Organic Electronics 15 (2014) 1362–1367

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Organic Electronics

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Performance improvement of pentacene-doped P3HT:PCBMinverted polymer solar cells with AZO nanorod array passivatedusing photoelectrochemical technique

http://dx.doi.org/10.1016/j.orgel.2014.04.0011566-1199/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +886 6 2082368.E-mail address: hylee@ee.ncku.edu.tw (H.-Y. Lee).

Hsin-Ying Lee ⇑, Hung-Lin HuangDepartment of Photonics, Research Center Energy Technology and Strategy, Advanced Optoelectronic Technology Center, National Cheng Kung University,Tainan 701, Taiwan, Republic of China

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 March 2014Received in revised form 1 April 2014Accepted 3 April 2014Available online 18 April 2014

Keywords:Al-doped zinc oxide nanorod arrayInverted polymer solar cellsPhotoelectrochemical method

The pentacene-doped P3HT:PCBM inverted polymer solar cells (IPSCs) with Al-doped zincoxide (AZO) nanorod array were fabricated. The AZO nanorod array could enhance the car-rier collection and carrier extraction capability. The AZO nanorod array formed by the laserinterference photolithography method and the wet etching process sequentially was usedas the carrier collection and carrier transportation layers. The defects on the sidewall sur-face of the AZO nanorods were passivated by using the photoelectrochemical (PEC)method. It was demonstrated that the better performance of the IPSCs was obtained byPEC treatment. Compared with the IPSCs without PEC treatment, the short current densityand power conversion efficiency of the IPSCs with PEC treatment for 60 s increased from14.56 mA/cm2 to 15.85 mA/cm2 and 5.45% to 6.13%, respectively.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction the polymer solar cells, the conversion efficiency of the

Over the past decades, various renewable energyresources have been extensively developed to solve prob-lems of energy depletion, such as wind power, hydraulicpower, and solar power [1–3]. The solar power attractedmost attention owing to that the solar power is a clean,non-noise, and non-pollution energy [4,5]. Recently,organic solar cells, polymer solar cells (PSCs) and invertedpolymer solar cells (IPSCs), have attracted great interest aspotential renewable energy. These organic solar cellsincluded many advantages, such as prepared on flexiblesubstrates, low cost, light weight and easy fabrication[6–8]. In general, the poly(3-hexylthiophene):[6,6]-phe-nyl-C61-butyric acid methyl ester (P3HT:PCBM) PSCs havethe problems of poor stability and lower efficiency [9].Although the P3HT:PCBM IPSCs improved the stability of

IPSCs was only close to 3% [10]. The lower conversion effi-ciency could be attributed to the lower carrier extractionand carrier collection efficiency. Many techniques weredeveloped to enhance the conversion efficiency of PSCsand IPSCs. For examples, the pentacene was doped inP3HT:PCBM absorption layer of PSCs to balance the elec-tron and hole mobility in the absorption layer [11].Consequently, the conversion efficiency was improvedfrom 3.46% to 4.42%. In our previous paper [12], the 100-nm-long Al-doped zinc oxide (AZO) nanorod arrays wereutilized to improve the carrier collection and carrierextraction capability. The conversion efficiency wasincreased from 3.12% to 4.19%. However, owing to the factthat the AZO nanorod arrays were fabricated using theinterference photolithography method and wet etchingprocess, more defects were generated on the surface ofthe AZO nanorods, which could degrade the performanceof the IPSCs. In general, there are some techniques, suchas hydrogen peroxide (H2O2) treatment [13], ammoniumsulfide ((NH4)2Sx) treatment [14], photoelectrochemical

H.-Y. Lee, H.-L. Huang / Organic Electronics 15 (2014) 1362–1367 1363

(PEC) treatment [15], and Al2O3 coating and hydrogenplasma treatment [16], were used to passivate the ZnOsurface.

To overcome the above drawbacks, in this work, thetechniques of utilizing the pentacene-doped P3HT:PCBMas an absorption layer and the AZO nanorod array as elec-tron transport layer were combined to fabricate the IPSCs.Furthermore, the photoelectrochemical method was alsoused to treat the surface of the AZO nanorods formed byinterference photolithography method and wet etchingprocess. It was demonstrated that the defects on the sur-face of the AZO nanorods were passivated and the betterperformance of the IPSCs was obtained.

2. Experimental procedure

Fig. 1(a) shows the schematic configuration of thepentacene-doped P3HT:PCBM inverted polymer solar cells(IPSCs) fabricated on a 250-nm-thick ITO-coated glasssubstrate, in which the electron transport layer is com-posed of an Al-doped zinc oxide (AZO) layer and an AZOnanorod array. The device fabrication procedures were asfollows. A 125-nm-thick AZO film was deposited on thesubstrates using a radio-frequency (RF) magnetron sput-tering system. The electronic concentration of the depos-ited AZO film was 8.41 � 1019 cm�3. The AZO nanorod

Fig. 1. (a) Schematic configuration of IPSCs with the AZO nanorod array.The inset figure shows the morphology of the AZO nanorod array. (b) TheSEM image of the cross-section of the fabricated IPSCs.

array with the rod-length of 100 nm was then formed onthe AZO film by using the laser interference photolithogra-phy method to define the rod pattern and subsequentlyusing the diluted HCl solution to etch the AZO film exceptthe defined AZO nanorod region that was protected withphotoresist. The detailed formation processes of the AZOnanorod arrays were shown in the previous publishedpaper [12]. The remaining 25-nm-thick AZO films togetherwith the 100-nm-long AZO nanorod arrays were used asthe electron transport layer. The morphology of the AZOnanorod array was recorded using an atomic layer micro-scope (AFM) and is shown in the inset of Fig. 1(a). Next,thus formed sidewall surfaces of the AZO nanorod arraywere passivated using the PEC method. Fig. 2 shows theschematic configuration of the PEC system. The sample tobe treated was fixed on the Pt anode electrode on Teflonstage and immersed into the ammonia (NH4OH) electro-lytic solution with a pH value of 8. Then the sample wasilluminated using a He–Cd laser (wavelength of 325 nmand power density of 10 mW/cm2) for various times of0 s, 30 s, 60 s, and 90 s. After the PEC treatment, a 200-nm-thick poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM):pentacene layer, asthe absorption layer of the IPSCs, was deposited on theAZO nanorod array by spreading the mixed solution ofP3HT, PCBM, and pentacene (1:0.8:0.065 by weight) in1,2-dichlorobenzene (DCB) using a spin-coating technique.Then the deposited absorption layer was thermally treatedin a nitrogen glove box at 110 �C for 20 min. Finally, theMoO3/Ag (10 nm/100 nm) anode electrode of the IPSCswas deposited on the absorption layer using a thermalevaporator. The absorption area of the IPSCs was 4 mm2.The cross-section image of the resulting IPSCs wasrecorded using a scanning electron microscope (SEM) andis shown in Fig. 1(b).

The chemical bonding situation of the AZO nanorodswith and without PEC treatment was analyzed usingX-ray photoelectron spectroscopy (XPS). The current den-sity–voltage (J–V) characteristics of the solar cells weremeasured at room temperature using a J–V curve tracerwith an AM 1.5G solar simulator (100 mW/cm2). The darkcurrent density–voltage characteristics of the solar cellswere measured using Agilent 4156C semiconductorparameter analyzer.

Fig. 2. Schematic configuration of the PEC system.

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3. Experimental results and discussion

To understand the PEC processes of the AZO, the banddiagram of the AZO and the NH4OH electrolyte is shownin Fig. 3. The work function (WNH4OH) of 4.72 eV for theelectrolytic solution with the pH value of 8 was estimatedby the formula of WNH4OH = 4.25 + 0.059 � pH value [17].The work function (WAZO) of 4.02 eV for the AZO was deter-mined by the formula of WAZO = qv + (Ec � EF), where qv of4.10 eV is the electron affinity of the AZO [18], (Ec � EF) of�0.08 eV is the separation between the conduction band(Ec) and the Fermi level (EF) of the AZO. Since the workfunction of the NH4OH electrolyte was larger than that ofthe AZO, a Schottky contact between AZO and NH4OH elec-trolyte was formed. As the sample was illuminated withHe–Cd laser, the electron–hole pairs were produced inthe AZO nanorods. The holes were moved to the sidewallsurface of the AZO nanorods, driven by the build-in electricfield within the surface region of the AZO nanorod contact-ing with the electrolyte solution, as shown in Fig. 3. At thesurface of the AZO nanorods, the holes reacted with the Zn/Al and electrolyte to form Zn(OH)2 and Al(OH)3. The reac-tion equations are shown as follows [15,19]:

Znþ 2OH� þ 2hþ ! ZnðOHÞ2 ð1Þ

Alþ 3OH� þ 3hþ ! AlðOHÞ3 ð2Þ

where h+ is the hole. In order to verify that the Zn(OH)2 andAl(OH)3 were formed on the sidewall surface of the AZOnanorods, X-ray photoelectron spectroscopy (XPS) was uti-lized to analyze the chemical bonding of the AZO nanorodswithout and with PEC treatment for 60 s. Fig. 4(a)–(c)shows the XPS spectra corresponding to the Zn 2p3/2, Al2p3/2, and O 1s core-levels, respectively. In Fig. 4(a), theZn–O bond (1021.9 eV) and the Zn–OH bond (1023.8 eV)were found for the AZO nanorod without and with PECtreatment [20]. In Fig. 4(b), the XPS spectra of the Al2p3/2 for the sample with and without PEC treatment con-tained two bands at 74.5 eV and 74.8 eV, which correspondto Al–O and Al–OH, respectively [21]. In Fig. 4(c), the XPSspectra of the O 1s for the treated and untreated AZO nano-rod were decomposed into four signals, including Zn–Obond (530.4 eV), Zn–OH bond (532.2 eV), Al–O bond(532.7 eV), and Al–OH bond (533.9 eV) [22,23]. As shownin Fig. 4, the peak intensity of the Zn–OH bond and the

Fig. 3. The band diagram of the AZO and the NH4OH electrolyte.

Al–OH bond for the PEC-treated AZO nanorods was largerin comparison with the untreated AZO nanorods. This phe-nomenon could be interpreted by the reactions of Eqs. (1)and (2) with which the Zn(OH)2 and Al(OH)3 were formedon the sidewall surface of the AZO nanorods in the PECtreatment process. In other words, the PEC treatment couldeffectively passivate the AZO nanorods and reduce thedangling bonds on the sidewall surface of the AZOnanorods.

To verify the function of the PEC treatment for the AZOnanorods, the P3HT:PCBM:pentacene IPSCs with AZOnanorod array treated for various times of 0 s, 30 s, 60 s,and 90 s by PEC method were fabricated. Fig. 5 shows thecurrent density–voltage (J–V) characteristics of the result-ing IPSCs with various PEC treating times. The series resis-tance (Rs) and the shunt resistance (Rsh) of the IPSCs wereestimated by fitting the J–V curve [24] as shown in theinset figure of Fig. 5 and were also listed in the inset tableof Fig. 5. The Rs of the IPSCs with various PEC treating timesof 0 s, 30 s, 60 s, and 90 s were 12.1 X cm2, 12.8 X cm2,13.3 X cm2, and 13.9 X cm2, respectively. In general, in aorganic solar cell, the Rs is related with the resistance ofthe organic absorption layer, electron transportation layer,hole transportation layer, anode electrode, and cathodeelectrode. In our situation, the Rs of the resulting IPSCsslightly increased with the PEC treating time. It could beattributed to the presence of more Zn(OH)2 and Al(OH)3

formed on the sidewall surface of the AZO nanorods bythe PEC treatment. Furthermore, the thickness of theZn(OH)2 and Al(OH)3 layer increased with the PEC treatingtime, which led to an increase in the resistance of the AZOnanorod array. In particular, the Rs increased from12.1 X cm2 to 13.9 X cm2 for the IPSCs treated by PECmethods for 90 s. The corresponding Rsh of the resultingIPSCs with various PEC treating times of 0 s, 30 s, 60 s,and 90 s were 351 X cm2, 479 X cm2, 518 X cm2, and560 X cm2, respectively. In general, the amount of thedefects in the solar cells affects the Rsh because the defectscaused carrier recombination and leakage current. In otherwords, the solar cells with lower defects have larger Rsh. Asshown in the inset table of Fig. 5, the Rsh increased consid-erably from 351 X cm2 to 479 X cm2 as the IPSCs treatedby PEC methods for 30 s. It could be attributed to thatthe PEC treatment effectively passivated the defects onthe sidewall surface of the AZO nanorod array. The currentdensity of the IPSCs was correlated with the Rs and Rsh. Theincrement of Rsh could elevate the current density of thecells and the increment of Rs would lead to a decrease inthe current density. As shown in Fig. 5, the current densityof the resulting IPSCs increased with the PEC treating time,the maximum Jsc of 15.85 mA/cm2 was obtained as theIPSCs treated by PEC methods for 60 s. The Jsc decreasedas the PEC treatment time was increased to 90 s, which isowing to the larger Rs of thus treated IPSCs. The open cir-cuit voltage (Voc) of the resulting IPSCs with various PECtreating times of 0 s, 30 s, 60 s, and 90 s were 0.604 V,0.611 V, 0.613 V, and 0.615 V, respectively. The Voc slightlyincreased with the PEC treating time. It was attributed toincrease in Rsh, which increased, as an example, to560 X cm2 as the IPSCs treated by PEC methods for 90 s.The other two important parameters, fill factor (FF) and

Fig. 4. The XPS spectra of the (a) Zn 2p3/2, (b) Al 2p3/2, and (c) O 1s core-levels for the AZO nanorods without and with PEC treatment for 60 s.

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Fig. 5. The current density–voltage characteristics of the resulting IPSCswith various PEC treating times. The inset figure shows the seriesresistance and the shunt resistance of the resulting IPSCs, estimated byfitting the J–V curve. The inset table shows the estimated series resistanceand the shunt resistance of the IPSCs with various PEC treating times.

1366 H.-Y. Lee, H.-L. Huang / Organic Electronics 15 (2014) 1362–1367

conversion efficiency (g), of the IPSCs with various PECtreating times of 0, 30 s, 60 s, and 90 s were estimated tobe 61.1%, 62.3%, 63.1%, and 62.2%, and 5.45%, 5.79%,6.13%, and 5.72%, respectively. The maximum FF and g ofthe IPSCs with the PEC treating time of 60 s was obtained.These phenomenons were similar with the Jsc as shownabove. According to the J–V characteristics of the resultingIPSCs, the defects on the sidewall surface of the AZO nano-rod array were passivated by PEC method and the PECtreating time of 60 s was a more suitable time than thatof 30 s and 90 s.

To further confirm the fact that the leakage current ofthe resulting IPSCs could be improved by using PECmethod to passivate the sidewall surface of the AZO nano-rods. The dark current density–voltage (Jdark–V) character-istics of the IPSCs with various PEC treating times of 0 s,30 s, 60 s, and 90 s were measured at the forward voltagerange from 0 V to 1.5 V and are shown in Fig. 6. The inset

Fig. 6. The dark current density–voltage characteristics of the IPSCs withvarious PEC treating times. The inset figure shows the equivalent circuitof the real solar cell.

figure of Fig. 6 shows the equivalent circuit of the real solarcell, which included an ideal diode, a series resistance (Rs)and a shunt resistance (Rsh). When the forward voltage waslarger than the build-in electric potential of the solar cell,the ideal diode was turned on. Therefore, in the high cur-rent density region the Jdark–V was influenced by the Rs.When the forward voltage was lower than build-in electricpotential, the diode was turned off and the Jdark was in alow current density level. In this case the Jdark–V character-istic was affected by the Rsh. As shown in Fig. 6, at lowerforward voltage, the dark current density decreased withthe PEC treating time owing to the Rsh increased with thePEC treating time, which corresponded to the lower carrierrecombination loss in the IPSCs. The opposite result wasfound at larger forward voltage, the dark current densitydecreased with the PEC treating time owing to the Rs

increasing with the PEC treating time. Consequently, inaddition to the PEC treatment, the PEC treating time wasanother impact factor for the performance of the IPSCs.As the treating time was too short, the defects on the side-wall surface of the AZO nanorods could not be effectivelypassivated. If the treating time was too long, the super-abundant Zn(OH)2 and Al(OH)3 were formed causing anincrease in the series resistance, which lead to the perfor-mance degradation of the IPSCs. Consequently, the bestperformances of the resulting IPSCs with AZO nanorodstreated for 60 s by PEC method were obtained.

4. Conclusions

In this work, the PEC method was utilized to passivatethe AZO nanorod array formed by the laser interferencephotolithography method and the diluted HCl solutionwet etching process sequentially. According to the XPSspectra of the Zn 2p3/2, Al 2p3/2 and O 1s core-level, the lar-ger amount of Zn(OH)2 and Al(OH)3 was formed on thesidewall surface of the AZO nanorods by PEC treatmentfor 60 s in comparison with the sample without PEC treat-ment. The Rs and Rsh of the resulting IPSCs were influencedby the PEC treating time. The Rsh increased from 351 X cm2

to 560 X cm2 as the PEC treating time increased from 0 s to90 s, which mean that the defects could be passivated andthe carrier recombination loss could be reduced by PECtreatment. The maximum Jsc of 15.85 mA/cm2, FF of63.1%, and g of 6.13% were obtained for the IPSCs withAZO nanorods treated by PEC method for 60 s.

Acknowledgments

The authors gratefully acknowledge the support fromthe Bureau of Energy, Ministry of Economic Affairs of theRepublic of China under contract no. TDPA 101-EC-17-A-08-S1-204, the National Science Council of Taiwan,Republic of China under contract no. NSC 102-3113-P-492-002, NSC 101-2628-E-006-017-MY3.

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