Journal of Energy and Power Engineering 6 (2012) 921-924
Effect of Temperature on the Organic Solar Cells Parameters
Belhocine-Nemmar Farida, Hatem Djediga, Boughias Ouiza and Belkaid Mohammed Said
Advanced Technologies of Electrical Engineering Laboratory (LATAGE), Mouloud Mammeri University, Tizi-Ouzou 15000, Algeria Received: February 14, 2011 / Accepted: June 09, 2011 / Published: June 30, 2012.
Abstract: The dependence of the organic solar cells parameters, e.g., the shirt circuit current I sc, open circuit voltage V oc, the fill factor FF and the efficiency eta on temperature is investigated. By expressing the different equations which link the parameters with temperature, it is obrved that the short circuit current I sc increas so monotonous with temperature and then saturates to a maximum value before decreasing at high temperatures. The open circuit voltage V oc decreas linearly with the increasing of the temperature. The fill factor FF and the efficiency eta which are directly related with short circuit current I sc and the open circuit voltage V oc follow their variations.
Key words:Cells parameters, organic materials, solar cells, temperature effect.
1. Introduction
The organic solar cells can be an alternative to the silicon bad photovoltaic cells [1]. They have recently acquired a big interest becau of their ea implementation and flexibility. They are made with a transparent mi-conductor as electrode, a mixture of organic mi-conductors in three dimensions, in continuous network as an active layer and a metal layer as against electrode. The best performance was obtained using a composite of two organic materials: a donor and an acceptor, in a bulk interpenetrated network. This concept is very investigated becau it allows a large interfacial surface which provides good excitants paration. The best efficiencies are allowed from the cells which u a mixture of (poly-3 hexylthiophene) P3HT and ([6-6]-phenyl C61 butyric acid methyl ester) PCBM, which allows an efficiency of 5% [2].
For the reliability of the devices in different operating conditions, the effect of various
Corresponding author: Belhocine-Nemmar Farida, Dr., rearch field: advanced technologies of electrical engineering laboratory(LATAGE).E-mail:************************.environmental parameters are investigated, e.g., temperature, humidity, electric and magnetic fields, sandstorm and illumination intensity on their performances.
In this paper, the effect of temperature on the organic solar cells parameters has been studied. The relations which link the shirt circuit current I sc, open circuit voltage V oc, the fill factor FF and the efficiency eta with temperature were expresd and their interpretations have been discusd.
2. Short Circuit Current I sc
In a solar cell, at any point depth z in the bulk of the device, the density of the current J(z) (which is the current per surface unity (s)) is proportional to the difference between the rate of generation of free charge carriers G(z) and the recombination rate R(z) according to the following equation:
)
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(
)
(
1z
G
z
R
dz
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dJ
q
-
=(1)
In the heterojunction structure devices, the
recombination in the bulk is of two types:
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雨下一整夜Effect of Temperature on the Organic Solar Cells Parameters
9222.1 The First Order Recombination
The first order recombination is due to the prence of impurities in the mixtures of the active layer of the solar cell. The first order recombination depends on the density of the impurities which are traps for the generated free carriers. In the ca of electrons, the first order recombination is given by Ref. [3]:
τ
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)()(z n z R =
(2)
where n (z ) reprents the density of electrons at a depth, z and τn is the lifetime of the electrons. 2.2 Bimolecular Recombination
The bimolecular recombination depends on the density of free carriers, electrons and holes. It is expresd by the following relation:
)()()(z p z n K z R = (3)
where n (z ) and p (z ) are the densities of electrons and holes respectively and K is the bimolecular coefficient. In the organic solar cells, the generation of the free carriers is the result of two process: generation of excitants in the donor material of the active layer and paration of the excitants at donor/acceptor interfaces. Thus, the generation rate G (z ) depends on the diffusion length of the excitants, the absorption coefficient of the active layer materials and the intensity of the incident light. The current density that flows in a PN junction under illumination is given by:
L J KT qV J J -⎥⎦
⎤
⎢⎣⎡-=1)exp(0 (4)
where, J L is the density of photogenerated current, K is the Boltzmann constant and J 0 is the saturation current density which is given by :
⎪⎪⎭
⎫ ⎝⎛+⎪⎭⎫ ⎝⎛-=p p n n c v tau p L tau n L KT Eg KT N N J exp 0 (5) N c and N v are the densities of states in the conduction and valence band respectively, p and n are the densities of the electrons and holes, L n and L p are the diffusion length, tau n and tau p are the lifetimes of electrons and holes.
At zero bias (the cell is short-circuited), the short circuit current density J sc generated by the cell can be
expresd as:
L sc J J -= (6)
In a study done by Monestier and Simon [4], the short circuit current is found to be dependent on te
mperature. They found that the short circuit current I sc increas with temperature and tends to be saturated at a maximum value followed by a decrea at high temperatures. This behaviour can be explained by considering that the current delivered by the cell is proportional to the number of the free charge carriers generated and their mobility. For the organic miconductors, the charge carrier transfer occurs via localized sites. The charge carrier transfer from one site to another nearby site is associated with phonons. The conductivity is thermally activated, otherwi say, increas with temperature. The increa of the mobility with temperature is suppod to be a hoping phenomenon enabled by the phonons. This hypothesis can be verified by considering that for organic materials, the conductivity is given by:
2exp(
0KT
E
delta sigma sigma -= (7) and the mobility by:
en
sigma
mu =
(8) where delta E is the activation energy of the process. The activation energy delta E is a contribution of two factors [5], gap energy value (the energy required to excite an electron from highest occupied molecular orbital HOMO to lowest unoccupied molecular orbital LUMO) and the delocalisation energy of the charge carriers (energy required for an electron to outside a trap).
At low temperatures, the probability of a phonon with an energy that allows a jump from a site to another nearby site is low; therefore, the mobility is low at low temperatures.
3. Open Circuit Voltage V OC
For photovoltaic cells bad on organic materials, the open circuit voltage depends directly on the band gap energy Eg . The open circuit voltage of a PN
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Effect of Temperature on the Organic Solar Cells Parameters
923
junction bad solar cell is given by:
)1ln(+=s sc oc I I q nKT
V (9) Taking into account that the short -circuit current I sc
is so greater than the saturation current I s , the open circuit voltage V oc will be written as:
)])(ln[(p
n oc ptau Lp
ntau Ln Icc Nv Nc q q nKT q nEg V +-=
(10) The slope of the equation expressing the variations
of the short circuit voltage as a function of temperature is negative, so the short circuit voltage V oc decreas linearly with temperature. From Eq. (10), it is clear that the open circuit voltage V oc depends greatly on the temperature, diffusion length, lifetime and the density of the charge carriers a
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nd it depends slightly on the materials ud for the cathodes.
4. The Fill Factor FF
View of the prospects for improving the efficiency of organic solar cells, the most critical factor for a
large impact on the different strategies depends on
better fill factor FF which is given by the relation:
max max oc sc
V I FF V I ⨯=
⨯ (11)
V max and I max are the voltage and the current at maximum energy delivered by the cell. Their values are deduced from the cell I-V curve under illumination reprented in Fig. 1.
The fill factor FF is the most nsitive parameter in a solar cell in comparison with the open circuit voltage V oc and the short circuit current I sc .
字母分类The fill factor FF depends on the active layer material properties (charge carriers mobility and their lifetime), the active layer morphology and the physical and chemical properties of the interface active layer/cathode. The important role of the active layer/cathode interface depends on the cathode deposit conditions which can cau a variety of chemical and physical defects, which will be subquently traps for charge carriers.
Fig. 1 Current-tension characteristic of a solar cell.
In organic solar cells, the free charge carrier photogeneration comes through the dissociation of the excitants at interfaces donors/acceptors with efficiency ηg . The charge carrier collection rate do not reach a unity becau the excitants in organic materials have a very limited lifetime, about 30 ns [6], which promotes the recombination of excitants before their paration.
5. The Efficiency eta
Adding to their limited lifetime, the efficiency eta of the organic solar cells is another factor which impedes their commercialization. It is given by the following relation:
in
oc sc P FF
V I eta =
(12)
where P in is the power of the incident light. It can be enhanced by the u of the polymer absorbing in the visible and infrared spectrum and the concept of the bulk heterojunction.
6. Conclusion
In this paper, the influence of temperature on the organic solar cells parameters has been studied.
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The open circuit voltage decreas linearly with temperature with a negative slope dV co /dT . The short circuit current at low temperatures I sc increas with temperature before reaching a maximum value of saturation and then decreas for higher temperatures. The fill factor FF and the efficiency eta follow the changes of the open circuit voltage V oc and the short
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Effect of Temperature on the Organic Solar Cells Parameters 924
circuit current I sc. The variations are explained by the charge carrier mobility behavior with temperature.
References
[1] A. Pivrikas, P. Stadler, Substituting the post production
treatement for bulk heterojonction solar cells using chemical additives, Organic Electronics 9 (5) (2008) 775-782.
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of poly (3-hexythiophene) on the performances of organic
photovoltaic devices, Polymer Inrnational 57 (5) (2008)
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[3]S. Amon, O. Haba, Head to tail regularity of poly
(3-hexylthiphene) in oxidative coupling polymerisation with FeCl3, Journal of Polymer Science 37 (13) (1999)
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[4] F. Monestier, J.J. Simon, Modeling the shorte ciruit
current density of polymer solar cells bad on P3HT:
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PCBM blend, Solar Energy Materials and Solar Cells 91
(5) (2007) 405-410.
[5] D. Chirva, Z. Chiguvare, M. Knipper, J. Parisi,
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France, 2004.
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