Circuital model validation for S-shaped organic solar cells by means of impedance spectroscopy

Organic solar cells (OSCs) based on Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5, 5-(4', 7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT):1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PCBM) have been electrically dc and ac characterized. Device structure is ITO/Poly(3,4-ethylenedioxy-wthiophene)-poly(4-styrene sulfonate) (PEDOT:PSS)/PCDTBT:PCBM/C60MA/Al. Devices show the so-called S-shaped current-voltage curve that has been modeled with a circuit including two diodes in forward and reverse bias. Impedance measurements show one semicircle at low bias ( V <; 0.4 V) and two semicircles at ( V <; 0.4 V). Results reveal that the high-frequency semicircle is associated with the forward diode-like behavior of the dc circuit, while the low-frequency semicircle is associated with the reverse diode that causes the S-shape. The analytical solution of the dc two-diode circuital model has been fitted to the experimental I-V, and dc circuital parameters have been obtained. The ac equivalent circuit was derived from the dc circuit, and small-signal parameters have been calculated from the fits to the experimental Cole-Cole.