Band-like temperature dependence of mobility in a

Report
Band-like temperature dependence of mobility
in a solution-processed organic semiconductor
Tomo Sakanoue and Henning Sirringhaus
University of Cambridge
What is the nature of charge carriers and transport in OSCs?
• In inorganic semiconductors charge carrier transport occurs via band-like transport. In
organic semiconductors transport is not well understood.
• Authors suggest that band-like transport does not occur in organics at room temperature and
that the charge carriers are localized.
• Polaron (charge and deformation of conjugated chain) – self trapped by the deformation
• Molecular vibrations can also destroy translational symmetry between neighboring
molecules – causing a mismatch in HOMO or LUMO alignment causing charge trapping
• Typically mobility temperature dependence in semicrystalline and polymeric films has
indicated the hopping mechanism, however increased mobility has been seen for very pure
crystalline small molecule films (rubrene) indicating band-like transport.
• Band-like transport is limited by scattering of the carriers (electrons or holes) on phonons
(thermally induced crystal lattice deformations). Therefore an indication of band-like transport
is increased mobility at low temperatures.
p – Channel FET
• Enhancement mode: a small gate voltage enhances the source – drain current
• Majority carriers (holes) flow from source to drain therefore in a p-channel FET drain potential must be lower than
source.
• As a reverse bias is applied at the gate, the negative potential induces a positive substrate channel immediately below
the gate, and the hole current increases as the reverse bias at the gate increases.
Device Architecture
P-channel field effect transistor, top gate, bottom contact architecture
low dielectric constant
polymer used to
insolate the gate
electrode
Gate
Insulator
Channel
Source / Drain
PFBT (Pentafluorobenzene thiol) / Au
Polyimide layer
inserted to control
wettability of glass
substrate
Polycrystalline organic
semiconductor, domain size
over 100 mm.
p – Channel FET, Room Temperature Properties
• Output characteristics of a device with relatively short channel length L = 5 mm, channel width W = 100 mm and a
120-nm-thick dielectric
• Majority carriers (holes) flow from source to drain therefore in a p-channel FET drain potential must be lower
than source (so negative drain potential applied)
• As a reverse bias is applied at the gate, the negative potential induces a positive substrate channel immediately
below the gate, and the hole current increases as the reverse bias at the gate increases. Current increases with
drain voltage until maximum output is reached (saturation region).
• Second graph displays gain (amplification, ratio of output signal to input) of current due to gate voltage.
Temperature Dependence
• For intermediate drain and gate voltages (-15V),
the FET current was nearly temp independent
between rt and 200 K, but then decreased with
decreasing T.
• For higher voltages (VD, G = -30 V) Current
increases by ~25% on cooling from r.t. to 140 K;
even at 4.3 K current remains same as r.t.
• Room temp to 140 K, the output I-V curves have
clear linear and saturation regions. However,
below 140K they acquire a positive curvature.
• The increase in transistor current between rt
and 140K demonstrates unambiguously that, in
contrast to other organic FETs, charge transport in
TIPS-pentacene FETs at sufficiently high applied
voltages improves with decreasing temperature.
Temp dependence of ID as function of VD, VG = -30 V
Field-effect Mobility
Square root of drain current as a function of gate voltage, with drain voltages of VD = -15 and -30 V. Temperature
dependence of the effective mobility of TIPS-pentacene FETs with different VD.
Ci = Capacitance of gate dielectric
VG = Gate Voltage, VT = gate voltage at
which channel conductance can begins
• Mobility can be extracted from the slope of the square root of the drain current in the saturation regime.
• Mobility in low voltage devices (VG,D = -15V), increases from 300K to 200 K but then decreases below 200 K.
Mobility is effected by trap states (energetic disorder pentacene/Cytop interface); at higher temperatures the more
mobile states higher in the density of states have been populated thermally and mobility increases with decreasing
temperature.
• Where high gate voltages are used (trap states are filled) the mobility in these more mobile states can be
estimated. Can see mobility increases with decreases temperature (down to 140 K) , reaching value of 2.5 cm2V-1s-1
Charge Modulation Spectroscopy
• Localized, polaronic charges in organic semiconductors have
characteristic optical absorptions differing from the neutral pp* absorption, such as the transitions between that HOMO
and SOMO and SOMO and LUMO of the radical cation.
• CMS modulates the charge density in the semiconducting
film by modulating the applied ac gate voltage . By measuring
the DT, the characteristic spectroscopic signature of the fieldinduced charge can be detected.
• Neutral TIPS-Pentacene (2.0 eV) and radical
cation (formed using FeCl3)
• Radical cation: 0.94-1.48 eV and 2.8 eV (0.94 eV
possibly SOMO-LUMO and 1.32-1.48 eV HOMOSOMO)
Charge Modulation Spectroscopy
• At low temperatures the spectral shape becomes very similar to that of the
isolated radical cation in solution.
• Resemblance to the absorption of the radical cation in solution, the charges
are not fully extended but remain localized over a certain N number of
molecules.
• CMS spectra in combination with T dependent mobility indicates charge
carriers in device are localized not by polaron self-localization, but dynamic
lattice disorder: small room temperature molecular motions that are sufficient
to destroy the translational symmetry of the electronic Hamiltonian
(representing a stack of molecules). – offsetting neighboring HOMO and LUMOs
from each other.
• T dependent CMS of FETs
(L = 40 mm, 450 nm thick dielectric, zero lateral electric field)
• Blue absorption of TIPS-pentacene thin film on polyimide
coated glass substrate.
CMS at 43 K (black) and difference between the
absorption spectra of a chemically doped and
undoped solution (red)
Conclusions
• At low drain and gate voltages trap states in TIPS-pentacene thin films inhibit
mobility but at high drain and gate voltages trap states are filled and more mobile
states can be accessed.
• In TIPS-pentacene polycrystalline p-channel FETs charge carriers show
spectroscopic signature similar to radical cation, but also a temperature dependent
mobility where m increases at T decreases.
• The charge mobility is therefore not band-like as in inorganic semiconductors, but
also not polaron trapped as in some polymer thin films. The charge carriers are
localized, but experience an increased mobility at low temperature due to freezing
of intermolecular motions.
Charge Modulation Spectroscopy
• The origin of the nonlinear dependence of the lowtemperature transport on the lateral electric field.
• Keeping lateral field constant (constant VD) carried out
low temp CMS.
• The sharp peak at 1.24 eV (trapped-charge absorption)
broadens with increasing VD.
• Spectrum at 100K and -15 V is very similar to the broad
charge-induced at 300K, VD = 0.
• This provides evidence that charges in single-molecule,
shallow trap states can be effectively detrapped into more
mobile states by application of the source-drain electric
field.
Drain-voltage dependence of the CMS spectra
at 100 K. For comparison, the corresponding
spectrum at 300K and VD = 0 V is also shown.
All spectra were acquired with VG = -20 V and a
modulation bias of ±2 V at 37 Hz.
What is the nature of charge carriers and the mechanism of charge
transport in organic semiconductors (OSCs) at room temperature?
Band-like Transport
In comparison to covalently bonded inorganic semiconductors,
OSCs are characterized by weak intermolecular van der Waals
bonds; this leads to relatively narrow bandwidths, stronger
electron lattice interactions, and more pronounced polaron
formation. Consequently, coherent band-like conduction is
expected only at low temperatures. As temperature increases,
electron-phonon coupling leads to enhancement of polaron
mass and further decrease in bandwidth, this may ultimately
result in localizing charge carriers and altering the transport
mechanism. Therefore band-like m should improve with
decreasing T.
Highly pure single crystals of small
molecule OSCs (i.e. rubrene)
electronic conduction improves with
decreasing temperature: charge
carriers occupy states that extend
over many molecules
Hopping
If transport is occuring via phonon assisted
hopping m will decrease with decreasing T.
In contrast electronic conduction in
many polymeric or polycrystalline
OSCs degrades exponentially with
decreasing T.
Field-effect Mobility
Square root of drain current as a function of gate voltage, with drain voltages of VD = -15 and -30 V. Temperature
dependence of the effective mobility of TIPS-pentacene FETs with different VD.
• Mobility can be extracted from the slope of the
W = channel width, Ci = capacitance of the gate
dielectric, VT = threshold voltage.
q = absolute electron charge, fB = difference between the
Fermi level and the intrinsic Fermi level (which is located
very close to midgap), ei = permittivity of the gate
dielectric.
square root of the drain current in the saturation
regime.
• Sq. root of the saturated drain current exhibits a
positive curvature at low gate voltages and the
threshold voltage shifts to more negative values. This
reflects filling up low-mobility trap states and
eventually populating more mobile states. Where
high gate voltages are used, the mobility in these
more mobile states can be estimated.
Field-effect Mobility
Square root of drain current as a function of gate voltage, with drain voltages of VD = -15 and -30 V. Temperature
dependence of the effective mobility of TIPS-pentacene FETs with different VD.
m (0) = mobility with zero applied field
q = absolute electron charge
F = magnitude of electric field
m (F) = field-effect mobility
Poole-Frenkel factor, b =(e/pee0) 1/2
k = Boltzmann constant
• Mobility can be extracted from the slope of the square root of the drain current in the saturation regime.
• Mobility in low voltage devices (VG,D = -15V), increases from 300K to 200 K but then decreases below 200 K.
Mobility is effected by trap states (energetic disorder pentacene/Cytop interface); at higher temperatures the more
mobile states higher in the density of states have been populated thermally and mobility increases with decreasing
temperature.
• Where high gate voltages are used (trap states are filled) the mobility in these more mobile states can be
estimated. Can see mobility increases with decreases temperature (down to 140 K) , reaching value of 2.5 cm2V-1s-1
+

similar documents