Table Of ContentUltrafast Conductivity Dynamics in Pentacene Probed using Terahertz Spectroscopy
∗
V. K. Thorsmølle, R. D. Averitt, X. Chi, D. J. Hilton, D. L. Smith, A. P. Ramirez, and A. J. Taylor
Los Alamos National Laboratory, Los Alamos, NM 87545.
We present measurements of the transient photoconductivity in pentacene single crystals using
optical-pumpTHz-probespectroscopy. Wehavemeasuredthetemperatureandfluencedependence
of the mobility of the photoexcited charge carriers with picosecond resolution. The pentacene
crystals were excited at 3.0 eV which is above the bandgap of ∼2.2 eV and the induced change
in the far-infrared transmission was measured. At 30 K, the carrier mobility is µ ≈ 0.4 cm2/Vs
and decreases to µ ≈ 0.2 cm2/Vs at room temperature. The transient terahertz signal reveals the
presence of free carriers that are trapped on the timescale of a few ps or less, possibly through the
formation of excitons, small polarons, or trappingby impurities.
4
0
The promise of organicsemiconductors for technologi- ductivitydynamics. Terahertztime-domainspectroscopy
0
cal applications in electronics and photonics has spurred (THz-TDS)withoutanopticalpumpisanultrafastopti-
2
an immense interest in these materials1. However, de- caltechniqueinwhichnearsingle-cyclefreespaceelectric
n
spite intense research efforts in recent years, the nature field transients of a duration of about 1 ps and spectral
a
J of charge transport and photoexcitations in conjugated width on the order of 1 THz are used to measure the
polymers and organic molecular crystals is still not well complex conductivity of the sample. This is a coher-
6
1 understood and remains controversial2. ent technique, and by varying the probe delay time with
respect to an optical excitation pulse the induced con-
Conductivities in undoped organicsemiconducters are
1 ductivity changes of the sample can be measured with
orders of magnitude smaller than in inorganic semi-
v sub-picosecond(sub-ps) resolution. Thus, THz-TDS has
conducters such as Si and GaAs. Polyacenes such as
6
become an important technique in the study of the far-
9 naphthalene (Nph) and pentacene (Pc) are conjugated
infraredconductivity in condensedmatter systems rang-
2 molecules with mobilities, µ, on the order of 1 cm2/Vs.
ing from carrier transport in semiconductors to the dy-
1 The main contributing factors to such small mobilities
0 are thought to be related to weak intermolecular inter- namics of fluids9,10,11,12,13,14,15,16,17.
4
actions. These factors include strong localization, po-
nd-mat/0 itltttnauhaarnlresoterc.hneeiesTsifoseolofhrlcwemiatrentyatrlyesteetmidoafis3nplirs.nemgarTisalnehetdoxuibsoprtnsehereeaarr(ispvTmoepbf)deienntedhtgtnoeaepslbaieenytentcvfrdiraiimdeebcanetupsnoctueercersedaiotsifitfnoeotrsth.hsaetienhtHtrgemealomeniowmsbpceipitervliyroieotarsrny---, mplitude [a.u.] 0112....5050 FFT Amplitude [a.u.]000000......0011220505050.0 F0.r5equ1e.0ncy1 .[5TH2z.]0 2.5 E(Dt)EE(((ttt)))xx5
o from a polaron hopping transport to a band-like trans- A
c portofcarriersat lowertemperatures describedby some eld 0.0
v: theoreticalmodels4,5 and seen by Schlein et al. in Nph6. c Fi-0.5
Xi Iµn∝soTm−en,mnat>er0ia.lsWtahretateemt pale.raotbusreervdeedptehnidsebnecheafvoilolorwins ectri-1.0
ar Nph until low temperature, ∼30 K, where it then levels El
-1.5
off at values greater than 100 cm2/Vs3,7. Clearly, the
-6 -4 -2 0 2 4 6
nature of charge carrier transport is still not completely Time [ps]
understood.
It is not cleareither whether electronic excitationsare
FIG. 1: Electric field E(t) of THz pulse transmitted through
best described by the molecular exciton model, or by
the semiconductor band model8. In the former case, Pccrystal,andinducedchangeinelectricfield∆E(t)atT =4
K. The inset shows the fast Fourier transformed amplitude
the excited states are localized and the primary pho-
spectrum of E(t).
toexcitations are excitons, which can dissociate into free
polarons. In the latter case, they are delocalized and
mobile polarons are created directly from free electron- Herewepresentoptical-pumpTHz-probeconductivity
hole pairs by the absorption of light. Ultrafast optical measurementsonPcsinglecrystals. HighqualityPcsin-
measurements are important in this regard in that the glecrystalsweregrowninaflowofinertgas. Thecrystals
dynamics of the photogenerated carriers can be tempo- used were typically 3 mm x 3 mm and approximately 50
rally resolved. While the optical-pump probe technique µm thick. The experiments utilized a commercial-based
is a noncontact technique and yields important informa- regeneratively amplified Ti:Al O system operating at 1
2 3
tion of the relaxationdynamics, optical-pump terahertz- KHz producingnominally 2.0mJ,sub-50fs pulses at1.5
probeisadirectprobeofthenonequilibriumcarriercon- eV. The THz pulses were generated and detected using
2
electrooptictechniques. The schematicofthe THz-setup fluence.
is shown in Ref.18. The Pc crystals were here excited at
3.0eVwhichisabovethebandgapof 2.2eV.Theexper- 01..90 (a) u.]6 2.5 (b)
iments were performed in transmission with the electric 0.8 a.4 2.0
fitshietelidoTno,HfaztnhbdeeTathmHezidspiauamplseeerstteuirnreotdfhetthoaeb2-ppmulammnpea.btTetahhmeedsisiaamm∼p3elteemrpmoo-f. A[a.u.] 100000.....34567 -DT/T [02 -4T0im4e 8[p1s2]16t [ps]111..05
0.2 0.5
ThesamplesweremountedinsideanopticalHecryostat.
0.1
The resultspresentedareobtainedfromseveralsamples. 0.0 0.0
0 50 100 150 200 250 300 0 50 100 150 200 250 300
Figure1showstheelectricfieldoftheTHzpulsetrans- Temperature [K] Temperature [K]
mitted through a Pc crystal without optical excitation,
comparedtotheinducedchangeinthe electricfieldwith
optical excitation. There is a decrease in the transmit- -1m] (c) 440 KK
ted electric field associated with the conductivity of the
c 80 K
mobile carriers (i.e. Drude-like response). No shift in - 140 K
the phase of the induced change of the THz pulse is ob- W[al 180 K
e
served. Thus it is valid to measure changes in the peak r
295 K
amplitude of the transmitted THz pulses to determine
Ds
the photoinduced conductivity (i.e. ∆σ ∝∆E/E).
Frequency [THz]
0.9
6.0
0.8
Carrier densi-
5445....5050 ty [1017 cm-3] t [ps]1 000000......234567 dFflteeuImcGeanp.yce3ert:aiomtTfueeIrme=v.peTer0sr.hua2ets1uitmrneesmJed/tpecespmhrea2ont.wdu(serane−).ceF∆(aocTsf)t/vTIdanerdaciouatuycTseadp=macprh4alaimtKnuge.det(eebrivn)seFarrsateuasastl
-30] 3.5 00..01 conductivityversus frequency at various temperatures.
1 3.0 0 2 4 6 8 10 12
T [ 2.5 Carrier Density [1017 cm-3]
T/ The differential transmission versus temperature dis-
2.0
plays a similar behavior where the peak of the transient
D-
1.5 signal, and hence the transient conductivity, increases
1.0 as the temperature is lowered. We attribute this to an
0.5 increase in carrier mobility. Figure 3(a) shows the fast
0.0 decay amplitude to be increasing with decreasing tem-
-4 -2 0 2 4 6 8 10 peratures,followingthemobility(seeFigure4). Thefast
Time [ps] decay time shown in Figure 3(b) is seen to increase with
increasing temperature which is not immediately obvi-
ous, but might be due to thermal population of exciton
FIG.2: DifferentialtransmissionofthepeakoftheTHzprobe levelsortrappedstateswithhighertemperatures. Figure
pulseversuspump-probedelaytimeatvariousfluencesatT = 3(c) shows the induced change in the real conductivity,
20 K.Theplots aredisplaced vertically for clarity. Theinset and it is seen to be increasing with decreasing tempera-
shows thefast exponential decay time versuspump fluence. tures. Theinducedchangeintheimaginaryconductivity
is close to zero. Thus, it was not possible to fit the data
Figure 2 shows the induced change in transmission of toaDrudemodelwithsuchsmallinducedconductivities.
the THz electric field −∆T/T at incident pump fluences Figure4showsthecarriermobilityversustemperature
rangingfrom0.12to0.83mJ/cm2 producingcarrierden- obtained directly from the time domain data, −∆T/T,
sitiesfrom1.6×1017to1.1×1018cm−3 atT =20K.The and the induced increase in conductivity, ∆σ = neµ13.
photoconductivitytransientincreaseswithincreasingflu- Here, n is the carrier density, e is the electronic charge,
ence indicating an increasing conductivity with increas- and µ is the effective carrier mobility. ∆σ is obtained
ing carrierdensity. The rise time is resolutionlimited by fromthe approximationfor a thin film ona semi-infinite
the THz setup. The initial exponential decay increases insulating media given by
withincreasingfluence(see insetto Figure2), indicating
some interaction between the carriers creating a bottle-
neck effect. This behavior could also simply be due to a ∆σ ∼= −∆T(1+N), (1)
saturationofthe trapdensity where carriersaretrapped T Zod
by defects. At higher fluences, an additionalexponential where N = 1.6 is the refractive index of the media
relaxationcomponentdevelopswithalongerlifetimelev- (unpumped crystal), d = 14 µm is the optical penetra-
eling off at ∼4 ps. −∆T/T is close to linear with pump tion depth at the pump wavelength, and Zo = 377 Ω is
3
dependenceintothehigh-temperatureregimeisnotcom-
0.4 pletely understood4. The continuation of the power-law
dependence into the high-temperature regime was also
0.35 observed in Nph by Warta et al.6,7. The fact that the
Vs] conductivity shown in Figure 3(c) is seen to be increas-
2m/ 0.3 ingwithdecreasingtemperaturedoes howeverindicate a
[cµ band-like transport.
y The fast rise time for −∆T/T of ∼0.5 ps suggests
obilit 0.25 free carriers or polarons to be photoexcited directly, be-
M
causeTHz-TDSissensitivetothepresenceoffreecharge
Carrier density, n = 2.8x1017 cm-3 carriers. This rise time is comparable to the response
Fluence, I = 0.21 mJ/cm2 time of the THz setup, and it is therefore likely that the
0.2 formation of mobile charge carriers occur on a shorter
50 100 150 200 250300
timescale. This same behavior was observed by Heg-
Temperature [K] mann et al. in functionalized Pc16. Another group have
reported the formation of polarons within ∼100 fs in a
FIG. 4: Photoinduced carrier mobility versus temperature.
The straight line is a fit toT−n, where n=0.27. conjugatedpolymer(PPV thin films)20,21, thus support-
ing the semiconductor band model. Yet another group
finds that polarons are first generated by electric-field-
assisted dissociation of primary excitons on a timescale
the impedance of free space. The carrier density decays
of 10 ps in light emitting diodes (m-LPPP thin films)22,
exponentially by d, but is assumedconstant overthe ex-
thus supporting the molecular exciton model. Clearly,
citation thickness.
further studies are needed to address this controversy.
At low temperatures, T = 30 K, the photoinduced
carrier mobility is µ ≈ 0.4 cm2/Vs and µ decreases to In conclusion, we have measured the transient photo-
µ ≈ 0.2 cm2/Vs at room temperature. These values for conductivity in Pc single crystals using THz-TDS. The
themobilityassumesa100%internalquantumefficiency rapid onset of photoconductivity suggests the primary
conversiontofreecarriers,whichmaybeanoverestimate photoexcitations to be mobile charge carriers. The pho-
ofthe carrierdensity. Theseresultsprovidealowerlimit toinduced response consists of a two-component expo-
on the mobility. These values are comparable to values nential relaxation. The initial fast component relaxes
found in Pc crystals by Bukto et al.19 in a field effect within a few ps or less, possibly through the formation
geometry, as well as by Hegmann et al. also using the of excitons, small polarons, or trapping by impurities.
sametechniqueofoptical-pumpTHz-probespectroscopy Photoinduced mobilities of at least µ = 0.4 cm2/Vs at
onfunctionalizedPccrystals16. Thetemperaturedepen- 30 K was obtained. The temperature dependence of the
dence of the mobility shown in Figure 4 follows the T−n photoinduced mobilities is found to follow a power-law,
power-law in the temperature interval 30−300 K, where T−n, where n=0.27.
n = 0.27± 0.05. The low value of n does not give a This research was supported by the Los Alamos Di-
clearindicationofapureband-liketransportatlowtem- rected Research and Development Program of the U.S.
pertures, and the continuation of the typical power-law Department of Energy.
∗ Electronic address: [email protected] Model World Scientific, Singapore, 1997).
1 M. A. Baldo, M.E. Thompson, and S.R. Forrest, Nature 9 V.K.Thorsmølle,R.D.Averitt,M.P.Maley,M.F.Hund-
403, 750 (2000). ley, A. E. Koshelev, L. N. Bulaevskii and A. J. Taylor,
2 F. A. Hegmann, Physics in Canada 59, 127 (2003). Phys.Rev.B 66, 012519 (2002).
3 W. Warta, R. Stehle, and N. Karl, Appl. Phys.A 36, 163 10 R.D.AverittandA.J.Taylor,J.Phys.: Condens.Matter
(1985). 14, R1357-R1390 (2002).
4 E. A. Silinsh, A. Klimk¯ans, S. Larsson and V. Cˇ´apek, 11 J. Shan,F. Wang, E. Knoesel, M. Bonn, and T. F. Heinz,
Chem. Phys. 198, 311 (1995). Phys. Rev.Lett. 90, 247401 (2003).
5 E. A. Silinsh and V. Cˇ´apek, Organic Molecular Crys- 12 M. C. Beard, G. M. Turner, C. A. Schmuttenmaer, Phys.
tals: Interaction, Localization, and Transport Phenomena Rev.B 62, 15764 (2000).
(American Instituteof Physics, New York,1994). 13 K. P. H. Lui and F. A. Hegmann, Appl. Phys. Lett. 78,
6 L. B. Schlein, C. B. Duke and A. R. McGhie, Phys. Rev. 3478 (2001).
Lett. 40, 197 (1978). 14 R. D. Averitt, A. I. Lobad, C. Kwon, S. A. Trugman,
7 W. Warta, and N. Karl, Phys. Rev. B 32, 1132 (1985). V. K. Thorsmølle, and A. J. Taylor Phys. Rev. Lett. 87,
(American Instituteof Physics, New York,1994). 017401 (2001).
8 N. S. Sariciftci, Primary Photoexcitations in Conjugated 15 T.-I.JeonandD.Grischkowsky,Phys.Rev.Lett.78,1106
Polymers: Molecular Exciton versus Semiconductor Band (1997).
4
16 F. A. Hegmann, R. R. Tykwinski, K. P. H Lui, J .E. Bul- 20 D.Moses, A.DogariuandA.J.Heeger,Phys.Rev.B.61,
lock and J. E. Anthony, Phys. Rev. Lett. 89, 227403 9373 (2000).
(2002). 21 P.B.Miranda,D.Moses, andA.J.Heeger,Phys.Rev.B.
17 C. Rønne, P.-O. ˚Astrand and S. R. Keiding, Phys. Rev. 64, 08120(R) (2001).
Lett. 82, 2888 (1999). 22 W. Grauper, G. Cerullo, G. Lanzani, M. Nisoli,
18 R. D. Averitt, G. Rodriguez, J. L. W. Siders, S. A. Trug- E. J. W. List, G. Leising and S. De. Selvistri, Phys. Rev.
manandA.J.Taylor,J.Opt.Soc.Am.B17,327 (2000). Lett. 81, 3259 (1998).
19 V. Y. Butko, X. Chi, D. V. Lang and A. P. Ramirez,
Preprint, cond-mat/0305402 (2003).
