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Electrodeposition zinc-oxide inverse opal and its application in hybrid photovoltaics

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Electrodeposition zinc-oxide inverse opal and its application in hybrid photovoltaics
  Solar Energy Materials & Solar Cells 92 (2008) 357–362 Letter Electrodeposition zinc-oxide inverse opal and its application inhybrid photovoltaics Mingqing Wang, Xiaogong Wang  Department of Chemical Engineering, Laboratory for Advanced Materials, Tsinghua University, Beijing 100084, PR China Received 5 August 2007; received in revised form 2 October 2007; accepted 3 October 2007Available online 19 November 2007 Abstract This article reports the preparation of three-dimensional (3D) mesoporous zinc oxide (ZnO) films and their application in solar cells.The films were obtained through electrochemical deposition in DMSO solutions by using PS colloidal crystal as templates. The ZnO filmswith inverse opal (IO) structure were obtained after removing the templates by thermolysis. The ordered porous ZnO films were used toprepare hybrid solar cells by infiltrating the films with poly(3-hexylthiophene) (P3HT) or P3HT:ZnO nanocomposite. Results showedthat the interpenetrating network of both ZnO(IO) and P3HT can form continuous pathways for electron and hole transport. Byinfiltrating a P3HT:ZnO nanocomposite into the porous ZnO films, the photocurrent of the solar cell can be dramatically improved. Thecell shows the  V  oc  and  I  sc  of 462mV and 444.3 m A/cm 2 , respectively. By using a 420nm cutoff filter, the cell retains about 80% and 50%of its srcinal  V  oc  and  I  sc  after continuous white-light illumination (100mW/cm 2 ) for 10h. Stability of the device under above conditionswas estimated to be 51h. r 2007 Elsevier B.V. All rights reserved. Keywords:  Solar cells; ZnO; Electrodeposition; Inverse opal; P3HT 1. Introduction Photovoltaic devices based on solution-processableconjugated polymers (CPs) are attractive for their low-costand scalable fabrication [1–5]. Compared with traditionalinorganic photovoltaics, performance of polymer-basedphotovoltaics is limited by weak absorption in the red,poor charge transport, and low stability, but improvementsare available through optimization of materials and devicestructures [6–11]. One of the most promising approaches toimprove the device efficiency is to build up interpenetratingnetwork of electron-accepting and hole-accepting compo-nents in the active layers of solar cells. This has beenachieved by using polymer blends [12,13], doping CPs with C 60  derivatives [14–16], and mixing CPs with inorganicnanocrystals such as TiO 2  [17 – 19], ZnO [20–24], and CdSe [25–28]. But for most bulk heterojunction devices, the CPsand electron acceptors are randomly interspersed through-out the films. The random distribution of electronacceptors could cause electron trapping on isolatedacceptors and incomplete photoluminesence quenching atthe interfaces [29].Fabrication of well-ordered bulk heterojunction of organic and inorganic semiconductors is a promising routeto increase the efficiency of polymer-based solar cells.Nanostructured oxide arrays, such as ZnO nanofibersvertically aligned on the substrate, have been used toconstruct bulk heterojunction [30]. This approach can beexpected to increase the donor–acceptor interfacial areaand create electron transport pathways toward the negativeelectrode. However, compared to ZnO nanoparticle-basedcells, the current density of the nanowire-based cells islower [31]. The lower efficiency could be caused by thedecreased surface area and low light-harvesting efficiencyof the nanowires as compared to nanoparticles. Anotherpromising approach is to use nanoporous films, such asinverse opal (IO) structures of inorganic-semiconductor.This architecture can increase interfacial area between CPs ARTICLE IN PRESS$-see front matter r 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.solmat.2007.10.001  Corresponding author. E-mail addresses:, (X. Wang).  and inorganic-semiconductor and form bicontinuouscharge-transfer channel in the heterojunction solar cells.In addition, the irradiated light will be scattered orreflected multiple times within the ordered porous struc-ture, which can enhance light absorption by increasingoptical path length in solar cells. Because of thesepromising characteristics for future applications, the useof inverse TiO 2  opals for solar cells has been investigated inrecent years [32,33].One of the main bottlenecks for these inverse opal-basedcells is the limited area of the ordered 3D arrays.Electrodeposition is an efficient method to fabricate largearea of inverse opal with high filling fraction. By thismethod, the morphology and thickness of the film can bereadily controlled by adjusting the electrochemical para-meters such as the electric current or potential. By usingcolloidal crystal as templates, electrodeposition can pro-duce inverse opal structures with a high filling fraction of the deposited material. Recently, it has been reported thatZnO inverse opal structure was prepared by electrochemi-cal deposition in water solution [34,35].In this work, ordered mesoporous ZnO films wereprepared through electrochemical deposition in a DMSOsolution of ZnCl 2  by using PS colloidal crystal astemplates, which were subsequently removed by heattreatment. By using DMSO instead of water as solventfor electrolyte during the ZnO deposition, the formation of compounds such as Zn x (OH)  y  could be avoided. High-quality inverse opal ZnO films were obtained and hybridsolar cells were prepared by infiltrating P3HT into theporous films. In order to further improve exciton dissocia-tion and electron transfer efficiency in the device, photo-voltaic devices were also prepared by infiltratingP3HT:ZnO nanocomposite into the ordered porous ZnOfilms. The preparation, characterization and solar cellperformance will be presented in following parts in detail. 2. Experimental The monodispersed polystyrene (PS) latex particles(with the diameter of 200nm) were prepared by emulsionpolymerization in this laboratory according to the litera-ture [36]. The 3D colloidal arrays were assembled on ITOsubstrates by vertical deposition method in a 45 1 C oven.A three-electrode system (CHI660A ElectrochemicalWorkstation) was used for the electrochemical depositionof the ZnO films. The ITO substrates covered with thecolloidal arrays were used as the electrodeposition workingelectrode and a saturated calomel electrode (SCE) was usedas a reference electrode. The ZnO mesoporous films wereelectrodeposited by the cathodic deposition carried out in aDMSO solution containing 0.02M ZnCl 2  at 45 1 C. 0.1MKCl was used as assistant electrolyte. After the colloidaltemplates were removed by thermolysis at 600 1 C in air for2h, ordered mesoporous films with interpenetratingchannels were obtained. A suitable amount of P3HT( M  w ¼ 35,148, PDI ¼ 1.32,  T  g ¼ 50 1 C,  T  m ¼ 175 1 C) wasdissolved in chloroform to form a 15mg/mL solution. Thesolution was spin-coated onto the top of the inverse ZnOopals and then the films were treated by thermal annealingat 180 1 C under vaccum in darkness. For device prepara-tion, the back electrodes (100-nm-thick Al films) wereevaporated in high vacuum. To prepare the devices usingZnO:P3HT nanocomposite as the infiltrated component,ZnO:P3HT composite (containing 50wt% of ZnO) wasprepared according to the reference [37] and was drop-coated onto the top of the inverse ZnO opals by using a15mg/mL chloroform/methanol solution.SEM measurement was performed on a field emissionmicroscope (JEOL JSM-6301F) with the acceleratingvoltage of 5kV. XRD was measured on a Bruker D8-Discover instrument. UV–vis spectra of the spin-coatedfilms were recorded on a Perkin-Elmer Lambda Bio-40spectrometer. Films for TEM observations were preparedby spin-coating on the 400 mesh copper grids. The TEMimages of the composite films were obtained by using aJEOL-JEM-1200EX microscope with an acceleratingvoltage of 120kV. Current–voltage ( I   –  V  ) measurementswere taken in air at room temperature using a Keithley 236high current source power meter under   100mW/cm 2 white-light illumination from a xenon lamp. 3. Results and discussion The mesoporous ZnO films with interconnected networkwere obtained by the cathode electrodeposition of ZnCl 2  inDMSO solution. Fig. 1 gives the SEM micrographs of theZnO inverse opals obtained at various cathodic currentdensities and deposition time periods. Fig. 1a and b showthe ZnO films prepared at the cathodic current density of 5and 1mA/cm 2 for 10min, respectively. The films possessperiodic, hexagonal, close-packed pores with smoothsurface, which can be observed over tens of squaremicrometers. Compared with Fig. 1a, some defects canbe seen in Fig. 1b. Fig. 1c and d show the films obtained after a longer deposition time. The channels connecting thepores can be clearly visible, which confirms the 3Dinterpenetrating pores formed in the structure. Comparedwith Fig. 1c, pores in Fig. 1d show a smoother surface. In the following discussion, the ZnO films were obtained bythe current density of 1mA/cm 2 and the deposition timeof 30min.Fig. 2 shows the X-ray diffraction of ZnO films afterheating at 600 1 C in air for 2h. Diffraction peaks arereadily indexed to the standard diffraction of hexagonalphase ZnO. The diffraction peaks at 2 y  equal to 31.77 1 ,34.42 1 , 36.25 1 , 47.54 1 , 56.60 1  and 62.78 1  are from thediffraction of (100), (002), (101), (102), (110), and(103) planes. It can be seen that the ZnO films obtainedfrom the electrodeposition possess a high crystallinity.Fig. 3 shows optical transmittance in the wavelengthrange from 300 to 900nm for both the porous ZnO film onITO glass and a thin film without the pores as reference.Compared with the thin film with similar thickness, the ARTICLE IN PRESS M. Wang, X. Wang / Solar Energy Materials & Solar Cells 92 (2008) 357–362 358  transmittance of the porous film is obviously lower in the400–550nm range. This is due to the interface scatteringcaused by the porous structure, which results in theincrease of the optical path length and the decrease of transmission near the shorter visible wavelength. Fordevice applications, this multiple scattering can produce alonger light path inside the porous ZnO films and the lightabsorption efficiency can be enhanced as a result.Two types of the photovoltaic devices were prepared byinfiltrating the mesoporous ZnO films with P3HT orP3HT:ZnO nanocomposite. The photovoltaic performanceof the ITO/ZnO(IO)/P3HT/Al and ITO/ZnO(IO)/P3HT:ZnO/Al devices was shown in Fig. 4. The ITO/ZnO(IO)/P3HT/Al device exhibits an open-circuit voltage( V  oc ) of 624mV and a short-circuit current ( I  sc ) of  ARTICLE IN PRESS Fig. 1. SEM micrographs of the ordered mesoporous ZnO films prepared with different cathodic current densities and depositing time: (a) 5mA/cm 2 ,10min; (b) 1mA/cm 2 , 10min; (c) 5mA/cm 2 , 30min; (d) 1mA/cm 2 , 30min. 20 30 40 50 60 70    I  n   t  e  n  s   i   t  y   (  a .  u .   ) 101102 1102 θ  (degrees) 002100103 Fig. 2. X-ray diffraction patterns of the electrodeposited ZnO on ITOglass. 300 500 600 700020406080100Compact ZnO filmPorous ZnO film    T  r  a  n  s  m   i   t  a  n  c  e   (   %   ) Wavelength (nm)400 800 900 Fig. 3. Optical transmittance spectra of ZnO thin films. M. Wang, X. Wang / Solar Energy Materials & Solar Cells 92 (2008) 357–362  359  80.5 m A/cm 2 . As ZnO films were obtained by growth in thevoids between PS colloids with diameter of 200nm, theP3HT phase possessed the characteristic dimensions asthe PS colloids. For high-efficiency bulk heterojunctioncells, the ideal size of the domains should be 10–20nm orless. But in this case, P3HT could hardly enter the voidsthrough capillary force. In order to enhance the dissocia-tion of the photogenerated excitons and increase  I  sc  basedon the 200nm void ZnO films, P3HT:ZnO nanocompositewas used as the infiltrated material. The performance of the ITO/ZnO(IO)/P3HT:ZnO/Al device is dramaticallyimproved, which shows the  V  oc  and  I  sc  of 462mV and444.3 m A/cm 2 , respectively. The incorporation of ZnO intothe P3HT resulted in a 5.5 times increase in  I  sc  over thedevice infiltrated only with P3HT, but  V  oc  is decreasedafter the addition of ZnO.The variation caused by using P3HT:ZnO nanocompo-site can be understood through the following analysis. TheTEM picture in Fig. 5a indicates the phase separation of the infiltrated P3HT:ZnO composite film. Because of thehigher electron density of the ZnO nanoparticles, contrastbetween ZnO-rich domains (dark) and P3HT-dominateddomains (bright) can be clearly seen. In the composite,ZnO particles with diameters of few nanometers are welldispersed in the polymer phase. According to the TEMobservation, a schematic illustration can be drawn for theactive layer morphology of the ITO/ZnO(IO)/P3HT:ZnO/Al solar cell (Fig. 5b). The ordered pores(ca. 200nm) in ZnO film was infiltrated by P3HT:ZnOnanocomposite. The interconnected channels can ensure alarge amount of active interfaces for exciton dissociationand form continuous pathways for charge carrier transfer.In every pore, ZnO nanoparticles of few nanometersare dispersed in the P3HT phase, which can formadditional interfaces between electron donor (P3HT)and electron acceptor (ZnO) and enhance the excitonseparation. This causes the significant increase of thephotocurrent observed in the system. The  V  oc  decreasecould be attributed to conducting channels formed by theZnO nanoparticles in the infiltrated P3HT phase. When theamount of ZnO nanoparticles increases, this effect could bemore significant. Based on the understanding, the photo-voltaic performance of the devices can be optimizedby adjusting the amount of ZnO nanoparticles in theinfiltrated phase.Performance stability under illumination is anotherimportant issue for hybrid solar cells [38–40]. Photoche-mical reactions of the n-type oxides could be one of thereasons to cause the deterioration of cell properties. UVlight can excite semiconductor oxides to generate activeoxygen species, which are known to interact with anddegrade organic polymers [41]. It has been reported that  I  sc of hybrid cells containing ZnO nanoparticles or ZnOfilms decreased in less than 2h under white-light irradi-ation [37,40]. Polymer bleaching and device degradationwere observed at the same time. The stability of theITO/ZnO(IO)/P3HT:ZnO/Al device was tested in ambientatmosphere under continuous illumination of 100mW/cm 2 white light. Fig. 6 shows  I  sc  and  V  oc  as a function of  ARTICLE IN PRESS 0.0 1.00.2 0.4 0.6 0.8-400-2000200400Voltage (V)    C  u  r  r  e  n   t   D  e  n  s   i   t  y   (      µ    A   /  c  m    2    ) ZnO IO/PTH ZnO IO/PTH:ZnO Fig. 4. Current density–voltage characterization of the ITO/ZnO(IO)/P3HT/Al and ITO/ZnO(IO)/P3HT:ZnO/Al photovoltaic devices.Fig. 5. (a) TEM image of the ITO/ZnO(IO)/P3HT:ZnO/Al photovoltaicdevices. (b) Schematic illustration of the morphology of ZnO(IO)/P3HT:ZnO active layer. M. Wang, X. Wang / Solar Energy Materials & Solar Cells 92 (2008) 357–362 360  illumination time, which was obtained by using a 420nmcutoff filter placed between the cell and the xenon lamp.The cell retains about 50% of its srcinal  I  sc  and 80% of itssrcinal  V  oc  after 10h. Stability of the solar cell under theabove condition, characterized by the decrease of   I  sc  toalmost zero, was measured to be 51h.In the current case, the stability is significantly improvedcompared with the early reports concerning ZnO andMEH–PPV hybrid cells [42]. The possible reason could bethat P3HT is not so easy to be bleached by light irradiationas are the PPV-based polymers. The light-induced bleach-ing of PPV-based polymers is well reported in the literatureand is normally attributed to chemical reactions leading tochain scission [43]. Upon photoexcitation, the singletexciton of PPVs can transfer to the excited triplet statethrough intersystem crossing. Singlet oxygen can then beproduced through energy transfer with the triplet. Thesinglet oxygen can oxidize the vinylene double bonds bycycloaddition reaction, which leads to the formation of carbonyl units and scission of the polymer backbone.Compared with PPV-based polymers, heterocycles inpolythiophenes are more stable under light irradiation[44]. This could play a very important role to enhance thestability of the ZnO/P3HT hybrid cells. 4. Conclusions 3D-ordered mesoporous ZnO films were fabricated byelectrodeposition in DMSO solution and characterized bySEM and XRD. The porous films were prepared by usingPS colloidal crystal as the templates. When using thecathodic current density of 1mA/cm 2 and depositing timeof 30min, the ZnO inverse opal showed a remarkable longrange of order and a smooth surface. The porous electrodehybrid solar cells were made by infiltrating P3HT orP3HT:ZnO composite into the ordered porous ZnO films.The photocurrent of the ITO/ZnO(IO)/P3HT/Al devicewas limited because of the large diffusion distance forexciton to reach ZnO frameworks. This was obviouslyimproved by using the P3HT:ZnO composite. A significanthigher photocurrent was observed owing to the enhancedexciton dissociation and electron transfer efficiency. Solardecay analyses showed lifetime of ITO/ZnO(IO)/P3HT:ZnO/Al device could be improved by the application of aUV filter. References [1] F.C. Krebs, J. Alstrup, H. Spanggaard, K. Larsen, E. Kold, Sol.Energy. Mater. Sol. Cells 83 (2004) 293.[2] J.R. Sheats, J. Mater. Res. 19 (2004) 1974.[3] F.C. Krebs, H. Spanggard, T. Kjaer, M. Biancardo, J. Alstrup,Mater. Sci. Eng. B 138 (2007) 106.[4] G. Dennler, C. Lungenschmied, H. Neugebauer, N.S. Sariciftic,A. Labouret, J. Mater. Res. 20 (2005) 3224.[5] C. Lungenschmied, G. Dennlera, H. Neugebauera, S.N. 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