Assessing the potential of silicon and germanium phthalocyanines in planar heterojunction organic photovoltaic devices
New article published in ACS Applied Materials & Interfaces. In this study we have assessed the potential application of dichloro silicone phthalocyanine (Cl2-SiPc) and dichloro germanium phthalocyanine (Cl2-GePc) in modern planar heterojunction organic photovoltaic (PHJ-OPV) devices. We have determined that Cl2-SiPc can act as an electron donating material when paired with C60 and that Cl2-SiPc or Cl2-GePc can also act as an electron acceptor material when are paired with pentacene. These two materials enabled the harvesting of triplet energy resulting from the singlet fission process in pentacene. However, contributions to the generation of photo current were observed for Cl2-SiPc with no evidence of photo current contribution from Cl2-GePc. The result of our initial assessment established the potential for the application of SiPc and GePc in PHJ OPV devices. Thereafter, bis(pentafluoro phenoxy) silicon phthalocyanine (F10-SiPc) and bis(pentafluoro phenoxy) germanium phthalocyanine (F10-GePc) were synthesized. Prior to characterization and device integration, F10-SiPc was purified by train sublimation where it was discovered that during the sublimation process, F10-SiPc underwent a reaction of unknown mechanism that resulted in the formation of small amounts of difluoro SiPc (F2-SiPc). Thermogravimetric analysis and x-ray photoelectron spectroscopy were utilized to identify that at a lower pressure and lower temperature the formation of F2-SiPc could be completely suppressed and therefore integration into devices F10-SiPc could be done without contamination from F2-SiPc. In contrast, when attempting to purify F10-GePc by train sublimation, regardless of the type of the carrier gases employed or the temperature used, 100% formation of F2-GePc was observed. It was determined that both F10-SiPc and its precursor Cl2-SiPc have very similar opto-physical properties in both solution and the solid-state. However, using single crystal x-ray diffraction it was determined that F10-SiPc has significantly enhanced π-π interactions compared with Cl2-SiPc having little to none. Unoptimized PHJ OPV devices based on F10-SiPc were fabricated and directly compared to those constructed from Cl2-SiPc and in all cases PHJ OPV devices based on F10-SiPc had improved device characteristics compared to Cl2-SiPc.
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