Conjugated polymer nanostructures for organic solar cell applications
Jiun-Tai Chen* and Chain-Shu Hsu*
Received 16th June 2011, Accepted 17th July 2011 DOI: 10.1039/c1py00275a
Recently, there has been tremendous progress in the development of polymer-based organic solar cells. Polymer-based solar cells have attracted a great deal of attention because they have the potential to be efficient, inexpensive, and solution processable. New materials, nanostructures, device designs, and processing methods have been developed to achieve high device efficiencies. This review focuses on the fabrication techniques of conjugated polymer nanostructures and their applications for organic solar cells. We will first introduce the fundamental knowledge of organic solar cells and emphasize the importance of nanostructures. Then we will discuss different strategies for fabricating conjugated polymer nanostructures, including topics such as polymer nanowires, nanoparticles, block copolymers, layer-by-layer deposition, nanoimprint lithography, template methods, nanoelectrodes, and porous inorganic materials. The effects of the nanostructures on the device performance will also be presented. Efficiencies higher than 10% are expected for polymer-based solar cells by using new materials and techniques.
Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin-Chu, 30049, Taiwan. E-mail: jtchen@mail.nctu.edu. tw; cshsu@mail.nctu.edu.tw; Fax: +886-3513-1523; Tel: +886-3513-1523
Jiun-Tai Chen
Jiun-Tai Chen received his B.S. degree in 1999 and M.S. degree in 2001 from the Department of Applied Chemistry at National
Chiao Tung University. He
joined Prof. Thomas Russell’s group in 2003 and completed his Ph.D. in 2008 at the University of Massachusetts, Amherst in
Polymer Science and
Engi-neering, where his thesis work
focused on template-based
nanomaterials. He then joined
the Center for Nano- and
Molecular Science and Tech-nology at the University of Texas at Austin with Prof. Paul F. Barbara as a postdoctoral fellow, where he worked on electrogenerated chemiluminescence of conjugated polymers. In the summer of 2010, he joined the Department of Applied Chemistry at National Chiao Tung University as an assistant professor. His research interests include the fabrication and characterization of polymer nanomaterials for optoelectronic applications.
Chain-Shu Hsu
Chain-Shu Hsu received his
Ph.D. degree from Case
Western Reserve University in
1987 and conducted
post-doctoral work at the National Tsing Hua University in Tai-wan. He joined the Department of Applied Chemistry of the National Chiao Tung Univer-sity, Taiwan in 1988 as an associate professor and was promoted to full professor in 1991. Currently he is serving as
a vice president and chair
professor of the National Chiao Tung University. His research interests include liquid crystalline polymers and conjugated poly-mers, polymer light-emitting diodes, and organic solar cells. He has published more than 200 research papers and 20 patents. He is currently on the international advisory board of Polymer and editorial boards of the Journal of Polymer Science, Polymer Chemistry, and the Journal of Polymer Research. He received the Excellent Research Award of the National Science Council, Tai-wan, in 1994, the Franco-Taiwan Scientific Award for nano-materials in 2006, Teco and Hou Chin Tui Awards in 2007, and an Academic Award of the Ministry of Education, Taiwan, in 2008.
Chemistry
Cite this: Polym. Chem., 2011, 2, 2707
www.rsc.org/polymers
REVIEW
1
Introduction
1.1 Organic solar cells
In recent years, energy-related issues have received considerable attention concerning the rising costs of fossils and growing global
greenhouse gas.1There is an urgent need to develop clean and
renewable energy technologies. The largest potential source of renewable energy is the solar energy incident on the Earth’s
surface.2 Solar cells are devices which convert solar energy
directly into electricity, and the most common material used for
solar cells is silicon.3 Although silicon-based solar cells exhibit
some of the highest power conversion efficiencies, they remain expensive because of the intensive processing techniques and the
high cost of purified silicon.4In this context, organic molecules
are alternative candidates for solar cells because of their low cost
and high processability.5,6
The most widely studied organic solar cells are polymer-based solar cells using conjugated polymers. Most conjugated polymers have high absorption coefficient and high percentage of absorbed
photons that can produce an excited state (>90%).7There are
other advantages for using the polymer-based organic solar cells. First, the photo and electronic properties of the conjugated polymers can be fine-tuned by changing the chemical structures
through advances in organic chemistry.8,9Second, simple coating
or printing processes can be used which will reduce the cost of the
fabrication process.10Third, the mechanical flexibility allows the
development of flexible devices.11 In addition to organic solar
cells, conjugated polymers have been used in other types of optoelectronic devices such as organic field-effect transistors
(OFETs) or organic light-emitting diodes (OLEDs).12,13
Although significant progress has been made in polymer-based solar cells, the maximum power conversion efficiencies (PCEs) are still not sufficient to be marketable. Also, the commerciali-zation of the polymer-based solar cells is limited by the lifetimes of the devices which are affected by the water and oxygen in the atmosphere. Therefore, more research effort will need to be devoted towards the commercialization of polymer-based solar
cells.14
1.2 Working principles of organic solar cells
The typical structure of an organic solar cell is shown in Fig. 1. A hole transport layer, poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS), is spin-coated on top of the
anode. The active layer comprising the donor and the acceptor is sandwiched between the cathode and the hole transport layer. The process for organic solar cells to convert sunlight into elec-tricity is described as follows: The light-absorbing material with a bandgap in the visible region absorbs photons that excite the electrons from the ground state to the excited state, and bound electron-hole pairs (excitons) are created. The excitons diffuse to the donor–acceptor interface where excitons dissociate into free charge carriers after overcoming the binding energies. The free charge carriers transport to the respective electrodes under the
internal electric fields, resulting in the generation of
photocurrent.
Power conversion efficiency (PCE) is used to evaluate the
performance of polymer solar cells.5The PCE of an organic solar
cell is determined by the following equation:
PCE¼ (FF Jsc Voc)/Pin (1)
where PCE is the power conversion efficiency, FF is the fill
factor, Voc is the open-circuit voltage, Jsc is the short-circuit
current, and Pinis the power density of the incident light. The
solar cells are usually tested under Air Mass (AM) 1.5G
condi-tions, 100 mW cm2. These conditions are experienced when the
sun is at an angle of about 48 and are considered to best
represent the Sun’s spectrum on the Earth’s surface.15Fill factor
is the ratio of the actual power limit to the theoretical power limit of a solar cell, which can be calculated from the division of the
largest power output (Pmax) by the product of Jscand Voc, as
shown in Fig. 2. Open-circuit voltage (Voc) is the maximum
possible voltage across a solar cell. The value of Vocis close to the
energy difference between the highest occupied molecular orbital (HOMO) of the electron donor and the lowest unoccupied
molecular orbital (LUMO) of the electron acceptor (see Fig. 3).16
The short-circuit current (Jsc) is the current through the solar cell
when the voltage across the solar cell is zero.
One of the most promising candidates for polymer organic solar cells is a blend of regioregular poly(3-hexylthiophene)
(rr-P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)
(Fig. 4).17Although the bandgap is only 2.0 eV, P3HT possesses
high hole mobility (up to 0.1 cm2V1s1) and great
self-organi-zation capability.18,19The mobility of P3HT is highly dependent
Fig. 1 Typical structure of an organic solar cell. PEDOT:PSS is spin-coated on top of the anode as a hole transport layer. The active layer is sandwiched between the cathode and the hole transport layer.
Fig. 2 Current–voltage (I–V) characteristics of an organic solar cell. Voc
is the open-circuit voltage and Jscis the short-circuit current. The points
where the product of current and voltage is maximized determine the largest power output (Pmax). The fill factor (FF) is obtained from the
division of Pmaxby the product of Jscand Voc.
on the crystallinity, the orientation of the polymer chains, and the molecular weight. PCBM has a high electron mobility and is soluble in common organic solvents. The combination of P3HT and PCBM dominates the current research in polymer solar cells. They also serve as good model materials to examine different effects on the device performance. In this review, many examples of solar cells based on conjugated polymer nanostructures are related to the fabrication of P3HT nanostructures followed by the deposition of PCBM. For example, P3HT nanorods can be made by a nanoporous template and function as the donor materials. Then PCBM can be deposited on the P3HT nanorods to form an ordered heterojunction.
1.3 Strategies to improve the device performance
After introducing the fundamental knowledge of organic solar cells, we will discuss some common strategies to improve the device performance of organic solar cells. As shown in Fig. 5, these strategies involve designing and synthesizing new materials, changing device structures, controlling morphology, and making nanostructures.
Although the Sun provides a vast amount of energy, only a small portion of the incident sunlight is absorbed because of the large bandgap of organic materials. The bandgap of typical conjugated polymers ranges between 2 and 3.5 eV, which limits the possible absorption of solar energy. For a material with
a band gap of 1.1 eV (1100 nm), 77% of the incident solar energy
on the Earth’s surface can be absorbed.5Therefore, it is necessary
to design and synthesize new low bandgap conjugated polymers that can absorb more of the solar energy. It also need to be noted that the device performance can be affected by not only the bandgap, but also the position of the HOMO and LUMO levels
of the conjugated polymers which can limit the Voc of the
device.16The absorption spectrum and energy levels of
conju-gated polymers can be tuned by functionalization. Many synthetic efforts have been made to develop new low bandgap
polymers.20,21
Towards the commercialization of organic solar cells, several issues with the standard organic solar cells have to be considered. First, the low work-function metal electrodes, such as calcium
and lithium, are unstable under ambient conditions.22Second,
the hole transport material, PEDOT:PSS, has been shown to react with the ITO electrode, resulting in the degradation of the
devices.23The acidic PEDOT:PSS layer is also detrimental to the
active layer. To resolve these issues, inverted device structures have been developed which allows the use of more stable high
work-function metals.24In the inverted structures, electrons and
holes exit the device in opposite directions, comparing with the normal device structures. ITO serves as the cathode and a more stable, high-work-function metal is used as the anode. The stability of the inverted device under ambient conditions is
dramatically improved, compared with the normal device.25
The morphology control of the active layer in solar cells is of great importance in improving the device efficiencies. Since exciton dissociation occurs at the interface of the donor and acceptor materials, a large interfacial area should allow
maximum exciton dissociation.26It has been shown that
post-treatments such as thermal annealing above the glass transition
temperature (Tg) of the active material are crucial for the device
performance, although low band-gap polymers often show
degraded device performance after thermal annealing.27,28 By
annealing, the active materials such as the P3HT chains can organize and self-assemble into a more regular, crystalline state, resulting in higher charge mobility. Acceptor materials such as PCBM will also diffuse and aggregate to form larger domains. The device performance is improved by the maximized donor– acceptor interfacial area and the higher degree of crystallinity. But the optimized morphology and phase segregation of the Fig. 3 Energy level diagram of an organic solar cell with a donor–
acceptor interface. The open-circuit voltage (Voc) is close to the energy
difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor.
Fig. 4 Chemical structures of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).
Fig. 5 Main research directions to improve the device performance of organic solar cells including materials, devices, morphology, and nanostructures.
active materials are at intermediate states that will form more
equilibrium morphologies upon further annealing.29With longer
annealing time, the donor–acceptor interfacial area decreases owing to larger size phase separation, resulting in lower effi-ciencies. Many efforts have been made to maintain the optimized morphology of the donor–acceptor heterojunction. Photo-crosslinkable P3HT copolymers or fullerene derivatives, for
example, are used to stabilize the bulk heterojuncitons.30–33The
morphology of the heterojunction can also be preserved by using a block copolymer containing oligothiophene and fullerene side
groups as a compatibilizer.26
Solar cells based on polymer heterojunction are considered to be better than single component polymer solar cells. The inter-facial area between the donor and acceptor materials can be increased and more exciton dissociation can occur to generate higher photocurrent. The concept of heterojunction was first introduced by Tang: bilayer heterojunction structures can be
used for creating efficient charge separation.34This concept was
then applied to polymer-based solar cells and the efficiency was dramatically increased, because of the large donor–acceptor
interfaces.35Bulk heterojunction polymer solar cells are usually
based on two components including an electron-donating material (donor) and an electron-accepting material (acceptor). Disordered structures on the nanoscale will result in poor device
efficiencies due to exciton recombination and poor mobility.36It
is necessary to control the morphology of the active materials on the nanoscale. There are several possible morphologies for the donor–acceptor heterojunction. The simplest case is the bilayer structures, where the electron-donor is first deposited on the anode, followed by the deposition of the electron-acceptor (see
Fig. 6a).37But the bilayer devices only allow excitons to
disso-ciate near the donor–acceptor interface, resulting in low device
efficiencies.38In order to improve the solar cell efficiencies, it is
critical to control the donor–acceptor interface in order to optimize charge separation and charge migration to the elec-trodes. The most common way to make bulk heterojunction organic solar cells is by blending donor and acceptor materials. The interfacial area between donor and acceptor significantly increase compared with the double layer devices, resulting in improved device efficiencies (see Fig. 6).
In order to achieve high device performance, the domain sizes of the donor and the acceptor need to be optimized. The optimal domain size is related to the diffusion length of excitons. Exciton diffusion length is the average distances the excitons travel before recombination, which depends on the lifetime and the diffusion coefficient of the excitons. The diffusion length of excitons in the polymer-based organic solar cells is usually around 5–10 nm. Therefore, the ideal size of the nanostructures should be close or equal to the diffusion length. A larger interface is produced by
reducing the domain sizes, allowing more exciton dissociation while reducing the recombination of excitons. Fig. 6c represents the ideal heterojunction morphology of organic solar cells, where the donor and the acceptor domains are aligned normal to the
electrode surfaces.39Also, there should be a continuous donor
film in contact with the anode and a continuous acceptor film in contact with the cathode. The thickness of the heterojunction also needs to be optimized. The light absorption depends on not only the bandgap of the materials, but also the thickness of the absorbing materials. The absorption coefficients of conjugated
polymers are relatively high (105cm1), and the film thickness
of 100–200 nm should allow efficient absorption.5,36 Higher
thickness will increase chances of recombination of the excitons, even though more photons might be absorbed. The crystallinity and charge mobilities can also be changed by the confinement effect at the nanoscale. For example, the hole mobility in regioregular P3HT was found to enhance by a factor of 20 when the polymers were infiltrated into straight nanopores of an
anodic alumina template.40In addition to the diffusion length of
excitons, the optimized domain sizes also depend on the packing of molecules, which affects the charge transport in devices,
concerning the p–p interaction between conjugated polymer
chains.
There are some excellent review articles on conjugated
poly-mers and polymer-based solar cells.5,6,8,11,17,24,41,42 This review
aims to highlight different fabrication techniques for polymer nanostructures for the application of organic solar cells. As shown in Fig. 7, these methods are divided into subjects including polymer nanowires, nanoparticles, block copolymers, layer-by-layer deposition, nanoimprint lithography, template methods, nanoelectrodes, and porous inorganic materials. Here we mainly focus on the nanostructures of the donor materials such as P3HT. In order to make heterojunction solar cells, the acceptor materials are usually deposited on the nanostructured donor materials. A typical example is to make P3HT nanorods using templates or nanoimprint lithography, then the PCBM can be deposited on the P3HT nanorods for constructing the donor– acceptor heterojunction. Although here we only focus on the discussion of donor nanostructures, these concepts can be
applied to make the acceptor nanostructures. For example, C60,
TiO2or ZnO nanorods can be generated by porous templates,
followed by the spin-coating of the P3HT.43–46
Fig. 6 Three possible donor–acceptor morphologies of organic solar cells. (a) Double layer morphology. (b) Phase-separated donor–acceptor blend morphology. (c) Ideal heterojunction morphology.
Fig. 7 Approaches to fabricate conjugated polymer nanostructures for applications in organic solar cells.
2
Strategies for fabricating polymer nanostructures
This part summarizes some common strategies and techniques for making polymer nanostructures for the application of organic solar cells. The device performance based on different nanostructures will also be presented.2.1 Polymer nanowires
One of the most common conjugated polymer nanostructures used in organic solar cells is conjugated polymer nanowires, for they provide percolation pathways for both electrons and holes, resulting in higher device efficiency. Nanowires are sometimes called nanocylinders, nanofibers, or nanowhiskers, meaning one-dimensional nanomaterials with high aspect ratios. The advan-tages of using the polymer nanowires approach for solar cell devices include (a) the morphology can be controlled; (b) the widths and lengths of polymer nanowires are matched to the exciton diffusion lengths; (c) the interfacial area between donor
and acceptor is large; (d) an electrically bicontinuous
morphology can be obtained; (e) high absorption coefficient and high carrier mobilities can be achieved; (f) devices on plastic substrates and devices with large areas can be easily produced; (g) the difficulties of blend phase-separation phenomena can be
avoided.47,48
Various techniques have been applied to prepare conjugated polymer nanowires. It has been found that the polymer
nano-wires can be simply observed by thermal annealing.28 For
example, P3HT nanowires were observed after annealing the
P3HT/PCBM mixture at 120 C for 60 min.28 The annealing
process increases the crystallinity of P3HT and enhances the demixing between P3HT and PCBM. Another simple method to
generate P3HT nanowires was studied by Sun et al.49They found
that P3HT nanowires and CdSe nanorods can be obtained by careful choice of the solvent used for spin-coating. 1,2,4-Tri-chlorobenzene (TCB), which has a high boiling point, was used as the solvent for P3HT and a fibrillar morphology was obtained. The power efficiency of the solar cell devices based on the composites of P3HT nanowires and CdSe nanorods was improved to 2.6% (AM 1.5G), compared with 1.8% where chloroform was used as the solvent and P3HT nanowires were
not formed.49
In addition to the previously mentioned methods, the two most common ways to make conjugated polymer nanowires for the applications in organic solar cells are the whisker method and the mixed-solvent method.
2.1.1 The whisker method. The whisker method was first
developed by Ihn et al.50 They reported that
poly(3-alkylth-iophene)s may readily crystallize from dilute solutions in rela-tively poor solvents in the form of ribbon-shaped whiskers. The formation of whiskers is dependent on the solvent quality, temperature, and the alkyl side-chain length. The widths of the whiskers are about 15 nm and their lengths often exceed tens of microns, so very high aspect ratios are observed for the whiskers. The polymer chains made by the whisker method were found to
pack with their backbones normal to the whisker direction.50
Fig. 8 shows an example using the whisker method to make P3HT nanowires. P3HT polymers are dissolved in p-xylene and
heated to 80 C. The solution is later cooled down to room
temperature and P3HT nanowires are formed.
To address the issue of what solvents are most suitable for nanofiber formation using the whisker method, Oosterbaan et al. performed systematic studies of the fiber formation of regiore-gular poly(3-alkylthiophene)s (P3ATs) with alkyl chain lengths
between 3 and 9 carbon atoms in several solvents.51 For the
aliphatic and (chlorinated) aromatic hydrocarbon solvents, the refractive index of the solvent was used to predict the feasibility of a particular solvent for the fiber formation. The effect of poly (3-alkylthiophene) (P3AT) crystallinity on the energy of the
intermolecular charge-transfer state (ECT) and open-circuit
voltage (Voc) in P3AT nanofibers:PCBM solar cells was also
investigated.52The P3AT crystallinity can be varied by
control-ling the temperature. The ECTwas found to increase slightly with
increasing side-chain length and the Vocfollowed the same trend
as ECTbecause of the morphological changes.52
Using the whisker method, P3HT nanofibers are most studied compared with other P3AT nanofibers. Berson et al. presented a new fabrication procedure to produce highly concentrated
solutions of P3HT nanofibers in p-xylene.53The concentration
range of up to 2% allowed the deposition of thick films and the obtained solutions were stable for several weeks. By mixing these nanofibers with an electron acceptor such as PCBM in solution, a highly efficient active layer for organic solar cells with a PCE of
up to 3.6% (AM 1.5G, 100mW cm2) was obtained without any
thermal post-treatment.53The maximum PCE was achieved with
the optimum composition of 75 wt% nanofibers and 25 wt% disorganized P3HT. It was proposed that the fraction of disor-ganized P3HT is probably necessary to fill the gaps in the nanostructures and intimate contact between the donor
nano-fibers and the acceptor domains can be ensured.53 The device
efficiency can also be improved by controlling the fiber content of the casting solution. Bertho et al. demonstrated that the fiber content of the P3HT-nanofiber:PCBM casting solution can be
easily controlled by changing the solution temperature.54 At
a solution temperature 45C, a 42% fiber content of the casting
solution was found and an optimal PCE of 3.2% was achieved,
which was linked to the morphology of the active layer.54
For the P3HT nanowires/PCBM system, Kim et al. tried to optimize solar cells based on the P3HT nanowires via solution
crystallization in DCM.55They studied the performances of the
solar cell devices based on the P3HT nanowires/PCBM composites as a function of solution ageing time. The PCE of 3.23% was achieved for the devices coated with a 60 h aged P3HT nanowires/PCBM blend solution. The ageing process was thought to increase both light absorption and charge balance. Fig. 8 Schematic illustration showing an example of using the whisker method to make P3HT nanowires. P3HT polymers are dissolved in p-xylene and heated to 80C. The solution is later cooled down to room temperature and P3HT nanowires are formed.
When a pure donor phase layer was inserted between the ITO/ PEDOT:PSS and P3HT nanowires/PCBM layers, an even higher
PCE of 3.94% was achieved.55
For solar cells based on the P3HT nanowires, other electron acceptors other than PCBM have also been used. Salim et al. reported the first application of the preassembled P3HT nano-wires with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT)
in all-polymer solar cells.56The role of the polymer nanowires on
the morphology of the polymer blends was investigated. Compared with as-cast blends, an enhancement in the
short-circuit current (Jsc) by a factor of 10 was achieved when P3HT
nanowires were added into the blends with polyfluorene copol-ymers. A higher PCE was achieved for the solar cells comprising the nanowire blends, even compared with the thermally annealed ones. The enhanced device performance was attributed to the enhanced optical absorption and charge transport originating
from the high crystallinity of the polymer nanowires.56
Inorganic nanorods have also been used in the solar cells based on P3AT nanowires. Jiu et al. demonstrated that the use of preformed poly(3-alkylthiophene) nanowires in hybrid solar cells
with CdSe nanorods achieved a PCE of 1%.57 The device
performance from nanowires prepared from poly(3-butylth-iophene) (P3BT) was better than that from poly(3-hexylth-iophene) (P3HT). The CdSe nanorods were found to be dispersed within the interpenetrated 3-D network of interconnected poly-mer nanowires, which improved electron and hole transport in the hybrid film. The maximum PCE of 1.01% (AM 1.5G,
100 mW cm2) was achieved with the optimum composition of
25 wt% P3BT nanowires and 75 wt% CdSe nanorods.57
Other P3AT nanowires-based solar cells other than P3HT have also been studied. By using poly(3-butylthiophene) nano-wires (P3BT-NW) as the donor and PCBM as the acceptor, Xin et al. reported that solar cells with 3.0% power conversion
effi-ciency (AM 1.5G, 100 mW cm2, 10 mm2device area) was
ach-ieved.47 The performance achieved was 1 order of magnitude
higher than that from thermally induced phase-separated P3BT: PCBM blend. In their approach, P3BT-NW/PCBM nano-composites exhibited an electrically bicontinuous morphology without going through the path of blend phase-separation phenomena. The results indicated that the P3BT NWs constitute an ideal donor component for enhanced exciton diffusion and
charge transport in solar cell devices.47Xin et al. also fabricated
bulk heterojunction solar cells based on blends of regioregular
poly(3-butylthiophene) (P3BT) nanowires and phenyl-C61
-butyric acid methyl ester (PCBM) using in situ self-assembly of
P3BT nanowires.48The approach of in situ self-assembly has the
advantages of simplifying and combining the previously separate process of preparing P3BT NWs and blending with PCBM into a single process. The P3BT NWs were found to self-assemble to an interconnected network in the presence of the PCBM. The device performance was found to depend strongly on the blend composition. A maximum PCE of 2.52% was achieved at a blend
ratio of 1 : 0.5 (wt : wt) P3BT : PCBM.48
Although a power conversion efficiency (PCE) of 3.0% is
achieved from the P3BT-NW/fullerene composite,47the device
structure was not optimized. Xin et al. systematically varied the morphology of P3BT-NW/fullerene composites by using
a combination of thermal and solvent annealing.58 The
photovoltaic performance was found to vary with the induced
structural variations. In unannealed devices, fullerene was found to disperse homogeneously in the P3BT-NW matrix and poor photovoltaic performances was obtained. The aggrega-tion of fullerene in the interstitial spaces of the nanowire network was induced by thermal annealing and improved photovoltaic performance was obtained. The authors sug-gested that the best device performance can be achieved when the ideal interpenetrating network of nanowires and fullerene is maintained while avoiding the device bridging of the
poly-mer nanowires.58
Of all the regioregular poly(alkylthiophene) nanowires, regioregular poly(3-pentylthiophene) (P3PT), which has
odd-numbered alkyl side chains (CnH2n+1, n¼ 5), is less studied. Wu
et al. first reported the fabrication of regioregular poly(3-pen-tylthiophene) (P3PT) nanowires and their applications in solar
cells.59The P3PT nanowires with a width of 16–17 nm and aspect
ratios of 70–465 were assembled from dichlorobenzene solution.
The power conversion efficiency (AM 1.5G, 100 mW cm2) of
bulk heterojunction solar cells based on P3PT nanowires/
PC71BM nanocomposites was 3.33%, while the power conversion
efficiency of bulk heterojunction solar cells based on P3PT/
fullerene (PC71BM) blend thin films was 3.70%.59
Block copolymer nanowires have also been fabricated using the whisker method. Ren et al. studied solution-phase self-assembled nanowires from diblock copolymer semiconductors,
poly(3-butylthiophene)-block-poly(3-octylthiophene).60 For the
copolymer nanowires, the authors tried to control the aspect ratio of solution phase assembled nanowires and studied the effects of the aspect ratio on their properties and device perfor-mances. The aspect ratio of the diblock copolymer nanowires was controlled by the copolymer composition. The vertical charge transport of the nanowires/fullerene thin films was found to be independent of aspect ratio of the nanowires, indicating a parallel orientation of the nanowires to the underlying
substrate.60But the power conversion efficiency of the solar cells
based on nanowires/PC71BM nanocomposites was found to be
dependent on the aspect ratio of the nanowires. A power conversion efficiency of 3.4% was achieved when the highest average aspect ratio of 260 was used. The improvement of device performance with the aspect ratio of nanowires was attributed to the increased exciton and charge photogeneration and collection
in the solar cells.60
2.1.2 The mixed-solvent method. The second common ways
to make conjugated polymer nanowires for the application of
solar cells is by using mixed solvents.61–66In the mixed-solvent
method, the polymer nanowires are driven by using a combina-tion of good and bad solvents. The polymers are usually dis-solved in a good solvent. Then a small quantity of bad solvent is added to the solution. The mixed solvent methods are based on the idea that the unfavourable interactions between the polymer chains and the bad solvent can induce the aggregation and self-assembly of the polymer chains. Therefore, the amount of the bad solvent in the solution will determine the degree of crystal-lization of the polymers. Fig. 9 shows an example of making P3HT nanowires using the mixed-solvent method. The P3HT polymers are first dissolved in a good solvent. After the bad solvent is added, P3HT polymers start to aggregate and P3HT nanowires are formed.
The mixed-solvent method was used by Kiriy et al. to fabricate one-dimensional polyalkylthiophene aggregation in dilute
solu-tion by adding a poor solvent to the solusolu-tion.61Hexane, a good
solvent for alkyl side chains but a poor solvent for polythiophene backbones, was used and the polyalkylthiophene formed ordered main-chain collapse driven by the solvophobic interaction. Length of the polyalkylthiophene aggregation can be adjusted by the concentration of polyalkylthiophenes or the solvent
composition.61 The solar cell performance of these
poly-alkylthiophene aggregations was not reported. Moule et al. then used a similar mixed-solvent method to determine the agglom-erated–amorphous ratio of P3HT and to control the degree of agglomeration/crystallinity of P3HT in the P3HT/PCBM solar
cell mixtures.62The advantage of this method is that the filtering
is not required to obtain pure agglomerated P3HT, and further heat-treatment of the polymer is not required. By adding nitro-benzene as the dipolar solvent, the ratio of the P3HT in the amorphous and aggregated phases was controlled and P3HT aggregates were formed. A power conversation efficiency of 4% (AM 1.5G) was achieved based on the P3HT/PCBM solar cell
mixtures with no pre- or post-treatment steps.62
The mixed solvent method was also used by Li et al. to make ordered aggregates of P3HT, and the crystallinity of P3HT was
substantially increased.63Hexane, a poor solvent for P3HT, was
titrated into well-dissolved P3HT-o-dichlorobenzene (ODCB) solution. A power conversion efficiency of 3.9% was achieved based on a P3HT:PCBM composite using this method, almost
four times than that of the pristine device.63 Zhao et al. also
prepared P3HT solution containing crystalline P3HT nanofibers
by adding a small amount of acetone into the solution.64 The
solvatochromic phenomenon in the P3HT was observed when acetone was added, and the solution color shifted gradually from orange to dark purple with more acetone content. The best power conversion efficiency of 3.60% was achieved based on the solar cell devices with the P3HT:PCBM composites prepared from the chlorobenzene solution containing 2.5% acetone. This value was higher than 3.45% from the control device, showing that the hole transport is enhanced by adding small amount of P3HT nanofibers because of the good connectivity of P3HT
nanofibers, and the demixing of PCBM is not influenced.64
Using the mixed solvent method, Kim et al. studied the
soni-cation-assisted self-assembly of P3HT nanowires with PC61BM
in a cosolvent system containing acetonitrile as the polar
solvent.67The self-assembly of P3HT nanowires was found to
depend on the regioregularity of P3HT, solvent polarity, and ultrasonic irradiation. A power conversion efficiency of 4.09%
was achieved based on the thermally annealed solar cells having
the self-assembled 98% regioregular P3HT nanowires/PC61BM
by the sonication-assisted self-assembly.67
In order to prepare well-controlled nanoscale morphologies in the P3HT nanowires/PCBM film, Kim et al. described a two-step
process.65 The first process was the in situ formation of
self-organized P3HT nanowires by adding the marginal solvent, cyclohexanone, to the blend solution in chlorobenzene. The second process was the nanoscale phase separation achieved by mild thermal annealing. This dual process effectively reduced the interference between P3HT crystallization and phase separation. The nanoscale PCBM domains were developed in the second step and bicontinuous percolation pathways between the P3HT and PCBM components were produced. The power conversion effi-ciency of 4.04% was achieved based on the P3HT nanowires/ PCBM film fabricated by the two-step process, with a
photo-current density of 10.9 mA cm2and a fill factor of 62.1%.65
Different from previous methods, Sun et al. reported an
unusual mixed-solvent approach to prepare P3HT nanofibers.66
P3HT was dissolved in a large quantity of marginal solvent with a small amount of good solvent (chlorobenzene). The effects of two marginal solvents, p-xylene and anisole, on the morphology and solar cell performances were investigated. At room temper-ature, anisole has a poorer solvent quality to P3HT compared with p-xylene and was found to promote a higher degree formation of P3HT nanofibers. A 50% improvement of the power conversion efficiency was observed for the P3HT nano-fibers by adding chlorobenzene into P3HT/anisole system than
the pure anisole system.66
2.2 Conjugated polymer nanoparticles
Conjugated polymer nanoparticles have also been applied to fabricate organic solar cells. There are several approaches for making polymer nanoparticles including reprecipitation, emul-sion polymerization, microfluidic-assisted synthesis, and mini-emulsion. The reprecipitation and miniemulsion methods are the two most commonly used methods to make conjugated polymer
nanoparticles.68–70
The reprecipitation method was first developed by Kasai
et al.71The particle formation is controlled by nucleation and
growth, or spinodal phase separation. The sizes of polymer nanoparticles are controlled by the water temperature and the concentration of the polymer solution. This method was used by
Kurokawa et al. to make nanoparticles of poly(thiophene).68In
their studies, polymer solution was injected into vigorously stir-red DI water by using a microsyringe. Szymanski et al. used a similar reprecipitation method to make different conjugated
polymer nanoparticles such as MEH-PPV.69,70Another modified
reprecipitation method was developed by Yabu et al. and regular
sized polymer nanoparticles were obtained.72 In the modified
method, a small amount of poor solvent (water) was slowly dropped into a polymer solution (polystyrene in THF). After the good solvent (THF) evaporated, the dissolved polymer
precipi-tated as fine particles.72
The second common approach to generate conjugated
poly-mer nanoparticles is by the miniemulsion process.73–77 In the
miniemulsion process, the polymers are first dissolved in a suit-able solvent, followed by adding of water containing a small Fig. 9 Schematic illustration of using the mixed-solvent method to make
P3HT nanowires. The P3HT polymers are first dissolved in a good solvent. After the bad solvent is added, P3HT polymers start to aggregate and P3HT nanowires are formed.
amount of a suitable surfactant. After sonication for a short time, a homogeneous size distribution of the droplets of the resulting miniemulsion is reached, and the whole mixture is sonicated. Finally, a stable, aqueous dispersion of polymer nanoparticles is obtained after the evaporation of the organic solvent by gentle heating.
The miniemulsion technique was used by Kietzke et al. to
prepare polyfluorene nanoparticle blends for solar cell devices.76
The polymer used are poly(9,9-dioctylfluorene-2,7-diyl-co-bis-N,
N0-(4-butylphenyl)-bis-N,N0-phenyl-1,4-phenylenediamine) (PFB)
and poly-(9,9-dioctylfluorene-2,7-diyl-co-benzothiadiazole) (F8BT). They applied two different approaches which both involve using thin spin-coated layers. For the first approach, heterophase solid layers were prepared from the mixing of two dispersions of single-component nanospheres. For the second approach, each individual particle contained both polymers. Polymer solar cells based on the polymer nanoparticles made by these two methods were fabricated, and device efficiencies were found to be comparable to those of solar cells prepared from solution. The device efficiencies were also found to be indepen-dent of the choice of solvent which was used in the miniemulsion
process.76Kietzke et al. also fabricated solar cells based on the
single- and dual-component polymer nanoparticles to further study the dependence of the solar cell efficiency on the layer
composition.77They found that an external quantum efficiency
of 4% was achieved for the solar cell device based on polymer blend nanoparticles containing PFB:F8BT at a weight ratio of 1 : 2 in each individual nanoparticles. The authors proposed that the small exciton diffusion length of F8BT and the tendency of F8BT to penetrate the PFB phase are the most important factors
for the efficiency of these devices.77
Using similar methods for generating polymer nanoparticles, Snaith and Friend reported an electroplating technique to form a continuous uniform film of polyfluorene nanoparticles on
conductive and polymer-coated substrates.78By spin-coating an
F8BT layer on top of the PFB:F8BT nanoparticle film, a multi-layer structure was formed. A solar cell device based on the multilayer structure was fabricated, and an open-circuit voltage of 0.75 V was observed for the device based on the electroplated
nanoparticles.78In addition to polyfluorene nanoparticles, Tada
and Onoda demonstrated the preparation of stable colloidal
suspensions of different materials such as C60, MEH-PPV, and
poly(3-alkylthiophene).79A small amount of solution of target
materials was injected into a large amount of nonsolvents. These organic particles can be collected by electrophoretic deposition
to make nanostructured films.79
2.3 Block copolymers
Block copolymers are polymers that consist of covalently-linked blocks, and they have aroused great interest in recent years
because of their nanoscale self-assembly.80,81Depending on the
number of blocks, the relative block volume fraction, and the interaction parameters between the blocks, block copolymers can self-assemble into a variety of ordered morphologies such as sphere, cylinder, or lamella. External fields such as electric fields or solvent annealing have been employed to orient the
micro-domains of block copolymers.82–84Block copolymers are very
useful in organic solar cell applications.85In this review, we will
focus on the use of diblock copolymers, which are comprised of two chemically distinct blocks. For the applications in organic solar cells, lamella and cylinder morphologies are usually the preferred morphologies, because they provide the percolation pathways for charge transport. As shown in Fig. 10, there are several strategies for using the block copolymers in the applica-tion of organic solar cells, which will be discussed below.
2.3.1 Donor–acceptor block copolymers. For the application
of organic solar cells, the simplest case is to use a donor–acceptor block copolymer. The self-assembly of the block copolymers create the interface needed for the electron dissociations. It was demonstrated by Lindner et al., who studied a single-active-layer solar cell device from block copolymers,
poly(bisphenyl-4-vinylphenylamine)-block-poly(perylene diimide acrylate)
(PvTPA-b-PPerAcr).86 The electronic functionalities were
attached as side groups instead of as main chains. The perfor-mance of the solar cell devices based on the block copolymers was increased by one order of magnitude with respect to effi-ciency compared with the devices based on the blend of the two
individual segment polymers.86This improved performance was
suggested to be caused by the larger phase-separated donor–
acceptor interface in the block copolymers.86,87
Later, Sommer et al. reported the synthesis and
characteriza-tion of two block copolymers, poly(bis(4-methoxyphenyl)-40
-vinylphenylamine)-block-poly(perylene diimide acrylat)
(PvDMTPA-b-PPerAcr) and poly(N,N0
-bis(4-methoxyphenyl)-N-phenyl-N0-4-vinylphenyl-(1,10-biphenyl)-4,40
-diamine)-block-poly(perylene diimide acrylat) (PvDMTPD-b-PPerAcr).87 The
hole-conducting blocks bis(4-methoxyphenyl) phenylamine
(DMTPA) and N,N0-bis(4-methoxyphenyl)-N,N0
diphenyl-(1,10biphenyl)-4,40diamine (DMTPD), are more electron-rich
than bisphenyl-4-vinylphenylamine (TPA). The new hole-con-ducting blocks were used to vary the HOMO of the donor and to improve the hole-carrier mobility. The power conversion effi-ciency of the solar cell based on PvDMTPA-b-PPerAcr was
improved to 0.32%.87
P3HT has also been synthesized into the block copolymers for use in organic solar cells. For example, Zhang et al. synthesized a donor–acceptor diblock copolymer in which the electron donor block is regioregular P3HT and the electron acceptor block is poly(perylene diimide acrylate) (PPDA), a polyacrylate with
pendent perylene diimide groups.88In the solid state, the
copol-ymers showed efficient photoluminescence quenching, indicating the charge separation. The power conversion efficiency of the solar cell based on the block copolymer was achieved at 0.49%
Fig. 10 Strategies for using block copolymers for organic solar cell applications.
using AM 1.5G solar simulation.88Lee et al. then synthesized
a new, well-defined diblock copolymer (P3HT-b-C60) based on
regioregular P3HT and fullerene.89 The P3HT-b-C
60 showed
phase separation when the block copolymer was thermally annealed. Complete photoluminescence quenching of the
P3HT-b-C60 film was observed, while the P3HT/PCBM blend still
showed some photoluminescence, indicating that the charge
transfer between P3HT and C60are more effectively in the
P3HT-b-C60diblock copolymer than that in the P3HT/PCBM blend.
Tao et al. reported the synthesis of rod–coil block copolymers poly(3-hexylthiophene)-b-poly(n-butyl acrylatestat-acrylate per-ylene) containing electron donor poly(3-hexylthiophene) and
acceptor perylene.90The self-assembled morphology of the block
copolymers and the performances of the solar cells based on the self-assembled block copolymers were studied. The authors concluded that the device performance can be significantly improved from well-defined interfaces but poorly organized
nanostructures.90
2.3.2 Block copolymer with one sacrificial block. The second
strategy for the application of block copolymers in organic solar cells is to use a diblock copolymer with one conjugated block and one sacrificial block. This strategy relies on the self-organization of block copolymers to generate idealized morphologies. Upon removal of the sacrificial block after the microphase separation, nanopores or nanogaps are formed with the presence of the
conjugated block (donor).91 Then the second active material
(acceptor) can be filled into the nanopores or nanogaps to form an ordered bulk heterojunction. The most common examples are based on the use of block copolymers with one block of P3HT
and a sacrificial block.92McCullough et al. discovered that
end-functionalized P3HTs can be synthesized with low polydispersity
using a catalyst-transfer polycondensation method.93,94 They
further showed that block copolymers of P3HT can be synthe-sized by using the end-functionalized P3HTs via atom transfer
radical polymerization (ATRP).95 Using a similar method
combined with anionic polymerization, Dai et al. synthesized copolymers consisting of P3HT and poly(2-vinylpyridine) (P2VP) that microphase-separated and self-assembled into different nanostructures of sphere, cylinder, and lamella. Different nanofiber structures were also observed depending on
the volume fraction of the P2VP block.96
For the application of organic solar cells, Botiz et al. demon-strated the synthesis of
poly(3-hexylthiophene)-block-poly-(L-lactide) (P3HT-b-PLLA) linear diblock copolymer as
a structure-directing agent for patterning active materials into
ordered nanostructures.97The block copolymers self-assembled
to form a lamellar morphology, determined by the molecular weights of the two blocks. The biodegradable PLLA block was selectively removed by NaOH solution after the ordered micro-phase-separated morphology was obtained. Acceptor material
fullerene hydroxide (C60) was then filled the gaps between the
P3HT domains by dip-coating technique.97One challenge using
this strategy is the lateral collapse of the resulting P3HT nano-structures after removing the PLLA. Botiz et al. further addressed this issue by proposing that collapse and the subse-quent aggregation of the P3HT domain is because of the
capil-lary forces present between the nanodomains.98 They applied
different drying approaches to control the surface tension forces
generated during the liquid evaporation process. A reduction in domain collapse was observed and the molecular ordering in the
plane perpendicular to the substrate was improved.98
2.3.3 Block copolymer with two sacrificial blocks. Block
copolymers can also be used simply as a template, where both blocks in a diblock copolymer are sacrificial blocks. One of the blocks will be selectively removed to create spaces that could be filled with the conjugated polymers as the donor. The second sacrificial blocks will then be removed, followed by the deposi-tion of the acceptor materials. Therefore, the block copolymers are not present in the final devices. The advantage of this strategy is that well-ordered block copolymers such as the common poly (styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) can be used and general methods to align the block copolymer domains can be applied. In addition, the problem that the crystalline behaviour of most conjugated polymers can affect the micro-phase-separation of block copolymers is avoided by using amorphous block copolymers as templates.
Conducting polymer nanorods were prepared by Lee et al.,
who used a porous diblock copolymer as a template.99In their
work, a mixture of poly(styrene)-block-poly(methyl methacry-late) (PS-b-PMMA) and a poly(methyl methacrymethacry-late) (PMMA) homopolymer is spin-coated onto a random copolymer-coated ITO substrate. Nanopores were generated after the PMMA homopolymer was removed using a selective solvent. The arrays of conducting polypyrrole (PPy) nanorods were fabricated directly on the indium-tin oxide coated glass by an
electro-polymerization.99The PPy nanorods were shown to have much
higher conductivity than that of thin PPy films, which is caused by the high degree of chain orientation. The block copolymer template was removed by using a suitable solvent, resulting in self-supporting arrays of conducting polymers oriented normal to the substrate surface. This method will be useful considering the well-controlled conjugated polymer nanorods. The block copolymer templates can be filled with other conjugated poly-mers such as P3HT, and solar cells devices can be made after the removing of the second sacrificial block, followed by the evap-oration of other active materials such as PCBM.
2.3.4 Block copolymers as compatibilizers. Instead of being
used as the active materials, block copolymers can be used as compatibilizers in organic solar cells. In the P3HT/PCBM system, a thermal annealing step above the glass transition
temperature (Tg) of the active material is usually applied to
improve the device performance.27,28But phase segregation also
occurs during the annealing process, affecting the resultant effi-ciency. The block copolymers can be used as compatibilizers to lower the interfacial energy between donor acceptor domains and can affect the phase segregation behaviour of the active materials.
Amphiphilic diblock copolymers incorporating fullerene and P3HT macromonomer was synthesized by Sivula et al. for their
uses as compatibilizers in P3HT/PCBM based solar cells.26Phase
segregation of P3HT and PCBM domains were found to be altered and even prevented by using the compatibilizers. Yang et al. also synthesized rod–coil block copolymers based on P3HT
containing C60chromophores (P3HT-b-P(SxAy)-C60) and used
the block copolymer as a surfactant for the P3HT/PCBM based
solar cells.100 The interfacial morphology between P3HT and PCBM domains was altered by adding a small amount of the
block copolymer (P3HT-b-P(SxAy)-C60). The efficiency of the
solar cell was improved to 3.5%, which was 35% increase compared with the standard device without adding the block
copolymer.100
2.4 Layer-by-layer deposition
Organic solar cells based on multi-layered small molecule films have been fabricated using vacuum deposition. For example, the power conversion efficiency of 3.6% was achieved from a simple donor–acceptor bilayer cell based on copper phthalocyanine
(CuPc) and fullerene (C60).101Even though the vacuum
deposi-tion technique is useful for precisely controlling the film thick-ness, this technique usually requires high-vacuum and high temperatures conditions, which is not suitable for polymer
materials.102Therefore, multi-layered solar cells based on
poly-mers are relatively less studied. In addition, it is usually difficult to find suitable selective solvents for making multi-layered polymer films to avoid the dissolving problem.
Layer-by-layer (LbL) approaches have been one of the best methods to generate conductive multilayer films based on
conjugated polymers.103–106These multilayer films are fabricated
in organic solvents and are useful in different optoelectronic applications such as the organic solar cells. Liang et al. reported the synthesis and fabrication of hybrid multilayer thin films of poly(p-phenylenevinylene)s (PPVs), and CdSe nanoparticles by
using the layer-by-layer assembly approach.106 This method
enables the preparation of hybrid thin films with great stability through covalent coupling reactions between polymers and nanoparticles. The power conversion efficiency of 0.71% (10 mW
cm2) was achieved from the solar cells based on the PPV/CdSe
nanocomposites.106Solar cells based on the self-assembled layers
of functional polymers and inorganic particles using the
layer-by-layer method was also demonstrated by Kniprath et al.107
Alternating layers of the polythiophene derivative,
poly(2,3-thienyl-ethoxy-4-butylsulfonate) (PTEBS), and TiO2
nano-particles were prepared using the layer-by-layer methods. Solar cells based on the composites films were fabricated and stable photovoltaic behavior was observed with photovoltages of up to
0.9 V.107Hybrid films of polymers and lead selenide nanocrystals
(PbSe-NCs) using the layer-by-layer deposition technique was
reported by Vercelli et al.108PbSe-NCs and sulfonate-,
carbox-ylate-, and pyridine-based polymers were alternately deposited on ITO-glass surfaces. The hybrid film was expected to be useful for organic-inorganic solar cells in the infrared region of the solar
spectrum.108
Benten et al. reported the preparation of poly(p-phenyl-enevinylene) (PPV) by using the layer-by-layer deposition of the PPV precursor cation and poly(sodium 4-styrenesulfonate) (PSS)
and studied their applications in solar cells.102Multilayer
poly-mer solar cells were fabricated based on the layer-by-layer assembled PPV layer. In their work, the PEDOT:PSS spin-coated film was cross-linked by gelation, yielding insoluble film in the polyelectrolyte aqueous solutions. The best power conversion efficiency of the solar cells based on PEDOT:PSS/
PPV/C60was achieved at 0.26% (AM 1.5G, 100mW cm2, air),
where the thickness of the PPV layer was 11 nm, comparable to
the diffusion length of the PPV singlet exciton.102
2.5 Nanoimprint lithography
As shown in Fig. 11, nanoimprint lithography (NIL) uses a mold with nanostructures that can be transferred into a polymer by
imprinting the mold onto the polymer.109The fabrication of the
nanostructures on the mold is mostly done by e-beam lithog-raphy and is time-consuming and expensive. But these molds can be repeatedly used, so the mold can be applied in an inexpensive and fast process. The NIL process can be used for small areas, but roll-to-roll processing and imprinting for large area are also feasible. NIL has been employed by several groups to produce
nanostructures in the application of organic solar cells.110–112For
polymer-based solar cell devices, the device performances are improved when nanoscale structures are imprinted into the donor polymer film, followed by the evaporating or spin-coating of acceptor materials.
Nanopatterns of polythiophene derivatives on flexible plastic were demonstrated by Kim et al. using the nanoimprint
lithog-raphy.110 Solar cell devices were fabricated after spin-coating
PCBM and evaporating Al. The short-circuit current of the nanoimprinted devices was shown to increase with the interfacial area of the structures. The fill factor and power conversion
effi-ciency also increased with the interfacial area.110
High density periodic P3HT nanopillars (1010 cm2) were
reported by Aryal et al. using nanoimprint lithography.111 A
large-scale nanoporous Si mold was obtained by using an anodic alumina template as a mask for a two-step plasma etching process. The pores in the Si mold were 50–80 nm wide and 100– 900 nm deep and the pore depth and profile can be controlled by
adjusting the plasma etching conditions such as Cl2: Ar ratio,
time, pressure, and bias power. Ordered and high density poly-mer nanopillars including poly(methyl methacrylate) (PMMA)
and P3HT were fabricated by this method.111 The imprinted
P3HT nanopillars were then used for organic solar cell devices followed by depositing PCBM on top of the nanostructures. For the purpose of comparison, double layer nonpatterned solar cell devices without the imprinting steps using similar process were fabricated. The power conversion efficiency (AM 1.5G, 100 mW
cm2) was improved by 78% from the solar cell devices built on
imprinted P3HT pillars compared with the devices of the non-patterned layer. The fill factor and short-circuit current also showed 38% and 20% increases, respectively. The authors concluded that the main reason for the improved performance of the solar cell devices was due to the well-controlled interdigitized
Fig. 11 Schematic illustration of using the nanoimprint lithography to make polymer nanostructures. A polymer film is first coated on a substrate. A master mold is used to imprint the polymer film at elevated temperatures to transfer the patterns. Polymer nanostructures are released after the demolding process.
heterojunction morphology which allowed more efficient charge
transport and exciton dissociation.111
For the imprinted P3HT nanostructures, Aryal et al., also studied the crystallization and chain orientation by using the
out-of-plane and in-plane grazing incident X-ray diffraction.113The
geometry (gratings or pillars) of these nanostructures was shown to have a strong influence on the 3D chain alignment of the nanostructures. The chain orientations inside nanoimprinted P3HT gratings and nanopillars were proved to be vertical by in-plane and out-of-in-plane grazing incidence X-ray diffraction
(GIXRD) measurement.113The chain alignment was proposed to
be induced by the nanoconfinement effect during the imprinting
process through p–p interaction and the hydrophobic
interac-tion between polymer chains and the surfaces of the molding materials. The vertical chain alignment of P3HT chains in both nanogratings and nanopillars also indicate their potentials for improving charge transport and solar cell devices compared with
bulk heterojunction solar cells.113
In addition to P3HT nanopillars, nanoimprinted P3HT
nano-gratings were demonstrated by Zhou et al.114The nanoimprinted
P3HT nanogratings were fabricated by imprinting a P3HT film using a Si mold gratings of 100 nm in width, 100 nm in depth, and 200 nm in pitch. PCBM was then spin-coated on top of the nanograting as the electron transport material. An orthogonal solvent (dichloromethane) that dissolves PCBM well but not the P3HT was used to allow the stacking of PCBM on top of the P3HT nanostructures, resulting in nondistortion of the nanostructures. For comparison studies, two types of nonpatterned films of P3HT and PCBM were also fabricated as solar cell devices. The first type of nonpatterned film was made by spinning a 50 nm PCBM layer on top of an 85 nm thick P3HT layer. Lee et al. have shown that this kind of film is not a bilayer, but an intermixed
hetero-junciton.37The second type of nonpatterned film was made by
spin-coating a mixture of P3HT/PCBM (1 : 0.9) onto the substrate. The power conversion efficiency (AM 1.5G, 100 mW
cm2) of the solar cell with P3HT nanogratings was higher than
those with the nonpatterned films.114 The fill factor and
short-circuit current also increased compared with those of the bilayer and blended solar cells. The authors concluded that the improved performance in P3HT nanograting devices was due to the better chain alignment in P3HT nanogratings and the increase in the interfacial area. The favorable 3D chain alignment enables high mobility, resulting in improved current density, fill factor, and efficiency of the nanoimprinted solar cells. The authors also proposed that better device performance can be expected by
increasing the density and the aspect ratio of nanogratings.114
To further improve the performance of P3HT nanogratings based solar cells, Yang et al. used oblique deposition to deposit
C60into the P3HT nanogratings to have good infiltration of C60
into the nanoimprinted P3HT nanogratings.115The evaporation
angle of C60was found to affect the uniformity and step coverage
of C60infiltration into the P3HT nanostructures. At the optimal
condition for the C60deposition filling, increased exciton
disso-ciation rate at the larger P3HT-C60interfacial area was achieved
and the power conversion efficiency was observed to increase 50%, compared with flat bilayer devices. The authors also expected that there will be 200% enhancement in power conversion efficiency of the solar cell devices if the P3HT gratings
can be twice as their width.115
Nanoimprinting was combined with lamination technique by
Wiedemann et al. to make nanostructured bilayer devices.116In
their work, P3HT was nanoimprinted using an anodic aluminum oxide as a stamp. PCBM was then deposited via a lift-off
lami-nation technique.116Instead of forming the P3HT nanostructures
first followed by depositing PCBM, Na et al. used a soft litho-graphic approach to prepare periodic patterned P3HT/PCBM
structures.117Photo-induced surface-relief gratings on azo
poly-mer films and poly(dimethylsiloxane) were used as a master and stamp. The power conversion efficiency of the patterned devices was 4.02%, compared with 3.56% from the unpatterned
devices.117 To avoid the nanoimprinting process at high
temperatures, Shih et al. used textured Si wafer as a stamp to nanoimprint P3HT:PCBM blends at room temperature, and the
Voc, Jsc, and FF of the nanoimprinted devices was found to
increase by 50%.118
To achieve the interpenetrating nanostructures, He et al. demonstrated a double nanoimprinting method to create nano-patterned polymer blends with features in the 25 to 200 nm size
range.119 The solar cell devices based on the nanostructured
polymer blends of poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis
(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-20,200-diyl) (F8TBT)/
poly(3-hexylthiophene) (P3HT) were fabricated and the influence of feature size on device performance was investigated. The solar
cell devices were shown to have extremely high densities (1014
mm2) of interpenetrating nanoscale columnar features in the
active polymer layer.119The power conversion efficiency of the
solar cell was improved to 1.9%, primarily due to the improved fill factor over that from the solution demixed device. The poorly controlled morphology formed from the demixed polymer blend layer was considered to cause the avoidance of locally trapped
photogenerated charges, resulting in lower efficiency.119 The
same technique was also applied to polymer-fullerene based solar
cell devices.112 The AFM and SEM results showed that the
features in the polymer mold used in the second imprinting process were only slightly distorted during imprinting steps. The performances of the solar cell devices using P3HT or F8TBT as the donor materials and PCBM as the acceptor material were investigated. In both cases, the performances of the devices were
influenced by the feature size and the interfacial area.112Different
from other imprinting methods, which have the disadvantages of rather large imprinted feature sizes and high temperature depo-sition process, the advantage of the new double nanoimprinting method is that the pure form of any materials can be used. The requirement for the donor and acceptor materials is that they
have a small difference in glass transition temperature (Tg),
melting temperature (Tm), or solubility in selective solvents.
Otherwise, the first imprint might be erased during the second imprinting step. An additional advantage of this double nano-imprint method is that both donor and acceptor layers can
completely cover the electrodes.112,119
The most popular choices for the donor and acceptor in the nanoimprinting method are still P3HT and PCBM. Instead of
using PCBM as the electron acceptor, N,N0
-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) was used by
Cheyns et al. They first obtained trenches of poly(3-hexylth-iophene) (P3HT) with feature size down to 50 nm and aspect
ratio up to 2 by nanoimprinting.109Then the electron acceptor
(PTCDI-C13) was deposited from the vapor phase. Zinc oxide
(ZnO) nanoparticles was later spin-coated on top to reduce the roughness of the layered structures. A 2.5-fold enhancement of the short-circuit current was observed in the nanoimprinted
devices compared with the planar devices.109 Another electron
acceptor 4,7-bis(2-(1-ethylhexyl-4,5-dicyano-imidazol-2-yl)
vinyl)benzo[c]1,2,5-thiadiazole (EV-BT) was used by Zeng
et al.120They applied nanoimprinting for bilayer solar cells using
P3HT as the electron donor and EV-BT as the electron acceptor. The power conversion efficiency of the devices was increased to 0.3% for the nanoimprinting devices, compared with 0.2% for the
simple bilayer device.120
2.6 Template methods
Nanoporous templates have been widely used to make
one-dimensional polymer nanostructures.121–127 Template-based
methods represent straightforward routes to make nano-materials, in which the template is simply used as a scaffold, and the materials are fabricated and shaped by the geometry of the template. Since the templates consist of cylindrical pores of uniform diameter, monodisperse nanocylinders of the desired material, whose dimensions can be carefully controlled, are obtained in high yield within the pores of the template
mate-rial.125Another advantage of using the template method is that
the placement and dimensions of different components of the nanomaterials can be controlled. Depending on the material, the chemistry of the pores wall, and other operating parameters, these nanocylinders may be solid (a nanorod) or hollow (a
nanotube).123 Fig. 12 describes the basic concept of using the
template method. Different materials, such as polymers, can be introduced into the nanopores of the templates by wetting the
pore walls of the porous template.128The surface energy of the
wall is usually high and the precursor materials cover the surface of the walls to reduce the surface energy by a simple wetting process. The nanomaterials can remain inside the pores of the templates or they can be released and collected by removing the template selectively.
Various materials have been used as the porous templates, but ion-track-etched membranes and anodic aluminum oxide (AAO)
templates are the most commonly employed.129 Ion-tracked
membranes are commercially available as filters in a wide variety
of pore sizes.130They are usually prepared from either polyester
or polycarbonate thin films with pore diameters ranging from 10 to 2000 nm. However, the track-etched membranes have low porosities and contain randomly distributed nanochannels across the membrane surface. In addition, the random nature of pore formation during this process can result in a large number of intersecting pores that will harmfully affect the homogeneity,
number, and sizes of the synthesized nanomaterials.125
Another important template is anodic aluminum oxide (AAO) template which is prepared electrochemically from aluminum. The popularity of the AAO template is because of its exceptional thermal stabilities, ease of fabrication, and its regular pore
diameter and pore length.131 The AAO templates typically
contain high porosities, and the pores are arranged in a
hexag-onal array. Pore densities as high as 1011pores per square
cen-timetre can be achieved, and typical membrane thickness can
range from 10 to 100mm.132AAO templates of many diameters
(5–267nm) can be synthesized following the two-step anodization process established by Masuda and co-workers in which
nano-sized pores are grown in an insulating oxide film of alumina.133,134
The template method usually requires removal of the template with selective etching solution such as weak acid or weak base. But a major challenge associated with these nanostructures is associated with their lateral collapse, which is related to the high aspect ratio of the nanostructures or the capillary force caused by
the evaporation of solvent.135,136A freeze drying technique has
been applied to remove the aqueous medium after the etching
process to prevent the lateral collapse of the nanostructures.137
Polythiophene nanotube films were prepared by Cao et al.
using the anodic aluminium oxide template.138A gold layer was
first deposited onto one side of the alumina membrane template with a pore size of 20 nm. Then an ITO layer was contacted closely to the gold layer as a current collector. By direct oxidation of thiophene in boron trifluoride diethyl etherate (BFEE) solu-tion, polythiophene nanotubes were synthesized in the
nano-pores of the AAO template.138After dissolving the template in
1M KOH solution, the aligned nanotube film was obtained. In their work, surface photovoltage spectrum (SPS) and field-induced surface photovoltage spectrum (FISPS) were used to investigate the electronic structure and the charge behaviour of the polythiophene nanotube films. They observed two extra photovoltaic responses in the near-IR region, besides the
intrinsic p–p* transition of polythiophene chains. From the
principle of FISPS and the band theory, the new responses were attributed to the charged surface electronic states, which were caused by the interaction between the nanotubes and the oxygen
absorbed on the surface.138Electrochemical polymerization was
used by Huesmann et al., to synthesize polythiophene with AAO
templates.139They showed that geometry of the prepared
nano-structures can be controlled by the hydrophobic side-chains, such as 3-hexyl and 3-(2-ethyl)hexyl.
Wang et al. reported the use of porous alumina templates to fabricate ordered arrays of core-shell P3HT and PCBM for the
use in organic solar cells.140The P3HT/PCBM film with a
thick-ness of 120 nm was first spin-coated onto an ITO substrate. An AAO template was then placed on top of the P3HT/PCBM film. After annealing in an oven under vacuum for 6 h, core-shell Fig. 12 Schematic illustration of using the template method to make
polymer nanostructures. A polymer film is first deposited on a substrate. Then a porous template is brought into contact with the template. Polymers are able to wet the surface of the nanopores of the template by a capillary force via thermal annealing. After removing the template by a selective etching solution, polymer nanostructures are generated.
