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

https://absimage.aps.org/image/MAR14/MWS_MAR14-2013-000770.pdf

Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber

2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Semiconductor-based Photocatalytic Hydrogen Generation

Mixed-halide perovskites (MHPs) have attracted attention as suitable wide-band-gap candidate materials for tandem applications owing to their facile band-gap tuning. However, when smaller bromide ions are incorporated into iodides to tune the band gap, photoinduced halide segregation occurs, which leads to voltage deficit and photoinstability. Here, we propose an original post-hot pressing (PHP) treatment that suppresses halide segregation in MHPs with a band gap of 2.0 eV. The PHP treatment reconstructs open-structured grain boundaries (GBs) as compact GBs through constrained grain growth in the in-plane direction, resulting in the inhibition of defect-mediated ion migration in GBs. The PHP-treated wide-band-gap (2.0 eV) MHP solar cells showed a high efficiency of over 11%, achieving an open-circuit voltage (Voc) of 1.35 V and improving the maintenance of the initial efficiency under the working condition at AM 1.5G. The results reveal that the management of GBs is necessary to secure the stability of wide-band-gap MHP devices in terms of halide segregation.

Suppressing Halide Segregation in Wide-Band-Gap Mixed-Halide Perovskite Layers through Post-Hot Pressing.

Interfacial engineering is extensively used to reduce the interfacial loss caused by surface recombination, improve the crystallinity of the active absorption layer, and enhance the long‐term stability of perovskite solar cells (PSCs). Solution processing techniques, such as spin coating and dip coating, are commonly used to deposit the interfacial layer because of their cost‐effectiveness and simplicity. Although determining suitable solutes for use in these processes is important, selecting appropriate solvents is also crucial. Herein, commonly used solvents are investigated to determine optimal solvents for solution processing by categorizing them into nonpolar and polar groups. The results suggest that the efficiency of the PSCs can be increased by simple solvent treatment. In particular, the efficiencies of systems subjected to hexane (nonpolar) and ethanol (polar) treatment are significantly improved (17.31% and 17.44%, respectively) compared with that of a control device (16.24%). Herein, the effects of pure solvents on the SnO2–perovskite interface are confirmed and an important direction for investigations that adopt solution processing to improve the efficiency of PSCs, such as research on interlayers and self‐assembled monolayers, is suggested.

Optimal Solvents for Interfacial Solution Engineering of Perovskite Solar Cells

Majority and minority carrier properties such as type, density and mobility represent fundamental yet difficult to access parameters governing semiconductor device performance, most notably solar cells. Obtaining this information simultaneously under light illumination would unlock many critical parameters such as recombination lifetime, recombination coefficient, and diffusion length; while deeply interesting for optoelectronic devices, this goal has remained elusive. We demonstrate here a new carrier-resolved photo-Hall technique that rests on a new identity relating hole-electron mobility difference ($\Delta\mu$), Hall coefficient ($h$), and conductivity ($\sigma$): $\Delta\mu=(2+d\ln h/d\ln \sigma)\,h\,\sigma$, and a rotating parallel dipole line ac-field Hall system with Fourier/lock-in detection for clean Hall signal measurement. We successfully apply this technique to recent world-record-quality perovskite and kesterite films and map the results against varying light intensities, demonstrating unprecedented simultaneous access to the above-mentioned parameters.

Carrier-Resolved Photo Hall Measurement in World-Record-Quality Perovskite and Kesterite Solar Absorbers

We synthesized thienyl-benzodithiophene and ester-substituted thiophene-based conjugated donor polymers (PTO2-urea series) functionalized with a urea group at the terminus of the side chain. When stretched, the prepared polymer film, PTO2-urea10, in which 10 mol% of monomers constituting the polymer are the urea-functionalized monomers, showed increased crystallinity in the in-plane direction of the film. In contrast, the unmodified PTO2 polymer exhibited anisotropic reorientation of polymer crystallites in the same direction. Experiments showed that stretching-induced temporary molecular alignment in the amorphous region of PTO2 recovered to the initial state when the applied strain was removed. In contrast, hydrogen bonding between the urea groups of the polymer alkyl chains effectively maintained the organized polymer packing structures which were induced under applied strain. As a result, whereas a PTO2-based thin film transistor (TFT) device showed a dramatic decrease in hole mobility from 1.0 × 10−2 to 5.6 × 10−3 cm2 V−1 s−1 because of morphological damage and cracking of the PTO2 film, the device with a PTO2-urea10 film maintained a relatively high TFT mobility of 5.8 × 10−3 cm2 V−1 s−1 in the direction perpendicular to the strain direction because of the increased crystallinity of the polymer.

Effects of stretching on the molecular packing structure of conjugated polymers with hydrogen bonding

Provided is a perovskite solar cell, and more particularly, a perovskite solar cell including an organometal halide layer having a perovskite structure; and a crystalline material layer stacked while forming an interface with the organometal halide layer, wherein a crystalline material of the crystalline material layer is a crystalline halide having a crystal structure different from the perovskite structure, and the crystalline halide has a band gap energy higher than a band gap energy of an organometal halide of the organometal halide layer, and has a valence band maximum energy level lower than a valence band maximum energy level of the organometal halide.

Jun Hong Noh

Papers

Suppressing Halide Segregation in Wide-Band-Gap Mixed-Halide Perovskite Layers through Post-Hot Pressing.

Optimal Solvents for Interfacial Solution Engineering of Perovskite Solar Cells

Carrier-Resolved Photo Hall Measurement in World-Record-Quality Perovskite and Kesterite Solar Absorbers

Effects of stretching on the molecular packing structure of conjugated polymers with hydrogen bonding

Perovskite solar cell with wide band-gap and fabrication method thereof