Abstract

Einladung
g zum Vo
ortrag von
v
Dr. Karste
en Hannew
wald
Fried
drich-Sch
hiller-Univ
versität, Jena
J
Donners tag, 24. März
M
2011
Hörsaa
al Physik, M
Montanun
niversität Leoben
L
11 Uhr c.tt.
Po
olaron Trransport in Organ
nic Crysttals: Theo
ory and Modellin
ng
An imp
portant class of org
ganic sem
miconduc
ctors are molecular
m
r crystals of high
purity. Due to their
t
high
h degree of structu
ural order, such crrystals are
e ideal
candid
dates for the investtigation o
of the intrin
nsic excita
ations and
d charge-carrier
transpo
ort phen
nomena in organ
nic solidss. A parrticularly importan
nt and
challenging top
pic is the
e understtanding and ab initio desscription of the
charge
e-carrier mobilities in these crystals, as the mobility
m
is a fundamental
materiial prope
erty and a centra
al quantity
y for the
e optimiza
ation of device
perform
mance.
The de
escription of charge transpo
ort in orga
anic crysta
als is a no
ontrivial ta
ask due
to the strong co
oupling of
o electron
nic and vibronic
v
degrees
d
o
of freedom
m. Very
often, one desscribes the
e resulting
g carrier mobility within eitther the limit of
localized small polarons or the limit of delocalized Bloc
ch wavess. Both
approa
aches, ho
owever, are
a not sa
atisfying and
a
also appear
a
to
o be som
mewhat
incompatible. While
W
elab
borations o
of Holstein
n's small-p
polaron m
model are usually
restrictted to hig
gh tempe
eratures a
and stron
ng electro
on-phono n couplin
ng, the
wide-b
band theo
ory based
d upon the
e Boltzma
ann equation can neither de
escribe
polaro
onic effectts nor hop
pping mottion and iss restricted
d to low te
emperatu
ures.
This situ
uation is particularl
p
y unsatisfy
ying since
e many in
nteresting organic systems
s
have strong electron-p
e
phonon c
coupling and, sim
multaneou
usly, also large
widths, i.e., strong electronic
e
c coupling
g. Hence, a perturb
bative trea
atment
bandw
of eith
her intera
action is not justiffied. Furth
hermore, the lackk of know
wledge
conce
erning the
e necessa
ary mate
erial parameters fo
or simulattions has to be
overco
ome by means
m
of first-princip
f
ples calcu
ulations off electron
ns, phonon
ns, and
their m
mutual inte
eraction.
In my talk, I will
w present a com bined the
eoretical and num
merical ab initio
descrip
ption of charge transport
t
in organ
nic crystals which overcom
mes the
above
e-mentioned limitattions. It ge
eneralizess Holstein's small po
olaron mo
odel to
polaro
ons of arb
bitrary size
e and all ows to calculate
c
the carrie
er mobilitty from
density
y-function
nal theory.
The generalized
d mobility expresssion treatts cohere
ent band
d transpo
ort and
therma
ally induc
ced hopp
ping on eq
qual footting and reproduc es the results of
previous theorie
es in the respectiive limits. As a prrototypica
al examp
ple, the
y is applie
ed to he
erringbone
e-stacked
d crystals where th
he tempe
erature
theory
depen
ndence of the mob
bilities is si mulated from
f
first principles
p
and com
mpared
to exp
perimental data. Finally, the mobility anisotrop
py is analyyzed by a novel
3D visu
ualization technique
e for the rrelevant trransport channels.
c
Der Vo
ortrag wird
d vom Leh
hrstuhl für Atomistic
c Modelling and De
esign of Materials
im Rahmen de
er SimNet Multisca
ale Lecturres und vom
v
Instit ut für Physik im
Rahme
en des Seminars au
us Halbleitterphysik und
u
Nanotechnolog
gie veran
nstaltet.