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

Following pioneering work, solution-processable organic-inorganic hybrid perovskites-such as CH3NH3PbX3 (X = Cl, Br, I)-have attracted attention as light-harvesting materials for mesoscopic solar cells. So far, the perovskite pigment has been deposited in a single step onto mesoporous metal oxide films using a mixture of PbX2 and CH3NH3X in a common solvent. However, the uncontrolled precipitation of the perovskite produces large morphological variations, resulting in a wide spread of photovoltaic performance in the resulting devices, which hampers the prospects for practical applications. Here we describe a sequential deposition method for the formation of the perovskite pigment within the porous metal oxide film. PbI2 is first introduced from solution into a nanoporous titanium dioxide film and subsequently transformed into the perovskite by exposing it to a solution of CH3NH3I. We find that the conversion occurs within the nanoporous host as soon as the two components come into contact, permitting much better control over the perovskite morphology than is possible with the previously employed route. Using this technique for the fabrication of solid-state mesoscopic solar cells greatly increases the reproducibility of their performance and allows us to achieve a power conversion efficiency of approximately 15 per cent (measured under standard AM1.5G test conditions on solar zenith angle, solar light intensity and cell temperature). This two-step method should provide new opportunities for the fabrication of solution-processed photovoltaic cells with unprecedented power conversion efficiencies and high stability equal to or even greater than those of today's best thin-film photovoltaic devices.

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Sequential deposition as a route to high-performance perovskite-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.

Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based first-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.

Efficient planar heterojunction perovskite solar cells by vapour deposition

Two organolead halide perovskite nanocrystals, CH3NH3PbBr3 and CH3NH3PbI3, were found to efficiently sensitize TiO2 for visible-light conversion in photoelectrochemical cells. When self-assembled on mesoporous TiO2 films, the nanocrystalline perovskites exhibit strong band-gap absorptions as semiconductors. The CH3NH3PbI3-based photocell with spectral sensitivity of up to 800 nm yielded a solar energy conversion efficiency of 3.8%. The CH3NH3PbBr3-based cell showed a high photovoltage of 0.96 V with an external quantum conversion efficiency of 65%.

Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells

Nanocrystalline lead bromide perovskite was synthesized by rapid self-organization on a mesoporous aluminum oxide film. With UV excitation, the film exhibited an intense green emission with a narro...

Highly Luminescent Lead Bromide Perovskite Nanoparticles Synthesized with Porous Alumina Media

PROBLEM TO BE SOLVED: To provide a solid-junction solar cell as an alternative to a dye-sensitized solar cell inferior in durability and an organic thin-film solar cell inferior in light resistance and moisture resistance.SOLUTION: A solid-junction photoelectric conversion element has a photoelectric conversion layer between a pair of transparent electrode substrates each comprising a transparent substrate and a transparent electrode layer formed on the transparent substrate. In the photoelectric conversion layer, a first semiconductor layer comprising a carbon nanotube layer on which an organic-inorganic hybrid perovskite compound A is deposited or adsorbed and a second semiconductor layer comprising a carbon nanotube layer on which an inorganic perovskite compound B and/or an organic-inorganic hybrid perovskite compound C is deposited or adsorbed are joined onto the electrode substrates.

Photoelectric conversion element using perovskite compound and method of manufacturing the same

Lead halide-based perovskite nanocrystals (quantum dot sizes) as visible light sensitizers achieved energy conversion efficiency of 3.8% on a mesoporous TiO2 photoelectrochemical cell. Lead halide (bromide and iodide)-based organic-inorganic perovskite nanocrystals (Fig. 1) were prepared on a surface of mesoporous TiO2 by spin-coating precursor liquid solutions of lead halide (for lead bromide type: a mixture of CH3NH3Br and PbBr2 in N,N-dimethylformamide) and subsequent selforganized crystallization. Formation of the perovskite was observed by development of a vivid yellow color (for lead bromide) from the colorless precursor solutions. X-ray diffraction analysis showed a perovskite form for CH3NH3PbBr3 with diffraction peaks at 14.77°, 20.97°, 29.95°, 42.9°, and 45.74°, assigned as (100), (110), (200), (220) and (300) planes, respectively, of a cubic perovskite structure with a lattice constant of 5.9 A. For the cell making, perovskite nanocrystals were coated on a TiO2 mesoporous film that was prepared by sintering of a nanocrystalline TiO2 paste on a glass substrate. A sandwich-type photoelectron-chemical cells were assembled with a platinum-coated counterelectrode by insertion of a bromide or iodide-containing organic electrolyte. Cell performance was measured by exposure to AM1.5 100 mW/cm sunlight with a solar simulator (Peccell Technologies, Inc). Measurements of incident-photon-to-current conversion efficiency (IPCE) exhibited action spectra covering visible wavelength regions up to 800 nm for the lead iodide perovskite (Fig. 2). The lead bromide perobskite-sensitized cell exhibits an IPCE response up to 580 nm and gives a high open-circuit voltage exceeding 0.95V. With a maximum of 65%, a plateau of IPCE indicates that incident photons are strongly absorbed by the thin film (8 μm) of perovskite, which is generally difficult with conventional organic sensitizers. In the electrolyte, a redox couple of halogen/halide was selected so that it is same as corresponding halogen in the perovskite composition. Work function analysis on photoelectron spectroscopy for the spin-coated polycrystalline films showed the valence band levels of the perovskite bromide and iodide being around 5.38 eV and 5.44 eV vs. vacuum level, respectively. Electrochemically, these valence band levels are considered to be more positive than oxidation potentials of the corresponding halide in electrolyte, which, depending on halide concentration, are 5.1-5.6 eV for Br2/Br, 4.5-5.0 eV for I2/I. Photocurrent-voltage (I-V) performance shows a conversion efficiency of 3.8% with the lead iodide perovskite sensitizer, which is a significantly high level compared to those so far obtained for photocells using non-organic sensitizers and quantum dots. The high Voc, 0.96V, obtained with the perovskite bromide is assumed to due to the bromide employed as a redox partner to couple with perovskite bromide; the electrochemically more positive potential of bromide compared to that of iodide expands the range of photovoltage. With Ru complex sensitizers and TiO2, the maximal Voc ever reported is in the range of 0.86–0.93 V. The inorganic sensitizer as studied in this experiment is promising in exploring the method to earn a high voltage in the TiO2 sensitization technology. Different organic-inorganic perovskite materials exhibit different energy gaps depending on the combination of metal and halogen. A series of organic-inorganic perovskite materials CH3NH3MX3 (M: Pb, Sn; X: halogen) exhibiting different energy gaps becomes also targets for optimizing the cell performance.

http://ma.ecsdl.org/content/MA2009-02/9/742.full.pdf

Lead Halide Perovskites as Quantum Dot Sensitizers for Mesoscopic TiO2 Photovoltaic Cells

A tin or niobium oxide layer (2) is formed on one surface of a layer (1) containing carbon nanotubes, the average diameter (Av) and the standard deviation (σ) of the diameters of which satisfy the relational expression 0.60 > 3σ/Av > 0.20.

Akihiro Kojima

Papers

Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells

Highly Luminescent Lead Bromide Perovskite Nanoparticles Synthesized with Porous Alumina Media

Photoelectric conversion element using perovskite compound and method of manufacturing the same

Lead Halide Perovskites as Quantum Dot Sensitizers for Mesoscopic TiO2 Photovoltaic Cells

Transparent conductive film, photoelectrode for dye-sensitized solar cells, touch panel and dye-sensitized solar cell