THE large-scale use of photovoltaic devices for electricity generation is prohibitively expensive at present: generation from existing commercial devices costs about ten times more than conventional methods1. Here we describe a photovoltaic cell, created from low-to medium-purity materials through low-cost processes, which exhibits a commercially realistic energy-conversion efficiency. The device is based on a 10-µm-thick, optically transparent film of titanium dioxide particles a few nanometres in size, coated with a monolayer of a charge-transfer dye to sensitize the film for light harvesting. Because of the high surface area of the semiconductor film and the ideal spectral characteristics of the dye, the device harvests a high proportion of the incident solar energy flux (46%) and shows exceptionally high efficiencies for the conversion of incident photons to electrical current (more than 80%). The overall light-to-electric energy conversion yield is 7.1-7.9% in simulated solar light and 12% in diffuse daylight. The large current densities (greater than 12 mA cm-2) and exceptional stability (sustaining at least five million turnovers without decomposition), as well as the low cost, make practical applications feasible.

A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films

Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

TiO2 photocatalysis and related surface phenomena

The iodide/triiodide redox shuttle has limited the efficiencies accessible in dye-sensitized solar cells. Here, we report mesoscopic solar cells that incorporate a Co(II/III)tris(bipyridyl)–based redox electrolyte in conjunction with a custom synthesized donor-π-bridge-acceptor zinc porphyrin dye as sensitizer (designated YD2-o-C8). The specific molecular design of YD2-o-C8 greatly retards the rate of interfacial back electron transfer from the conduction band of the nanocrystalline titanium dioxide film to the oxidized cobalt mediator, which enables attainment of strikingly high photovoltages approaching 1 volt. Because the YD2-o-C8 porphyrin harvests sunlight across the visible spectrum, large photocurrents are generated. Cosensitization of YD2-o-C8 with another organic dye further enhances the performance of the device, leading to a measured power conversion efficiency of 12.3% under simulated air mass 1.5 global sunlight.

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Porphyrin-Sensitized Solar Cells with Cobalt (II/III)–Based Redox Electrolyte Exceed 12 Percent Efficiency

Understanding the mechanism of ionic transport in organic–inorganic halide perovskites is crucial for the design of future solar cells. Here, Eames et al. undertake a combined experimental and computational study to elucidate the ion conducting species and help rationalize the unusual behaviour observed in these perovskite-based devices.

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Ionic transport in hybrid lead iodide perovskite solar cells

Transparent titanium dioxide membranes (thickness 2.7 {mu}m) were prepared by sintering of 8-nm colloidal anatase particles on a conducting glass support. The dynamics of charge recombination following electron injection from the excited state of RuL{sub 3} (L = 2,2{prime}-bipyridine-4,4{prime}-dicarboxylic acid) into the conduction band of the semiconductor were examined under potentiostatic control of the electric field within the space charge layer of the membrane. Biasing the Fermi level of the TiO{sub 2} positive of the flat-band potential sharply reduced the recombination rate, a 1,000-fold decrease being associated with a potential change of only 300 mV. Photoelectrochemical experiments performed with the same RuL{sub 3}-loaded membrane in NaI-containing water show the onset of anodic photocurrent to occur in the same potential domain. Forward biasing of the membrane potential impairs photosensitized charge injection turning on the photoluminescence of the adsorbed sensitizer.

Vectorial electron injection into transparent semiconductor membranes and electric field effects on the dynamics of light-induced charge separation

Ion migration has been proposed as a possible cause of photovoltaic current–voltage hysteresis in hybrid perovskite solar cells. A major objection to this hypothesis is that hysteresis can be reduced by changing the interfacial contact materials; however, this is unlikely to significantly influence the behaviour of mobile ionic charge within the perovskite phase. Here, we show that the primary effects of ion migration can be observed regardless of whether the contacts were changed to give devices with or without significant hysteresis. Transient optoelectronic measurements combined with device simulations indicate that electric-field screening, consistent with ion migration, is similar in both high and low hysteresis CH3NH3PbI3 cells. Simulation of the photovoltage and photocurrent transients shows that hysteresis requires the combination of both mobile ionic charge and recombination near the perovskite-contact interfaces. Passivating contact recombination results in higher photogenerated charge concentrations at forward bias which screen the ionic charge, reducing hysteresis. Ion migration has been related to hysteresis in perovskite solar cells, but not all perovskite cells exhibit a hysteresis. Here, Caladoet al. show that ion migration occurs regardless of hysteresis, but photogenerated carriers screen the effects of ionic charge for some solar cell architectures.

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Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis

In this review, we address the materials design parameters that control the processes of charge separation, and thereby device efficiency, in dye-sensitized photoelectrochemical solar cells. The review starts with an overview of the structure, energetics and kinetics of dye-sensitized solar cells. It then goes on to consider in more detail the parameters determining the efficiency of the two primary charge separation steps in these devices: electron injection from the dye excited state into the metal oxide electrode, and regeneration of the dye ground state by the redox electrolyte. We consider the kinetic competition between these desired charge separation steps and the undesired loss pathways of excited state decay to ground and electron recombination with dye cations. The review avoids detailed mathematical and spectroscopic discussion, but rather tries to summarize the key conclusions relevant to materials design. A recurring theme of the review is the energy cost of achieving charge separation, and h...

Electron Transfer Dynamics in Dye-Sensitized Solar Cells

In this paper we focus upon the electron injection dynamics in complete nanocrystalline titanium dioxide dye-sensitized solar cells (DSSCs) employing the ruthenium bipyridyl sensitizer dye N719. Electron injection dynamics and quantum yields are studied by time-resolved single photon counting, and the results are correlated with device performance. In typical DSSC devices, electron injection kinetics were found to proceed from the N719 triplet state with a half-time of 200 ± 60 ps and quantum yield of 84 ± 5%. We find that these injection dynamics are independent of presence of iodide/triiodide redox couple and of the pH of the peptization step used in the synthesis of the TiO2 nanoparticles. They are furthermore found to be only weakly dependent upon the application of electrical bias to the device. In contrast, we find these dynamics to be strongly dependent upon the concentration of tert-butylpyridine (tBP) and lithium cations in the electrolyte. This dependence is correlated with shifts of the TiO2 co...

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Brian C. O’Regan

Papers

Ionic transport in hybrid lead iodide perovskite solar cells

Vectorial electron injection into transparent semiconductor membranes and electric field effects on the dynamics of light-induced charge separation

Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis

Electron Transfer Dynamics in Dye-Sensitized Solar Cells

Parameters influencing the efficiency of electron injection in dye-sensitized solar cells.