-Research on photovoltaic energy conversion has recently received great impulse due to the growing demand for low carbon dioxide emission energy sources. In particular, the high manufacturing cost of crystalline silicon and the latest advancements on semiconducting polymer design and synthesis in recent years have directed the attention of the scientific community towards Organic Solar Cells (OSCs). In this paper, the electromagnetic field induced on OSCs has been investigated to evaluate the potential of the optical efficiecy at several frequencies of OSCs. Index Terms-FEM; Organic solar cells; Electromagnetic field
The electronic devices more efficient and economics able
to work as solar cells can be developed using several types
of material with different structural architectures [
In order to be competitive for the production of solar energy on an industrial scale, this technology must however dissolve a crucial node. Compared to inorganic semiconductors, the organic materials have a lower charge transfer coefficient, namely the electron transfer speeds in the energy system. Although thinner and flexible, the organic photovoltaic films fail to capture an equally large portion of the solar spectrum with a lower amount of energy based on the silicon technologies.
However, the mechanical reliability of full cell packaging
is rarely concerned. As solar cell often appears to be a part
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of structure, stress will inevitably act on the package [
The researchers have tried to solved these problems by
increasing the ability to transport electrons polymer using
polymer mixture, studying the morphology of the organic
photovoltaic and new solutions to increase the charge transfer
coefficient. During the last years the development of
organic photovoltaics (OPV) focuses on inexpensive materials,
promising a simple processing and integration on different
substrates with high performance. In this regard the equivalent
circuit modeling of photovoltaic devices can be useful for
understanding the performance and the optimization of the
design solar cells [
Doo-Hyun Ko et al. [
Thomas Rieks et al. [
Gholamhosain Haidari et al. [
From optical and electrical simulations, increasing the
thickness of the organic solar cells device due to the low mobility
of the organic materials, non-geminate recombination rate
enhances and causes the power conversion efficiency reduction
[
The optical performance of the P3HT:PCBM solar cell has
been simulated by AMPS-1D to study the J -V characteristics
and electric field with active layer thickness in [
The processes controlling the efficiency of hybrid planar
devices comprising two semiconducting donor polymers and
amorphous silicon,charge generation and the distribution of
electric fields in a-Si:H/organic-hybrid solar cells have been
investigated by [
By design and simulation of the OSCs devices is possible to provide a simple model for the performance analysis of extending the high frequency range. The electromagnetic field induced on solar devices has been investigated to evaluate the interactions of the optical model at several frequencies using as tool COMSOL to calculate the magnetic field effects starting from a sample with OSCs carried out in collaboration with Optoelectronic Organic Semiconductor Devices Laboratory (OOSDL), Department of Electrical and Computer Engineering Ben-Gurion University of the Negev, Israel.
A bulk heterojunction solar cell consists of an active organic
part with a donor and an acceptor material, the metal
electrodes and the substrate. The substrate usually glass and
transparent ensures that the light reaches the active material. The
positive and negative electrodes respectively indium tin oxide
and aluminum are responsible for the transport of the charges
from the organic material to the electrical connections. The
absorption of photons from incident light generates excitons
namely bound electron-hole pairs, due to the stronger Coulomb
attraction in an organic materials. To generate electrical
energy it is necessary to separate the charges to contribute to
photocurrent in an organic cell. The exciton with a random
process based on concentration gradients can diffuse at this
donor/acceptor interface to dissociate and convert into free
charge carriers, electrons and holes. To generate electrical
energy it is necessary to separate the charges to contribute
to photocurrent in an organic cell. The photons energy excites
the electron from the HOMO to the LUMO band, where the
HOMO and LUMO are equivalent to the valence band and
conduction band known from classical physics. The exciton
with a random process based on concentration gradients can
diffuse at this donor/acceptor interface to dissociate and
convert into free charge carriers, electrons and holes, when the
interface is within the exciton diffusion length, typically is not
very long some 10 nm depending on the material in organic
semiconductors. However excitons can recombine or decay
back to the ground state, during the diffusion process before
reaching the donor/acceptor interface, leading to absorbed
photons that do not contribute to the current. In this case,
the recombination rate depends on the charge density, as
the probability of two charges meeting increases with higher
carrier density. For an efficient exciton dissociation it has to
be energetically favorable for the electron to be transferred to
the LUMO of the acceptor or for the hole to the HOMO of
the donor material, respectively. Finally, the charges due to
an internal electric field and a gradient in the electrochemical
potential are collected and transported towards the respective
electrodes, electrons and holes can be extracted by cathode and
anode, leading to current’s generation to the external circuit
[
The procedure for fabrication of organic solar cell device is carried out inside the glovebox at low temperature 22 C compatible with their sensitivity and degradation very fast under normal air conditions, performed under a dry nitrogen atmosphere with an O2 concentration of 1:7 ppm and an H2O concentration smaller than 0.1 ppm. The fabrication of polymer solar cells is based on four steps. Before the first step is the cleaning of IT O coated glass substrate to eliminate particles, contaminates and imperfections, in acetone, methanol and isopropanol bath for 15 minutes each one through an ultrasonic bath (53 kHz at room temperature). Afterwards the substrate is dipped into distilled water and put into a vacuum oven; the plasma cleaning need to remove of remaining oxygen molecules. The glass substrate is sized 12 mm 12 mm 0:7 mm and is coated in the middle with a rectangular section 6 mm 12 mm and 90 nm 10 nm of transparent thick ITO layer with resistance of 20 Ohm=m2. The ITO is the anode with high transparency, conductivity but it is fragile and susceptible to deterioration. P EDOT : P SS is a transparent conjugated polymer with thickness layer of 30 nm. Inside the glove box, a solution of photo-active polymer P3HT and PCBM is prepared. The P3HT:PCBM layer is deposited onto cleaned substrates by spin coating the samples at 5000 rpm (revolutions per minute) and an
Fig. 5. Extinction coefficient of P3HT:PCBM blend acceleration of 7000 rpm=s for one minute, resulting in a layer thickness of approximately 200 nm. The acceptor material is the PCBM and P3HT is the donor material. Devices were completed by evaporation of aluminum 80 nm in a vacuum, through a shadow mask used to create the preferred shape of aluminum cathodes.To connect the thin film to electrical wire the Silver Conductive Epoxy was deposited on small part of ITO removed locally and Aluminum layer.
The model, shown in Figg. 1 and 2, consists of an embedded structure sample with domain size of 12 12 mm and layers thicknesses of: 80 nm for metal electrode aluminum, 200 nm of active layer pcbm, 30 nm of pedot: pss, 90 nm of ITO and glass substrate with layer of 0:7 mm. The sample contains F/m nm four organic solar cells with different lengths, each of them has size of 4:50 mm, 5:50 mm, 6:50 mm and 7:50 mm.
It was presented an OSC simulated model in a large area of high frequency (1014 3:75 1014 H z). The model is based on solving solves the Maxwell equation using the finite element method. The simulation electromagnetic effects in 2D and 3D were carried out within Comsol Multiphysics. The Maxwells equations in the time domain are: 5 5 5 D 5 B
E H = = = =
Under varying assumptions, these equations are solved with a set of boundary conditions related to material for modeling perfect electrically conducting surfaces within the RF Module of COMSOL Multiphysics using the finite element method to solve for the electromagnetic fields within the modeling domains.
The optical properties of the organic thin lm materials such as poly(3 hexylthiophene) : poly(6; 6 phenylC61 butyricacidmethylester)(P 3H T : P CBM )andP EDOT : P SS=I T O are described by complex wavelength-dependent refraction index n. The values of the complex refractive indexes specified as a function of wavelength and used in the simulation were taken from the literature for P3HT, PCBM, ITO and Al. In Figg. 3, 4 and 5 are displayed the plots, as function of the wavelength, of the optical constants such as P3HT:PCBM and PEDOT in particular, the refractive index and the extinction coefficient are related by phisical materials properties. The refractive index of the environment is equal to the refractive index of air (n=1). In the table the optical and electrical values set used for the FEM analysis: The simulation was carried out at wavelengths in the range of absorption of the material. Then, parametric analysis of COMSOL was used with frequency as the changing parameter, ranging from 1014 Hz to 3:75 1014 Hz. The Perfectly matched layers (PMLs) are used to enclose the model domain. The methodology is consisted to define the equations to solve, creating the model geometry, define the material properties, setting up the boundaries. During the meshing step the model space has been discretized using finite elements. Solving a set of linear equations that describe the electric fields. The useful information are extracted from the computed electric and magnetic fields.
Fig. 6. Electic field in 2D, view from above of sample structure with OSCs
An optical wave is propagated in the system determining the degradation of electric field amplitude due to interference by optical properties in each active layer. To better understand the interaction of the each OSCs, the simulation was repeated for a range of wavelength from 300 to 800 nm, where the incident light has an acceptable intensity on the optical models with different layers. The magnetic field distribution indicates the absorption of electromagnetic wave externally applied depending on the refractive index,wavelength and width of layers. The magnetic field is calculated on sample device with different OSCs located on top of the glass support. The structure and the electric field configuration, bidimensional and tridimensional, are represented in Fig. 6 and in Fig. 7 respectively.
As shown in Fig. 8, the magnetic field decrease along y axis illustrating the dependence with the layers and materials optical properties. Magnetic field results are reported in Fig. 9 for different OSCs, with the same trend. It is necessary to consider several different factors which affect the absorption of any organic photovoltaic device defined as number of photons absorption,exciton dissociaton. Therefore, more excitons are able to diffuse to the heterojunction and dissociate into free charges, moving to the electrodes where they are extracted and establish a photocurrent. An interesting extension to this work could be to examine and explore the relationship between efficiency and device thickness considering an ideal morphology organic solar cell.
The results obtained by these simulations can promote to investigate the polymer materials attitudes examining a large range of frequencies and effects under external electric and magnetic field. An optical model relative to organic solar cells sample model is proposed by means the electromagnetic field simulation at large range of frequency. The simulations in this work were based upon finite element method with 3D Surface plot of Electic field applied on sample structure with 2D geometry. The simulations were carried out using the RF module of COMSOL Multiphysics software package
This work has been supported by the BGU-ENEA joint lab and the ILSE-Joint Italian-Israeli Laboratory on Solar and Alternative Energies.In particular, the author would like to thank the Optoelectronic Organic Semiconductor Devices Laboratory (OOSDL) , Department of Electrical and Computer Engineering Ben-Gurion University of the Negev, Israel.
Magnetic field of the sample containing the OSCs