Circuital modelling of S-shape removal in the current-voltage characteristic of TiOx inverted organic solar cells through white-light soaking
Résumé
In this work light activation phenomenon in inverted bulk heterojunction (BHJ) organic solar cells (OSC) has been electrically modelled with a two-diode equivalent circuit. OSC are based on poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PCBM) with a titanium oxide (TiOx) sublayer. Current-voltage (I-V) characteristics show a highly pronounced S-shape that is gradually removed during light activation process. The circuit used to model I-V curves includes two diodes in forward and reverse bias together with two parallel resistances, RP 1 and RP 2. The parallel of the reverse bias diode and its corresponding resistance RP 2 models the electrical behaviour of the TiOx interlayer. This interlayer has been thermally treated at different temperatures, from 80 °C up to 180 °C, reducing the activation time from 400 s for unbaked devices down to 30 s for devices annealed at temperatures higher than 80 °C. The S-shape shown in the I-V characteristic is completely removed after a few minutes of white-light illumination. I-V curves recorded during the activation process have been fitted with the analytical solution of the two-diode circuit based on W-Lambert function. A decrease of the subcircuit 2 equivalent resistance has been found to be the cause of S-shape removal. This resistance diminishing is in good agreement with the increase of TiOx conductance with baking temperature and white-light exposure time found by other authors.