Zhiliang Ku

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.

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.

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.

Organic/inorganic self-doping controlled crystallization and electronic properties of mixed perovskite solar cells

Organic and inorganic molecules/atoms located at different lattice positions in hybrid perovskite films play various roles in crystallization and carrier transport for perovskite films. For efficient and stable solar devices, high quality crystals and the proper dispersion of organic/inorganic species are required. In a preferably mixed perovskite system (a blend of MA, FA, I, Br, etc.), however, the effects of inorganic/organic ratios on the performances and electrical properties of perovskite solar cells have not been fully understood. Here, we present perovskite solar cells with self-doped organic and inorganic components to investigate in detail their effect on crystallization and electrical properties. The organic component enhances the conductivity and reduces the hysteresis, while the inorganic component promotes crystal growth. A critical composition is beneficial to perovskite crystallization, with larger crystals and a pinhole-free morphology, and the corresponding device achieved a high efficiency over 19.14% (0.16 cm2, mask area) and 16.2% at an area of 1.21 cm2. Thus, evaluation of the organic/inorganic variation effects provides an understanding of self-doping in mixed perovskites and is beneficial to pursue high-performance perovskite devices.

Low-Temperature Presynthesized Crystalline Tin Oxide for Efficient Flexible Perovskite Solar Cells and Modules

Organic-inorganic metal halide perovskite solar cells (PSCs) have been emerging as one of the most promising next generation photovoltaic technologies with a breakthrough power conversion efficiency (PCE) over 22%. However, aiming for commercialization, it still encounters challenges for the large-scale module fabrication, especially for flexible devices which have attracted intensive attention recently. Low-temperature processed high-performance electron-transporting layers (ETLs) are still difficult. Herein, we present a facile low-temperature synthesis of crystalline SnO2 nanocrystals (NCs) as efficient ETLs for flexible PSCs including modules. Through thermal and UV-ozone treatments of the SnO2 ETLs, the electron transporting resistance of the ETLs and the charge recombination at the interface of ETL/perovskite were decreased. Thus, the hysteresis-free highly efficient rigid and flexible PSCs were obtained with PCEs of 19.20 and 16.47%, respectively. Finally, a 5 × 5 cm2 flexible PSC module with a PCE of 12.31% (12.22% for forward scan and 12.40% for reverse scan) was fabricated with the optimized perovskite/ETL interface. Thus, employing presynthesized SnO2 NCs to fabricate ETLs has showed promising for future manufacturing.

Efficient and stable mixed perovskite solar cells using P3HT as a hole transporting layer

Inorganic–organic hybrid perovskite solar cells (PSCs) have drawn great attention in the past several years. As the stability of PSCs has been a major obstacle for their commercialization, a lot of work has been focused on improving the long-term stability. Here, we reported a meso-structured mixed PSC employing poly(3-hexylthiophene-2,5-diyl) (P3HT) as a hole transport layer (HTL) showing a stable performance even after one year of exposure to air. The impact of different HTL thicknesses on the power conversion efficiency (PCE) of the PSCs has been investigated. The performance of the PSCs was further improved by doping Li-salt/4-tert-butylpyridine into the P3HT HTL. A maximum PCE of 17.55% was achieved, superior to the 14.30% PCE of the pristine P3HT-based devices. The effect of the dopants on the stability of the devices has also been studied.

Structural and Chemical Changes to CH3NH3PbI3 Induced by Electron and Gallium Ion Beams

Organic-inorganic hybrid perovskites, such as CH3 NH3 PbI3, have shown highly promising photovoltaic performance. Electron microscopy (EM) is a powerful tool for studying the crystallography, morphology, interfaces, lattice defects, composition, and charge carrier collection and recombination properties at the nanoscale. Here, the sensitivity of CH3 NH3 PbI3 to electron beam irradiation is examined. CH3 NH3 PbI3 undergoes continuous structural and compositional changes with increasing electron dose, with the total dose, rather than dose rate, being the key operative parameter. Importantly, the first structural change is subtle and easily missed and occurs after an electron dose significantly smaller than that typically applied in conventional EM techniques. The electron dose conditions under which these structural changes occur are identified. With appropriate dose-minimization techniques, electron diffraction patterns can be obtained from pristine material consistent with the tetragonal CH3 NH3 PbI3 phases determined by X-ray diffraction. Radiation damage incurred at liquid nitrogen temperatures and using Ga+ irradiation in a focused ion beam instrument are also examined. Finally, some simple guidelines for how to minimize electron-beam-induced artifacts when using EM to study hybrid perovskite materials are provided.

Alleviate the J–V hysteresis of carbon-based perovskite solar cells via introducing additional methylammonium chloride into MAPbI3 precursor

The hysteretic phenomenon commonly exists in the J–V curves of perovskite solar cells with different structures, especially for carbon-based mesoscopic perovskite solar cells without hole-conductor (carbon-based PSCs). By adding moderate amounts of methylammonium chloride (MACl) into MAPbI3 perovskite precursor, we found the J–V hysteresis of carbon-based PSCs could be significantly alleviated and the crystallinity of MAPbI3 perovskite could also be influenced. With the increasing amount of MACl, MAPbI3 perovskite showed better and better crystallinity until the MACl came to 0.45 M. The champion device with 0.45 M of additional MACl exhibited a preferable PCE of 14.27% for reverse-scan (RS) and 14.50% for forward-scan (FS), significantly higher than that of the pristine device (8.74% for RS and 4.80% for FS). Whats more, the J–V hysteretic index of the device gradually decreased along with the increasing amount of MACl, and kept at low value even when the crystallinity of MAPbI3 perovskite became poor. Through XRD and PL analysis, we demonstrated that the recombination rate of the accumulated charges at the perovskite/TiO2 interface is the main reason for photocurrent hysteresis in carbon-based PSCs. High quality of perovskite crystals is an important contributing factor for high-performance PSCs with low hysteresis, but there is no necessary correlation between low hysteresis and good crystallinity. This research presents an effective way to fabricate carbon-based PSCs with low-hysteresis, and at the same time, provides evidence for investigating the origin of J–V hysteresis of PSCs.