Photovoltaic effect in symmetrical cells of a liquid crystal porphyrin

Abstract

the organizing element which promoted complex formation was the polymer, to which pyrochlorophyll was bound by ligation and DTNB by counterion attraction. In the present case, the organizing element is evidently the cationic micelle.Chl and its relatives have been examined in almost every conceivable kind of model system in efforts to imitate photosynthesis. To our recollection, however, this is the first report of its presence in cationic inverted micelles. Chl has been reported in anionic inverted micelles of aerosol OTIS though these may have been more in the nature of microemulsions, with a clearly defined internal aqueous phase. Inverted micelles have been promoted as models of the interiors of proteins or lipoproteins, where numerous biological reactions, including the primary processes of (18) Pileni, M. P.; Lerebours, B.; Brochette, P.; Chevalier, Y. J . Photochem. An unusual photovoltaic effect has been observed in symmetrical cells consisting of indium-tin oxide (ITO) electrodes and a liquid crystal porphyrin (LCP, zinc octakis(8-octyloxyethy1)porphyrin). The illuminated electrode acts as a photoanode; the direction of current flow reverses upon reversal of the direction of illumination. Stable photocurrents of up to ca. 0.4 mA/cm2 have been measured under illumination with a 150-W Xe lamp (intensity ca. 150 mW/cm2). The photocurrentincreases linearly with incident light intensity (I,) at all wavelengths up to I, > 1OIs photons s-I cm-2. This photovoltaic effect is interpreted as resulting from exciton dissociation at the illuminated electrode leading to the preferential photoinjection of electrons into the IT0 electrode and holes into the porphyrin. This appears to be the first unambiguous example of a photovoltaic cell controlled entirely by interfacial kinetics. The predominance of the photoinjection process in these capillary-filled liquid crystal (solid-phase) films, and its relative absence in similar cells containing evaporated films of porphyrins and phthalocyanines, is attributed to the single-crystal-like character of the LCP films. The wavelength dependence of the photocurrent (action spectrum) is a function of the cell thickness, correlating with the absorption spectrum for thin cells, while it is inversely correlated for thicker cells. This is attributed to an effective cell resistance that is determined by the number and spatial distribution of carriers photogenerated in the bulk LCP. An approximate mathematical model of these two processes, photoinjection at the interface coupled with photoconductivity in the bulk LCP, is proposed which reproduces the experimental results. (20) (a) Chandrasekhar, S. Mol. Cryst. Liq. Cryst. 1981, 63, 171. (b) Destrade, C.; Tinh, N. H.; Gasparoux, H.; Malthete, J.; Levelut, A. M. Mol. Tran-Thi, T.-H.; Briois, V.; Simon, J.; Ohta, K.

Photovoltaic effects in porphyrin polymer films and heterojunctions

“…We therefore assume that the overall conductivity is determined by the integral of the local resistivity ͑determined by the local photocarrier concentrations͒, rather than averaging the total amount of formed photocarriers over the total film thickness as is the case for high mobilities. Analogue to Gregg, Fox, and Bard, 30 we deduce for the conductivity…”

“…The explanation is thus found in the very low mobility of carriers in organic materials. 17,18,30 A value as low as 8 ϫ10 Ϫ9 cm 2 /V s for the effective mobility is inferred by Savenije and co-workers 24 from measurements of the diffusion coefficient of carriers in the porphyrin polymer films.…”

A Life in Electrochemistry

“…There was a belief that a single homogeneous film without any asymmetry would not show photogenerated electron-hole separation. In fact, it did, because of differential rates at the interfaces, but the efficiency was not good (47). We later showed that if one applied an electrical field to the film during the cooling stage, the photocurrent was much improved, at least initially (48).…”

Liquid crystals in photovoltaics: a new generation of organic photovoltaics

“…Gregg et al 74 and Petritsch et al 75 were the first ones to demonstrate the PV effect in porphyrin-and Pc-based DLCs. However, the seminal work of Schmidt-Mende et al 6 on well-defined discotic PV solar cell prepared from a hexa-perihexabenzocoronene (HBC)-based DLC, reported in 2001, generated tremendous interest in this field.…”