Dear Bipin Rijal , This parallel behaviour or S-like nature of J–V curve is often found in the solar cells generally due to charge accumulation and dipole formation on the electrodes. This S-like behaviour limits the device performances and can be eliminated by optimizing the device processing parameters. You may change the PCBM concentration and thickness of the electrode to get the required results. You may get some more informations from these works also-

References-

1. Anand Pandey et al 2021 J. Phys. D: Appl. Phys. 54 275502

2.Sims L, Hörmann U, Hanfland R, Mackenzie R C I, Kogler F R, Steim R, Brütting W and Schilinsky P 2014 Investigation of the s-shape caused by the hole selective layer in bulk heterojunction solar cells Org. Electron. 15 2862–7

3. Sesa E, Vaughan B, Feron K, Bilen C, Zhou X, Belcher W and Dastoor P 2018 A building-block approach to the development of an equivalent circuit model for organic photovoltaic cells Org. Electron. 58 207–15.

4. Gupta D, Bag M and Narayan K S 2008 Correlating reduced fill factor in polymer solar cells to contact effects Appl. Phys. Lett. 92 093301

Regards,

Anand