Photovoltage Polarity Inversion (PVPI) In Phthalocyanine-based Semiconductor Composites

Recently, high-performance photovoltaic devices, which constructed by composites with different structures and functions, have become the focus in the area of organic photovoltaic materials. The fundamental researches into the mechanisms underlying the photo-electronic processes observed in organic composites are potential to result in the discovery of new phenomena and the origination of new concepts. The photovoltage polarity inversion (PVPI) is a representative example, which was found by our group in elucidation the physical origin of photo/electronic interaction in the organic semiconducting composite materials by the surface photovoltage spectroscopy (SPS). A systematic study on PVPI will contribute unique evidences and experiences to theoretical progress and practical innovation in the field of organic semiconductor materials and organic photovoltaic devices.Based on the PVPI found in the titanyl phthalocyanine (TiOPc) / 4,4 -2-{N-[p-hydroxyl-y-(p-methoxylanilino)naphthyl] }azodiphenyl (DCRL) composites, we designed and synthesized4,4 -2-{N-[p-hydroxyl-y-(m-nitroanilino)naphthyl] Jazodiphenyl (DCBS) and 4,4-2-{N-[p-hydroxyl-y-naphthyl]}azofluorinone (ASRL), then they were blended with TiOPc and their photovoltage properties were investigated by SPS and electric field-induced SPS (FISPS). The results showed that higher intensity PVPI in wider component range was found in the composites composed with DCRL and TiOPc than composites of DCBS and TiOPc. When the ASRL with more pronounced acceptor group of fluorinone were blended with TiOPc, we also observed higher intensity PVPI in wider component range of these composites. These data indicated that PVPI in TiOPc/AZO was associated with the photo-induced charge transfer process in these composites. At the same time, the facts that all these PVPI were recorded in the near-IR region suggested the correlation between the PVPI and the local states transition. FISPS data revealed that the photoinduced charge transfer between donor and acceptor molecules were responsible to the formation of the localized states.Based on the above results, the typical n-type organic semiconductors of3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and its derivatives were used to fabricate a series of composites with TiOPc, respectively. Comparing with TiOPc/AZOs composites, the higher intensify PVPI was found in the UV-vis region and also in the near-IR in the TiOPc/PTCDA and TiOPc/N, N -(4-methyl-phenyl)-3,4,9,10-tetracarboxylicperylenediimide(ToPTCDI) composites. The PVPI in the UV-vis region maybe due to the energy transfer from perylene to TiOPc chromophores, and that in near-IR could be ascribed to the photo-induced charge transfer between the two composites. When the 4-methyl phenyl was introduced to the perylene backbone, the energy transfer would be increased, therefore the higher intensity PVPI was observed in the TiOPc/ToPTCDI composites. On the other hand, the introducing of methyl phenyl is favorable for the charge transfer, so the PVPI in the near-IR of the TiOPc/ToPTCDI composites was weaker than the TiOPc/PTCDA composites in the same region. For the N, N-(4-trifluoromethyl-2,3,5,6-pentafluorophenyl)-3,4,9,10-tetracarboxylicperylenedii mide(F-ToPTCDI), when it was blended with TiOPc, the space charge regions and the built-in field of the TiOPc/F-ToPTCDI composites were dramatically reduced because the hydrophobic and oilyphobic character of the material. Furthermore, they could not form uniform composites. Hence, the PVPI could not be observed both in the UV-vis and near-IR regions. These evidences indicated that the PVPI phenomena correlated not only to characteristics of the p-type and n-type of the components, but also to the space charge region and the built-in field formed in the composites.Considering the complementary of organic and inorganic semiconductor, n-type compound semiconductors such as TiO2 and CdS were introduced into the composites. The nano-scale mate...