F Jean

G Juška

J.-M Nunzi

K Arlauskas

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Vol. 107 (2005) ACTA PHYSICA POLONICA A No. 2
Proceedings of the 12th International Symposium UFPS, Vilnius, Lithuania 2004
Slowup of Bimolecular Recombination
in Organic Polymer Solar Cells
J.-M. Nunzia, G. Juškab,∗, F. Jeana and K. Arlauskasb
aEquipe de Recherche Technologique Cellules Solaires Photovoltäıques Plastiques
Université d’Angers 2, boulevard Lavoisier, 49045 Angers, France
bDepartment of Solid State Electronics, Vilnius University
Sauletekio 9, build. 3, 01513 Vilnius, Lithuania
For the retrieval of best polymers for effective organic solar cells as test-
ing parameter we have used the coefficient of bimolecular recombination (B).
The B measurement demonstrated that in pure MEH-PPV layers its value
is close to the value of Langevin recombination coefficient (BL), while in
case of perylene/MEH-PPV junction layers the ratio B/BL ∼= 0.013, and in
blends of MEH-PPV with PCBM B/BL < 0.01.
PACS numbers: 72.80.Le, 72.20.Jv
1. Introduction
Recently, due to probable low-cost and technological simplicity, the inter-
est in organic polymer solar cells (OSC) has grown up. However, in layers of
organic polymers the charge carrier mobility µ, which is prevailed by hopping,
is low (≤ 10−4 cm2/(V s)). To obtain the energy conversion coefficient, and,
herewith, current density, comparable with ones of crystalline solar cells (SC),
the density of photogenerated pairs must be very high. On the other hand, the
low charge carrier mobility of organic polymers is caused by short, comparing to
Coulomb radius, hopping distance. The latter, in turn, causes decreased pho-
togeneration quantum efficiency (Onsager-type) and bimolecular recombination
(Langevin-type). The Langevin recombination is predetermined by probability for
electron and hole to meet in coordinate space, and the coefficient of recombina-
tion BL = e(µn + µp)/εε0, where the symbols have their standard meaning [1, 2].
Recently at least two possibilities have been known, how to reduce bimolecular re-
combination by creating separate paths for electrons and holes. The first of them is
∗corresponding author; e-mail: gytis.juska@ff.vu.lt
(377)
378 J.-M. Nunzi et al.
based on the formation of hetero-junction of electron and hole transporting mate-
rials [3, 4]. Created by light, the excitons are destructed at the junction interface,
and so created electrons and holes diffuse in different transporting materials. In
organic polymers the typical diffusion length does not exceed 0.1 µm. Thus, the
thickness of OSC is limited, and, herewith, the amount of absorbed light is limited
as well. The second possibility is based on usage of blends of electron and hole
transporting materials [1].
The aim of this work is to compare the features of ultrafast bimolec-
ular recombination (described by coefficient B) in pure poly(2-methoxy-5-(2’-
-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV), perylene/MEH-PPV junc-
tion structure and [6,6]-phenyl C60 butyric acid methyl ester (PCBM)/MEH-PPV
blend and to select the optimal structure for solar cells. The magnitude of B/BL
ratio was chosen as a testing parameter.
2. Method
In Ref. [5] the more detailed description of estimation of B using time-of-
-flight (TOF) method is given. The method is based on measurement of amount
of extracted charge from the reservoir created by short pulse of high intensity
light (L). When the amount of photogenerated charge in the reservoir is higher
than CU (here C is the geometric capacitance of sandwich-type sample, U is the
voltage applied onto sample’s electrodes), the electric field (F ) inside of reservoir
is screened quickly. The extraction of charge carriers thins thickness of reservoir
dL, and the reservoir vanishes at the extraction time te. In case of thick layer
(d À dL) the magnitude of extraction current is limited by space charge, while in
case of thin layer (d ∼= dL) it is limited by resistance of electrodes or/and loading
resistor. When L →∞, the amount of extracted charge (Qe(L)) saturates Qs [4]:
Qs =
∫ te
0
j(t)dt =
edLS
Bte
or
B
BL
=
CU
Qs
ttr
te
dL
d
. (1)
When B = BL and if d À dL, Qs = CU [6]. In case of monomolecular re-
combination te ∼ ln L, while when recombination is absent te ∼ L. Thus, the
measurements of Qe(L) and te(L) allow us to estimate the type of recombination,
and evaluate B.
3. Experiment and results
The sandwich-type layers have been formed by deposition of perylene on
glass substrate covered by ITO with following coating by MEH-PPV. The MEH-
-PPV/PCBM blends of various ratios where coated on glass substrates covered
by ITO. The semi-transparent Al or Au electrode was deposited on the top of
structures.
Slowup of Bimolecular Recombination . . . 379
The measurements have been provided by conventional TOF method. The
standard ND:YAG laser (pulses of light hν = 2.3 eV, halfwidth 25 ps, and intensity
1 mJ/cm2) has been used and the electric signals has been recorded by Tektronix
TDS3032 oscilloscope.
Qe has been estimated by integrating transients of photocurrent, and te from
these transients as te = [t1/2(L → ∞) − t1/2(L → 0)] (here t1/2(L) is halfwidth
of photocurrent transient at given intensity of light). The typical transients of
TOF photocurrent are demonstrated in Fig. 1 and µp = 1.3 × 10−6 cm2/(V s)
(F = 105 V/cm) mobility (field) of holes has been evaluated.
Fig. 1. Photocurrent transients of perylene/MEH-PPV junction structure at high (1)
and low (2) intensity of exciting pulse of light L.
Fig. 2. Extracted charge dependences on intensity of exciting pulse of light L
(× — pure MEH-PPV, solid triangle — perylene/MEH-PPV junction, ◦ — blend of
MEH-PPV/PCBM).
The investigation result of pure MEH-PPV was that Qs = CU (see Fig. 2),
therefore B ∼= BL = 6.9×10−13 cm3/s, as it was expected in case of single organic
polymer [7].
380 J.-M. Nunzi et al.
Fig. 3. Dependences of halfwidth of photocurrent transient on intensity of exciting
pulse of light L (× — pure MEH-PPV, solid triangle — perylene/MEH-PPV junction,
◦ — blend of MEH-PPV/PCBM).
In perylene/MEH-PPV (dL = 0.1 µm, d = 2.7 µm) layer the extraction cur-
rent has been estimated to be limited by space charge. The tendency to saturation
of Qe(L) and te(L) with intensity of light is clearly seen in Figs. 2 and 3. In this
case B = 0.013BL = 1.3×10−12 cm3/s, which causes that the mobility of electrons
in perylene is much higher than holes in MEH-PPV (µn = 2.4× 10−4 cm2/(V s)).
From investigation results of MEH-PPV/PCBM blends it follows that Qs
much exceeds CU , and the value B = 6 × 10−10 cm3/s < 0.01BL was estimated.
It shows that the mobility of electrons in blends is even higher, but we could not
estimate it, because of high dispersivity of transport.
Acknowledgments
This work was partly supported by joint Lithuania-France research project
“Gilibert”(“FONEPC”) and by Lithuanian VMSF Grant No.T-68/04.
References
[1] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11, 15 (2001).
[2] M. Pope, C.E. Swenberg, Electronic Processes in Organic Crystals and Polymers,
Oxford University Press, New York 1999.
[3] J.-M. Nunzi, C.R. Physique 3, 523 (2002).
[4] N. Nekrašas, G. Sliaužys, G. Juška, K. Arlauskas, J. Stuchlik, J. Kočka, Acta
Phys. Pol. A 107, 373 (2005).
[5] G. Juška, M. Viliūnas, K. Arlauskas, J. Kočka, Phys. Rev. B 51, 16668 (1995).
[6] G. Juška, Lithuanian J. Phys. 29, 600 (1989).
[7] H. Bässler, Polym. Adv. Technol. 9, 402 (1998).


Slowup of Bimolecular Recombination in Organic Polymer Solar Cells

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Slowup of Bimolecular Recombination in Organic Polymer Solar Cells

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