Submicron-scale manipulation of phase separation in organic solar cells
Fang-Chung Chen, Yi-Kai Lin, and Chu-Jung Ko
Citation: Applied Physics Letters 92, 023307 (2008); doi: 10.1063/1.2835047
View online: http://dx.doi.org/10.1063/1.2835047
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/2?ver=pdfcov Published by the AIP Publishing
Articles you may be interested in
Driving vertical phase separation in a bulk-heterojunction by inserting a poly(3-hexylthiophene) layer for highly efficient organic solar cells
Appl. Phys. Lett. 98, 023303 (2011); 10.1063/1.3541648
Solution processed inverted tandem polymer solar cells with self-assembled monolayer modified interfacial layers
Appl. Phys. Lett. 97, 253307 (2010); 10.1063/1.3530431
Electrical characterization of single-walled carbon nanotubes in organic solar cells by Kelvin probe force microscopy
Appl. Phys. Lett. 96, 083302 (2010); 10.1063/1.3332489
The use of thermal initiator to make organic bulk heterojunction solar cells with a good percolation path Appl. Phys. Lett. 93, 043304 (2008); 10.1063/1.2965468
Photovoltaic enhancement of organic solar cells by a bridged donor-acceptor block copolymer approach Appl. Phys. Lett. 90, 043117 (2007); 10.1063/1.2437100
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 140.113.38.11 On: Wed, 30 Apr 2014 23:17:39
Submicron-scale manipulation of phase separation in organic solar cells
Fang-Chung Chena兲 and Yi-Kai Lin
Department of Photonics, and Display Institute, National Chiao Tung University, Hsinchu, Taiwan 300, Republic of China
Chu-Jung Ko
Institute of Electro-optical Engineering, National Chiao Tung University, Hsinchu, Taiwan 300, Republic of China and National Nano Device Laboratories, Taiwan 300, Republic of China
共Received 12 November 2007; accepted 20 December 2007; published online 17 January 2008兲 This paper describes a method for controlling the submicron-scale phase separation of poly共3-hexylthiophene兲 and 共6,6兲-phenyl-C61-butyric acid methyl ester in organic solar cells. Using
microcontact printing of self-assembled monolayers on the device buffer layer to divide the surface into two regimes having different surface energies, an interdigitated structure aligned vertical to the substrate surface is achieved after spontaneous surface-directed phase separation. The power conversion efficiency increases upon decreasing the grating spacing, reaching 2.47%. The hole mobility increased as a consequence of improved polymer chain ordering, resulting in higher device efficiency, while smaller pattern sizes were used. © 2008 American Institute of Physics.
关DOI:10.1063/1.2835047兴
Organic photovoltaic devices are receiving increasing at-tention because their favorable properties, such as light weight, fabrication at low temperature, low cost, and me-chanical flexibility, are attractive for application in solar en-ergy conversion.1–4The “bulk heterojunction” structure pre-pared through the blending of conjugated polymers and fullerenes is commonly used for preparing polymer photo-voltaic cells on account of its simple device structure and ease of fabrication.1–4The two material phases form a ran-domly interpenetrating network having a large interfacial area that ensures the highly efficient dissociation of excitons. Because the separated electrons and holes must move through their two respective phases to the electrodes without significant recombination, the morphology of the polymer blend, which affects not only the charge separation but also the charge transportation, plays an important role in deter-mining the device efficiency.
The ideal morphology would be one that is vertically separated with an average interspacial distance equal to or less than the exciton diffusion length. Furthermore, the inter-digitated structure must be aligned perpendicular to the elec-trodes to provide direct pathways for efficient charge transportation.1,2 To achieve such an ideal structure, several groups have attempted to fabricate polymer solar cells with controllable vertical phase separation. Arias et al. reported that the use of organic solvents having high boiling points enhanced the vertical phase separation of a polymer mixture.5In addition, Lindner et al. designed a block copoly-mer to control the phase separation vertically.6 More re-cently, Kim et al. used an imprint method to construct a submicron-scale ordered structure on a polymer thin film. Depositing the electron acceptor onto the artificial structure led to vertical interpenetrative phase separation.7In this pa-per, we report an effective method for inducing self-organized phase separation in polymer photovoltaic devices. We obtained an interdigitated structure vertical to the sub-strate surface through spontaneous phase separation directed
by patterning the device buffer layer, using microcontact printing 共CP兲, into two regimes having different surface energies.
Figure 1 presents the process flow of this approach. Polydimethylsiloxane共PDMS兲 共Sygard184兲 was used as the mold to transfer the pattern. The patterns on the PDMS molds, including grating sizes of 1.0, 0.75, and 0.5m, were transferred from silicon wafers, which were fabricated using standard semiconductor processes. Using CP, the PDMS mold defined a patterned area of a self-assembled monolayer 共SAM兲 prepared from 3-aminopropyltriethoxysilane 共APTES兲 on a layer of poly共3,4-ethylenedioxythiophene兲:poly共styrenesulfonate兲 共PEDOT:PSS兲, which had been deposited on the indium tin oxide 共ITO兲 substrate. Before CP, the PEDOT:PSS was pretreated with UV ozone for 15 min and some dangling bonds of oxygen were, therefore, produced on the surface. When aminosilane contacted with the oxidized surface, the SAM molecules were spontaneously linked onto the surface via a dehydration reaction. For the preparation of the active layer, poly共3-hexylthiophene兲 共P3HT兲 and 共6,6兲-phenyl-C61-butyric acid methyl ester共PCBM兲 共1:1, w/w兲 were
dis-solved in 1,2,4-trichlorobenzene. The solution were spin coated at 600 rpm on the SAMs patterned PEDOT:PSS layer.
a兲Electronic mail: [email protected]. FIG. 1.study. 共Color online兲 Fabrication processes of polymer solar cells in this
APPLIED PHYSICS LETTERS 92, 023307共2008兲
0003-6951/2008/92共2兲/023307/3/$23.00 92, 023307-1 © 2008 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
The thickness of the active layer was 260⫾10 nm. After spin coating, the active layer was spontaneously dried in a covered Petri dish for 20 min to induce lateral phase separation.4Then, the polymer film was heated at 110 ° C for 15 min to remove the residue solvent. Finally, Al and Ca were deposited on the active layer as the cathode and the whole device was placed in a glove box filled with nitrogen gas. The dark and light current density-voltage共J-V兲 curves were measured using a Keithley 2400 source meter. The pho-tocurrent was obtained under illumination from a Thermal Oriel solar simulator共AM1.5G兲. The illumination intensity was calibrated using a standard Si photodiode detector equipped with a KG-5 filter.8
The SAM grating pattern on the PEDOT:PSS layer were firstly examined by using atomic force microscopy 共AFM兲. Figures 2共a兲–2共d兲 display SAM patterns of varying sizes. When the grating size was 0.5m, the edge of the grating was slightly unclear as a result of defects in the template. After coating the P3HT/PCBM blend on the patterned sur-face, phase separation clearly occurred following the grating pattern after solvent annealing关cf. Figs.2共b兲,2共d兲, and2共e兲兴. Unlike the conventional phase separation of P3HT and PCBM which is usually controlled through thermal treatment or solvent evaporation,3,4the phase separation induced by the underlying pattern occurs through a process known as pattern-directed spinodal decomposition.9Driven by the two different surface free energies presented on the buffer PEDOT:PSS layer, the two phases adhered spontaneously and selectively onto their preferred individual surfaces.
Several authors have reported that spinodal decomposi-tion occurs efficiently in polymer-blend films on SAM-patterned surfaces.9–11We note that well-defined phase sepa-ration in our system occurred only after manipulation of the SAM pattern. Furthermore, we utilized scanning electron mi-croscopy to examine the variations in thickness of the P3HT and PCBM thin films while dipping the patterned sub-strates into the solutions. We found that P3HT adhered fa-vorably onto the APTES surface pattern rather than on the PEDOT:PSS surface. In contrast, the adhesion of PCBM on the SAM-patterned surface was quite limited. Thus, we infer that the P3HT-rich regime was located on the APTES-patterned areas and the PCBM-rich phase on the exposed PEDOT:PSS.
Figure 3 displays the J-V characteristics of the PV de-vices incorporating the various grating patterns. The device efficiency increased upon decreasing the grating size. Each device had a similar open-circuit voltage共Voc兲. The device covered with the SAM without patterning exhibited poor per-formance. The calculated power conversion efficiency共PCE兲 was 1.43%. When using grating diameters of 1.00 and 0.75m, the values of the PCE increased to 2.22% and 2.41%, respectively. At a grating spacing of 0.5m, the val-ues of JSCand PCE improved to 7.76 mA/cm2 and 2.47%,
respectively. Figure3provides a summary of the overall de-vice performance with respect to the grating period. We ob-serve that the short circuit current density increased upon decreasing the grating spacing.
To determine the effect on the film internal resistance respect of the grating spacing, we derived device series re-sistances共Rs兲 from the slopes of the J-V curves obtained in
the dark.12,13 We found that the series resistance gradually reduced from 3.21 to 3.04⍀ cm2 when the grating size
de-creased from 1.00 to 0.50m. The nonpatterned device had an even larger resistance. The major contributions to the value of Rsinclude the bulk resistance共Rs,bulk兲 and the
con-tact resistance 共Rs,contact兲. The value of Rs,contact originates
from the interface between the electrodes and the active layer. The value of Rs,bulkarises from the bulk resistance of
the organic layers 共P3HT:PCBM blends and PEDOT:PSS兲 and the electrodes 共ITO and cathode metals兲. Because the contact area at the electrodes was similar in each device, the value of Rs,contact was constant, irrespective of the grating
size. Therefore, any series resistance variation probably arose from the reduction of the bulk resistance. Conse-quently, we infer that our polymer chains became more or-dered when the smaller grating size was used.
To further characterize the thin film morphologies, we extracted the values of the hole mobilities from the current density-voltage characteristics of the hole-only devices in the dark关Fig.4共a兲兴.14We fabricated these devices using a high-work-function material, molybdenum oxide 共MoO3兲, as the
buffer layer at the cathode to block the injection of electrons. The hole mobility was calculated at the regime of the space-charge-limited current regime according to the equation14
J = 90V2/8L, 共1兲
where0is the permittivity of the polymer,is the carrier
mobility, and L is the device thickness. From Fig.4共a兲, we know that the hole mobility of the device prepared without
FIG. 2. 共Color online兲 Tapping mode AFM images of the SAM-patterned PEDOT:PSS films关共a兲, 共c兲, and 共d兲兴 and the P3HT:PCBM films on the SAM-patterned PEDOT:PSS 关共b兲, 共d兲, and 共f兲兴. The grating sizes were 1.00m in共a兲 and 共b兲, 0.75m in共c兲 and 共d兲, and 0.50m in共e兲 and 共f兲.
FIG. 3. Current density vs voltage共J-V兲 characteristics of the devices with various SAM-patterned grating sizes measured under illumination 共simu-lated AM1.5G, 100 mW cm−2兲.
023307-2 Chen, Lin, and Ko Appl. Phys. Lett. 92, 023307共2008兲
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 140.113.38.11 On: Wed, 30 Apr 2014 23:17:39
SAM patterning was 2.53⫻10−8 m2/V s. On the other hand,
for the devices prepared with grating sizes of 1.00, 0.75, and 0.50m, the mobilities increased to 3.94⫻10−8, 4.10
⫻10−8, and 4.23⫻10−8m2/V s, respectively. The increased
hole mobility observed upon reducing the grating size sug-gests that pattering the SAMs through contact printing helps to align the P3HT phase.
It has been reported that the presence of PCBM in P3HT hampers the crystallization of the polymer.15 In our device prepared using the larger grating pattern, although phase separation occurred, the two components did not segregate perfectly. Even though PCBM has very high diffusivity in P3HT, some PCBM remained in the P3HT-rich domain prior to solidification of the films. In contrast, when we used the smaller grating size, both molecules had a shorter distance for diffusion. As a result, the degree of phase separation was more complete. In other words, the amount of PCBM in the P3HT-rich domain was lower for the film blend when the grating size was shorter. Consequently, we believed that P3HT aligned in a more orderly manner in the devices con-taining the smaller grating size.
To confirm this hypothesis, we examined the absorp-tion spectra of the P3HT:PCBM films patterned on the
PEDOT:PSS layer at various grating sizes as Fig.4共b兲. In all cases, we clearly observe vibronic features arising from P3HT.16Furthermore, the absorption shoulders became more pronounced upon decreasing the grating size. These results are consistent with the improved hole mobility. Both phe-nomena suggest that the patterned SAMs assisted with the organization of the main chains and reduced the presence of irregularities in the blend film. Hence, improved- stack-ing enhanced the charge mobility.
In summary, we have utilized SAM patterning to control the phase separation of P3HT and PCBM in a film blend on the path to fabricating polymer solar cells. With a smaller pattern size, the two-component phase separation occurred more effectively. Because the presence of PCBM hampers the alignment of P3HT, the polymer chains became more ordered when the distant for diffusion became shorter, lead-ing to better phase separation. We suggest that the hole mo-bility increased because of improved - stacking of the polymer chains on the patterned PEDOT:PSS, resulting in higher device efficiency.
We thank the National Science Council 共NSC-95-2221-E-009-305 and NSC-96-ET-7-009-001-ET兲 and the MOE ATU program for financial support.
1K. M. Coakley and M. D. McGehee, Chem. Mater. 16, 4533共2004兲. 2S. Gunes, H. Neugebauer, and N. S. Sariciftci, Chem. Rev.共Washington,
D.C.兲 107, 1324 共2007兲.
3W. L. Ma, C. Y. Yang, X. Gong, K. Lee, and A. J. Heeger, Adv. Funct.
Mater. 15, 1617共2005兲.
4G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery, and Y.
Yang, Nat. Mater. 4, 864共2005兲.
5A. C. Arias, N. Corcoran, M. Banach, R. H. Friend, J. D. MacKenzie, and
W. T. S. Huck, Appl. Phys. Lett. 80, 1695共2002兲.
6S. M. Lindner, S. Huttner, A. Chiche, M. Thelakkat, and G. Krausch,
Angew. Chem., Int. Ed. 45, 3364共2006兲.
7M. S. Kim, J. S. Kim, J. C. Cho, M. Shtein, L. J. Guo, and J. Kim, Appl.
Phys. Lett. 90, 123113共2007兲.
8V. Shrotriya, G. Li, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, Adv.
Funct. Mater. 16, 2016共2006兲.
9A. Karim, J. F. Douglas, B. P. Lee, S. C. Glotzer, J. A. Rogers, R. J.
Jackman, E. J. Amis, and G. M. Whitesides, Phys. Rev. E 57, R6273 共1998兲.
10J. K. Cox, A. Eisenberg, and R. B. Lennox, Curr. Opin. Colloid Interface
Sci. 4, 52共1999兲.
11P. Cyganik, A. Bernasik, A. Budkowski, B. Bergues, K. Kowalski, J. Rysz,
J. Lekki, and M. Lekka, Vacuum 63, 307共2001兲.
12W. D. Johnston, Jr., Solar Voltaic Cells共Dekker, New York, 1980兲. 13D. W. Sievers, V. Shrotriya, and Y. Yang, J. Appl. Phys. 100, 114509
共2006兲.
14V. Shrotriya, Y. Yao, G. Li, and Y. Yang, Appl. Phys. Lett. 89, 063505
共2006兲.
15A. Swinnen, I. Haeldermans, M. Vande Ven, J. D’Haen, G. Vanhoyland, S.
Aresu, M. D’Olieslaeger, and J. Manca, Adv. Funct. Mater. 16, 760 共2006兲.
16M. Sundberg, O. Inganas, S. Stafstrom, G. Gustafsson, and B. Sjogren,
Solid State Commun. 71, 435共1989兲. FIG. 4.共a兲 Dark J-V curves of the hole-only devices with various grating
sizes.共b兲 The UV-vis spectrum of the P3HT:PCBM films on PEDOT:PSS patterned with different grating sizes.
023307-3 Chen, Lin, and Ko Appl. Phys. Lett. 92, 023307共2008兲
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 140.113.38.11 On: Wed, 30 Apr 2014 23:17:39
