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

Narrow band gap semiconductors are attractive materials as novel visible light sensitizers for the dyesensitized solar cell (DSSC). We have previously reported that a lead-bromide perovskite compound, CH3NH3PbBr3, works as a visible light sensitizer for TiO2 electrode in a photoelectrochemical cell, showing a good energy conversion efficiency of 2.19%. However, amplitude of photocurrent is not sufficient because the absorption band of CH3NH3PbBr3 is comparatively narrow (less than 570nm). To improve the photocurrent and photovoltaic performances, we examined a lead-iodide perovskite compound (CH3NH3PbI3) as the visible light sensitizer for TiO2 electrode. This compound has a wider absorption band in the visible light region, which is expected to improve the light harvesting efficiency and the photocurrent. In this study, we will report the optical properties of the lead-iodide perovskite compound and photovoltaic performance of the TiO2/CH3NH3PbI3 photoelectrode. Titania paste (Solaronix SA Ti-Nanoxide D) was spread on FTO glass by squeeze method and sintered at 400°C for 1h in the air. CH3NH3PbI3 precursor solution consisted of CH3NH3I and PbI2 in acetonitrile. The TiO2/CH3NH3PbI3 photoelectrode was prepared by spin coating of the precursor solution on nano-porous TiO2 electrode. A Pt coated glass was used as counterelectrode. Electrolyte solution was composed of LiI, I2, and isobutyronitrile as solvent. Photovoltaic performance was measured under 100 mW cm irradiation with I-V measurement system (Peccell Technologies, Inc.). We estimated the energy levels of valence band for the prepared perovskite compounds. The work function of the CH3NH3PbI3 was determined as 5.22eV (vs. vacuum) by the photoelectron spectroscopy measurement for the spin-coating film, which corresponded to the valence band level of the compound. The result indicated that the valence band level of CH3NH3PbI3 is lower than the oxidation-reduction potential of I/I3 redox mediator. Fig.1 shows an incident photon-to-current conversion efficiency (IPCE) spectrum of the photovoltaic cell using TiO2/CH3NH3PbI3 electrode in comparison with UV-VIS absorption spectrum of CH3NH3PbI3 spin-coated thin film on quartz glass substrate. The optical absorption edge corresponds to 1.43eV and the energy level of conduction band was estimated as 3.79eV. The result showed that electron injection from excited CH3NH3PbI3 into the TiO2 conduction band is possible. The IPCE spectrum shows a photocurrent response over a wide visible light region up to 800 nm with efficiencies exceeding 20%. Absorption and IPCE spectra showed good agreement, indicating that CH3NH3PbI3 acts as visible light sensitizer for TiO2. Fig.3 shows the I-V carve of the cell sensitized by lead iodide perovskite compound. The short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (FF), and energy conversion efficiency (η) are 1.98 mA cm, 0.43 V, 0.43, and 0.36 %, respectively (Table 1). The result shows that lead-iodide perovskite compounds are capable of sensitizing TiO2 in visible region. However, in the above example, photocurrent density is lower than the cell using TiO2/CH3NH3PbBr3 electrode. The cause of low photocurrent is now under considerations. Optimizations for experimental conditions, such as preparation method of the photoelectrode, composition of the electrolyte solution, and so on, are expected to improve the sensitization performance of CH3NH3PbI3.

https://iopscience.iop.org/article/10.1149/MA2008-02/1/27/pdf

Kenjiro Teshima

Papers

Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells

Highly Luminescent Lead Bromide Perovskite Nanoparticles Synthesized with Porous Alumina Media

Novel Photoelectrochemical Cell with Mesoscopic Electrodes Sensitized by Lead-Halide Compounds (2)

Conductive polymer-carbon-imidazolium composite : a simple means for constructing solid-state dye-sensitized solar cells

Improvement in durability of flexible plastic dye-sensitized solar cell modules