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.

/pdf/electron-hole-diffusion-lengths-exceeding-1-micrometer-in-an-3rhimkkmrj.pdf

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

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

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

Advancing perovskite solar cell technologies toward their theoretical power conversion efficiency (PCE) requires delicate control over the carrier dynamics throughout the entire device. By controlling the formation of the perovskite layer and careful choices of other materials, we suppressed carrier recombination in the absorber, facilitated carrier injection into the carrier transport layers, and maintained good carrier extraction at the electrodes. When measured via reverse bias scan, cell PCE is typically boosted to 16.6% on average, with the highest efficiency of ~19.3% in a planar geometry without antireflective coating. The fabrication of our perovskite solar cells was conducted in air and from solution at low temperatures, which should simplify manufacturing of large-area perovskite devices that are inexpensive and perform at high levels.

http://science.sciencemag.org/content/sci/345/6196/542.full.pdf

Interface engineering of highly efficient perovskite solar cells

Ju Mei,†,‡,∥ Nelson L. C. Leung,†,‡,∥ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen Research Institute, Hi-Tech Park, Nanshan, Shenzhen 518057, China ‡Department of Chemistry, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

Aggregation-Induced Emission: Together We Shine, United We Soar!

The band gap of formamidinium lead iodide (FAPbI3) perovskites allows broader absorption of the solar spectrum relative to conventional methylammonium lead iodide (MAPbI3). Because the optoelectronic properties of perovskite films are closely related to film quality, deposition of dense and uniform films is crucial for fabricating high-performance perovskite solar cells (PSCs). We report an approach for depositing high-quality FAPbI3 films, involving FAPbI3 crystallization by the direct intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in PbI2 with formamidinium iodide. This process produces FAPbI3 films with (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual PbI2. Using films prepared by this technique, we fabricated FAPbI3-based PSCs with maximum power conversion efficiency greater than 20%.

/pdf/high-performance-photovoltaic-perovskite-layers-fabricated-13hmgypipo.pdf

High-performance photovoltaic perovskite layers fabricated through intramolecular exchange

It is shown experimentally that the ketene dithioacetal group is very effective as an electron-donating group in the design and synthesis of thermally stable and efficient second-order nonlinear optical chromophores.

Ketene Dithioacetal as a π-Electron Donor in Second-Order Nonlinear Optical Chromophores.

Self-assembly, the spontaneous organization of parts into ordered arrays and structures, is an omnipresent process in nature. Our group explores the use of self-assembly as an engineering concept to construct functional devices across the size scale. In this paper we present two examples of self-assembly. In the first example, we show how self-assembled monolayers of anthryl phosphonic acid can be used to form a nano-scale charge conduction channel. The molecular monolayer forms on a silicon dioxide surface and is placed between two metal electrodes. We have synthesized the molecules, verified the formation of the monolayer with X-ray photoemission spectroscopy and atomic force microscopy, and characterized the lateral charge carrier transport properties of the molecules. This molecular monolayer provides a facile way for integration of a nano-scale electronic device with conventional circuitry. In the second example, we demonstrate how microfabricated components can self-assemble into an electrical network. We have developed a microfabrication process for parallel production of 100 mm hexagonal silicon parts each carrying a portion of an electrical network. We functionalize the parts either with a low-melting point alloy or with a polymer and allow for their self-assembly into an ordered lattice at an air-water interface due to capillary forces.

Using self-assembly for the construction of nanoscale lateral-transport molecular electronic devices and microscale silicon-based networks

Hydrogen-bond-bridged intermediate for perovskite solar cells with enhanced efficiency and stability

We demonstrate electro-optic modulation at 81 GHz, 108 GHz, and 133 GHz using traveling-wave electro-optic polymer modulators based upon ring resonators. We show that ring resonator type modulators have significant advantages for high-speed modulation.

Alex K.-Y. Jen

Papers

Ketene Dithioacetal as a π-Electron Donor in Second-Order Nonlinear Optical Chromophores.

Using self-assembly for the construction of nanoscale lateral-transport molecular electronic devices and microscale silicon-based networks

Hydrogen-bond-bridged intermediate for perovskite solar cells with enhanced efficiency and stability

Millimeter-wave ring resonator based electro-optic polymer modulator

Thermally stable poled polymers by using heteroaromatic chromophores in high-temperature polyimides