Impact of recombination mechanisms on the capacitance-voltage characteristics in bulk heterojunction organic solar cells

Low frequency Capacitance- voltage (C-V) measurements have been employed to investigate the effect of thickness variation of poly (3-hexylthiophene (P3HT) and fullerene [6,6],-phenyl-C61-butyric acid methyl ester(PCBM) based Bulk Heterojunction (BHJ) photo absorbing layer in inverted organic solar cell (OSC) configuration, ITO|ZnO|P3HT:PCBM|MoO3|Ag and are corelated with different recombination mechanisms. The normalized capacitance (CCgeom) with voltage curves at different P3HT: PCBM blend thicknesses such as 130, 100, 80 and 70 nm demonstrate an increase in peak height with thickness as well as the presence of negative capacitance (NC) beyond peak voltage for higher thicknesses. To understand both peak height variation and the occurrence of NC, a defect sensitive simulation study has been performed by employing the radiative and non-radiative recombination processes. Based on the analysis of simulation results we confirm the role of Langevin bimolecular recombination rate in determining the peak height with thickness; however, the presence of NC in C-V curves can be understood through a competition between the Langevin bimolecular, and trap assisted recombination rates. Study confirms that peak height reduces due to increase in the rate of Langevin bimolecular recombination, whilst the trap assisted recombination resurges the NC beyond peak voltage. Later, the utilities of D-D based1C23 vs V analysis to calculate built-in voltage (Vbi) have been performed for the experimental and simulation data at different thicknesses emphasizing the strength of this approach to characterize threshold voltage of organic diodes.