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Inverted Type Polymer Solar Cells with Self-Assembled MonolayerTreated ZnO

Ye Eun Ha, Mi Young Jo, Juyun Park, Yong-Cheol Kang, Seong Il Yoo, and Joo Hyun Kim*,

Department of Polymer Engineering and Department of Chemistry, Pukyong National University, Busan 608-739, Korea

ABSTRACT: The work function and surface property of ZnO can besimply tuned by the self-assembled monolayer (SAM) molecules derivedfrom benzoic acid such as 4-methoxybenzoic acid (MBA), 4-tert-

butylbenzoic acid (BBA), and 4-uorobenzoic acid (FBA), which havedifferent dipole orientation and magnitude. MBA, BBA, and FBA treatedZnO layers were used as an electron injection/transporting layer forinverted type polymer solar cells (PSCs) with a structure of ITO/SAMtreated ZnO/active layer (P3HT:PC61BM)/MoO3/Ag. The powerconversion efficiency (PCE) of PSCs based on MBA and BBA treatedZnO reaches 3.34 and 2.94%, respectively, while the PCE of the device

based on untreated ZnO is 2.47%. In contrary, the PCE of the devicewith FBA treated ZnO is 1.81%. The open circuit voltage (Voc) of thedevice with MBA, BBA, and FBA treated ZnO is 0.63 and 0.62 V,respectively, while the Voc of PSC with untreated ZnO is 0.60 V.Contrarily, theVocof the device with FBA treated ZnO is 0.53 V. The PCE and Vocof PSCs based on MBA and BBA treatedZnO are better than those of the other devices. This seems to be related with the direction of dipole moment of benzoic acidderivatives. Also, the morphology of the active layer seems to be affected by the substituent on the 4-position of benzoic acid.The active layer on MBA treated ZnO shows optimized morphology, and its device shows the best performances. Wedemonstrate that the work function and morphology of the active layer can be controlled by SAM treatment of the ZnO surface

with different dipole orientation and a substituent on the 4-position of benzoic acid. These are very simple and e ffective methodsfor improving the performances of PSCs. The results provide an alternative strategy to improve the interface property betweeninorganic and organic materials in organic electronic devices.

1. INTRODUCTION

Polymer solar cells (PSCs) based on -conjugated polymers areconsidered as an energy source because they can be fabricated

by a cost-effective, large area printing and coating process onexible substrates.14 The bulk heterojunction (BHJ) solar cells

based on -conjugated polymer and fullerene derivative blendlayers sandwiched between a transparent conducting electrodeand a low work function metal electrode are effective structuresof polymer solar cells.57 In the past few years, tremendousresults have been reported to improve the performances ofPSCs by the development of new materials811 andoptimization of morphologies by processing methods.1215

The photoinduced charge separation, transporting, andcollection properties are very important factors for inuencingthe performances of PSCs. Thus, the interfacial properties

between the active layer and the cathode or anode are a crucialfactor for governing performances as well16 because seriesresistance (Rs) of PSCs is an important parameter forperformances of PSCs and determined by the electricalresistivity of each layer and the contact resistance betweenlayers. The charge collections from the active layer to eachelectrode are one of the fundamental steps, which are stronglyrelated with the contact resistance. A thin layer of poly(3,4-ethylenedioxylenethiophene):poly(styrenesulfonic acid) (PE-DOT:PSS)17 on ITO, cross-linkable arylamine derivatives1822

onITO, and self-assembled monolayers (SAMs) modied onITO23,24 are mainly used for improving the interface properties

between ITO and the active layer. As for the cathode,introducing a thin layer of LiF,2527 poly(ethyleneoxide),28

water-soluble -conjugated polymers,2931 alcohol-solubleneutral conjugated polymers,32 and water-soluble nonconju-gated polyelectrolyte based on viologen33 between the activelayer and metal cathode has been used for decreasing the workfunction of the cathode. These improve the device perform-ances through the formation of a favorable interface dipoleacross the junction. In addition, TiOx

3436 and ZnO3743

prepared by the solgel process between the active layer andmetal cathode have been used for improving the deviceperformance. The surfaceproperty of a thin lm of ZnO can beeasily tuned by SAMs.44 By using this advantage, the contactproperties between ZnO/cathode and ZnO/active layer can bemodied by SAMs. The direction of the interface dipole

between ZnO/metal can be modied by the conjugatedcarboxylic acid derivatives.12 To control contact properties

between ZnO and the TiO2/active layer in inverted type PSC(ITO/ZnO/active/MoO3/Ag), mixed SAMs with a different

Received: November 10, 2012Revised: January 2, 2013Published: January 21, 2013

Article

pubs.acs.org/JPCC

2013 American Chemical Society 2646 dx.doi.org/10.1021/jp311148d|J. Phys. Chem. C2013, 117, 26462652
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molecular species were applied to the ZnO surface;45 carboxylicacid derived from a fullerene SAM was applied to the ZnOsurface;24 and various materials were applied to ZnO or TiO 2surfaces.46,47 The optimum surface energy for the optimumBHJ morphology can be controlled by the mixed SAM withoutsacricing the work function of ZnO. The performances ofinverted type PSCs are improved by insertion of ultrathin layersof TiO2 (


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the sample and the KP tip. The KP tip work function was 5.2030.011 eV. TEM images of the P3HT:PCBM active layer wereobtained with a JEM-2010 using an accelerating voltage of 80kV. The active layer was delaminated from the ITO substrate

by dissolving the ZnO layer in HCl solution. The typicalthickness of delaminated lms for TEM was ca. 200 nm. The

AFM topography images were taken using a Digital Instru-ments (MultiMode SPM) operated in the tapping mode. Thecurrent densityvoltage measurements under 1.0 sun (100mW/cm2) condition from a 150 W Xe lamp with a 1.5 G lter

were performed using a KEITHLEY model 2400 source-measure unit. A calibrated Si reference cell with a KG5 ltercertied by the National Institute of Advanced IndustrialScience and Technology was used to conrm 1.0 sun condition.The incident photon to collected electron efficiency (IPCE),external quantum efficiency, was calculated by

=

J

IIPCE (%) 1240 /

SCP

where Jsc (A/cm2) is the short circuit current density

measured at the wavelength (nm) and IP (W/m2).

Fabrication of PSCs. For fabrication of PSCs with astructure of ITO/before and after SAM treated ZnO/activelayer/MO3/Ag, a layer of 40 nm thick ZnO lm on precleanedand UV/O3 treated ITO (sheet resistance = 13 ohm/square)

was deposited by using the solgel process. The solgelsolution was prepared with 0.164 g of zinc acetate dihydrateand 0.05 mL of ethanolamine dissolved in 1 mL ofmethoxyethanol. The solution was stirred for 30 min at 60C prior to deposition. The thin lm of ZnO precursor wascured at 300 C for 10 min to partly crystallize the ZnO lm,

which is prepared by the literature procedures.44,47 To depositself-assembled molecules, a 1.0 mg/mL solution of benzoic acidderivative in methanol was spin-coated on the ZnO lm at 4000rpm for 60 s. To remove physically absorbed molecules, the

SAM treated ZnO surface was washed using pure methanol andthen dried by the stream of nitrogen. The active layer was spincast from the blend solution of P3HT/PCBM (20 mg of P3HTand 20 mg of PCBM were dissolved in 1 mL of o-dichlorobenzene (ODCB)) at 600 rpm for 40 s and dried ina covered Petri dish for 1 h. Prior to spin coating, the activesolution was ltered through a 0.45 m membrane lter. Thetypical thickness of the active layer was 200 nm. Beforedeposition of MoO3/Ag, the active layer was thermallyannealed at 150 C for 20 min in the glovebox (N2atmosphere). Finally, 20 nm thick MoO3 and 100 nm thick

Ag were deposited successively onto the top of the active layerthrough a shadow mask with a device area of 0.13 cm2 at 2 106 Torr.

3. RESULTS AND DISCUSSION

Characterization of SAM Modied ZnO. XPS (X-rayphotoelectron spectroscopy) spectra were measured to conrmcovering by the SAM of the ZnO surface. As shown in Figure 2,the oxygen peaks in XPS are asymmetric, indicating that thereare two oxygen species in the ZnO surface. The peaks at 542and 543 eV are attributed to oxygen in ZnO (Figure 2(a)). Thepeak at 542 eV is due to oxygen in the ZnO crystal lattice, andthe peak at 543 eV corresponds to chemisorbed oxygen caused

by surface hydroxyl.51 As shown in Figure 2(b)(d), thepositions of the peaks corresponding to oxygen in SAMmodied ZnO are shifted to lower binding energy. This

indicates that the ZnO surface is fully covered by BAderivatives.

For polymer solar cells (PSCs), the charge injection barrier isa very important factor for improving the device performances.

As illustrated in Figure1, the electron injection barrier can bemodied by the formation of an interface dipole between ZnOand the active layer. The interface dipole is induced by theSAM molecule with permanent dipole moment. To conrm theformation of the interface dipole by SAM treatment, wemeasure the effective work function of ZnO and SAM treatedZnO surface by using Kelvin Probe Microscopy (KPM). Asshown in Figure3, the effective work function of ZnO/FBA is

4.31 0.04 eV, which is larger than that of ZnO (4.17 0.01eV). The effective work functions of ZnO/BBA and ZnO/BBAare 3.970.01 and 3.94 0.04 eV, which are smaller than thatof untreated ZnO. The direction of the interface dipole across

the junction depends on the permanent dipole orientation ofBA derivatives. The dipole orientation of FBA and BBA (orMBA) is the exact opposite direction. For FBA, the interfacedipole between ZnO and the active layer is directed towardZnO. As for BBA and MBA, the interface dipole is directedaway from ZnO. Therefore, the effective work function of ZnOtreated with FBA is larger than that of ZnO. In contrary, theeffective work function of BBA and MBA treated ZnO issmaller than that of untreated ZnO. It is known that the surfacepotential of an inorganic semiconductor such as CdTe,CdInSe2, CdSe, and GaAs4850 can be controlled by theadsorption of a series of 4-substituted benzoic acid derivativeson the semiconductor surface. The surface potential difference

Figure 2. XPS spectra of (a) ZnO, (b) FBA treated ZnO, (c) BBAtreated ZnO, and (d) MBA treated ZnO.

Figure 3. Effective work function (square) and the surface energy(triangle) of ZnO/FBA, ZnO, ZnO/BBA, and ZnO/MBA.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp311148d|J. Phys. Chem. C2013, 117, 264626522648


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is directly proportional to the effective work function difference.The change in the semiconductor surface potential varieslinearly with the electron affinity of the substituent of the

benzoic acid derivatives. The dipole moment of benzoic acidderivatives reects the electron-withdrawing and -donatingpower of the substituent. The strong electron-donating powersubstituent on benzoic acid reduces the surface potential and

work function of the semiconductor. Very similar correlationsare observed in SAM modied ZnO with benzoic acidderivatives. From the KPM results, we conrm that directionof the formation of the interface dipole and variation of the

work function of ZnO illustrated in Figure1are reasonable. Asshown in Figure3, the surface energy reects the substituent onthe benzoic acid derivative. The hydrophobic uorine and tert-

butyl substituent decrease the surface energy of the ZnOsurface.

Photovoltaic Properties.Figure4shows current densityvoltage curves of inverted type PSCs under AM 1.5G simulated

illumination with an intensity of 100 mW/cm2 and under thedark condition. The photovoltaic parameters and efficiency ofthe best PSCs with various SAM treated ZnO are summarizedin Table1and Figure5. As shown in Table1and Figure5(a),the Voc data of the device based on ZnO/MBA are 0.63 V,

which is higher than those of PSC with ZnO/BBA (0.62 V),untreated ZnO (0.61 V), and ZnO/FBA (0.53 V). This is

because the effective work function of ZnO treated with MBAshows the smallest value than the others. As for ZnO/FBA, Vocis smaller than that of the device based on untreated ZnO. This

is due to the formation of an unfavorable interface dipolebetween ZnO and the active layer. The power conversionefficiency (PCE) of PSC with ZnO/MBA reaches 3.34%, whichis higher than that of the device based on ZnO/FBA (1.81%),ZnO (2.49%), and ZnO/BBA (2.94%). As seen in Table 1andFigure5(b), theJscand FF values of PSCs with MBA/ZnO arehigher than those of the other devices as well.

As seen in Figure5(b) and Table1, short circuit current (Jsc

)data of the devices with ZnO/FBA, ZnO, ZnO/BBA, andZnO/MBA are 7.55, 7.58, 7.88, and 8.77 mA/cm2,respectively. The device with ZnO/MBA and ZnO/BBA shows

betterJscdata than that of the device with untreated ZnO. Onthe contrary, the Jsc data of PSC with ZnO/FBA are verycomparable to that of the device based on untreated ZnO. Thell factor (FF) data of the device with ZnO/FBA are 45.1%,

which is lower than that of the device based on untreated ZnO(51.5%). The FFs of PSCs with ZnO/BBA and ZnO/MBA are60.4 and 60.5%, respectively, which are higher than that of thedevice based on untreated ZnO. Brabec et al. reported that thediodes ideality factor (n) and saturation current density (Jo)reect the performances of PSCs as well. The diode s idealityfactor (n) reects the density of donor/acceptor interfaces in

which recombination processes take place. Therefore, n isrepresentative of the morphology between the polymers andthe fullerenes. The saturation current density (Jo) reects thenumber of charges that can overcome the barriers under reverse

bias. Therefore, Jorepresents the minority charge density inthedonor/acceptor interface of bulk heterojunction solar cells.52Asfor PSCs with FBA treated ZnO, then and J0show the highest

values. This is presumably due to that the best performances ofPSCs with MBA treated ZnO arise from the lower idealityfactor and saturation current. However, there are still lots ofdebates about the relationship between n, Jo, and theperformance of PSCs. Figure6shows the incident photon tocollected electron efficiency (IPCE) of the PSCs in thisresearch, which show a maximum of IPCE at 540 nm. Among

the devices, PSC with MBA treated ZnO shows the highestvalue of 65.1%, which is higher than that of PSC without SAM(62.9%), with FBA treated ZnO (60.9%), and BBA treatedZnO (64.7%). The IPCE results also strongly demonstrate howthe photovoltaic parameters are related to the performances ofPSCs with various SAM treated ZnO.

The series resistance (Rs) and parallel resistance (Rp) ofPSCs are important parameters of PSCs. The Rs and Rp werecalculated from the inverse slope near the high current regimeand the slope near the lower current region in the dark JVcurves (Figure4(b)).53 As shown in Figure5(c) and Table1,the Rs values of the device based on ZnO, ZnO/FBA, ZnO/BBA, and ZnO/MBA are 13.7, 5.17, 3.15, and 2.82 cm2,respectively. TheRsreduces in the devices with ZnO/BBA and

ZnO/MBA, whileRsis increased in the device based on ZnO/FBA. TheRsof the device with ZnO/FBA is much higher thanthe other devices. Moreover, parallel resistance (Rp) of the PSC

with FBA/ZnO is 1.03 kcm2, which is much smaller than thePSC without SAM (2.45 kcm2), with BBA treated ZnO (6.22k cm2) and with MBA treated ZnO (10.1 kcm2). The Vocdata in ZnO/FBA are much different from the device withuntreated ZnO as change of the effective work function of FBAtreated ZnO, whereas theVocdata of PSCs with BBA and MBAtreated ZnO are not much different from the device withuntreated ZnO regardless of a sharp change in the effective

work function of ZnO compared to those of ZnO/BBA andZnO/MBA. The high drop ofVocdata in ZnO/FBA compared

Figure 4. Current densityvoltage curves of inverted type PSCs (a)under AM 1.5G simulated illumination with an intensity of 100 mW/cm2 and (b) under the dark condition (square, FBA SAM modied;

circle, without SAM; triangle, BBA SAM modied; inverted triangle,MBA SAM modied).




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to that of the device with untreated ZnO seems to be attributedto the highest Rsand lowestRpvalue of the device with ZnO/

FBA.54 This is due to the formation of an unfavorable interfacedipole between ZnO and the active layer in FBA treated ZnO.FBA SAM with ZnO forms an unfavorable dipole across theZnO, and the active layer results in Schottky contact and showspoor device performance. On the contrary, BBA and MBAtreated ZnO have a favorable dipole across the ZnO and activelayer and generate better contact so that the devices show

better performances.Morphology of the Active Layer. We conrm that the

Vocdata vary by the direction of interface dipole, which is easily

tuned by the SAM molecules with diff

erent dipole moment.However, the variation ofJscand FF data is not explained by thechange of interface dipole. Among the photovoltaic parameters,the PCE of PSC with ZnO/MBA is signicantly higher thanthat of the device with untreated ZnO regardless of the smallchange inVocdata of PSCs with ZnO/MBA compared to thatof PSCs with untreated ZnO. This is due to the big change of

Jscand FF value, which are strongly related with morphologicalproperty of the active layer.55 For efficient charge separationand transporting in PSCs, it shouldhave phases of P3HT andPCBM in the order of 1020 nm.1,56 To obtain morphology ofthe active layer, transmission microscopy (TEM) and atomicforce microscopy (AFM) were taken to investigate themorphology of the active layer on SAM treated ZnO. Figure7 shows TEM images of active layers. The active layer was

delaminated from the ITO substrate by dissolving a ZnO layerin HCl solution. The bright region in TEM images (Figure 7)indicates a P3HT-rich local phase. Figure 7(a) shows a verygood interpenetrating network and P3HT:PCBM phaseseparated morphology. However, the size of PCBM aggregatesis 80130 nm, and the TEM image shows very big size ofP3HT domains. The maximum size of the PCBM aggregate is

Table 1. Best Photovoltaic Parameters and Efficiencies of PSCs with Various SAM Treated ZnOa

Voc (V) Jsc (mA/cm2) FF (%) PCE (%)

Rs(cm2)b

Rp(kcm2)c nd

J0(A/cm2)e

FBA 0.53 (0.55 0.01) 7.55 (7.23 0.15) 45.1 (45.0 0.43) 1.81 (1.78 0.03) 13.7 1.03 2.37 0.93

no SAM 0.60 (0.61 0.01) 7.83 (7.80 0.07) 53.0 (51.6 1.14) 2.49 (2.44 0.07) 5.17 2.45 2.22 0.091

BBA 0.62 (0.62 0.004) 7.88 (8.11 0.17) 60.4 (57.6 1.40) 2.94 (2.88 0.05) 3.15 6.22 1.77 0.033

MBA 0.63 (0.62 0.004) 8.77 (8.51 0.13) 60.5 (59.8 0.82) 3.34 (3.16 0.06) 2.82 10.1 1.66 0.059a

The averages for photovoltaic parameters of each device are given in parentheses with mean variation. b

Series resistance (estimated from the devicewith best PCE value). cParallel resistance (estimated from the device with best PCE value). dIdeality factor (estimated from the device with best PCEvalue). eSaturation current density (estimated from the device with best PCE value).

Figure 5.(a)Voc(square) and PCE (triangle), (b) Jsc(square) and FF(triangle), and (c) Rs (square) and Rp (triangle) vs with and withoutSAM treated ZnO and (d) Voc vs effective work function of SAMtreated ZnO.

Figure 6.IPCE spectra of PSCs with SAM modied ZnO.

Figure 7.TEM images of the active layer deposited on (a) ZnO/FBA,(b) ZnO, (c) ZnO/BBA, and (d) ZnO/MBA.




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very close to the thickness of the active layer (200 nm). Ther.m.s. roughness of the active layer on ZnO/FBA (13.39 nm)(Figure8(a)) also supports that a FF of the device with ZnO/

FBA is much lower than that of the device with untreated ZnO.Figure7(c) and (d) shows TEM images of the active layer onZnO/BBA and ZnO/MBA, respectively. The size of the PCBMaggregates of ZnO/BBA and ZnO/MBA are 3040 nm and1020 nm, respectively, which are smaller than those of theactive layer on untreated ZnO (4060 nm) and FBA treatedZnO. Moreover, the boundaries between PCBM aggregates andP3HT domains of active layers on ZnO/BBA and ZnO/MBAare sharper than those of the active layer on untreated ZnO.

This indicates that the FF data of ZnO/BBA and ZnO/MBAare much higher than those of the other devices. Thedistribution of PCBM aggregates on ZnO/MBA is moreuniform than the case of ZnO/BBA. Moreover, the size ofPCBM aggregates on ZnO/MBA is very close to the optimumcondition, indicating that the Jsc and FF of the device withZnO/MBA are signicantly improved compared to the otherdevices. Even though TEM images do not provide exactinformation about vertically phase separated structures across

both electrodes, we conrm that ZnO/MBA exhibit optimizedphase separated morphology among the devices by TEMimages. As shown in Figure8, the r.m.s. roughness of the activelayer on ZnO/MBA is 4.24 nm, which is not much differentfrom the r.m.s. roughness data of the active layer on ZnO (2.89

nm) and ZnO/BBA (1.62 nm). The performances of the deviceseem to be unaffected by the surface energy of SAM modiedZnO. However, the morphology of the active layer seems to beaffected by the substituent on the 4-position of benzoic acid.The effective work function data and morphological changes ofthe active layer strongly support that the device based on ZnO/MBA exhibits the best performances.

4. CONCLUSION

We have fabricated inverted polymer solar cells with a series ofbenzoic acid derivative SAM treated ZnO as the electroninjection/transporting layer. The work function and surfaceproperty of ZnO can be successfully tuned by the interfacial

modication with a series of benzoic acid derivative SAMtreatments. The work function of ZnO depends on theorientation of the dipole moment of SAM molecules. Also, wehave observed that the substituent on the 4-position of benzoicacid affects the morphology of the active layer. Theperformances of inverted type polymer solar cells can beimproved by the appropriate choice of SAM molecule. Ourresults in this paper provide an alternative strategy to improvethe performances of PSCs by the control of interface property

between inorganic and organic materials in polymer solar cells.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Notes

The authors declare no competing nancial interest.

ACKNOWLEDGMENTS

This research was supported by Converging Research CenterProgram through the Ministry of Education, Science and

Technology (2012K001279) and Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science andTechnology (2012-0001356).

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