HIGHLY PHOTOACTIVE MOLECULAR SEMICONDUCTORS - DETERMINATION OF THE ESSENTIAL PARAMETERS THAT LEAD TO AN IMPROVED PHOTOACTIVITY FOR MODIFIEDCHLOROALUMINUM PHTHALOCYANINE THIN-FILMS

Thin films of chloroaluminum, chlorogallium, and chloroindium phthaloc yanines (ClAlPc, ClGaPc, and ClInPc) have been sublimed on SnO2 substr ates maintained during sublimation at temperatures ranging from -130 t o 190 degrees C. Using this procedure, it is possible to obtain molecu lar semiconductor layers with a structure varying from amorphous to po lycrystalline. These layers were immersed in KI3/KI or KCl solutions a t pH = 3.0. This treatment was found to improve drastically the photoe lectrochemical activity of ClAlPc thin films. Short-circuit photocurre nts J(sc) = 0.75 +/- 0.25 mA/cm(2) were obtained, using polychromatic illumination (35 mW/ cm(2)), after immersion of ClAlPc into KCl soluti ons while lower J(sc) values (0.3 +/- 0.1 mA/cm(2)) were obtained for KI3/KI solutions. No change in the photoactivity was observed either f or ClGaPc or for ClInPc when they were immersed in the same solutions. Both molecular semiconductors provided lower short-circuit photocurre nts (J(sc) less than or equal to 0.15 +/- 0.03 mA/cm(2) for ClGaPc; J( sc) less than or equal to 0.20 +/- 0.02 mA/cm(2) for ClInPc). The char acterization of the chloro-trivalent metal phthalocyanine films indica tes that the hydrolysis of the metal-Cl bond is essential for the occu rrence of the physicochemical transformation leading to improved photo activity. The Al-Cl bond of ClAlPc hydrolyzes, but this reaction does not occur for ClGaPc or for ClInPc. In contact with KI3/KI or KCl solu tions at pH = 3.0, bulk hydrolysis occurs for ClAlPc, only if both H3O + and an anion could diffuse from the solution into the material. The large I-3(-) anion is prevented from doing so for polycrystalline ClAl Pc films obtained by sublimation on SnO2 substrates maintained at 180 degrees C. However, it can diffuse easily in more disorganized films o btained at lower substrate temperatures. Powders of the chloro-trivale nt metal phthalocyanines as well as bromoaluminum phthalocyanine (BrAl Pc) were used to quantify anion incorporation in these materials. Afte r complete hydrolysis of BrAlPc (powder) and ClAlPc (films) there are ca. 50-85% of the anions, generated in situ by the hydrolysis reaction or diffusing from the solution as a consequence of the hydrolysis rea ction, that remain in the Pc material. Thus, ca. 50-85% of the protons released by the hydrolysis either protonate the macrocycles or react with Pc(+)O(2)(-) already present in the film. In both cases, anions a re necessary to neutralize the excess of positive charges. H2O is also found in the modified films. The presence of protonated Pcs, of anion s, and of H2O into what is now HOAlPc (after ClAlPc hydrolysis) modifi es the structure of the material as well as its photoactivity.