Mapping of Grain Orientation <i>In Situ</i> of 2D Perovskite Thin Films with Low-Frequency Polarized Raman Microspectroscopy

Recent developments in two-dimensional (2D) Ruddlesden–Popper perovskites have ushered in a new phase in optoelectronics through their promising physical, chemical, and optical properties. Thin films of 2D perovskites used in most optoelectronic applications contain many grains oriented either horizontally or vertically with respect to the substrate on which they grow; the orientation of such grains plays important roles, not yet fully understood, in their functionalities. Techniques to map the grain orientation in situ in 2D perovskite thin films are hence highly desirable. Here we report mapping of the grain orientation of thin films of the horizontal 2D perovskite, BA2PbI4, in which BA is butylammonium, with low-frequency polarized Raman microspectroscopy. We established a relation between the intensities of low-frequency (10–60 cm–1) polarized Raman lines of BA2PbI4 originating primarily from the PbI6 octahedra and varied orientations of a plate-like BA2PbI4 single crystal. This relation served to determine unambiguously the orientation of grains of micrometer size in a spin-coated BA2PbI4 thin film. Although this study is a case study of BA2PbI4, it opens a new window to obtain crystallographic information about 2D perovskite thin films in general and hence to understand their properties and functions

Interface- and Temperature-Sensitive Linear Electric Field Effects on Exciton Absorption of CH₃NH₃PbI₃ Perovskite Films

The influence of applied electric field (FA) on the absorption spectra of a methylammonium lead tri-iodide (MAPbI3) crystalline film sandwiched between a fluorine-doped tin oxide (FTO) layer and a polymer film of poly(methyl methacrylate) (PMMA) (Sample I) and sandwiched between a compact layer of titanium oxide (cp-TiO2) and a PMMA film (Sample II) has been investigated by first harmonic modulation spectroscopy on various temperatures in the range of 290–60 K. The linear electroabsorption spectra, EA(1f), of the MAPbI3 film in Sample I showed very different behaviors in the temperature range above and below 200 K. EA(1f) spectra show a shape similar to the second derivative of the exciton absorption band having a Gaussian profile, and switching between positive and negative signs is generated at 290 K on alternating polarity of FA. With decreasing temperature, the same tendency of FA polarity-dependent EA(1f) switching was maintained until ∼200 K. At temperatures below 200 K, the inversion of the EA(1f) signal was found with the same field direction of FA, and the signal intensity increased with decreasing temperature. In sample II, the polarity-dependent EA(1f) signal was negligibly small at room temperature but became prominent with decreasing temperature and followed the same trend as that in Sample I observed at temperatures below 200 K, indicating that the substrate on which the MAPbI3 film was coated played a conclusive role as the origin of the polarity-dependent EA(1f) spectra. The second-derivative-like shape of the interface- and temperature-sensitive EA(1f) spectra is interpreted in terms of the polarity-dependent linear Stark shift of the exciton absorption band whose peak position depends on the temperature and substrate on which the MAPbI3 film is coated

Highly Efficient HTM-Free Tin Perovskite Solar Cells with Outstanding Stability Exceeding 10000 h

The bottleneck in the rapid development of tin-based perovskite solar cells (TPSCs) is the inherent chemical instability. Although this is being addressed continuously, the device performance has not improved further due to the use of PEDOT:PSS as the hole-transport material (HTM), which has poor long-term stability. Herein we have applied commercial ITO nanoparticles over ITO glass substrates and altered the surface chemistry of the ITO electrode via a simple two-step thermal annealing, followed by a UV–ozone treatment. These surface-modified ITO electrodes display promising interfacial characteristics, such as a suitable band alignment owing to significantly reduced surface carbon contamination, increased In–O bonding, and reduced oxygen vacancies, that enabled fabrication of an HTM-free TPSC device according to a two-step method. The fabricated device possessed an outstanding power conversion efficiency (PCE) of 9.7%, along with a superior long-term stability by retaining over 90% of the initial PCE upon shelf storage in a glovebox for a period of over 10000 h. The application of ITO nanoparticles led to effective interfacial passivation, whose impacts on the long-term durability were assessed using electrochemical impedance spectroscopy, time-resolved photoluminescence decay profiles, and femtosecond transient absorption spectroscopy techniques

Design and Characterization of Heteroleptic Ruthenium Complexes Containing Benzimidazole Ligands for Dye-Sensitized Solar Cells: The Effect of Thiophene and Alkyl Substituents on Photovoltaic Performance

We designed heteroleptic ruthenium complexes RD16–RD18 containing fluoro-substituted and thiophene-based benzimidazole ligands for dye-sensitized solar cells. Whereas the substitution of only fluorine in the RD12 device has an effect of enhancing the open-circuit voltage (VOC), additional substitution of thiophene in the RD16–RD18 sensitizers showed a slightly decreased VOC. Systematic enhanced short-circuit current density (JSC) and efficiency (η) of power conversion of the devices had the order RD18 > RD17 > RD16 > RD12 > N719, attributed to the increasing light-harvesting ability and the broadened spectral features with thiophene-based ligands. Measurements of charge extraction and intensity-modulated photovoltage spectra indicate that thiophene substitution shifts downward the TiO2 potential and accelerates charge recombination, but inclusion of a long hexyl chain on the thiophene moiety retards charge recombination to account for the variation of VOC in the series. For a duration test of device performance at ambient temperature, only ∼2% degradation of cell performance was found for the devices of RD18 and RD12 over 2000 h, but a 10% decrease in overall efficiency was observed in the N719 device

Photoisomerization Dynamics of Azobenzene in Solution with S₁ Excitation: A Femtosecond Fluorescence Anisotropy Study

Measurements of anisotropy of femtosecond fluorescence after direct excitation of the S1(n,π*) state of azobenzene in hexane and ethylene glycol solutions have been carried out to address the controversy about inversion and rotation in the mechanism of photoisomerization. The observed anisotropies in hexane decay to a nonzero asymptotic level with a relaxation period the same as that for slow decay of the corresponding biexponential transient; this effect is attributed to involvement of the out-of-plane CNNC-torsional motion on approach to a twisted conical intersection along the “rotation channel” that depolarizes the original in-plane transition moment. In contrast, when the rotational channel becomes substantially hindered in ethylene glycol, the anisotropies show no discernible decay feature, but the corresponding transients show prominent decays attributed to involvement of in-plane symmetric motions; the latter approach a planar-sloped conical intersection along a “concerted inversion channel” for efficient internal conversion through vibronic coupling. The proposed mechanism is consistent with theoretical calculations and rationalizes both results on quantum yields and ultrafast observations

Illumination Power-Dependent Electroabsorption of Excitons in a CH₃NH₃PbI₃ Perovskite Film

Electroabsorption (E-A) spectra of methylammonium lead triiodide (MAPbI3) are found to depend on the power density of the illumination light (IL-D) in both tetragonal and orthorhombic phases when the modulation frequency (M-F) of the applied electric field (F) is low. As IL-D increased, the E-A intensity decreased and the shape of the E-A spectra altered from that similar to the second derivative of the exciton absorption band having a Gaussian profile to the one similar to the first derivative, when the M-F of F is low. When the M-F of F is high, E-A spectra were independent of the power density of illumination. Orientational polarization of excitons is considered to be induced by F having a low M-F with strong photoirradiation, following ion migration of MA+ and I– along the applied field direction enhanced by photoirradiation

Label-Free Optical Microscope Based on a Phase-Modulated Femtosecond PumpProbe Approach with Subdiffraction Resolution

A far-field optical microscope (OM) is a powerful noninvasive, nondestructive tool to study sub-micrometer structures and organisms, which has been used for decades to study the interactions between light and matter in the spatial domain. We report here a sophisticated label-free OM method with superspatial resolution to visualize ZnO nanoparticles. Of three femtosecond pulses, two served as pumps at 1000 nm and the other one served as a probe at 774 nm. The two pumps (one of Gaussian shape and the other of toroidal shape) were generated with a phase difference of 180°. When the conventional pump–probe approach was used in the absence of a second toroidal pump, a ZnO nanoparticle was observed to show a particle size of 445 nm because of the limit of diffraction. In contrast, when the second toroidal pump was applied out of phase, the obtained OM image showed a ZnO nanoparticle down to 96 nm. We demonstrated for the first time that the reported phase-modulated pump–probe approach has an ability for spatial resolution beyond its optical diffraction limit and a potential for label-free imaging applications in nanomaterials and life sciences

Photoisomerization Dynamics of Azobenzene in Solution with S₁ Excitation: A Femtosecond Fluorescence Anisotropy Study

Measurements of anisotropy of femtosecond fluorescence after direct excitation of the S1(n,π*) state of azobenzene in hexane and ethylene glycol solutions have been carried out to address the controversy about inversion and rotation in the mechanism of photoisomerization. The observed anisotropies in hexane decay to a nonzero asymptotic level with a relaxation period the same as that for slow decay of the corresponding biexponential transient; this effect is attributed to involvement of the out-of-plane CNNC-torsional motion on approach to a twisted conical intersection along the “rotation channel” that depolarizes the original in-plane transition moment. In contrast, when the rotational channel becomes substantially hindered in ethylene glycol, the anisotropies show no discernible decay feature, but the corresponding transients show prominent decays attributed to involvement of in-plane symmetric motions; the latter approach a planar-sloped conical intersection along a “concerted inversion channel” for efficient internal conversion through vibronic coupling. The proposed mechanism is consistent with theoretical calculations and rationalizes both results on quantum yields and ultrafast observations

Energy and Charge Transfer Dynamics in Red-Emitting Hybrid Organo-Inorganic Mixed Halide Perovskite Nanocrystals

We report time-resolved spectral properties of highly stable and efficient red-emitting hybrid perovskite nanocrystals with the composition FA0.5MA0.5PbBr0.5I2.5 (FAMA PeNC) synthesized by using the hot-addition method. The PL spectrum of the FAMA PeNC shows a broad asymmetric band covering 580 to 760 nm with a peak at 690 nm which can be deconvoluted into two bands corresponding to the MA and FA domains. The interactions between the MA and FA domains are shown to affect the relaxation dynamics of the PeNCs from the subpicosecond to tens of nanoseconds scale. Time-correlated single-photon counting (TCSPC), femtosecond PL optical gating (FOG), and femtosecond transient absorption spectral (TAS) techniques were employed to study the intercrystal energy transfer (photon recycling) and intracrystal charge transfer processes between the MA and the FA domains of the crystals. These two processes are shown to increase the radiative lifetimes for the PLQYs exceeding 80%, which may play a key role in enhancing the performance of PeNC-based solar cells

Simultaneous Observation of an Intraband Transition and Distinct Transient Species in the Infrared Region for Perovskite Solar Cells

Solar cells based on organometal-halide perovskites such as CH₃NH₃PbI₃ have emerged as a promising next-generation photovoltaic system, but the underlying photophysics and photochemistry remain to be established because of the limited availability of methods to implement the simultaneous and direct measurement of various charge carriers and ions that play a crucial role in the operating device. We used nanosecond time-resolved infrared (IR) spectroscopy to investigate, with high molecular specificity, distinct transient species that are formed in perovskite solar cells after photoexcitation. In CH₃NH₃PbI₃ planar-heterojuction solar cells, we simultaneously observed infrared spectral signatures that are associated with an intraband transition of conduction-band electrons, Fano resonance, and the spiro-OMeTAD cation having an exceptionally short lifetime of 1.0 μs (at ∼1485 cm^–1). The present results show that the time-resolved IR method offers a unique capability to elucidate these important transients in perovskite solar cells and their dynamic interplay in a comprehensive manner