Organic Photovoltaic Solar Cells: A Molecular Perspective
Corinna Cisneros, S.M. Golam Mortuza, and Soumik Banerjee
School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99163
Introduction
The depletion of fossil fuel resources has lead to global
energy crisis with an increasing effort in the scientific
community to develop renewable energy technologies.
Solar cell technology has generated significant
commercial and scientific interest because they provide
an easy means to harvest freely available solar energy
and can be used to power several applications including
small electronic devices and can be used on the tops of
carport canopies, bus stops and large buildings.
Specifically, organic photovoltaic (OPV) solar cells have
several advantages, including ease of manufacturing,
relative low costs, light weight, flexibility and low
operating temperature. OPVs are also becoming more
attractive for commercial uses, ideal for uneven terrains
and even preferred more safe in earthquake zones.1
Configuration
Computational model for polymers
P3HT
PCBM
Indane
Toluene
Polymers
The polymers that we choose to simulate were poly(3-hexylthiophene)
(P3HT), and poly [methoxy, 5-(2’-ethyloxy)-p-phenylenevinylene)](MEHPPV).
P3HT
• The most popular polymer due to its high power conversion efficiency
(PCE) of 10%. 4
• The most promising and most researched OPV polymer. 4
• P3HT contains side chains which make them soluble in common
organic solvents . 1
• Most importantly P3HT absorbs a significant fraction of the sun light
(95%) depending on its thickness. 1
The key component of an OPV is the photoactive layer,
which comprises an electron acceptor (organic
nanoparticle) and a light harvesting material (conjugated
polymer). The photons with energy greater than the band
gap are absorbed by the photoactive layer resulting in
exciton generation. The excitons diffuse to the donoracceptor interface and dissociate into electrons and
holes that are then collected at the anode and cathode
respectively generating electricity. The power conversion
efficiency (PCE) for current OPVs are limited to 10%
partly due to recombination losses that are directly
determined by the morphology of the photoactive layer.
Hence, it is important to improve the morphology, and
arrange the photoactive layer in a way that will produce
the highest efficiency for OPV solar cells.2
MEH-PPV
• This polymer also contains side chains that also make them soluble. 1
• Like P3HT, MEH-PPV also absorbs a large fraction on sunlight. 1
• This polymer has a high electron affinity and chemical stability. 1
• MEH-PPV can be used in conjunction with PCBM. 3
• More studies need to be done on this material to learn more about its
performance. 3
• The simulation of the chosen polymer requires specification of its
molecular structure and a description of atomistic interaction
parameters.
• The initial molecular structure was built and optimized using the
Avogadro molecular modeling package. The resulting pdb file
specifies the polymer structure without hydrogens and includes
the atom types, residues names and x,y,z coordinates of the
various components of the polymer.
• Next, an hdb file was manually created. An hdb file tells Gromacs
exactly where each hydrogen is located, which atom it is
attached to, how many atoms that are attached to that atom, and
its shape (such as planar or tetrahedral).
• The parameters and functional form for both intra-molecular and
inter-molecular interactions are included in a carefully
constructed topology file.
• For the present project the OPLS-aa force field was used.
• The molecular modeling package Gromacs 4.5.5 was used for
the simulations and hence the topology was build using the
pdb2gmx module such that it is compatible with gromacs.
• An rtp file that contains residue type and number, all atoms
including hydrogen, OPLS force field parameters, and respective
atomic charge was generated.
• Using gromacs’ pdb 2gmx module with the pdb, hdb and residue
files as inputs a topology file was created. The topology file
contains all interatomic interaction and intratomic (bonds, angles,
dihedrals) parameters that are required to run molecular
dynamics simulations of the system.
• Finally using the topology file and structure of a single polymer, in
gromacs, we can create a polymeric system by replicating the
polymer in various directions. The energy of the resulting
polymeric system has to be minimized followed by molecular
dynamics simulation to obtain appropriate density.
Previous results of PCBM-solvent systems
P3HT Oligomer
MEH-PPV Oligomer
Future work
MD snapshots of PCBM in toluene (left) and indane (right) at 310 K
The photoactive layer, which comprises an electron
acceptor [6-6]-phenyl-C61-butyric acid methyl ester
(PCBM) in a conjugated polymer poly(3-hexylthiophene)
(P3HT) is obtained through spin coating of the dispersed
system in solvents such indane and/or toluene. The
understanding of such systems is therefore critical to
improving the efficiency of OPVs. 3
Objectives
• To investigate the mechanisms that govern molecular
rearrangement, agglomeration and transport of
polymers using molecular dynamics (MD)
simulations.
• To correlate the morphology of the photoactive layer
with key parameters such as polymer concentration,
temperature and PCBM- solvent interactions.
Here,
P1T1: 1:1 PCBM-toluene
P1T1: 1:2 PCBM-toluene
P1I1: 1:1 PCBM-indane
P1I2: 1:2 PCBM-tndane
P1TI1: 1:1 PCBM-toluene
indane mixture
P1TI2: 1:2 PCBM-toluene
indane mixture
• Larger PCBM clusters
are dominant in systems
comprising toluene.
• Greater number of
monomers and dimers
of PCBM occur in
indane than in toluene
and toluene-indane
mixture.
• The sharp peak of radial
distribution function (RDF)
indicates that fullerenes have
a tendency to agglomerate in
solvents and form clusters
• The number of fullerenes (not
shown here) that surround a
single fullerene is more in
toluene than indane.
• Our next goal is to include polymer with PCBM and solvents
and simulate a comprehensive system to understand the
molecular mechanisms that govern the formation of
nanoparticle agglomerates in the photoactive layer.
• Evaluate the nature of association of PCBM with respect to
the polymer and solvent molecules.
References
1. Coakley, Kevin M. "Conjugated Polymer Photovoltaic Cells." Chem. Mater..
16. (2004): 4533-4542.
2. Yang, Xiaoniu. "Nanoscale Morphology of High-Performancs Polymer Solar
Cells." Nano Letters. 5.4 (2005): 579-583.
3. Saunders , Brain R. Saunders. "Nanopartical-polymer photovoltaic cells."
Advances in Colloid and Interface Science. 138. (2008): 1-23.
4. Krebs, Frederik. Polymeric Solar Cells: Materials, Design, manufacture.
Landcaster, PA: DEStech Publications, Inc., 2010. 1-66.
Contact information
Please contact:
corinna.cisneros@email.wsu.edu or soumik.banerjee@wsu.edu
for further information.
***This work was supported by the National Science Foundation’s
REU program under grant number EEC-1157094