478
18 Organic Semiconductors
Fig. 18.8 Historic
development of the
experimentally achieved
mobility of organic
semiconductors at room
temperature
18.5 Transport Properties
Transport in organic semiconductors is characterized by
• strong polaronic effects,
• hopping conduction,
• low mobility, low drift saturation velocity.
The interaction of charges with lattice deformations leads to the formation of polarons [1572]. In
organic materials these are often ’small‘, i.e. the extension of the deformation is on atomic scale.
Such self-trapping of charges reduces their mobility. Two charges can share the same deformation
(bipolaron) or oppositely charged polarons can attract (similar to an exciton). If these charges are on
the same (neighboring) polymer chain, the polaron is called intrachain (interchain).
The conductivity within a molecule, e.g. a long polymer chain, and the conductivity between different molecules have to be distinguished. The conduction between different molecules occurs via
hopping. Typically, the conductivity is thermally activated according to
σ = σ 0 exp
−
E a
kT
,
(18.2)
where E a is an energy of the order of 1 eV. Such activation also pertains to the mobility alone, e.g.
E a = 0.48 eV for PPV [1573].
The maximum low-field mobility of many crystalline organic semiconductors at room temperature
is around 1 cm
2 /Vs with a weak temperature dependence [1574]. Such mobility is much smaller than
that of crystalline silicon and rather comparable to that of amorphous silicon. Improved purity and
handling of organic semiconductors has allowed to achieve intrinsic material properties (Fig. 18.8).
The mobility increases at low temperatures, e.g. below 100 K in naphtalene [1575]. This has been
attributed to the freeze-out of phonons and the transition from hopping to band transport.
The drift velocity at higher fields shows saturation but the values, even at low temperature, are much
smaller than in silicon (Fig. 18.9). An analytical model for the described main features of transport in
organic semiconductors has been given in [1576].
18 Organic Semiconductors
Fig. 18.8 Historic
development of the
experimentally achieved
mobility of organic
semiconductors at room
temperature
18.5 Transport Properties
Transport in organic semiconductors is characterized by
• strong polaronic effects,
• hopping conduction,
• low mobility, low drift saturation velocity.
The interaction of charges with lattice deformations leads to the formation of polarons [1572]. In
organic materials these are often ’small‘, i.e. the extension of the deformation is on atomic scale.
Such self-trapping of charges reduces their mobility. Two charges can share the same deformation
(bipolaron) or oppositely charged polarons can attract (similar to an exciton). If these charges are on
the same (neighboring) polymer chain, the polaron is called intrachain (interchain).
The conductivity within a molecule, e.g. a long polymer chain, and the conductivity between different molecules have to be distinguished. The conduction between different molecules occurs via
hopping. Typically, the conductivity is thermally activated according to
σ = σ 0 exp
−
E a
kT
,
(18.2)
where E a is an energy of the order of 1 eV. Such activation also pertains to the mobility alone, e.g.
E a = 0.48 eV for PPV [1573].
The maximum low-field mobility of many crystalline organic semiconductors at room temperature
is around 1 cm
2 /Vs with a weak temperature dependence [1574]. Such mobility is much smaller than
that of crystalline silicon and rather comparable to that of amorphous silicon. Improved purity and
handling of organic semiconductors has allowed to achieve intrinsic material properties (Fig. 18.8).
The mobility increases at low temperatures, e.g. below 100 K in naphtalene [1575]. This has been
attributed to the freeze-out of phonons and the transition from hopping to band transport.
The drift velocity at higher fields shows saturation but the values, even at low temperature, are much
smaller than in silicon (Fig. 18.9). An analytical model for the described main features of transport in
organic semiconductors has been given in [1576].