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Investigation of Nanostructured Organic Solar Cells with Transmission
Electron Microscopy
Golap Kalita, Koichi Wakita and Masayoshi Umeno
Department of Electronics and Information Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai-shi 487-8501,
Japan
Author address: Phone no.: +81-568-51-1111 Fax no.: +81-568-51-1478 E-mail: [email protected]
Organic photovoltaic devices (OPVs) based on conjugated polymers (donor) and fullerene or fullerene derivatives
(acceptor) is one of the most researched topics from the last decade. OPVs have the great potential as the future generation
of solar cells with abundant materials, simple manufacturing process and flexibly. Bulk heterojuction OPVs are most
efficient system with homogenous mixing of conjugated donor and acceptor. Solar cells performances are highly
dependent on the solid state nanoscale morphology of the donor and acceptor in the photoactive layer. The proper
understanding and characterization of nanostructure morphology is most important to optimize high device performance.
Transmission electron microscope (TEM) is an important tool to investigate the nanoscale morphology which is discussed
elaborately by giving suitable examples.
Keywords: Organic solar cells, transmission electron microscopy, nanostructured morphology
1. Donor acceptor based organic solar cells
The discovery of ultra fast photo induced charge transfer at donor- acceptor interface has brought lot of interest in OPVs
based on conjugated polymer and fullerene (C60) or C60 derivatives. [1,2] Donor-acceptor based organic solar cells
fabricated with either spin-coated or printed offer an alternative to the conventional inorganic solar cells with such
potential advantages as low-cost, ease of manufacturing, non-toxicity, and lightness [3-5]. The idea behind a
heterojunction of donor and acceptor is to use two materials with different electron affinities and ionization potentials.
In heterojuction organic solar cells the donor acceptor interface is most important, the resulted potentials at the interface
are strong enough to favor exciton dissociation. With separation of charges the electron will be accepted by the material
with the larger electron affinity and the hole by the material with the lower ionization potential, provided that the
differences in potential energy are larger than the exciton binding energy. [6,7] Separated charges are transported
through the donor and acceptor material and collected at the electrode to obtain a photo-generated current completing
the photovoltaic process. The following figure (1) presents a schematic view of donor acceptor based solar cells with
corresponding energy level of donor, acceptor and electrodes as well as the charge transfer process.
Figure 1 Schematic diagram of a donor-acceptor based organic solar cell and corresponding energy band diagram presenting the
charge (electron and hole) transportation
In a donor-acceptor based solar cell sunlight is absorbed and excites the donor molecule, leading to the creation of
excitons. The created excitons start to diffuse (diffusion length 10 nm) within the donor phase and if they encounter the
interface with the acceptor then a fast dissociation takes place leading to charge separation (hole and electron). [8,9]
Subsequently, the separated free electrons and holes are transported with the aid of the internal electric field towards the
cathode and anode, respectively where they are collected by the electrodes and driven into the external circuit.
Glass/plastic
Light
Cathode
Anode
Donor:Acceptor
-3.7 eV
e-
-6.1 eV
-4.8 eV
-4.2 eV
h+
-2.85 eV
e-
-5.25 eV
Anode Donor Acceptor Cathode
Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)
354 ©FORMATEX 2010
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However, the excitons can decay, yielding e.g. luminescence, if they are generated far from the interface. Thus, the
excitons should be formed within the diffusion length of the interface. Since the exciton diffusion lengths in organic
materials are much shorter than the absorption depth of the film, this limits the width of effective light-harvesting layer.
[10-13] In figure (2) the most widely investigated conjugated polymer as donor and fullerene derivative as acceptor is
presented.
Figure 2 Representation of (a) conjugated polymer poly (3-hexylthiophene) (b) fullerene derivative PCBM.
A revolutionary development in OPVs came in the mid 1990’s with the introduction of the bulk heterojunction,
where the donor and acceptor material are blended together [14]. If the length scale of the blend is similar to the exciton
diffusion length, the exciton decay processes is dramatically reduced since in the proximity of every generated exciton
there is an interface with an acceptor where fast dissociation takes place. Hence, charge generation takes place
everywhere in the active layer. Now the important issue to carry this generated charges to the respective electrode so
that no charges recombine again. With existence of continuous pathway in each material from the interface to the
respective electrodes, the photon-to-electron conversion efficiency and, hence, the photosensitivity is dramatically
increased. This determine by how the donor and acceptor material is mixed and the nanostrucutred morphology. The
ordered polymer chain can collect hole to the anode and nonostructured fullerene percolation path can take the electron
to the cathode. In the following figure (3) a sketch of an OPV device with nanostructured morphology of donor and
acceptor in between the anode and cathode is presented.
Figure 3 Sketch of OPV devices with nanostructured morphology of donor and acceptor in between the anode and cathode.
2. Transmission electron microscopy
Transmission electron microscopy (TEM) is a microscopy technique, where a beam of electrons is transmitted through
an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of
Poly (3-hexylthiophene) (6,6)-phenyl-C61-butyric
acid methyl ester
(a) (b)
Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)
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the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a
fluorescent screen, photographic film, or to be detected by a sensor such as a charge coupled device (CCD) camera.
TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small de
Broglie wavelength of electrons. This enables the instrument to be able to examine fine detail; even as small as a single
column of atoms, which is tens of thousands times smaller than the smallest resolvable object in a light microscope.
TEM forms a major analysis method in different scientific field to investigate in nano scale order.
The TEM consists of an emission source, which may be a tungsten filament, or a lanthanum hexaboride (LaB6)
source. For tungsten, this will be of the form of either a hairpin-style filament, or a small spike-shaped filament. LaB6
sources utilize small single crystals. By connecting this gun to a high voltage source (typically ~100-300 kV) the gun
will, given sufficient current, begin to emit electrons either by thermionic or field electron emission into the vacuum.
This extraction is usually aided by the use of a Wehnelt cylinder. Once extracted, the upper lenses of the TEM allow for
the formation of the electron probe to the desired size and location for later interaction with the sample.
Manipulation of the electron beam is performed using two physical effects. The interaction of electrons with a
magnetic field will cause electrons to move according to the right hand rule, thus allowing for electromagnets to
manipulate the electron beam. The use of magnetic fields allows for the formation of a magnetic lens of variable
focusing power, the lens shape originating due to the distribution of magnetic flux. Additionally, electrostatic fields can
cause the electrons to be deflected through a constant angle. Coupling of two deflections in opposing directions with a
small intermediate gap allows for the formation of a shift in the beam path, this being used in TEM for beam shifting,
subsequently this is extremely important to scanning TEM (STEM). From these two effects, as well as the use of an
electron imaging system, sufficient control over the beam path is possible for TEM operation.
The lenses of a TEM allow for beam convergence, with the angle of convergence as a variable parameter, giving the
TEM the ability to change magnification simply by modifying the amount of current that flows through the coil,
quadrupole or hexapole lenses. The quadrupole lens is an arrangement of electromagnetic coils at the vertices of the
square, enabling the generation of a lensing magnetic fields. Typically a TEM consists of three stages of lensing. The
stages are the condensor lenses, the objective lenses, and the projector lenses. The condensor lenses are responsible for
primary beam formation, whilst the objective lenses focus the beam down onto the sample itself. The projector lenses
are used to expand the beam onto the phosphor screen or other imaging device, such as film. Imaging systems in a TEM
consist of a phosphor screen, which may be made of fine (10-100 µm) particulate zinc sulphide, for direct observation
by the operator. Optionally, an image recording system such as film based or doped YAG screen coupled CCDs.
Typically these devices can be removed or inserted into the beam path by the operator as required. Detailed discussion
on the TEM instrument, its operation and capabilities and its usage is out of scope of this chapter and the readers are
requested to see some of the dedicated books on the transmission electron microscopy. [15-18]
For investigation of naostrucutred organic solar cells, TEM is one of the important tools to have very good idea
about the morphology. The formation of domain structure with acceptor and donor materials is most important in
organic solar cells to obtain maximum device performance. TEM study can provide us in depth knowledge of the
nanostructured morphology of the active material in solar cells, which enable to fabricate device with optimize device
structures.
3. Importance of TEM in investigation of organic solar cells
Let us see how important TEM observation in nanostructured morphology of the donor acceptor based organic solar
cells. As discussed previously, C60 and its derivatives were used as electron acceptor material due to its high electron
affinity in bulk heterojunction solar cells. The morphology of donor acceptor based solar cells are dependent on
different parameters of device fabrication such as solvent, solubility, annealing and fabrication process etc. In the
beginning of bulk heterojunction solar cells C60 molecules directly blend with the polymer, but solubility of C60 in some
of the organic solvent is poor. How the poor solubility influence on the no scale morphology is discussed in the
following. As C60 shows poor solubility, C60 derivatives with better solubility are used in blend. Improved in solubility
of C60 derivative (PCBM), allowed the incorporation of a larger fraction of C60 into polymer blend films. The same
holds for the conjugated polymers, which become only soluble due to side-chain addition. Thus it is clear that the part
of the chemical structure that provides the solubility is an important parameter for reaching well-blended bulk
heterojunctions of the constituents at the nano scale. Another factor is the chemical compatibility between the polymer
and the C60 is thermal annealing as required for better efficiency, which resulted in a phase separation of nano ordered
between polymer and C60. Now the nano scale phase separation of the polymer and C60 in the blend is most influencing
parameters which control the device performance. [19-23]
Solvent processing and annealing of the donor-acceptor blend material determine the domain structure formation. In
the blend film, PCBM molecules may diffuse and form larger aggregates with an at least partially crystalline structure.
Thereby, the polymer domains also form a more ordered phase. For example: whilst poly[2-methoxy-5-(3′,7′-
dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) : PCBM blend films yielded generally the best results when
spin cast from chlorobenzene, this solvent could not deliver similar good results for an MDMO-PPV : C70-PCBM blend.
It has been shown that changing the fullerene to C70-PCBM required the use of ortho-dichlorobenzene instead of
Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)
356 ©FORMATEX 2010
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chlorobenzene to reach a small scale of phase separation. Thus chemical compatibility can be readily tuned by changing
the solvent, and C60 blend system and its phase separation. It has been observed that with direct addition of C60
Figure 4 Transmission electron micrographs of (a) P3OT/C60 and (b) P3OT/plasticizer/C60 blends. Due to the plasticizer the average
C60 domain decreased drastically in size (N. Camaioni et al.[24]).
to P3OT solvent there are large aggregate of C60 due to less solubility, as shown in figure 4(a). To overcome the large-
scale phase separation in P3OT-C60 blends plasticizers were added. Figure 4(b) shows a TEM image of the P3OT-C60
blend with plasticizers to increase the compatibility between the two components, as reported by N. Camaioni et al.
[24]. From image it is quite clear that the size of the C60 crystallites was reduced by a large extent and more
homogeneous blends resulted. So the explanation on the nano scale morphology show the importance of the TEM study
and how we can have the idea of phase separation and domain formation in OPVs blend film. [24,25]
In the figure (5) a TEM image of well blend poly (3-hexylthiphene) and PCBM is presented. The dark spot in the
image present PCBM domain which are formed through out the blend film. The other feature of the image is fiber like
structures, which are crystalline P3HT. Now the TEM image is discussed in a pictorial presentation as shown in the
figure to have a better perception. The PCBM in the polymer composite after the fabrication film with necessary
processing thshows domain formation in nano scale order which are surrounded with polymer fibers. The formation of
highly ordered region-regular P3HT fibers and nano scale PCBM domain help better photo exciton dissociation and
charge transportation and thereby improve device performance can be achieved. [26,27]
(a) (b)
(a) (b)
Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)
©FORMATEX 2010 357
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Figure 5 (a) TEM image of a polymer and PCBM composite film showing crystallization of the two components (b) Schematic
diagram of phase separated blend of polymer and PCBM. (X. Yang et al.et al. [24]).
For better understanding of nanostructured order and domain formation let us discuss in a pictorial representation.
Figure 6(a) presents a pictorial representation of large domain acceptor (dark red), with larger domain there will not be
efficient charge separation. The diffusion length of exciton form in the donor materials is of the order of 10 nm; hence
the exciton formed far away from the acceptor domain can not diffuse to interface of acceptor and for free charges. The
acceptor domain large aggregates in certain are of the blend and not properly distributed to have efficacy charge
transportation. As well as larges aggregate of the acceptor facilitate recombination of separated charges. Figure 6(b)
presents a pictorial representation of small domain acceptor (dark red), well distributed through out the blend film. The
small domains provide large surface area and well distributed within the donor to have efficient charge separation. The
small domain can be within the diffusion length of the excition and can provide the interface for dissociation in separate
charges. As well as, the nano scale acceptor domain can form percolation path for efficient electron transportation
thought the blend,thereby achieving charge separation with out recombination. In the following, the nanostructured
morphology of the acceptor in P3HT:PCBM solar cells and how electron percolation path formation can help in device
performance is elaborately discussed.
Figure 6 A pictorial presentation of domain size for the acceptor (dark red) in the blend film with (a) large domain size and (b) well
distributed small domain size for efficient photo-excitation.
4. Nanostructured morphology of P3HT:PCBM solar cells
In OPV research, P3HT has been most widely used as electron donor along with PCBM, as electron acceptor to form
the photoactive layer of organic solar cells. Such type of organic solar cells performance is increasing with every
passing years due to better understanding of working principle, device structure and nonostructured morphology of the
photoactive layer. Here, we discuss about nanostructured morphology of the particular P3HT:PCBM system due to its
immense interest in scientific community.
The nanostructured morphology of the P3HT: PCBM composite layers have great influence on the device
performances. In recent works, it has been reported about the relationship between the nanostructured morphology of
the organic photoactive layer and the performance of the polymer solar cells. The morphology of this layer can be
strongly affected by the processing conditions used such as the donor–acceptor composition, the solvent and the thermal
annealing of the organic layer as discussed earlier. In order to achieve high efficiency organic solar cells there should be
increase in mesoscopic order and crystallinity in the P3HT:PCBM bulk heterojunction network. With enhanced
crystallinity of the composite, there can be better percolation of charges by reducing internal resistance. Morphology of
P3HT: PCBM blend films have been studied by using bright-field TEM images and selected-area electron diffraction
(SAED) patterns. As well as, HRTEM and atomic force microscopic studies were done to explore the morphology of
P3HT: PCBM composite layers. [28,29]
In a study of P3HT:PCBM bulk heterojunction solar cells were fabricated with annealing or unanealing and varying
the concentration ratio to observe the differences in morph local structures. Nanostructured morphology of the P3HT:
PCBM composite films were analyzed by HRTEM studies to correlate the device performances. It was found that for
annealed and unanealed films the morphology is quite homogenous even at high resolution. Whereas, it has been
recently been shown that P3HT:PCBM films were composed of homogeneously distributed PCBM nanocrystals.
[30,31] In the following figure (7) presents HRTEM image of unannealed and annealed films with different
concentration ration of P3HT and PCBM. HRTEM image of the unannealed film with 1:1 weight ratio (Figure 7a)
shows almost no crystallization of PCBM, although some atomic planes were observed. On the other hand, when these
(a) (b)
Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)
358 ©FORMATEX 2010
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Figure 7 High resolution TEM images of P3HT: PCBM film with (a) 1:1 concentration ratio before annealing (unannealed device)
(b) 1:0.5 concentration ratio with annealing (c) 1:1 concentration ratio with annealing, formation of PCBM nancrystals and its
elongated structure (d) 1:2 concentration ratio with annealing (PCBM nanocrystals were denoted with arrow marks).
films were annealed significant transformation in morphology was observed. As shown in figure 7(b) for annealed film
with 1:0.5 concentration ratio of P3HT: PCBM, there are formation of small nanocrystals homogeneously distributed
throughout the composite film. There are differences in the brightness in certain regions of the PCBM nanocrystals that
are due to depth of the nanocrystals in the P3HT composites. Figure 7(c) presents nanostructure morphology of the
P3HT:PCBM composite film with 1:1 concentration ratio. Elongated nanocrystals were clearly visible and these
nanocrystals are homogeneously distributed in the composite film. The elongated nanostructures are composed of
several nanocrystals having different plane orientations and d spacing was observed to be around 0.42 nm. The
formation of elongated PCBM nanocrystals which is not observed in the unannealed and low PCBM concentrated film
provides efficient percolation path for the electron. With better electron transportation in the composite film there are
improved photocurrent and thereby better device performance. Figure 7(d) shows nanostructured morphology of the
P3HT:PCBM composite film with 1:2 concentration ratio. PCBM nanocrystals were observed in these films as well, but
in this case much larger nanocrystals were observed. The densities of the nanocrystals are much higher then that of the
previous composite films, this due to the fact that PCBM concentration is much higher in this film. Though the density
and size of the nanocrystals increase with increase in PCBM concentration, device performance significantly reduced
then that of device fabricated with 1:1 weight concentration of P3HT:PCBM. The good performance of the device with
1:1 weight concentration of P3HT:PCBM is attributed to an optimized morphology that enables close intermolecular
packing of P3HT chains. Inferior performance for the 1:0.5 compositions is attributed to poorer intermolecular packing
with increased PCBM content. These results show clearly that it is very much important to consider the morphology of
the PCBM nanocrystals in order to enhance the efficiency of P3HT:PCBM bulk heterojunction solar cells.[32]
2 nm
a
2 nm
b
2 nm
PCBM nanocrystal
PCBM nanocrystal
c PCBM nanocrystal
PCBM nanocrystal
d
Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)
©FORMATEX 2010 359
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5. Concluding remarks
Importance of transmission electron microscopy (TEM) in investigation of nanostructured organic solar cells is
discussed elaborately with different examples. Solar cells performances are highly dependent on the solid state nano
scale morphology of the donor and acceptor in the photoactive layer. TEM gives direct insight into nano scale
morphology of the donor acceptor composite film. TEM study revel how important the phase separation study in bulk
heterojunction solar cells. Phase separation of the donor acceptor materials is analyzed with pictorial representation to
have a clear overview on nanostructured morphology. It is the one of the most important and most reliable technique for
correctly identifying the nature and formation of blend of different materials in organic solar cells. It can be used for
imaging, electron diffraction and chemical analysis of different material in organic solar cells. With different
advantages of TEM, it has become a versatile and comprehensive analysis tool for characterizing the chemical and
electronic structure at nano scale material systems.
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Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)
360 ©FORMATEX 2010
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