  We carry out a comprehensive theoretical and experimental study of charge
injection in Poly(3-hexylthiophene) (P3HT) to determine the most likely
scenario for metal-insulator transition in this system. We calculate the
optical absorption frequencies corresponding to a polaron and a bipolaron
lattice in P3HT. We also analyze the electronic excitations for three possible
scenarios under which a first-- or a second--order metal--insulator transition
can occur in doped P3HT. These theoretical scenarios are compared with data
from infrared absorption spectroscopy on P3HT thin film field-effect
transistors (FET). Our measurements and theoretical predictions suggest that
charge-induced localized states in P3HT FETs are bipolarons and that the
highest doping level achieved in our experiments approaches that required for a
first-order metal--insulator transition.
