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Assessing the Potential Roles of Silicon and Germanium Phthalocyanines in Planar Heterojunction Organic Photovoltaic Devices and How Pentafluoro Phenoxylation Can Enhance π–π Interactions and Device Performance

Lessard, Benoît H., White, Robin T., AL-Amar, Mohammad, Plint, Trevor, Castrucci, Jeffrey S., Josey, David S., Lu, Zheng-Hong, Bender, Timothy P.
ACS Applied Materials & Interfaces 2015 v.7 no.9 pp. 5076-5088
X-ray diffraction, energy, germanium, harvesting, heat treatment, photovoltaic cells, silicon
In this study, we have assessed the potential application of dichloro silicon 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 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 photocurrent were observed for Cl₂-SiPc with no evidence of photocurrent 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 and characterized. During thermal processing, it was discovered that F₁₀-SiPc and F₁₀-GePc underwent a reaction forming small amounts of difluoro SiPc (F₂-SiPc) and difluoro GePc (F₂-GePc). This undesirable reaction could be circumvented for F₁₀-SiPc but not for F₁₀-GePc. Using single crystal X-ray diffraction, it was determined that F₁₀-SiPc has significantly enhanced π–π interactions compared with that of Cl₂-SiPc, which had 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 significantly improved device characteristics compared to Cl₂-SiPc.