Photophysics and Applications in Plastic Solar Cells of Conjugated Polymer/Fullerene Composites

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1 Photophysics nd Applictions in Plstic Solr Cells of Conjugted Polymer/Fullerene Composites Photophysics nd Applictions in Plstic Solr Cells of Conjugted Polymer/Fullerene Composites Hongzhen Lin, Hongmin Hung, Qingguo He, Fenglin Bi Lbortory for Orgnic Solids, Center for Moleculr Sciences, Institute of Chemistry, Chinese Acdemy of Sciences, Beijing, Chin Received: 30 December 2002 Accepted: 24 April 2003 SUMMARY The photovoltic pplictions of conjugted polymer/fullerene composites hve ttrcted much ttention from the scientific community. The photoinduced electron trnsfer nd other relted photophysicl processes occurred in the composites re the centrl issues. Severl kinds of conjugted polymers hve been synthesized in our lbortory. Studies of their composites with fullerene re reviewed. We observed strong fluorescence quenching of these conjugted polymers on mixing them with C 60 in dilute solution. Anion rdicl species of C 60 were observed in ESR mesurements s result of photoinduced chrge seprtion. The quenching constnts vry with different conjugted polymers nd thus provide quntum mesurement for the interction between the donor nd cceptor. Solr cells hve been fbricted from these conjugted polymer/fullerene composites. Energy conversion efficiency up to 1.1% hs been chieved. 1. INTRODUCTION During the pst few yers, photoinduced electron trnsfer nd chrge seprtion in semiconducting conjugted polymer/fullerene composites hve been extensively studied. Sriciftci nd co-workers firstly reported the photovoltic effect of the bilyers films consisting of poly (2-methoxy-5-(2 -ethylhexoxy-pphenylene vinylene (MEH-PPV nd C After tht, high energy-conversion efficiency up to 3% hs been chieved through control the morphology of the composites into n interpenetrting network. 2 The reltionship between the photoinduced electron trnsfer nd other photophysic processes tht occur in the semiconducting polymer hs lso been explined. Besides MEH-PPV, mny other kinds of conjugted polymer hve been designed nd synthesized for photovoltic pplictions. It is importnt to study the photophysics of conjugted polymer/fullerenes to understnd their interction mechnism nd their pplictions in devices. It is ssumed tht the ction rdicl resulting from the chrge seprtion cn be stbilized nd deloclized on the semiconducting polymer bckbone. The migrtion rtes of the seprted chrges in the corresponding phses ply n importnt role in the chrge collection process. High hole-migrtion rtes cn be chieved by introducing hole-trnsporting groups such s triphenylmine (TPA to the bckbone of conjugted polymer. 3 In our lbortory, series of TPA-contining conjugted polymers hs been synthesized nd chrcterized. Replcing TPA by nother hole-trnsporting group, i.e. crbzole (Cz, we hve obtined nother series of conjugted polymers. 4 Scheme 1 shows the chemicl structures of the triphenylmine-contining nd crbzolecontining conjugted lternting copolymers (TPA- PArV nd Cz-PArV. (PArV=pr romtic vinylene. These polymers my serve s ctive mterils in photovoltic devices. The liner structure of conjugted polymer cn be expnded to three-dimensionl, hyperbrnched structure. Severl kinds of poly (phenylene vinylene- bsed hyperbrnched conjugted polymers (HPVs hve been synthesized by our group (see Scheme 2. 5 By chnging or modifying the core, the connecting units nd the end-cpped groups, the physicl nd chemicl properties of HPVs cn be tuned for different pplictions. These polymers possess high photoluminescence efficiency, high therml stbility nd good processbility. Therefore they re promising mterils for opticl devices such s light emitting diodes nd photovoltic cells. Polymers & Polymer Composites, Vol. 11, No. 8,

2 Hongzhen Lin, Hongmin Hung, Qingguo He, Fenglin Bi Scheme 1 Chemicl structures of TPA-contining (TPA-PArV nd crbzole-contining (Cz-PArV lternting copolymers Before fbricting devices from conjugted polymer/ fullerene composites, we usully investigte the interction between the polymer nd fullerene in dilute solution t first. Bsed on these results, some of the conjugted polymers would be selected to mke photovoltic cells. There re severl issues we re concerned bout. Does photoinduced electron trnsfer occur? How would it be influenced by the structure of the conjugted polymer? And wht bout the relted photophysicl process? To solve these problems, fluorescence spectroscopy nd other techniques such s ESR nd electrochemicl mesurements were employed. In this rticle, we summrize our studies on both the liner conjugted polymers contining hole-trnsporting groups nd the hyperbrnched conjugted polymers. Preliminry results bout devices re lso included. 2. EXPERIMENTAL 2.1 Mterils The conjugted lternting copolymers nd the hyperbrnched conjugted polymers (see Scheme 1 nd Scheme 2 were synthesized in our lbortory ccording to previously reported methods. 4,5 For convenience, n bbrevited nme ws used for ech polymer, s shown in Scheme 1 nd Scheme 2. Detils bout the synthesis nd chrcteriztion of these polymers hve been described in the corresponding report. 4,5 MEH-PPV ws obtined from Aldrich. C 60 (99%, provided by Peking University, ws used s suggested. Benzene, A. R. regent obtined from Beijing Chemicl Plnt, ws used s received or fter purifiction. 680 Polymers & Polymer Composites, Vol. 11, No. 8, 2003

3 Photophysics nd Applictions in Plstic Solr Cells of Conjugted Polymer/Fullerene Composites Scheme 2 Chemicl structures of HPVs with different end-cpped groups 2.2 Spectroscopic Mesurements The UV-vis bsorption spectr were recorded on Hitchi U-3010 spectrometer. The fluorescence spectr were obtined on Hitchi F-4500 spectrometer. The fluorescence lifetimes were mesured by monitoring the emission decy curves on Horib NAES-1100 ns spectrometer. ESR experiments were performed on Bruker ESP-300 spectrometer nd hlogen lmp (400W ws used s the irrdition source with emission monochromticity t wvelength c. 450 nm. ESR spectrometer settings were s follows: sweep width: 20 x 10-4 T, microwve power: 12.9 x 10-3 W, modultion mplitude: 1.0 x 10-4 T, receiver gin: 8.00 x 10 5, time constnt: x 10-3 s, conversion time: x 10-3 s. 2.3 Device Fbriction nd Chrcteriztion Photovoltic cells were fbricted with the device rchitecture of indium tin oxide (ITO/poly (3,4-ethylene dioxythiophene (PEDOT/Active lyer/b/al. TPA- CNPPV (see Scheme 1, TPA-CNPPV/C 60 composite, MEH-PPV nd MEH-PPV/ C 60 composite were used s the ctive lyers, respectively. The thickness of ll the lyers ws 1000 Å. Photocurrent ws mesured under the illumintion of tungsten lmp or AM 1.5 fter being pckged using simple glss fixed with UV curing epoxy resin. With clibrted Si used s the stndrd, the photosensitivity ws chieved on commercil photomodultion spectroscopic setup (MERLIN digitl Lock-in, ORIEL including xenon lmp, n opticl chopper, monochromtor nd lockin mplifier operted by computer. 3. INTERACTION BETWEEN CONJUGATED POLYMER AND C 60 IN DILUTE SOLUTION 3.1 Irreversible Photoinduced Electron Trnsfer in TPA-PAV/C 60 Composites Efficient photoinduced chrge seprtion hs been obtined in composite films of conjugted polymer/ fullerene. The initil steps occurring before chrge seprtion must be clrified for deeper understnding of the whole energy conversion process. However, little evidence concerning the precursor of chrge trnsfer hs been proposed in conjugted polymer/ C 60 composites. In our previous report, 6 the composition between TPA-PAV nd C 60 ws investigted nd fluorescent exciplex ws observed. Further chrge seprtion process my occur in the exciplex, which hs been proved by ESR experiments. Polymers & Polymer Composites, Vol. 11, No. 8,

4 Hongzhen Lin, Hongmin Hung, Qingguo He, Fenglin Bi Figure 1 displys the bsorption spectr of TPA-PAV, with nd without C 60 in benzene. The bsorption spectrum of the mixture is simple superposition of the two components, which shows tht no new complex is formed between TPA-PAV nd C 60 in the ground stte in dilute solution under these experimentl conditions. We note tht similr results were obtined for ll the other conjugted polymers presented in this pper. Intrmoleculr chrge trnsfer (ICT from the triphenylmine moiety to the nthrcene unit cn occur in TPA-PAV. 7 Upon excittion t longer wvelength such s 430 nm, the fluorescence emission of TPA-PAV is minly from its ICT sttes. Figure 2 shows the fluorescence spectr of TPA-PAV quenched by C 60. A new structureless bnd with n isoemission point emerged in the long wvelength region, indicting tht n exciplex ws formed between the ICT stte of TPA-PAV nd C 60 upon photoexcittion. Figure 1 The bsorption spectr of C 60 nd TPA-PAV solution in benzene. (- - TPA-PAV solution (2.0 x 10-5 M without C 60, ( C 60 in benzene (4.2 x10-4 M, ( the mixture of TPA-PAV nd C 60 Figure 2 The fluorescence spectr of TPA-PAV (2.0 x 10-5 M in benzene with incresing the concentrtion of C 60. The concentrtion of C 60 (x10-4 M: 0, 0.0; 1, 1.3; 2, 2.5; 3, 4.2; 4, 6.7; 5, Polymers & Polymer Composites, Vol. 11, No. 8, 2003

5 Photophysics nd Applictions in Plstic Solr Cells of Conjugted Polymer/Fullerene Composites The ESR spectrum of the mixture of TPA-PAV nd C60 in benzene is shown in Figure 3 (b. An ESR signl with g = ppered fter irrdition of the mixture t wvelength of c. 450 nm. This is in greement with Konishi s result 8, demonstrting tht C is produced in this system. The control experiment ws done in the solution of TPA-PAV without C 60 under the sme conditions nd no signl ws observed, s shown in Figure 3 (. We note tht no ESR signl of rdicl ctions ws observed under our experimentl conditions, which my be due to the much less stbility of the positive polrons t room temperture. Figure 3 ESR spectr of TPA-PAV (2.0 x 10-5 M ( nd its mixture with C 60 (8.4 x 10-4 M (b in benzene, detected fter being irrdited t 450 nm The ESR signls of the chrge seprtion species formed in MEH-PPV/C 60 composites cn only be observed t reltive low temperture such s 80 K due to the reversibility of the ultrfst electron trnsfer process. 1 However, in our present system, the chrge seprtion species is stble even t room temperture, which suggests tht n irreversible photoinduced electron trnsfer from TPA-PAV to C 60 tkes plce. It is ssumed tht the formtion of n intrmoleculr chrge trnsfer (ICT stte in TPA-PAV nd the formtion of n exciplex between the ICT stte nd C 60 prevents the bck electron trnsfer nd stbilizes the seprted chrges (see Figure 4. Figure 4 The schemtic representtion for photoinduced chrge trnsfer from TPA-PAV to C 60. F: Fluorescence, P: Phosphorescence, IC: Intersystem Crossing, ICT: Intrmoleculr Chrge Trnsfer, NR: Non-Rdition. [(TPA-PAV-C 60 ] S* nd [(TPA-PAV-C 60 ] T* stnd for the exciplex in the singlet nd triplet stte, respectively 3.2 Fluorescence Quenching Mechnisms for TPA-PArV nd Cz-PArV by C 60 The photoinduced electron trnsfer from the conjugted polymer to the fullerene will compete with other dectivtion pthwys of its excited sttes. The fluorescence quenching mechnism nd the corresponding quenching constnts my reflect some detils of the photoinduced electron trnsfer nd other relted photophysicl processes. For ll the TPA-contining conjugted polymers shown in Scheme 1, the fluorescence quenching by C 60 cn be well described by the Stern-Volmer eqution 9, i.e. F 0 /F = 1 + K SV [C 60 ] = 1 + k q τ 0 [C 60 ] (1 where F 0 nd F re the fluorescence intensities of the polymer in the bsence nd presence of C 60, respectively, K SV is the Stern-Volmer constnt, τ 0 is the fluorescence lifetime of the conjugted polymer without C 60 nd k q is the bimoleculr quenching rte constnt. As n exmple, Figure 5 shows the Stern- Volmer plot for the TPA-PPV/C 60 system. All the K SV, τ 0 nd k q vlues for TPA-contining conjugted polymers re listed in Tble 1. Tble 1 The quenching constnts for TPA-PArV/C60 in benzene Polymer T PA-PPV T PA-PNV T PA-PAV T PA-CNPPV T PA-MEHPPV See Scheme 1 K SV ( 10 3 M τ ( ns k q 2 ( 10 1 M-1 s Polymers & Polymer Composites, Vol. 11, No. 8,

6 Hongzhen Lin, Hongmin Hung, Qingguo He, Fenglin Bi Figure 5 Stern-Volmer plot for TPA-PPV quenched by C 60 in benzene solution As shown in Tble 1, ll the bimoleculr quenching rte constnts re round M -1 s -1, which re t lest one order of mgnitude bove the moleculr diffusion rte in solution. The fluorescence lifetimes of these conjugted copolymers remin unchnged in the presence of C 60. Therefore, dynmic quenching mechnism cn be ruled out in the present system. As no new complex is formed in the ground stte, the non-dynmic quenching is ttributed to n ultrfst formtion of exciplex species between the excited stte nd the djcent C 60 molecule upon excittion. Therefore, k q cn provide quntum mesurement for the exciplex formtion. Subsequent processes such s chrge seprtion cn occur, which re even fster thn the exciplex formtion. Ln (F 0 /F = NV[C 60 ] (2 where V is the volume of the sphere-of-ction nd N is Avogdro s constnt. Figure 6 shows typicl plot ccording to Eq. 2 for Cz-PPV. The fluorescence lifetimes of these polymers re unffected by ddition of C 60, indicting tht the quenching within the sphere-ofction is lso non-dynmic in the present cse. The rdius (R of the sphere-of-ction cn be obtined from its volume (V. All these quenching constnts nd the fluorescence lifetimes of the five Cz-contining copolymers re collected in Tble 2. The rdius of the Chnging the romtic conjugted unit from benzene to nphthlene or nthrcene does not exert much influence on the bimoleculr quenching rte constnt (k q of TPA-PArV/C 60. Introduction of substituent to the benzene ring lso mkes little difference to k q. The k q vlue my be chiefly determined by the introduction of triphenylmine group with its strong electron donting bility. On replcing TPA by crbzole group, the fluorescence quenching mechnism chnged drmticlly. For Cz-PArV/C 60 in dilute solution, the fluorescence quenching is best described by sphereof-ction mechnism 10, i.e. Tble 2 The fluorescence lifetimes nd rdii of phere-of-ction for Cz-PArV/C in benzen s e 60 Copolymer C z-ppv C z-pbpv C z-dmppv C z-pnv C z-pav Nv / (10 2 M - 1 R / nm 0 τ /ns , 7. 3 b 2 See Scheme 1 b The longer lifetime is for the ICT sttes formed in Cz-PAV 684 Polymers & Polymer Composites, Vol. 11, No. 8, 2003

7 Photophysics nd Applictions in Plstic Solr Cells of Conjugted Polymer/Fullerene Composites Figure 6 Dependence of the reltive fluorescence intensity of Cz-PPV on the concentrtion of C 60 sphere-of-ction is relted to the fluorescence lifetime of the copolymer. We ssume tht n excittion diffusion step occurs prior to the photoinduced electron trnsfer. In fct, s the excited stte lifetime of the conjugted polymer is very short, it is difficult for moleculr diffusion-controlled quenching to tke plce in dilute solution. The excittion diffusion vi energy trnsfer is much fster thn the moleculr diffusion process, so it is possible for n excited species to encounter C 60 molecule vi excittion diffusion. Therefore, the rdius of the sphere-ofction depends on the excittion diffusion length in the polymer, which cn be well relted to the lifetime of the excited stte. 11,12 Such quenching mechnism hs not been observed in the TPA-contining copolymer/c 60 system. This cn be explined by the fct tht the exciplex formtion between TPA-contining copolymer nd C 60 is much fster thn the excittion diffusion process. 3.3 Interction Between HPVs nd C 60 : Effect of End-cpped Groups As shown in Scheme 2, series of hyperbrnched conjugted polymers (HPVs with different endcpped groups ws synthesized in our lbortory. 5 The fluorescence of these mcromolecules cn lso be quenched by C 60. The quenching in dilute solution cn be well described by the Stern-Volmer eqution. Their fluorescence lifetimes mesured in solution re lso independent of the mount of C 60. By compring their bimoleculr quenching rte constnts, we could investigte the effects of n end-cpped group on the interction between HPV nd C 60. Tking Es-HPV s n exmple, the fluorescence spectr of the polymer without nd with vrious mounts of C 60 re displyed in Figure 7. The corresponding Stern-Volmer plot is given in Figure 8. All the quenching constnts nd fluorescence lifetimes of these HPVs re listed in Tble 3. The end-cpped group plys n importnt role in the interction between HPV nd C 60. Unlike liner Tble 3 The quenching constnts for HPVs/C i 0 benzene Polymer E s-hpv P y-hpv D MA-HPV B Py-HPV K SV ( 10 3 M See Scheme 2 6 n τ 0 ( ns k q 2 ( 10 1 M-1 s Polymers & Polymer Composites, Vol. 11, No. 8,

8 Hongzhen Lin, Hongmin Hung, Qingguo He, Fenglin Bi Figure 7 The fluorescence spectr of Es-HPV (~5 x 10-3 mg/ml in benzene with incresing concentrtion of C 60. The concentrtion of C 60 ( 10-4 M: 0, 0.00; 1, 0.52; 2, 1.05; 3, 1.75; 4, 2.45; 5, 3.15; 6, 3.85; 7, 4.55; 8, 5.25 Figure 8 Stern-Volmer plot for Es-HPV quenched by C 60 in benzene solution conjugted polymer, the steric interction between brnches mkes hyperbrnched polymer threedimensionl structure. For HPV with reltively high moleculr weight, sphericl geometry is expected nd cve-like spces my be formed mong the brnches. 13 The end-cpped groups my serve s medi between the inner prts of HPV nd the quencher. Therefore, the quenching constnts re dependent on the interction between the end groups nd C 60. We especilly note tht the bimoleculr quenching rte constnt for BPy-HPV/C 60 is nerly three times s lrge s the others, resulting from the strong π-π interction between the bipyridyl group nd fullerene. Further work is being done. 686 Polymers & Polymer Composites, Vol. 11, No. 8, 2003

9 Photophysics nd Applictions in Plstic Solr Cells of Conjugted Polymer/Fullerene Composites 4. APPLICATION IN PHOTOVOLTAIC CELLS Orgnic photovoltic devices bsed on composites of conjugted polymer/fullerene hve number of potentil dvntges such s low cost, light weight, mechnicl flexibility nd the fcility to be fbricted to lrge res. So they re clled plstic solr cells. 14 Two points re of gret importnce for high performnce photovoltic semiconducting polymers: (1 low bndgp; (2 high chrge crrier mobility. Low bnd gp semiconducting polymers cn be obtined by introducing electron donors nd cceptors lterntely into the skeleton of conjugted polymer. 15,16 In TPA-CNPPV (see Scheme 1, such n lternting donor-cceptor system is estblished. At the sme time, the introduction of TPA groups to the min chin of the polymer my fvour the trnsport of photogenerted holes. Therefore, TPA-CNPPV is good cndidte for photovoltic pplictions. The UV-Vis bsorption spectrum of TPA-CNPPV in film is shown in Figure 9. The spectrum covers wide rnge from 300 nm to nerly 600 nm, which helps to hrvest the sun s rys. The bnd gp of the polymer ws estimted to be 2.2 ev ccording to the onset of the bsorption spectrum, much smller thn tht of unsubstituted TPA-PPV (2.6 ev. 17 Photovoltic cells with TPA-CNPPV nd TPA- CNPPV/C 60 composites (weight rtio: 2/1 s ctive lyers were fbricted with the structure of ITO/ PEDOT/Active Lyer/B/Al. Drk current nd photocurrent under illumintion (100 mw/cm 2 with tungsten lmp were mesured, s shown in Figure 10. Photovoltic cells mde of MEH-PPV were fbricted under similr condition. The photosensitivity (PS of devices mde from MEH-PPV nd MEH-PPV/C 60 re lso displyed in Figure 9. The vlues were much lower thn those of TPA-CNPPV. The chrcteristics of the devices re given in Tble 4, which shows tht energy conversion efficiencies (ECE t single wvelength of 500 nm for TPA-CNPPV were much higher thn those for MEH-PPV, wherever in the pristine polymer or in the composites. The better performnce of TPA-CNPPV my be explined by its higher hole mobility. For photovoltic cells mde from TPA-CNPPV/C 60 composites, n ECE vlue up to 1.1% ws obtined under AM 1.5 illumintion (78.2 mw/cm 2. Much higher efficiency cn be expected when PCBM is used insted of C 60. CONCLUSIONS Two types of conjugted polymer hve been synthesized in our lbortory. One type ws liner conjugted lternting copolymer contining holetrnsporting groups (e.g. triphenylmine nd N- Figure 9 Photosensitivity of the photovoltic cells (open circle for TPA-CNPPV nd closed circle for TPA-CNPPV/C 60 ; open squre for MEHPPV nd closed squre for MEHPPV/C 60 nd UV-Vis spectrum of TPA-CNPPV (line Polymers & Polymer Composites, Vol. 11, No. 8,

10 Hongzhen Lin, Hongmin Hung, Qingguo He, Fenglin Bi Figure 10 Drk current (open squre for TPA-CNPPV nd open circle for TPA-CNPPV/C 60 nd photocurrent (closed squre for TPA-CNPPV nd closed circle for TPA-CNPPV/C 60 of the photovoltic cells Tble 4 Performnces of photovoltic cells V o c ( V I sc ( ma/cm 2,c F F (% P S (A/W E CE (% TPA-CNPPV TPA-CNPPV/C60 b MEHPPV b MEHPPV/C V : open circuit voltge, I : short circuit current, FF: fill fctor, PS: photosensitivity, ECE: energy conversion oc sc efficiency b The weight rtios of TPA-CNPPV/C60 nd MEH-PPV/C60 re kept t 2/1 c PS ws mesured t 500 nm substituted crbzole in the bckbone, nd the other ws PPV-bsed hyperbrnched conjugted polymer. The composites of these polymers with fullerene were investigted. The fluorescence of these conjugted polymers cn be strongly quenched by C 60. No complex between the conjugted polymer nd C 60 ws formed in the ground stte in dilute solution. Exciplex my be formed in the quenching system, nd further chrge seprtion process cn tke plce. The ESR signl of the C 60 nion rdicl species ws observed in TPA- PAV/C 60 mixed solution upon the excittion of TPA- PAV. The ESR signl could be observed even t room temperture, indicting tht n irreversible photoinduced electron trnsfer process occurred. The chrge seprtion species might be stbilized by the intrmoleculr chrge trnsfer (ICT stte in TPA- PAV nd the met-stble exciplex formtion. The fluorescence quenching mechnisms for TPA- PArV nd Cz-PArV differed drmticlly. The nondynmic Stern-Volmer quenching of TPA-PArV/C 60 is ttributed to ultrfst exciplex formtion between the excited moiety nd the djcent C 60 upon irrdition. The sphere-of-ction quenching mechnism for Cz-PArV/C 60 cn be relted to the excittion diffusion long the polymer chins. The corresponding quenching constnts provide quntum mesurement for the interction between the conjugted polymer nd fullerene. The interction between C 60 nd the hyperbrnched conjugted polymers cn be tuned by vrying the end-cpped groups of the polymer. A reltively lrge quenching constnt ws obtined for BPy-HPV/C 60, showing this polymer to be good cndidte for photovoltic pplictions. 688 Polymers & Polymer Composites, Vol. 11, No. 8, 2003

11 Photophysics nd Applictions in Plstic Solr Cells of Conjugted Polymer/Fullerene Composites Solr cells hve been fbricted from conjugted polymer/fullerene composites. An energy conversion efficiency of 1.3% hs been chieved in TPA-CNPPV/ C 60 composites, which is much higher thn tht obtined from MEH-PPV/C 60 under the sme device condition. The superior performnce of TPA-CNPPV is explined by the higher hole mobility of this conjugted polymer. ACKNOWLEDGEMENTS This work is finncilly supported by the Ntionl Nturl Science Foundtion of Chin (NSFC (No , 973 (No. 2002CB nd 863 foundtion. REFERENCES 1. N. S. Sriciftci, L.Smilowitz, A. J. Heeger, nd F. Wudl, Science, 258 (1992, G. Yu, J. Go, J. C. Hummelon, F. Wudl, nd A. J. Heeger, Science, 270 (1995, D. Hertel, H. Bässler, U. Scherf, nd H.-H. Horhöld, J. Chem. Phys., 110 (1999, F. Bi, M. Zheng, G. Yu, nd D. Zhu, Thin Solid Films, 363 (2000, Q. He, T. Lin, nd F. Bi, Chin. Sci. Bull., 45 (2000, J. Yng, F. Bi, H. Lin, M. Zheng, Y. Zhng, Y. Li, J. Sun, Y. Liu, nd D. Zhu, Mcromol. Chem. nd Phys., 202 (2001, J. Yng, H. Lin, M. Zheng, nd F. Bi, Mcromol. Chem. & Phys., 202 (2001, T. Konishi, Y. Sski, M. Fujitsuk, Y. Tob, H. Moriym, nd O. Ito, J. Chem. Soc. Perkin Trns. II (1999, J. R. Lkowicz, Principles of Fluorescence Spectroscopy, New York:Plenum Press ( I. M. Frnk, nd S. I. Vvilov, Z. Phys. Chem. (Munich, 69 (1931, F. Bi, C. H. Chng, nd S. E. Webber, Mcromolecules, 19 (1986, F. Bi, C. H. Chng, nd S. E. Webber, Mcromolecules, 19 (1986, J. Yng, H. Lin, Q. He, L. Ling, C. Zhu, nd F. Bi, Lngmuir, 17 (2001, C. J. Brbec, N. S. Sriciftci, nd J. H. Hummelen, Adv. Funct. Mter., 11 (2001, E. E. Hving, W. ten Hoeve, nd H. Wynberg, Polym. Bull., 29 (1992, E. E. Hving, W. ten Hoeve, nd H. Wynberg, Synth. Met., 55 (1993, M. Zheng, F. Bi, Y. Li, G. Yu, C. Yng, nd D. Zhu, Synth. Met., 102 (2001, Polymers & Polymer Composites, Vol. 11, No. 8,

12 Hongzhen Lin, Hongmin Hung, Qingguo He, Fenglin Bi 690 Polymers & Polymer Composites, Vol. 11, No. 8, 2003