Journal ArticleSilicon bridged PT polymer (PTSi) is a new soluble polymer with a strong luminescence in the infrared band (0-0 transition at 1.6 eV). We have applied various spectroscopies such as absorption, electroabsorption (EA), photo induced absorption (PA), electromodulated PA (EPA), photoluminescence (PL) and photo induced absorption detected magnetic resonance (PADMR) to pristine and C60- doped PTSi thin films in order to eluvidate the photoexcitation and electronic states this  material.. The absorption spectrum consists of three bands, one in the visible with a peak at 2 eV and the others in the UV at 4.5 eV and 6.2 eV respectively. The PL band at 1.6 eV contains also three pronounced phonon replica, each 150 meV apart. An electric field reduces the luminescence due to dissociation of excitons into polarons. The PA spectrum is dominated by a broad PA band peaked at 1.1 eV which is due to triplet-triplet transitions, with a shoulder at 1.7 eV due to polaron  pairs; there is also a distinctive band at 0.3 eV due to both, polarons and polaron pairs. Upon doping with C60- the 1.1 eV triplet band loses intensity, whereas the 0.3 eV feature dramatically increases and the typical signature of C60- HOMO to LUMO transition appears at 1.15 eV

Liess, M.

Vardeny, Zeev Valentine

English

The University of Utah: J. Willard Marriott Digital Library

Optical probes of pristine and C60 -doped Silicon bridged PT polymer; a novel highly luminescent, low bandgap polymer. M. Liessa* and Z. V. Vardenya, W. Chenb and T. J. Bartonb 80epartment of Physics, University of Utah, Salt Lake City. Utah 84112, USA bChemistry Department, Iowa State University, Ames, Iowa 500 11 , USA ABSTRACT Silicon bridged PT polymer (PTSi) is a new soluble polymer with a strong luminescence in the infrared band (0-0 transition at 1.6 eV). We have applied various spectroscopies such as absorption, electroabsorption (EA), photo induced absorption (PA), electromodulaled PA (EPA), photoluminescence (PL) and photo induced absorption detected magnetic resonance (PADMR) to pristine and C60 doped PTSi thin films in order to eluvidate the photoexcilalion and electronic slates this material.. The absorption spectrum consists of three bands, one in the visible with a peak at 2 eV and the others in the UV at 4.5 eV and 6 .2 eV respectively. The PL band at 1.6 eV contains also three pronounced phonon replica, each 150 meV apart. An el(!{;tric field reduces the luminescence due 10 dissociation of excitons into polarons. The PA spectrum is dominated by a broad PA band peaked at 1.1 eV which is due 10 triplet-triplet transi tions, with a shoulder at 1.7 eV due to polaron pairs; there is also a distinctive band at 0.3 eV due to both, polarons and polaron pairs. Upon doping with C60 the 1. 1 eV triplet band loses intensity, whereas the 0.3 eV featu re dramatically increases and the typical signature ofC60- HOMO to LUMO transition appears at 1.15 eV. Keywords: Silicon bridged PT polymer, low bandgap p-conjugated polymer, C60 dopi ng, luminescence, eI(!{;lric field induced exciton dissociation I. INTRODUCTION Recently luminescent conductive polymers (LCP) have been the focus of extensive research efforts. The drive for these efforts are the potential applications such as molecular light emitting diodes (LED). large panel display and perhaps even "plastic lasers" . The bright photoluminescence (PL) or the order of 50% is the most important property of LCP . The PL band in LCPs has been reported to span the visible spectral range from red to blue. However PL in the near lit range has nOi been yet reported. We present in this communication a new type of LCr with Ihe emitting PL band in the IR range. The new polymer is the Silicon bridged PT polymer (PTSi) of which repeat unil is shown in Figure I inset. • Current amlialion : Istitilio di SpcctroSCOpill Molecuilire. CNR, 1-40 129, Bologna, Italy SPtE Vot . 3145 . 0271-786X/97/$10.00 415 The strong luminescence in PTSi and the factthal the absorpt ion and PA bands are at least 0.4 eV away from the PL (Figure I), reduce self absorption in PTSi. This makes PTSi promissing for infrared LED and possibly also laser applications. The low bandgap of PTSi is also useful to improve understanding of theoretical models describing the visible and UV absorption bands. The photoexcilation dynamics and the electronic structure in PTSi are also interesting and have been studied using a variety of optical techniques. Elcctroabsorption (EA) measures the change in absorption due to the application of an electric field . The analysis of signal is based on a X(3) process and allows TO determine the excitonic levels and their strongly coupled vibrations. Experimental setup and analysis of EA spectroscopy are described elsewhere in more detail {I , 2] . Electric field s in the order oflOS Vlcm and sample temperatures or80K were used in this study. Photoinduced absorption (PA) spectroscopy measures the change in absorption of a infrared probe beam due to pumping of the material with a Slrong light beam of photon energies above the absorption edge. In Ihis study, the 0.1 mW of the UV lines of an Ar+ laser were mechanically chopped at 400 Hz to periodically pump the material. The pump induces excitation which are delected with the probe beam. A more complete description of the experimemal technique and the analysis of the spect ra is given by Vardeny et al. [3J . As probe a tungsten lamp was used and the signal was spectrally resolved using a monochromator and infrared filters . Spectra were taken at 80 and 300 K. PTSi a 0 -0 .8 ::; <i. -'" PA ~ c 0 .6 Il ~ " c E => ...J 0.4 -t:::: b -M 0.2 0 « Cl. o 0 . 5 1 Figure 1: Spectra of PA, PL, and optical density or PTSi 416 PL 1.5 Energy (eV) Hex S( 00 2 Hex 2.5 Photoinduced absorption detected magnetic resonance (PADMR) measures the change in PA due to magnetic resonance. The spectra were taken at 10K using a microwave frequency oD GHz and magnetic fields at around 500 G and at around 1000 G. The experimental setup is described elsewhere [3-5) . The measurements anow to characterize the spin quantum number of PA and active excitations. 2. EXPERIMENTA L TECHNIQUES EPA and EPL measures the change in PA and PL respectively due 10 exposure of the material to an electric fi eld. The effects can have many different origins as discussed elsewhere in this volume (6] . In PTSi an electric field leads to an enhanced dissociation of exci tons into polarons. Thus the EPA shows thc field related generation of polarons. while EPL indicated the field relatcd reduction of I Bu excitons. To measurc EPA, PA active photocxcilation were generated using 100 mW of the 514 nm linc of an Ar+laser. distributed ovcr 112 cm2 area of the samplc. The sa mple was exposed to an electric fields in the order of 105 V/cm. The electric field induced change in IR transmission of the sample indicates a modulation of the PA. EPL measurements were done similarly by spectrally detecting the field induced changes of photoluminescence. A temperatures of80K was used for these measurements. 3.EXPERIMENTALRESULTS 3.1 LUMIN ESCENSE AND SU8GAP PIIOTO EXCITATIONS The PL band at 1.6 eV cont ains three pronounced phonon replica, each I SO meV apan (Figure 2). Electromodulated photoluminescence (EPL) spectroscopy shows that an electric field of 105 Vlcm applied to the film reduces the PL by a factor of 1 + 112000 (Figure 2). The PA spect rum of the pristine PTSi fitms (Figure 3 (a) is dominated by a broad PA band which peaks at 1.1 eV with a shoulder at 1.65 eV; there is also a dist inctive band at 0.3 eV. An appl ied electric field enhances the bands at 0.3 eV and ahove 1.2 eV, while it reduces the PA bands al 1.1 eV as shown elsewhere in this proceedings (6) . Also upon doping wi th C60 ' the 1.1 eV band loses relative intensity. whereas the 0.3 eV and 1.65 eV bands dramatically increase, and the typical signature ofC60- HOMO to LUMO transition appears al 1.1 5 cV ( Figure 3(b». It is important to OOte that the PA between 0.7 eV and 0.9 eV is clearly above zero in the pristine sample due to the long tai l of the broad absorpt ion band at 1. 1 eV. Upon C60 doping the PA goes down to zero at 0 .8 eV. This is an indication thaI the excitation which causes the broad feature al 1. 1 eV gels reduced by C60 doping while another excitation with a much sharper absorption signature appears at the same energy. This excitation consists of three correlated bands at 0.3 eV 1.15 (C60- -ion HOMO-LUMO transition) and 1.65 eV due to polaron pair complexes (p' C60- ). Additional evidence for this interpretation is given by ADM R spectroscopy. The ADMR speclrum (Figure 4) of the undoped material shows a reduction of the PA due to magnetic resonance. For spin- 1/2 excitations (g=2), this reduction peaks at 1.1 eV (polarons) and 1.65 eV (bipolarons). For 8~ the PADMR shows only one broad band due to lriplet excitons that peaks at 1.1 eV. The fact that in PTSi three 417 ~ =i 400 <i. ~ ~ " =i .. m c <i. " u ~ m " 200 " c m f' c " => u -' m " "0 C 0 0 .f' " u => => -' "0 ." -0 .2 "0 .<!! ~ 0.75 , 1.25 1.5 , .75 2 Energy (eV) Figure 2 : PL and electro-modulated change in PL (EPL) in pristine PTSi: An electric field reduces the luminescence slightly. 4 3 ~ 2 ~ , ~ -<aJ r\,;st;ne PTSi ! C> 0 <t: ~ - , e 60 doped -<bJ PTSi , - \ r\. !::: ~ 0.5 ~ C> it 0 -0.5 o 0.5 , 1.5 2 2.5 En ergy(eV) Figure J : PA orCa) pristine and (b) C60 doped PTSi at 80 K, excited with 0. 1 W of514 om laser light. 418 4 3 ( a) -t-- 2 --.. t--~ 1 cYl C> « 0 CL - 1 4 - -t::: 2 2 t::: '=? '=? Lr> 0 0 <.D C> C> '" ""'""" II -2 II 0) -2 0) = = ::a ::a 0 -4 -4 0 « « -6 -6 0.5 1 1 .5 2 -,,_"'0"; >\<? Energy (eV) Figure 4: P A and PADMR spectra of pristine PTSi at 10K. At g=:!, the P ADMR spectrum shows s= 1/2 (doublet) excitations and at g=4 it shows s=1 (triplet) excitations. ditTerent excitations (polarons, C60- ,and the triplet exciton) have a transition at the sampe energy ('" 1.1 eV) has to be taken into account for the interpretation of the spectra. 3.2. THE EXCITONIC BANDS IN PTSI r;~ i. PT polymers show three major absorption bands. Figure 5 compares the absorption spectra of PTSi, Poly(3-butyl thiophene) P3BT and Poly (3-0ctyl thiophene) P30T (The spectrum of P30T was taken with permission from leglinski [2].) In the low gap polymer PTSi, the absorption bands peak at 2 eV, 4.5 eV and 6.3 eV. The comparison with the absorption bands of other polymers shows that the first band at 2 e V due to the 1 Bu exciton occurs at a different energies in different PT polymers, while the other bands at 4.5 eV and 6.2 eV appear consistently at the same energy in all PT polymers (Figure 5). ~ The EA spectrum of PTSi shows a feature at l. 95 eV that resembles the first derivative of the linear absorption (Fib'Ure 6). As an effect of the conjugation length distribution, this signature appears in the EA at lower photon energies as the corresponding band in the absorption [1]. At higher energies, however a field induced absorption band appears at 2.55 eV, which has no correspondent band in the linear absorption. This band can therefore be assigned to the optically forbidden mAg ~ "ate whieh gm", o,dU",o, ""0sth ;0 EA due to the 'l""motry b",aldog ; .. e"c';oo of the h;gh "«trie fidd. At highe< 419 3,----------------.r-, PTS I .. 2 8 1 o .j....e!f.?-,~~:..._,,___._,___..___l 1 2 3 4 5 Energy (eV) Figure 5: Optical absorption of different PT polymers from 1.5 to 6.S eV. 4 3 ~ 2 t::: I-<l 1 , ~ <t 0 , 0 -' {\ -« l1J - 1 - 2 -3 , , , 2 3 4 Energy (eV) Figure 6 : Elettroabsorption ofPTSi, measured al SOK with 105 V/cm 420 6 7 5 photon energies only at 4.5 eV a small derivative shaped feature appears where an band can be seen in the linear absorption. The ract, that the EA signature is extremely weak indicates the relatively low polarizability or the corresponding exciton. 4. DISCUSSION 4.1 . PHOTOEXCI1'ATIONS WITH SIGNATURES BELOW THE GAP: The broad PA band between 0.9 and 1.8 eV (Figure 4 (a)) is composed or optical transitions or triplet excilons (I eV), polaron pairs (1 .65 eV), and free polarons ( 1.1 eV). The rollowing evidence supports this interpretation: I) A strong electric field reduces the luminescence of the pristine material (Figure 2). This might be due to field~ induced dissociation or singlet excitons into polarons and polaron pairs, which in tum also reduces the percentage or the singlet excitons that may decay into the triplet manifold. EPA shows a bleaching close to the maximum of the triplet absorption band and an increase or the polaron and polaron-pair absorption bands at the low and high energy end or the spectrum due to an electric field , as discussed elsewhere in this proceedings [6] This supports the hypothesis or field-induced dissociation or singlet excitons. Obviously, the band which is to be assigned 10 the triplet state behaves in a complementary rashion to the polaron bands in EPA. 2) C60 -doping is known to quench singlet excitons and to rorm polaron pairs. Thererore this path indirectly decreases the triplet density. C6Q -doping decreases the main broad triplet-triplet absorption at 1.1 eV. At the same time it increases reatures due to polaron pairs at 1.6 eV and the C60- absorption at 1. 15 eV. This indicates red uction of the singlet to triplet intersystem crossing due to enhancement or a competing decay route. An additional electric field increases due to singlet exciton separation the effect rurther. The ract that the signatures of polarons, C60' and polaron pairs partially overlap with the triplet band obscures thie process or exciton dissociation. 3.) The PADMR spectrum or pristine PTSi (Figure 4.22) measured at g=4 clearly indicates the presence of a very broad triplet excitation peaking at 1.15 eV. The R'''2 PADMR spectrum indicates that the photoexcitations with transitions below 0.5 eV and at J.I eVand 1.65 eV are sensitive 10 a spin-lIZ resonance. These pholoexcitations are thererore polarons with bands at below ... 0.5 eVand I.J eV, which are spin- 112 excitations and polaron pairs at 1.65 eV. Even though polaron pairs are spinless, their signature shows up in the spectrum due 10 the modulation of polarons. 4.2. EXCITON BANDS IN PTSI The linear optica lly active I Bu exci lon lies at 1.95 eV as determined by EA . The maximum of the absorpt ion is at approximately 2.2 eV, while the maximum of the luminescence can be round at 1.8 eV. The reason for this shift in energy is the conjugati on lengt h related distribution orthe states in energy, as weU as phonon sidebands which merge in the absorption spectrum to one big blue shifted absorption band. The PL however is emitted ITom excitons, which diffused to the I Bu stales Which have the longest conjugation length and are thus low lying in energy. 421 The EA data indicates that the polarizability oflhe excito n 3t4.5 eV is much lower than the polarizability of the I Bu exciton. The energy of the delocalized IBu exciton depends highly on its subst ituents. This is discussed in more detail by Hotta f7] . The other twO bands in the UV appear in all polyt hiopenes at the same energy. From this we can conclude that the bands in the UV are int rinsic to the thiophene ring and thus not largely innuenced by different substituent s. 5. CONCL.USIONS Using various opticaltcchniques, the photoexcitations in PTSi and C60 doped PTSi have been determined (Table I). C60 doping induces the dissociation of singlet excitons into charge transfer complexes. Four singlet excitonic levels were found in PTSi : the IBu exciton and two higher energy optically allowed excitons wit h a low po larizabilily, probably intrinsic excitations o f the thiophene ri ng; the optically forbidden mAg state was found al about 2.6 eV. We found than in PTSi an electric fi eld can enhance the dissociation of of the 1 nu singlet excitons into polarons and polaron pairs. Because of high exciton binding energies, this effect most likely takes place al defects and impurities. Table 1: Energy levels ofexeito ns and photoexeitations and their transitions assembled fro m EA, PA, EPA, EPL and PADMR spectra 0 ( PTS' .. Excitons Optical gap (1 B • exciton) 1.95 eV Lumineseense (relaxed 18 - exciton) 1. 65 eV rn A exciton 2.55 eV rnA J • exci ton - 2 .5 eV I B 3 _ exciton - 1.4 eV Triplet-Triplet transition 1.1 eV Localized 8 - stales 4.5 eV , 6.3 eV Si~natures of polarons and related photocxcitations p - transi tion 0.4 eV p - transi tion 1.1 eV Po laron pairs: PP - transi tion 1.65 eV C;" (HOMO -> LUMO) 1.1 5 eV REFERENCES M. Liess, P. A. Lane, S. Jeglinski, Z. V. Vardeny, A three essential states model for electroabsorption in nonluminescent conjugated polymers, Synthetic Metals, 84 (1996) ~[2] S. Jeglinski, Electroabsorption spectroscopy of conjugated polymers and carbon fullerenes and conjugated polymer light emitting devices, (PhD Thesis, University of Utah, 1996). [3] Z. V. Vardeny, X. Wei, Optical Porbes of Photoexcitations in Conducting Polymers in Handbook of conducting polymers A. Skotheim , R. Elsenbaumer (eds.) (to be published in 1997). [4] X. Wei, (PhD Thesis, University of Utah, Salt Lake City, 1994). [5] X. Wei, Z. V. Vardeny, N. S. Saricifti, A. 1. Heeger, Absorption-detected magnetic-resonance studies of photoexcitations in conjugated-polymer/C60 composits, Phys. Rev. B, 53 (1996) 2187. [61 Liess, P. A. Lane, Z. H. Kafafi, M. Hamaguchi, M. Ozaki, K. Yoshino, Z. V. Vardeny, Electromodulated photoinduced absorption: a new spectroscopy in p-conjugated polymer/C60 blends, (SPIE proceedings, San Diego, 1997). S. Hottea, S. Rughooputb, A. 1 Heeger, A. 1. Wudl, Macromolecules, 20 (1987) 212. ,:~ .. ~ , , J , , ' ~, •• ,d '" 423 

Optical probes of pristine and C60-doped silicon-bridged PT polymer; a novel highly luminescent, low bandgap polymer

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Optical probes of pristine and C60-doped silicon-bridged PT polymer; a novel highly luminescent, low bandgap polymer

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