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the PV layer and the electrodes also has to be optimized. To achieve

electrodes. At the indium tin oxide (ITO) anode side, a buffer layer

has been used to tune the electrical contact of the ITO. At the

iatede thescalent ofork-

and high lowest-unoccupied-molecular-orbital (LUMO) (2.1 eV)

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Contents lists available at ScienceDirect

.els

Solar Energy Mater

Solar Energy Materials & Solar Cells 94 (2010) 11521156improve the performance of organic light-emitting diodes [15].E-mail address: bcpark@kw.ac.kr (B. Park).of poly(3,4-ethylene dioxylene thiophene):poly(styrene sulfonicacid) (PEDOT:PSS) [9] or self-assembled monolayers [10],

levels of PTE [15], it may be surmised that the nanoscaleinterfacial layers of (PTE/Al:Li) can modify the charge carrierinjection and transport at the electrode interface. In previouswork, nanoscale double interfacial layers (DILs) were also used to

0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.solmat.2010.02.045

Corresponding author. Tel.: +8229405237; fax: +8229433208.better electrical contact, it has been common practice tomodify thefunction metal alloy (Al:Li) layer, as shown in Fig. 1. Because ofthe low highest-occupied-molecular-orbital (HOMO) (8.1 eV)such as the roll-to-roll process (abbreviated R2R) [7,8]. However,for practical applications, the efciency of PSCs needs to beimproved further with good environmental stability. In order toachieve this goal, the electrical contact at the interfaces between

remain inadequate. Hence, to improve PSCs further, we initwork related to the interfacial layer. In this study, to improvcollection of electrons at the cathode side, we used nanointerfacial layers, which consisted of (i) a ultrathin surfactapoly(oxyethylene tridecyl ether) (PTE) and (ii) a low-w(P3HT) and phenyl C61-butyric acid methyl ester (PCBM), PSCswith a power conversion efciency (PCE) of 34% were demon-strated [4,5]. More recently, it was reported that the efciency canbe increased dramatically up to 6:77% by using a low-band gappolymer cell [6]. The use of these PSC technologies should resultin much lower costs for high-speed solution-coating processes,

the dipolar layer [14]. However, the electrical properties ofinorganic lms depend strongly on the conditions of processingand their fabrication is complex. These may be limiting factors forfabricating highly efcient devices. Thus, the improvements madeso far with respect to the efcient transfer of electrons in PSCs1. Introduction

Recently, important research hdevelopment of polymer solar ceefciency in generating electrical poAmong the desired developmentsefcient photoinduced charge generlayer is the bulk heterojunction Pcomposed of a conjugated polymer a

Using the bulk heterojunction stpost-thermal annealing of PV layeen conducted on theCs) for realizing highy absorbing light [15].teresting one for thef the photovoltaic (PV)ucture [2,3], which islerene composite layer.e together with pre- oroly(3-hexylthiophene)

cathode side, an ultrathin LiF [11] or poly(ethylene oxide) [12]layer has been placed between the PV layer and the cathode toform a favorable dipole layer, which results in improved electroncollection. As a representative example, an efciency of 3:3%was reported for PSCs with a LiF interfacial layer [11]. Recently,instead of a LiF interfacial layer, a multifunctional inorganic layerof TiOx [13] or ZnO [14] was provided as a hole-blocking barrier,an optical spacer, and oxygen barrier. The use of these inorganiclms resulted in signicantly improved device efciency andstability. A PCE of 4.2% was reported when using ZnO lm withImproved photovoltaic effect of polymeinterfacial layers

Young In Lee a, Mina Kim a, Yoon Ho Huh a, J.S. Lima Department of Electrophysics, Kwangwoon University, Wolgye-Dong, Nowon-gu, Seoub Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT

a r t i c l e i n f o

Article history:

Received 3 November 2009

Received in revised form

26 February 2010

Accepted 28 February 2010Available online 21 March 2010

Keywords:

Polymer

Photovoltaic

Interfacial layers

a b s t r a c t

In poly(3-hexylthiophene)

introduced nanoscale inter

layers were made of ultrat

metal of Al:Li layers. It w

performance: increasing sh

PV cells with the nanoscale

was achieved, compared t

journal homepage: wwwsolar cells with nanoscale

Sung Cheol Yoon b, Byoungchoo Park a,

9-701, Republic of Korea

epublic of Korea

thanofullerene bulk-heterojunction polymer photovoltaic (PV) cells, we

ial layers between the PV layer and the cathode. The nanoscale interfacial

poly(oxyethylene tridecyl ether) surfactant and low-work-function alloy

found that the nanoscale interfacial layers increase the photovoltaic

-circuit current density with ll factor and improving device stability. For

terfacial layers, an increase in power conversion efciency of 4.1870.24%at of the control devices (3.89 70.08%).

& 2010 Elsevier B.V. All rights reserved.

evier.com/locate/solmat

ials & Solar Cells

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shows strong absorption spectra with absorption peak centered ataround 507nm with the pronounced three vibronic absorptionpeaks at ca. 600nm. This absorption in the visible region isattributed mainly to the P3HT polymer because PCBM showsstrong absorption in the UV region. It is noted that theimprovement in the ordering of P3HT polymer with thermalannealing can be conrmed from the pronounced three vibronicabsorption peaks. As shown in the gure, these results are almost

A PTE (1~2 nm)

( O

HO1

or t

Fig. 2. Normalized UVvis absorption spectra of PV layers of P3HT:PCBM without(black dotted curve) and with (blue solid curve) a PTE interfacial layer after

heating treatment at 150 1C for 10min.

Y.I. Lee et al. / Solar Energy Materials & Solar Cells 94 (2010) 11521156 11532. Experimental details

For the experiments, P3HT (Aldrich, regiospecic ratio Z99:5%),as the electron donor and PCBM (Rieke Metals, Inc.), as the electronacceptor were used as received. The chemical structures of P3HT andPCBM are shown in elsewhere [4,5]. P3HT (1.20wt%) and PCBM(0.88wt%) were added to a solvent of 1, 2-dichlorobenzene to blenda PV solution [16]. An ITO layer (80nm, 30O=square) on glasssubstrate was used as an anode, as shown in Fig. 1. After routinecleaning of the substrate using ultraviolet-ozone treatment, theblended PV solution was spin-coated on top of the ITO, precoatedwith PEDOT:PSS buffer layers. The PV layer was about 85 nm thick.In order to form the rst ultrathin interfacial layer of PTE (C13H27(OCH2 CH2)12OH, Aldrich), a PTE solution (0.1wt% in distilled water)was further spin-coated (12nm) on top of the underlying PV layer.(Details of the thickness of the PTE layer will be reported elsewhere.)After spin-coating, all coated lms were baked at 120 1C for 3min.For the second interfacial layer, a 1nm thick Al:Li alloy(Li: 0.1wt%) layer was formed on the PTE layer via thermaldeposition (0.5nm/s) at a base pressure below 2 106Torr.Finally, a pure Al ( 50nm thick) cathode layer was formed on theinterfacial layers under vacuum. Thus, the sample PSC devicefabricated has the structure of ITO/PEDOT:PSS/P3HT:PCBM/PTE(12nm)/Al:Li (1nm)/Al with active area of 3 3mm2. Forcomparison, we also fabricated a control device without the PTEinterfacial layer, i.e. ITO/PEDOT:PSS/P3HT:PCBM/Al:Li (1nm)/Al.For post-thermal annealing, the fabricated PSC was annealedat 150 1C for 10min to induce the crystallization of the PV layer[4,5]. It should be noted that, except for the PTE interfacial layer,the control device was fabricated in exactly the same way as thesample cell. The optical properties of the PV layers were

Glass substrate

ITOPEDOT:PSS

P3HT:PCBM PV LayerV

Fig. 1. (Left) Device structure and (right) energy band diagram fAl:Li (1 nm) / Al PTEinvestigated via UV-vis spectrometry at room temperature with aCary 1E (Varian) UVvis spectrometer. The performance of the PSCswas measured under an illumination intensity of 100mW/cm2

generated by an AM1.5 light source (Newport, 96000 SolarSimulator). The photocurrentvoltage (JV) characteristics weremeasured with a source meter (Keithley 2400) and calibrated byusing a reference cell (Bunkoh-keiki, BS-520). The incident photon-to-current collection efciency(IPCE) spectra were measured usingan IPCE measurement system (Titan Electro-optics Co.,QE-IPCE 3000).

3. Results and discussion

First, the optical characteristics of the PV layers were observedby UVvis absorption spectroscopy. Fig. 2 shows the normalizedabsorption spectra of PV layers after thermal annealing at 150 1Cfor 10min. It is clear from the gure that the P3HT:PCBM PV lmAlAl:L

i

P3HT

PCBM

PED

OT:

PSS

ITO 4.24.4~4.7

5.1~5.4

3.0

4.9

3.7

6.1

PTE

2.1

8.1

3.1(2

he studied polymer solar cells with nanoscale interfacial layers.identical to those of the P3HT:PCBM PV lm with the PTEinterfacial layer. Thus, it indicates that the PTE interfacial layerdoes not alter the optical absorption characteristics of the PV layerof P3HT:PCBM.

Next, the effect of the interfacial layers on the performance ofthe current versus voltage (JV) characteristics for the PSCs wasinvestigated, as shown in Fig. 3. Fig. 3(a) shows the dark currentcharacteristics of the PSCs under study. Both of the devices thatwe tested show an excellent rectication ratio and thus goodcoverage for the organic layers. However, as shown in the gure,the tested PSCs show a small but clear difference in current ow,which implies that there is a clear difference in the extraction ofcharge carriers through the interfacial layers. Thus, to see the PVeffect of the studied devices, JV curves under illumination werealso observed, as shown in Fig. 3(b). For the case of the control PSCdevices without the PTE interfacial layer, fairly good performancewas observed with an open-circuit voltage (VOC) of 0.6170.01Vand a short-circuit current density (JSC) of 11.1170.17mA/cm

2

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Y.I. Lee et al. / Solar Energy Materials & Solar Cells 94 (2010) 115211561154with a ll factor (FF) of 57.34 71.11%. This corresponds to a PCEof 3.8970.08%, which is comparable to that reported in previousstudies [4]. By contrast, for the sample cells with nanoscaleinterfacial layers comprised of (PTE/Al:Li), the VOC was0.6370.01V and the JSC increased to 11.2270.29mA/cm

2 witha FF of 59.7771.09%. Thus, the efciency of the sample solar cellswith interfacial layers comprised of (PTE/Al:Li) increases to4.1870.24%. These values are the averages and standard

Fig. 3. The JV characteristics of the PSCs with different interface layer structures(a) in the dark and (b) under light illumination. (c) The JV curves of theilluminated photocurrent for extended reverse bias. The linear lines represent

linear ts.deviations of four to ve individual PSCs on independentsubstrates for both device congurations. From these results,one can see that the device with the nanoscale interfacial layersyields increases in JSC and FF. These increases lead to animprovement of PCE by up to 107%. Note that, for thenanoscale interfacial layers, when the thickness of each layer,i.e., the PTE layer and Al:Li layer, changes from 122nm theefciency begin to decrease. We also focused on losses of chargesby recombination, which has a critical inuence on the chargeseparation and transport. The losses in the JSC that are due tomonomolecular recombination were estimated from thedependence of the current on negative voltage [17], as shown inFig. 3(c). For the control device and the sample cells with(PTE/Al:Li) interfacial layers, the losses in the JSC were estimatedat 3.4871.47% and 2.6072.50%, respectively. These losses of JSCare relatively small, which indicates that little monomolecularrecombination occurred in both devices. This yields evidence thatthere is no signicant difference in recombination mechanismsbetween the studied PSC devices.

The performance of the interfacial layers was also evaluated bymeasuring the IPCE spectra. The observed IPCE spectra of the PSC

Fig. 4. IPCE spectra of P3HT:PCBM solar cells without (dotted curve) and with(solid curve) the PTE interfacial layer.devices are shown in Fig. 4. Both PSCs shows the well-knownspectral response of its BHJ composite and are in good agreementwith the absorption spectra of the PV composites (Fig. 2). Asshown in Fig. 4, the PSC device structure of ITO / PEDOT:PSS/P3HT:PCBM/PTE/Al:Li/Al has the higher IPCE with a maximum ofca. 64.1% at 480nm than that of ca. 62.8% for the control device.This interesting result is the substantial contribution to the IPCEof the PTE interfacial layer on the PV layer. Given that the UVVisabsorption results (Fig. 2) show no signicant differences betweenthe sample and control device structures, this increased IPCE canbe attributed to the improvement of the carrier injection andtransport at the electrode interface. By coating PTE interfaciallayer on the PV layer, the oriented dipole effect [14] of the dipolarOH groups in the PTE molecules along normal to the surface maycontribute to the efcient carrier injection and transport at theinterface via the reduction of the potential barrier between the PVlayer and the Al cathode.

Next, in order to understand the electrical performance of thePTE interfacial layer, the owing current density versus appliedelectric eld (JE) characteristics for hole-only and electron-onlydevices with the PTE interfacial layers were investigatedat T300 1K, as shown in Fig. 5. For hole-only devices, weused a (ITO/PEDOT:PSS/P3HT:PCBM/Au) structure and a

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Y.I. Lee et al. / Solar Energy Materials & Solar Cells 94 (2010) 11521156 1155(ITO/PEDOT:PSS/P3HT:PCBM/PTE/Au) structure. As shown inFig. 5(a), the hole-only device with the PTE interfacial layershows a greatly reduced hole-current ow, which conrms thatthe presence of the PTE interfacial layer on the PV layer can blockthe hole current efciently. In order to estimate the electrontransfer, we also tested electron-only devices with structures of(Al/P3HT:PCBM/Al:Li/Al) and (Al/P3HT:PCBM/PTE/Al:Li/Al). Asshown in Fig. 5(b), the electron-only device with the PTEinterfacial layer shows an increased electron-current ow. Thus,charge carriers can be selectively transported through the PTEinterfacial layer, blocking the holes and transporting theelectrons. Thus, these electric effects of the PTE layer at theinterface between the PV layer and the cathode can contribute tothe collection of electrons, thereby improving PSC performance.

Next, we also observed atomic force microscopy (AFM)topographic images of the surfaces for the PV layers formed onat glass substrates by scanning in the static force mode. Fig. 6shows the AFM surface images for the spin-coated P3HT:PCBMlayer (upper gure) and the P3HT:PCBM layer after coating thePTE interfacial layer (lower gure). Compared with the smoothsurface of the P3HT:PCBM layer, which has roughness at thenanoscale, it is clear that a fairly smooth surface with thenonoscale roughness was also formed on the P3HT:PCBM layercoated with a PTE surfactant layer. This is in contrast to that

Fig. 5. The JE characteristics of hole-only devices (a) and electron-only devices(b) with different interfacial layers.which was observed by AFM with respect to the behavior of PTEsurfactant on polymer lm [18]; there was no spike orientation ofPTE on the PV lm in this study due to the difference in surfaceinteractions. Thus, it is also easy to judge that, coating the PV layerwith the PTE surfactant does not change the surface quality of theP3HT:PCBM PV layer.

Finally, we also investigated the storage lifetime of the PSCwith different interfacial layers. Fig. 7 shows a linear-log plot ofthe storage lifetime for the studied PSCs. Immediately afterfabrication, the tested PSCs were encapsulated in a nitrogenglovebox (r2ppm of H2O and O2) with a glass cap that wassealed with an epoxy resin. A desiccant was incorporated insidethe package. The devices were kept in the dark under open-circuitcondition between measurements. The operational lifetimes of

Fig. 6. 3-D topographical AFM images of P3HT:PCBM lms without (upper) andwith (lower) the PTE interfacial layer after annealing at 150 1C.

Fig. 7. Comparison of the relative PCE as a function of the storage time for the PSCswith (open circles) and without (closed circles) the PTE interfacial layer. The

dotted and solid curves are least-squares ts of the stretched exponential decays.

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the encapsulated devices were measured under intermittentillumination (solar simulator, 100mW/cm2, AM1.5G) at atemperature of 26.172.0 1C and a relative humidity of74.779.7%. As shown in gure, the storage lifetime of the PSCclearly exhibits the stretched-exponential relaxation [19],indicating a broad distribution of relaxation times. Moreover, itis clear that the lifetime of a PSC with a PTE interfacial layer ismuch longer than that of the control device, which implies thatthe PTE surfactant interfacial layer may lead to increased devicestability. Because the encapsulation of the tested devicessignicantly inhibits the degradation process that is due tooxygen and water [20,21], the degradation of the tested device

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Y.I. Lee et al. / Solar Energy Materials & Solar Cells 94 (2010) 115211561156Acknowledgments

This work was supported by technology development projectof new and renewable energies of the Ministry of KnowledgeEconomy of the Republic of Korea (2009). This work wassupported by the Brain Korea 21 Project 2009.4. Conclusions

In summary, we demonstrated an efcient PSC with the(PTE/Al:Li) nanoscale interfacial layers between the PV layer andcathode. As an experimental result, with the interfacial layers, thePSCs show clear improvements in power conversion efciency byup to 107%. This improvement can be attributed to theimproved electron collection at cathode electrode. Moreover, itwas shown that the PSCs with the nanoscale interfacial layers aremore stable than the PSCs without the interfacial layers. Combiningthe device reported here with the highly efcient materialsreported elsewhere will surely lead to highly efcient PSCs.protect the PV active layer effectively against the diffusion ormigration of Al (and/or Li) atoms into the PV layer through theinteraction between OH group of PTE and metal atoms (OH:Aland/or OH:Li). The above results conrm that the deviceperformance due to the (PTE/Al:Li) nanoscale interfacial layers isimproved and that the fabrication of the interfacial layers mayalso increase the storage lifetime of the device.[8] F.C. Krebs, S.A. Gevorgyan, J. Alstrup, A roll-to-roll process to exible polymersolar cells: model studies, manufacture and operational stability studies,J. Mater. Chem. 19 (2009) 54425451.

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[21] M. Jorgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solarcells, Sol. Energy Mater. Sol. Cells 92 (2008) 686714.degradation [21]. Thus, the PTE surfactant interfacial layer may394412.may be caused by other factors, such as diffusions of cathodematerials into the active layer. The diffusion of Al (and/or Li,perhaps) cathode atoms and impurities into the PV active layermay act as a recombination site and hence accelerate device

Improved photovoltaic effect of polymer solar cells with nanoscale interfacial layersIntroductionExperimental detailsResults and discussionConclusionsAcknowledgmentsReferences