부산대학교 도서관

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/C 60 MA/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 (${\bf V} < {\bf 0}.{\bf 4}$ V) and two semicircles at ${\bf V} \geq {\bf 0}.{\bf 4}$ 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.