The effect of exciton interfacial dissociation on transient photocurrent (TPC) in a single-layer organic solar cell is investigated within a time-dependent device model. The spike observed in TPC experiments is attributed to exciton dissociation at the electrode/organic interface. In comparison with the observed negative signal of transient photovoltage (TPV), the spike more directly reflects the charge processes at the interface. Moreover, numerical results show that the spike of TPC is sensitive to the voltage applied on the device and the hole mobility of the organic semiconductor. Further investigation on the spike by the favorable TPC technique is suggested to provide details about the exciton and carrier processes at the interface

Chang-Qin Wu

De-Li Li

Wei Si

Wen-Chao Yang

Xiao-Yuan Hou

Yao Yao

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Physics Letters A 376 (2012) 227–230Contents lists available at SciVerse ScienceDirect
Physics Letters A
www.elsevier.com/locate/pla
Spike in transient photocurrent of organic solar cell: Exciton dissociation at
interface
De-Li Li, Wei Si, Wen-Chao Yang, Yao Yao, Xiao-Yuan Hou, Chang-Qin Wu ∗
Department of Physics and State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China
a r t i c l e i n f o a b s t r a c t
Article history:
Received 10 August 2011
Received in revised form 24 November 2011
Accepted 26 November 2011
Available online 30 November 2011
Communicated by R. Wu
Keywords:
Exciton dissociation
Transient photocurrent
Electrode/organic interface
Organic solar cell
The effect of exciton interfacial dissociation on transient photocurrent (TPC) in a single-layer organic
solar cell is investigated within a time-dependent device model. The spike observed in TPC experiments
is attributed to exciton dissociation at the electrode/organic interface. In comparison with the observed
negative signal of transient photovoltage (TPV), the spike more directly reflects the charge processes at
the interface. Moreover, numerical results show that the spike of TPC is sensitive to the voltage applied
on the device and the hole mobility of the organic semiconductor. Further investigation on the spike by
the favorable TPC technique is suggested to provide details about the exciton and carrier processes at the
interface.
© 2011 Elsevier B.V. All rights reserved.1. Introduction
With the introduction of bulk heterojunction structure, organic
solar cells are proved to be promising candidates for low cost en-
ergy sources [1]. The dissociation of light-induced exciton at the
various interfaces in these cells is identified as one of the essential
processes for charge generation [2]. A comprehensive understand-
ing of these processes is required to improve the efficiency. Re-
cently, the transient photovoltage (TPV) techniques were applied to
investigating some single layer organic solar cells. An abnormal po-
larity change from negative to positive was observed shortly after
switching on the laser pulse [3–5]. The phenomenon was realized
to be related to the exciton dissociation at the electrode/organic
layer interface [6].
In comparison with TPV techniques, time-of-flight experiment
(one of the transient photocurrent techniques) is used more fre-
quently to measure the carrier’s mobility [7]. Normally, the time-
resolved signal exhibits an initial sharp spike followed by a wide
platform until its decay. The mobility is obtained through deter-
mining the transit time ttr, which denotes the moment at which
the signal begins to decay [7–9]. TPC seems to be more sensitive
to the charge carriers movements in the device. Thus, it is sug-
gestive to investigate the effects of the exciton dissociation at the
electrode/organic layer interface on the TPC.
Theoretically, the Monte Carlo method, which describes the
dynamic processes of carriers in the bulk, has been adopted to
* Corresponding author.
E-mail address: cqw@fudan.edu.cn (C.-Q. Wu).0375-9601/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.physleta.2011.11.055simulate the TPC. The method treats the transit time as the aver-
age travel time of the photo-generated carrier from one electrode
to the other [11]. However, the application of the Monte Carlo
method is inconvenient, as the Poisson equation and role of the
circuit must be treated carefully. Fortunately, the time-dependent
device model provides a suitable means. Recently, we successfully
extended the model and simulated the TPV signal. We obtained an
excellent results compared with related experimental results [6].
In this Letter, this method is modified to investigate the interfacial
effects on the TPC under the short-circuit condition.
2. Device model method
The model quantitatively addresses charge processes, exciton
processes, the role of the contact interface and the evolution of the
electric field by Poisson equation. The details of the model were
presented in one of our previous works [6]. However, in order to
simulate the transient photocurrent, some modification should be
made because of its sensitivity to the circuit under such condition.
To this end, we consider a model that contains a sensing resistor R
connected in series to the organic layer. The voltage drop on the
sensing resistor reflects the electric processes in the device. Simi-
lar with the treatment of Macdonald for one-dimensional current
transport equation [12], the dynamic equation for E(x) is given
as
∂
∂t
E(x, t) = 1
ε0ε
[
V eff
R S
− J (x, t)
]
, (1)
where ε0 is the static dielectric constant, ε the relative permit-
tivity, R the external resistor, S the area of the organic solar cell,
228 D.-L. Li et al. / Physics Letters A 376 (2012) 227–230J (x, t) the current density in the device, and V eff [≡ V app − V bi +∫ L
0 E(x, t)dx] the voltage drop on the organic device with V app the
applied voltage and V bi the build-in voltage. Thus, the Poisson
equation changes into a time-dependent equation of the electric
field E(x, t), in which both the effects of conduction current den-
sity J (x, t) and displacement current density V eff/R S are included.
Besides, in the open-circuit condition, we simulate V eff, which cor-
responds to the measured TPV. While near the short-circuit condi-
tion, the current transient, that goes through the external circuit,
corresponds to the observed TPC. In the simulation, it is obtained
by
I(t) = V eff
R
. (2)
By including a resistance, the RC time constant should be
treated carefully. During the TPC experiments, the load resistor R
and the area of the cells are adjusted to ensure that the time con-
stant RC of the circuit is at least 20 times shorter than the carrier
transit time [10]. In our simulation, we set the load resistor R to
be 50  and a device area, 1 mm2, which generate a time constant
RC of about 5 ns. A time constant RC of 5 ns does not significantly
influence the issues concerned in this Letter.
In the device we considered, excitons mainly dissociate at the
interface, while some of them may dissociate in the bulk under the
influence of defects and electric field [13,14]. Exciton dissociates at
the interface mainly by injecting the electron into the electrode
that works as an acceptor [2,5]. Although it is indicated that the
dissociation is under sub-nanosecond, a detail knowledge of the
process has not been achieved. In contrast, the process of exciton
dissociation in the bulk is slow (microsecond) due to the absence
of the donor/acceptor structure and of a high internal electric field.
To address the issue, we define two dissociation rates, η1 at the
interface, where excitons undergo a fast dissociation within ∼1 nm
away from the anode [15], and η2 in the bulk, which is assumed to
be uniform across the device [14]. The exciton dynamic equation
reads
∂
∂t
ρex − D ∂
2
∂x2
ρex = G(x, t) − ρex
τ
− η(x)ρex, (3)
where ρex is the density of excitons, D the diffusion constant, and
η(x) = η1 at the interface or η2 in the bulk. The exciton genera-
tion function G(x, t) follows the form of the input laser pulse in
experiments, say,
G(x, t) =
{
G0 exp(−x/La), ton  t  toff,
0, otherwise,
(4)
where G0 is the generation rate of singlet excitons depending on
the intensity of light, La the absorption length, and ton (≡ 0) and
toff the time when the laser is switched on and off, respectively.
The boundary condition of the above equation is ∂ρex/∂x = 0.
3. Results and discussion
In the simulation, we take a single-layer device with the
structure of ITO (indium tin oxide)/NPB (N,N-bis(l-naphthyl)-N,N-
diphenyl-1,l-biphentl-4,4-diamine)/Al. The parameters are set as
follows: the thickness of the device is 400 nm, the mobility
of hole and electron μp = 1.0 × 10−2 cm2/V s and μn = 5.0 ×
10−4 cm2/V s respectively, the lifetime of exciton 1 ns [13,16].
G0 = 1.0 × 10−7 nm−3 ns −1 and the absorption length La =
50 nm. The value of η2 is several order smaller than that of η1
due to the low exciton dissociation rate in the bulk [14]. We turn
on the “laser pulse” at 0 ns and turn it off at 2 ns.
We first simulate the TPC response after switching on the laser
pulse. A positive voltage of 1.0 V is applied to the ITO electrodeFig. 1. The transient photocurrent for different exciton interfacial dissociation rates
η1 under a positive applied voltage of 1.0 V.
Fig. 2. The determination of the transit time ttr by the intersect of the asymptotes
in logarithm photocurrent versus time plots.
corresponding to the applied voltage of time-of-flight experiments
in determining the charge carrier mobility. The exciton interfacial
dissociation rate η1 is varied, while η2 is fixed at 1.0 × 103 s−1. As
can be seen in Fig. 1, TPC curve is obtained with an initial sharp
spike followed by a wide platform, which qualitatively agrees with
the experimental results [8–10]. In the standard time-of-flight ex-
periment, the mobility is derived from the transit time denoted by
the starting point of the decay of the TPC. The equation and its
correction are μ = L2/V ttr and ttr/ttr = (2kT /eV ) 12 respectively,
where k is the Boltzmann constant, T the temperature [17]. To
verify our simulation results, Fig. 2 shows the transit time ttr de-
fined as the intersect of the asymptotes in logarithm photocurrent
versus time plots for the case η1 = 0.4 ns−1 [17]. The transit time
given in Fig. 2 is the same as the one of about 260 ns derived from
above equations.
As also can be seen in Fig. 1, the most significant effect is that
the initial spike gets smaller when we decrease the exciton interfa-
cial dissociation rate. Then, we changed η2 by almost two orders of
magnitude, while fix η1. The simulated results is shown in Fig. 3.
As illustrated, the magnitude of the TPC platform increases when
η2 rises. However, the initial spike is not sensitive to the exci-
ton dissociation rate in the bulk. Thereupon we infer that the fast
exciton interfacial dissociation rate induces an observable current
spike that qualitatively agrees with the one observed in TPC exper-
iments.
D.-L. Li et al. / Physics Letters A 376 (2012) 227–230 229Fig. 3. The transient photocurrent for different bulk dissociation rates η2 under
a positive applied voltage of 1.0 V.
The origin of the spike is the same as the origin of the ultrafast
negative signal of TPV [5,6]. Both of them are related with the ex-
citon and charge carrier processes at the interface, which are more
complicated than these in the bulk. Firstly, as mentioned above,
the electrons and holes are separated by the interface during the
exciton dissociation [2,5,13]. Secondly, the generated holes can dif-
fuse into the bulk, drift under the electric field or recombine with
the electrons in the anode. The diffusion and drift of the holes lead
to a displacement of charges, which induces the observable TPV or
TPC depending on the experiment conditions. Besides, the recom-
bination takes away most of the generated holes and thus weakens
the signals. From this model, no polarity change is expected for
TPC when a positive voltage is applied, which is different from the
TPV signals [5,6]. Under such condition, the holes, generated by ex-
citon dissociation at the interface and in the bulk, transport to the
other electrode under the internal electric field, which induces a
transient current platform as shown in Fig. 1. The conduction cur-
rent is in the same direction with the induced current by exciton
dissociation at the interface.
The transient conductivity investigation on the subject offers
the possibility of revealing the exciton and charge carrier physi-
cal processes at the interface as well as in the bulk. To compare
with the case of TPV [5,6], we simulate the TPC of the above
device without applied voltage. Under such condition, the build-
in electric field drives the charge carriers towards the electrodes,
which is the core of the mechanism of organic solar cells. Fig. 4
shows the simulation results, where we varied the exciton inter-
facial dissociate rate η1 from 0.1 up to 1.0 ns−1. The TPV curves,
obtained under the open circuit condition with η1 = 1.0 ns−1 and
η1 = 1000.0 ns−1 are also presented in the inset for comparison.
Focusing on Fig. 4, we find that the dependence of the spike
on the interfacial dissociation rate is the same as that in Fig. 1.
The inset shows the TPV curves with η1 = 1.0 ns−1 and η1 =
1000.0 ns−1. The curves agree with the results of experiments very
well, which further proves the relation of the negative signal with
the exciton interfacial dissociation [5]. Another significant result is
that TPC shows more transient characteristics by presenting a po-
larity change and a fast decay. The fast decay is absent in the TPV
curves of the inset in Fig. 4. The fast decay of TPC is caused by the
gradual extraction of charges by the electrode under a negative in-
ternal electric field. Moreover, the origin of the polarity change of
TPC is also different with that of TPV. It has been indicated that the
polarity change was caused by the competition between the nega-
tive voltage due to charges generated at the interface and positive
one induced by the build-in potential [5,6]. As the case for TPC,
however, a polarity change is presented even there is no internalFig. 4. The transient photocurrent for different exciton interfacial dissociation rates
η1 without applied voltage. The inset shows the TPV curve with η1 = 1.0 ns−1 (the
short dot line, red in the web version) and 1000.0 ns−1 (the black solid line) for
comparison.
Fig. 5. Transient photocurrent under (a) positive and (b) negative applied voltages.
The absolute value of the voltages in (b) is the same with that in (a), correspond-
ingly. The inset of (b) shows the maximum and the minimum value of the pho-
tocurrent during evolution as a function of applied voltage V app. η1 = 1000 ns−1.
electric field. The conclusion is easily seen from the comparison
of curve of η1 = 0.1 ns−1 with the one η1 = 1 ns−1 and from the
curve with 0.5 V applied voltage in the lower plot of Fig. 5, when
the internal voltage is counteracted by the applied voltage. The po-
larity change of TPC is closely related to the competition between
the diffusion and recombination of the charge carrier at the inter-
face. Thereupon, TPC more directly reflects the charge processes at
the interface and reveals more transient characteristics in compar-
ison with TPV.
230 D.-L. Li et al. / Physics Letters A 376 (2012) 227–230Fig. 6. TPC with different hole mobility μp. The inset shows the maximum and the
minimum value of the curves as a function of μp. η1 = 1000 ns−1.
TPC is more convenient for investigating the interfacial effects
in that the environment of charge carriers in the device can be
changed by the applied voltage. In the following, we simulate TPC
of devices by changing the external voltage, and the mobility of
the organic materials. The results are shown in Fig. 5 and Fig. 6
respectively. As expected, the magnitude of spike increases with
increasing voltage. While under large voltage, the platform be-
comes slope, which is due to drift current accelerated by the large
electric field. In Fig. 5(b), the cases of negative applied voltage are
shown. Unlike the positive biased voltage condition, the spike is re-
strained under higher negative voltage. The inset of Fig. 5(b) shows
the dependence of the maximum value of the TPC, Imax, and the
minimum value, Imin, on the applied voltage. In Fig. 6, we show
the influence of hole’s mobility on TPC under negative external
voltage, since the TPC is mostly contributed by the diffusion of
holes as discussed above. We find that the spike fades away when
the mobility is smaller than 1.0 × 10−3 cm2/V s. However, it will
logarithmically increase with increasing mobility. From this sim-
ulation, we realize that the spike could only be observed in the
organic material with high enough mobility.
Although the moments, at which the polarity changes of TPV
and TPC take place, dependent on the mobility of the material, the
applied voltage as well as the exciton interfacial dissociation rate,
we pay less attention to them in our simulation due to its com-
plexity. However, we believe that valuable information of chargecarrier and exciton processes could be extracted from these time
points by further modified simulation and delicate experiments.
Besides, we assumed fast exciton dissociation rates and a simple
way of exciton dissociation as mentioned above, which also needs
further investigation.
4. Summary
In summary, we have investigated the influence of exciton in-
terfacial dissociation on the transient photocurrent by a modified
time-dependent device model. The simulation results reveal an es-
sential role of exciton interfacial dissociation on the TPC spike. The
spike depends a lot on the applied electric field as well as the
mobility of the organic semiconductor. We suggest measuring the
favorable TPC of a device with high hole’s mobility and with an
external voltage that could counteract the build-in potential.
Acknowledgements
The authors would like to acknowledge the financial support
from the National Natural Science Foundation of China and the
National Basic Research Program of China (2012CB921401 and
2009CB929204).
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Spike in transient photocurrent of organic solar cell: Exciton dissociation at interface

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Spike in transient photocurrent of organic solar cell: Exciton dissociation at interface

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