Fuzhi Huang

Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites

The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I−/Br−) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.

A novel quadruple-cation absorber for universal hysteresis elimination for high efficiency and stable perovskite solar cells

Organic–inorganic metal halide perovskite solar cells (PSCs) have made a striking breakthrough with a power conversion efficiency (PCE) over 22%. However, before moving to commercialization, the hysteresis of PSCs, characterized as an inconsistent photovoltaic conversion property at varied electric fields, should be eliminated for stable performance. Herein, we present a novel quadruple-cation perovskite absorber, KxCs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 (labeled as KCsFAMA), with which the hysteresis in PSCs can be fully eliminated irrespective of the electron transportation layers. The incorporation of potassium intensively promotes the crystallization of the perovskite film with a grain size up to ∼1 μm, doubled compared to the K free counterparts. Further characterization revealed that a lower interface defect density, longer carrier lifetime and fast charge transportation have all made contributions to the hysteresis-free, stable and high PCE (20.56%) of the KCsFAMA devices. Moreover, we present a 6 × 6 cm2 sub-module with the KCsFAMA composition achieving a high efficiency of 15.76% without hysteresis. This result suggests that the quadruple-cation perovskite is a highly attractive candidate for future developments of efficient and stable PSC modules.

Effect of the Microstructure of the Functional Layers on the Efficiency of Perovskite Solar Cells

The efficiencies of the hybrid organic-inorganic perovskite solar cells have been rapidly approaching the benchmarks held by the leading thin-film photovoltaic technologies. Arguably, one of the most important factors leading to this rapid advancement is the ability to manipulate the microstructure of the perovskite layer and the adjacent functional layers within the device. Here, an analysis of the nucleation and growth models relevant to the formation of perovskite films is provided, along with the effect of the perovskite microstructure (grain sizes and voids) on device performance. In addition, the effect of a compact or mesoporous electron-transport-layer (ETL) microstructure on the perovskite film formation and the optical/photoelectric properties at the ETL/perovskite interface are overviewed. Insight into the formation of the functional layers within a perovskite solar cell is provided, and potential avenues for further development of the perovskite microstructure are identified.

Optimizing semiconductor thin films with smooth surfaces and well-interconnected networks for high-performance perovskite solar cells

An efficient solution-based technique to grow homogeneous TiO2 nanoparticle (TNP) thin films with tunable crystal size, porosity and roughness on a transparent conducting oxide (TCO) substrate is demonstrated that can direct the evolution of an optimal perovskite morphology. Combining a gas-assisted spin-coating and a subsequent blended vapor-assisted annealing protocol, highly crystalline perovskite films with a flat surface, reduced pin-holes and micrometer-scale grains were formed. The blended vapor was produced from a mixed N,N-dimethylformamide (DMF) and chlorobenzene (CBZ) solution. In the mixed vapor-assisted annealing, the DMF vapor molecules dissolved small perovskite grains and the anti-solvent CBZ facilitated rapid precipitation of perovskite crystals filling the gaps and pores among adjacent grains to produce high quality perovskite films. Perovskite solar cells with consistent power conversion efficiencies exceeding 15% and little cell-to-cell variability have been obtained. This work shows that fine tuning of the semiconducting networks by simple chemistry methods, can potentially lead to the fabrication of high-performance and cost-effective thin film solar cells.

Temperature dependent optical properties of CH3NH3PbI3 perovskite by spectroscopic ellipsometry

Mixed organic-inorganic halide perovskites have emerged as a promising new class of semiconductors for photovoltaics with excellent light harvesting properties. Thorough understanding of the optical properties of these materials is important for photovoltaic device optimization and the insight this provides for the knowledge of energy band structures. Here we present an investigation of the sub-room temperature dependent optical properties of polycrystalline thin films of CH3NH3PbI3 perovskites that are of increasing interest for photovoltaics. The complex dielectric function of CH3NH3PbI3 in the energy range of 0.5–4.1u2009eV is determined between 77u2009K and 297u2009K using spectroscopic ellipsometry. An increase in optical permittivity as the temperature decreases is illustrated for CH3NH3PbI3. Optical transitions and critical points were analyzed using the energy dependent second derivative of these dielectric functions as a function of temperature.

Robust transparent superamphiphobic coatings on non-fabric flat substrates with inorganic adhesive titania bonded silica

The technological implementation of superamphiphobic surfaces has been largely hindered by the stability issues caused by surface abrasion, corrosion, contamination, etc. Robustness still remains the major challenge for a well-performing superamphiphobic coating. In this study, the simple route of spraying inks containing pre-designed silica, cetyltrimethylammonium bromide (CTAB) and titanium diisopropoxide bis-2,4-pentanedionate (TAA) is presented to prepare micro–nanostructure films. The mechanical properties of the films are significantly strengthened by titania after the pyrogenic decomposition of TAA, and the films are able to withstand a standard 2H pencil scratching and sand flow impact. The as-made films exhibit excellent super-repellency to various liquids after treatment with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS). The static contact angles (SCAs) for water (surface tension 72.1 mN m−1) and dodecane (surface tension 25.3 mN m−1) can reach 166° ± 3° and 153° ± 3°, respectively. On controlling the thickness of the films, the optical transmittance of the films (400 nm thick) can come close to that of glass. Moreover, efficient photocatalytic decomposition of an organic substance attached on the surfaces is demonstrated; this decomposition enables the recovery of the superamphiphobic property of the contaminated films. Thus, the unique properties of robustness, transparency and self-healing, etc., combined with the relatively low cost fabrication, make these superamphiphobic coatings promising in various applications.

Improved air stability of perovskite hybrid solar cells via blending poly(dimethylsiloxane)–urea copolymers

A new kind of PDMS–urea co-polymer has been synthesized and successfully incorporated into the fabrication process of perovskite solar cells. Such a polymer possesses both a flexible and hydrophobic PDMS backbone and urea groups capable of hydrogen binding. Scanning electron microscopy showed that the morphology of the perovskite layer was greatly improved after addition of 10 mg ml−1 PDMS–urea into perovskite precursor solution. As a result, the short circuit current of the devices was improved by 10% and the energy conversion efficiency by 27%, reaching 16.15% under 1 sun simulated sunlight. Steady-state photoluminescence spectra reveal a much improved photoluminescence intensity after the introduction of PDMS–urea into the perovskite layer. The devices with the perovskite-PDMS–urea (20 mg ml−1) hybrid demonstrate an almost unchanged efficiency for 2500 h, proving its remarkable effect on improving the stability of perovskite solar cells.

High efficiency solid-state dye-sensitized solar cells using a cobalt(II/III) redox mediator

Improvement in the mass transport of the considerably large sized cobalt(II/III) complexes, particularly in a high viscosity electrolyte, is crucial for solid-state dye-sensitized solar cells (SS-DSCs) to reach a reliable high efficiency for practical applications. In this study, titania nanorod aggregates (TNA) with a large specific surface area and well-developed crystalline network were utilized as photoanode building blocks for application in a cobalt(II/III) tris(2′2-bipyridine) complexes-based solid-state electrolyte. An initial efficiency excess of 7.1% and a long term stable efficiency of approximately 8.0% were achieved under full sun illumination by the freshly assembled and aged TNA SS-DSCs, respectively. This represents dramatic enhancements of nearly 35% and 100% against the SS-DSCs prepared from two standard TiO2 nanoparticle samples – CCIC (approx. 5.9%) and Degussa P25 (approx. 4.0%), correspondingly. The TNA is characterised by a combination of mesopores within each aggregate and macro inter-aggregate voids, a high specific surface area and great light scattering ability; such features make the aggregates superior photoanode building blocks for their application in bulky cobalt redox mediator based SS-DSCs systems.

An efficient, flexible perovskite solar module exceeding 8% prepared with an ultrafast PbI 2 deposition rate

Large-area, pinhole-free CH3NH3PbI3 perovskite thin films were successfully fabricated on 5u2009cmu2009×u20095u2009cm flexible indium tin oxide coated polyethylene naphthalate (ITO-PEN) substrates through a sequential evaporation/spin-coating deposition method in this research. The influence of the rate-controlled evaporation of PbI2 films on the quality of the perovskite layer and the final performance of the planar-structured perovskite solar cells were investigated. An ultrafast evaporation rate of 20u2009Åu2009s−1 was found to be most beneficial for the conversion of PbI2 to CH3NH3PbI3 perovskite. Based on this high-quality CH3NH3PbI3 film, a resultant flexible perovskite solar sub-module (active area of 16u2009cm2) with a power conversion efficiency of more than 8% and a 1.2u2009cm2 flexible perovskite solar cell with a power conversion efficiency of 12.7% were obtained.

Enhancing the performance and stability of carbon-based perovskite solar cells by the cold isostatic pressing method

The cold isostatic pressing method was used as a post-treatment process for enhancing the power conversion efficiency and stability of carbon-based perovskite solar cells without hole transport materials.