A guide to use fluorinated aromatic bulky cations for stable and high-performance 2D/3D perovskite solar cells:The more fluorination the better?

While serious stability issues impede the commercialization of perovskite solar cells(PSCs),two-dime nsional(2D)perovskites based on fluorinated bulky cations have emerged as more intrinsically stable materials.However,the influence of fluorination degree of the bulky aromatic cation on the per-formance of resulting PSCs has not been scrutinized.Here,2D perovskites(FxPEA)_(2)PbI_(4)(x=1,2,3,5)are grown in situ on the surface of the three-dime nsion al(3D)perovskite and dem on strate effective passivation of the surface defects of 3D perovskite.The power conversion efficiency(PCE)of the optimized devices were boosted from 20.75%for the control device to 21.09%,22.06%,22.74%and 21.86%for 2D/3D devices treated with 4-fluorophenethylamine iodide,3,5-difluorophenylethylamine iodide,2,4,5-trifluoroethylphenylethylamine iodide,and 1,2,3,4,5-pentafluorophenylethylamine iodide,respectively.We firstly reported two unexplored RP-type layered perovskites with F_(2)PEAI and F_(3)PEAI as bulky cations.The combined experimental and theoretical analysis revealed the reasons behind the various morphology,device performances,dynamic behavior,and humidity stability.The best performing F_(5)PEAI-treated device retaining 95.0%of its initial PCE under ambient atmosphere(with RH of 60%±5%)without encapsulation for 300 h storage.This work provides useful guidance for selecting fluorinated bulky cations with different molecular electronic properties,which will play an essential role in further improving the performance/stability of PSCs for the sake of further commercialization.

Electrodeposited NiSe2 on carbon fiber cloth as a flexible electrode for high-performance supercapacitors

A flexible electrode of nickel diselenide/carbon fiber cloth(NiSe/CFC) is fabricated at room temperature by a simple and efficient electrodeposition method. Owing to NiSecharacter of nanostructure and high conductivity, the as-synthesized electrodes possess perfect pseudocapacitive property with high specific capacitance and excellent rate capability. In three-electrode system, the electrode specific capacitance of the NiSe/CFC electrode varies from 1058 F gto 996.3 F gat 2 A gto 10 A grespectively, which shows great rate capability. Moreover, the NiSeelectrode is assembled with an active carbon(AC) electrode to form an asymmetric supercapacitor with an extended potential window of 1.6 V. The asymmetric supercapacitor possesses an excellent energy density 32.7 Wh kgwith a power density 800 W kgat the current density of 1 A g. The nanosheet array on carbon fiber cloth with high flexibility, specific capacitance and rate capacitance render the NiSeto be regarded as the promising material for the high performance superconductor.

Surface passivation using pyridinium iodide for highly efficient planar perovskite solar cells

Perovskite solar cells have developed rapidly in the past decades.However,there are large amounts of ionic defects at the surface and grain boundaries of perovskte films which are detrimental to both the efficiency and stability of perovskite solar cells.Here,an organic halide salt pyridinium iodide(PyI) is used in cation-anion-mixed perovskite for surface defect passivation.Different from the treatment with Lewis base pyridine(Py) which can only bind to the under-coordinated Pb ions,zwitterion molecule PyI can not only fill negative charged iodine vacancies,but also interact with positive charged defects.Compared with Py treatment,PyI treatment results in smoother surface,less defect densities and nonradiative recombination in perovskite,leading to an improved VOC, negligible J-V hysteresis and stable performance of devices.As a result,the champion PyI-treated planar perovskite solar cell with a high VOC of 1.187 V achieves an efficiency of 21.42%,which is higher than 20.37% of Py-treated device,while the pristine device without any treatment gets an efficiency of 18.83% at the same experiment conditions.

Chromium trioxide modified spiro-OMeTAD for highly efficient and stable planar perovskite solar cells

Hole transporting materials(HTMs)play an unparalleled role in heightening the stability and photovoltaic performance of perovskite solar cells(PSCs).The organic small molecule spiro-OMeTAD is frequently utilized for HTM in PSCs.However,the raw spiro-OMeTAD without dopant would be harmful to the development of highly efficient PSCs,due to its unsatisfied hole mobility and conductivity.Therefore,we introduce an inorganic dopant(chromium trioxide,CrO_(3))into the lithium-salt doped spiro-OMeTAD.Because of the exclamatory oxidizability of CrO_(3),it can accelerate the oxidation of spiro-OMeTAD and thereby enhancing the hole mobility of HTM.The introduction of CrO_(3) not only substantially decreases the density of defects,but also adjusts spiro-OMeTAD energy band,and thus effectively suppresses the hysteresis and improving stability of PSCs.In the end,we obtained a power conversion efficiency(PCE)as high as 22.6%after doping CrO_(3) in spiro-OMeTAD.The facile,low cost and outstanding photovoltaic performance render CrO_(3) an excellent dopant for HTMs in PSCs.

Hydrothermal synthesis of CoMoO_4/Co_(1-x)S hybrid on Ni foam for high-performance supercapacitors

Cobalt molybdate/non-stoichiometric cobalt sulfide（CoMoO4/Co1-xS） hybrid was in situ grown on nickel foam by a simple two-step hydrothermal process. The as-prepared CoMoO4/Co1-xS hybrid electrode possessed core-shell nanostructure, large surface area and high specific capacitance of 2250 F g-1 at a current density of 1 A g-1. Using the hybrid as anode and activated carbon（AC） as cathode, an asymmetric supercapacitor of CoMoO4/Co1-xS//AC was fabricated. The optimized supercapacitor had large potential window of 1.6 V and high capacitance of 112 F g-1, resulting in high power density of 804.5 W kg-1 and energy density of 39.8 Wh kg-1. Furthermore, the supercapacitor exhibited an excellent long cycle life along with 86.4% specific capacitance retained after 5000 cycles. The superior performances and good stability of the asymmetric supercapacitor can be attributed to the unique structure of the two components in hybrid, and the positive synergistic effects of the hybrid electrodes. The facile preparation process and excellent performance presented here render the CoMoO4/Co1-xS hybrid as a promising candidate for energy storage device.

High-Performance and Hysteresis-Free Perovskite Solar Cells Based on Rare-Earth-Doped SnO_(2) Mesoporous Scaffold

Tin oxide(SnO_(2)),as electron transport material to substitute titanium oxide(TiO_(2))in perovskite solar cells(PSCs),has aroused wide interests.However,the performance of the PSCs based on SnO_(2) is still hard to compete with the TiO_(2)-based devices.Herein,a novel strategy is designed to enhance the photovoltaic performance and long-term stability of PSCs by integrating rare-earth ions Ln^(3+)(Sc^(3+),Y^(3+),La^(3+))with SnO_(2) nanospheres as mesoporous scaffold.The doping of Ln promotes the formation of dense and large-sized perovskite crystals,which facilitate interfacial contact of electron transport layer/perovskite layer and improve charge transport dynamics.Ln dopant optimizes the energy level of perovskite layer,reduces the charge transport resistance,and mitigates the trap state density.As a result,the optimized mesoporous PSC achieves a champion power conversion efficiency(PCE)of 20.63%without hysteresis,while the undoped PSC obtains an efficiency of 19.01%.The investigation demonstrates that the rare-earth doping is low-cost and effective method to improve the photovoltaic performance of SnO_(2)-based PSCs.

Effective iron-molybdenum-disulfide counter electrodes for use in platinum-free dye-sensitized solar cells

Nanocomposites of iron disulfide（FeS2） and molybdenum disulfide（MoS2） with nanosheets structure were successfully grown on the flexible titanium foils through a facile one-step hydrothermal process, and then worked as counter electrodes（CEs） in the dye-sensitized solar cells（DSSCs）. X-ray diffraction, scanning electron microscopy,transmission electron microscopy, and energy dispersive spectrometer were employed to characterize the microstructure and composition of the FeS2/MoS2. Cyclic voltammogram reveals that the catalytic activity of FeS2/MoS2 CE is higher than that of FeS2, MoS2, and platinum（Pt） CEs towards triiodide/iodide（I3^–/I^–） redox electrolyte, owing to the superior carrier transfer properties of vertical array structure and abundant catalytic active sites of FeS2/MoS2 nanosheets.Moreover, the FeS2/MoS2 CE maintains its activity after 500 cycles, exhibiting excellent electrochemical stability. Furthermore, the power conversion efficiency（PCE） of FeS2/MoS2 CE reaches 8.67%, which is higher than that of the FeS2（7.20%）,MoS2（7.38%） and Pt（8.16%） CEs.

Preparation of titanium dioxide-double-walled carbon nanotubes and its application in flexible dye-sensitized solar cells

Titanium dioxide-double-walled carbon nano- tubes （TiO2-DWCNTs） with DWCNTs/TiO2 of 20 wt.% is prepared by a conventional sol-gel method. Doping the TiO2-DWCNTs in TiO2 photoanode, a flexible dye- sensitized solar cell （DSSC） is fabricated. The sample is characterized by scanning electron microscopy, X-ray diffraction （XRD）, Fourier transform infrared spectroscopy （FTIR） absorption, ultraviolet-visible spectroscopy （UV- vis） absorption spectra , electrochemical impedance spec- troscopy （EIS） technique and photovoltaic measurement. It is found that adding a certain amount of TiO2-DWCNTs can efficiently decrease the resistance of charge transport, improve dye adsorption. Under an optimal condition, a flexible DSSC contained with 0.50 wt.% TiOz-DWCNTs achieves a light-to-electric energy conversion efficiency of 3.89% under a simulate solar light irradiation of 100 mW. cm^2.

NaHCO_(3)-induced porous PbI_(2) enabling efficient and stable perovskite solar cells

Driven by their many unique features,perovskite solar cells(PSCs)have become one of the most promising candidates in the photovoltaic field.Two-step preparation of perovskite film is advantageous for its higher stability and reproducibility compared to the one-step method,which is more suitable for practical application.However,the incomplete conversion of the dense lead iodide(PbI_(2))layer during the sequential spin-coating of formamidinium/methylammonium(FA^(+)/MA^(+))organic amine salts severely affect the performance of PSCs.Herein,sodium bicarbonate(NaHCO_(3))is used to induce the formation of porous PbI_(2),which facilitates the penetration of the FA^(+)/MA^(+)ions and the formation of a perovskite film with high crystallinity and large grain microstructure.Meanwhile,the introduction of Na^(+)not only improves the energetic alignment of the PSC,but also increases the conductivity via p-doping.As a result,the optimized NaHCO_(3)-modified PSC achieves a champion power conversion efficiency of 24.0% with suppressed hysteresis.Moreover,the significant reduction in defect density and ion migration as well as a mild alkaline environment enhance the stability of device.The unencapsulated NaHCO_(3)-modified PSCs maintain over 90% of their original efficiency upon storage in ambient air(30%–40% relative humidity)for 2160 h.We have demonstrated an ingenious strategy for controlling the quality of perovskite and improving the performance of device by low-temperature foaming of simple inorganic molecules of NaHCO_(3).