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Highly flexible and bendable carbon nanosheets as transparent conducting electrodes for organic solar cells
Accepted Manuscript Highly flexible and bendable carbon nanosheets as transparent conducting electrodes for organic solar cells Su-Young Son, Jin-Mun Yun, Yong-Jin Noh, Sungho Lee, Hae-Na Jo, Seok-In Na, Han-Ik Joh PII: DOI: Reference:
S0008-6223(14)00954-3 http://dx.doi.org/10.1016/j.carbon.2014.09.089 CARBON 9385
To appear in:
Carbon
Received Date: Accepted Date:
7 July 2014 29 September 2014
Please cite this article as: Son, S-Y., Yun, J-M., Noh, Y-J., Lee, S., Jo, H-N., Na, S-I., Joh, H-I., Highly flexible and bendable carbon nanosheets as transparent conducting electrodes for organic solar cells, Carbon (2014), doi: http:// dx.doi.org/10.1016/j.carbon.2014.09.089
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Highly flexible and bendable carbon nanosheets as transparent conducting electrodes for organic solar cells Su-Young Son,a,b† Jin-Mun Yun,a,c† Yong-Jin Noh,a,b Sungho Lee,a Hae-Na Jo,a,b Seok-In Nab and Han-Ik Joha*
a
Carbon Convergence Materials Research Center, Institute of Advanced Composite
Materials, Korea Institute of Science and Technology, San 101, Eunha-ri, Bongdoungeup, Wanju-gun, Jeollabuk-do 565-905, Korea b
Professional Graduate School of Flexible and Printable Electronics and Polymer
Materials Fusion Research Center, Chonbuk National University, 664-14, Deokjin-dong, Deokjin-gu, Jeonju-si, Jeollabuk-do 561-756, Korea c
Radiation Research Division for Industry and Environment, Advanced Radiation
Technology Institute, Korea Atomic Energy Research Institute (KAERI), 29 Geumgugil, Jeongeup-si, Jeollabuk-do 580-185, Korea
*Corresponding author: Tel: +82-63-219-8137; Fax: +82-63-219-8269, E-mail address: [email protected] †
S.-Y. Son and J.-M. Yun contributed equally to this work.
Abstract We synthesized highly flexible carbon nanosheets (CNSs) as transparent conducting electrodes (TCEs) suitable for flexible organic solar cells (OSCs). The flexible electrodes were obtained by transferring CNSs, which were synthesized on recyclable silicon wafers using a catalyst-free process, onto a polyethylene terephthalate substrate. The TCEs exhibited a sheet resistance of ~2.5 kohm/sq. and a transmittance of 61% even after 200 bending cycles, leading to a high power conversion efficiency of approximately ~1.33%, which is very high compared to that of flexible OSCs composed of pristine graphene generated by chemical vapor deposition or chemical exfoliation methods.
1. Introduction Transparent and flexible electronic devices that can be bent, curved, rolled, and folded have been extensively studied due to their portability and ergonomic convenience. All components of these devices are required to maintain their flexibility and durability, even under harsh operating conditions. Organic solar cells (OSCs), a representative electronic device, are primarily made from flexible organic polymers, with the exception of indium tin oxide (ITO), which is used as a substrate for transparent conducting electrodes (TCEs). ITO, which possesses high transparency and low sheet resistance, has been widely used for TCEs in rigid electronic devices. However, the mechanical properties of the ITO, such as a fracture strain of ~ 1.2%, cannot satisfy the requirements for flexible devices.[1] When ITO-based flexible devices are bent or rolled, formation of microcracks, channeling, and debonding of the ITO occur due to the mechanical strains, which leads to non-conducting materials.[1,2] Furthermore, there are several drawbacks to the use of ITO, including the shortage of indium, high production costs, and poor mechanical properties.[2] To overcome these issues, considerable research has been conducted to replace ITO with flexible materials such as graphene,[3-7] conducting polymers,[8] or metal nanowires.[9-14] Among these alternatives, graphene has attracted great interest as a promising flexible material for use in transparent electrodes due to its high fracture strain of > 15%, optical transmittance of ~ 97.7%, sheet resistance of ~ 30 2
s.[15-17] ohm/sq, and electron mobility of 150,000 cm /V•s. Two representative methods for preparing graphene-based flexible TCEs are solution processing of graphene oxide (GO) and transferring graphene films by chemical vapor deposition (CVD). GO-based TCEs can be easily obtained by coating GO on a flexible substrate and performing reduction processes.[3,4] However, the severe oxidative conditions in this method result in defects on the basal plane and at the edges of the GO,
which increases the sheet resistance. TCEs composed of chemically and laser-induced GO were fabricated with sheet resistances of 3.2 kohm/sq (at 65% transparency) and 1.6 kohm/sq (at 70% transparency), respectively. Power conversion efficiencies (PCE) of OSCs made from these TCEs were 0.78 and 1.10%, respectively.[3,4] To fabricate highly conducting graphene-based TCEs, graphene was grown on metal films by CVD and then transferred onto transparent substrates. The well-ordered hexagonal structure of the graphene improved the electrical conductivity and transmittance of the TCEs due to the catalytic effects compared to the GO-based TCEs.[5-7] The sheet resistance of graphene-based TCEs was ~ 850 ohm/sq. (at ~ 90% transparency), leading to OSCs with a highly efficient performance of ~ 1.77%.[6] In previous studies, we reported a novel and facile method for the synthesis of carbon nanosheets (CNSs) with similar properties to graphene using polymeric carbon sources. These CNSs can be easily obtained by spin-coating and heat-treating the source, and their properties can be controlled via alterations in source concentration, source type, and substrate. Among these parameters, changing the substrate from quartz to Si wafers has large effects on the properties of the CNSs, as shown in Table S1.[18-21] Namely, CNSs prepared using polyacrylonitrile (PAN) and Si wafers exhibit higher electrical conductivities and transmittances compared with those prepared with pitch, polymer of intrinsic microporosity-1, and quartz. These improvements in properties are attributed to enhanced compatibility of the source and the substrate. Notably, the substrates play one of the most important roles in improving the properties of the CNSs. In this study, we synthesize novel flexible CNS-based TCEs and demonstrate their use as flexible electrodes in OSCs. Flexible TCEs were fabricated using the transfer process of CNSs, which were prepared using PAN and Si wafers as the source and substrate, respectively, as illustrated in Figure 1. The CNSs can be easily transferred from the Si wafer to polyethylene terephthalate (PET) through spin-coating of a protective layer and
sub bsequeent etcching off the Si S oxid o de lay yer wiith buffferr oxid de eetch han nt. The T e Si S waf w ferss caan theen be b reu r sed d affterr su ucceessfful traansfferrring g off th he C CNS to th he dessireed sub s straate,, ass sh how wn in Fig F guree S1. T Thee CNS C S-baseed TCE T Es on PE ET weere use u ed as a flex f xiblle elec e ctro odes in n IT TO-freee OS SCss.
Figurre 1. Scchemattic illu ustraatio on of o th he ITO I O-freee flex f xiblee O OSC C wiith the t flex xible CNS C S-baseed TCE T E.
2. Exp E perrimenttal 2.11 Syynthhesiis of C CNSSs T Thee CNS C Ss wer w re syn s ntheesizzed usiingg a PA AN prrecuursoor (Sig ( gma-A Aldrrich h) with w h an a aveeragge moolecculaar weig w ghtt (M Mw) off 1550,00000 g//mo ol. Thee 1.0, 1.55, and a d 2.0 wt% w % PAN P N sooluutionns werre preeparred in dim metthyllforrmaamide (DM MF F, Fish F her) and were w e sppin--coated oonto o cleann Sii suubsttratees witth a 10 00-nnm--SiO O2 layyer. Thhe spinn-cooateed PAN P N film f ms were w e sttabiilizeed aat 270 2 0 °C C foor 2 h in i air a andd thhen carrbonized at up u to t 11150 °C C uunder a H2/A Ar atmo a osp pherre. 2.22 Faabricationn off fleexibble CN NS-bbassed TC CEs
To transfer the synthesized CNSs onto flexible PET substrates, polymethyl methacrylate (PMMA, Sigma-Aldrich) was spin-coated onto the CNSs. Then, the SiO2 layer was etched using a buffered oxide etch (BOE) solution and rinsed several times with deionized water. Subsequently, the PMMA/CNS films were transferred onto cleaned PET substrates and then dried with N2 gas. The CNSs prepared using 1.0, 1.5, and 2.0 wt% PAN solution and transferred on the PET substrates were designated as TCE 1.0, 1.5, and 2.0, respectively. Finally, the flexible CNS-based TCE was obtained by removing the PMMA layer with acetone. ITO-coated PET (Sigma-Aldrich, 40 Ohm/sq) was used as a flexible electrode control. 2.3 Fabrication of flexible OSCs To fabricate flexible OSCs, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Clevios) was utilized as the hole transfer layer (HTL). It was spin-coated onto CNS- or ITO-coated PET substrates and then dried at 110 °C for 10 min. A solution with 50 mg of poly(3-hexylthiophene) (P3HT, Rieke Metals) and 50 mg of [6,6]-phenyl-C61 butyric acid methyl ester (PCBM, Nano-C) in 2 ml of 1,2-dichlorobenzene was spin-coated onto the resulting surface in a N2 glove box. The surface then underwent solvent-annealing for 2 h and thermal-annealing at 110 °C for 10 min. Finally, Ca/Al (20 nm/100 nm) layers were thermally deposited at 10-6 torr using a thermal evaporator system. 2.4 Characterization of CNSs and OSCs The thickness and roughness of the CNSs were measured by atomic force microscopy (AFM, Dimension 3100, Veeco) in tapping mode. The sheet resistance and transmittance of the CNSs were measured using a four-point probe (MCP-T610, Mitsubishi chemical analytech) and ultraviolet-visible spectroscopy (UV-vis, V-570, Jasco), respectively. To analyze the performance of the OSCs, current density-voltage (J-V) curves of the cells were
measured using a Keithley 1200 instrument under 100 mWcm-2 illumination with air mass (AM) 1.5 global (G) conditions.
3. Results and discussion The optical transparency of the flexible CNS-based TCEs was measured using UVVIS spectroscopy, as shown in Fig. 2(a). The transmittances of the TCEs at a wavelength of 550 nm were ~76%, ~61%, and ~50% at 1.0 wt%, 1.5 wt%, and ~ 2.0 wt% PAN solution, respectively. As shown in the inset of Fig. 2(a), the TCEs had an optical transparency in the visible wavelength region and the red letters “KIST” under the TCEs with ~ 61% transmittance were clearly identified. The sheet resistances (Rsheet) of the CNSs before and after the transfer process, indicated as CNSs and TCEs, respectively, were measured using a four-point probe, as shown in Fig. 2(b). The Rsheet of the CNSs and the TCEs were inversely proportional to the PAN content due to the increase in the thickness of the CNSs that occurred as the PAN concentration increased (Figure 2(c)).[18,19] The electrical conductivities of the CNSs and the TCEs calculated using these results were greater than 800 and 500 S/cm, respectively. The TCE prepared with 1.0 wt% PAN solution showed the highest conductivity value of ~ 950 S/cm. The conductivities of these flexible TCEs were superior to those reported for CNSs on quartz, graphene-based, and rGO-based TCEs as shown in Figure 2(d).[3,4,19-21] The conductivities of the TCEs were changed within the error range when the CNSs synthesized on the Si wafer were transferred onto the rigid and thermostable substrate and followed by annealing due to the improvement in the contact property and removal of the attached surface functional group during the transfer process (Figures S2 and S3).
Fig guree 2.. (aa) Tran T nsm mittaancee ass a fun nctiion of PA AN con nceentraatio on. The in nsett sh how ws the t fleexib ble CN NS-b baseed TC T Es witth the t difffereent PA AN con c ncen ntraation ns. (b)), (cc) Chan C ngees in n Rsheet and th hick kneess of CN NSss beeforre and a d affterr traanssfer.. (d d) Com C mpaarisson off electrricaal and a op pticcal pro operrties for fo graapheene(rG GO/C CV VD grap g pheene)) an nd CNS C S reeporrted d in thee litteratturee.
Li et al.. reeported d thatt so omee smaall air gaaps rem maain bettweeen n grraph hen ne and d th he dessireed sub bstrratee surffacee aafteer the t p cesss, lleadin ng to po oor co ontaact beetw ween n thes t se traanssferr proc lay yerss.[2 22] To o prreveent thiis o occcurrrencce, theey sug ggeest that t t an n ad ddiitio onall cu urin ng pro p ocesss be b utillizeed to t mec m chaaniccallly rela r ax tthe un nderrlyiing graph hen ne film f ms aand d alllow w fo or bett b ter con ntaact bettweeen thee grrap phen ne and a d th he sub s straate.. F Fig guree 3 sho ows th he surf s face mor m rpho olo ogy and ro oug ghn nesss off th he Si S w waffer, PE ET, CN NSss (o on Si waaferr), and a TC CEss (o on PET P T) m measu ured d by y atom a micc forrcee miicro osccopy y (A AF FM)). The T CN NSss
Fig guree 3. AF FM M im mages ((1 X 1 ㎛) of o CNS C Ss and a d TC CEss ass fu uncttion ns of o subs s straate ttypee an nd PA AN con ncen ntraatio on: (a) ( barre S Si wafe w er, (b) ( CN NS 1.0, (cc) CNS C S 1.5, (d) ( CN NS 2.0, 2 , (e) baare PE ET, (f) TC CE 1.0 0, (g g) TCE T E 1.5 5, and a (h)) TC CE 2.0
and dT TCE Es were fully cov vered wiith a unif u form m mo morph holo ogy y an nd roo ot-m mean--squ uarre (rm ( ms) rou ugh hnesss valu v uess off ~0 0.39 and a d ~1 1.28 8 nm, n respeectiivelly. Th he TCE T E ssurffacees wer w re rou r ugheer thaan thos t se of o CN CNSs on n Si waaferrs due d e to o thee high h h rm ms rou r ughnesss valu v ue of o PET P T (~2..71 nm m) relativ ve to t tha t t off Si wafe w ers (~0 0.43 nm) n ). The T rough h su urfa facee off PE ET ind ducces po oor con ntaact CNSs,, reesullting in n lo oweer con c nducctiv vity y off TC CE Es on o PET P T co om mparred d wiith thaat of o CNS C Ss witth CN on Sii wafe w ers. How weveer, consiiderring g th he hig gh rep portted d rm ms rou ugh hness valluess of o rGO r Od TC CEss, C CNS S-b baseed TC TCEs haave relativ vely y sm mo ooth h su urfaaces.[3 3,4]] bassed T To ev valu uatee th he pro p perrtiees of o th he TC CEss, fllexiblee OSC O Cs were fab briccateed usin u ng polly(3 3hex xylthio oph hen ne): [6,6]--ph heny yl-C C61 b buty yricc accid meethy yl este e er (P3 ( HT T:PC CB BM)) ass an n acctiv ve lay yer, ass sh how wn in Fig guree 1. Figu F uress 4 (aa)-(d d) disp d plaay the t cu urreent den nsitty-v voltag ge ((J-V V) chaaraccterristticss off TC CE-baased d OCS O Ss rela r ativ ve to t ITO O-baaseed OSC O C cont c trolls mea m asu ured d att 10 00 mW Wcm m-2 illu um minaatio on und u der AM M 1.5 1 G con c ndittion ns. Th he corr c resp pon ndin ng dev vicee paraameeterrs, incclud ding g th he opeen-circcuiit voltaagee (V V), sho ort--cirrcuit curr c ren nt denssity y (JJsc), filll faacto or ((FF F), and d PCE P E, arre sum s mm marizzed d in n Taablee 1. As A sh how wn in Fig guree 4 an nd Tab T ble 1, the t Voc fo or TCE T E
Fig guree 4. Efffectts o of beend ding g cy yclees on J--V chaaraccteriisticcs o of flexi fl iblee OSCs: (a) TCE T E 1.0, (b) TC CE 1.5 5, (cc) TCE T E 2.0, and a d (d)) IT TO ano odes. T The inssetss in (a)) sh how w thee beend ding g ex xperrim mentt of fleexib ble OS SCs.. (f)) Ch han ngess in Rshheet of o TCE T Es befo b ore and a d aft fter ben ndin ng. Thee in nsett in (f) sho owss thee beent TC CE.
5 an nd 2.0 2 weere app pro oxim mattely y id dentticaal valu v ues (~0 0.5 54 V), V and d th hey y diiffered d fro om m thee Voc 1.5 forr TCE T E 1.0 1 (0 0.49 9 V V). Conssideerin ng thaat all deevicess wer w e ccom mpo oseed of thee sam s me pho oto oacttivee laayerr, P3H P HT:PC CBM M, thiss deviiatio on can nb be attri a ibu uted d to o th he roug ghn nesss of th he TC CEss beecaausee th he Vocc iss dete d erm mineed by b thee d difffereencee betw b weeen thee hiigh hestt occcu upieed mo olecculaar orb o bital (H HO OMO O) off th he d don nor an nd the t loweest uno occcupied d mole m ecu ular orrbittal (LU UM MO)) off th he acccepttor.[23 3] T Thiis phe p enomeenon n is occa o asio onaally ob bserrveed in i tran t nsfeerreed graaph henee elec e ctro odes o on fleexib ble su ubsttrattes, w with h th hinn nerr CNS C S fiilm ms bei b ng prrim marilly afffectted..[24 4] K Kim m eet al. a hav h ve rece r entlly rep r orted thaat th he num mb ber of graaph henee laayeers useed as tran nsp pareent co ondu ucting g electtrod de in i OSC O Cs hav ve an infflueence on o the t Vocc (0 0.48 89 ~ 0.53 0 35 5]. In parrticculaar, the t e OS SC fab briccateed ontto m mon nollayeer grap g pheenee eleectrrod de exh e ibitted da V) [25 or Vocc off 0.48 0 89 V, wh hicch is i cau c used d b by its hig gh sh heett reesisstan nce an nd forrmaatio on of o poo miccro ocraack ks or piinh holees duri d ing g traansfferrring g grap g phene ontto a flexiiblee su ubsttratte. The T ereforre, a sligh htly y decrreassed d Voco of TCE T E 1..0-b based celll with w h th he thin nnesst ffilm m am mong alll thee CNS C Ss
might be attributed to a high sheet resistance and formation of microcracks/pinholes as well as high surface roughness of TCE 1.0. The FF values in devices fabricated with TCEs increased as the PAN concentration increased. Changes in series and shunt resistance (Figure S4) of the flexible TCEs in the J-V curve are similar to those of quartz based TCEs as reported in previous study.19 Number of bending cycles
Electrode
0 100 200 0 100 200 0 100 200 0 100 200
TCE 1.0
TCE 1.5
TCE 2.0
ITO
Voc (V) 0.49 0.48 0.49 0.54 0.54 0.54 0.53 0.53 0.53 0.57 0.38 0.35
Jsc 2
(mA/cm ) 6.02 5.99 5.85 4.76 4.73 4.71 4.17 4.16 4.11 8.96 0.007 0.008
FF
PCE
(%) 33.26 33.37 33.80 52.07 52.20 52.58 54.02 54.11 54.30 65.92 66.18 53.83
(%) 0.97 0.97 0.97 1.33 1.32 1.33 1.19 1.19 1.18 3.39 0.002 0.002
Table 1. Summary of photovoltaic parameters of ITO-free flexible OSCs with CNS-based TCEs and a control ITO-based flexible OSC following exposure to different bending cycles.
Considering the resistances and FF values, the improvement of FF from ~33% for TCE 1.0 to ~54% for TCE 2.0 can be attributed to a reduction in the Rsheet as the CNS thickness was increased. However, the Jsc of the OSC using TCE 2.0 showed the lowest value of 2
4.17 mA/cm , despite TCE 2.0 having the highest FF value among the TCEs. Jsc is inversely proportional to the film thickness of the electrode because an electrode with higher transmittance can help to produce more excitions and carriers than an electrode with lower transmittance.[19] Thus, optimization between FF (or the Rsheet of the electrode) and Jsc is inevitably required to obtain high photovoltaic performance. Considering these relationships, OSCs produced with TCE 1.5 as the anode had the best performance characteristics with a Voc of 0.54 V, Jsc of 4.76 mAcm-2, FF of 52.07%, and
PCE of 1.33%, which is the highest value compared to OSCs using only pristine graphene on flexible PET substrates by chemical vapor deposition (1.18 – 1.77%) or chemical exfoliation (0.78 – 1.10%) methods (Table S2).[3-6] To investigate the application potential of the CNS-derived TCEs as a flexible device, we measured the J-V characteristics of TCE- and ITO-based OSCs as a reference after bending cycles. The PCEs of all TCE-based OSCs were almost identical, even after 200 bending cycles, due to the Rsheet remaining unchanged before and after the cycles, whereas the ITObased OSC exhibited a PCE of ~ 0% after less than 100 bending cycles even though the asfabricated OSCs exhibit a high efficiency of ~ 3.39% (Figure 4(e) and (f)). It has previously been reported that the poor flexible performance of ITO-based OCSs can be attributed to increases in Rsheet that are caused by severe crack and defect formation in the ITO electrode during bending, which eventually results in device failure.[8] Hence, considering the flexibility and stability of the TCEs derived from the CNSs, the TCEs are highly practical for flexible and bendable OSCs, and this novel approach will lead to the widespread commercialization of OSCs.
4. Conclusions In conclusion, we have demonstrated highly flexible and bendable TCEs obtained by transferring CNSs that were synthesized on silicon wafers through a catalyst-free process onto a PET substrate. TCE 1.5 exhibited a sheet resistance of ~2.5 kohm/sq. and a transmittance of 61% even after 200 bending cycles, leading to a high power conversion efficiency of approximately ~1.33% although the CNSs were laid on PET with a highly rough surface. The identical properties, such as the conductivity and morphology, of the TCEs before and after the bending cycles reflected the stable performances of the TCE-based OSCs under flexing and bending conditions. Therefore, they are likely to be potential
materials as flexible TCEs for organic devices, although the TCEs should be modified to improve the PCE to the level of ITO-based OSCs.
Acknowledgement The authors acknowledge the financial supports of this work from the Korea Institute of Science and Technology, Republic of Korea.
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S0008-6223(14)00954-3 http://dx.doi.org/10.1016/j.carbon.2014.09.089 CARBON 9385
To appear in:
Carbon
Received Date: Accepted Date:
7 July 2014 29 September 2014
Please cite this article as: Son, S-Y., Yun, J-M., Noh, Y-J., Lee, S., Jo, H-N., Na, S-I., Joh, H-I., Highly flexible and bendable carbon nanosheets as transparent conducting electrodes for organic solar cells, Carbon (2014), doi: http:// dx.doi.org/10.1016/j.carbon.2014.09.089
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highly flexible and bendable carbon nanosheets as transparent conducting electrodes for organic solar cells Su-Young Son,a,b† Jin-Mun Yun,a,c† Yong-Jin Noh,a,b Sungho Lee,a Hae-Na Jo,a,b Seok-In Nab and Han-Ik Joha*
a
Carbon Convergence Materials Research Center, Institute of Advanced Composite
Materials, Korea Institute of Science and Technology, San 101, Eunha-ri, Bongdoungeup, Wanju-gun, Jeollabuk-do 565-905, Korea b
Professional Graduate School of Flexible and Printable Electronics and Polymer
Materials Fusion Research Center, Chonbuk National University, 664-14, Deokjin-dong, Deokjin-gu, Jeonju-si, Jeollabuk-do 561-756, Korea c
Radiation Research Division for Industry and Environment, Advanced Radiation
Technology Institute, Korea Atomic Energy Research Institute (KAERI), 29 Geumgugil, Jeongeup-si, Jeollabuk-do 580-185, Korea
*Corresponding author: Tel: +82-63-219-8137; Fax: +82-63-219-8269, E-mail address: [email protected] †
S.-Y. Son and J.-M. Yun contributed equally to this work.
Abstract We synthesized highly flexible carbon nanosheets (CNSs) as transparent conducting electrodes (TCEs) suitable for flexible organic solar cells (OSCs). The flexible electrodes were obtained by transferring CNSs, which were synthesized on recyclable silicon wafers using a catalyst-free process, onto a polyethylene terephthalate substrate. The TCEs exhibited a sheet resistance of ~2.5 kohm/sq. and a transmittance of 61% even after 200 bending cycles, leading to a high power conversion efficiency of approximately ~1.33%, which is very high compared to that of flexible OSCs composed of pristine graphene generated by chemical vapor deposition or chemical exfoliation methods.
1. Introduction Transparent and flexible electronic devices that can be bent, curved, rolled, and folded have been extensively studied due to their portability and ergonomic convenience. All components of these devices are required to maintain their flexibility and durability, even under harsh operating conditions. Organic solar cells (OSCs), a representative electronic device, are primarily made from flexible organic polymers, with the exception of indium tin oxide (ITO), which is used as a substrate for transparent conducting electrodes (TCEs). ITO, which possesses high transparency and low sheet resistance, has been widely used for TCEs in rigid electronic devices. However, the mechanical properties of the ITO, such as a fracture strain of ~ 1.2%, cannot satisfy the requirements for flexible devices.[1] When ITO-based flexible devices are bent or rolled, formation of microcracks, channeling, and debonding of the ITO occur due to the mechanical strains, which leads to non-conducting materials.[1,2] Furthermore, there are several drawbacks to the use of ITO, including the shortage of indium, high production costs, and poor mechanical properties.[2] To overcome these issues, considerable research has been conducted to replace ITO with flexible materials such as graphene,[3-7] conducting polymers,[8] or metal nanowires.[9-14] Among these alternatives, graphene has attracted great interest as a promising flexible material for use in transparent electrodes due to its high fracture strain of > 15%, optical transmittance of ~ 97.7%, sheet resistance of ~ 30 2
s.[15-17] ohm/sq, and electron mobility of 150,000 cm /V•s. Two representative methods for preparing graphene-based flexible TCEs are solution processing of graphene oxide (GO) and transferring graphene films by chemical vapor deposition (CVD). GO-based TCEs can be easily obtained by coating GO on a flexible substrate and performing reduction processes.[3,4] However, the severe oxidative conditions in this method result in defects on the basal plane and at the edges of the GO,
which increases the sheet resistance. TCEs composed of chemically and laser-induced GO were fabricated with sheet resistances of 3.2 kohm/sq (at 65% transparency) and 1.6 kohm/sq (at 70% transparency), respectively. Power conversion efficiencies (PCE) of OSCs made from these TCEs were 0.78 and 1.10%, respectively.[3,4] To fabricate highly conducting graphene-based TCEs, graphene was grown on metal films by CVD and then transferred onto transparent substrates. The well-ordered hexagonal structure of the graphene improved the electrical conductivity and transmittance of the TCEs due to the catalytic effects compared to the GO-based TCEs.[5-7] The sheet resistance of graphene-based TCEs was ~ 850 ohm/sq. (at ~ 90% transparency), leading to OSCs with a highly efficient performance of ~ 1.77%.[6] In previous studies, we reported a novel and facile method for the synthesis of carbon nanosheets (CNSs) with similar properties to graphene using polymeric carbon sources. These CNSs can be easily obtained by spin-coating and heat-treating the source, and their properties can be controlled via alterations in source concentration, source type, and substrate. Among these parameters, changing the substrate from quartz to Si wafers has large effects on the properties of the CNSs, as shown in Table S1.[18-21] Namely, CNSs prepared using polyacrylonitrile (PAN) and Si wafers exhibit higher electrical conductivities and transmittances compared with those prepared with pitch, polymer of intrinsic microporosity-1, and quartz. These improvements in properties are attributed to enhanced compatibility of the source and the substrate. Notably, the substrates play one of the most important roles in improving the properties of the CNSs. In this study, we synthesize novel flexible CNS-based TCEs and demonstrate their use as flexible electrodes in OSCs. Flexible TCEs were fabricated using the transfer process of CNSs, which were prepared using PAN and Si wafers as the source and substrate, respectively, as illustrated in Figure 1. The CNSs can be easily transferred from the Si wafer to polyethylene terephthalate (PET) through spin-coating of a protective layer and
sub bsequeent etcching off the Si S oxid o de lay yer wiith buffferr oxid de eetch han nt. The T e Si S waf w ferss caan theen be b reu r sed d affterr su ucceessfful traansfferrring g off th he C CNS to th he dessireed sub s straate,, ass sh how wn in Fig F guree S1. T Thee CNS C S-baseed TCE T Es on PE ET weere use u ed as a flex f xiblle elec e ctro odes in n IT TO-freee OS SCss.
Figurre 1. Scchemattic illu ustraatio on of o th he ITO I O-freee flex f xiblee O OSC C wiith the t flex xible CNS C S-baseed TCE T E.
2. Exp E perrimenttal 2.11 Syynthhesiis of C CNSSs T Thee CNS C Ss wer w re syn s ntheesizzed usiingg a PA AN prrecuursoor (Sig ( gma-A Aldrrich h) with w h an a aveeragge moolecculaar weig w ghtt (M Mw) off 1550,00000 g//mo ol. Thee 1.0, 1.55, and a d 2.0 wt% w % PAN P N sooluutionns werre preeparred in dim metthyllforrmaamide (DM MF F, Fish F her) and were w e sppin--coated oonto o cleann Sii suubsttratees witth a 10 00-nnm--SiO O2 layyer. Thhe spinn-cooateed PAN P N film f ms were w e sttabiilizeed aat 270 2 0 °C C foor 2 h in i air a andd thhen carrbonized at up u to t 11150 °C C uunder a H2/A Ar atmo a osp pherre. 2.22 Faabricationn off fleexibble CN NS-bbassed TC CEs
To transfer the synthesized CNSs onto flexible PET substrates, polymethyl methacrylate (PMMA, Sigma-Aldrich) was spin-coated onto the CNSs. Then, the SiO2 layer was etched using a buffered oxide etch (BOE) solution and rinsed several times with deionized water. Subsequently, the PMMA/CNS films were transferred onto cleaned PET substrates and then dried with N2 gas. The CNSs prepared using 1.0, 1.5, and 2.0 wt% PAN solution and transferred on the PET substrates were designated as TCE 1.0, 1.5, and 2.0, respectively. Finally, the flexible CNS-based TCE was obtained by removing the PMMA layer with acetone. ITO-coated PET (Sigma-Aldrich, 40 Ohm/sq) was used as a flexible electrode control. 2.3 Fabrication of flexible OSCs To fabricate flexible OSCs, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Clevios) was utilized as the hole transfer layer (HTL). It was spin-coated onto CNS- or ITO-coated PET substrates and then dried at 110 °C for 10 min. A solution with 50 mg of poly(3-hexylthiophene) (P3HT, Rieke Metals) and 50 mg of [6,6]-phenyl-C61 butyric acid methyl ester (PCBM, Nano-C) in 2 ml of 1,2-dichlorobenzene was spin-coated onto the resulting surface in a N2 glove box. The surface then underwent solvent-annealing for 2 h and thermal-annealing at 110 °C for 10 min. Finally, Ca/Al (20 nm/100 nm) layers were thermally deposited at 10-6 torr using a thermal evaporator system. 2.4 Characterization of CNSs and OSCs The thickness and roughness of the CNSs were measured by atomic force microscopy (AFM, Dimension 3100, Veeco) in tapping mode. The sheet resistance and transmittance of the CNSs were measured using a four-point probe (MCP-T610, Mitsubishi chemical analytech) and ultraviolet-visible spectroscopy (UV-vis, V-570, Jasco), respectively. To analyze the performance of the OSCs, current density-voltage (J-V) curves of the cells were
measured using a Keithley 1200 instrument under 100 mWcm-2 illumination with air mass (AM) 1.5 global (G) conditions.
3. Results and discussion The optical transparency of the flexible CNS-based TCEs was measured using UVVIS spectroscopy, as shown in Fig. 2(a). The transmittances of the TCEs at a wavelength of 550 nm were ~76%, ~61%, and ~50% at 1.0 wt%, 1.5 wt%, and ~ 2.0 wt% PAN solution, respectively. As shown in the inset of Fig. 2(a), the TCEs had an optical transparency in the visible wavelength region and the red letters “KIST” under the TCEs with ~ 61% transmittance were clearly identified. The sheet resistances (Rsheet) of the CNSs before and after the transfer process, indicated as CNSs and TCEs, respectively, were measured using a four-point probe, as shown in Fig. 2(b). The Rsheet of the CNSs and the TCEs were inversely proportional to the PAN content due to the increase in the thickness of the CNSs that occurred as the PAN concentration increased (Figure 2(c)).[18,19] The electrical conductivities of the CNSs and the TCEs calculated using these results were greater than 800 and 500 S/cm, respectively. The TCE prepared with 1.0 wt% PAN solution showed the highest conductivity value of ~ 950 S/cm. The conductivities of these flexible TCEs were superior to those reported for CNSs on quartz, graphene-based, and rGO-based TCEs as shown in Figure 2(d).[3,4,19-21] The conductivities of the TCEs were changed within the error range when the CNSs synthesized on the Si wafer were transferred onto the rigid and thermostable substrate and followed by annealing due to the improvement in the contact property and removal of the attached surface functional group during the transfer process (Figures S2 and S3).
Fig guree 2.. (aa) Tran T nsm mittaancee ass a fun nctiion of PA AN con nceentraatio on. The in nsett sh how ws the t fleexib ble CN NS-b baseed TC T Es witth the t difffereent PA AN con c ncen ntraation ns. (b)), (cc) Chan C ngees in n Rsheet and th hick kneess of CN NSss beeforre and a d affterr traanssfer.. (d d) Com C mpaarisson off electrricaal and a op pticcal pro operrties for fo graapheene(rG GO/C CV VD grap g pheene)) an nd CNS C S reeporrted d in thee litteratturee.
Li et al.. reeported d thatt so omee smaall air gaaps rem maain bettweeen n grraph hen ne and d th he dessireed sub bstrratee surffacee aafteer the t p cesss, lleadin ng to po oor co ontaact beetw ween n thes t se traanssferr proc lay yerss.[2 22] To o prreveent thiis o occcurrrencce, theey sug ggeest that t t an n ad ddiitio onall cu urin ng pro p ocesss be b utillizeed to t mec m chaaniccallly rela r ax tthe un nderrlyiing graph hen ne film f ms aand d alllow w fo or bett b ter con ntaact bettweeen thee grrap phen ne and a d th he sub s straate.. F Fig guree 3 sho ows th he surf s face mor m rpho olo ogy and ro oug ghn nesss off th he Si S w waffer, PE ET, CN NSss (o on Si waaferr), and a TC CEss (o on PET P T) m measu ured d by y atom a micc forrcee miicro osccopy y (A AF FM)). The T CN NSss
Fig guree 3. AF FM M im mages ((1 X 1 ㎛) of o CNS C Ss and a d TC CEss ass fu uncttion ns of o subs s straate ttypee an nd PA AN con ncen ntraatio on: (a) ( barre S Si wafe w er, (b) ( CN NS 1.0, (cc) CNS C S 1.5, (d) ( CN NS 2.0, 2 , (e) baare PE ET, (f) TC CE 1.0 0, (g g) TCE T E 1.5 5, and a (h)) TC CE 2.0
and dT TCE Es were fully cov vered wiith a unif u form m mo morph holo ogy y an nd roo ot-m mean--squ uarre (rm ( ms) rou ugh hnesss valu v uess off ~0 0.39 and a d ~1 1.28 8 nm, n respeectiivelly. Th he TCE T E ssurffacees wer w re rou r ugheer thaan thos t se of o CN CNSs on n Si waaferrs due d e to o thee high h h rm ms rou r ughnesss valu v ue of o PET P T (~2..71 nm m) relativ ve to t tha t t off Si wafe w ers (~0 0.43 nm) n ). The T rough h su urfa facee off PE ET ind ducces po oor con ntaact CNSs,, reesullting in n lo oweer con c nducctiv vity y off TC CE Es on o PET P T co om mparred d wiith thaat of o CNS C Ss witth CN on Sii wafe w ers. How weveer, consiiderring g th he hig gh rep portted d rm ms rou ugh hness valluess of o rGO r Od TC CEss, C CNS S-b baseed TC TCEs haave relativ vely y sm mo ooth h su urfaaces.[3 3,4]] bassed T To ev valu uatee th he pro p perrtiees of o th he TC CEss, fllexiblee OSC O Cs were fab briccateed usin u ng polly(3 3hex xylthio oph hen ne): [6,6]--ph heny yl-C C61 b buty yricc accid meethy yl este e er (P3 ( HT T:PC CB BM)) ass an n acctiv ve lay yer, ass sh how wn in Fig guree 1. Figu F uress 4 (aa)-(d d) disp d plaay the t cu urreent den nsitty-v voltag ge ((J-V V) chaaraccterristticss off TC CE-baased d OCS O Ss rela r ativ ve to t ITO O-baaseed OSC O C cont c trolls mea m asu ured d att 10 00 mW Wcm m-2 illu um minaatio on und u der AM M 1.5 1 G con c ndittion ns. Th he corr c resp pon ndin ng dev vicee paraameeterrs, incclud ding g th he opeen-circcuiit voltaagee (V V), sho ort--cirrcuit curr c ren nt denssity y (JJsc), filll faacto or ((FF F), and d PCE P E, arre sum s mm marizzed d in n Taablee 1. As A sh how wn in Fig guree 4 an nd Tab T ble 1, the t Voc fo or TCE T E
Fig guree 4. Efffectts o of beend ding g cy yclees on J--V chaaraccteriisticcs o of flexi fl iblee OSCs: (a) TCE T E 1.0, (b) TC CE 1.5 5, (cc) TCE T E 2.0, and a d (d)) IT TO ano odes. T The inssetss in (a)) sh how w thee beend ding g ex xperrim mentt of fleexib ble OS SCs.. (f)) Ch han ngess in Rshheet of o TCE T Es befo b ore and a d aft fter ben ndin ng. Thee in nsett in (f) sho owss thee beent TC CE.
5 an nd 2.0 2 weere app pro oxim mattely y id dentticaal valu v ues (~0 0.5 54 V), V and d th hey y diiffered d fro om m thee Voc 1.5 forr TCE T E 1.0 1 (0 0.49 9 V V). Conssideerin ng thaat all deevicess wer w e ccom mpo oseed of thee sam s me pho oto oacttivee laayerr, P3H P HT:PC CBM M, thiss deviiatio on can nb be attri a ibu uted d to o th he roug ghn nesss of th he TC CEss beecaausee th he Vocc iss dete d erm mineed by b thee d difffereencee betw b weeen thee hiigh hestt occcu upieed mo olecculaar orb o bital (H HO OMO O) off th he d don nor an nd the t loweest uno occcupied d mole m ecu ular orrbittal (LU UM MO)) off th he acccepttor.[23 3] T Thiis phe p enomeenon n is occa o asio onaally ob bserrveed in i tran t nsfeerreed graaph henee elec e ctro odes o on fleexib ble su ubsttrattes, w with h th hinn nerr CNS C S fiilm ms bei b ng prrim marilly afffectted..[24 4] K Kim m eet al. a hav h ve rece r entlly rep r orted thaat th he num mb ber of graaph henee laayeers useed as tran nsp pareent co ondu ucting g electtrod de in i OSC O Cs hav ve an infflueence on o the t Vocc (0 0.48 89 ~ 0.53 0 35 5]. In parrticculaar, the t e OS SC fab briccateed ontto m mon nollayeer grap g pheenee eleectrrod de exh e ibitted da V) [25 or Vocc off 0.48 0 89 V, wh hicch is i cau c used d b by its hig gh sh heett reesisstan nce an nd forrmaatio on of o poo miccro ocraack ks or piinh holees duri d ing g traansfferrring g grap g phene ontto a flexiiblee su ubsttratte. The T ereforre, a sligh htly y decrreassed d Voco of TCE T E 1..0-b based celll with w h th he thin nnesst ffilm m am mong alll thee CNS C Ss
might be attributed to a high sheet resistance and formation of microcracks/pinholes as well as high surface roughness of TCE 1.0. The FF values in devices fabricated with TCEs increased as the PAN concentration increased. Changes in series and shunt resistance (Figure S4) of the flexible TCEs in the J-V curve are similar to those of quartz based TCEs as reported in previous study.19 Number of bending cycles
Electrode
0 100 200 0 100 200 0 100 200 0 100 200
TCE 1.0
TCE 1.5
TCE 2.0
ITO
Voc (V) 0.49 0.48 0.49 0.54 0.54 0.54 0.53 0.53 0.53 0.57 0.38 0.35
Jsc 2
(mA/cm ) 6.02 5.99 5.85 4.76 4.73 4.71 4.17 4.16 4.11 8.96 0.007 0.008
FF
PCE
(%) 33.26 33.37 33.80 52.07 52.20 52.58 54.02 54.11 54.30 65.92 66.18 53.83
(%) 0.97 0.97 0.97 1.33 1.32 1.33 1.19 1.19 1.18 3.39 0.002 0.002
Table 1. Summary of photovoltaic parameters of ITO-free flexible OSCs with CNS-based TCEs and a control ITO-based flexible OSC following exposure to different bending cycles.
Considering the resistances and FF values, the improvement of FF from ~33% for TCE 1.0 to ~54% for TCE 2.0 can be attributed to a reduction in the Rsheet as the CNS thickness was increased. However, the Jsc of the OSC using TCE 2.0 showed the lowest value of 2
4.17 mA/cm , despite TCE 2.0 having the highest FF value among the TCEs. Jsc is inversely proportional to the film thickness of the electrode because an electrode with higher transmittance can help to produce more excitions and carriers than an electrode with lower transmittance.[19] Thus, optimization between FF (or the Rsheet of the electrode) and Jsc is inevitably required to obtain high photovoltaic performance. Considering these relationships, OSCs produced with TCE 1.5 as the anode had the best performance characteristics with a Voc of 0.54 V, Jsc of 4.76 mAcm-2, FF of 52.07%, and
PCE of 1.33%, which is the highest value compared to OSCs using only pristine graphene on flexible PET substrates by chemical vapor deposition (1.18 – 1.77%) or chemical exfoliation (0.78 – 1.10%) methods (Table S2).[3-6] To investigate the application potential of the CNS-derived TCEs as a flexible device, we measured the J-V characteristics of TCE- and ITO-based OSCs as a reference after bending cycles. The PCEs of all TCE-based OSCs were almost identical, even after 200 bending cycles, due to the Rsheet remaining unchanged before and after the cycles, whereas the ITObased OSC exhibited a PCE of ~ 0% after less than 100 bending cycles even though the asfabricated OSCs exhibit a high efficiency of ~ 3.39% (Figure 4(e) and (f)). It has previously been reported that the poor flexible performance of ITO-based OCSs can be attributed to increases in Rsheet that are caused by severe crack and defect formation in the ITO electrode during bending, which eventually results in device failure.[8] Hence, considering the flexibility and stability of the TCEs derived from the CNSs, the TCEs are highly practical for flexible and bendable OSCs, and this novel approach will lead to the widespread commercialization of OSCs.
4. Conclusions In conclusion, we have demonstrated highly flexible and bendable TCEs obtained by transferring CNSs that were synthesized on silicon wafers through a catalyst-free process onto a PET substrate. TCE 1.5 exhibited a sheet resistance of ~2.5 kohm/sq. and a transmittance of 61% even after 200 bending cycles, leading to a high power conversion efficiency of approximately ~1.33% although the CNSs were laid on PET with a highly rough surface. The identical properties, such as the conductivity and morphology, of the TCEs before and after the bending cycles reflected the stable performances of the TCE-based OSCs under flexing and bending conditions. Therefore, they are likely to be potential
materials as flexible TCEs for organic devices, although the TCEs should be modified to improve the PCE to the level of ITO-based OSCs.
Acknowledgement The authors acknowledge the financial supports of this work from the Korea Institute of Science and Technology, Republic of Korea.
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