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Nano Energy (2015) 16, 122–129

http://dx.doi.org/12211-2855/& 2015 E

nCorresponding auE-mail addresses

kseo@unist.ac.kr (K.1J. H. Seo and H.-2Present address

Institute of Technolo

RAPID COMMUNICATION

Low-temperature solution-processed flexibleorganic solar cells with PFN/AgNWs cathode

Ji Hoon Seoa,1, Han-Don Uma,1, Atul Shuklaa,2, Inchan Hwanga,Juyun Parkb, Yong-Cheol Kangb, Chang Su Kimc,Myungkwan Songc,n, Kwanyong Seoa,n

aDepartment of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Republic of KoreabDepartment of Chemistry, Pukyong National University, 45, Yongso-ro, Nam-Gu, Busan 608-737, Republicof KoreacSurface Technology Division, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea

Received 27 April 2015; received in revised form 17 June 2015; accepted 18 June 2015Available online 27 June 2015

KEYWORDSFlexible solar cells;Inverted organic solarcells;Low-temperatureprocess;Interfacial layers

0.1016/j.nanoen.2lsevier Ltd. All rig

thors.: smk1017@kims.rSeo).D. Um contribute: Polymer Sciengy Roorkee, Malv

AbstractWe report on highly efficient flexible inverted organic solar cells (IOSCs) fabricated by low-temperature solution process on polyethylene terephthalate (PET) substrate. In general, IOSCshave been required to use an annealed (4200 1C) zinc oxide (ZnO) as an electron transportlayer. However, any twisting of the flexible substrate during heat treatment leads to poordevice performance. To overcome this issue, we developed a novel low temperature process forflexible IOSCs using an alcohol-/water-soluble conjugated polymer, namely poly [(9,9-bis(30-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN) instead of the ZnO.Using this low-temperature process, we successfully demonstrate highly efficient flexible IOSCsthat are proven to be capable of the power conversion efficiency (PCE) of 6.17% which retain96% of its efficiency at a bending radius of RE5 mm or less. To the best of our knowledge, thisPCE 6.17% is the best result among the reported values so far for flexible OSCs fabricated onPET substrate.& 2015 Elsevier Ltd. All rights reserved.

015.06.013hts reserved.

e.kr (M. Song),

d equally to this work.ce and Technology, Indianiya Bhawan, A-334, India.

Introduction

Flexible organic solar cells (OSCs) are one of the mostpromising devices in solar energy research because of theadvantages they offer in terms of material lightness and lowcost, as well as their simple manufacturing processes [1–3].

Figure 1 (a) Comparison of conversion efficiencies for flexible solar cells using various transparent conducting electrodes [14–17]and our device. (b) Schematic illustration of flexible IOSCs, in which a photoactive layer is sandwiched between PFN/AgNWs cathodeand a top Ag film anode.

123Organic solar Cells with PFN/AgNWs cathode

Furthermore, the mass production of flexible OSCs ispossible using printing methods such as the roll-to-rollprocess, and wearable devices that are readily attachableto curved surfaces can also be fabricated [4,5]. In particu-lar, a structure of inverted organic solar cells (IOSCs) hasbeen developed that uses silver as an anode, resulting inbetter long-term stability against chemical oxidation com-pared to that of conventional organic solar cells (COSCs)using aluminum electrodes [6]. Moreover, this invertedstructure has been reported to be superior to conventionalstructures in terms providing efficient charge collection,which is attributed to the two-phase separation mechanisminside a photoactive layer [7].

Despite the many advantages of IOSCs, few studies havebeen conducted on their flexible application owing to thedifficulties associated with their processing that typicallystem from the use of sol-gel based zinc oxide (ZnO) as anelectron transport material [8]. Creating a more efficientelectron transport layer (ETL) requires converting the sol–gel–type ZnO into nanoparticle form through annealing athigh temperatures (4200 1C); however, commonly usedflexible polymer substrates such as polyethylene terephtha-late (PET) cannot endure such processing temperatures dueto a low glass transition temperature (Tg) of around 70–120 1C [9]. To overcome this problem, researchers havedeveloped IOSCs in which a ZnO layer is coated onto plasticsubstrates with a high Tg such as polyethersulfone (PES) orpolyimide [10]. These polymer substrates, however, havetheir own shortcomings in terms of high cost, high coeffi-cient of thermal expansion (CTE), high moisture absorption,and poor solvent resistance, all of which present a signifi-cant barrier to the inexpensive mass production of suchdevices [9]. This is particularly true of IOSCs that use silvernanowires (AgNWs) as a transparent conducting electrode(TCE), as any twisting of the plastic substrate during heattreatment can create a short circuit inside the devicethrough direct contact between the AgNWs and anode [11].

This paper attempts to overcome the problems associatedwith using ZnO as a surface modification material byreplacing it with an alcohol-/water-soluble conjugatedpolymer, namely poly [(9,9-bis(30-(N,N-dimethylamino) pro-pyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN)[12].This is based on the premise that as a PFN layer can beformed using a simple spin coating process without anneal-ing, flexible IOSCs can be created from a PFN/AgNWs hybrid

cathode on an inexpensive, low-Tg PET substrate. To allowlow-temperature device fabrication, {poly[4,8-bis[(2-ethyl-hexyl)oxy]benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)car-bonyl]thieno[3,4–b]thiophene-diyl] (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester(PC71BM)} were used as the photoactive layer [13]. Althoughcarbon nanotubes (CNTs) [14], graphene and PEDOT:PSShave all been previously used as TCEs, they have provendifficult to implement efficiently due to their relatively lowtransmittance and conductivity when compared to indiumtin oxide (ITO) [15,16]. More recently, COSCs using veryflexible AgNWs/PET TCEs have demonstrated electrical andoptical characteristics equivalent to those of rigid ITO, butthe highest reported power conversion efficiency (PCE) hasso far only been 5% [17]. In contrast, the IOSCs fabricated atlow temperature in this study exhibited a 22% improvementin efficiency over existing conventional structures that useAgNWs as a cathode (Figure 1a), while bending testsconfirmed that the device has excellent flexibility charac-teristics (Figure S1). This low-temperature processing tech-nology is therefore considered an important step toward thepractical realization of inexpensive and flexible solar cellsusing a roll-to-roll manufacturing method, thereby allowingfor inexpensive substrates and the mass production ofdevices such as electronic paper in the future.

Experimental section

Fabrication of inverted organic solar cells (IOSCs)

To obtain a solution suitable for use in the spin-castingequipment, PFN (1-material Chemscitech) was first dis-solved in methanol to a concentration of 1 mg ml�1, towhich 2 μl of acetic acid was added to ensure completedissolution of the PFN. Next, 8 mg of PTB7 and 12 mg ofPC71BM (1-material Chemscitech and Solemme BV) weredissolved in 1 ml of chlorobenzene to act as an electronacceptor and electron donor material, respectively. Thissolution was mixed with 0.03 ml of 1,8-diiodooctane (SigmaAldrich) and stirred for 12 h at 50 1C inside a nitrogen-filledglove box. To create the cathodes for the IOSCs, a PETsubstrate was first cleaned with oxygen plasma for 5 min,and then spin coated with a liquid dispersion of AgNWs(Nanopyxis lnc.) for 40 s at a speed of 600–1200 rpm. The

J.H. Seo et al.124

film of AgNWs formed on the PET substrate in this way wasthen annealed for 5 min at 120 1C in air. Spin-coating of thePFN interlayer was performed using the same procedure asthat used for the AgNWs cathode, but to create a photo-active layer, a bulk-heterojunction material was passedthrough a 0.45 μm PTFE syringe filter inside the glove boxto create a thin (150 nm) film after spin-coating at 1000 rpmfor 40 s. To provide a hole transport layer and metal anodelayer, films of MoO3 (7 nm) and Ag (100 nm) films wereformed on the top portion of the device through thermalevaporation using a shadow mask. The area of the finaldevice specified as an anode area was 0.23 cm2.

Bend testing

To assess the mechanical flexibility of the fabricated IOSCs,a custom-made bending test system was used to not onlydetermine the severe radii resistance characteristics of thePET/ITO and PET/AgNWs/PFN, but also the efficiencycharacteristics of the fabricated flexible devices (PET/ITO,PET/AgNWs/PFN). Figure S1 shows an example of a non-encapsulated device to which pressurized stress has beenapplied.

Characterization

The current-voltage characteristics of the devices in thedark were investigated using a semiconductor parameteranalyzer (4200-CSC, Keithley). The photovoltaic propertiesof the solar cells were investigated under AM 1.5G illumina-tion using a solar simulator (Class AAA, Oriel Sol3A, New-port). Incident flux was measured using a calibrated powermeter, and double-checked using a NREL-calibrated solarcell (PV Measurements, Inc.). The EQE was measured using aXe light source and a monochromator to produce wave-lengths in the range of 400–1100 nm. Optical reflectionmeasurements were performed over these same wave-lengths using a UV–vis/NIR spectrophotometer (Cary 5000,Agilent) equipped with a 110 mm integrating sphere toaccount for the total light (diffuse and specular) reflectedfrom the samples. The surface morphologies of the PFN/AgNWs and AgNWs were characterized by field-emissionscanning electron microscopy (FE-SEM, Hitachi S-4800) andatomic force microscopy (AFM, Veeco microscope). Thework functions of the electrodes were measured throughultraviolet photoelectron spectroscopy (UPS) using a gasdischarge UV source embedded in an ESCALAB MKII(hν=21.22 eV).

Results and discussion

An IOSCs based on a PFN/AgNWs was fabricated on PET,giving a final structure of PET/AgNWs/PFN/photoactivelayer/molybdenum oxides (MoO3)/Ag film (Figure 1b) anda device area of 0.23 cm2. Various characteristic changes inthese AgNWs-based IOSCs at different bending radii werecompared against ITO-based COSCs (Figure 2). Of the manyphotovoltaic parameters, it is the fill factor (FF) in Figure 2athat shows the most noticeable change with bending radius;the FF of ITO-based COSCs reduces dramatically withincreased bending radius, whereas the PFN/AgNWs-based

IOSCs exhibit only a relatively small reduction in the FF.These changes in the FF can be explained by the resistancechange ratio (ΔΩ �Ω0

�1) of the flexible TCEs (Figure S2),which is defined as the change in resistance (ΔΩ) afterbending compared to the initial resistance (Ω0). The highlyflexible PFN/AgNWs maintain a ΔΩ �Ω0

�1 value of around�7.5172.3%, even at an extreme bending radius RE0.43mm, which corresponds to a bending strain (ε) of �15%(ε=hs/(2R), where hs is the substrate thickness) [18]. On theother hand, an extreme change in resistance occurs at abending radius of RE5 mm (εE1.3%) and smaller in thecase of commercial ITO films fabricated on PET. This can beattributed to the fact that although the crystalline structureof ITO film can relieve mechanical compressive stress duringbending through the generation of cracks and defects, thesedivide the film into numerous pieces and result in a highcontact resistance to the movement of electrons [17,19,20].Consequently, the resistance of the ITO film increasesrapidly with bending, and this in turn increases the seriesresistance and reduces the FF inside the element. AgNWs,on the other hand, demonstrate excellent compressivestress relief in comparison, with very little change inresistance even at a low bending radius. It is this superiorbending stability of IOSCs based on PFN/AgNWs that allowsthem to maintain their original FF.

Much like the FF, the short circuit current (JSC) of the ITO-based COSCs is inverse proportional to their change inbending radius, experiencing a reduction in value of about34% at RE1.5 mm when compared to their initial value(Figure 2b). In contrast, the JSC of the PFN/AgNWs-basedIOSCs does not begin to change until a bending radius ofRE4.8 mm, with value reducing by just 7% at RE1.5 mmwhen compared to the initial value. Although this reductionrate is less than that of the ITO-based COSCs, the change ismore pronounced than the FF reduction (3%) at RE1.5 mm.This suggests an issue in the Ag film anode of the IOCSsrather than any sort of instability in the PFN/AgNWscathode, as the separation of the electron–hole pair doesnot usually occur smoothly in OSCs owing to the high excitonbinding energy of the photoactive material [21]. In otherwords, the destruction of the Ag film anode by extremebending creates an electrically independent region (FigureS3), with the impossibility of collecting any carriers gener-ated in this independent region being responsible for thedegradation in JSC.

The open circuit voltage (VOC) of the ITO-based COSCs andPFN/AgNWs-based IOSCs is not greatly influenced by thebending radius, as shown in Figure 2c. Indeed, even if cracksoccur in the ITO cathode or Ag film anode, VOC is stilldetermined by the relative difference between the lowestunoccupied molecular orbital (LUMO) of the donor materialand highest occupied molecular orbital (HOMO) of the acceptormaterial [22], meaning that a constant VOC value generallymaintained regardless of any changes in the cathode or anode.Figure 2d presents the change in efficiency of the ITO-basedCOSCs and PFN/AgNWs IOCSs with different bending radii, fromwhich it can be seen that the latter retains 96% of its efficiencyat a bending radius of RE5 mm or less, and still maintains 90%of its efficiency at an extreme bending radius of RE1.5 mm.This reduction in PCE can be attributed to an increase inresistance caused by cracks in the Ag film anode, which alsorepresents the main reason for the reduction in JSC and FF. In

Figure 2 (a) Measured FF, (b) JSC, (c) VOC, and (d) PCE values for flexible COSCs based on a PET/ITO film (black sphere) and IOSCs basedon a PET/AgNWs/PFN (blue sphere) as a function of their bending radius during compressive bending (normalized to the initial value).

125Organic solar Cells with PFN/AgNWs cathode

other words, it is an increase in the area from which carriercollection is impossible that is responsible, not deformation ofthe PFN/AgNWs. As there is clearly a need to find ways tobetter secure the metal film anode, research is currentlyunderway into developing a highly flexible and stable anodestructure as well.

To better understand the role of the PFN/AgNWs insidethe fabricated flexible device, a variety of analyses wereconducted, as shown in Figure 3. This included determiningthe change in the electrical and optical characteristics ofAgNWs in solution with various spin speeds, the results ofwhich are summarized in Table 1. These AgNWs were formedon a PET substrate by spin-coating at a speed of 800 rpm,with subsequent annealing at 120 1C in air producing anelectrode with an electrical resistance of RsheetE32.8 Ωsq�1. Commercial ITO/PET substrate with 150-nm-thick ITOfilm has an electrical resistance of RsheetE14 Ω sq�1. In thescanning electron microscope (SEM) images obtained(Figure 3a), there is a clear difference in contrast betweenthe AgNWs and the PFN/AgNWs on PET that can beexplained by the fact that the PFN film is a conjugatedpolyelectrolyte (CP) with a relatively low conductivity [23].In other words, the loss of contrast indirectly shows that thePFN material was coated over the AgNWs, with such acoating helping to reduce the typically high surface rough-ness of a AgNWs when it is formed on a PET substrate.Without this surface smoothing, it is far more likely thatAgNWs will come into direct contact with the cathode oranode, making electrical short circuits a more frequentoccurrence. Thus, in order to maximize the manufacturingyield, the surface roughness of the AgNWs needs to bereduced as much as possible. Atomic force microscopy (AFM)

verified that although the AgNWs has an initial root meansquare (RMS) surface roughness of 19.7 nm, this wasreduced to just 16.3 nm by the PFN film formed on itssurface (Figure S4). In addition to morphology, the opticaltransmittance of the PFN film was also analyzed andcompared against a ZnO film (Figure 3b). The opticaltransmittance of AgNWs is higher than that of the commer-cial ITO substrate (Figure S5). When the ETLs are coatedonto the AgNWs, the results show that PFN and ZnO filmsmainly differ in transmittance in the ultraviolet (UV) andvisible wavelength range; a PFN/AgNWs has a lower UVtransmittance than a ZnO/AgNWs, but a higher transmit-tance in the 425 nm to near-infrared range. This low UVtransmittance can be explained by the fact that PFN has amuch stronger optical absorption at 400 nm than a ZnOnanoparticle film, which has an absorption peak at 288 nm.However, considering that the 400 to 700 nm range is themain absorption spectra of the PTB7/PC71BM-blend filmused as a photoactive layer, the use of a PFN/AgNWs isbeneficial in terms of JSC and device efficiency [13].

Figure 3c provides information as to the work functions ofAgNWs, ZnO/AgNWs, and PFN/AgNWs, which were deter-mined through UV photoelectron spectroscopy (UPS) bytaking the binding energy of the secondary electron cutoffand Fermi level measured from the injected energy(21.22 eV in our experiment). Using the equation:φm=hν�(EF�ESC), the work functions of the AgNWs andPFN/AgNWs were calculated as �4.1 and �4.33 eV. Thiswork function tuning is made possible by the chargealignment of the photoactive materials, in that the perma-nent interfacial dipole PFN interlayer is located at theAgNWs surface [12]. Thus, the PFN/AgNWs exhibits an

Figure 3 (a) Schematic illustration of a PFN/AgNWs-coated substrate. SEM images show the AgNWs cathode with (right top) andwithout (right bottom) a PFN layer. (b) Total transmittance spectra of AgNWs (black line), ZnO/AgNWs (red line), and PFN/AgNWs(blue line) coated onto PET substrates. (c) UPS spectra of AgNWs (black line), ZnO/AgNWs (red line), and PFN/AgNWs (blue line).

Table 1 Key parameters for the various AgNWs electrodes fabricated at different spin speeds on PET.

Spin-speed[rpm]

Thickness[nm]

Sheet resistance[Ω sq�1]

Specular transmittance[%, at 550 nm]

Total transmittance[%, at 550 nm]

600 70.1 19.3 90.2 94.4800 45.3 32.8 91.8 95.8

1000 38.5 35.7 95.1 96.71200 33.8 44.4 95.1 97.1

J.H. Seo et al.126

energy level that is closer to the LUMO level of the PC71BMused as an acceptor material inside the photoactive mate-rial than it is to the LUMO level of the AgNWs. This meansthat the PFN facilitates electron transport from the photo-active material to cathode, while also fulfilling the role ofelectron collection. The work function value of the ZnO/AgNWs (4.31 eV) is also close to that of PFN/AgNWs,indicating that PFN is just as effective as an ETL as themore commonly used ZnO.

As a reference device for comparing the performance ofPET/AgNWs/PFN film-based flexible IOSCs, a glass/ITO/PFNfilm-based IOSC was fabricated (Figure 4). Figure 4a and bshows the current density–voltage (J–V) characteristics andexternal quantum efficiency (EQE) curves of the two devicesunder AM 1.5 G illumination conditions. From the J-V char-acteristics, the device performance characteristics of JSC, VOC,FF and PCE were determined, which as shown in Table 2,revealed that the PFN/AgNWs-based IOSC has a JSC of

12.8 mA � cm�2, VOC of 0.740 V, FF of 65.3% and a PCE of6.1370.09%. The fact that this flexible device has a relativelylower JSC than the glass based device (JSC: 16.5 mA cm�2, VOC:0.753 V, FF: 66.6%, PCE: 8.2270.11%) can be explained by thedifference in the thickness of the photoactive layer. That is, aphotoactive layer �100 nm thicker than the optimal thicknessof �100 nm was used inside the flexible device to preventshort circuits created by the AgNWs. This creates a longerdiffusion path length than in the glass-based device, resultingin a comparatively low JSC (Figure S6). Similarly, a lowerquantum efficiency was observed in the PFN/AgNWs-basedIOSC over the entire spectral range, which agrees well with thetrend in Jsc. If the thickness of this photoactive layer can beoptimized in the future without unduly affecting the surfaceroughness of the AgNWs, then the efficiency can be expectedto improve by up to 8%; putting it on a par with glass/ITO/PFNfilm-based IOSCs. More important than this though, is the factthat the change in efficiency of the PET/AgNWs/PFN-based

Figure 4 (a) J–V curves of IOSCs fabricated on glass/ITO/PFN(black sphere with line) or PET/AgNWs/PFN (blue sphere withline) with PTB7:PC71BM photoactive layers under AM 1.5 Gillumination (100 mW/cm2). (b) EQE spectra of IOSCs fabricatedon glass/ITO/PFN (black sphere with line) or PET/AgNWs/PFN(blue sphere with line) with PTB7:PC71BM photoactive layers.(c) Change in efficiency with repeated cycles of bending to aradius of 4.8 mm.

Table 2 Average photovoltaic properties of IOSCsunder AM 1.5 G illumination (100 mW/cm2). Values inbrackets were obtained from the champion device.

Jsc[mA/cm2]

Voc[V]

FF[%]

PCE[%]

Glass/ITO/PFN 16.5 0.753 66.6 8.2270.05 (8.27)PET/AgNWs/PFN 12.8 0.740 65.3 6.1370.04 (6.17)

127Organic solar Cells with PFN/AgNWs cathode

IOSCs with repeated bending to a radius of RE4.8 mm(Figure 4c) shows it to have excellent bending characteristics;with a retained efficiency of �93.8% after 1000 cycles beingvery close to the initial bending result of �96.2%.

Conclusion

In conclusion, flexible IOSCs have been successfully fabri-cated and demonstrated to possess excellent stability interms of their power conversion efficiency, even at an

extreme bending radius. A PFN interlayer used to createan interfacial dipole between an AgNWs and a photoactivelayer facilitates electron extraction, allowing the AgNWs tofunction as an ETL without the need for the high tempera-ture annealing required when using ZnO. Furthermore,flexible IOSCs in which a PFN/AgNWs was applied to a lowTg PET substrate achieved a high efficiency of up to 6.17%,with only a 4% reduction in efficiency when bent to a radiusof RE5 mm or smaller. This slight loss in efficiency isattributed to a reduction in JSC due to an increaseddifficulty in carrier collection created by the destructionof the Ag film anode. Future work should therefore focus inimproving the flexibility of the metal film anode to preventsuch a reduction in JSC, which should allow for the devel-opment of inexpensive, and highly efficient flexible OSCswith excellent bending stability.

Acknowledgment

This research was supported by NRF through Basic ScienceResearch Program (2014R1A1A1004885) from the Ministry ofScience, ICT & Future Planning. It was also supported byFuture Strategic Fund (1.140013.01) of Ulsan NationalInstitute of Science and Technology (UNIST) and theresearch fund (No. 2015-PNK 4291) of the Korea Instituteof Materials Science.

Appendix A. Supplementary Information

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.nanoen.2015.06.013.

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Ji Hoon Seo received his B.S. (2010) degreefrom the department of control & instrumen-tation engineering at Pukyong National Uni-versity and his master's degree (2013) fromthe interdisciplinary program in hybrid mate-rials solution at Pusan National University. Heis currently a Ph.D. student in the departmentof energy engineering, Ulsan National Instituteof Science Technology (UNIST). His researchfocuses on fabrication of flexible/large area

organic solar cell and perovskite solar cell.

Han-Don Um received his BS (2006) inChemical Engineering and Ph. D (2013) inBio-Nano Technology from Hanayng Univer-sity. He is currently a Post-doctoralresearcher in Energy Engineering at UlsanNational Institute of Science and Technology(UNIST). His areas of expertise includenano/microstructure Si and ultra-thin crys-talline Si solar cells.

Atul Shukla is currently undergraduate studentat the Polymer Science and Technology at theIndian Institute of Technology Roorkee in India.His research has been focused on the under-standing of extremely bendable, efficient poly-mer solar cells and synthesis of divalent of themolecular double plug-socket system.

Inchan Hwang received his B.S. (2013) degreefrom the department of fine chemical engi-neering and applied chemistry at ChungnamNational University. He is currently a masterstudent in the department of energy engi-neering, Ulsan National Institute of ScienceTechnology (UNIST). His research focuses onfabrication of silicon 3D structure and flexiblesilicon application.

Yong-Cheol Kang is currently a full professor atPukyong National University, Korea. Hereceived his Ph.D. degree in Surface Physics,from University of Akron in 2002. His currentresearch interests include the surface analysisby using electron emission spectroscopy (XPS,UPS, AES), fabrication and characterization ofthin films and nanofibers.

Juyun Park received her Ph.D degree inChemistry from Pukyong National University(PKNU) in 2015. She continued at PKNU forher post-doctoral research under the super-vision of Prof. Yong-Cheol Kang. Theresearch in her laboratory mainly focuseson surface modification and investigation ofnano-materials, such as metal oxide thinfilms by rf co-sputtering and nanofibers byelectrospinning

Chang Su Kim is currently a seniorresearcher in the Korea Institute of Materi-als Science (KIMS). He received his Ph. D.degree in Department of materials scienceand engineering from Yonsei University, in2008. He was a Post-Doctoral Associate inDepartment of chemical Engineering at thePrinceton University from 2008 to 2009.Since 2010, he has been a Senior Researcherwith the Korea Institute of Materials

Science. His current research interests include flexible electronicsand thin film solar cells.

Myungkwan Song is currently a seniorresearcher in the Korea Institute of Materi-als Science (KIMS). He received his Ph. D.degree in chemistry material from PusanNantional University in 2011. His researchinterests include thin film (organic/inor-ganic) photovoltaic, transparent conductingelectrodes (TCEs) with nanowire, metalmesh and their nanostructured sensingdevices.

129Organic solar Cells with PFN/AgNWs cathode

Kwanyong Seo is currently an assistant profes-sor in the school of energy and chemicalengineering at UNIST, Korea. He received hisPh.D. degree in chemistry from KAIST, Korea in2008. He joined UNIST in 2013. His currentresearch interests mainly focus on nano/microstructure based energy devices includingnovel functional energy devices such as flex-ible solar cells, transparent solar cells, andPV-battery hybrid system.