Based on my research the effects of the holes, electrons, acceptor and donor concentration is telling you the direction of the polarization and the conventional sign of the electric potential generated. But up until now, i don't know what is the best material to used for piezoelectric material. is it n-type or p-type semiconductor ?(assuming that this semiconductor have high piezoelectric coefficient). Is conductivity and resistivity affects the generated potential? if so please enlighten me and send me a link about your reference papers! thank you!