Selective observation of photo-induced electric fields inside different materialcomponents in bulk-heterojunction organic solar cellXiangyu Chen, Dai Taguchi, Takaaki Manaka, and Mitsumasa Iwamoto

Citation: Applied Physics Letters 104, 013306 (2014); doi: 10.1063/1.4861620 View online: http://dx.doi.org/10.1063/1.4861620 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Investigating the origin of S-shaped photocurrent-voltage characteristics of polymer:fullerene bulk-heterojunctionorganic solar cells J. Appl. Phys. 115, 124504 (2014); 10.1063/1.4869661 Analyzing photo-induced interfacial charging in IZO/pentacene/C60/bathocuproine/Al organic solar cells byelectric-field-induced optical second-harmonic generation measurement J. Appl. Phys. 111, 113711 (2012); 10.1063/1.4728225 Analyzing photovoltaic effect of double-layer organic solar cells as a Maxwell-Wagner effect system by opticalelectric-field-induced second-harmonic generation measurement J. Appl. Phys. 110, 103717 (2011); 10.1063/1.3662914 Analysis of interface carrier accumulation and relaxation in pentacene/C60 double-layer organic solar cell byimpedance spectroscopy and electric-field-induced optical second harmonic generation J. Appl. Phys. 110, 074509 (2011); 10.1063/1.3642964 Modeling the temperature induced degradation kinetics of the short circuit current in organic bulk heterojunctionsolar cells Appl. Phys. Lett. 96, 163301 (2010); 10.1063/1.3391669

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Selective observation of photo-induced electric fields inside differentmaterial components in bulk-heterojunction organic solar cell

Xiangyu Chen, Dai Taguchi, Takaaki Manaka, and Mitsumasa Iwamotoa)

Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1, S3-33 O-okayama, Meguro-ku,Tokyo 152-8552, Japan

(Received 22 September 2013; accepted 16 December 2013; published online 9 January 2014)

By using electric-field-induced optical second-harmonic generation (EFISHG) measurement at two

laser wavelengths of 1000 nm and 860 nm, we investigated carrier behavior inside the pentacene

and C60 component of co-deposited pentacene:C60 bulk-heterojunctions (BHJs) organic solar cells

(OSCs). The EFISHG experiments verified the presence of two carrier paths for electrons and holes

in BHJs OSCs. That is, two kinds of electric fields pointing in opposite directions are identified as a

result of the selectively probing of SHG activation from C60 and pentacene. Also, under

open-circuit conditions, the transient process of the establishment of open-circuit voltage inside the

co-deposited layer has been directly probed, in terms of photovoltaic effect. The EFISHG provides

an additional promising method to study carrier path of electrons and holes as well as dissociation

of excitons in BHJ OSCs. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4861620]

Among a variety of organic solar cells (OSCs), bulk-

heterojunctions (BHJs) OSCs based on soluble compounds

or small molecules are increasingly being investigated.1–5 In

BHJs OSCs, electron donor and acceptor molecules are

mixed to introduce larger contacting area for excitons to be

separated into electrons and holes efficiently.3,5 Recently,

important research progresses have been made to acquire a

better understanding of BHJs OSCs,6–8 where various experi-

mental techniques have been employed to characterize the

carrier behavior of these devices. Among these techniques,

there are time-of-flight, photo-induced absorption spectros-

copy, charged carrier extraction by linearly increased voltage

method, and so forth.9–12 However, many of these experi-

mental techniques are only applicable to thick organic film

(not real devices). Consequently, we need a technique that is

capable of probing carrier motion and electric field distribu-

tion in actual BHJs OSCs for getting the whole picture of

carrier behaviors.

The optical electric-field-induced second-harmonic gen-

eration (EFISHG) measurement is very useful to investigate

fundamental processes such as carrier injection, accumula-

tion, transport, and recombination.13–16 We have previously

shown that the EFISHG measurement is available as a tool

for probing photo-voltage generation process in bilayer (dou-

ble active layer) OSCs.17–20 These studies motivated us to

further apply the EFISHG measurement for analyzing BHJs

OSCs. The generation of the EFISHG strongly depends on

the materials properties of targeted sample. Consequently, it

is possible to observe and study different material compo-

nents in the BHJ layer individually, by choosing two appro-

priate laser wavelengths. This is an advantage of the

EFISHG for the study of carrier path in BHJ OSCs. In this

Letter, we showed that the internal electric fields formed in

pentacene and C60 regions of the co-deposited pentacene:

C60 layer can be selectively investigated by the EFISHG

measurement, using a laser with wavelengths of 1000 nm

and of 860 nm.

We prepared BHJs OSCs with an indium zinc oxide

(IZO)/pentacene:C60/Al structure. Figure 1(a) illustrates the

sample structure as well as the energy diagram of the used

materials. The pentacene:C60 co-deposited layer with a total

FIG. 1. Experimental set up and I-V results (a) experimental setup for the

EFISHG measurement and the energy diagram of the different materials and

two electrodes (b) I-V characteristics of the OSCs sample.a)E-mail: iwamoto@pe.titech.ac.jp

0003-6951/2014/104(1)/013306/5/$30.00 VC 2014 AIP Publishing LLC104, 013306-1

APPLIED PHYSICS LETTERS 104, 013306 (2014)

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thickness of 60 nm were co-deposited onto the UV/ozone

treated IZO surface in vacuum. The co-deposition was per-

formed from two spatially separated sources, and the deposi-

tion rate was kept to be 4:1 (pentacene:C60). The deposited

film thickness (60 nm) was monitored with a quartz crystal

microbalance. Finally, Al electrodes were deposited with a

thickness of 100 nm onto the surface of pentacene:C60 blend.

Prepared BHJs OSCs were sealed with a dry agent to

avoid degradation during measurements. We also prepared

single-layer IZO/pentacene (50 nm)/Al and IZO/C60

(50 nm)/Al devices as reference samples in the EFISHG

measurement. The current–voltage (I–V) characteristics of

the OSCs were recorded under illumination and also in dark

(see Fig. 1(b)), where Voc¼ 0.4 V, Jsc¼�0.012 mA/cm2,

and FF¼ 0.21. Devices based on the co-deposited films

exhibit lower Jsc and higher Voc, due to the amorphous prop-

erties of the active layers.21,22 In the measurement, a red

light from a light-emitting diode (wavelength of 630 nm and

intensity of 10 mW/cm2) was used as a light source to pro-

vide illumination pulse (repetition rate of 10 Hz, duration of

50 ms, switching on and off time within 50 ns) to OSCs.

Note that pentacene and C60 layers absorb light at a wave-

length of 630 nm,17,18 and generate excitons inside the or-

ganic layers.

Figure 1(a) also portrays the arrangement of the

EFISHG measurement for probing the electric field in OSCs.

A pulsed laser is used as a probing light (repetition rate of

10 Hz, average power of 1 mW/cm2, duration of 4 ns), which

is generated from an optical parametric oscillator pumped

with the third-harmonic light of Q-switched Nd:YAG laser.

The delay time of the laser pulse corresponds to the initial

time of the illumination pulse. A p-polarized pulsed laser

beam is focused onto the sample surface at an incident angle

of 45�. The SHG light generated from the sample is detected

using a photomultiplier tube, and its intensity is recorded

with a digital multimeter. In our EFISHG experiment,

EFISHG is activated due to the coupling of electrons in C60

(pentacene) molecules and electro-magnetic waves of the

incident laser beam E(x) in the presence of local electro-

static field E(0), where E(x) is the electric field of the

p-polarized light, and E(0) is the average electric field across

the sample. The square root of the absolute difference of

generated EFISHG intensity (I 2xð Þ) is in proportion to the

E(0) formed in the targeted organic layer,15,16,23 which isffiffiffiffiffiffiffiffiffiffiffiffiIð2xÞ

p/ je0vð3Þ�Eð0ÞEðxÞEðxÞj. E(0) is given as Ee

þEb1þEb2þEs, Here, Eb1 (Eb2) is the background internal

electric field established in the pentacene (C60) region of the

co-deposited layer, due to the presence of trapped carriers,

work-function difference, etc. Ee is the electric field origi-

nated from charges �Qe(t) on Al and þQe(t) induced on IZO

electrodes by applying an external voltage Vex, and Es is the

electric field originated from accumulated charges Qs(t) at

the pentacene/C60 interface. Accordingly, we can discuss

carrier behaviors in the OSCs by probing the transient

EFISHG.

The generation of EFISHG is material dependent and it

shows wavelength dependence of incident laser beam.

Accordingly, C60 layer provides strong SHG signal at a

wavelength of k2x¼ 500 nm, whereas pentacene shows no

SHG response at this wavelength.17–19 Hence, we can use a

laser beam with a wavelength of kx¼ 1000 nm as incident

light (EFISHG signal: k2x¼ 500 nm), to selectively measure

the electric field of C60 parts inside the co-deposited layer.

Meanwhile, it has also been found that both pentacene

and C60 response to laser signal with a wavelength of

kx¼ 860 nm (EFISHG signal: k2x¼ 430 nm). In order to

clarify the generation of EFISHG signal under the laser

wavelength of kx¼ 860 nm, we carried the EFISHG mea-

surement by applying AC square-wave voltage pulse (Vex) to

the single layer samples of IZO/pentacene or C60/Al in

dark. Figure 2(a) shows the square-root of generated

EFISHG from single layer pentacene and C60 samples, where

Vex¼þ0.3 V with respect to Al electrode. Only one relaxa-

tion process was identified during the EFISHG measurement,

corresponding to the charge accumulation on the two electro-

des induced by the applied external voltage (electrode charg-

ing (Ee)).20 The time-constants as well as the presence of this

charge relaxation process can be identified by using a

curve-fitting method based on a filtering technique to re-plot

the EFISHG signal in time domain.17,24,25 Hence, the

FIG. 2. (a) EFISHG measurements of the single layer sample of

IZO/pentacene/Al and IZO/C60/Al using voltage pulse, where EFISHG is

collected at the wavelength of 430 nm. (b) The sketch of the suggested

potential drop of two single layer samples.

013306-2 Chen et al. Appl. Phys. Lett. 104, 013306 (2014)

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step-shift of the EFISHG signal with a response time of

around 10�6 s indicates the establishment of the internal

electric field. As shown in Fig. 2(a), the intensity change of

EFISHG signal (DEe) observed in two different samples are

comparable, indicating that both pentacene and C60 response

equally to the laser signal with a wavelength of 860 nm.

On the other hand, the application of Vex¼ 0.3 V results

in the increase of EFISHG signal for the C60 single layer

sample, while it leads to the decrease of EFISHG signal for

the pentacene single layer sample, as shown in Fig. 2(a).

These results indicate that the background electric fields,

induced possibly by the work function difference between

two electrodes, are pointing in the direction from Al to IZO

electrode for the pentacene sample (Eb1), and from IZO to

the Al electrode for the C60 sample (Eb2). Here, the work

function of IZO is WIZO¼ 4.8 eV, while that of Al is

WAl¼ 4.2 eV. Noteworthy that work function of Al could be

modified by evaporating a thin C60 layer on Al, resulting in a

value about W0Al¼ 5.2 eV’26–28 greater than the work func-

tion of IZO. Hence, the presence of two opposite background

electric fields (Eb1, Eb2) can be explained by the different

work function of Al electrode in these two samples. Figure

2(b) shows the suggested potential drop across two kinds of

single layer samples. Our observation is in good agreement

with other researchers’ measurement.26–28 Thus, for the

pentacene:C60 co-deposited layer, we can probe the whole

layer as a single layer sample by collecting the generated

EFISHG signal at the wavelength of 430 nm, and selectively

probe the contribution of C60 by the use of generated SHG

light at 500 nm. That is, by observing both the EFISHG sig-

nals at the wavelength of k2x¼ 500 nm and k2x¼ 430 nm,

we can selectively study the internal electric field inside C60

component only or inside pentacene:C60 layer together,

which can be of great help to distinguish the carrier behavior

inside different material scope in the co-deposited BHJs

OSCs.

Figure 3(a) shows the EFISHG generated from the

IZO/pentacene:C60/Al BHJs OSC in response to the AC

square-wave voltage pulse (10 Hz,þ 0.5 V), where the wave-

lengths of probing laser were k2x¼ 500 nm and

k2x¼ 430 nm, respectively. One significant relaxation pro-

cess was identified during each EFISHG measurement, cor-

responding to the electrode charging (Ee),20 which resulted

in the Ee across the Al and IZO electrodes. For the laser

wavelength k2x¼ 500 nm, the EFISHG signal increased

under the applied Vex of 0.5 V, suggesting that the back-

ground electric field across the C60 component (Eb2) mostly

points in the direction from IZO to Al electrode. The direc-

tion of this background electric field is opposite with the

photo current and thus photo current will be decreased by

this background field. Accordingly, the role of the buffer

layer for this kind of BHJ OSCs was proposed to break this

contact and thus a suitable background electric field could be

established for better separation and collection of the free

carriers. Meanwhile, the EFISHG signal (k2x¼ 430 nm)

decreased under the same Vex, suggesting that the back-

ground electric fields across the whole pentacene:C60

co-deposited layer points in the direction from Al to IZO

electrode, namely, opposite with the background electric

field inside C60 component. It is necessary here to note that

EFISHG intensity is proportional to the local electric field as

well as the portion of the material components. Since the ra-

tio of the co-deposition of C60 and pentacene is 1:4, the

observed EFISHG signal at such laser wavelength is mostly

generated from pentacene component. Hence, we argue that

in the co-deposited layer, the background electric field inside

pentacene and C60 components are opposite with each other,

and EFISHG measurement is capable of distinguishing the

electric fields in different materials components. Moreover,

this opposite background electric fields distribution also sug-

gests that the distributed phase separation of pentacene and

C60 in the co-deposited layer may not be random, but both

two components may have distinctive domain formed in the

horizontal direction (see Fig. 3(b)). Recently, detailed

observation of component distribution in BHJs OSCs has

been intensively investigated using several different

methods.29–32 Results showed that in the organic BHJ layer,

phase separation leads to the formation of clusters and

islands structure with a diameter of approximately 20–50 nm

in the blended BHJ layer.33,34 Accordingly, it is quite possi-

ble that in the region close to Al electrode, the distribution of

C60 component had some common domains in the horizontal

direction and in the three-dimensional region of the whole

co-deposited film most of C60 component shared the same

background electric field (Eb2). Figure 3(b) displays the sug-

gested morphology profile and background electric field in

the OSC, where Eb2 of C60 component points in the direction

from IZO to Al, whereas Eb1 of pentacene component points

in the opposite way. The suggested thin layer morphology in

Fig. 3(b) is strongly supported by the work of Salzmann

et al.35 In their study, the pronounced phase separation of

FIG. 3. (a) EFISHG measurements of the BHJs OSCs using voltage pulse

and different EFISHG wavelength. (b) The sketch of the suggested morphol-

ogy profiles and background electric field of pentacene and C60 components.

013306-3 Chen et al. Appl. Phys. Lett. 104, 013306 (2014)

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pentacene and C60 in the co-deposited films can be revealed

by using atomic force microscopy and x-ray diffraction. The

crystalline growth of pentacene forms crystallites that

exceeded the nominal film thickness by an order of magni-

tude, whereas C60 was crystalline only if grown on the penta-

cene pre-covered substrates.35,36

To illustrate the photo-induced carrier behavior inside

the co-deposited OSCs, the EFISHG measurements using

illumination pulse were applied to the OSCs, where the sam-

ples were activated continuously by an illumination pulse

(sample was illuminated firstly and subsequently under dark

condition) with a frequency of 10 Hz under open-circuit con-

ditions, as shown in Fig. 4. As described in previous section,

for the open-circuit condition, the charging 6QeðtÞ on the

two electrodes results in the change of the EFISHG signal

under the illumination DE(0) (Eð0Þ ¼ Ee / QeðtÞ, where

Qeð1Þ ¼ Vex

A Ctotal, with Vex: external voltage, and A: elec-

trode area). As seen in Fig. 4, the EFISHG signal showed a

step change with the photo illumination, and this changing of

the EFISHG signal (see Fig. 4) is the result of the establish-

ment of the internal electric field caused by the photovoltaic

effect. First, the incident laser wavelength is chosen as

860 nm (EFISHG wavelength: k2x¼ 430 nm), which allows

us to probe the average electric field formed in the whole

co-deposited organic layer. After the saturation of SHG in-

tensity, the electric field across the whole sample layer

results in the decrease of the EFISHG intensity �4 (abs.

unit). Comparing with the voltage pulse application’s case

(see Fig. 3(a), where Vex¼ 0.5 V), this photo-induced

EFISHG intensity changing corresponds to the external elec-

tric field induced by Vex � 0.4 V¼Voc. Hence, with the

appearance of photoillumination, the internal electric field,

which matched to open circuit voltage, was established in

the OSCs by photovoltaic effect. This result also proved no

charge accumulated on the donor-acceptor interfaces, in a

manner similar to the bilayer OSCs sample studied in the

previous paper.20 Meanwhile, EFISHG signal from the BHJs

OSCs under open-circuited condition was also measured by

using laser wavelength of kx¼ 1000 nm (SHG wavelength

of k2x¼ 500 nm). For the laser wavelength of kx¼ 1000 nm,

EFISHG measurement selectively probes the internal electric

field inside the C60 component only. In comparison with the

measurement using laser wavelength of kx¼ 860 nm (SHG

wavelength of k2x¼ 430 nm), EFISHG signal increased with

the illumination, which is opposite with the case of

k2x¼ 500 nm. The intensity changes of the EFISHG signal

in these two cases are almost identical, indicating that the

same internal electric field, corresponding to open-circuit

voltage, exists inside both C60 and pentacene components.

However, even though the same internal electric field is

established in the OSCs, the changing directions of the

EFISHG signals under two cases are opposite with each

other. The reasonable conclusion is that the background elec-

tric fields in pentacene (Eb1) and in the C60 (Eb2) are opposite

with each other. This result is in good agreement with that of

the voltage application’s measurement, as shown in

Fig. 3(a). On the other hand, for the bilayer OSCs with com-

parable thickness, the transient time str(the time for charges

to reach the electrodes) is on the order of 10�6 s,20 which is

about ten times smaller than that in BHJs OSCs. These

results may attribute to the ambiguous current path in the

co-deposited layer and the induced interference between

photo-generated charges.

In conclusion, by applying the EFISHG measurements

to the IZO/pentacene:C60/Al co-deposited OSCs, we studied

the photo-induced carrier behavior, in terms of photovoltaic

effect. The EFISHG experiments showed that the back-

ground electric fields inside pentacene and C60 components

of the co-deposited layer have opposite directions.

Moreover, the EFISHG also probes dynamics of the photo-

voltaic generation in BHJs OSCs under open-circuit condi-

tion. Hence, the EFISHG technique provides a way to probe

carrier paths as well as interfacial carrier behavior in BHJs

OSCs by taking an advantage of selectively probing.

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