, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japanc 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 i n f o

Article history:Received 6 August 2013Received in revised form 12 September2013Accepted 15 September 2013Available online 27 September 2013

a b s t r a c t

We investigate the electronic structure of [6,6]-phenyl-C71-butyric acid methyl ester(PC70BM) films by ultraviolet photoelectron and inverse photoemission spectroscopy. Wediscuss the electronic structure of PC70BM in comparison with C70 and [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM). The molecular orbitals around the energy gap ofPC70BM are broadly distributed due to the low symmetry of the molecular structure.Consequently, the energy gap of PC70BM is smaller than that of C70 and PC60BM. The filmdeposition method affects the polarization energy between the PC70BM molecules, andthus affects the electronic structure of the films.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Fullerene and its derivatives have been used in organicphotovoltaic (OPV) devices having high-efficient photo-electric conversion characteristics, and serve as indispens-able n-type semiconductor materials [1]. OPVperformance, especially optical absorption and carrierinjection and transport, strongly depends on the electronicstructure of the organic thin film, and on the organic/metaland organic/organic interfaces. In particular, the energiesof the highest occupied molecular orbital (HOMO) andlowest unoccupied molecular orbital (LUMO) [2,3] playan important role with respect to optical and transportproperties.

[6,6]-Phenyl-C61-butyric acid methyl ester (PC60BM) iscommonly used as the n-type semiconductor in OPVs.The combination of regioregular poly(3-hexylthiophene)(P3HT) and PC60BM has been extensively studied in effortsto optimize cell performance [4–7]. In the context of suchoptimization studies, Akaike et al. reported the electronic

structures of C60, PC60BM, and the bis-adduct of PC60BM(bis-PC60BM) on the basis of ultraviolet photoelectronspectroscopy (UPS), inverse photoemission spectroscopy(IPES), and molecular orbital (MO) calculations, respec-tively [8,9]. Their results show that PC60BM and bis- PC60-

BM are weaker acceptors than C60, and suggest that theside chain of such C60 derivatives strongly influences thecorresponding electronic structure. On the other hand,OPVs using [6,6]-phenyl-C71-butyric acid methyl ester(PC70BM) have recently achieved higher power conversionefficiency (over 8%) than those using other acceptor mate-rials because PC70BM has stronger absorption in the visibleregion than PC60BM [7,10–12]. These results provideguidance for future synthesis of more efficient acceptormolecules, and in this context, we investigated theoccupied and unoccupied electronic structure in thevicinity of the energy gap (Eg) of PC70BM.

In this letter, we report the electronic structure ofPC70BM on the basis of UPS and IPES (UPS/IPES). We dis-cuss the effects of the side chain of PC70BM on its electronicstructure by comparison with the electronic structures ofC70 and PC60BM. MO calculations based on densityfunctional theory (DFT) provide a basis for comparingexperiment with theory. We also focus on the differencein the electronic structure between vacuum-deposited

A. Nogimura et al. / Organic Electronics 14 (2013) 3222–3227 3223

and spin-coated PC70BM films. The Eg value of the formerwas smaller than that of the latter by �0.2 eV. To under-stand why the film deposition method caused such a dif-ference in Eg, we investigated the surface morphology ofthese films by atomic force microscopy (AFM).

Fig. 1. (a) UPS spectra of C70 film (20), vaccum-deposited PC70BM film (40),and spin-coated PC70BM film (50). Bottom axis is binding energymeasured from the Fermi level (EF). Vertical bars in (20), (40), and (50)indicate the secondary electron cutoffs of C70 film, vacuum-depositedPC70BM film, and spin-coated PC70BM film, respectively. (b) UPS/IPES

2. Experimental and theoretical procedures

PC70BM (99%) and C70 (99%) were purchased from Al-drich and used as received. Thin films of PC70BM werespin-coated from chlorobenzene solutions of PC70BM(0.4 wt%) in a glovebox filled with N2 atmosphere at roomtemperature. These films were spin-coated onto indium tinoxide (ITO)-coated glass substrates at 1500 rpm for 30 sand transferred to the vacuum chamber using thecontainer filled with dry N2 to avoid contamination by airexposure. Vacuum deposition of PC70BM and C70 was car-ried out in vacuum chambers. The thickness of these filmswere determined with Veeco model Dektak 150 surfaceprofiler, and the thickness was more than 8 nm for all thesamples in this work. We cleaned the ITO substrates priorto use with acetone (3 min � 3) and isopropanol(3 min � 2) in an ultrasonic bath, followed by UV/ozonetreatment for 15 min.

UPS spectra were acquired with a SCIENTA spectrome-ter (SES200) using the He I resonance line. The basepressures of the preparation and analysis chambers in boththe UPS and IPES instruments were 1 � 10�7 and8 � 10�8 Pa, respectively. We performed IPES measure-ments in isochromat mode using a laboratory-made appa-ratus. The photon detector and electron gun were installedin the analysis chamber. A band-pass photon detector forhv = 9.2 eV was used, consisting of a channeltron coatedwith NaCl and placed behind a SrF2 window. A low-energyelectron gun produced a mono-energetic electron beam.These samples are subjected to electron beam damagesduring IPES measurements. Therefore, these samples weremeasured with changing the probing area on the samplesurface to avoid the beam damage. Sample current is lessthan 1 lA at 20 V of electron energy, and it had been care-fully adjusted to obtain reproducibility of the data. The en-ergy resolution of the UPS (IPES) spectrometer, as deducedfrom the Fermi edge of the vacuum-deposited Au film, was0.1 eV (0.4 eV). We investigated the surface morphology ofthe thin films by AFM using an Agilent Technologiessystem in acoustic mode, and acquired AFM images innoncontact mode.

MO calculations were performed with the Gaussian03package. After structural optimization of isolated C70 andPC70BM molecules by the Hartree–Fock method using the6-31G basis set, DFT calculations were performed usingthe B3LYP exchange–correlation function. We obtainedsimulated UPS/IPES spectra by convoluting delta functionsat MO energies, with a Gaussian function having a 0.3 eVfull width at half-maximum.

spectra of C70 film (2), vacuum-deposited PC70BM film (4), and spin-coated PC70BM film (5), along with corresponding simulated spectra ofC70 (1) and PC70BM (3). Vertical bars in (2), (4), and (5) indicate spectralonsets. Vertical bars below (1) and (3) indicate MO energy positions of C70

and PC70BM, respectively. Fig. 2 shows the A–F MO character labels in (1)and (3). Molecular structures above (2) and (5) are those of C70 andPC70BM.

3. Results and discussion

Fig. 1 shows the UPS/IPES results for C70 and PC70BMfilms on ITO substrates. Vertical bars in Fig. 1a indicate

the secondary electron cutoffs in the UPS spectra. Fromthe energy of the secondary electron cutoffs, the vacuumlevels (Ev) for vacuum-deposited C70, vacuum-depositedand spin-coated PC70BM films were found to be �4.18,�3.94, and �3.93 eV, respectively. Fig. 1b shows the exper-imental UPS/IPES spectra of these films as well as the MOsand corresponding simulated spectra. The UPS/IPES spectrafor the PC70BM films were broader than those for the C70

film. Vertical bars at 1.84 ± 0.02 and �0.11 ± 0.05 eVindicate the experimental UPS/IPES spectral onsets, respec-tively, for the C70 film; these onset values were 1.57 ± 0.06and �0.06 ± 0.03 eV, respectively, for the vacuum-depos-ited PC70BM film. Here, it should be noted that there is lar-ger experimental error at estimating the onset of IPESspectra due to worse energy resolution of IPES comparedwith UPS. The threshold ionization energies (I) for the C70

film and the vacuum-deposited PC70BM film were6.02 ± 0.06 and 5.51 ± 0.06 eV, respectively. The I value ofPC70BM was less than that of C70 by 0.51 eV. DFT calcula-tion results indicate that I of PC70BM is smaller than thatof C70 by 0.32 eV. These calculations were qualitativelyconsistent with the experimental results. The electron

Table 1Threshold ionization energy I, threshold electron affinity A, and energy gap Eg of C60, PC60BM, C70, and PC70BM. These data of C60 and PC60BM are cited from thereference [8].

C60 [8] (eV) PC60BM [8] (vacuum-deposited) (eV) C70 (eV) PC70BM (vacuum-deposited) (eV) PC70BM (spin-coated) (eV)

I 6.45 ± 0.02 5.96 ± 0.02 6.02 ± 0.06 5.51 ± 0.06 5.55 ± 0.06A 4.5 ± 0.1 3.9 ± 0.1 4.07 ± 0.10 3.88 ± 0.10 3.74 ± 0.10Eg 2.0 ± 0.1 2.1 ± 0.1 1.95 ± 0.10 1.63 ± 0.10 1.81 ± 0.10Ical 6.25 5.93 6.12 5.80 –Acal 3.38 3.12 3.41 3.27 –

Ecalg

2.87 2.81 2.71 2.53 –

Fig. 2. Representative MO patterns of C70 and PC70BM in the upper occupied and lower unoccupied MOs. A, B, C, D, E and F are corresponding to the MOs inFig. 1.

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A. Nogimura et al. / Organic Electronics 14 (2013) 3222–3227 3225

affinities (A) of the C70 film and the vacuum-depositedPC70BM film were determined to be 4.07 ± 0.10 and3.88 ± 0.10 eV from the energy difference between the Ev

value and the spectral onsets. The A value of PC70BM wasless than that of C70 by 0.19 eV. DFT calculation resultsindicate that A of PC70BM was smaller than that of C70 by0.14 eV. These calculations were also qualitatively consis-tent with the experimental results. The Eg value can beobtained from the equation Eg = I � A, which gives a valuefor PC70BM that is less than that for C70 by �0.3 eV.Table 1 shows the observed I, A, and Eg values.

We performed DFT calculations on isolated molecules,whereas the experimental values came from measure-ments on solid films, and it is known that DFT calculationsgenerally cannot reproduce the value of Eg. Nevertheless,DFT calculations are useful for qualitative confirmation ofthe experimental results, especially with respect to the Iand A values. By introducing a side chain, and therebyreducing the symmetry of the C70 molecule, the HOMO(A in Fig. 1b) and LUMO (B in Fig. 1b) of C70 were raisedwith respect to those of PC70BM. Consequently, as seen inTable 1, the I and A values of PC70BM were much smallerthan those of C70.

The spectral shapes obtained by DFT calculation were inexcellent agreement with the observed UPS/IPES spectra(see Fig. 1b). We next examined the MOs of PC70BM byusing DFT. The C70 backbone greatly contributed to theMO labeled C (1.86 eV) (see Fig. 1b), which correspondsto the HOMO of PC70BM (MO pattern C in Fig. 2), in theDFT-simulated UPS spectrum. Analogously, the C70 back-bone also greatly contributed to the MO labeled F(�0.67 eV), which corresponds to the LUMO of PC70BM,in the DFT-simulated IPES spectrum. On the other hand,the side chain of PC70BM contributed many MOs in thebinding energy region (above approximately 4 eV). Forinstance, as shown in Fig. 2, the side chain strongly influ-enced the MOs labeled D (HOMO�14, 3.67 eV) and E(HOMO�35, 5.77 eV).

The DFT calculations show that the partial electricalcharge donated from the side chain to the C70 backboneof PC70BM lowers both I and A, indicating that PC70BM isa weaker acceptor than C70 (see Table 1). That can be ex-plained by the calculated Mulliken charges of PC70BM(Fig. 3). The two carbon atoms of the C70 backbone bound

Fig. 3. The calculated Mulliken charges in a C70 and a PC70BM molecules. R

to side chain indicated by an arrow are charged negatively(red). This mean that the side chain donates electron to theC70 backbone. From the calculation, amount of the chargedonated to the backbone is �0.23 e. Here, e represents ele-mentary charge. Akaike et al. reported analogous effects onthe I and A values of PC60BM, which is composed of a C60

backbone and the same side chain. The I and A values ofPC60BM were less than those of C60 because of the smallextent of electron donation from the side chain to the C60

backbone of PC60BM [8]. The A value of the fullerene deriv-ative PCxBM (x = 60, 70) was greater than that of fullerenedue to the side chain electron-donation effect. Otherreports indicate that there is a correlation between the Avalue of the acceptor molecule and the open circuit voltage(Voc) of the OPV cell. Chu et al. reported that Voc of a PC60-

BM /copper phthalocyanine cell is 0.10 eV greater than thatof the corresponding C60/copper phthalocyanine cell [13].The increased A value of PC60BM explains the correspond-ingly increase in Voc. We therefore expect analogous resultswith respect to PC70BM and C70.

Differences in the fullerene backbone of PC60BM andPC70BM affect device properties. Yasuda et al. havereported that the conversion efficiency of a PC70BM/poly(benzothiadiazole–triphenylamine) cell (2.65%) isabout twice that of a PC60BM /poly(benzothiadiazole–triphenylamine) cell (1.34%) [14]. In contrast, they reportthat the corresponding Voc changes little and the shortcircuit current (Jsc) of the former is 3.59 mA/cm2 higherthan that of the latter. As shown in Table 1, the observedA of PC70BM was almost the same as that of PC60BM, whichwas consistent with the similar Voc values. On the otherhand, the I and Eg values of PC70BM were less than thoseof PC60BM by 0.45 and 0.47 eV [8], respectively, due tothe widely distributed MOs around the Eg of PC70BM. Thesmaller Eg of PC70BM enhanced light absorption and energyconversion efficiency, and in turn improved exciton gener-ation and charge separation, in the PC70BM cell relative tothe PC60BM cell.

We also investigated the impact of the film formationmethod on the electronic structure and morphology ofPC70BM. Regarding the electronic structure, spectra (4)and (5) in Fig. 1b shows the experimental UPS/IPES spectraof vacuum-deposited and spin-coated PC70BM films,respectively. Although we observed little effect on the

ed and green mean negatively and positively charged, respectively.

Fig. 4. (a) AFM images of C70 film, (b) vacuum-deposited film, and(c) spin-coated PC70BM.

3226 A. Nogimura et al. / Organic Electronics 14 (2013) 3222–3227

UPS spectral shapes, we did observe a small differenceabove EF in the IPES spectrum. Table 1 shows the I, A, andEg values for the vacuum-deposited and spin-coated PC70-

BM films. The respective values of Eg, which are definedas the difference between I and A, were 1.63 ± 0.10 and

1.81 ± 0.10 eV. The Eg value for the vacuum-deposited filmwas smaller than that of the spin-coated film by 0.18 eV.

Regarding film morphology, Fig. 4a–c respectively showAFM images of the C70 film, the vacuum-deposited and thespin-coated PC70BM films. The C70 film vacuum-depositedon an ITO substrate was composed of grains about100 nm in length. The film morphology of the PC70BM film(Fig. 4b and c) was markedly different from that of the C70

film. Fig. 4b shows that the surface of the vacuum-deposited PC70BM film was rough and exhibited scatteredsmall grains. On the other hand, the surface of the spin-coated PC70BM film was smooth (surface roughness of lessthan 10 nm). This spin-coated film appeared to be in anamorphous state. The deposition technique clearly affectedthe film morphology. Polarization energy, which is a func-tion of the molecular number density and polarizability[15], directly influences the Eg value. Since the depositionmethod does not affect molecular polarizability, it mustinstead affect the molecular number density. Our AFMresults indicate that vacuum deposition and spin coatingimparted a close-packed and amorphous morphology,respectively, in the PC70BM thin films. Vacuum depositionalso imparted a greater polarization energy, which maylargely explain the smaller Eg value of the vacuum-deposited PC70BM film.

4. Conclusion

We investigated the electronic structure of C70 andPC70BM thin films by UPS, IPES, and DFT. The C70 backbonelargely contributed to the occupied and unoccupied MOs ofPC70BM around the HOMO and LUMO. The I value of PC70-

BM was smaller than that of C70 because of HOMO destabi-lization, which was caused by electron donation into theC70 backbone from the side chain. Similarly, the A valueof PC70BM was also smaller than that of C70, indicating thatPC70BM was a weaker acceptor than C70. The Eg value forthe PC70BM film was much smaller than that of C70 andPC60BM films. The small Eg of PC70BM enhancesphoto-absorption of active layer in OPV cell, leading to anincrease in Jsc as compared with PC60BM and PC70BM. Vary-ing the film formation method affected film morphology,which in turn affected the Eg value for the PC70BM film.The electronic structure of the PC70BM films revealed inthis work will be helpful for understanding the underlyingprinciples of high-efficiency OPV devices and designingnew OPV acceptor molecules, with the goal of improvingOPV electronic properties and device performance.

Acknowledgements

This work was supported by a Grant-in-Aid for Scien-tific Research (Grant No. 24350013) from the Ministry ofEducation, Culture, Sports, Science and Technology ofJapan (MEXT).

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