A modified equivalent circuit model for Organic Solar Cells based on voltage dependent series resistance.

A single-diode circuit model with a voltage-dependent series resistance is introduced for Organic Solar Cell (OSC). This modified equivalent circuit model benefits from simplicity of the equivalent circuit while it predicts the device behavior accurately. Voltage dependency of series resistance of OSCs is mainly related to the internal processes of charge transport. The received charges at electrodes are influenced by space charge effect and many other physical parameters including exciton lifetime, dissociation rate, and free carrier recombination. The carrier transport process from donor/acceptor interface to electrodes is governed by its mobility. Also, the carrier mobility of organic material depends on the field variation. The voltage-dependent series resistance is not often used in today’s OSC circuit models due to the lack of a reliable field-dependent extraction method. This refined model expands its application in finding optimum bias, behavior analysis and efficiency improvement of OSCs. In this paper, as the first step, voltage-dependent series resistance is modeled. Then a new algorithm is proposed for extracting the voltage-dependent series resistance. Finally, the consistency of the extracted field dependent series resistance with previous literature is discussed. It is shown that the average mean square errors (MSE) of the predicted J-V characteristics by the new approach for five cells under test are improved by a factor of 106 in comparison with those of a constant value Rs approach.