Evolution with annealing of solar cell parameters modeling the S-shape of the current-voltage characteristic

In this work we use a modified equivalent circuit to simulate the S-shape found in current density-voltage (J-V) characteristics of organic solar cells (OSC) based on P3HT:PCBM. The modification of the traditional model includes a second diode opposite to the main one together with a parallel resistance, R-P2. The transcendental equation derived from the circuit is solved exactly without approximations. The S-shape (so called kink), usually attributed to a poor quality of polymer/cathode interface, can be removed after three slow annealing treatments. The evolution of the J-V curves with the annealing process has been quantified by extracting the equivalent circuit parameters of each curve (pristine and annealed) by fitting the model to the experimental data. Results show that kink annihilation is mostly due to a strong decrease of R-P2, which diminishes the reverse diode effect on the J-V curve. (C) 2012 Elsevier B.V. All rights reserved.