Download Department of Physics, Fudan University, Shanghai, 200433, PRC

Exciton Dissociation at Metal-Organic Interface and
The induced Surface Loss in BHJ Solar Cells
Wenchao Yang, Deli Li and Chang-Qin Wu
Department of Physics, Fudan University, Shanghai, 200433, PRC
The pursuit of new energy resources that is clean and renewable is always a hot topic of research. Organic solar cells attracts very much attention due to their desirable
properties like inexpensive, flexible over their inorganic counterparts. Bulkhetrojunction (BHJ) solar cell emerges as the most efficient and prominent type of organic solar
cells in recent years. Experiments of transient photovoltage (TPV) find that there is possibly intensive exciton dissociation occurring at the anode (metal)-organic interface,
which leads to an innegligible loss of free electrons, resulting the so-called surface loss. Starting from the device model and including the exciton dynamics as well, we
calculate the J-V curves in different interfacial dissociation rates, and evaluate the magnitude of the surface loss and its influence to the short circuit current, open circuit
voltage and the efficiency of the device. The role of bimolecular recombination is also investigated.
Background: Bulk Heterojunction
Organic Solar Cells
Models and Equations for Simulation
N ex
 2 N ex
D
 k0 N ex   ( x) N ex  G ( x)  R
t
x 2
p
1 J p

 Gc  R
t
e x
n 1 J n

 Gc  R
t e x
E
1 V
1 
1 L



J

J
(
x
)
dx


t
L t  0 r 
L 0

1
k0  , G ( x)  g 0 exp( x La ),
g 0   I 0 La
interface
Two types of organic solar cells: (a) the bilayer
device and (b) the recently popular bulk
heterojunction one. Both of them have a
sandwiched structure, which consists of a
transparent anode of ITO, an active layer of
organic semiconducting material, and a metal
cathode, like Al.
illumination
The schematic illustration of the
mechanism and electronic process of BHJ
solar cell. (1) Excitons are generated in
the donor under illumination. (2) They
diffuse to the donor/acceptor interface
and experience a charge transfer to
convert into Coulomb bounded e-h pairs.
(3) The pairs can dissociate into free
carriers. These free electrons and holes
are extracted under the built-in field by
the electrodes, giving the photocurrent.
Current Voltage Characteristics
D
Gc   ( x) N ex ,
R   (np  ni2 )
ITO Organic Cathode
To consider the interfacial dissociation
explicitly, the active layer is divided into the
two parts of interface (red) and the bulk.
They are of different exciton dissociation
rates.
Device model combined with the exciton
evolution equations. The four coupled equations
are solved to obtain the steady state current,
under the boundary condition of
N ex ( x)
0
x x 0, L
J n / p  J th  J re
Interfacial Dissociation Proportion
The current-voltage curves are shown when
changing the interfacial dissociation rates
(left-upper corner) or bulk dissociation rates
(right). The short circuit current is decreased
with increasing interfacial dissociation rate or
decreasing bulk dissociation rate. The upper
figure gives a quantitative relation between
the short circuit current and the interfacial
dissociation time using different interface
thickness, suggesting the interfacial effect is
restricted to 1nm away from the anode.



d
0
L
0
 ( x) Nex ( x)dx
 ( x) Nex ( x)dx
.
Exciton density distribution
under short circuit condition
The depletion of excitons in the interface at large dissociation rates
result in the saturation of interfacial dissociation proportion .
Interfacial Dissociation at
Open Circuit Condition
Compensation Voltage decreasing
Induced by Surface Loss
Photocurrent versus the applied voltage. In Ohmic contacts, the compensation
voltage of the photocurrent decreases with the increasing of interfacial
dissociation rates. This effect is more obvious than the decreasing of short
circuit Current.
Interfacial dissociation proportion decreases at high bias voltage, for
the bimolecular free carrier recombination play an important role,
which increases the exciton density in the bulk region .
Conclusions & References
• The short circuit current decreasing slightly with increasing of interfacial dissociation rate, until saturation at very large rate or small dissociation time. The
decreasing of bulk dissociation rate can lead to the decreasing of short circuit current as well, and the effect is much more obvious .
• The open circuit voltage dose not change with the increasing of interfacial dissociation rate in Schottky contact device, while it decreases at Ohmic Contact
one, this can be seen more clearly from the plot of net photocurrent versus the applied voltage.
• The interfacial dissociation proportion increases with the interfacial dissociation rate, varying conversely with the short circuit current. Its saturation can be
understood with the help of exciton density profile. The excitons are equivalently quenched at the interface.
• The bimolecular recombination can reduce the interfacial dissociation proportion at high bias voltage in Ohmic contact device.
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