Page 155 - Organic Electronics in Sensors and Biotechnology

132 Cha pte r F o u r

The semicrystalline nature of polymer ferroelectrics covers three
different types of polarization sources. There is frozen-in polarization
from dipoles in the amorphous phase, ferroelectric (thermodynamic
stable) polarization in the crystalline phases, as well as charge-
induced polarization from compensation charges that accumulate at
the interfaces between amorphous and crystalline regions, to com-
pensate for the large depolarization fields and to stabilize the ferroe-
lectric polarization.

Preparation of P(VDF-TrFE) Thin Films and of the Reference
Capacitance Structures
It has been reported 25, 26 that lyophilized gels of PVDF using γ-
butyrolactone as solvent lead to stable gels favoring the formation of
TT conformation resulting in β-phase PVDF. Therefore it was decided
to transfer the γ-butyrolactone-based sol-gel process for PVDF to the
P(VDF-TrFE) copolymer, which seems to be the most promising way
to obtain highly crystalline, sufficiently flat, ferroelectric thin films by
using a low-temperature spin-on process. 27, 28
In this process γ-butyrolactone is heated up to 180°C using a
reflux condenser to avoid solvent evaporation. A known amount of
P(VDF-TrFE) pellets with a VDF:TrFE composition of 55:45, 65:35,
and 76:24 is added to the solvent and dissolved. The solution is kept
at this temperature for approximately 2 h. After cooling down, the
solution is poured into a glass bottle and kept at room temperature
for gelation. Prior to spin-on, viscid gels are transferred to a low-
viscous sol by slow heating up. Depending on the spin-speed and the
solid phase content, layer thickness values between 300 nm and 3 μm
can be achieved easily. 27, 28 The dielectric constants of materials with
VDF:TrFE composition of 55:45, 65:35, and 76:24 typically are 15 to 17
for materials with 55% VDF content and 10 to 12 for the others.
For the electrical characterization of the ferroelectric properties of
the polymer layers, capacitancelike structures were prepared by
means of standard process steps. Aluminum layers with a thickness
of 50 nm are sputtered on glass or PET substrates (Melinex) serving
as the bottom electrode onto which the sol is spin-coated. Subse-
quently a calcination step is carried out at 110°C, which increases the
crystallinity of the dielectric thin films. The samples are kept at this
temperature for 5 h and then slowly cooled down to room tempera-
ture. Finally top electrodes are deposited by thermal evaporation of
50 to 80 nm silver via shadow masks. The hysteresis measurements for
the determination of the remnant polarization were done in a Sawyer-
Tower configuration.

Ferroelectric Characteristics of P(VDF-TrFE)
In Fig. 4.7 at left, a typical hysteresis loop measurement of a P(VDF-
TrFE) capacitor is shown. The different curves correspond to loop
measurements with increasing maximum electric field. The coercive