Unreacted lead iodide is commonly believed to be beneficial to the efficiency of methylammonium lead iodide perovskite based solar cells, since it has been proposed to passivate the defects in perovskite grain boundaries. However, it is shown here that the presence of unreacted PbI2 results in an intrinsic instability of the film under illumination, leading to the film degradation under inert atmosphere and faster degradation upon exposure to illumination and humidity. The perovskite films without lead iodide have improved stability, but lower efficiency due to inferior film morphology (smaller grain size, the presence of pinholes). Optimization of the deposition process resulted in PbI2‐free perovskite films giving comparable efficiency to those with excess PbI2 (14.2 ± 1.3% compared to 15.1 ± 0.9%) Thus, optimization of the deposition process for PbI2‐free films leads to dense, pinhole‐free, large grain size perovskite films which result in cells with high efficiency without detrimental effects on the film photostability caused by excess PbI2. However, it should be noted that for encapsulated devices illuminated through the substrate (fluorine‐doped tin oxide glass, TiO2 film), film photostability is not a key factor in the device degradation.
Large earthquakes initiate chains of surface processes that last much longer than the brief moments of strong shaking. Most moderate‐ and large‐magnitude earthquakes trigger landslides, ranging from small failures in the soil cover to massive, devastating rock avalanches. Some landslides dam rivers and impound lakes, which can collapse days to centuries later, and flood mountain valleys for hundreds of kilometers downstream. Landslide deposits on slopes can remobilize during heavy rainfall and evolve into debris flows. Cracks and fractures can form and widen on mountain crests and flanks, promoting increased frequency of landslides that lasts for decades. More gradual impacts involve the flushing of excess debris downstream by rivers, which can generate bank erosion and floodplain accretion as well as channel avulsions that affect flooding frequency, settlements, ecosystems, and infrastructure. Ultimately, earthquake sequences and their geomorphic consequences alter mountain landscapes over both human and geologic time scales. Two recent events have attracted intense research into earthquake‐induced landslides and their consequences: the magnitude M 7.6 Chi‐Chi, Taiwan earthquake of 1999, and the M 7.9 Wenchuan, China earthquake of 2008. Using data and insights from these and several other earthquakes, we analyze how such events initiate processes that change mountain landscapes, highlight research gaps, and suggest pathways toward a more complete understanding of the seismic effects on the Earth's surface.
High‐quality hole transport layers are prepared by spin‐coating copper doped nickel oxide (Cu:NiO) nanoparticle inks at room temperature without further processing. In agreement with theoretical calculations predicting that Cu doping results in acceptor energy levels closer to the valence band maximum compared to gap states of nickel vacancies in undoped NiO, an increase in the conductivity in Cu:NiO films compared to NiO is observed. Cu in Cu:NiO can be found in both Cu+ and Cu2+ states, and the substitution of Ni2+ with Cu+ contributes to both increased carrier concentration and carrier mobility. In addition, the films exhibit increased work function, which together with the conductivity increase, enables improved charge transfer and extraction. Furthermore, recombination losses due to lower monomolecular Shockley‐Read‐Hall recombination are reduced. These factors result in an improvement of all photovoltaic performance parameters and consequently an increased efficiency of the inverted planar perovskite solar cells. A power conversion efficiency (PCE) exceeding 20% could be achieved for small‐area devices, while PCE values of 17.41 and 18.07% are obtained for flexible devices and large area (1 cm2) devices on rigid substrates, respectively.
Both conductivity and mobility are essential to charge transfer by carrier transport layers (CTLs) in perovskite solar cells (PSCs). The defects derived from generally used ionic doping method lead to the degradation of carrier mobility and parasite recombinations. In this work, a novel molecular doping of NiO hole transport layer (HTL) is realized successfully by 2,2'-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ). Determined by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, the Fermi level (E ) of NiO HTLs is increased from -4.63 to -5.07 eV and valence band maximum (VBM)-E declines from 0.58 to 0.29 eV after F6TCNNQ doping. The energy level offset between the VBMs of NiO and perovskites declines from 0.18 to 0.04 eV. Combining with first-principle calculations, electrostatic force microscopy is applied for the first time to verify direct electron transfer from NiO to F6TCNNQ. The average power conversion efficiency of CsFAMA mixed cation PSCs is boosted by ≈8% depending on F6TCNNQ-doped NiOx HTLs. Strikingly, the champion cell conversion efficiency of CsFAMA mixed cations and MAPbI -based devices gets to 20.86% and 19.75%, respectively. Different from passivation effect, the results offer an extremely promising molecular doping method for inorganic CTLs in PSCs. This methodology definitely paves a novel way to modulate the doping in hybrid electronics more than perovskite and organic solar cells.
Metal oxide materials, such as TiO 2 , ZnO, etc., are of significant interest for photocatalytic applications. Although it is less commonly studied and has inferior stability compared to TiO 2 , 1 ZnO is more efficient for degradation of various organic compounds. 1À3 Consequently, there is an interest in the study of photocatalytic activity of ZnO, which has been investigated for a range of different morphologies. 1À19 It has been shown that both activity and stability could be affected by changing the morphology of ZnO. 1,2 Improvements in photocatalytic activity and/or stability have been attributed to different terminating crystal facets, 1,4,9,11 preparation method, 15 higher crystallinity, 2,3,8 crystallite size, 8 lower defects, 2,8 increased surface defects, 4À7,14,19 and increased surface area. 5,18,19 Higher surface area would typically lead to an increased photocatalytic activity, although there have been demonstrations of increased activity of different morphologies of ZnO in spite of the lower surface areas. 4,11 In addition, contradictory effects of native defects have been reported, with higher 4À7,14 and lower 2,8 concentrations having beneficial effects. In addition, it was proposed that there is an optimal surface oxygen vacancy concentration that would result in an increased photocatalytic activity, while lower and higher concentrations compared to an optimal one would result in reduced photocatalytic activity. 13 Photoluminescence (PL) measurements are a convenient method to study the native defects in ZnO and investigate their effect on the photocatalytic activity. 14,17 However, the relationship between the PL, defects, and photocatalytic activity is likely to be inherently complex, especially in metal oxides, which have rich defect chemistry. The presence of different defects could result in either improvement or worsening of the photocatalytic activity, depending on the type and location of native defects.
We examined different encapsulation strategies for perovskite solar cells by testing the device stability under continuous illumination, elevated temperature (85 °C) and ambient humidity of 65 %. The effects of the use of different epoxies, protective layers and the presence of desiccant were investigated. The best stability (retention of ∼80 % of initial efficiency on average after 48 h) was obtained for devices protected by a SiO film and encapsulated with a UV-curable epoxy including a desiccant sheet. However, the stability of ZnO-based cells encapsulated by the same method was found to be inferior to that of TiO -based cells. Finally, outdoor performance tests were performed for TiO -based cells (30-90 % ambient humidity). All the stability tests were performed following the established international summit on organic photovoltaic stability (ISOS) protocols for organic solar cell testing (ISOS-L2 and ISOS-O1).
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