For performing dielectric studies on pellet samples it is necessary to coat both faces of the pellet with a conductive material. What kind of material can be used?
Shiddaling....If you want to measure dielectric properties of the sample than you must coat it with any conducting material (silver, gold carbon..paint ) only than it can be treated as a capacitor (two electrical conductors separated by a dielectric )..gold coating is best but due to low price silver paste is commonly used...
Will silver not get oxidized when exposed to the room air and change the conductivity of the electrode? I am curious to know if I can use silver paste for organic solar cells also.
Yes definitely you can use external electrodes but it must be so much adhesive with the sample otherwise air will fill in between and your sample may behave like a multilayer capacitor.
First of all ensure that your sample is more than 95% theoretical density. Otherwise any conducting paste that you might apply will, would flow through the crevices, worst it would short the sample. I would prefer the conducting paste to be cured at room temperature. Alternatively, you may opt for sputtering to coat the surface. This will be the best method if you are trying make electrodes on SINGLE crystals. Pastes will simply peal off. I would NOT prefer to have another sheet of conducting materials as it would lead to poor contact. Certainly no Al as its surface gets oxidized to Al2O3. You add more complication.