Light-induced structural dynamics plays a vital role in the physical properties, device performance, and stability of hybrid perovskite–based optoelectronic devices. We report that continuous light illumination leads to a uniform lattice expansion in hybrid perovskite thin films, which is critical for obtaining high-efficiency photovoltaic devices. Correlated, in situ structural and device characterizations reveal that light-induced lattice expansion benefits the performances of a mixed-cation pure-halide planar device, boosting the power conversion efficiency from 18.5 to 20.5%. The lattice expansion leads to the relaxation of local lattice strain, which lowers the energetic barriers at the perovskite-contact interfaces, thus improving the open circuit voltage and fill factor. The light-induced lattice expansion did not compromise the stability of these high-efficiency photovoltaic devices under continuous operation at full-spectrum 1-sun (100 milliwatts per square centimeter) illumination for more than 1500 hours.

https://science.sciencemag.org/content/sci/360/6384/67.full.pdf

Light-induced lattice expansion leads to high-efficiency perovskite solar cells

X-ray diffraction analysis by Williamson-Hall, Halder-Wagner and size-strain plot methods of CdSe nanoparticles- a comparative study

Integral breadth methods for line profile analysis are reviewed, including modifications of the Williamson–Hall method recently proposed for the specific case of dislocation strain broadening. Two cases of study, supported by the results of a TEM investigation, are considered in detail: nanocrystalline ceria crystallized from amorphous precursors and highly deformed nickel powder produced by extensive ball milling. A further application concerns a series of Fe–Mo powder specimens that were ball milled for increasing time. Traditional and modified Williamson–Hall methods confirm their merits for a rapid overview of the line broadening effects and possible understanding of the main causes. However, quantitative results are generally not reliable. Limits in the applicability of integral breadth methods and reliability of the results are discussed in detail.

Line broadening analysis using integral breadth methods: a critical review

The Williamson-Hall (W-H) equation, which has been used to obtain relative crystallite sizes and strains between samples since 1962, is revisited. A modified W-H equation is derived which takes into account the Scherrer equation, first published in 1918, (which traditionally gives more absolute crystallite size prediction) and strain prediction from Raman spectra. It is found that W-H crystallite sizes are on average 2.11 ± 0.01 times smaller than the sizes from Scherrer equation. Furthermore the strain from the W-H plots when compared to strain obtained from Raman spectral red-shifts yield factors whose values depend on the phases in the materials – whether anatase, rutile or brookite. Two main phases are identified in the annealing temperatures (350 °C–700 °C) chosen herein – anatase and brookite. A transition temperature of 550 °C has been found for nano-TiO2 to irreversibly transform from brookite to anatase by plotting the Raman peak shifts against the annealing temperatures. The W-H underestimation on the strain in the brookite phase gives W-H/Raman factor of 3.10 ± 0.05 whereas for the anatase phase, one gets 2.46 ± 0.03. The new βtot2cos2θ-sinθ plot and when fitted with a polynomial yield less strain but much better matching with experimental TEM crystallite sizes and the agglomerates than both the traditional Williamson-Hall and the Scherrer methods. There is greater improvement in the model when linearized – that is the βtotcos2θ-sinθ plot rather than the βtot2cos2θ-sinθ plot.

Strain and grain size of TiO2 nanoparticles from TEM, Raman spectroscopy and XRD: The revisiting of the Williamson-Hall plot method

Hybrid perovskites are on a trajectory toward realizing the most efficient single-junction, solution-processed photovoltaic devices. However, a critical issue is the limited understanding of the correlation between the degree of crystallinity and the emergent perovskite/hole (or electron) transport layer on device performance and photostability. Here, the controlled growth of hybrid perovskites on nickel oxide (NiO) is shown, resulting in the formation of thin films with enhanced crystallinity with characteristic peak width and splitting reminiscent of the tetragonal phase in single crystals. Photophysical and interface sensitive measurements reveal a reduced trap density at the perovskite/NiO interface in comparison with perovskites grown on poly(3,4-ethylene dioxy thiophene) polystyrene sulfonate. Photovoltaic cells exhibit a high open circuit voltage (1.12 V), indicating a near-ideal energy band alignment. Moreover, photostability of photovoltaic devices up to 10-Suns is observed, which is a direct result of the superior crystallinity of perovskite thin films on NiO. These results elucidate the critical role of the quality of the perovskite/hole transport layer interface in rendering high-performance and photostable optoelectronic devices.

Critical Role of Interface and Crystallinity on the Performance and Photostability of Perovskite Solar Cell on Nickel Oxide

Wie in Fig. 2 gezeigt, kann der Neutronenleiter aus zwei entgegengesetzt gekriJmmten Teilstficken 'einfacher direkter Sichtweite' zusammengesetzt werden. Durch die Richtungsanderung treffen Neutronen, die im ersten Abschnitt noch unter sehr kleinen Winkeln nur an der gekrfimmten Aussenfl~iche reflektiert wurden (girlandenreflektiert), im zweiten Abschnitt mit zu grossem Winkel auf und gelangen nicht mehr durch den Leiter. Die fiber den Querschnitt gemittelte Durchl~issigkeit far den doppelt gekrfimmten Neutronenleiter, zeigt ebenfalls Fig. 1. Fiir k,_> l/2k,, m ist die Durchl~issigkeit Null. W~ihlt man k~=0,8 k . . . . dann ist der Verlust gegenfiber einem geraden Neutronenleiter nur 30 Vo. Die in z-Richtung austretenden Neutronen haben am linken und rechten Rand die maximale Wellenzahl k, = k .... in der Mitte die maximale Wellenzahl k, = I/2k ..... Die Impulsraumbreite in der k=-Richtung ist ebenfalls in der Mitre des Strahls am gr6ssten. Der Neutronenstrahl ist beziiglich seiner Impulsraumverteilung symmetrisch zur Mitte des Strahls, im Gegensatz zum einfach gekrfimmten Neutronenleiter (siehe auch Maier-Liebnitz, 1965). Wie man aus Fig. 2 ersieht, gelangen schnelle Neutronen und ~,-Strahlen bestenfalls nach zweimaliger Streuung aus dem Neutronenleiter. Die Anordnung k6nnte symmetrisch verktirzt werden bis zu (1⁄2 + 1/1⁄4)Lu, jedoch ist dieser Gewinn nicht wesentlich und aus Abschirmungsgrfinden ist es auf jeden Fall zweckm~issig die Baul/inge gleich Lu zu belassen, auch gilt die Kurve in Fig. 1 exakt nur ftir L >_ LII. Ein Dimensionierungsbeispiel ffir 4/~ Neutronen m6ge den Sachverhalt noch veranschaulichen: Gewtinscht sei eine Strahlbreite yon 3 cm. Mit k~ = 0,8 k .... berechnet sich nach voriger Gleichung der Krtimmungsradius und damit nach Fig. 2 die L~inge zu 45 Meter. Der Divergenzwinkel am Ausgang des Neutronenleiters ist ffir beide Richtungen senkrecht zum Strahl 50 Winkelminuten bei 4 A Neutronen.

C. N. J. Wagner

Papers

Separation of particle size and lattice strain in integral breadth measurements