Modification of PEDOT:PSS films using ZnI2 additive for power conversion efficiency enhancement of organic solar cells

Organic solar cells (OSCs) fabricated with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) films often have limited performance due to high sheet resistances since commercial PEDOT:PSS contains a high insulating PSS to conducting PEDOT ratio. To resolve this issue, zinc iodide (ZnI2) additive was utilized to modify PEDOT:PSS films, which was carried out by mixing ZnI2 with PEDOT:PSS solution. The mixture was deposited on a fluorine-doped tin oxide substrate to derive the modified PEDOT:PSS films for application as a hole-transporting layer in OSCs. This resulted in an enhanced power conversion efficiency for OSCs fabricated with modified PEDOT:PSS films. The enhancement was primarily due to the improved PEDOT:PSS/PCDTBT:PC70BM (poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] – PCDTBT, [6,6]-phenyl-C71-butyric acid methyl ester – PC70BM) interfaces, which facilitates enhanced hole collection performance and results in a high current density for OSC devices. Moreover, the ZnI2 plays a role in the depletion of insulating PSS from the film’s surface. This behavior causes lower sheet resistance and results in an increased Jsc and Voc for OSC devices. Therefore, the improved interfacial contact and depletion of PSS are considered synergistic functions for OSC enhancement.