Kinetics of occupancy of defect states in poly(3-hexylthiophene): fullerene solar cells

Abstract

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-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester solar cells, varying the temperature. The analysis reveals the presence of defect bands (shallow acceptors) centered at E0 = 35 meV above the highest-occupied molecular orbital level of P3HT. The total density of defects results of order 1016 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.