Solvent effects on ZnPc thin films and their role in fabrication of nanostructured organic solar cells

Abstract

Fabrication of interdigitated organic photovoltaic (OPV) devices commonly involves filling a nanostructured thin film using solution-based methods. In these cases, the sensitivity of the nanostructured films to the solvents used needs to be studied in order to optimize the interface. Here, we study the ability of chlorobenzene (CB) and dichlorobenzene (DCB) to dissolve and recrystallize zinc phthalocyanine (ZnPc) thin films, while fabricating interdigitated active layers composed of ZnPc and [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM). Using glancing angle deposition (GLAD), we are able to construct slanted nanorod arrays of ZnPc, with nanorod diameters of nominally 40 nm and lengths up to 450 nm. We find that these films are recrystallized upon direct exposure to CB and DCB, regardless of initial morphology, yielding a variety of possible nanoscale formations. The crystallinity and absorbance of these recrystallized films changes as well, depending on solvent choice and exposure time. Through control of initial ZnPc film morphology and solvent choice for PCBM casting, we exploit the sensitivity of ZnPc to these organic solvents to optimize the photovoltaic performance of ZnPc/PCBM devices. Optimal OPV device performance is achieved with a 30 nm thick GLAD-textured ZnPc film while using DCB as the PCBM solvent. In this case, power conversion efficiencies are up to 3.0%, compared to an average of 2.3% when using CB as the solvent on the same film, and 2.3% also for bilayer devices when using DCB as the solvent. A higher degree of material mixing at the ZnPc/PCBM interface is shown when using DCB over CB as the PCBM solvent, which may be the primary mechanism for the photovoltaic improvements seen in these devices.