Page 323 - Organic Electronics in Sensors and Biotechnology

300 Cha pte r Ei g h t

applications in memory elements and sensor systems concludes the
chapter.

8.1 Functional Organic Materials

8.1.1 Organic Semiconductors
The requirements for organic semiconductors in organic electronics
are rigorous. A large number of equally important criteria determine
the performance of these devices.
• Organic semiconductors need to exhibit good chemical sta-
bility when exposed to ambient conditions.
• They need to be stable under bias stress in typical operating
conditions.
• They must be compatible with gate dielectrics to form inter-
faces with low interface trap density.
• Organic semiconductors must allow for efficient charge injec-
tion and low contact resistance, when in contact with metal
electrodes.
• They need to exhibit charge carrier mobilities exceeding 0.1 or
2
even 1 cm /(V . s) in practical applications to be comparable with
inorganic competing technologies such as amorphous silicon.
• They should be sustainable to bending in flexible displays,
circuits, and sensors.

Since charge transport in organic field-effect transistor (OFET)
devices takes place at the interface between the organic semiconductor
film and the gate dielectric, charge transport depends on achieving
close intermolecular stacking throughout the length scale of the OFET
channel. One of the key tasks is to achieve long-range efficient charge
transport by self-organization schemes. This long-range order is often
prevented by side groups that allow for producing solution processabil-
ity in molecular semiconductors and polymers. Impurities and by-prod-
ucts from synthesis often hinder achieving high mobilities and result in
low “off” currents. These impurities are also a potential reason for the
1
slow degradation of devices, as often observed in experimental inves-
tigations. Traps, e.g., due to impurities, may easily immobilize charge
carriers and thereby decrease charge carrier mobility.
There is an intense ongoing research on high mobility organic
semiconductors with all the aforementioned desirable properties.
Indeed, significant recent advancement has been made in developing
both p-type and n-type semiconductors with recent reviews describing
1–7
in detail what has been achieved. Among these reviews one can divide
1
research efforts into synthetic aspects of organic semiconductors,