Thin film all polymer capacitor and method of making

An all polymer, electrochemically formed, thin film capacitor is provided thout use of any metal foil and found to yield reasonable capacitance values. The capacitor includes an electrochemically formed insulating polymer sandwiched between two electrochemically formed conductive polymers.

This invention relates in general to a thin film all polymer capacitor and 
to its method of making, and in particular to such a capacitor including a 
flat layer of an electrochemically formed insulating polymer sandwiched 
between two flat layers of electrochemically formed conducting polymers. 
BACKGROUND OF THE INVENTION 
Preparation of thin film capacitors has generally required that an 
insulating free standing dielectric film be sandwiched between thin metal 
foils. The dielectric film and metal foil have then been spirally wound 
together to produce a device. One major disadvantage of this technique has 
been that metal foils use a large percentage of the available weight and 
volume in the fabricated device. Since energy capacity of the device is 
directly related to surface areas of insulating dielectric, a reduction in 
the volume of the contact through use of vanishingly thin, conductive 
polymeric films should increase the energy capacity of the resulting 
device. In addition, since the density of conducting polymers are 
generally between 1 and 1.5 g/cm while aluminum is 2.7 g/cm , a 
substantial reduction in energy capacity per unit weight can also be 
realized. Therefore, if an insulating dielectric can be readily placed on 
a thin conductive material, this would be of great interest. 
Another major disadvantage of the standard technique has been that pinholes 
in the insulating dielectric can have catastrophic effects on the 
resulting device. That is, when the capacitor is stressed with a high 
voltage and a pinhole is present, a tremendous current density passes 
through the pinhole causing heat generation and subsequent deterioration 
of the insulating dielectric. However, if the conductor were a conducting 
polymer, heat build-up from the current would quickly flash off the 
conducting polymer, shut off the current and isolate the pinhole. This 
self-healing process from pinholes would also be of great interest. 
SUMMARY OF THE INVENTION 
The general object of this invention is to provide a thin film all polymer 
capacitor that can be made without the use of any metal foil. A more 
particular object of the invention is to provide such a capacitor that 
will be lightweight and easily produced. A further object of the invention 
is to provide such a capacitor that will be substantially free from 
stressing by pinholes. 
It has now been found that the aforementioned objects can be attained using 
a flat layer of an electrochemically formed insulating polymer sandwiched 
between two flat layers of electrochemically formed conducting polymer. 
The newly designed, all polymer construction enables very thin, lightweight 
capacitors to be easily produced. A practical capacitor uses 
poly-n-methylpyrrole (PNP) as the insulating dielectric sandwiched between 
two sheets of conductive polypyrrole (PP) to form the capacitor.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A conducting flat layer of PP is first electrochemically prepared on an 
indium tin oxide coated glass electrode at a constant current in an 
aqueous solution of 0.1 molar sodium dodecylbenzenesulfonate and 0.1 mole 
of pyrrole. The resulting polymer is peeled off the electrode and washed 
in water. A flat insulating layer of (PNP) is then electrochemically 
deposited on the conducting polymer at a constant current in an aqueous 
solution of 0.1 molar sodium sulfate and 0.05 molar N-methyl pyrrole. The 
resulting deposit is then washed in distilled water and dried under 
dynamic vacuum. A second conducting deposit is then prepared as above and 
mechanically pressed onto the other side of the deposit. 
As a result of carrying out the foregoing embodiment, the following 
experimental results are shown for a laboratory scale device 
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Capacitor Capacitance Loss 
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130.19 pF/100 Hz 
7.638/100 Hz 
PP/PNP/PP 143.29 pF/1k Hz 
0.8234/1k Hz 
(A = 2.0 cm.sup.2) 
141.55 pF/10k Hz 
0.0453/10k Hz 
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PP = polypyrrole (C.sub.12 H.sub.25 C.sub.6 H.sub.4 SO.sub.3 
dopedconductive) 
PNP = polyn-methyl-pyrrole (SO.sub.4.sup.-2 dopedinsulating). 
The results clearly show that a working device can be prepared by this 
method. 
Dielectric breakdown of the device was also investigated. In this 
connection it was found that a PP/PNP/PP capacitor prepared for test 
purposes held off a voltage of approximately 100V before dielectric 
breakdown occurred. This result indicates that the electrochemically 
formed insulator layer is truly acting as a static energy storage device. 
In the aforedescribed embodiment, the thickness of the prepared capacitor 
is about 110 microns of which 15 microns is the thickness of the 
insulating layer and 45 microns is the thickness of each conductive layer. 
I wish it to be understood that I do not desire to be limited to the exact 
details as described for obvious modifications will occur to a person 
skilled in the art.