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

The energy costs associated with separating tightly bound excitons (photoinduced electron-hole pairs) and extracting free charges from highly disordered low-mobility networks represent fundamental losses for many low-cost photovoltaic technologies. We report a low-cost, solution-processable solar cell, based on a highly crystalline perovskite absorber with intense visible to near-infrared absorptivity, that has a power conversion efficiency of 10.9% in a single-junction device under simulated full sunlight. This "meso-superstructured solar cell" exhibits exceptionally few fundamental energy losses; it can generate open-circuit photovoltages of more than 1.1 volts, despite the relatively narrow absorber band gap of 1.55 electron volts. The functionality arises from the use of mesoporous alumina as an inert scaffold that structures the absorber and forces electrons to reside in and be transported through the perovskite.

https://science.sciencemag.org/content/sci/early/2012/10/03/science.1228604.full.pdf

Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites

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

Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI(3-x)Cl(x)) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.

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Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber.

The surface science of titanium dioxide

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

Solid-state dye-sensitized photovoltaic cells have been fabricated with TiO2 as the electron conductor and CuSCN as the hole conductor. The cells show photocurrents of ≈8 mA/cm2, voltages of ∼600 mV, and energy efficiencies of ≈2% at 1 sun. The CuSCN was deposited into the pores of the nanoparticulate TiO2/dye film from dilute solution in propylsulfide. The degree of pore filling achieved is near 100% for TiO2 films <2-μm thick and falls to ≈65% for films near 6 μm. The final drying step after the CuSCN deposition is shown to be critical; drying in vacuum or argon is required for photocurrents above 2 mA/cm2. The photocurrent IVs of these cells are fit to a single diode equation and the results are discussed and compared to those for equivalent photoelectrochemical cells, and similar solid cells composed of ZnO/dye/CuSCN.

A Solid-State Dye-Sensitized Solar Cell Fabricated with Pressure-Treated P25−TiO2 and CuSCN: Analysis of Pore Filling and IV Characteristics

Photocurrent action spectra and current/voltage characteristics are presented for transparent film of p-type CuSCN immersed in ethanol electrolytes containing cyanine dyes. The action spectra show that the photocurrent is caused by hole injection from adsorbed dyes. Of the dyes tested, those with oxidation potentials {>=}0.7 V vs SCE photoinject holes into CuSCN with quantum efficiencies greater than 50%. The highest quantum efficiency measured was {>=}80%. From the threshold oxidation potential for injection and the photocurrent onset potential, the valence band edge of CuSCN in ethanol/TBABr{sub 4} is estimated to lie between 0.55 and 0.75 V vs SCE. The results suggest that CuSCN is a promising candidate material for the hole-conducting portion of dye sensitized heterojunctions for solar photovoltaic applications. 36 refs., 5 figs., 1 tab.

Efficient photo-hole injection from adsorbed cyanine dyes into electrodeposited copper(I) thiocyanate thin films

Wide-band-gap, dye-sensitized, heterojunctions of the composition n-TiO2/Ru-dye/p-CuSCN undergo large increases in efficiency when subjected to low-power UV illumination (<40 mW/cm2) for periods between 10 and 30 min. Ru-dye refers to any of several ruthenium polypyridyl dyes. The UV illumination increases the incident-photon-to-current-efficiency for light absorbed by the dye by a factor of 5−10 and increases the open circuit voltage by 100−300 mV. This beneficial effect is stable for months after the UV illumination is terminated. Investigation of the effects of varying the UV wavelength and the applied bias show that the reaction(s) responsible for the increase in efficiency begin with the oxidation of some interface species by a photoexcited hole in the TiO2 valence band. Treatment of the TiO2/dye/CuSCN cells with oxidized thiocyanate in solution causes a similar increase in the efficiency of dye sensitization. Results from TiO2 and CuSCN photoelectrochemical cells are also presented. The combined res...

Large Enhancement in Photocurrent Efficiency Caused by UV Illumination of the Dye-Sensitized Heterojunction TiO2/RuLL‘NCS/CuSCN: Initiation and Potential Mechanisms

We report the fabrication and performance of a dye‐sensitized p‐n heterojunction formed from a planar interface between two wide‐band‐gap semiconductors, n‐TiO2 and p‐CuSCN, which contains an intervening monolayer of a sulforhodamine B dye. When exposed to visible light, the photoexcited dye molecules transfer electrons to the n‐TiO2 and holes to the p‐CuSCN. The absorbed‐photon‐to‐current efficiency (APCE) is ≳70% and the open circuit voltage is ≊500 mV. This heterojunction is the solid‐state analog of the dye‐sensitized photoelectrochemical interfaces used in photography and photovoltaics. The high quantum efficiency and voltage show that it is possible to simultaneously optimize both the dye/n‐type and dye/p‐type interface for efficient forward charge injection and slow charge combination in a solid‐state device.

Brian O'Regan

Papers

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

A Solid-State Dye-Sensitized Solar Cell Fabricated with Pressure-Treated P25−TiO2 and CuSCN: Analysis of Pore Filling and IV Characteristics

Efficient photo-hole injection from adsorbed cyanine dyes into electrodeposited copper(I) thiocyanate thin films

Large Enhancement in Photocurrent Efficiency Caused by UV Illumination of the Dye-Sensitized Heterojunction TiO2/RuLL‘NCS/CuSCN: Initiation and Potential Mechanisms

Efficient dye‐sensitized charge separation in a wide‐band‐gap p‐n heterojunction