Electrochemically assembled planar hybrid poly(3-methylthiophene)/ZnO nanostructured composites

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Electrochimica Acta 81 (2012) 83– 89

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Electrochimica Acta

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Electrochemically assembled planar hybrid poly(3-methylthiophene)/ZnOnanostructured composites

Dmytro Sydorova,1, Petro Smertenkoc, Yuri Piryatinskid, Tsukasa Yoshidab,∗, Alexander Puda,∗,1

a Institute of Bioorganic Chemistry and Petrochemistry, NAS of Ukraine, Kharkivske Shosse, 50, Kyiv 02160, Ukraineb Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University, Jonan 4-3-16, Yonezawa, Yamagata 992-8510, Japanc Institute of Semiconductor Physics, NAS of Ukraine, pr. Nauki 45, Kyiv 03028, Ukrained Institute of Physics, NAS of Ukraine, pr. Nauki 46, Kyiv 03028, Ukraine

a r t i c l e i n f o

Article history:Received 30 April 2012Received in revised form 20 July 2012Accepted 21 July 2012Available online 27 July 2012

Keywords:Electrochemically synthesized layersZnOPhotoelectrochemical polymerizationPoly(3-methylthiophene)Hybrid heterostructure with photovoltaicability

a b s t r a c t

Step by step electrochemical assembling layers of compact ZnO (ZnOcomp), ZnO nanorods (ZnONR)and poly(3-methylthiophene) (P3MT) has been applied to form the photovoltaically active hybridorganic–inorganic heterostructure. Photoluminescence and photoconductivity characterization of thesynthesized ZnOcomp and ZnONR layers display their different defect and doping states resulting in anon-linear I–V behavior of the assembled complex ZnOcomp/ZnONR layer structure for account of for-mation of a p–n junction at the interface. We find some healing of the defect states at surface ofthe ZnOcomp layer after its covering with ZnONR layer. The photoelectrochemical deposition of thestable P3MT on surface of the complex ZnOcomp/ZnONR structure is realized under light irradiationwith wavelength close to near-band-edge (NBE) emission of ZnO (∼380 nm) when using the specialmixed voltammetry–potentiostatic–galvanostatic mode. This mode is found to minimize overoxida-tion/degradation of the forming P3MT. After the synthesis the formed P3MT layer is electrochemicallyreduced to move it to a semiconductor state. The resulting hybrid ZnOcomp/ZnONR/P3MT heterostructureis photovoltaically active and displays open-circuit voltage about 0.45 V.

© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Currently hybrid organic–inorganic nanocomposites are consid-ered as materials being suitable for a wide range of applications.Dependently on the task these can be photovoltaic [1] orlight-emitting devices [2]; catalytic [3], electrocatalytic [4],electrochromic [5], sensory [6] and membrane systems [7]; super-capacitors [8]; drug delivery [9], etc. Special effects can beexpected from combination of two or three or even more smartorganic/inorganic components resulting in a hybrid material withsynergetic properties. Specifically, in the photovoltaic field theepoch of such materials began since the seminal paper by O’Reganand Grätzel [10] on photoelectrochemical dye sensitized solar cells(DSSCs). The efficient DSSC work, in fact, strongly depends on thequality of the highly porous layer made of TiO2 particles and adye contacting with organic electrolyte solution containing I−/I3−

redox couple. Due to a promising high conversion efficiency of thesecells (up to 11.3% [11]) giant efforts of investigators and industry are

∗ Corresponding authors. Tel.: +380 44 5597003; fax: +380 44 5732552.E-mail addresses: [email protected] (T. Yoshida),

[email protected], [email protected] (A. Pud).1 ISE member.

currently directed on their optimization and improvement not onlyfrom the efficiency point but also from the technological, econom-ical and ecological positions. Specifically, much attention has beendevoted to a replacement of TiO2 with ZnO having higher electronmobility and easier synthesis [12,13].

A lot of expectations are connected with more technologi-cal solid state DSSCs [10–12] and hybrid organic–inorganic solarcells using as a photoactive layer a bulk heterojunction (BHJ) ofintrinsically conducting polymer (ICP) like e.g. MEH-PPV [14] orpoly(3-alkylthiophenes) [13,15,16] with inorganic semiconductornanocrystals (CdSe, PbS, TiO2, ZnO, etc.). The important issues hereare realization of not only an effective charge separation but alsogood conditions for a charge delivery to electrodes [16,17]. In thisconnection some approaches are devoted to formation of high qual-ity porous nanostructured metal oxide (TiO2, ZnO) layers and theirfilling with donor ICP [18,19]. Enhanced effectiveness is expectedin case of vertically grown ZnO nanorod arrays on conducting sub-strates and with 10 nm pores that match the exciton diffusionlength [e.g. 13]. Hopefully, these acceptor arrays are the good pre-requisites of an unhindered, fast and direct electron transfer tothe electrode [20]. However, this is a necessary but not sufficientcondition for the effective work of the device and needs to be com-pleted at least with a good contact and interaction between theZnO acceptor and ICP donor counterparts. The crucially important

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84 D. Sydorov et al. / Electrochimica Acta 81 (2012) 83– 89

demand here is complete filling the array pores with the polymer.To respond this demand the infiltration is realized through spin-casting a solution of ICP (typically poly(3-hexylthiophene (P3HT))on the ZnO porous layer surface for some time (from a few tensof seconds [13] till 45 min [21,22]) followed by annealing or evenmelting the P3HT layer for a better infiltration and the polymercrystallization [21,22]. However, the completeness of the infiltra-tion strongly depends on the porous layer morphology and sizeof its pores [13], on the solvent, concentration and viscosity ofthe solution, as well as on the molecular weight and structure ofthe polymer. To minimize these dependences a chemical or pho-tosensitized polymerization of a corresponding monomer insidethe porous layer can be probably useful [23]. Recently [24] thisapproach was realized on the example of electrochemical poly-merization of 3-methylthiophene (3MT) and formation of insolublepoly(3-methylthiophene) (P3MT) doped with ClO4

− on the sur-face of electrochemically grown ZnO nanorods layer. The formedP3MT phase improved electroluminescence of the nanorods signif-icantly. In general, the application of electrochemical methods tosynthesize the hybrid nanocomposite of both ZnO nanorods andP3MT counterparts seems to be not only cost effective but alsovery promising from the point of a good quality of the formingmaterial due to controlled electrochemical techniques. Our workexpands this interesting electrochemical approach to create PV-active hybrid heterojunction of P3MT with ZnO nanostructuredlayer formed at a thin compact ZnO layer precipitated on a con-ducting fluorinated tin oxide (FTO) glass. This porous ZnO/compactZnO double layer structure has been demonstrated as the inorganiccounterpart of effective DSSCs [12]. We have found, however, thata simple electrochemical polymerization of 3MT cannot run at thisdouble layer structure in a wide range of potentials even up to 1.6 Vvs. Ag/Ag+ because of its high dark resistance. To bypass this prob-lem we have used the known photoelectrochemical approach toallow a polymerization process for account of photon absorptionand charge generation/separation in a n-type semiconductor elec-trode (e.g. TiO2, GaAs, ZnO) followed by transfer of electrons toconduction band and holes to monomer molecules in a solution[25–28]. To estimate a separate input of each ZnO part in this dou-ble layer structure we investigated changes of their conductivityand suitability for the 3MT photoelectrochemical polymerizationalso.

2. Experimental

2.1. Electrodeposition of the compact and nanostructured ZnOlayers

The FTO coated conductive glass (Asahi Glass, Asahi-U, 10 �/sq.)was ultrasonically cleaned subsequently in a detergent, acetone,2-propanol and finally rinsed with water. It was used in a configura-tion of a rotating disc electrode (RDE) using an originally designedattachment that was set to a commercial RDE system (Autolab).The size of the electrode was regulated to a diameter of 20 mmusing a masking tape. A single compartment cell equipped with theFTO glass RDE working, Zn wire counter and an Ag/AgCl referenceelectrodes was used for the electrolysis. Potentiostatic electrolysisin an O2 saturated 0.1 M KCl (Merck) aqueous solution at −1.0 V,ω = 500 rpm, 70 ◦C and for 20 min was conducted to activate theFTO surface for O2 reduction that promotes nucleation of ZnOto yield dense and flat growth of ZnO [29]. Then, a concentratedsolution of ZnCl2 (Merck) was added to yield its concentrationof 5 mM in the bath. Potentiostatic electrolysis under the sameconditions but for 5 min yielded a compact ZnO layer of about300 nm thickness. A concentrated solution of eosin Y disodium salt(Kanto) was further added to the bath to yield its concentration of

50 �M to electrodeposit nanostructured ZnO/eosin Y hybrid thinfilms with ca. 3 �m thickness for 20 min over the compact ZnO. Theelectrodeposited multilayer was eventually soaked in a dilute KOHsolution (pH 10.5) at room temperature for overnight to extractthe eosin Y molecules from the hybrid layer to convert it into aporous ZnO with a vertically oriented nanowire structure. Afterdrying in air, such ZnO layers were used for further experiments.

2.2. Photoelectrochemical polymerization of 3-methylthiophene

The (photo)electrochemical polymerization of 3MT (Merck) andprecipitation of P3MT in/at ZnO layers was run in the dry 0.1 M3MT solution in 0.1 M LiClO4 in acetonitrile (Merck) under light illu-mination of different wavelength both in a typical three-electrodetwo compartment glass cell and in a home-made thin layer elec-trochemical cell shown in Fig. 1.

Working solutions were prepared from previously thoroughlydried LiClO4 and acetonitrile in a dry glove box under nitro-gen atmosphere. The background electrolyte quality was checkedby cyclic voltammetry at Pt electrode: the background cur-rents at potentials of 3MT polymerization (1.2–1.5 V) were below0.01 mA/cm2. The polymerization was performed both in galvano-static mode at 0.1 mA/cm2 and in a complex mode by combinationof polymerization in CV mode, followed by potentiostatic modethen by galvanostatic mode. The working electrode potentials in allelectrochemical experiments were measured vs. Ag/Ag+ separatedfrom the reaction medium by a membrane.

As working electrodes we used the above described complexlayered (ZnO nanorods/compact ZnO)/FTO, (compact ZnO)/FTO andporous (ZnO nanorod)/FTO structures. Conductivity of these ZnOlayers was enhanced by their illumination through a quartz lightguides transmitting the light from an external light-emitting diode(LED) to the electrode surface. There were used LEDs which illumi-nated light with peak wavelengths at 460 nm, 405 nm, 395 nm and380 nm.

In the case of electrochemical studies the synthesized P3MT lay-ers were used as received, but for photoluminescent (PL) studiesand estimation of photovoltaic (PV) activity most of the sampleswere electrochemically reduced directly in the photoelectrochem-ical polymerization cell.

2.3. Methods and characterization

Current–voltage (I–V) characteristics of the dry ZnO structuresunder light of the above mentioned LEDs and PV responses ofthe ZnO/P3MT heterostructures were preliminary measured withquasi-DC measurements using a standard automated tester 14 TKS-100 (Russia) in the dark and under illumination using a halogenlamp with power radiation being equivalent to 0.03 sun underambient air conditions. The indium pad (S = 1 mm2) served as a toppressure ohmic contact to the structures under investigation (theohmic nature was proved by the linear I–V curves of the separateZnO structures). Voltage sweep was applied to the heterostruc-ture step by step with the duration of each step 250 ms and themeasurement time 90 ms.

The photoluminescence (PL) spectra were registered by thehomemade setup [30]. The emission was excited by nitrogen laserradiation (�e = 337 nm) with the pulse width 9 ns. The power ofpulse emission was 5 kW and the repetition rate was 70 Hz. PL wasregistered through a MDR-12 grate monochromator equipped witha photomultiplier tube counter, an amplitude discriminator and acoincidence circuit. The final spectra were corrected for the spectralsensitivity of the registering system.

Effect of light on electrochemical properties of the synthesizedP3MT was estimated by cyclic voltammetry (CVA) with the help ofthe digital potentiostat IPC-Pro (NTF Volta, Russia).


D. Sydorov et al. / Electrochimica Acta 81 (2012) 83– 89 85

Fig. 1. Hermetic thin-layer cell for electrochemical deposition of P3MT between/over ZnO-nanostructures given (a) artist’s view of the structural section; (b) bottom sideview; (c) side view: 1 – working electrode (different ZnO layers at FTO, anode), 2 – quartz waveguides, 3 – reference electrode Ag/Ag+, 4 – auxiliary electrode (cathode, copperfoil), 5 – silicon gaskets, 6 – current contact.

The surface morphology of the prepared samples was visualizedby optical microscopy and scanning electron microscopy (SEM)using JEOL JSM-6060 LV.

3. Results and discussion

3.1. Photoluminescence and photoconductivity properties of theZnO structures

Room temperature PL measurements of the electrochemicallysynthesized ZnO layers showed their different optical propertiesto be strongly depended on the layer morphology (Fig. 2). Specif-ically, the compact layer displays in UV-region a near-band-edge

Fig. 2. PL spectra of the ZnO layers: 1 – compact ZnO layer, 2 – ZnO nanorods, 3 –complex ZnOcomp/ZnONR structure.

(NBE) emission at 375 nm being very weak due probably to its non-radiative quenching at the defects, which in turn give an intensiveblue-green-orange emission (Fig. 2, curve 1).

It should be emphasized here that spectra of ZnO PL are typi-cally characterized with a weakly structured wide band in the visualblue-red region [31]. Unlike this, in the case of the compact con-ducting ZnO layer, one can see three distinct overlapping bands at560 nm and 600 nm in yellow region, at 654 nm in red region, clearshoulders at 490 nm and 525 nm in green region and at 706 nm inred region (Fig. 2, curve 1). These features testify obviously to anexistence in this sample of a few kinds of defects with well definedenergies (e.g. different oxygen vacancies or oxygen surplus at ZnOsurface, etc. [32]).

The ZnO nanorods, synthesized in the presence of the dye, alsoshowed visible broad green-yellow-red band centered at 580 nm,located practically in the same PL region and with the similar inten-sity as in the case of the compact layer but being structureless (Fig. 2,curve 2). Unlike the case of the compact layer, the shape of thisdefect PL band is typical of different ZnO nanostructures [31–34].However, these nanorods do not display NBE emission that in com-bination with the visual band shape suggests that they have anotherset of defects than that of the compact ZnO layer.

The situation was completely changed when ZnO nanorods wereelectrochemically grown over the compact ZnO layer. Specifically,the defect visual bands of the both layers were strongly quenched(Fig. 2, curve 3) and, moreover, NBE emission appeared also withratio IUV/IVis ∼ 2.1 testifying obviously to the improved quality ofthis complex ZnOcomp/ZnONR structure as compared with the sep-arate layers [33,35]. This result suggests at least two reasons of theimprovement. The first one can be obviously precipitation duringelectrodeposition of the ZnO nanorods of the precursor Zn(OH)2



Fig. 3. Effect of the light wavelength (given at peak intensities – wavelengths of LEDs) on: a resistance of ZnO layers: (a) compact ZnO; (b) ZnO nanorods; (c) complex bilayerZnOcomp/ZnONR structure; and (d) voltammetric behavior of bilayer ZnOcomp/ZnONR structure in the background electrolyte 0.1 M LiClO4 in CH3CN, cyclic voltammogramswere recorded at 100 mV/s.

and ZnO in the defect states of the compact layer surface leading tochanges in different oxygen and zinc vacancies, etc. and thereforeto healing some part of the defect states. The second reason canbe probably some orientation/quasi-seeding effect of the compactlayer surface on the nanorods growth.

The differences in optical properties of these three electro-chemically synthesized structures match with peculiarities of theirphotoconductivity behavior. As one can see from Fig. 3a, the ZnOcompact layer is characterized with a practically linear I–V depen-dencies and a low resistance (∼50 Ohm/sample) which nearly doesnot change with light wavelength, i.e. ZnO in the compact layer isalmost photoinactive.

Comparison of the above-mentioned PL spectrum (Fig. 2)of the compact layer with these conductivity features (Fig. 3a)suggests that they are probably due to a quite high level of dopingfor account of some kinds of defects which are not only surfacelocalized along grain boundaries in this polycrystalline compactlayer but also distributed in the bulk of the grains. This leads inturn to the quite high intrinsic conductivity of this layer.

The ZnO nanorods appeared to be much more resistive and reallyphotoconductive with strong dependence on the wavelength light(Fig. 3b). Specifically, they are practically insulating in dark butunder light strongly reduce their photoresistance with its lowestvalue 0.9 MOhm under illumination by LED with peak light wave-length 380 nm (Fig. 3b). The difference with the above-mentionedcompact layer behavior stems probably from differing set of defectspresenting in the nanorods and observed in their PL spectrum(Fig. 2).

Unlike the single layer ZnO structures, the complex bilayerZnOcomp/ZnONR one displayed pronounced non-linear I–V charac-teristics, intermediate resistance and more complicated photocon-ductivity behavior (Fig. 3c). Specifically, the conductivity increasesunder UV illumination and in the visual light it is practically thesame as in dark that suggests a possibility of application of such astructure as a UV-detector. On the other hand, this bilayer struc-ture under low voltages (Voc ∼ 1 V) keeps resistance almost on theconstant high level ∼1 MOhm due to probably high resistance ofthe ZnONR layer. The non-linear electrical behavior of this structure



Fig. 4. Cyclic voltammograms of P3MT precipitated at/in the bilayer ZnOcomp/ZnONR

structure under irradiation with different light wavelength (a) synthesis of P3MT,0.1 M 3MT solution in 0.1 M LiClO4 in acetonitrile, 3 first cycles in the CVA mode; (b)electrochemical cycling of the synthesized P3MT in the background electrolyte.

stems obviously from the nonidentity and different doping states ofthe ZnO compact and nanorods layers that result in appearance ofp–n junction at the interface. The similar I–V behavior was observedby Nakamura et al. for ZnO/ZnO single crystal contacts [36].

Importantly, in electrochemical conditions being close to thecase of DSSC this bilayer structure is more sensitive to the lightwavelength than in the above electrical case. Specifically, back-ground currents strongly increased under light comparatively tothe dark one in up to about two orders of magnitude at the electrodepotential 1.5 V and light source with wavelength 380 nm (Fig. 3d).This phenomenon can probably be explained not only through theabove described increase of the ZnO structure conductivity butalso through some photoactivation of background electrochemi-cal processes occurring at the interface of semiconductor electrodeand electrolyte like above mentioned photoelectrochemical poly-merization due to the increment of charge carriers (holes) in theelectrode and facilitation of their transfer to a depolarizer [25–28].

3.2. Electrochemical polymerization of 3MT at the bilayerZnOcomp/ZnONR structure

A facilitation of the 3MT polymerization process in CVA modeunder illumination is clearly observed at the photoactive bilayerZnOcomp/ZnONR structure (Fig. 4). Specifically, under the visuallight (430–510 nm or at peak maximum 460 nm) the polymeriza-tion proceeds very slowly that results in an appearance of a verysmall quantity of the polymer at the electrode only after multiple

Fig. 5. Cyclic voltammograms of P3MT layers synthesized at bare FTO under lightwith different wavelength.

potential cycling even under anodic polarization up to 1.5 V. Unlikethis, in the case of irradiation with a shorter light wavelength(380 nm) being close to the band edge absorption of ZnO, one canclearly observe from the second cycle appearance and growth ofthe polymer electrochemical activity, which becomes apparent byan increase of the current density, by gradual shifting of the oxi-dation potential onset (e.g. from ∼1.3 V of the first anodic scanto ∼1 V) as well as by appearance and growth of a typical pair ofanodic/cathodic peaks of P3MT.

Similarly to electrochemical polymerization of 3MT at conduct-ing electrodes [37] the both peak potentials shift from cycle tocycle to their respective higher values due to a growth of the poly-mer layer thickness that results in polarization phenomena foraccount of changes in heterogeneous electron transfer kinetics, theIR drop across the layer, compacting of the film and hence impedinga mobility of a counter-ion and a decrease in the film conduc-tivity [38]. This suggests that a specificity of the electrochemicalpolymerization of 3MT at the photoactive bilayer semiconduc-tor ZnOcomp/ZnONR structure does not change qualitatively underlight irradiation comparatively to the same process at a conducting

Fig. 6. Chronopotentiogram (a) and chronoamperogram (b) of the process ofthe 3MT photoelectrochemical polymerization at the bilayer semiconductorZnOcomp/ZnONR structure in the combined mode.



Fig. 7. Photo (a) and SEM (b) images of the ZnOcomp/ZnONR/P3MT heterostructure.

electrode without irradiation. However, the P3MT formed at thisstructure under irradiation with wavelength 380 nm is oxidizedeasier (anodic peak at 653 mV) than one formed under irradiationwith higher wavelengths 395 and 460 nm (anodic peak at 755 mV)(Fig. 4b). This suggests higher molecular weight of the former P3MTwhich in turn facilitates the less oxidation potential.

An important point in photoelectrochemical polymerization is astability of the formed polymer under UV irradiation [27]. To eval-uate possible changes in the forming P3MT under UV irradiationwe polymerized 3MT on ITO anode under illumination of the abovedescribed light sources and compared cyclic voltammograms of theformed polymer. As one can see the cyclic voltammograms dis-play that the synthesized P3MT layers almost do not differ in theirelectrochemical behavior (Fig. 5).

The observed very weak differences in height and shape of theP3MT anodic/cathodic peaks pair are obviously in the frames of anexperimental error. This P3MT behavior suggests its stability underthe experimental conditions and therefore confirms a possibility offormation of the stable P3MT at/inside the ZnO nanostructure underlight irradiation.

Fig. 8. PL quenching (a) and photovoltaic activity (b) in the ZnOcomp/ZnONR/P3MTheterostructure: (a) 1 – PL spectrum of the ZnOcomp/ZnONR structure, 2 – PL spectrumof the ZnOcomp/ZnONR/P3MT heterostructure.

At the same time, there is an electrochemical instability of P3MTin this system when repeating cycling of the heterostructure tohigh potentials up to the up-switch potential 1.5 V. Specifically,this is accompanied with some decrease of the P3MT peak fromcycle to cycle (Fig. 4b) that obviously stems from the overoxi-dation/degradation processes known for polythiophenes at highpotentials [39]. To minimize these processes we have run the pho-toelectropolymerization process mainly at lower potentials exceptthe initial stage. This was realized through a change of the aboveCV synthesis method to the combined one including linear sweepvoltammetry mode (part I) to scan the potential up to 1.5 V for 15 sand to activate the process (scan rate of 100 mV/s) (Fig. 6a) followedby keeping this potential for 20 s in the potentiostatic mode (part II)(Fig. 6a) to generate 3MT oligomers facilitating the polymerizationprocess at lower potentials [40,41].

Changes of the chronoamperogram in the potentiostatic mode(part II) (Fig. 6b) are typical of diffusion limited processes [42] anddisplay an oxidation of the monomer during initiation stage of thepolymerization that is followed by a formation of the oligomericproducts having the oxidation potential lower than 3MT [40,41].This mode was then commutated on a galvanostatic mode at cur-rent density 0.1 mA/cm2 that allowed the potential dropping up tothe ∼1.075 V (stage III). This potential was in fact constant for next450 s (Fig. 6a) and in total all the photoelectrochemical polymer-ization time was 485 s. As one can see from SEM and OM images(Fig. 7), this process allowed the formation of a quite homogeneous



P3MT layer inside/over that part of the ZnO structure, which wasunder irradiation.

The synthesized P3MT layer at the ZnOcomp/ZnONR structure wasthen electrochemically reduced at −0.5 V to move this polymer toa semiconductor state and to estimate photovoltaic activity of thewhole heterostructure. As one can see from Fig. 8a, the weak PL ofthe ZnOcomp/ZnONR structure is practically quenched in this hybridZnOcomp/ZnONR/P3MT heterostructure.

Only low PL intensity in red region is observed that can corre-spond both to PL of some defect states of ZnO and to not completelyquenched PL of the reduced solid P3MT. This PL quenching sug-gested charge separation in the heterostructure and a possibility ofits PV activity. These points are obviously confirmed with voltam-metric measurements of the heterostructure, specifically withopen-circuit voltage of ∼0.45 V, which is clearly observed in theI–V curve in Fig. 8b. However, as one can see from these measure-ments, a short-circuit current of this heterostructure is very lowthat can be caused by presence in the system of oxygen hinderingcharge transfer [43] during measurement conditions and probablyby some unclear for the moment factors resulting in total with highserial resistance of the cell.

Nevertheless, the used approach testified to a real possi-bility of the formation of the photovoltaically active hybridZnOcomp/ZnONR/P3MT heterostructure using quite simple andtechnological electrochemical techniques. The problem of thehigh serial resistance is under study and will be discussedelsewhere.

4. Conclusions

The photoactive hybrid organic–inorganic planar heterostruc-ture consisting of layers of compact ZnO, ZnO nanorods andpoly(3-methylthiophene) have been successfully assembled at sur-face of a transparent FTO electrode when using only the applicableelectrochemical deposition and polymerization techniques, respec-tively. Due to PL and I–V measurements we have known that theseparately synthesized ZnOcomp, and ZnONR layers have differentdefect and doping states. These differences resulted in formation ofa p–n junction and non-linear voltammetric behavior in the subse-quently mounted ZnOcomp/ZnONR heterojunction. Importantly thatelectrochemical deposition of ZnO nanorods at surface of the ZnOcompact layer leads to a partial healing of the defect surface statesof the latter.

The photoelectrochemical deposition of the stable P3MT onsurface of the complex ZnOcomp/ZnONR structure was effec-tively realized only under light irradiation with wavelength(∼380 nm) close to the near-band-edge (NBE) emission of ZnO.Naturally that visual light was practically ineffective in thephotosensibilization of the ZnO layers and therefore in the elec-trochemical polymerization of 3-methylthiophene. We have foundthat possibility of the overoxidation of the forming P3MT underconditions of the investigation can be minimized due the spe-cial mixed voltammetry–potentiostatic–galvanostatic mode. Thismode allows short-time initiation of the polymerization at highpotential (1.5 V vs. Ag/Ag+) through formation of oligomers duringmixed voltammetry–potentiostatic part followed by fast droppingof the polymerization potential up to ∼1.075 V in the galvanostaticconditions.

In order to invert the synthesized doped P3MT layer to asemiconductor dedoped state and to facilitate a photovoltaicactivity of the whole heterostructure, this polymer was then elec-trochemically reduced at potential −0.5 V. The resulting hybridZnOcomp/ZnONR/P3MT heterostructure is photovoltaically activeand displays open-circuit voltage about 0.45 V.

Acknowledgments

The authors are grateful to Mr. K. Hiramatsu from Environmentaland Renewable Energy Systems Division, Graduate School of Engi-neering Gifu University for technical assistance with preparation ofthe ZnO structures at FTO.

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