Molecular Architecture and Electrical Properties in Evaporated Films of Cobalt Phthalocyanine

Abstract

Thin films of cobalt phthalocyanine (CoPc) were deposited onto solid substrates through physical vapor deposition (PVD) by thermal evaporation up to 60 nm thick to determine their molecular architecture and electrical properties. The growth was monitored using UV-Vis absorption spectroscopy, revealing a linear increase for absorbance versus thickness. PVD films were found in the crystalline alpha phase and with the CoPc molecules forming ca. 45 degrees in relation to the substrate surface. The film surface was fairly homogeneous at the micro and nanoscales, with the roughness at ca. 3 nm. DC and AC electrical measurements were carried out for devices built with distinct structures. Perpendicular contact was established by depositing 60 nm CoPc PVD films between indium tin oxide (ITO) and Al, forming a sandwich-type structure (ITO/CoPc/Al). The current versus DC voltage curve indicated a Schottky diode behavior with a rectification factor of 4.2. The AC conductivity at low frequencies increased about 2 orders of magnitude (10(-9) to 10(-7) S/m) with increasing DC bias (0 to 5 V) and the dielectric constant at 1 kHz was 3.45. The parallel contact was obtained by depositing 120 nm CoPc PVD film onto interdigitated electrodes, forming an IDE-structured device. The latter presented a DC conductivity of 5.5 x 10(-10) S/m while the AC conductivity varied from 10(-9) to 10(-1) S/m between 1 Hz and 1 MHz, respectively, presenting no dependence on DC bias. As proof-of-principle, the IDE-structured device was applied as gas sensor for trifluoroacetic acid (TFA).

Supramolecular Organization-Electrical Properties Relation in Nanometric Organic Films

“…This change in conductivity is probably due to the reduction of the interfacial barriers 56 (organic semiconductor/Al) and subsequent increase of the charge carrier injection in the device. In a previous work, we have demonstrated a σ dc increase about 2 orders of magnitude for another organic macromolecule, the thin film of cobalt phtalocyanine, for DC bias from 0 to 5 V. 57 For the IDE device, the increase of the current as a function of the DC voltage (I vs V) is linear, as seen in Figure 7a, 58 The film resistance (R) can be obtained from this plot, calculated directly from y = a + bx equation, where the slope b is 1/R. Using a model from Olthuis et al, 59 it is possible to obtain the conductivity of the device from its resistance.…”

Metallic Phthalocyanines: impact of the film deposition method on its supramolecular arrangement and sensor performance

“…The molecular organization can be determined considering the FTIR spectra combined with the selection rules (Debe 1987), which has been widely applied to determination of molecular organization of MPc films (Alessio et al 2012, Furini et al 2013, Martin et al 2018, Rubira et al 2017. In short, this method is based on the scalar product between radiation intensity (I) and electric field (E) as , being µ the dipole moment (Debe 1987).…”

Supramolecular arrangements of an organometallic forming nanostructured films

“…Specific molecular orientation can be determined by combining FTOR data and surface selection rules 41,42 . Slightly differences are seen when the transmission and reflection spectra of both LB and LS ZnPPOX-DME films are compared.…”