Carl C. Wamser

Porphyrins and phthalocyanines in solar photovoltaic cells

This review summarizes recent advances in the use of porphyrins, phthalocyanines, and related compounds as components of solar cells, including organic molecular solar cells, polymer cells, and dye-sensitized solar cells. The recent report of a porphyrin dye that achieves 11% power conversion efficiency in a dye-sensitized solar cell indicates that these classes of compounds can be as efficient as the more commonly used ruthenium bipyridyl derivatives.

Reaction of dichloromethane with pyridine derivatives under ambient conditions.

Pyridine derivatives and dichloromethane (DCM) are commonly used together in a variety of different applications. However, DCM slowly reacts with pyridine and a variety of other representative pyridine derivatives to form methylenebispyridinium dichloride compounds under ambient conditions. The proposed mechanism (two consecutive S(N)2 reactions) was studied by evaluating the kinetics of the reaction between 4-(dimethylamino)pyridine and DCM. The second-order rate constants for the first (k(1)) and second (k(2)) substitutions were found to be 2.56(+/-0.06) x 10(-8) and 4.29(+/-0.01) x 10(-4) M(-1) s(-1), respectively. Because the second substitution is so much faster than the first, the monosubstitution product could not be isolated or detected during the reaction; it was synthesized independently in order to observe its kinetics.

Solar cells with porphyrin sensitization

Porphyrins have been studied as photosensitizers in two different types of solar cells. In one type of cell, a p-n junction was created with the system ITO/5,10,15,20-tetra(o-nitrophenyl)porphyrinatozinc/Alq 3 /Al. In another type of cell, porphyrins with carboxyphenyl and aminophenyl substituents were used as photosensitizers for a nanoparticulate TiO 2 (Gratzel) cell in which photoelectropolymerization of aniline created polyaniline as a hole transport medium. Optimum results from such cells gave an open-circuit voltage (V oc ) of 0.6 V, a short-circuit current (J sc ) of 0.2 mA/cm 2 , a fill factor of 0.7, and a conversion efficiency of 2.4% under 4.4 mW/cm 2 of simulated solar intensity. Such cells offer a promising new approach to a solid-state Gratzel cell.

Steric effects in singlet and triplet electronic energy transfer to azo compounds

Rate constants have been determined for the solution-phase quenching of singlet and triplet excited states of a vari- ety of sensitizers (aromatic hydrocarbons, aryl and alkyl ketones, a-diketones) by a series of azo compounds with varying steric properties. Both singlet and triplet quenching are attributed to electronic energy transfer by the collisional, electron- exchange mechanism. Steric hindrance to exothermic triplet energy transfer is significant, with azo-n-butane a better accep- tor than azo-tert-butane by a factor of 3.6-10.7. Steric hindrance to singlet energy transfer is less pronounced (comparable steric factors are 1.7-2.9), apparently because diffusion rather than energy transfer is rate limiting in solution. For a given donor-acceptor pair, singlet energy transfer is two to three times faster than triplet energy transfer. The triplet energy level of the azobutanes is estimated to be 53 f 1 kcal/mol.

Gold Tris(carboxyphenyl)corroles as Multifunctional Materials: Room Temperature Near-IR Phosphorescence and Applications to Photodynamic Therapy and Dye-Sensitized Solar Cells

Two amphiphilic corroles-5,10,15-tris(3-carboxyphenyl)corrole (H3[mTCPC]) and 5,10,15-tris(4-carboxyphenyl)corrole (H3[pTCPC])-and their gold complexes have been synthesized, and their photophysical properties and photovoltaic behavior have been investigated. Like other nonpolar gold corroles, Au[mTCPC] and Au[pTCPC] were both found to exhibit room temperature phosphorescence in deoxygenated solutions with quantum yields of ∼0.3% and triplet lifetimes of ∼75 μs. Both compounds exhibited significant activity as dyes in photodynamic therapy experiments and in dye-sensitized solar cells. Upon irradiation at 435 nm, both Au corroles exhibited significant phototoxicity against AY27 rat bladder cancer cells while the free-base corroles proved inactive. Dye-sensitized solar cells constructed using the free bases H3[mTCPC] and H3[pTCPC] exhibited low efficiencies (≪1%), well under that obtained with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, H2[pTCPP] (1.9%, cf. N719 9.5%). Likewise, Au[pTCPC] proved inefficient, with an efficiency of ∼0.2%. By contrast, Au[mTCPC] proved remarkably effective, exhibiting an open-circuit voltage (Voc) of 0.56 V, a short-circuit current of 8.7 mA cm(-2), a fill factor of 0.72, and an efficiency of 3.5%.

Spectroscopy of protonated tetraphenylporphyrins with amino/carbomethoxy substituents: hyperporphyrin effects and evidence for a monoprotonated porphyrin.

Spectrophotometric titrations for a full series of para-amino/carbomethoxy-substituted tetraphenylporphyrins were carried out using methanesulfonic acid in DMSO to study the hyperporphyrin effect across different substitution patterns. The series included zero, one, two (cis and trans), three, and four amino groups, with the remaining para substituents carbomethoxy groups. With increasing numbers of aminophenyl groups, the relative basicity increased and the hyperporphyrin effect increased, marked by a strong red band and a split Soret band. The cis diamino derivative showed a stronger hyperporphyrin effect compared to the trans isomer, which can be explained based on simple resonance forms. For the monoamino derivative, an initial increase in the Soret band upon acid titration along with well-defined isosbestic points provided evidence for a monoprotonated porphyrin, distinct from the diprotonated and triprotonated states. The relative stability of this unusual intermediate is proposed to be due to charge delocalization of the first cation to the single amino group and destabilization of the second protonation by the electron-withdrawing carbomethoxy substituents.

Syntheses and optoelectronic properties of amino/carboxyphenylporphyrins for potential use in dye-sensitized TiO2 solar cells

New mixed-substituent amino/carboxyphenylporphyrins for a dye-sensitized TiO2 solar cell were prepared using several synthetic routes. The reaction of 4-carbomethoxy- and 4-acetamidobenzaldehydes with pyrrole in propionic acid under aerobic conditions afforded mixtures of mixed amide/ester substituted tetraphenylporphyrins which were separated using centrifugal chromatography then deprotected to give the target compounds. Condensation of p-nitrophenyl- dipyrromethane with 4-carbomethoxybenzaldehyde in CH2Cl2 catalyzed by trifluoroacetic acid, followed by oxidation with dichlorodicyanoquinone gives trans-dicarbomethoxy/ dinitrophenylporphyrin, which when treated with SnCl2 and HCl affords the trans-diamino/dicarboxy derivative, trans-TA2C2PP. Commercially available tetrakis-5,10,15,20-(4-carboxyphenyl)porphyrin (TCPP) was converted to mixtures of mixed amino/carbomethoxyphenylporphyrins using hydroxylamine hydrochloride in polyphosphoric acid with methanol workup. Relative yields and product distributions from each route are discussed and the optoelectronic characteristics of the synthesized porphyrins were studied using UV-visible spectroscopy and cyclic voltammetry. Copyright

Water Cooling Method to Improve the Performance of Field-Mounted, Insulated, and Concentrating Photovoltaic Modules

The installation rate of crystalline silicon photovoltaic (PV) modules worldwide is at an all-time high and is projected to continue to grow as the cost of PV technology is reduced. It is important to note that PV power generation is heavily influenced by the local climate. In particular, for crystalline silicon-based PV devices, as the operating temperature of the panel increases, the efficiency decreases. Higher operating temperatures also lead to accelerated material and mechanical degradation, potentially compromising system effectiveness over the lifetime of the panels. In addition, atmospheric pollution can cause particle deposition on the surface of PV modules (soiling), reducing the amount of solar irradiance that reaches the PV material and reducing panel efficiency. Various cooling and cleaning methods have been proposed in the literature to mitigate these problems. In this study, a uniform film of water was continuously recirculated by pumping over the surface of a solar panel using an emitter head attached to the top of the panel. The water cooling technique was able to maintain panel temperature below 40 °C while adjacent untreated panels were operating near 55 °C. Besides the efficiency improvements due to cooling, the film of water also kept the panels clean, avoiding any reduced power output caused by panel soiling. Additional studies were carried out with artificially chilled cooling fluid, insulating materials, and side mirrors to examine the cooling system performance under different installation scenarios. Water cooling is concluded to be an effective means of increasing the efficiency of monocrystalline silicon photovoltaic panels. Under normal operating conditions, the increased energy output from the panels is more than sufficient to compensate for the energy required to pump the water.

Hyperporphyrin Effects in the Spectroscopy of Protonated Porphyrins with 4-Aminophenyl and 4-Pyridyl Meso Substituents

Spectrophotometric titrations for a full series of 4-aminophenyl/4-pyridyl meso-substituted porphyrins were carried out using methanesulfonic acid in DMSO to study the hyperporphyrin effect across different substitution patterns. The series included zero, one, two (cis and trans), three, and four meso(4-aminophenyl) groups, with the remaining meso substituents being 4-pyridyl groups. The peripheral pyridyl groups consistently protonate before the interior porphyrin pyrrole nitrogens, which protonate before the aminophenyl groups. Aminophenyl substituents increase the basicity of the pyrrole nitrogens and lead to distinctive hyperporphyrin spectra with a broad Soret band and a strong red absorption. The structure proposed to give rise to these spectra is the previously proposed charge-transfer interaction between the aminophenyl and the protonated pyrrole. A novel hyperporphyrin structure involving charge-transfer interactions between two peripheral substituents is also proposed in one case, the triply protonated (+3) porphyrin with three aminophenyls and one pyridyl substituent; two of the aminophenyl groups delocalize the charges on the interior nitrogens, while the third aminophenyl group delocalizes with the protonated pyridyl.

Asymmetric Polyporphyrin Films by Interfacial Polymerization

Abstract Interfacial polymerization of porphyrins derivatized with appropriate reactive functional groups yields thin polymeric films consisting of a network of linked porphyrins. A typical example is the reaction of tetra(p–chlorocarbonylphenyl)porphyrin (TCCPP) in chloroform with tetra(p–hydroxyphenyl)porphyrin (THPP) in aqueous base. The films display a unique chemical asymmetry, in the sense that opposite surfaces of the films show distinctive differences in the concentration and type of unreacted functional groups that are present. When placed between semitransparent electrodes and irradiated with either steady–state broad–band light or a pulsed laser, these films develop directional photopotentials (up to 25 mV), whereby the film surface that was prepared in contact with the TCCPP solution (the acid surface) develops a more negative potential than the opposite surface. We consider the directional photopotentials to be a manifestation of the chemical asymmetry of these interfacial films – that is, ph...