Exciton Delocalization in H2OBPc1–xMOBPcx (M = Co, Cu, Ni, Mn) Crystalline Thin-Film Organic Alloys
journal contributionposted on 20.05.2016, 00:00 by Lane W. Manning, Naveen Rawat, Cody Lamarche, Rory Waterman, Randall L. Headrick, Madalina Furis
Novel solution-processing deposition techniques in tandem with chemical synthesis design of small-molecule soluble derivatives represent a viable avenue for exploring organic analogues of semiconductor alloyed systems, where excitonic properties are tunable through alloy concentration. Here these properties are explored using absorption, grazing incidence X-ray diffraction (GIXRD), and temperature-dependent/time-resolved photoluminescence spectroscopy (TRPL) in a series of crystalline thin film alloys of metal-free (H2OBPc) and metal (MOBPc) octabutoxy-phthalocyanine, H2OBPc1–xMOBPcx (M = Co, Cu, Ni, or Mn) where 0.5 ≥ x ≥ 0.001. Films are fabricated using a solution-processed, novel hollow pen-writing technique that results in millimeter-sized crystalline grains with long-range macroscopic order for all concentrations. The spectroscopy experiments produce two important results that offer great insight into the fundamental quantum mechanics of delocalized excitons in small-molecule semiconductors. First, they indicate that the delocalization of bandgap excitons previously observed in pure H2OBPc films extends over approximately ten molecules, and second they reveal that the presence of the MOBPc molecule inhibits the formation of this delocalized exciton for x > 0.09. Furthermore, the MOBPc molecule introduces a highly localized state with a strong photoluminescence signature.