10.1117/2.1200805.1149
Spontaneous phase separation
for efficient polymer solar cells
Fang-Chung Chen
Inducing self-organized phase separation in polymer-fullerene blends
by using patterned substrates to modify surface energies improves the
performance of organic photovoltaics.
Polymer solar cells offer a potentially inexpensive way to produce electricity from sunlight and are therefore receiving an increasing amount of attention.1–4 Many of these cells have a ‘bulk
heterojunction’ structure, which is produced by blending conjugated polymers with the spherical carbon molecules known
as fullerenes (C60 ).1–4 The conjugated polymers and fullerenes
have different electrical properties. The polymer is a p-type conductor, with positive ‘holes’ as its major charge carrier, while the
fullerenes are an n-type conductor, with negative electrons as
their major charge carrier. These two materials naturally form a
randomly interpenetrating network with a large interfacial area,
which ensures the highly efficient dissociation of excitons. These
are electron-hole pairs that form when a photon of light hits the
solar cell. They subsequently separate into individual holes and
electrons when they encounter a junction between the two types
of conductors.
Because the separated electrons and holes must then move
through two different materials to the electrodes—the polymer for the holes and the fullerenes for the electrons—the precise structure of the polymer-fullerene blend plays an important role in determining the efficiency of the solar cell. Ideally,
the conjugated polymer and fullerenes would be vertically separated, with an average interspacial distance that is equal to or
less than the exciton diffusion length (10–20nm). Furthermore,
the polymer-fullerene blend should be aligned perpendicular to
the electrodes to provide direct pathways for efficient charge
transportation.1, 2
Several research groups have attempted to fabricate polymer
solar cells that possess this kind of vertical separation between
the conjugated polymer and the fullerenes. The group of Arias
reported that organic solvents with high boiling points enhanced
the vertical phase separation of a polymer mixture.5 Kim and
colleagues used an imprint method to generate an ordered struc-
Figure 1. Illustration of the fabrication process for polymer solar
cells with spontaneous surface-directed phase separation. PEDOT:PSS: Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate).
ITO: Indium tin oxide. u-CP: Microcontact printing. PCBM: (6,6)Phenyl-C61 -butyric acid methyl ester. P3HT: Poly(3-hexylthiophene).
ture on a polymer thin film. By depositing an n-type conductor
onto the artificial structure, the researchers managed to induce
vertical separation.6
Recently, we reported an effective method for inducing selforganized phase separation in polymer solar cells, leading to an
ideal structure and an improved power conversion efficiency.7
This phase separation was achieved by using microcontact printing (µ-CP) to pattern the device buffer layer into two regimes
with different surface energies.
Pattern-directed spinodal decomposition has been reported in
polymer blends with patterned surfaces of self-assembled monolayers (SAMs).8 These SAM materials can modify the surface
energy of the substrate, which controls local boundary interactions between the polymer blends and the substrate, resulting
in directed phase separation. In other words, phase separation
in the polymers can be induced by patterning the substrates.
Continued on next page
10.1117/2.1200805.1149 Page 2/3
Figure 3. A plot of current density versus voltage characteristics of
the devices with various SAM-patterned grating sizes measured under
illumination.
Figure 2. Tapping mode AFM images of the self-assembled monolayer
(SAM)-patterned PEDOT:PSS films (a, c, e) and the P3HT:PCBM
layer on the SAM-patterned PEDOT:PSS films (b, d, f). The grating
sizes were 1.00µm in (a) and (b); 0.75µm in (c) and (d); and 0.50µm
in (e) and (f).
Figure 1 illustrates the process. Using µ-CP, a polydimethylsiloxane mold produces a defined pattern in a SAM prepared from 3-aminopropyltriethoxysilane on a layer of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS),
which has been deposited on an indium tin oxide (ITO) substrate.
To produce the active layer, poly(3-hexylthiophene) (P3HT)
and the fullerene derivative (6,6)-phenyl-C61 -butyric acid
methyl ester (PCBM) (1:1, w/w) are dissolved in 1,2,4trichlorobenzene. This solution is then spin-coated onto the
SAM-patterned PEDOT:PSS layer. After spin coating, the active
layer is spontaneously dried to induce lateral phase separation,
and the polymer film is then heated at 110◦ C for 15min to remove the residue solvent. Finally, aluminum and calcium are
deposited onto the active layer to form the cathode.
We first examined the SAM grating pattern on the PEDOT:PSS
layer using an atomic force microscope (AFM). Figure 2, panels
(a), (c), and (d) show AFM images of SAM patterns of varying
sizes. When the grating size was 0.5µm, the edge of the grating
was slightly unclear due to defects in the template. After coating the P3HT/PCBM blend onto the patterned surface, phase
separation clearly occurred in line with the grating pattern, as
shown in Figure 2(b), (d), and (f). It appears that the underlying
pattern induced the self-organized phase separation.8 Driven by
the two different surface free energies on the PEDOT:PSS layer,
P3HT and PCBM adhered spontaneously and selectively onto
their preferred individual surfaces.
Figure 3 displays the electrical characteristics of devices incorporating the various grating patterns, showing that the device efficiency increases as the grating size decreases. The device
covered with the SAM with no patterning showed poor performance, with a calculated power conversion efficiency (PCE) of
1.43%. For grating diameters of 1.00 and 0.75µm, the PCE values
increased to 2.22 and 2.41%, respectively. At a grating spacing of
0.5µm, the PCE improved to 2.47%.
We are now exploring a number of different approaches for
further improving the efficiency of these devices. These include
reducing the dimensions of the grating to the diffusion length
of excitons in order to suppress exciton quenching. We also plan
to use a dot-like pattern, rather than the striped patterns that we
employed here to demonstrate the concept. With such a 3D interpenetrating structure, a dot-like pattern would increase the area
of the p-n junctions and might further improve the efficiencies
of charge separation and transportation. In addition, inserting
hole- and electron-blocking layers between the cathode and anode, respectively, and the active layer might inhibit the recombination of charges at the electrodes.2
The author would like to acknowledge financial support from the National Science Council and the Ministry of Education ATU Program.
Continued on next page
10.1117/2.1200805.1149 Page 3/3
Author Information
Fang-Chung Chen
Department of Photonics
National Chiao Tung University
Hsinchu, Taiwan
Fang-Chung Chen received his PhD in materials science and engineering from the University of California, Los Angeles. He is
currently an assistant professor in the department of photonics
and the Display Institute at National Chiao Tung University. His
research interests include polymer light-emitting diodes, plastic solar cells, organic thin-film transistors, and microlens arrays
and their application in flat panel displays.
References
1. K. M. Coakley and M. D. McGehee, Conjugated polymer photovoltaic cells, Chem.
Mater. 16, pp. 4533–4542, 2004. doi: 10.1021/cm049654n
2. S. Gunes, H. Neugebauer, and N. S. Sariciftci, Conjugated polymer-based organic
solar cells, Chem. Rev. 107, pp. 1324–1338, 2007. doi: 10.1021/cr050149z
3. W. L. Ma, C. Y. Yang, X. Gong, K. Lee, and A. J. Heeger, Thermally stable, efficient
polymer solar cells with nanoscale control of interpenetrating network morphology, Adv.
Funct. Mater. 15, pp. 1617–1622, 2005. doi: 10.1002/adfm.200500211
4. G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, Highefficiency solution processable polymer photovoltaic cells by self-organization of polymer
blends, Nat. Mater. 4, pp. 864–868, 2005. doi: 10.1038/nmat1500
5. A. C. Arias, N. Corcoran, M. Banach, R. H. Friend, J. D. MacKenzie, and
W. T. S. Huck, Vertically segregated polymer-blend photovoltaic thin-film structures
through surface-mediated solution processing, Appl. Phys. Lett. 80, pp. 1695–1697,
2002. 10.1063/1.1456550
6. M. S. Kim, J. S. Kim, J. C. Cho, M. Shtein, L. J. Guo, and J. Kim, Flexible conjugated
polymer photovoltaic cells with controlled heterojunctions fabricated using nanoimprint
lithography, Appl. Phys. Lett. 90, p. 123113, 2007. doi: 10.1063/1.2715036
7. F. C. Chen, Y. K. Lin, and C. J. Ko, Submicron-scale manipulation of phase separation
in organic solar cells, Appl. Phys. Lett. 92, p. 023307, 2008. doi: 10.1063/1.2835047
8. A. Karim, J. F. Douglas, B. P. Lee, S. C. Glotzer, J. A. Rogers, R. J. Jackman, E.
J. Amis, and G. M. Whitesides, Phase separation of ultrathin polymer-blend films on
patterned substrates, Phys. Rev. E 57, pp. R6273–R6276, 1998. doi: 10.1103/PhysRevE.57.R6273
c 2008 SPIE