Methanofullerene elongated nanostructure formation for enhanced organic solar cells

Abstract

Using transmission electron microscopy (TEM) and Z-contrast imaging we have demonstrated elongated nanostructure formation of fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) within an organic host through annealing. The annealing provides an enhanced mobility of the PCBM molecules and, with good initial dispersion, allows for the formation of exaggerated grain growth within the polymer host. We have assembled these nanostructures within the regioregular conjugated polymer poly(3-hexylthiophene) (P3HT). This PCBM elongated nanostructure formation maybe responsible for the very high efficiencies observed, at very low loadings of PCBM (1:0.6, polymer to PCBM), in annealed photovoltaics. Moreover, our high resolution TEM and electron energy loss spectroscopy studies clearly show that the PCBM crystals remain crystalline and are unaffected by the 200-keV electron beam.

Full text

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Methanofullerene Elongated Nanostructure Formation for Enhanced
Organic Solar Cells
M. Reyes-Reyes1*, R. López-Sandoval2, J. Arenas-Alatorre3, R. Garibay-Alonso2, D. L.
Carroll4 and A. Lastras-Martinez1
1Instituto de Investigación en Comunicación Optica, Universidad Autónoma de San Luis
Potosí, Alvaro Obregón 64, San Luis Potosí, Mexico.
2Instituto Potosino de Investigación Cientifíca y Tecnológica, Camino a la presa San José
2055, CP 78216. San Luis Potosí, Mexico.
3Instituto de Física, UNAM, Apartado Postal 20-364, 01000, México, D.F., Mexico
4Center for Nanotechnology and Molecular Materials, Department of Physics. Wake Forest
University, Winston-Salem NC 27109, USA.

*Corresponding author. Instituto de Investigación en Comunicación Optica, Universidad
Autónoma de San Luis Potosí, Alvaro Obregón 64, San Luis Potosí, Mexico. Tel.: +52-444-
825-0183; fax: +52-444-825-0198.
E-mail address: reyesm@cactus.iico.uaslp.mx (M. Reyes-Reyes).

Abstract
Using transmission electron microscopy (TEM) and Z-contrast imaging we have
demonstrated elongated nanostructure formation of fullerene derivative [6,6]-phenyl-C61-
butyric acid methyl ester (PCBM) within an organic host through annealing. The annealing
provides an enhanced mobility of the PCBM molecules and, with good initial dispersion,
allows for the formation of exaggerated grain growth within the polymer host. We have
assembled these nanostructures within the regioregular conjugated polymer poly(3-
hexylthiophene) (P3HT). This PCBM elongated nanostructure formation maybe responsible
for the very high efficiencies observed, at very low loadings of PCBM (1:0.6, polymer to
PCBM), in annealed photovoltaics. Moreover, our high resolution TEM and electron energy

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loss spectroscopy studies clearly show that the PCBM crystals remain crystalline and are
unaffected by the 200-keV electron beam.

Keywords: Organic Solar Cells; P3HT; PCBM nanocrystals

1. Introduction
It is known that the polymer photovoltaics promise great flexibility, great processability and
ever increasing efficiencies [1-3]. The fullerenes are efficient acceptor materials when they
are blended with most conducting polymers. Yet, to realize efficiencies close to the
theoretical maxima (set by the spectral overlap) in these bulk heterojunctions, internal
resistance must be substantially lowered. The electron transport within the fullerene phase,
i.e. between the interpenetrating network components, must be improved. Therefore, it is
important to increase the mesoscopic order and the crystallinity of these interpenetrating
networks. In the case of holes, this can be achieved with polymers undergoing crystallization
with annealing, e.g. poly(3-hexylthiophene (P3HT). The P3HT [4] and fullerene derivative
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) [5] crystallinities have been reported
independently in blended film of PCBM:P3HT [4] and in PCBM film [5], respectively.
Note that by combining both phases simultaneously, the charge balance can be maintained
and in this way the internal resistance can be lowered by controlling the crystallinity in both
phases simultaneously.
On the other hand, the enhanced crystallinity of the two components P3HT and
PCBM in annealed blends has been also reported [2,6]. In a recent work [6] the morphology
of PCBM:P3HT blend films has been studied by using bright-field (BF) transmission

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electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns.
In that work different P3HT-PCBM concentration and solvent have been used than those we
report here. Moreover, these results are related to low power conversion efficiency (2.7%) in
comparison with previous results from our group [1,2] and W. Ma et al. [3], where 5%
power conversion efficiencies were reported. We consider that the crystallinity of both
materials is a very important feature as well as the morphology of PCBM crystals (i.e.,
agglomerates or elongate shaped PCBM nanocrystals). We think that the reported
differences in efficiencies can be related to differences in the morphologies. The present
report is focused to elucidate, using advanced techniques of electron microscopy, the
morphology in films of high efficiency (5%) [2].

2. Experimental details
Photovoltaic devices were prepared by blending PCBM (American Dye Source) and P3HT
(Aldrich: regio-regular Mw=87 kgmol-1, without further purification) in chlorobenzene [1-
2]. After that, the organic components were dissolved and filtered. The deposition of the
solutions was carried out on glass/indium tin oxide (ITO, delta technologies Rs=20 Ohm-
cm-1) substrates. These substrates were thoroughly cleaned by using organic solvents and
exposed to ozone for 90 min. The first film deposited by spin coating with a thickness of
approximately 80 nm consist in poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS, Baytron P). After drying this layer at 80 °C for 10 min, a second layer was
deposited. The active layer was deposited through spin casting at 1500 rpm. These films
were made with different P3HT:PCBM weight ratios 1:1, 1:0.6 and 1:0.4 corresponding to
15 mg of P3HT in 1 ml of chlorobenzene. The blend was continuously stirring for ~12 h.

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The thickness of the blend layer for the P3HT:PCBM weight ratios 1:1, 1:0.6 and 1:0.4
corresponded approximately to 123 nm, 88 nm and 86 nm, respectively. Film thickness was
measured by using a Tencor Alpha-step 500 surface profiler. The devices (without Al and
LiF electrodes) were annealing in inert atmosphere (dry nitrogen glove box), after their
cooling, they were prepared for an electron microscopy study. Note that the kinetic of the
PCBM crystals can be reduced when annealing is performed on a real device with metal
electrodes [3,6]. The samples for TEM measurements were prepared by floating the layers
(PEDOT:PSS/P3HT:PCBM) onto a water surface, and transfer to a TEM grid. The
photovoltaic characteristics for unannealed devices and for the optimally loaded annealed
device have been previously reported [2].
BF TEM images and SAED patterns were recorded on a JEOL JEM-1230
transmission electron microscope operated at 100 kV. Z-contrast imaging, energy electron
loss spectroscopy (EELS) and high-resolution transmission electron microscopy (HRTEM)
were conducted on a JEOL 2010 FEG FAS transmission electron microscope operated at
200 kV (resolution 0.19 nm and spherical aberration 0.5 mm). X-ray diffraction (XRD)
patterns were recorded using a Rigaku, DMAX 200 diffractometer using Cu Kα radiation.
For the study of the thin films in grazing incidence (GI) diffraction geometry [4], the angle
between the film surface and the incident beam was fixed at 0.3°. The detector scans in a
plane defined by the incident beam and the surface normal.

3. Results and discussion
Recently Reyes-Reyes et al. [2] have varied the loading of PCBM in P3HT films and have
shown that the maximal efficiencies in P3HT:PCBM bulk heterojunction organic solar cells

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are found for very low PCBM loading (1:0.6). These devices with the optimal loading were
annealed at 155ºC between two and three minutes resulting in an efficiency increase of up to
120% (from 2.4% to 5.2%). Several characteristics of the annealed and unannealed devices
with different PCBM loading were analyzed to understand the great increase in their
efficiency. In this paper, by using transmission electron microscopy, Z-contrast imaging and
HRTEM, it is examined the morphology and meso-order of the PCBM nanostructures in
these films.
First, we have observed that for all P3HT:PCBM weight ratios, 1:1, 1:0.6 and 1:0.4
corresponding to a PCBM weight percentage of 50, 37, 28, respectively, BF TEM images
(not shown) show the morphology of the unannealed and annealed films to be quite
homogeneous even at very high magnification (transmission electron microscope operated at
100 kV), similar results were presented by W. Ma et al. [3]. After that, we have performed a
SAED analysis on unannealed films made at different concentration of PCBM (50, 37 and
28 wt %, Fig. 1a, b and c, respectively), three weak broad Debye-Scherrer diffraction rings
can be distinguished with average d-spacings of 0.42, 0.25 and 0.13 nm. These same
distances between adjacent planes are also observed in the annealed sample with optimal
concentration (37 wt % of PCBM, Fig. 1d). There are several possible origins for the
diffraction patterns in such films. However, it has recently been shown that these films are
composed of homogeneously distributed PCBM nanocrystals [2]. Furthermore, through
Near-Field Scanning Optical Microscope analysis, it has been proposed that these crystals
take some kind of elongated shapes or large aggregations of PCBM [2].
In order to clarify the last assumption, a deeper and more elaborated study is
necessary. The morphology of the films was observed by using techniques such as HRTEM,