photo-physical and lasing characterisation of a polyparaphenylenevinylene (ppv) neat film

Photo-physical and lasing characterisation of a

polyparaphenylenevinylene (PPV) neat film

W . H O L Z E R 1 , A . P E N Z K O F E R 1 , * , S . S C H R AD E R 2 A ND B . G R IMM 2

1Institut II - Experimentelle und Angewandte Physik, Universitat Regensburg, Universitatsstrasse 31,

D-93053, Regensburg, Germany2Institut fur Physik, Universitat Potsdam, Am Neuen Palais 10, D-14469, Potsdam, Germany

(*author for correspondence: e-mail: [email protected])

Received 25 August 2004; accepted 17 March 2005

Abstract. The wave-guided travelling-wave laser action (amplified spontaneous emission) of a neat film of

poly(p-phenylenevinylene) (PPV) on a quartz glass substrate prepared by a sulfinyl precursor technique is

studied. The samples are transversally pumped with picosecond excitation pulses (wavelength 347.15 nm,

duration 35 ps). Lasing occurs at 550 nm. The optical constants of the neat films are determined by

transmittance measurements exploiting the multiple beam interference in the transparency region. A

fluorescence spectroscopic characterisation is carried out determining the fluorescence quantum distri-

bution, fluorescence quantum yield, degree of fluorescence polarisation, and fluorescence lifetime. The

emitting chromophore size (emitting singlet exciton extension) is determined by the ratio of exciton

radiative lifetime to repeat-unit based radiative lifetime. The obtained size of about two repeat units is

discussed in a disordered solid-state polymer model.

Key words: amplified spontaneous emission, emitting exciton size, fluorescence lifetime, fluorescence

quantum yield, luminescent polymers, optical constants, PPV, wave-guided travelling-wave lasing

1. Introduction

Poly(p-phenylenevinylene) (PPV) is the luminescent polymer used for the firstpolymer light emitting diode which operated successfully at low drivingvoltage (Burroughes et al. 1990; Braun and Heeger 1991). As a prototypeorganic semiconductor it was investigated extensively (for reviews see Friendet al. 1987, 1997; Greenham and Friend 1995, 2004; Rothberg et al. 1996;Pope and Swenberg 1999). Wave-guided amplified spontaneous emission(travelling-wave lasing) (Denton et al. 1997), and micro-cavity lasing (Tessleret al. 1996; Denton et al. 1997) was achieved under air-free conditions. Thephotoluminescence and laser properties strongly depend on the thin-filmpreparation procedures (Yan et al. 1994; Son et al. 1995; Rothberg et al.1996; Friend et al. 1997; Jeoung et al. 1999). PPV itself is not soluble. The filmpreparation applies a soluble precursor polymer which is then transformed toPPV by heat treatment (Wessling and Zimmermann 1968; Wessling 1985;Bradley et al. 1986). PPV derivatives with appropriate side-groups are

Optical and Quantum Electronics (2005) 37:475–494 � Springer 2005

DOI 10.1007/s11082-005-4225-2

solution processible (spin-coating from liquid solution) and emission wave-length tunable (Moses 1992; Brouwer et al. 1995; Hide et al. 1997; McGeheeand Heeger 2000).

Here the refractive index spectrum, the absorption coefficient spectrum, theabsorption cross-section spectrum, the fluorescence quantum distribution,fluorescence quantum yield, degree of fluorescence polarisation, and fluo-rescence lifetime of a PPV neat film on a fused-silica substrate are deter-mined. All measurements were performed at room temperature underambient conditions. An average emitting chromophore size (emitting singletexciton size) of two repeat units is extracted. Wave-guided travelling-wavelaser action for this sample is achieved by transverse pumping with pico-second laser pulses. The laser performance is characterised. The obtainedresults are compared with previous publications and are interpreted.

2. Experimental

The PPV alkylsulfinyl precursor polymer was synthesised by polymerisationof 1-(chloromethyl)-4-[(n-octylsulfinyl)methyl]benzene (van Bremen et al.1999). The preparation scheme is shown in Scheme 1. Step 1: 1,4-Dichlo-roxylene reacts with tetrahydrothiophene to give 1,4-bis(tetrahydrothio-pheniomethyl)xylene dichloride (I). Step 2: Compound I reacts withn-Octanethiol to give 1-(chloromethyl)-4-[(n-octylsulfanyl)methyl]benzene(II). Step 3: The oxidation of II by H2O2/TeO2 yields 1-(chloromethyl)-4-[(n-octylsulfinyl) methyl]benzene (III). Step 4: The polymerisation reaction iscarried out with NaOtBu as a base to give the precursor polymer poly{[1,4-phenylene]-[1-(n-octylsulfinyl)ethylene]} (IV). Step 5: This polymer wasdissolved in toluene and spin-coated onto a quartz glass substrate. It wasannealed at 40 �C to remove the solvent. Then it was transformed to PPV (V)by heating up to 200 �C in high vacuum (10)6 Torr) for 2 h (Louwet et al.1992; Flueraru et al. 2000). Good optical quality fluorescent yellow colouredfilms were obtained. Using atomic force microscopy a root mean squareroughness value of 1.9 nm was determined. The sample was stored in thedark at room temperature. All experimental studies have been carried out atroom temperature under ambient conditions (exposure to air).

The film thickness was measured with a Dektak thickness profiler (Dektak3 from Veeco Instruments). The film transmission is measured with aconventional spectrophotometer (Beckman type ACTA M IV, blank fusedsilica substrate was in reference beam path). The refractive index dispersion isextracted from the wavelength dependent transmission modulation in thetransparent spectral region due to thin-film multiple reflections.

The technique of fluorescence quantum distribution and fluorescencequantum yield measurement is described in (Holzer et al. 1999) and the

476 W. HOLZER ET AL.

experimental arrangement is shown there. As reference fluorescence solutionCoumarin 314T in ethanol was used (fluorescence quantum yield /F,R =0.87[technical data sheet of Kodak]). The fluorescence lifetime measurementshave been carried out by transverse excitation of the sample with attenuatedpicosecond laser pulses (wavelength 347.15 nm, duration 35 ps) below laserthreshold and collecting the emission emitted normal to the film surface inbackward direction. The signal is detected with a fast micro-channel-platephotomultiplier (Hamamatsu type R1564U-01) and a fast real-time digitaloscilloscope (LeCroy type 9362).

The absorption coefficient spectrum, a(k), is determined from the transmis-sion spectrum, T(k), and the film thickness, df, according to a=)ln(T)/df. Therepeat-unit based absorption cross-section spectrum is determined from theabsorption coefficient, a, and the PPV repeat unit number density,NRU=qNA/MRU, according to ra=a/NRU, whereby q is the mass density, NA is the Avo-gadro constant, and MRU is the molar mass (MRU=102.14 gmol)1 for PPV).

Cl ClH

HH

HS

+S

+H

H

H

H Cl-

S

Cl-

S+

S

H

H

H

HR

HSR

Cl-

S

_

S

H

H

H

HRCl

H2O2 / TeO2

S

H

H

H

HCl

O

R

NaOtBuS

O

R

n

n

200oC

S

_I

II

III IV

V

Scheme 1. Synthesis of PPV via a sulfinyl precursor polymer route, R = n-octyl.

PHOTO-PHYSICAL AND LASING CHARACTERISATION OF A PPV NEAT FILM 477

The mass density of PPV was determined by use of a gas displacement pyc-nometerAccupyk 1330V2.01 (fromMicromeritics) underHe atmosphere. Theexpansion volumewas 0.6918 cm3, the cell volumewas 1.2626 cm3, and 20 flowcycles have been used. The determined density of the precursor polymer isq=1.1030 g cm)3, and the density of PPV after the conversion processamounts to q=1.153 g cm)3 giving a PPV repeat unit number density ofNRU

� 6.8 · 1021 cm)3. The size of the luminophore, i.e. of the emitting chromo-phore (emitting singlet exciton) expressed by the number mem of repeat unitsforming a coherently emitting entity, is extracted from the ratio of the repeatunit based Strickler–Berg radiative lifetime (Strickler and Berg 1962; Birks andDyson 1963), srad,RU, to the experimentally determined radiative lifetime,srad=sF//F, according to (Holzer et al. 2001)mem=srad,RU/srad, where sF and/F are the fluorescence lifetime and the fluorescence quantum yield, respec-tively.

The lasing studies are carried out by transversally exciting the sample withsingle picosecond pulses of an active and passive mode-locked ruby laser(Weidner and Penzkofer 1993) (laser pulse duration DtL �35 ps, wavelengthDkL=347.15 nm, pulse energy up toWL=1 mJ). The experimental setup andthe experimental procedure have been described in (Holzer et al. 2001).

3. Results

3.1. SPECTROSCOPIC CHARACTERISATION

The film thickness of the investigated PPV sample was measured to bedf=190±20 nm. The transmission spectrum of the film is shown in Fig. 1a.It was measured in a two-beam arrangement with a fused silica plate in thereference path. The minimum transmission is Tmin=0.0019 at kmin=416 nmcorresponding to an absorption coefficient of amax ¼ �lnðTminÞ=df ¼ 3:3�105 cm�1:

The limited transmission relative to the reference (fused silica) in thetransparent region of k > 500 nm is due to larger reflection of the filmsample compared to the blank substrate. The transmission modulation is dueto thin-film interference (Jenkins and White 1958). The transmission mini-mum at kcon=543 nm is due to constructive reflection interference occurringat 2df½n2 � sin2ðhiÞ�1=2 ¼ ðmþ 1=2Þkcon leading to n (543 nm)=2.15 using theinterference order m=1 and the angle of incidence hi =10�. The transmissionmaximum at kdes=762 nm is due to destructive reflection interferenceoccurring at 2df½n2 � sin2ðhiÞ�1=2 ¼ ðmþ 1Þkdes leading to n (762 nm) =2.01using the interference order m=0 and the angle of incidence hi = 10�. Therefractive index dispersion in the transparency region is determined by fittingthe single oscillator model (Penzkofer et al. 1982).


n2ðkÞ � 1

n2ðkÞ þ 2¼ j

1=k20 � 1=k2; ð1Þ

to the refractive indices at 762 nm and 543 nm. The resulting refractive indexdispersion is shown in Fig. 1b, and the fit parameters j and k0 are given in thelegend there.

The repeat-unit based absorption cross-section spectrum of PPV is dis-played in Fig. 2. It is extracted from the transmission spectrum via theabsorption coefficient spectrum as described above. The first absorption bandis rather broad and unstructured. The absorption maximum occurs at415 nm. A quantum chemical collective-electronic oscillator calculation(Mukamel et al. 1997) indicates a strong transition (approximately 85% ofthe absorption strength) and two weak transitions within the first absorptionband. This is taken into account in Fig. 2 by the position, ku, of the upperlimit of the S0–S1 absorption band.

The repeat-unit based stimulated emission cross-section spectrum is in-cluded in Fig. 2. It is calculated from the absorption cross-section spectrum,

Fig. 1. (a) Transmission spectrum of a PPV neat film on fused silica substrate measured versus a blank

fused silica reference plate. Film thickness is 190±20 nm. (b) Refractive index spectrum of a PPV neat

film on fused silica substrate. Circles are extracted from transmission modulation in Fig. 1a. The curve is

a single oscillator model fit to experimental data (Equation 1 with j=1.0522 · 109 and k0=210.2 nm).


ra(k), and the fluorescence quantum distribution, E(k), by use of the relations(Peterson et al. 1971; Deshpande et al. 1990)

remðkÞ ¼k4

8pc0n2Fsrad;RU

EFðkÞRem EFðk0Þdk0

; ð2Þ

and (Strickler and Berg 1962; Birks and Dyson 1963)

srad;RU ¼8pc0n3FnA

Rem EFðkÞdk

Rem EFðkÞk3dk

Z

abs

raðkÞdkk

" #�1

; ð3Þ

where the integrals extend over the S1–S0 fluorescence region (em) and theS0–S1 absorption region (abs, upper limit is ku). The stimulated emissioncross-section spectrum shows a well resolved vibronic structure with a levelspacing of about 1300 cm)1. The spectral half-width of the first absorptionband, D~ma � 9840 cm�1; is considerably larger than the spectral half-width ofthe stimulated emission cross-section spectrum, D~mem � 3650 cm�1: There-fore, the peak stimulated emission cross-section is larger than the peak S0–S1absorption cross-section.

Fig. 2. Repeat-unit based absorption cross-section spectrum and stimulated emission cross-section spec-

trum of the investigated PPV sample. ku represents the used upper border of the S0–S1 absorption band.


The fluorescence quantum distribution of the investigated PPV neat filmis shown in Fig. 3a. The obtained fluorescence quantum yield is /F=�emE(k)dk � 0.15±0.01. This small number indicates dominant non-radiativedecay.

The spectral distribution of the degree of fluorescence polarisation,PFðkÞ ¼ ½SF;jjðkÞ � SF;?ðkÞ�=½SF;jjðkÞ þ SF;?ðkÞ� is displayed in Fig. 3b.Thereby, SF;jjðkÞ is the fluorescence signal at wavelength k which is polarisedparallel to the excitation light, and SF;?ðkÞ is the fluorescence signal atwavelength k which is polarised perpendicular to the excitation light. For thesame molecular orientation of the absorption and emission transition mo-ment and no transition dipole moment re-orientation within the excited-statelifetime the degree of fluorescence polarisation would be PF=0.5. In the casethat reorientation of the transition dipole moment is fast compared to thefluorescence lifetime a degree of fluorescence polarisation around PF=0 isexpected (Lakowicz 1983). Here a value of PF � 0.07 is determined. This lowvalue indicates a fast transition dipole moment re-orientation. In the neatfilms no fast molecular repeat-unit re-orientation is expected, but there oc-curs fast energy transfer from an excited chromophore to a differently

Fig. 3. Fluorescence quantum distribution (a) and degree of fluorescence polarisation (b) of the investi-

gated PPV neat film.


oriented nearby non-excited chromophore (exciton migration). Assumingparallel orientation of the absorption and of the emission transition dipolemoments the relation between reorientation time and degree of fluorescencepolarisation is given by (Parker 1968)

sor ¼1=PF;0 � 1=3

1� PF=PF;0PFsF; ð4Þ

where PF,0=0.5, and sF is the fluorescence lifetime. Using sF=250 ps (seebelow) a re-orientation time of sor =34 ps is obtained.

The temporal fluorescence signal behaviour due to single picosecond pulseexcitation is depicted in Fig. 4. The solid-line connecting circles show thefluorescence trace. The dashed-line connecting triangles show the instru-mental response function. It was obtained by strongly attenuating the pico-second excitation pulse and directing it directly to the micro-channel-platephotomultiplier detection system. The dotted curve is a Gaussian functionwhich approximates the instrumental response function and which is used in

Fig. 4. Temporal fluorescence signal of the investigated PPV neat film. Circle-connecting line is the

experimental fluorescence signal. Triangle-connecting line is the experimental response function. The

dotted curve is the Gaussian response function applied in convolution fits. Solid curves are calculated

convolutions (Equation 5) belonging to single exponential fluorescence decay times of (1) sF=100 ps,

(2) sF=200 ps, (3) sF=300 ps, and (4) sF=500 ps.


the convolution simulations. The solid curves are convolution signals,SF,con(t), according to (Islam et al. 2003)

SF;conðtÞ ¼ S0

Z t

�1p�1=2 expð�t02=t2resÞ exp½�ðt� t0Þ=sF�dt0: ð5Þ

In the upper part of Fig. 4 (SF(t)/SF,max > 0.4) a good fit to the experi-mental fluorescence trace is obtained for sF=250 ps. In the lower part theexperimental signal curve decreases slower than the corresponding theoreticalconvolution curve because there the experimental response function de-creases slower than the Gaussian response function used in the convolutioncalculations.

The experimental fluorescence lifetime of sF=250 ps and the experimentalfluorescence quantum yield of /F=0.15 determine a luminophore radiativelifetime of srad=sF//F=1.7 ns. The repeat-unit based radiative lifetime,srad,RU, determined from the repeat-unit based absorption cross-sectionspectrum and the fluorescence quantum distribution according to Equation 1gives srad,RU � 3.7 ns. These values give an average luminophore (emittingchromophore) size of mem=srad,RU/srad � 2.2, i.e. the emitting excitonextension is restricted to approximately two repeat units (Table 1).

3.2. WAVE-GUIDED TRAVELLING WAVE LASER ACTION

The wave-guided travelling wave laser (wave-guided amplification of spon-taneous emission) performance is documented and analysed in Figs. 5 and 6.

In Fig. 5a the spectra Sem(k) of the collected edge emission are shown forseveral excitation energy densities. The dotted curve at an input pump pulsepeak energy density of w0L ¼ 2� 10�5 J cm�2 is due to wave-guided spon-taneous emission. Its spectral shape is somewhat different from the shape of

Table 1. Spectroscopic and lasing parameters of the investigated PPV neat film

Parameter Value Comment

NRU (cm)3) 6.8 · 1021 See text

/F 0.15±0.01 Fig. 3a

PF 0.07±0.01 Fig. 3b

sor(ps) 34±5 Equation 4

sF (ps) 250±50 Fig. 4

srad (ns) 1.7±0.2 srad=sF//F

srad,RU (ns) 3.7±0.2 Equation 3

mem 2.2±0.2 mem=srad,RU/sradwL,th (J cm)2) (1±0.3) · 10)4 Fig. 6a

wL,g,sat (J cm)2) 0.0025 Fig. 6a

wL,p,sat (J cm)2) 0.012 Equation 10


the fluorescence quantum distribution (dash-double-dotted curve) because ofspectral filtering in the film plane. Above laser threshold, wL;thP1� 10�4 J cm)2, a spectral peak for travelling wave lasing aroundkTWL=550 nm builds up. At high excitation energy density the rise inemission saturates.

In order to show the spectral changes with excitation energy density, inFig. 5b the edge-emission spectra, Sem(k), of Fig. 5a are divided by thespontaneous emission spectrum, Ssp(k), at the lowest excitation energy(dotted curve in Fig. 5a). Only in the wavelength range from 540 to 560 nm

Fig. 5. Spectra of travelling-wave laser edge emission of the investigated PPV neat film on fused silica

substrate. Film thickness is 190 nm. Exposed film area: 5 mm · 0.13 mm. Acceptance angle Dh=34�.(a) Spectral energy distribution of edge emission Sem(k) for several input pump pulse peak energy den-

sities w0L. The dotted curve at w0L=20 lJ cm)2 represents edge emitted spontaneous emission Ssp(k).(b) Ratio of spectral energy distribution of wave-guided edge emission to spectral energy density distri-

bution of edge emitted fluorescence Sem(k,w0L)/Ssp(k,w0L=20 lJcm)2). (c) Edge-emission efficiency spec-

tra [Sem(k,w0L)/w0L]/[Ssp(k,w0L=20 lJcm)2)/20 lJcm)2].


amplification of spontaneous emission is observed (peak region of Sem/Ssp).In this region the stimulated emission cross-section is maximal (highestvibronic peak, see Fig. 2).

The reduction of edge-emission efficiency at high pump pulse energydensity is illustrated in Fig. 5c where the spectra of Fig. 5b are divided bytheir corresponding pump pulse excitation energy densities. Without satu-ration/attenuation effects, the ratios should be 1 in the regions where onlyspontaneous emission occurs, and in the region of amplified spontaneousemission the ratio should rise continuously with excitation energy density.

In Fig. 6a the edge-emitted output energy, Wem, is plotted (circles) versusthe input pump pulse peak energy density, w0L. The emission is composed ofwave-guided spontaneous emission, Wsp, and wave-guided travelling-wavelaser emission, WTWL. The dashed curve resembles the spontaneous emissioncontribution. It is calculated by the following relation:

Wsp ¼ jFw0L expð�w0L=wL;g;satÞ; ð6Þwhere jF is a proportionality constant, and wL,g,sat is the characteristic pumppulse energy density of gain saturation. The fit parameters, jF and wL,g,sat,are listed in the figure caption. The dash-dotted curve resembles the travellingwave laser contribution. It is calculated by use of

WTWL ¼WTWL;max 1� exp � w0L � wL;th

wL;g;sat � wL;th

� ��

; ð7Þ

wherewL,th is the pump pulse energy density at laser threshold. The solid curve is

Wem ¼Wsp þWTWL: ð8Þ

Thefit gives a laser thresholdpumppulse energy density ofwL;th � 10�4 J cm�2:In Fig. 6b the edge emission efficiency is shown. The output emission en-

ergy, Wem, is divided by the applied pump pulse energy, WL, to the aperturedfilm region. The normalised fit curves of Fig. 6a are included. The onset oflaser action at wL,th, the reduction of the spontaneous emission efficiency, andthe increase and decrease of laser efficiency with rising pump pulse excitationare clearly seen.

The dependence of the wavelength peak position, kem,max, on the pumppulse energy density is shown in Fig. 6c. Due to travelling-wave laser actionthe peak position changes from 558 nm below laser threshold to 550 nmabove laser threshold. The changeover occurs at w0L � 7 · 10)5 J cm)2.In Fig. 6a a better fit was obtained for a laser threshold ofwL,th = 1 · 10)4 J cm)2. The spectral narrowing of the edge-emission withexcitation energy density is displayed in Fig. 6d. The spectral width narrowsfrom Dem � 28 nm below laser threshold to Dkem � 6 nm above laserthreshold.


4. Discussion

The determination of the absorption coefficient spectrum, a(k), has beenrealised via measurements of the transmittance T(k) and of the film thickness,df. The refractive index dispersion, n(k), was obtained from the transmittancemodulation in the transparency region (T=1 ) R, where R is the reflec-tance). The obtained refractive indices are in good agreement with previousmeasurements on TE modes of PPV waveguides (Burzynski et al. 1990). Therefractive index nTE was found to be isotropic (independent of propagationdirection). The refractive index of the TM modes was found to be

Fig. 6. Wave-guided travelling-wave laser performance of the investigated PPV neat film. Film thick-

ness is 190 nm. Exposed film area is 5 mm · 0.13 mm. (a) Collected emission signal Wem versus pump

pulse energy density w0L. Full acceptance angle of emitted laser light is 34�. Circles are measured and

curves are calculated. Dashed curve, contribution of edge emitted spontaneous emission Wsp calculated

by use of Equation 6 with jF=850 cm2, wL,g,sat=2.5 mJ cm)2. Dash-dotted curve, contribution of

edge-emitted amplified spontaneous emission calculated by use of Equation 7 with a threshold pump

pulse energy density wL,th=100 lJ cm)2, a pump pulse energy density of gain saturation

wL,g,sat=2.5 mJ cm)2, and a maximum output energy density WTWL,max=2.8 nJ. Solid curve, sum of

edge-emitted spontaneous emission and amplified spontaneous emission, Wem=Wsp+WTWL (Equa-

tion 8). (b) Edge-emission efficiency, Wem/WL. Data of part (a) are divided by pump pulse energy

inputted to exposed film area. (c) Peak wavelength of wave-guided emission, kem,max, versus input

pump pulse energy density, w0L. (d) Spectral linewidth (FWHM) of wave-guided edge emission, Dkem,versus input pump pulse energy density, w0L.


considerably smaller (nTM � 1.6) (Burzynski et al. 1990). This indicates thatthe polymer chains are lying unordered parallel to the film plane and that theS0–S1 transition dipole moments are directed along the chain direction (largevalue of nTE determined by first absorption band).

The absorption cross-section spectrum was extracted from the absorptioncoefficient spectrum by using a mass density for PPV of 1.153 g cm)3.Knowing the repeat-unit based absorption cross-section spectrum the repeat-unit based radiative lifetime could be calculated. On the other side, the trueradiative lifetime of the emitting chromophore was determined from themeasured fluorescence lifetime and the measured fluorescence quantum yield.The emitting chromophore size was determined by the ratio of the radiativelifetimes of the repeat-unit and the luminophore giving an average excitonextension over mem � 2.2 repeat units. Reported absorbing chromophoresizes range from mabs=3 (Drefahl et al. 1970; Horhold and Opfermann 1970;Pomerantz et al. 1989) to mabs ‡ 50 (Hagler et al. 1994; Pakbaz et al. 1994).In Horhold and Opfermann (1970) and Drefahl et al. (1970) the value ofm=3 is estimated from the experimental energy gap dependence on thenumber of repeat units. In Pomerantz et al. (1989) this number is extractedfrom calculations of the energy gap as function of the number of repeat units.In Heun et al. (1993) a value of m ‡ 20 is estimated for ordered stretched PPVfilms using the particle in a box model, and in Rauscher et al. (1990) a valueof m=11 is extracted from a non-stretched film applying the same model. Aconjugation extent of m = 15–20 is estimated in Pichler et al. (1993) bycomparing phenylene–vinylene oligomers with PPV. A similar comparisonbetween oligomers and PPV led to the conclusion of m = 7–10 in Tian et al.(1991). An exciton extension over m = 6 repeat units is expected in Grahamet al. (1991) by considering the spectral dependence of the luminescencespectra on the oligomer size. The Raman spectra in Graham et al. (1991) givesome indication of m £ 3 (no vibrational frequency shifts for oligomer size ‡ 3repeat units). Electroabsorption measurements led to an exciton extensionover m � 2 repeat units in Greenham and Friend (1995), Friend et al. (1987)and Halliday et al. (1993). In Pakbaz et al. (1994) and Hagler et al. (1994)anisotropic electroabsorption measurements on highly oriented and struc-turally ordered samples indicated an instantaneous excited state wavefunc-tion delocalisation over at least 50 unit cells. The triplet exciton size of PPVwas found to be extended over not much more than one repeat unit (Friendet al. 1987; Halliday et al. 1993; Greenham and Friend 1995).

The broad smooth first absorption band of conjugated polymers like PPVis generally discussed in terms of a semiconductor-like band structure(Heeger et al. 1988; Pakbaz et al. 1994; Greenham and Friend 1995; Sariciftci1997; Moses et al. 2000) or an inhomogeneous distribution of neutral exci-tations of polymer chain segments of different conjugation length (excitonextension, chromophore size) (Friend et al. 1987, 2004; Samuel et al. 1993;


Greenham and Friend 1995; Hayes et al. 1995; Sariciftci 1997; Arkhipovet al. 2004). Depending on the PPV preparation the PPV films may be nearlyamorphous favouring a variable-length chain-segment description (Rauscheret al. 1990; Samuel et al. 1993; Greenham and Friend 1995, 2004; Hayeset al. 1995), or they may be a mixture of microcrystalline and amorphousstructure favouring a semiconductor-like band structure description withpolarons and self-localised excitons (Yan et al. 1994; Rothberg et al. 1996).After excitation there occurs a fast relaxation to the lowest energetic statesfrom where emission occurs. In the variable-length chain segment model it isthought that the lowest lying states belong to the intrachain excitons with thelargest delocalisation. The photoluminescence is, therefore, thought to be theemission of the excited intrachain singlet excitons with the largest delocali-sation (Friend et al. 1987; Samuel et al. 1993; Greenham and Friend 1995,2004; Hayes et al. 1995; Sariciftci 1997). In the semiconductor-like bandstructure description there occurs a fast excitation relaxation in the con-duction band followed by self-trapped exciton formation (excited-state latticerelaxation) (Toyozawa and Lumin 1976; Heeger et al. 1988; Yan et al. 1994;Greenham and Friend 1995; Rothberg et al. 1996; Sariciftci 1997). Theexciton size may extend over many unit cells (Wannier-type exciton, weakelectron-phonon coupling), it may be restricted to one unit cell (Frenkel-typeexciton, strong electron–phonon coupling), or it may extend over a few re-peat units (charge-transfer exciton) (Greenham and Friend 1995; Pope andSwenberg 1999).

Our radiative exciton lifetime analysis gives a small extension of theemitting unit over approximately two repeat units because the emittingchromophore size is on the average limited to about 2.2 repeat units. Thesmall extension of the emitting excitons is in agreement with the well-struc-tured vibronic emission spectrum as it is observed for single organicmolecules. A huge coherent delocalisation of the excitation over many re-peat-units would be expected to lead to a narrow J-aggregate like emissionwithout resolved vibronic structure (Fidder et al. 1993; Moll 1995; Kobay-ashi 1996). Our results are in agreement with the semiconductor-like polymermodel with a self-trapped exciton size of about 2 repeat units (Frenkelexcitons to charge-transfer excitons).

The findings here can be explained in the framework of a disordered solid-state polymer model. In solid materials, the Pauli-exclusion principle requiresa level splitting to bands (tight-binding approach in solid state physics (Yuand Cardona 1996). In crystals the periodicity causes the Brillouin zone bandstructure with reciprocal lattice vector selection rules for electronic transi-tions between the bands. In disordered solid-state polymers reciprocal latticevector selection rules are lost. The closely spaced pair interaction (Lu andPenzkofer 1986) is thought to cause a stronger inhomogeneous line broad-ening (Moerner 1988) on the smaller-size excitons than on the larger-size


excitons, because larger excitons average-out the inhomogeneous localinteraction forces. Coming from isolated excitons to solid-state density it isthought that the larger excitons are to some extend red-shifted but less dis-torted (smaller inhomogeneous width) than the smaller excitons (largerinhomogeneous width) and the band broadening is mainly caused by thesmall excitons. After excitation a fast relaxation (energy transfer Agranovichand Galanin 1982), dielectric relaxation (Moerner 1988; Kinoshita 1989;Weidner and Penzkofer 1995), and lattice relaxation (Greenham and Friend1995) to the lowest energetic excitons occurs. The emission occurs from theselowest lying small excitons. An illustration of this disordered solid-statepolymer model is presented in Fig. 7.

Semi-empirical quantum chemical AM1 calculations (program packageHyperchemTM from Hypercube, Inc. is used) have been carried out todetermine excited-state singlet energy levels and oscillator strength for para-phenylenevinylene oligomers (PV)n with n = 1, 2, 3, 4. The obtained S0–S1

Fig. 7. Illustration of disordered solid-state polymer model. (a) Tight-binding model of level splitting

to bands for small-size and large-size chromophores. (b) Absorption and emission band structure. A:

absorption band due to transition from disordered valence band (inhomogeneous HOMO band) to

conduction band (LUMO band). E: emission band due to transition from self-trapped exciton band

(relaxed excited chromophores) to valence band (ground state). rel: intra-band relaxation and lattice

relaxation.

Table 2. Excitation wavelengths and oscillator strengths calculated for paraphenyenevinylene oligomers

(PV)n using semi-empirical AM1 calculations

n kS0–S1(nm) f kS0–S2(nm) f

1 326.8 0.336 250.9 0.597

2 402.2 1.268 234.4 1.126

3 457.8 2.129 256.2 0.104

4 473.3 2.721 357 0.075


and S0–S2 excitation wavelength and corresponding oscillator strengths arecollected in Table 2. The isolated paraphenylenevinylene dimer (PV)2 givesan S0–S1 excitation wavelength of kS0–S1=402 nm with an oscillator strengthof f=1.268. The S0–S2 transition wavelength is at kS0–S1=234 nm, and thecorresponding oscillator strength is f=1.126. The large energy separationbetween the S1 and S2 level indicates that the broad absorption band of PPVfrom 320 to 500 nm only belongs to S0–S1 excitation, as it was assumed in thecalculation of the repeat-unit based radiative lifetime srad,RU (Equation 3).The calculated wavelength position of the S0–S1 transition of (PV)2 is in themiddle of the broad experimental S0–S1 absorption band of PPV (Fig. 2)making it reasonable that the determined chromophore extension over about2.2 repeat units on the average is sufficient to form the observed broad firstabsorption band at neat thin-film solid-state density.

For the PPV sample studied here a fluorescence quantum yield of/F=0.15±.02 was measured together with a fluorescence lifetime ofsF=250±50 ps. The resulting radiative lifetime is srad=sF//F=1.7±.5 ns.Very high fluorescence quantum yields between /F=0.6 and /F=0.8 werereported in (Tessler et al. 1996). In carefully prepared films a fluorescencelifetime of /F=0.27 and a fluorescence lifetime of sF=320 ps were obtained(Greenham et al. 1995). A fluorescence lifetime distribution analysis wasperformed in Samuel et al. (1993) giving a lifetime distribution around 240 ps.A fluorescence lifetime of sF=25 ps was measured on a PPV sample (Yanet al. 1994) which showed femtosecond laser excited stimulated emission on asubpicosecond timescale. The fluorescence lifetime and fluorescence quantumyield strongly depend on the film preparation (Papadimitrakopoulos et al.1994; Denton et al. 1997). A luminescence enhancement was achieved byintroduction of disorder into PPV (Son et al. 1995). The fluorescence effi-ciency was found to depend strongly on the carbonyl content in the polymer(Papadimitrakopoulos et al. 1994). Thermally induced oxidation (Rothberget al. 1996) and photo-oxidation were reported to strongly reduce the fluo-rescence efficiency (Bradley and Friend 1989; Rothberg et al. 1996; Friendet al. 1997). Our film was thermally converted at 200 �C under high vacuum.It was stored in the dark in a refrigerator at 4 �C over a long period of time(more than half a year) and the spectroscopic and laser experiments werecarried out at room temperature under normal air conditions. No film deg-radation was observed due to long-time storage and due to the performedoptical, spectroscopic, and laser action measurements. For the smooth ratherthick film (df=190 nm) the contact to air seems to have no negative influenceon the film behaviour.

Wave-guided travelling-wave laser action was achieved for our investigatedPPV sample as can be seen in Figs. 5 and 6. The travelling-wave laserthreshold of wL,th � 100 lJ cm)2 is higher than in other laser active PPVderivatives like MEH-PPV where a value of wL,th�10 lJ cm)2 was measured


(Holzer et al. 2004). The higher travelling-wave laser threshold is likely tobe due to a shorter effective amplification length, ‘gain; because of strongscattering of the PPV film. A strong scattering of a PPV film is also reportedin Tessler et al. (1996). For our sample we measured scattering lossesof about 3% in the case of transverse illumination at a wavelength of632.8 nm. Assuming that the effective stimulated emission cross-section,rem,eff=rem ) rex, at the peak amplified spontaneous emission wavelength(kTWL,max=550 nm) is equal to the to stimulated emission cross-section, rem,at this wavelength ðrem ¼ 8:6� 10�17 cm2Þ; i.e. neglecting the excited-stateabsorption rex, then the experimental laser threshold requires a gain length of‘gain ¼ 10 lm (calculated by use of Equations (11) and (12d) in Holzer et al.(2001)). For a smaller effective stimulated emission cross-section the gainlength has to be correspondingly longer.

With rising pump pulse energy density the edge-emitted emission com-posed of spontaneous emission, Wsp and travelling-wave laser emission,WTWL, saturates above wL,g,sat � 2.5 mJcm)2. This saturation is assumed tobe due to exciton–exciton annihilation at high density of excitation (Kepleret al. 1996; Holzer et al. 2002). In picosecond pump and probe experimentsthe fast decay of stimulated emission of PPV films at high excitation densitywas found to be due to exciton–exciton annihilation (Denton et al. 1997). AtwL,g,sat the number density of excited chromphores, Nchr,exc, at the end ofpump pulse excitation is

Nchr;exc ¼wL;g;sataL

hmL¼ wL;g;satra;LNRU

hmL; ð9Þ

giving Nchr;exc � 7:5� 1020 cm�3: The average distance, dchr, between twoexcited chromophores at wL,g,sat is dchr � N

�1=3chr;exc � 1:1 nm:

The characteristic laser energy density, wL=wL,p,sat, needed for ground-state depopulation of chromophors is given (Hercher 1967)

wL;p;sat ¼hmL

mabsra;L: ð10Þ

This relation gives a value of wL;p;sat � 1:2� 10�2 J cm�2 by usingmabs=mem=2.2. It is wL,p,sat � wL,g,sat indicating that the gain saturation isnot due to ground-state population depletion.

Photo-oxidation was shown to reduce the stimulated emission efficiency(Denton et al. 1997). Competition between picosecond photo-inducedabsorption by ‘spatially indirect’ singlet excitons (oppositely charged polaronpairs, charge-transfer excitons) and stimulated emission due to singlet exci-tons is reported in Yan et al. (1994) where stimulated emission was limited tothe subpicosecond range. Femtosecond time-resolved transmission


pump-probe measurements indicated pump pulse induced stimulated emis-sion which decayed within about 10 ps (Jeoung et al. 1999).

In PPV samples with high photo-luminescence efficiency laser action wasachieved in microcavities (Tessler et al. 1996; Friend et al. 1997), and trav-elling-wave laser arrangements (Denton et al. 1997). In PPV samples ofmoderate to low photo-luminescence efficiency light amplification around thefluorescence peak was found to occur only on a subpicosecond to a fewpicosecond timescale (Denton et al. 1997, Yan et al. 1994, Jeoung et al.1999).

In our experiments and in reports of other groups (Tessler et al. 1996;Denton et al. 1997; Friend et al. 1997) the laser action was restricted to thefirst vibronic emission peak (S1,v=0 to S0,v=1 transition) in the wavelengthregion from 540 to 560 nm where the stimulated emission cross-section islargest.

5. Conclusions

Travelling-wave laser action was achieved using a PPV neat film on a quartzglass substrate prepared from a sulfinyl precursor by spin coating and ther-mal transformation. The laser performance parameters have been deter-mined. Rather strong film scattering is thought to be the reason of a ratherhigh laser threshold, and singlet exciton – singlet exciton annihilation at highpump laser excitation is thought to limit the travelling-wave laser output.

The laser active PPV sample was characterised optically and spectro-scopically. The absorption strength and fluorescence properties lead to anaverage emitting chromophore size extending over about two repeat units inagreement with the molecule-like vibronic emission spectrum. The absorp-tion and emission behaviour agrees with Frenkel-like self-trapped excitonformation in the semiconductor-like band structure description of PPV. Itagrees with the proposed disordered solid-state polymer model where thesmall-size (Frenkel-like) excitons are inhomogeneous distributed over thewhole first absorption band, while larger size excitons are less distorted andtherefore spectrally more confined.

Acknowledgement

The authors would like to thank Prof. Dieter Weiss, University of Regens-burg, Germany, for enabling PPV film thickness measurement on the Dek-tak profilometer.


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photo-physical and lasing characterisation of a polyparaphenylenevinylene (ppv) neat film

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