P29S

23BU005

BL-6

Molecular orientation in polymer/fullerene blend films
studied by NEXAFS
Y. Hanakia, H. Yoshidab,c and T. Sekitanib,caFaculty of Science, Hiroshima University
Graduate School of Advanced Science and Engineering, Hiroshima Universityc
Hiroshima Synchrotron Radiation Center, Hiroshima Universityb

Keywords: fullerene, polymer, solar cell, NEXAFS

With the increasing demand for renewable energy, organic photovoltaics have attracted muchattention due to the low cost, lightweight and flexibility in recent years. Polymer solar cells based on bulk
heterojunction (BHJ) consisting of interpenetrating networks of conjugated polymers as electron donors
and fullerene derivatives as electron acceptors, are being developed for their potential application [1]. They
provide a large interfacial area where efficient exciton dissociation can occur. The morphology of BHJ
plays an important role in obtaining high efficiency organic photovoltaic devices. The order andorientation of the polymer backbone in the bulk heterojunction can influence device properties, such as theexciton diffusion, and charge carrier transport.
In this study, the molecular orientation of polymer/fullerene blend film was examined by near edge
X-ray absorption fine structure (NEXAFS) spectroscopy.
Poly-{bi(dodecyl)thiophenethieno[3,4-c]pyrrole-4,6-dione} (PBTTPD) was used as electron donor, and (6,6)-phenyl-C61-butyric acidmethyl ester (PCBM) is used as electron acceptor. Figure 1 show the molecular structures of PBTTPD andPCBM. The blend of poly-3-hexylthiophene (P3HT) and PCBM has been used as a model system in BHJ
solar cells. Compared with P3HT, PBTTPD is co-polymer which has donor and acceptor in the main chainand the higher power conversion efficiency of PBTTPD blend film is reported [2].
Samples for NEXAFS measurement were spin-coated on the gold coated silicon substrate from eachsolution. Pure PBTTPD PCBM, and blend of PBTTPD:PCBM (1:1.5 by weight) were dissolved inchloroform. NEXAFS measurements were performed at BL6 of Hiroshima Synchrotron Radiation Center.
Oxygen K-edge NEXAFS spectra were measured for PBTTPD, PCBM and PBTTPD: PCBM blendfilms. Figure 2 shows the NEXAFS spectra of blend films. Red line shows spectrum of normal incidence
and black line shows that of grazing incidence. First strong peak come from *C=O resonant excitations ofboth PBTTPD and PCBM. NEXAFS spectrum of blend film was well reproduced by the superposition ofPBTTPD and PCBM. From the angular dependence of *C=O excitation, orientation angle of eachmolecule was obtained. Both PBTTPD of pure and blend films show “edge-on” orientation, which has
the plane of polymer backbone perpendicular to the substrate. The effect of the coexistence of PCBM is
considered small for polymer orientation which is important for charge transport process.

(a)(b)FIGURE 1.

Molecular structures of (a) PBTTPD and (b) PCBM.

20deg90deg

2.5Intensity（a.u.）

21.5
10.50-0.5520525530

535540545550555560

Photon Energy（eV）
FIGURE 2. Oxygen K-edge NEXAFS spectra of PBTTPD:PCBM blend film measured at different incidence angle.

REFERENCES
1. G. Yu, J. Gao, J. C. Hummulen, F. Wudl, A.J. Heeger, Science 270, 1789 (1995).
2. M. S. Su, C. Y. Kuo, M. C. Yuan, U. S. Jeng,; C. J. Su, K.-H. Wei, Adv. Mater. 23, 3315 (2011)