Energetics and kinetics of defects in the effective band gap of organic bulk heterojunctions are determined by means of capacitance methods. The technique consists of calculating the junction capacitance derivative with respect to the angular frequency of the small voltage perturbation applied to thin film poly(3-heicylthiophene) (P3HT) and [6,6]-phenyl C-61-butyric acid methyl ester solar cells, varying the temperature. The analysis reveals the presence of defect bands (shallow acceptors) centered at E-0 = 35 meV above the highest-occupied molecular orbital level of P3HT. The total density of defects results of order 10(16) cm(-3). Characteristic frequency is obtained to be situated within the range of 1-10 Hz. Defect bands acting as negatively charged levels are responsible for the p-doping of the active layer and the band-bending near the cathode contact, as derived from Mott-Schottky capacitance-voltage analysis. (C) 2011 Elsevier B.V. All rights reserved.