Tony Khoury

Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion

Single-threshold solar cells are fundamentally limited by their ability to harvest only those photons above a certain energy. Harvesting below-threshold photons and re-radiating this energy at a shorter wavelength would thus boost the efficiency of such devices. We report an increase in light harvesting efficiency of a hydrogenated amorphous silicon (a-Si:H) thin-film solar cell due to a rear upconvertor based on sensitized triplet–triplet-annihilation in organic molecules. Low energy light in the range 600–750 nm is converted to 550–600 nm light due to the incoherent photochemical process. A peak efficiency enhancement of (1.0 ± 0.2)% at 720 nm is measured under irradiation equivalent to (48 ± 3) suns (AM1.5). We discuss the pathways to be explored in adapting photochemical UC for application in various single threshold devices.

Real-time single-molecule imaging of oxidation catalysis at a liquid-solid interface

Many chemical reactions are catalysed by metal complexes, and insight into their mechanisms is essential for the design of future catalysts. A variety of conventional spectroscopic techniques are available for the study of reaction mechanisms at the ensemble level, and, only recently, fluorescence microscopy techniques have been applied to monitor single chemical reactions carried out on crystal faces and by enzymes. With scanning tunnelling microscopy (STM) it has become possible to obtain, during chemical reactions, spatial information at the atomic level. The majority of these STM studies have been carried out under ultrahigh vacuum, far removed from conditions encountered in laboratory processes. Here we report the single-molecule imaging of oxidation catalysis by monitoring, with STM, individual manganese porphyrin catalysts, in real time, at a liquid-solid interface. It is found that the oxygen atoms from an O2 molecule are bound to adjacent porphyrin catalysts on the surface before their incorporation into an alkene substrate.

Molecular electrocatalysis for oxygen reduction by cobalt porphyrins adsorbed at liquid/liquid interfaces.

Molecular electrocatalysis for oxygen reduction at a polarized water/1,2-dichloroethane (DCE) interface was studied, involving aqueous protons, ferrocene (Fc) in DCE and amphiphilic cobalt porphyrin catalysts adsorbed at the interface. The catalyst, (2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p-amino-phenylporphyrin) cobalt(II) (CoAP), functions like conventional cobalt porphyrins, activating O(2) via coordination by the formation of a superoxide structure. Furthermore, due to the hydrophilic nature of the aminophenyl group, CoAP has a strong affinity for the water/DCE interface as evidenced by lipophilicity mapping calculations and surface tension measurements, facilitating the protonation of the CoAP-O(2) complex and its reduction by ferrocene. The reaction is electrocatalytic as its rate depends on the applied Galvani potential difference between the two phases.

Dye-Sensitized Solar Cell with Integrated Triplet-Triplet Annihilation Upconversion System.

Photon upconversion (UC) by triplet-triplet annihilation (TTA-UC) is employed in order to enhance the response of solar cells to sub-bandgap light. Here, we present the first report of an integrated photovoltaic device, combining a dye-sensitized solar cell (DSC) and TTA-UC system. The integrated device displays enhanced current under sub-bandgap illumination, resulting in a figure of merit (FoM) under low concentration (3 suns), which is competitive with the best values recorded to date for nonintegrated systems. Thus, we demonstrate both the compatibility of DSC and TTA-UC and a viable method for device integration.

Proton-Coupled Oxygen Reduction at Liquid−Liquid Interfaces Catalyzed by Cobalt Porphine

Cobalt porphine (CoP) dissolved in the organic phase of a biphasic system is used to catalyze O(2) reduction by an electron donor, ferrocene (Fc). Using voltammetry at the interface between two immiscible electrolyte solutions (ITIES), it is possible to drive this catalytic reduction at the interface as a function of the applied potential difference, where aqueous protons and organic electron donors combine to reduce O(2). The current signal observed corresponds to a proton-coupled electron transfer (PCET) reaction, as no current and no reaction can be observed in the absence of either the aqueous acid, CoP, Fc, or O(2).

Oxygen Reduction Catalyzed by a Fluorinated Tetraphenylporphyrin Free Base at Liquid/Liquid Interfaces

The diprotonated form of a fluorinated free base porphyrin, namely 5-(p-aminophenyl)-10,15,20-tris(pentafluorophenyl)porphyrin (H(2)FAP), can catalyze the reduction of oxygen by a weak electron donor, namely ferrocene (Fc). At a water/1,2-dichloroethane interface, the interfacial formation of H(4)FAP(2+) is observed by UV-vis spectroscopy and ion-transfer voltammetry, due to the double protonation of H(2)FAP at the imino nitrogen atoms in the tetrapyrrole ring. H(4)FAP(2+) is shown to bind oxygen, and the complex in the organic phase can easily be reduced by Fc to produce hydrogen peroxide as studied by two-phase reactions with the Galvani potential difference between the two phases being controlled by the partition of a common ion. Spectrophotometric measurements performed in 1,2-dichloroethane solutions clearly evidence that reduction of oxygen by Fc catalyzed by H(4)FAP(2+) only occurs in the presence of the tetrakis(pentafluorophenyl)borate (TB(-)) counteranion in the organic phase. Finally, ab initio computations support the catalytic activation of H(4)FAP(2+) on oxygen.

Photochemical Upconversion Enhanced Solar Cells: Effect of a Back Reflector

Photochemical upconversion is applied to a hydrogenated amorphous silicon solar cell in the presence of a back-scattering layer. A custom-synthesized porphyrin was utilized as the sensitizer species, with rubrene as the emitter. Under a bias of 24 suns, a peak external quantum efficiency (EQE) enhancement of ~2 % was observed at a wavelength of 720 nm. Without the scattering layer, the EQE enhancement was half this value, indicating that the effect of the back-scatterer is to double the efficacy of the upconverting device. The results represent an upconversion figure of merit of 3.5 × 10–4 mA cm–2 sun–2, which is the highest reported to date.

Entropically driven photochemical upconversion.

Conventional photochemical upconversion (UC) through homo-geneous triplet-triplet annihilation (TTA) is subject to several enthalpic losses that limit the UC margin. Here, we address one of these losses: the triplet energy transfer (TET) from the sensitizer to the emitter molecules. Usually, the triplet energy level of the emitter is set below that of the sensitizer. In our system, the triplet energy level of the emitter exceeds that of the sensitizer by ∼600 cm(-1). Choosing suitable concentrations for the sensitizer and emitter molecules, we can exploit entropy as a driving force for the migration of triplet excitation from the sensitizer to the emitter manifolds. Thereby we obtain a new record for the peak-to-peak TTA-UC energy margin of 0.94 eV. A modified Stern-Volmer analysis yields a TET rate constant of 2.0 × 10(7) M(-1) s(-1). Despite being relatively inefficient, the upconverted fluorescence is easily visible to the naked eye with irradiation intensities as low as 2 W cm(-2).

A porphyrin-hexa-peri-hexabenzocoronene-porphyrin triad: synthesis, photophysical properties and performance in a photovoltaic device

Hexa-peri-hexabenzocoronene (HBC) is a discotic polycyclic aromatic hydrocarbon with good charge transport characteristics while the light harvesting property of porphyrins is well-known. In order to take advantage of the properties of both materials, a porphyrin-hexa-peri-hexabenzocoronene-porphyrin triad 6 was synthesised for the first time. In photoluminescence studies, the emission from the higher energy HBC unit was completely quenched by the porphyrin unit indicating efficient energy transfer. This novel hybrid material 6 was tested in bulk heterojunction photovoltaic devices reaching a power conversion efficiency of 1.2%.

Three-Metal Coordination by Novel Bisporphyrin Architectures

The synthesis and characterization of a new type of bisporphyrin system is reported where the two macrocycles are linked in a cofacial arrangement by a substituted carbazole bridge. The three nitrogen atoms of the carbazole bridge in the compounds may complex a metal ion and thus provide a new parameter for varying the physical properties and flexibility of the dyad after formation of a three-metal system. In the present study, four bis-metalloporphyrin complexes were synthesized and examined by electrochemistry and thin-layer spectroelectrochemistry in CH(2)Cl(2) and PhCN. Two of the examined compounds contain Cu(II) or Zn(II) porphyrins and a carbazole linker with a bound Cu(II) ion, giving a three metal system, while the other two examined compounds contained the same porphyrins but with a carbazole bridge which lacks the Cu(II) component. Since carbazoles and Cu(II) ions are both electroactive, redox properties of several unlinked carbazoles with and without bound Cu(II) ions were also examined as to their electrochemical behavior under the same solution conditions as the dyads to better understand the redox reactions which may occur at the carbazole group linking the two porphyrin macrocycles. Several mononuclear porphyrins with structures related to macrocycles in the dyads were also investigated.