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PPV/TiO 2 hybrid composites prepared from PPV precursor reaction in aqueous media and their application in solar cells

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PPV/TiO 2 hybrid composites prepared from PPV precursor reaction in aqueous media and their application in solar cells
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  PPV/TiO 2  hybrid composites prepared from PPV precursor reactionin aqueous media and their application in solar cells Mingqing Wang, Xiaogong Wang*  Department of Chemical Engineering, Laboratory for Advanced Materials, Tsinghua University, Beijing 100084, PR China Received 10 September 2007; received in revised form 23 January 2008; accepted 25 January 2008Available online 1 February 2008 Abstract In this work, poly(phenylene vinylene) (PPV) and TiO 2  nanocomposites containing different amounts of TiO 2  were prepared through PPVprecursor reaction in aqueous media. The TiO 2  components were introduced into the systems by two methods, i.e. through in situ sol e gel re-action or by mixing commercially available TiO 2  nanoparticles with the PPV precursor before reaction. The composite prepared by mixing com-mercially available TiO 2  nanoparticles shows perfect crystal character of the anatase TiO 2 , but TiO 2  particles severely agglomerate in the PPVmatrix. The composite prepared by introducing TiO 2  nanoparticles through the sol e gel reaction shows uniform nanoscale dispersion of anataseTiO 2  in PPV matrix. The UV e vis and FL spectroscopic analyses confirm the formation of the TiO 2  /PPV composites and reveal the enhanced PLquenching effect as the TiO 2  content increases. The PPV/TiO 2  composites can show significant photovoltaic response. Better photovoltaicperformance is observed for the solar cells prepared by using the in situ sol e gel reaction method.   2008 Elsevier Ltd. All rights reserved.  Keywords:  PPV/TiO 2 ; Nanocomposite; Photovoltaics 1. Introduction Recently, nanocomposites of conjugated polymers (CPs)and inorganic compounds have been intensively investigatedfor the applications in devices such as light emitting diodes,photodiodes, sensors, smart microelectronic, and photovoltaiccells [1 e 3]. The nanocomposites can show novel synergiceffects as well as enhanced optical and electronic properties.Hybrid CPs  SN (semiconductor nanocrystal) solar cells cancombine interesting properties from CPs and bulk inorganicmaterials. The nanocomposites can show processing benefitsof polymer-based materials such as the solution-processibilityand low-temperature chemical synthesis. These advantagesmake them good candidate materials for clean, renewableand low-cost energy resources. The nanocomposites for hybridsolar cells have employed different SNs such as TiO 2  [4 e 6],ZnO [7 e 11], and CdSe [12 e 15].To obtain high efficiency, it is ideal to have a bicontinuousinterpenetrating network of electron-accepting and electron-donating components within the devices. Generally, the activelayers of the polymer-based devices are prepared by usingtheir solutions or dispersions. Conjugated polymers such asP3HT, P3OT, MEH e PPV and MDMO e PPV can be welldissolved in organic solvents and have been widely appliedas the electron-donating and hole-transferring components inthe photovoltaic materials. On the other hand, due to theincreasing concern on environmental and safety problems re-lated to the organic solvents, processing conjugated polymersby using aqueous media is getting increased importance.Recently, a water-soluble polythiophene (sodium poly(2-(3-thienyl)ethoxy-4-butylsulfonate), PTEBS) has been investi-gated for organic photovoltaics [16 e 19]. It is well knownthat PPV precursor has good solubility in aqueous media andcan be converted into PPV by heating at high temperature (theWessling e Zimmerman route) [20]. Therefore, PPV precursorcould be used for preparing electron-donating component inaqueous media. To enhance the performance of PPV-based * Corresponding author.  E-mail address:  wxg-dce@mail.tsinghua.edu.cn (X. Wang).0032-3861/$ - see front matter    2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.polymer.2008.01.061  Available online at www.sciencedirect.com Polymer 49 (2008) 1587 e 1593www.elsevier.com/locate/polymer  composites, studies have been carried out on composites madeof the polymers and nano-oxides recently [21,22].In this work, PPV/TiO 2  composites were prepared throughPPV precursor reaction in aqueous media. TiO 2  was used asthe electron-accepting material because of its non-toxicityand abundance availability [23]. The TiO 2  components wereintroduced into the composites through in situ sol e gel reac-tion or by direct mixing of TiO 2  nanoparticle dispersionswith the PPV precursor solutions. The PPV/TiO 2  compositeswere tested for photovoltaic device applications by sandwich-ing the active layers between a thin PEDOT layer and analuminum top electrode. The preparation, characterization andphotovoltaic properties of the nanocomposite materials willbe presented in following sections in detail. 2. Experimental  2.1. Synthesis of PPV precursor  The PPV precursor was prepared according to the literature[24]. The monomer  p -xylene-bis(tetrahydrothiophenium chlo-ride) was prepared by reaction of dichloro-  p -xylene (0.25 M)with excess tetrahydrothiophene (0.75 M) at 50   C in metha-nol for 12 h. The product was purified by concentrating thereaction solution and then precipitating the condensed solutionin cold acetone (0   C). The solid was collected by filtrationand dried thoroughly in vacuum oven. The PPV precursorwas prepared by addition of 30 mL of 0.1 M NaOH solutioninto 30 mL of   p -xylene-bis(tetrahydrothiophenium chloride)aqueous solution (0.1 M). Both solutions were cooled to0   C in an ice bath before mixing. The reaction proceeded at0   C for 1 h and then was terminated by the addition of 0.1 M HCl aqueous solution to neutralize the reaction solution.After the solution was concentrated, the PPV precursor aque-ous solution was dialyzed against deionized water for severaldays. The PPV precursor was then freeze-dried and dissolvedin water (20 mg/mL) for the following experiments.  2.2. Preparation of TiO  2  and PPV precursor mixtures Method 1 : TiO 2  dispersion with the solid content of 50 mg/ mL was prepared by ultrasonication of nanoparticles (DegussaP25, average particle size 20 nm) in deionized water for 8 h.PPV precursor (20 mg/mL) was mixed with the TiO 2  disper-sion in different ratios. The weight percentages of TiO 2  inPPV are given in Table 1. After mixing, the mixtures werefurther sonicated for 4 h to disperse the TiO 2  powder andprevent the dispersed powder from aggregating.  Method 2 : Ti(OC 3 H 7 ) 4  (20 mL) was slowly dropped into120 mL aqueous nitric acid solution (0.1 M) with strong stir-ring. The mixture was vigorously stirred at 80   C for 8 h untila translucent solution was obtained. The solution was filtratedthrough a G4 filter to remove a small amount of yellow oil-likeliquid and insoluble conglomerate. The sol solution containingTiO 2  particles was concentrated to 100 mg/mL and mixed withthe PPV precursor solution (20 mg/mL) in different volumeratios. The weight percentages of TiO 2  in PPV are given inTable 2.  2.3. Substrate preparation ITO glass slides covered with 50 nm thick poly(3,4-ethyl-enedioxythiophene) (PEDOT) were used as the solar cellsubstrates. The PEDOT films were prepared by electrodeposi-tion in a three-electrode system (CHI660A ElectrochemicalWorkstation). The ITO substrates were used as the electrode-position working electrode and a saturated calomel electrode(SCE) was used as a reference electrode. PEDOT was depos-ited by the anodic deposition carried out in 50 mL borontrifluoride ether complex solution containing 0.355 g EDOTat 25   C. Deposition current density and deposition timewere 0.1 mA/cm 2 and 60 s, respectively.  2.4. Fabrication of solar cells The mixtures prepared as above were cast-coated onto thePEDOT-covered ITO glass slides. After drying in an oven at80   C for 2 h, the composite films were subsequently placedin a vacuum oven at 200   C for 6 h. In the process, PPVprecursor was converted to PPV. For the sol e gel approach,the TiO 2  nanoparticles were formed in situ at the sametime. The thickness of PPV/TiO 2  layers were measured byatomic force microscopy (AFM) and the results are givenin Tables 1 and 2. Aluminum counter electrodes were depos-ited on the film surfaces by vacuum evaporation. The activeareas of the cells were controlled to be 0.09 cm 2 with theadhesive transparent tapes before the thermal evaporation of aluminum.  2.5. Characterization Composite films prepared by spin-coating on quartz sub-strates were used for the XRD and spectroscopic characteriza-tions. XRD was performed on a Bruker D8-Discoverinstrument. Films for TEM observation were prepared byspin-coating on a 400 mesh copper grid. The TEM images Table 1The current e voltage characteristics of the hybrid photovoltaic cells withdifferent TiO 2  contents prepared by direct mixingTiO 2  (wt%) Thickness ( m m)  I  sc  ( m A/cm 2 )  V  oc  (V) FF  h  (%)20 1.22 17.55 1.15 0.21 0.00440 1.26 35.1 1.18 0.21 0.00960 1.32 58 0.86 0.2 0.010Table 2The current e voltage characteristics of the hybrid photovoltaic cells withdifferent TiO 2  contents prepared by in situ sol e gel reactionTiO 2  (wt%) Thickness ( m m)  I  sc  ( m A/cm 2 )  V  oc  (V) FF  h  (%)20 1.14 36.9 0.56 0.23 0.004840 1.16 139.7 0.46 0.26 0.01860 1.18 118.5 0.35 0.22 0.0091588  M. Wang, X. Wang / Polymer 49 (2008) 1587  e 1593  of the composite films were obtained by using a JEOL-JEM-1200EX microscope with an accelerating voltage of 120 kV.SEM measurement was performed on a field emission micro-scope (JEOL JSM-6301F) with the accelerating voltage of 5 kV. UV e vis spectra of the spin-coated films were recordedon a Perkin e Elmer Lambda Bio-40 spectrophotometer. Photo-luminescence measurements were performed by using a Hita-chi F-4500 flourescence spectrophotometer. Current e voltage(  I  e V  ) measurements were carried out in air at room tempera-ture using a Keithley 236 high current source power meterunder white light with intensity of 100 mW/cm 2 . 3. Results and discussion The structure and schematic energy level diagram for thephotovoltaic device are illustrated in Fig. 1. In the design,the PPV component was used as the electron donor and alsofor hole-transport. The PPV/TiO 2  composites were preparedfrom the PPV precursor in aqueous media through theWessling e Zimmerman approach. The TiO 2  componentswere introduced into the composites through two differentmethods. In one of the methods, the composite was preparedthrough a modified sol e gel technique. In typical sol e gel ap-proaches, TiO 2  /polymer composites were prepared by usingorganic solvents [25,26]. In the current study, TiO 2  precursorwas first hydrolyzed in aqueous solution and then mixed withPPV precursor in aqueous media. The applied method allowsthe inorganic materials to be synthesized at low temperatureswithout degrading the organic functional groups or polymers.This in situ formation scheme is designed to better dispersethe nanostructured components. For comparison, PPV/TiO 2 composite was also prepared by mixing TiO 2  nanoparticledispersions with PPV precursor solutions and then the PPVprecursor was converted to PPV.Fig. 2 shows the XRD patterns of the hybrid films with60 wt% TiO 2  prepared by the two methods. In the figure,TiO 2  diffraction peaks can be assigned to the diffraction planesof the anatase phase according to the standard diffractionindex. The peak at 2 q  ¼ 25.3  corresponds to the (101) crystalplane of anatase, others peaks at 38.1  , 48.1  , and 54.2  corre-spond to the anatase (112), (200), (211) crystal planes. As PPVcomponent has a low crystallization degree, a broad diffractionpeak related to the PPV component, which appears at2 q ¼  21.06  [27], can be seen in the XRD figures (Fig. 2(a) and (b)). For the composite prepared by directly mixingTiO 2  nanoparticles with PPV precursor (Method 1), the crys-tallization of TiO 2  is perfect and the XRD curves is very sim-ilar to that of the pristine nanoparticles. Theaveragecrystallitesize of TiO 2  phase was calculated to be 22 nm from the (211)reflection peak by the standard XRD software equipped on theinstrument. For the composite prepared by the modified sol e gel method (Method 2), XRD curve also shows characteristicpeaksoftheanataseTiO 2 ,butthepeaksarebroader.Inthisprep-aration process, PPV precursor was blended with TiO 2  particlesoland the PPV/TiO 2 composite filmswere obtained byheatingthemixtureinavacuumovenat200   Cfor6 h.Duringtheheat-ing treatment, PPV precursor was converted to PPV and thecrystallized TiO 2  was also formed at the same time.In order to understand the XRD peak broadening of TiO 2  inthe composites prepared by the sol e gel method, TiO 2  powderalso was prepared by the sol e gel method under the similarconditions without the PPV-precursor/PPV matrix. Fig. 2(c)shows the XRD spectrum of the TiO 2  powder. Except the dif-fraction peak due to PPV, Fig. 2(b) and (c) shows very similarcharacteristic peaks of the anatase crystal. It can be seen thatTiO 2  powder also shows the broad peaks similar to those givenin Fig. 2(b). The result suggests that the PPV-precursor/PPVmatrix shows little influence on the crystal structure of theTiO 2 . The peak broadening is mainly due to the smaller crys-tallite domain and lower crystallization degree of TiO 2 , whichcould be attributed to the low sol e gel reaction temperatureused here. The average crystallite domain size of TiO 2  pre-pared by sol e gel method was calculated to be 12 nm fromthe (211) reflection peak by the standard XRD software.The morphology and particle dispersion of the compositeswere investigated by TEM. Since the TiO 2  nanoparticleshave the higher electron density, contrast between TiO 2 -richdomains (darker) and PPV domains (lighter) can be clearlyseen (Fig. 3). TEM images of the composites containing40 wt% TiO 2  are given here as typical examples. Fig. 3(a)shows the PPV/TiO 2  composite prepared by direct mixing(Method 1). It can be seen that the TiO 2  nanoparticles aggre-gate severely to form clusters of ten to hundred nanometers.On the contrary, for PPV/TiO 2  composite prepared by in situsol e gel reaction (Method 2), the TiO 2  particles are more uni-formly dispersed in the PPV phase and show typical particlesizes in range from 10 to 20 nm. The size is in good agreementwith the crystallite domain size estimated from XRD. It indi-cates that the TiO 2  nanoparticles are better dispersed in thePPV matrix by the sol e gel method. Fig 4 shows SEM imagesof PPV/TiO 2  composites with different contents (20, 40, and ITO electrodeActive layer  Al electrodeCopper lead3.9 eV+5.0 eVhvPEDOTTiO 2 7.1 eV5.1 eVPPV Al b 4.3 eV AlPPV/TiO 2 PEDOTITO coated glass a 2.7 eV c  _  Fig. 1. (a) Layout of the PPV/TiO 2  photovoltaic devices, in which active layeris sandwiched between a PEDOT layer and aluminum top electrode. (b) Sche-matic energy level diagram for the device, where energy levels are given in eVrelative to the vacuum level. (c) Schematic structure of one piece of solar celldevice.1589  M. Wang, X. Wang / Polymer 49 (2008) 1587  e 1593  60 wt%) of TiO 2  nanoparticles prepared by direct mixing(Method 1). It can be seen that the TiO 2  phase in the hybridmaterials are composed of the aggregates in significantamount.Different dispersion states of TiO 2  nanoparticles in bothkinds of the composites are related with the structures at thepolymer/inorganics interface and the formation process of the TiO 2  nanoparticles. For the commercially available TiO 2 nanoparticles, some active bonds of TiO 2  on surfaces mightreact with each other during the last annealing phase of theTiO 2  nanoparticle preparation. In the aqueous dispersions,the nanoparticles will agglomerate to decrease the surfacearea of the dispersed phase. This process is also favorable inenergy as the van der Waals interaction between the nanocrys-tals is stronger compared to the weak interaction between TiO 2 nanoparticle and PPV precursor. When TiO 2  nanopartilces areformed in the in situ sol e gel process, TiO 2  precursor was par-tially hydrolyzed in aqueous solution, which will have stronginteraction with PPV precursor in the media. This in situ for-mation nature can efficiently prevent TiO 2  nanoparticles fromagglomeration.UV e vis spectroscopy was used to investigate the effect of the TiO 2  amount and preparing method on the light absorptionof composite films. Fig. 5 shows the UV e vis absorption spec-tra of PPV, PPV precursor, and spin-coated films of a series of PPV/TiO 2  nanocomposites. The PPV component shows ab-sorption band with  l max  around 400 nm (Fig. 5(a)) and TiO 2 absorbs light mainly in the wavelength range from 200 to300 nm. As it can be seen from Fig. 5(b), the PPV/TiO 2 composites prepared by both methods show the overlappingabsorption bands of PPV and TiO 2 . The absorption intensityof TiO 2  increases with the increase of the TiO 2  content. Bycomparing the spectra, some differences can be seen fromthe UV e vis spectra of the composites obtained through differ-ent methods. For PPV/TiO 2  composites obtained by Method 1,the absorption band around 400 nm shows no difference as the Fig. 3. TEM photographs of PPV/TiO 2  hybrid composites with 40 wt% TiO 2 prepared by (a) direct mixing and (b) in situ sol e gel reaction. 102030405060211112200101 a    I  n   t  e  n  s   i   t  y   (  a .  u .   ) 2   (degrees)2   (degrees)2   (degrees) c 102030405060211200112101 b    I  n   t  e  n  s   i   t  y   (  a .  u .   ) PPV 110102030405060211200112101    I  n   t  e  n  s   i   t  y   (  a .  u .   ) Fig. 2. X-ray diffraction of PPV/TiO 2  composites with 60 wt% TiO 2  prepared by (a) direct mixing, (b) in situ sol e gel reaction and (c) X-ray diffraction of TiO 2 nanoparticles prepared by sol e gel reaction.1590  M. Wang, X. Wang / Polymer 49 (2008) 1587  e 1593  Fig. 4. SEM images of PPV/TiO 2  composites with different contents of TiO 2  nanoparticles prepared by direct mixing: (a) 20 wt%, (b) 40 wt%, (c) 60 wt%. 3004005006007000.00.10.20.30.40.5 a Wavelength (nm)    A   b  s  o  r   b  a  n  c  e   (  a .  u .   ) PPV precursor PPV2003004005006007000.00.51.01.52.02.5 b 20wt% TiO 2 40wt% TiO 2 60wt% TiO 2 Wavelength (nm)    A   b  s  o  r   b  a  n  c  e   (  a .  u .   ) 2003004005006007000.00.40.81.21.6 c 20wt% TiO 2 40wt% TiO 2 60wt% TiO 2 Wavelength (nm)    A   b  s  o  r  p   t   i  o  n   (  a .  u .   ) Fig. 5. UV e vis absorption spectra of PPVand PPV precursor (a), and PPV/TiO 2  nanocomposites with different TiO 2  contents prepared by direct mixing (b) andin situ sol e gel reaction (c).1591  M. Wang, X. Wang / Polymer 49 (2008) 1587  e 1593
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