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aDepartment of Electrical and Compute

Singapore, 10 Kent Ridge Crescent, S

elezhucx@nus.edu.sgbDepartment of Chemistry, National Univ

Singapore 117543, SingaporecInstitute of Materials Research and Enginee

117602, Singapore. E-mail: jiangc@imre.a-s

† Electronic supplementary informationdevices. See DOI: 10.1039/c3ra46702c

Cite this: RSC Adv., 2014, 4, 6646

Received 15th November 2013Accepted 20th December 2013

DOI: 10.1039/c3ra46702c

www.rsc.org/advances

6646 | RSC Adv., 2014, 4, 6646–6651

Enhanced inverted organic solar cell performanceby post-treatments of solution-processed ZnObuffer layers†

Zhenhua Lin,ac Jingjing Chang,b Changyun Jiang,*c Jie Zhang,c Jishan Wubc

and Chunxiang Zhu*a

The effects of post-treatments (150 �C-thermal, humidity, and vacuum) on the cathode buffer layer of

aqueous-solution-processed zinc oxide (ZnO) films on the performance of the inverted organic solar

cells (OSCs) are investigated, based on poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl

ester (P3HT/PC61BM) as the active layers. The devices with the ZnO buffer layers that underwent thermal

and vacuum post-treatments exhibited 17% and 15% increments in the power conversion efficiency

(PCE) as compared to that of the cell without post-treatment on the ZnO layer, mainly due to the

increases of the short circuit current (Jsc). It was found that the thermal and vacuum post-treatments

reduced the defects and increased the electron mobility in the ZnO buffer layers, improving the electron

extractions in the inverted OSCs. Both the 150 �C-thermal and vacuum post-treatments are compatible

with plastic substrates, showing a potential way to further improve the film properties of the low-

temperature processed ZnO buffer layers.

1. Introduction

Recently, inverted organic solar cells (OSCs) have attractedmuch attention due to the improved stability and compatibilitywith roll-to-roll fabrication processes as compared with theconventional OSCs.1–4 In most conventional OSCs, anacidic poly(3,4-ethylenedioxithiophene):poly(styrenesulfonate)(PEDOT:PSS) hole transport layer is coated on ITO as the anode,and a low-work-function metal such as aluminum (Al) orcalcium (Ca) is used as the cathode. However, the low-work-function metal can be easily oxidized in air conditions, and thePEDOT:PSS, since it is acidic in nature, can cause the etching ofITO and indium diffusion into the active layer, resulting ininterface instability.5–7 In the inverted structure, the low-work-function metal and the interface of ITO/PEDOT:PSS can beavoided by using an n-type metal oxide lm as the electronselective layer (ESL) on ITO for the cathode and an air-stablehigh-work-function metal, such as Ag or Au, as the top anode,resulting in better air stability of the devices.8–10

r Engineering, National University of

ingapore 119260, Singapore. E-mail:

ersity of Singapore, 3 Science Drive 3,

ring, A*STAR, 3 Research Link, Singapore

tar.edu.sg

(ESI) available: The stability of the

ESL is crucial for achieving efficient inverted OSCs, becausethe nature of the electrical contact between the active layer andITO cathode has signicant effects on the device performance,while the ESL is used for tuning this electrical contact proper-ties. Several metal oxides used for the ESL have been studied,such as zinc oxide (ZnO),11,12 titanium oxide (TiOx),13,14 andaluminum oxide (Al2O3).15 ZnO lm has been considered as agood candidate due to its high electron mobility and hightransparency in the visible wavelength range which allow it tobe an effective electrons transporter and excellent wave guidefor light.11,16 Meanwhile, ZnO is a cheap and environmentallyfriendly material which can be synthesized with high purity atlow temperature, and ZnO lms can be easily processed via low-temperature solution method which makes it compatible withroll-to-roll fabrication onto plastic substrate.12,17

Solution processed ZnO is mainly obtained by sol–gel, nano-crytalline ZnO colloid (Nc-ZnO) or aqueous solution route. Sol–gel based ZnO lms usually require high annealing temperature($200 �C) to remove carbon impurities, which is not compatiblewith exible substrates.7,12 The Nc-ZnO colloid processed lmspossess large surface area, which results in an environmentallyvery sensitive electrical-performance of the lm.8,18 However,the method of aqueous solution with ammine–hydroxo zinccomplex could obtain dense ZnO thin lms by low temperatureannealing.19 Previously, ZnO ESL lms deposited with aqueoussolutions of ammine–hydroxo zinc complex have been investi-gated.20,21 Kim et al. has reported the effects of ZnO-layerannealing temperature on the inverted OSC performance.20

However, studies on the effects of various low-temperature post-

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treatments on ZnO ESL lm have not been reported. Such low-temperature processes may be important for realizing highefficient OSC on plastic substrates.

In this paper, we investigated the effects of thermal (150 �C),humidity, and vacuum post-treatments on the aqueous-solu-tion-processed ZnO ESL lms on the lm properties andinverted-OSC performance. It was found that thermal andvacuum post-treatments reduced the defects and increased theelectron mobility in the ZnO lm (up to 0.36 cm2 V�1 s�1).Enhanced photovoltaic performances were achieved from theinverted OSC based on the thermal and vacuum post-treatedZnO buffer layers, as the results of improved electrons extrac-tions in the devices.

2. Experimental2.1. Inverted solar cell fabrication and characterization

The inverted OSC has a layer structure of Glass/ITO/ZnO/active-layer/MoO3/Ag. ITO-coated glass substrates were cleaned by aroutine solvent ultrasonic cleaning, sequentially with detergent,de-ionized water, acetone, and isopropyl alcohol (IPA) in anultrasonic bath for 15 minutes each. For the ZnO lms prepa-ration, rstly, ZnO nanopowders (Sigma Aldrich) were directlydissolved in ammonia solution (0.1 M). The solution was stirredovernight to yield a homogenous, clear, and transparent solu-tion. Aer the ZnO particles totally dissolved, the ZnO solutionwas spin-coated on top of ITO substrates at 3000 rpm for 30 s.Then, the as-spun ZnO lms were directly thermally annealed at150 �C for 10min. The resulted ZnO lms are referred to as ZnO-ref (without post treatment). Other three kinds of ZnO lms arethat the lms were rstly prepared similar as ZnO-ref and thenunderwent further post-treatments by thermal (at 150 �C in anoven for 12 h) (ZnO-therm), humidity (in RH 80% air for 12 h)(ZnO-humi) and vacuum (in 1 � 10�6 torr high vacuumchamber for 12 h) (ZnO-vac).

To fabricate inverted OSCs, an active layer was deposited ontop of the ZnO layer by spin-coating a solution of the P3HT andPC61BM blend with a weight ratio of 1 : 1 in 1,2-dichloroben-zene (40 mgml�1) at 500 rpm for 130 s in N2 glove box (�180 nmthickness), and the active layers were dried for 2 hours and thenpre-annealed at 140 �C for 10 minutes. For fabrication of the cellwith poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b0]dithio-phene-2,6-diyl][3-uoro-2[(2 ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]] (PTB7) and [6,6]-phenyl-C71-butyric acid methylester (PC71BM) blend as the active layer, the solution of thePTB7:PC71BM blend with weight ratio of 1 : 1.5 in 1,2-dichlo-robenzene (25 mg ml�1 with 3% 1,8-diiodooctane) was spincoated on top of the ZnO layer at 1000 rpm for 100 s (�70 nmthickness), and the active layer was dried for 2 hours. Finally, aMoO3 layer (6 nm) and an Ag layer (100 nm) were deposited onthe active layers by using vacuum thermal evaporation. Thedevice area is dened as 9 mm2 by using a shadow mask. Foreach condition, four devices on one substrate were prepared togive an average value. The current density versus voltage (J–V)characteristics of the devices were measured using a Keithley2400 parameter analyzer in the dark and under a simulatedlight (AM 1.5G) with intensity of 100 mW cm�2 calibrated via a

This journal is © The Royal Society of Chemistry 2014

silicon reference cell. IPCE measurements were performedunder short-circuit conditions with a lock-in amplier (SR510,Stanford Research System) at a chopping frequency of 280 Hzduring illumination with a monochromatic light from a Xe arclamp.

2.2. Electrical characterization of ZnO lms

To evaluate the charge transport properties of ZnO lms, ZnOthin-lm transistors (TFT) with bottom-gate top-contactstructure were employed. For the device fabrication, a heavilydoped p-type Si wafer (purchased from Silicon Quest Interna-tional, Inc.) was served as the gate electrode and 200 nm ofthermally grown SiO2 was used as the dielectric layer. Prior tospin coating ZnO precursors, the Si/SiO2 substrates were rstlycleaned with acetone, IPA, and de-ionized water, and thentreated with Ar plasma to facilitate the thin lm formation.Aer that, ZnO lms of ZnO-ref, ZnO-therm, ZnO-humi andZnO-vac were fabricated on top of the Si/SiO2 substrates.Finally, Al layers, as the source and drain electrodes (W ¼1000 mm, L ¼ 100 mm), were deposited on the ZnO thin lmwith a shadow mask. The transistors were characterized withKeithley 4200 parameter analyzer in the N2-lled glove box.The eld-effect mobility of the fabricated transistor wasextracted using the following equation in the saturation regionfrom the gate sweep: ID ¼ W/(2L)Cim(VG � VT)

2, where ID is thedrain current in the saturation region, m is the eld-effectmobility, Ci is the capacitance per unit area of the gatedielectric layer (SiO2, 200 nm, Ci ¼ 17 nF cm�2), VG and VT aregate voltage and threshold voltage, and W and L are channelwidth and length, respectively.

2.3. ZnO lm characterization

Surface properties of the ZnO lm were characterized by X-rayphotoelectron spectroscopy (XPS) (VG ESCALAB-220i XL).Ultraviolet photoelectron spectroscopy (UPS) experimentswhich were carried out at the Escalab 220i, and He I (21.2 eV) asthe excitation sources. The transmittance spectra of the ZnOlms deposited onto quartz were characterized using an UV-3600 Shimadzu UV-VIS-NIR Spectro photometer. The surfacemorphology and roughness of the ZnO lms deposited on ITOsubstrate were measured by tapping-mode Atomic ForceMicroscopy (TM-AFM) which was performed on a BrukerICON-PKG atomic force microscopy (AFM). The photo-luminescence (PL) spectra of the ZnO lms were excited with aHe–Cd laser (l ¼ 320 nm) using LS 55 uorescence spectrom-eter (PerkinElmer).

3. Results and discussion3.1. Effects of ZnO post-treatments on cell performance

The J–V characteristics of the P3HT:PC61BM devices incorpo-rating ESL lms of ZnO-ref (A), ZnO-therm (B), ZnO-humi (C)and ZnO-vac (D) are shown in Fig. 1a, and the extracted deviceperformance parameters of open-circuit voltage (Voc), shortcircuit current density (Jsc), ll factor (FF) and power conversionefficiency (PCE) are summarized in Table 1. Compared to the

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Fig. 1 (a) J–V characteristics of P3HT:PC61BM inverted OSCs incorporating with ESL films of ZnO-ref (A), ZnO-therm (B), ZnO-humi (C) andZnO-vac (D). (b) IPCE spectra of the P3HT:PC61BM OSCs. (c) J–V characteristics of PTB7:PC71BM inverted OSCs incorporating with ZnO-ref (E)and ZnO-therm (F) as the ESL. (d) IPCE spectra of the PTB7:PC71BM OSCs.

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reference device A, the ZnO-therm and ZnO-vac based cells (Band D) show slight increases in Voc and large increases in Jsc.The Jsc increased from 8.93 mA cm�2 (cell A) to 10.29 mA cm�2

upon thermal treatment on ZnO (cell B), and 9.92 mA cm�2

upon vacuum treatment (cell D). The resulting PCEs of cell Band D are 3.58% and 3.51%, exhibiting 17% and 15% incre-ments, respectively, as compared to the reference cell A (3.05%).The performance of ZnO-humi based cell (C) has almost noimprovement as compared to cell A. Incident photon-to-currentconversion efficiency (IPCE) spectra of these four cells are

Table 1 Optical band gap (Eg), work-function (Wf) and electronmobilityof the P3HT:PC61BM (device A, B, C and D) and PTB7:PC71BM (device E

Device

ESL lm characteristics

ESL Eg (eV) Wf me (cm2 V�1 s

A ZnO-ref 3.418 3.83 0.225B ZnO-therm 3.354 3.78 0.360C ZnO-humi 3.406 3.86 0.183D ZnO-vac 3.398 3.80 0.311E ZnO-refF ZnO-therm

6648 | RSC Adv., 2014, 4, 6646–6651

shown in Fig. 1b. The IPCEs of cell B and D show higher valuesas compared to that of cell A in the wavelength range from 350to 620 nm, and a maximum value of 71% (cell B) is obtained atthe wavelength of 550 nm. The devices were tested again aerstored in nitrogen-lled glove box for about half a year. ThePCEs of the devices were noted to retain more than 85% of theiroriginal values (Fig. S1 in ESI†), and it was found that thedevices with post-treated ZnO have better stability than the ZnO-ref device, indicating that the interfacial stability was improvedby the post-treatments.

(me) of the ZnO ESL films, and the photovoltaic performance parametersand F) inverted solar cells incorporating these ZnO films as the ESL

Device characteristics

�1) Jsc (mA cm�2) Voc (V) FF h (%)

8.93 0.56 0.61 3.05 � 0.0410.29 0.58 0.60 3.58 � 0.029.01 0.56 0.61 3.07 � 0.069.92 0.58 0.61 3.51 � 0.02

13.76 0.73 0.68 6.83 � 0.0614.34 0.74 0.67 7.10 � 0.10

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Fig. 3 (a) The transmittance spectra, and (b) plot of (ahy)2 vs. photonenergy of ZnO films (on quartz substrates) with different post-treatments.

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Inverted OSCs based on PTB7/PC71BM system incorporatingwith a ZnO-ref lm (cell E) and a ZnO-therm lm (cell F) as theESL have also been fabricated and characterized. The J–Vcharacteristics and photovoltaic parameters of these twodevices are shown in Fig. 1c and Table 1. A high PCE of 7.1%was obtained from the ZnO-therm based cell (F) with Voc of 0.74V, Jsc of 14.34 mA cm�2 and FF of 0.67. The Jsc is slightly higherthan that of the ZnO-ref based cell E (13.76 mA cm�2). The IPCEspectra of these two devices (see Fig. 1d) also show clearlyincreased IPCE from the cell F as compared to the cell E, whichis in consistent with the measured Jsc.

3.2. Surface properties of ZnO lms with different post-treatments

The surface morphologies of all the ZnO lms (on ITO coatedglass substrates) without and with different post-treatmentswere closely similar, with the root mean square (RMS) rough-ness values at 2.36–2.58 nm as measured by AFM, as shown inFig. 2. This means less likely that there are morphology changeseither at the interfaces or in the active layers of the devicesaffecting the photocurrent.

Fig. 3a shows the optical transmittance spectra of ZnO lmscoated on quartz substrates. It shows that all the ZnO thin lmsshow similarly high transmittance (>99%) over the visiblewavelength range. To obtain the optical band gap (Eg) of eachZnO lm, the absorption coefficient as a function of photonenergy has been plotted in Fig. 3b according to the trans-mittance spectra. The absorption coefficient a can be calculatedusing T ¼ A exp(�ad), where T is the transmittance of the ZnOlm, A is a constant and approximately unity, and d is the lmthickness.22 The value of optical band gap can be calculatedusing the Tauc model23 and the Davis Mott model24 in the highabsorbance region: ahy ¼ D (hy � Eg)

n, where hy is the photonenergy, Eg is the optical band gap, D is a constant, and n equalsto 1/2. In Fig. 3b, the relationship between (ahy)2 and hy isplotted. The Eg value can be obtained by extrapolating the linearportion to the photon energy axis. The optical band gap valuesobtained are summarized in Table 1. It can be seen that theband gaps of the ZnO lms decreased aer post-treatments,from 3.418 eV (ZnO-ref) to 3.354 eV (ZnO-therm), 3.406 eV (ZnO-

Fig. 2 AFM images (25 � 25 mm) of the ZnO films deposited on ITOcoated glass substrates without (a), and with thermal (b), humidity (c),and vacuum post-treatment (d), respectively.

This journal is © The Royal Society of Chemistry 2014

humi), and 3.398 eV(ZnO-vac). The narrowing band gap shouldbe due to increased crystallinity caused by the post-treatments,especially of the thermal and vacuum treatments. Similar band-gap decrease and crystallinity increase were also observed whenincrease the thermal annealing temperatures.22 The changes ofthe band-gap and crystallinity in the ZnO lms may inuencethe electric properties at the ZnO/active-layer interfaces, such asthe energy band alignments, surface defects/recombination orelectron transport resistance.

Fig. 4 shows the UPS measurements of the valence band andsecondary electron cut-offs of ZnO lms with different post-treatments. The work function values determined from theelectron cutoffs of the UPS spectra are 3.83, 3.78, 3.86, and 3.80eV for ZnO-ref, ZnO-therm, ZnO-humi and ZnO-vac, respec-tively. Such low work functions make the ZnO lms suitable forbeing used as ESL for efficient electron extractions. The valenceband maximum (VBM) relative to Fermi level (EF) determinedfrom the valence band region for all the ZnO lms are around3.3 eV, and the conduction band minimum (CBM) of the ZnOlms is estimated to be located near the EF and LUMO of PCBMbased on the values of Eg and VBM. The work function and CBMdata show very small differences among all these ZnO lms,thus it is not likely that the work function causes the changes ofelectron-extraction rate of the devices.

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Fig. 4 UPS spectra of ZnO films with different post-treatments.

Fig. 6 O 1s XPS spectra of ZnO films with different post-treatments.

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The defect conditions in the ZnO lm will inuence thecharge carrier mobility and the interface recombination rates.Room temperature PL spectra were rstly measured for the ZnOlms to exam the defects levels in the lms. Fig. 5 shows the PLspectra of the ZnO lms without (ZnO-ref) and with post-treat-ments (ZnO-therm, ZnO-humi, ZnO-vac). Strong emission at�370 nm was observed for all the samples, which correspondsto the near band-edge emission.25 This band is usually assignedto radiative recombination from free excitons or unspeciedlocalized states.25 Usually, a broad green emission peak is alsopresented in the ZnO PL spectrum, which is commonly referredto as a deep-level trap-state emission. The green emission hasbeen attributed to the singly ionized oxygen vacancy in ZnO lmresulting in radiative recombination of a photogenerated holewith an electron occupying the oxygen vacancy.26 Here, thegreen emission peaks of all the samples are very weak, indi-cating a low level defects contents in each of the ZnO lms.27

In order to further understand the differences of the defectsin the ZnO lms, XPS measurements were carried out to studythe components structures on the lm surfaces. All atomicspectra were calibrated by taking hydrocarbon C 1s peak at284.6 eV as a reference. Fig. 6 shows core level XPS spectra of O1s for the ZnO lms with different post-treatments. The O 1s

Fig. 5 The PL spectra of ZnO films of ZnO-ref, ZnO-therm, ZnO-humiand ZnO-vac.

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XPS spectra exhibit asymmetric line shapes as shown in Fig. 6(black color curves). The main peak with lower binding energy(529.8 eV) corresponds to oxygen lattice in ZnO matrix, whileanother two peaks (at 531.1 eV and 531.9 eV) correspond to thelattice oxygen in oxygen-decient regions (oxygen vacancy) andzinc hydroxide (Zn(OH)2), respectively.28,29 By tting the threepeaks of the XPS spectra, the percentage of each single peakcomponent to the whole spectrum was calculated as shown inFig. 6. The percentage value of the 531.9 eV-peak component(refer to the blue tting curves) decreased from 13.0% (ZnO-ref)to 9.5%, 10.0%, and 9.7% for ZnO-therm, ZnO-humi and ZnO-vac lms, respectively, which infers that the zinc hydroxidecomponent decreased with post-treatments due to conversionfrom metal hydroxide to oxides. The 531.1 eV-peak component(refer to the green tting curves), representing the contributionsfrom oxygen vacancy, shows also clearly a decreased intensity inthe ZnO-therm and ZnO-vac lms, as compared to ZnO-ref lm.The decreased metal hydroxide component and oxygen vacancyin the ZnO-therm and ZnO-vac lms indicates more completedconversion from zinc hydroxide into ZnO, and reduced defectsites in the lm.

3.3. Electrical properties of the ZnO lms

The charge transport properties of the ZnO lms under variouspost-treatments were studied using thin lm transistors withstructure as shown in Fig. 7a. The measured ID–VG character-istics of four TFT devices based on lms of ZnO-ref, ZnO-therm,ZnO-humi and ZnO-vac, respectively, are shown in Fig. 7b, andthe calculated electron mobilities (me) of these ZnO lms arealso summarized in Table 1. It shows that all the lms under-went different post-treatments showed good transistor charac-teristics. The ZnO-therm and ZnO-vac lms, with thermal andvacuum post-treatment, have me of 0.36 cm2 V�1 s�1 and 0.311cm2 V�1 s�1, respectively, which are substantially higher thanthat of the ZnO-ref lm (0.225 cm2 V�1 s�1). The increase in

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Fig. 7 (a) Device structure of the ZnO TFT used in this study. (b) Thesource–drain current (IDS) versus gate voltage (VG) characteristics offilms.

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charge mobility is most possibly the results of reduced impurity(zinc hydroxide) defects and increased crystallinity in the ZnO-therm and ZnO-vac lms as discussed above. The increasedelectron mobility in the ESL lms is favorable for the electronextraction in inverted OSC, which may account mainly for theincrease of Jsc in the ZnO-therm and ZnO-vac based OSCs.

4. Conclusions

In summary, both the thermal and vacuum post-treatments onthe ZnO buffer layers improved the Jsc and PCE considerably inthe inverted OSCs. The increase in Jsc was mainly the result ofincreased electron mobility and reduced recombination centers(defects) in the ZnO-ESL buffer layers with thermal and vacuumpost-treatments, which increase the electron-extraction effi-ciency in the inverted OSCs. The results indicated that 150 �C-thermal post treatment showed the most effective way toimprove the ZnO buffer layer, while vacuum treatment is alsouseful since it can improve the lm properties at roomtemperature.

Acknowledgements

This work was nancially supported by IMRE Core Funding(IMRE/12-1P0902), IMRE Core Funding (IMRE/10-1P0508) andA*STAR SERC TSRP Grant (Grant #102 170 0137).

Notes and references

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