Air-Stable Inverted Organic Solar Cells with Pentacene Anode Buffer Layer

Tatsuya Oida, Tatsuhiro Naito, Yuki Miyagawa, Muneo Sasaki1, and Kenji Harafuji

Department of Electrical and Electronic Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan1Industrial Research Center of Shiga Prefecture, Ritto, Shiga 520-3004, Japan

Received January 27, 2011; accepted May 3, 2011; published online August 22, 2011

A small-molecular organic solar cell with an inverted structure of indium–tin oxide (ITO)/bathocuproine (BCP)/fullerene (C60)/copper

phthalocyanine (CuPc)/pentacene/Ag is reported. Although the Ag electrode usually acts as the cathode because of its low work function, the Ag

electrode appropriately works as the anode by inserting a pentacene thin layer between Ag and the active layer. The unencapsulated device

shows a power conversion efficiency of 0.28% under 100mW/cm2 AM1.5G simulated illumination, and a lifetime of 700 h. The lifetime is 700

times longer than that of a conventional device with a structure of ITO/CuPc/C60/BCP/Ag. The long lifetime is attributed to the inverted structure

in which the top electrode acts as the anode because this structure could effectively protect C60 from the diffusion of oxygen. By substituting

naphthalene-1,4,5,8-tetracarboxylic dianhydride for BCP as a cathode buffer, the power conversion efficiency and the lifetime are improved to

0.40% and more than 1200 h, respectively. # 2011 The Japan Society of Applied Physics

1. Introduction

Organic solar cells (OSCs) have received attention as arenewable energy resource because of its low cost and easeof fabrication. In 1986, Tang presented a concept of ‘‘donor–acceptor heterojunction (D/A heterojunction)’’, and accom-plished a power conversion efficiency of almost 1%.1)

Further efficiency increases have been achieved throughthe use of new materials,2–4) cathode buffer layers5) andmixed D/A heterojunctions.6,7) On the basis of these ideas,the efficiency of OSCs has increased steadily.8–10) However,the lifetime remains short. The short lifetime of OSCs hasbeen attributed to the following reasons. The first is thedecrease in conductivity of fullerene (C60) acceptor layer byoxygen diffusion.11) The second is the chemical change ofthe interface between an electrode and an active layer.12) Thethird is the degradation of organic materials by illumina-tion.13,14)

The degradation of C60 is focused on in this study. Ina conventional device structure of glass-substrate/anode[indium–tin oxide (ITO)]/donor/acceptor (C60)/cathode,C60 lies near the device top surface. Therefore, C60 is opento the diffusion of oxygen, which causes a decrease in theconductivity of C60. As a result, OSCs quickly degrade.Recently, in order to protect C60 from the diffusion ofoxygen effectively, an inverted structure of bottom electrode(ITO)/acceptor (C60)/donor/top electrode has been re-ported.15–17) According to these studies, in this type ofinverted OSC, the work function of the top electrode shouldbe higher than that of ITO in order that the top electrodeplays a role of an anode. Researchers of these studiesinsisted that the electric field produced by built-in potentialin OSCs points from one electrode with a lower workfunction to the other electrode with a higher work function.Gold and platinum have a higher work function than ITO,and satisfy this condition as the top electrode.

As an example of this type of OSC, an inverted OSC wasreported with a structure of ITO/tris(8-hydroxy-quinoli-nato)aluminum (Alq3)/C60/copper phthalocyanine (CuPc)/Au. The OSC showed a longer lifetime than that of a con-ventional structure device.15) Its long lifetime was attributedto the effective protection of C60 from oxygen diffusion andto the stability of the Au electrode. Gold is, however, veryexpensive and hinders low-cost fabrication. Silver andaluminum, which are usually used as top electrode, are less

expensive than gold. However, they cannot work as an anodebecause of their low work function.

In order to resolve the above problem, a structure of‘‘bottom electrode/acceptor/donor/anode buffer layer/topelectrode’’ has been developed by several researchers. Insuch a structure, the anode buffer layer modifies the workfunction of the top electrode, and the top electrode worksas an anode. So far, C60/Al

16) and MoO3/Al17) composited

anodes have been reported for this type of OSC. The workfunction of Al is 4.3 eV,18) which is lower than that of ITO(4.7 eV).19) By evaporating C60 on Al, the work function ofAl becomes 5.2 eV.20) By evaporating MoO3 on Al, the workfunction of Al becomes 5.5–6.9 eV, which depends on thethickness of MoO3.

17) As a result, the work function of Albecomes higher than that of ITO, and the Al top electrodeworks as the anode. These anodes, however, have severaldrawbacks at the same time. In the C60/Al compositedanode, the ‘‘CuPc (donor)/C60 (anode buffer layer)’’ hetero-junction plays the role of a counter p–n junction against theD/A heterojunction. This may have a negative effect on thedevice performance. In the MoO3/Al composited anode,MoO3 is very harmful to human health.

In this paper, an inverted device with the structure ofITO/bathocuproine (BCP)/C60/CuPc/pentacene/Ag is pro-posed. The pentacene layer plays a role of the anode bufferlayer instead of C60 or MoO3. In x2, experimental methodsof fabricating organic solar cells are described in detail.In x3, experimental results are shown. In x4, the physicalmechanism for the inverted device nature is discussed withemphasis on the role of the pentacene buffer layer in theinverted OSC. The device stability is also discussed. In x5,concluding remarks are given.

2. Experimental Methods

Figure 1(a) shows a schematic of a proposed invertedstructure of OSC with the pentacene anode buffer layer.Hereafter, this OSC is called Device A. The other two kindsof OSCs, i.e., Devices B and C, are prepared to examine therole of pentacene layer in Device A, as shown in Figs. 1(b)and 1(c), respectively. Both Devices A and B have theinverted structure whose anode is the Ag top electrode. Theyconsist of a 20-nm-thick CuPc layer (Aldrich, >99%) asthe donor, a 40-nm-thick C60 layer (SES Research, 99.9%) asthe acceptor, and a 5-nm-thick BCP layer (Tokyo Kasei,refined product) as the cathode buffer layer. The thickness

Japanese Journal of Applied Physics 50 (2011) 081601

081601-1 # 2011 The Japan Society of Applied Physics

REGULAR PAPERDOI: 10.1143/JJAP.50.081601

of the Ag top electrode is 100 nm. The sole differencebetween Devices A and B is the existence of a 2-nm-thickpentacene layer.

Device C is a conventional structure with ITO as theanode. The materials and thicknesses of the donor andacceptor layers are the same as those of Devices A and B.The BCP buffer layer in Device C is made 5 nm thickerthan the Devices A and B. This is to compensate for thepenetration of Ag atoms into the BCP layer during the Agdeposition on BCP.

All the devices are fabricated on ITO precoated on aglass substrate (Sanyo Vacuum Industry). The thickness ofITO is 1450� 100 �A and its sheet resistance is less than15�/square. ITO substrates are cleaned by ultrasonictreatment in detergent, and successively deionized water,acetone and ethanol for 5min each. No further treatmentof ITO (for instance, ultraviolet ozone treatment) isperformed. Organic materials are evaporated on the ITOsubstrates at a rate of 0.5 �A/s, under a pressure of lessthan 1:0� 10�3 Pa (ULVAC KIKO VFR-200M/ERH). Thedeposition rate is monitored by an oscillation quartzmicrobalance (ULVAC CRTM-6000). All organic materialsare used as purchased. The Ag top electrode is thermallyevaporated through a stainless mask with 6mm2 activearea of 2:4� 2:5mm2. During the deposition, the devicesare exposed to the atmosphere 2 or 3 times in order tochange the deposition sources, and to equip the stainlessmask.

All the devices are illuminated through the trans-parent ITO electrode. The photovoltaic current density–voltage (J–V ) measurements are carried out using anelectrometer (ADC 6241A) and an electric automatic shutter(Agilent 34970A/34903A). Xenon lamp illumination atan intensity of 100mW/cm2 is performed and an air mass1.5 global (AM1.5G) spectrum is obtained using a solarsimulator (SAN-EI Electric XES-40S1). The intensity of thesimulator output is calibrated using crystalline siliconsolar cells with a 320–1100 nm broadband (Axis NetANS-002A). Data acquisition software (Sunrise W32-R6244S0L3-R) is used to evaluate the J–V characteristics.The investigated devices are kept in air at room temperatureduring the entire measurement procedure without anyencapsulation.

3. Results

3.1 Current–voltage characteristics of proposed inverted

devices

Figure 2 shows the typical J–V characteristics of DevicesA–C under illumination. The horizontal axis indicates theapplied voltage. The positive region of the voltage meansthat a plus voltage is applied to the Ag electrode for DevicesA and B, and to the ITO electrode for Device C. The verticalaxis indicates the current density. Positive current meansthat positive carriers flow from the anode to the cathode. Thedevice parameters are summarized in Table I, where Jscis the short-circuit current density, Voc is the open-circuitvoltage, FF is the fill factor, and �p is the power conversionefficiency.

Device C with the conventional structure shows typicalJ–V characteristics of OSC, which is a diode characteristicwith a minus current density bias. The J–V characteristic ofDevice A also shows a diode characteristic. This indicatesthat the Ag top electrode appropriately works as the anodein Device A. On the other hand, Device B, which has theinverted structure without pentacene, does not show suchdiode characteristics.

The sole difference between Devices A and B is theexistence of a 2-nm-thick pentacene layer, so the differencein the J–V characteristics between them is attributed to theexistence of pentacene.

3.2 Further insights into the role of pentacene layer

To obtain insights into the role of the pentacene layer, thefollowing two experiments are conducted: The first is theanalysis of the interfacial condition between pentaceneand Ag. The second is the examination of the role of thepentacene layer in combination with ITO electrode.

pentacene 2 nm Ag (anode) Ag (cathode)

Inverted Device (Ag is anode)Conventional Device

(ITO is anode)

Device BDevice A Device CAg (anode)

C60 40 nm

CuPc 20 nm

C60 40 nm

CuPc 20 nm

C60 40 nm

BCP 10 nm

ITO

BCP 5 nm

ITO

BCP 5 nm CuPc 20 nm

ITO

glass

(c)(a) (b)

glass glass

Fig. 1. Schematics of the structure of organic solar cells; (a) proposed

inverted structure with pentacene buffer layer (Device A), (b) inverted

structure without pentacene buffer layer (Device B), and (c) conventional

structure (Device C).

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Device ADevice BDevice C

Fig. 2. Current density–voltage (J–V ) characteristics under illumination

of Device A with pentacene buffer layer (solid line), Device B without

pentacene buffer layer (dotted line), and Device C of conventional device

(dashed line).

Table I. Photovoltaic parameters for Devices A–C based on current–

voltage characteristics.

JSC(mA/cm2)

VOC

(V)FF

�p(%)

Device A 1.97 0.34 0.43 0.28

Device B 0.78 0.29 0.18 0.04

Device C 3.22 0.42 0.40 0.53

T. Oida et al.Jpn. J. Appl. Phys. 50 (2011) 081601

081601-2 # 2011 The Japan Society of Applied Physics

Firstly, the interfacial condition between pentacene andAg is studied using X-ray photoelectron spectroscopy (XPS;ULVAC-PHI 5400). Figure 3 shows the schematic of twokinds of samples. In Sample I, a 30-nm-thick Ag layer isdeposited on the glass substrate. In Sample II, a 30-nm-thickpentacene layer is firstly deposited on the glass substrate,and then a very thin Ag layer with a thickness of 0.6 nm isdeposited. The deposition is performed by the evaporationmethod at a rate of 0.5 �A/s under a pressure of less than1:0� 10�3 Pa.

Figure 4 shows the Ag 3d XPS profiles of Sample I withglass/Ag and of Sample II with glass/pentacene/Ag. It isclear that the peak of Ag on pentacene shifts to a higherbinding energy than that of pure Ag. The peak of a pureAg 3d orbital is 368.3 eV. The peak at a binding energylower than 368.3 eV indicates the existence of Ag oxide.On the other hand, the peak of Ag on pentacene shifts to ahigher binding energy compared with a peak of pure Ag.This indicates the possible existence of a Ag–O–C chemicalbond. The role of this chemical bond will be discussed in x4.

Secondly, the role of the pentacene layer for anotherelectrode material is examined. Concretely speaking, therole of the pentacene layer at the interface between the ITOanode and the donor layer is examined. For this purpose,Device D in Fig. 5(a) is fabricated. Device C is also shownfor comparison in Fig. 5(b). In Device D, there exists a2-nm-thick pentacene layer between ITO and CuPc. DevicesC and D are fabricated at the same time except for thepentacene layer to remove the other factors. The onlydifference between Devices C and D is the existence of thepentacene layer.

Figure 6 shows the J–V characteristics of Devices C andD under both illumination and dark conditions. Device

parameters are summarized in Table II. It can be understoodthat Jsc is enhanced by inserting the pentacene layer. InFig. 6, the dark curve of Device D shifts by �0:15 eVcompared with that of Device C.

4. Discussion

4.1 Role of the pentacene layer

The role of the pentacene layer in the inverted OSC isdiscussed. Firstly, the effect of pentacene layer in Device Dis examined. As shown in Table II, Jsc is enhanced byinserting the pentacene layer. Devices C and D have thesame active layer, so the quantity of photocarriers thatgenerate at the D/A interface for the two devices are thesame. Therefore, the difference in Jsc between the twodevices is attributed to the difference in carrier collectionefficiency. The pentacene layer is inserted between theanode and the donor layer, so it is considered that thepentacene layer improves hole collection efficiency. Inaddition, as shown in Fig. 6, the dark curve of Device Dshifts by �0:15 eV compared with that of Device C. Thisindicates that the pentacene layer enhances the efficiency of

Sample Sample

Ag 0.6 nmAg 30 nm pentacene 30 nm

glassglass

(a) (b)

Fig. 3. Schematics of sample structures for XPS analysis: (a) Sample I

and (b) Sample II.

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0.8

1

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Inte

nsity

Ag on pentaceneAg only

368.3 eV

375 373 371 369 367 365Binding Energy [eV]

Fig. 4. X-ray photoelectron spectra of Ag (solid line) and Ag on

pentacene (dashed line).

Ag BCP 10 nm

Device D Device C

Ag

CuPc 20 nm

C60 40 nm

ITOITO pentacene 2 nm

glass

(b)

glass

(a)

Fig. 5. Schematics of the structure of organic solar cells; (a) Device D

with pentacene anode buffer, and (b) Device C.

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0.15 V

Fig. 6. J–V characteristics of Devices C and D both under illumination

and in the dark.

Table II. Photovoltaic parameters for Devices C and D based on current–

voltage characteristics.

JSC(mA/cm2)

VOC

(V)FF

�p(%)

Device C 3.22 0.42 0.40 0.53

Device D 3.77 0.45 0.42 0.71

T. Oida et al.Jpn. J. Appl. Phys. 50 (2011) 081601

081601-3 # 2011 The Japan Society of Applied Physics

hole injection from the anode. In other words, the pentaceneanode buffer layer decreases the height of the hole barrierbetween the anode and the donor layer.

In the next step, the mechanism of the decrease in theheight of the hole barrier is examined using the simplifiedenergy level diagrams shown in Fig. 7. The Fermi level(EF), highest occupied molecular orbital (HOMO), andvacuum level (Evac) are depicted. The work function of Agis cited from ref. 18, and the HOMOs of CuPc and pentaceneare cited from refs. 8 and 21, respectively. Figure 7(a)shows the energy level diagram after the contact between Agand the CuPc layer. The work function of Ag is 4.3 eV. SinceCuPc is a p-type semiconductor, the work function of CuPcis slightly lower than its absolute value of HOMO (5.2 eV).Therefore, the work function of Ag is lower than that ofCuPc. When the two materials come into contact, electronsflow from Ag to CuPc until the Fermi levels equilibrate eachother. As a result, band bending takes place in CuPc. Thisband bending brings about a thick hole barrier. The thickhole barrier causes the deformation J–V characteristics ofthe inverted device without the pentacene layer (Device B)shown in Fig. 2. Figure 7(b) shows the energy level diagramof Ag/pentacene/CuPc when the Ag–O–C chemical bonddoes not exist at the Ag/pentacene interface. Pentacene is ap-type semiconductor, so the electron redistribution similarto that in Ag/CuPc takes place. In this case, however,pentacene is fairly thin, so electron redistribution occursamong Ag, pentacene, and CuPc. Band bending occursin HOMOs of the whole pentacene layer and part of theCuPc layer. As a result, a thick hole barrier is formed in thesame manner as that of Ag/CuPc contact, as shown inFig. 7(a).

On the other hand, Fig. 7(c) shows the energy leveldiagram of Ag/pentacene/CuPc when a Ag–O–C chemicalbond exists. The electronegativity of carbon is lower thanthat of oxygen, so the carbon side is charged positively andthe silver–oxygen side is charged negatively. The Ag–O–Cchemical bond plays a role as an interfacial dipole. Thedipole hinders electron flow from Ag to the organic layerwhen the Ag comes into contact with the organic layer. Bythe effect of the interfacial dipole, the contact between Agand pentacene reaches non-thermodynamic equilibrium.In other words, the vacuum level and Fermi level are

discontinuous at the interface between Ag and pentaceneowing to the existence of the dipole. As a result, the AgFermi level does not affect the energy-level structure ofpentacene and CuPc. This is because the electron redistribu-tion does not occur at the interface between Ag and theorganic layer. The band bending of CuPc depends on thecondition of the pentacene/CuPc contact. Both pentaceneand CuPc are of the p-type, so the difference between theirwork functions is small. Therefore, band bending almostdoes not take place in the HOMO of CuPc. Strictly speaking,there also exist dipoles at the electrode/CuPc interface.19,22)

The magnitude of the dipole is, however, much lower thanthat of the electrode/pentacene interface.21) Therefore, it isconsidered that the effect of the dipole can be neglected atthe electrode/CuPc contact.

In Fig. 7(a), the cause of the hole barrier is the bandbending of the CuPc layer. In Fig. 7(c), the pentacene thinlayer itself plays a role as the hole barrier. The hole barrierformed by the band bending of CuPc layer is high and thick.Only a small amount of carriers can surmount the Ag/CuPcinterface. On the other hand, the hole barrier formed bypentacene layer is low and thin. Therefore, holes passthrough the hole barrier by the tunneling effect. As a result,ohmic-like contact is formed at the interface between Ag andCuPc.

To confirm this hypothesis, Device A in Fig. 1 with a 4-nm-thick pentacene layer is fabricated. This pentacene layeris twice as thick as that of the original Device A. Theperformance of the device is as follows: Jsc is 1.74mA/cm2,Voc is 0.23V, FF is 0.29 and �p is 0.12%. In particular, FFdiminishes markedly. This indicates that a thick pentacenelayer hinders the tunneling effect and increases seriesresistance.

4.2 Comparison with other studies

Hole barrier lowering by a buffer layer similar to thephenomenon shown in x4.1 was reported by severalresearchers. Cattin et al. presented a MoO3 anode bufferlayer in OSC.23) Wang et al. presented a C60 anode bufferlayer in an organic light-emitting device.24) These research-ers argued that the anode buffer layer adjusted the positionalrelation between the anodic Fermi level and HOMO of theorganic layer. As a result, holes pass through the buffer layerby the tunneling effect, and ohmic contact between theanode and the organic layer is formed. On the other hand,MoO3 and C60 anode buffer layers were also used in theinverted OSC.16,17) In these articles, the inverted nature wasdiscussed from the viewpoint of the work function of the topelectrode. In other words, these researchers argued thatMoO3 and C60 buffer layers increase the work function ofthe top electrode and then the polarity in OSC is reversed.This hypothesis can also explain the phenomena reported inrefs. 23 and 24. However, Watkins et al. showed that thework function of Ag on pentacene was lower than that ofpure Ag.21) The mechanism of our inverted OSC cannot beexplained by the hypothesis that the anode buffer layerincreases the work function of the top electrode. Therefore,it is considered that the pentacene layer does not increasethe work function of the top electrode, but decreases theheight of the hole barrier between the top electrode and thedonor layer.

(b)(a)

pentacene

Ag

CuPc

dipole

Ag-O C

4.3 eV 5.2 eV

4.9 eV

HOMOAg CuPc

4.3 eV 5.2 eV

pentacene

4.9 eV

HOMO

EF

AgCuPc

4.3 eV 5.2 eV

thick hole barrier

HOMO

(c)

EF EF EF EF

EF

Evac Evac Evac

thick hole barrierhole

Fig. 7. (a) Energy diagram at the Ag/CuPc interface. (b) Energy diagram

at the Ag/pentacene/CuPc interface when the Ag–O–C chemical bond does

not exist. (c) Energy diagram at the Ag/pentacene/CuPc interface when the

Ag–O–C chemical bond exists. EF is the Fermi level, Evac is the vacuum

level, and HOMO is the highest occupied molecular orbital.

T. Oida et al.Jpn. J. Appl. Phys. 50 (2011) 081601

081601-4 # 2011 The Japan Society of Applied Physics

4.3 Stability of OSC

The device stability in air is investigated as a function oftime. Figure 8(a) shows the degradation of power conversionefficiency �p in Devices A, C, D, and E. Device E will beexplained in x4.4. The horizontal axis is the time that passesafter the devices are completed. The vertical axis is �pnormalized to its initial value. The lifetime of OSCs isdefined as the time at which �p decreases to half its initialvalue. The investigated devices are kept in air at roomtemperature during the whole measurement procedurewithout any encapsulation. Figure 8(b) shows the degrada-tion of �p in Devices C and D in a short-time range.Figures 9(a), 9(b), and 9(c) show the J–V characteristics ofDevices A, C, and D at typical times after the fabrication ofthe devices, respectively. The J–V curve in Device A doesnot change so much with time in the fourth quadrant. On theother hand, the J–V curve in Device C changes markedlywith time in the fourth quadrant.

It can be understood in Figs. 8(a) and 8(b) that the lifetimeof Device A reaches approximately 700 h, while the lifetimeof Device C is only 1 h. In the first phase up to 30 h, thenormalized �p of Device A becomes greater than 1.0, andthen decreases gradually. In the conventional structure ofITO/CuPc/C60/BCP/Ag, oxygen diffuses first into the C60

(n-type) layer. It is reported that oxygen acts as a p-typedopant of organic semiconductors.11,25) Thus, the conductiv-ity of C60 deteriorates and �p decreases rapidly. In theinverted structure of ITO/BCP/C60/CuPc/pentacene/Ag,oxygen diffuses first into the CuPc (p-type) layer. Theconductivity of CuPc increases at the same time with the

effective protection of C60 from oxygen. As a result, thenormalized �p of Device A temporarily exceeds 1.0.

The difference between Devices C and D is discussed. InFig. 8(b), Device D shows a longer lifetime than Device C.The structural difference between these devices is only thepentacene buffer layer. In other words, the distance fromthe atmosphere to the C60 layer in Device D is the same asthat in Device C. Therefore, the difference in degradationfeatures between the two devices is not due to the differencein C60 protection. The difference is attributed to a deg-radation of ITO/CuPc contact. In Device D, CuPc is not indirect contact with ITO. The degradation of the ITO/CuPccontact is suppressed by the pentacene buffer layer inDevice C. Therefore, the long lifetime of Device A isattributed not only to the effective protection of C60 fromoxygen, but also to the suppression of the electrode/CuPccontact degradation.

4.4 Further improvement of OSC

Further improvement of the inverted device is examinedfrom the viewpoint of the cathode buffer material. In thepreceding sections, the degradation in OSC is discussedmainly with respect to the material stability of C60. How-ever, the degradation of BCP is also an ineligible factor.26)

Therefore, naphthalene-1,4,5,8-tetracarboxylic dianhydride(NTCDA; Tokyo Kasei, purified by sublimation) is used asthe cathode buffer instead of BCP because NTCDA is stablein air. NTCDA is an n-type organic semiconductor. In thepast, it has been used as an electron transport layer27,28)

which lies between the acceptor layer and the cathode.Device E with a cathode buffer of NTCDA is separatelyfabricated. The structure of Device E is the same as that of

0

1.3N

orm

aliz

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p

Device A (BCP buffer)Device C (conventional)Device D Device E (NTCDA buffer)

0.5

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200Time [h]

0 400 600 800 1000 1200

(a)

0

1.2

Device C

Device D

1.0

0.5

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mal

ized

ηp

Time [h]0 1 2 3 4

(b)

Fig. 8. (a) Normalized power conversion efficiency (�p) of Devices A, C,D, and E. (b) Normalized power conversion efficiency (�p) of Devices C and

D in a short-time range.

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169 h

645 h

1056 h

(d)

Fig. 9. Evolution of J–V characteristics for the four kinds of devices at

typical times; (a) Device A of inverted structure with pentacene buffer layer,

(b) Device C of conventional structure, (c) Device D of conventional

structure with pentacene buffer layer, and (d) Device E of inverted structure

with NTCDA cathode buffer layer.

T. Oida et al.Jpn. J. Appl. Phys. 50 (2011) 081601

081601-5 # 2011 The Japan Society of Applied Physics

Device A except for the cathode buffer layer. The thicknessof the NTCDA layer is 15 nm. Because NTCDA has a higherconductivity than BCP, a larger Jsc is expected as well. Theperformance of the device is as follows: Jsc is 2.77mA/cm2,Voc is 0.36V, FF is 0.40, and �p is 0.40%. The evolutionof its J–V characteristics with time is shown in Fig. 9(d).The J–V curve does not change so much with time.The degradation of the normalized �p of Device E isshown in Fig. 8(a). This device shows a higher �p of0.40% and a longer lifetime of 1200 h than Device A. Thisenhancement in device performance is attributed to thehigher stability and higher conductivity of NTCDA thanBCP.

5. Conclusions

A small-molecular organic solar cell with an inverted devicestructure of ITO/BCP/C60/CuPc/pentacene/Ag is pro-posed. Although the Ag electrode usually acts as the cathodebecause of its low work function, the Ag top electrodeappropriately works as the anode. Interfacial dipoles atthe pentacene/Ag interface prevent electron redistributionbetween Ag and CuPc when the Ag comes into contact withthe organic layer. As a result, the band bending of CuPc,which interferes with hole collection, is suppressed. TheCuPc band bending plays a role as the thick hole barrierwhen there is no pentacene layer. The pentacene layer itselfplays a role as the hole barrier. Since the pentacene layeris very thin, holes pass through the layer by the tunnelingeffect. As a result, holes effectively collect at the Agtop electrode. That is, the Ag top electrode appropriatelyworks as the anode. This device shows a power conversionefficiency of 0.28% under 100mW/cm2 AM1.5G simulatedillumination and a lifetime of 700 h without encapsulation.This long lifetime is mainly attributed to the invertedstructure that effectively protects C60 from the diffusionof oxygen. In addition, by substituting BCP for NTCDA asthe cathode buffer, the power conversion efficiency andthe lifetime are enhanced to 0.40% and more than 1200 h,respectively. Further improvement will be achieved byoptimizing the thickness of the active layer.

Acknowledgement

One of the authors (K. Harafuji) wishes to thank ProfessorM. Hiramoto and his colleagues of the Institute forMolecular Science for fruitful discussions and technicalassistance. This work was partly performed in Nanotechnol-ogy Support Project in Central Japan (Institute for Molecular

Science), financially supported by Nanotechnology Networkof the Ministry of Education, Culture, Sports, Science andTechnology (MEXT), Japan (Contract Nos. A122, B113).Thanks are also given to Professor S. Imai for advice duringthis study.

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