Noticeable Chiral Center Dependence of Signs and Magnitudes in Circular Dichroism (CD) and Circularly Polarized Luminescence (CPL) Spectra of <i>all</i>-<i>trans</i>-Poly(9,9-dialkylfluorene-2,7-vinylene)s Bearing Chiral Alkyl Side Chains in Solution, Aggregates, and Thin Films

Effects of chiral alkyl side chains in poly­(9,9-dialkyl-fluorene-2,7-vinylene)­s [PFVs, (<i>S</i>)-3-methylpentyl (3mpe), (<i>S</i>)-4-methylhexyl (4mhex), (<i>S</i>)-5-methylheptyl (5mhep), (<i>S</i>)-6-methyloctyl (6moct), and (<i>S</i>)-3,7-dimethyloctyl (dmo)] toward aggregation-induced circular dichroism (AICD) and circularly polarized luminescence (AICPL) and CD/CPL spectra in solution and in thin film have been explored. The (<i>all</i>-<i>trans</i>) PFV samples with similar conjugation repeat units containing well-defined (vinyl) end groups were prepared by acyclic diene metathesis polymerization using Ru catalyst. The PFV aggregates, prepared <i>in situ</i> in a mixed solution of CHCl₃/MeOH, showed clear CD signals ascribed to AICD, whereas these samples showed CD-silent in the CHCl₃ solution. The absolute magnitude (<i>g</i>_CD value) was affected by the chiral side chains (without obey the even–odd rule) and increased in the order 3mpe, 5mhep < 4mhex < dmo < 6mcot. Both the 6-moct and dmo aggregates showed clear CPL signals ascribed to AICPL, whereas signals of the others were not obvious. The λ_max values in the UV–vis spectra red-shifted depending upon kind of alkyl side chains due to formation of <i>J</i>-type aggregates. These results suggest that the optically active aggregates adopt certain helical supramolecular ordered structures induced by an interpolymer interaction through chain entanglement. The <i>g</i>_CD values in the drop casted thin film (prepared from the CHCl₃ solution) were lower than those in the aggregates, and the value increased in the order 5mhep, dmo < 4mhex, 6mcot < 3mpe. The λ_max values in their UV–vis spectra red-shifted but were not affected by the side chain. These results suggest that supramolecular structures formed by aggregate and film are different, and the formation in film could be induced by an interpolymer π-stacking. In contrast, the basic characteristics were preserved in the thin film prepared from the PFV-6moct aggregate (CHCl₃/MeOH); the film showed high both <i>g</i>_CD and <i>g</i>_CPL values close to those in the original aggregate

Interfacial Charge-Carrier Trapping in CH₃NH₃PbI₃‑Based Heterolayered Structures Revealed by Time-Resolved Photoluminescence Spectroscopy

The fast-decaying component of photoluminescence (PL) under very weak pulse photoexcitation is dominated by the rapid relaxation of the photoexcited carriers into a small number of carrier-trapping defect states. Here, we report the subnanosecond decay of the PL under excitation weaker than 1 nJ/cm² both in CH₃NH₃PbI₃-based heterostructures and bare thin films. The trap-site density at the interface was evaluated on the basis of the fluence-dependent PL decay profiles. It was found that high-density defects determining the PL decay dynamics are formed near the interface between CH₃NH₃PbI₃ and the hole-transporting Spiro-OMeTAD but not at the CH₃NH₃PbI₃/TiO₂ interface and the interior regions of CH₃NH₃PbI₃ films. This finding can aid the fabrication of high-quality heterointerfaces, which are required improving the photoconversion efficiency of perovskite-based solar cells

Photocarrier Recombination and Injection Dynamics in Long-Term Stable Lead-Free CH₃NH₃SnI₃ Perovskite Thin Films and Solar Cells

We investigated the near-band-edge optical responses and photocarrier dynamics of encapsulated long-term stable CH₃NH₃SnI₃ (MASnI₃) thin films and solar-cell devices. The MASnI₃ thin film prepared with SnF₂ exhibited a bandgap of 1.25 eV, while the film without SnF₂ had a significantly blueshifted absorption edge. On the contrary, the PL peak energies were not influenced by the addition of SnF₂. These observations indicate that the blueshift of the absorption edge in the SnF₂-free MASnI₃ sample is due to the Burstein–Moss shift induced by a significant unintentional hole doping. Furthermore, time-resolved photoluminescence measurements revealed that by adding SnF₂ the photocarrier lifetime of the film increased by one order of magnitude, which enables improved device performance of solar cells. We clarified that in the MASnI₃ solar cells the short-circuit current stays significantly below the ideal value due to a large nonradiative recombination rate in the perovskite layer, resulting in a small photocarrier-injection efficiency into the charge-transport layers

Dynamic Optical Properties of CH₃NH₃PbI₃ Single Crystals As Revealed by One- and Two-Photon Excited Photoluminescence Measurements

The dynamic optical properties of perovskite CH₃NH₃PbI₃ single crystals were studied by means of time-resolved photoluminescence (PL) spectroscopy at room temperature. The PL peak under one-photon excitation exhibits a red-shift with elapsing time, while two-photon PL is time-independent and appears at lower energy levels. The low-energy two-photon PL can be attributed to emissions from the localized states because of strong band-to-band absorption and photon re-absorption of the emitted light in the interior region. We revealed that the PL behaviors can be explained by the diffusion of photocarriers generated in the near-surface region to the interior region. The excitation fluence dependence of the one-photon PL dynamics is also discussed in terms of the electron–hole radiative recombination and carrier diffusion effects

Formation of M‑Like Intermediates in Proteorhodopsin in Alkali Solutions (pH ≥ ∼8.5) Where the Proton Release Occurs First in Contrast to the Sequence at Lower pH

Proteorhodopsin (PR) is an outward light-driven proton pump observed in marine eubacteria. Despite many structural and functional similarities to bacteriorhodopsin (BR) in archaea, which also acts as an outward proton pump, the mechanism of the photoinduced proton release and uptake is different between two H⁺-pumps. In this study, we investigated the pH dependence of the photocycle and proton transfer in PR reconstituted with the phospholipid membrane under alkaline conditions. Under these conditions, as the medium pH increased, a blue-shifted photoproduct (defined as M_a), which is different from M, with a p<i>K</i>_a of ca. 9.2 was produced. The sequence of the photoinduced proton uptake and release during the photocycle was inverted with the increase in pH. A p<i>K</i>_a value of ca. 9.5 was estimated for this inversion and was in good agreement with the p<i>K</i>_a value of the formation of M_a (∼9.2). In addition, we measured the photoelectric current generated by PRs attached to a thin polymer film at varying pH. Interestingly, increases in the medium pH evoked bidirectional photocurrents, which may imply a possible reversal of the direction of the proton movement at alkaline pH. On the basis of these findings, a putative photocycle and proton transfer scheme in PR under alkaline pH conditions was proposed

Composition–Property Mapping in Bromide-Containing Tin Perovskite Using High-Purity Starting Materials

The wide band gaps of bromide-containing tin perovskites, ASnI3–xBrx, make them attractive materials for use as the top-layer absorber in tandem solar cells, as well as in single-junction solar cells for indoor applications. In the present work, a series of ASnI3–xBrx films was systematically fabricated by varying the A-site (FA+, MA+, and Cs+) and X-site (I– and Br–) ions. The use of a solvent-coordinated SnBr2 complex as a high-purity source of bromide combined with Sn(IV) scavenging treatment helps to ensure that the optimal film quality across the compositional space is realized. The energy levels and electronic properties of the films were characterized by photoemission yield spectroscopy and photoluminescence measurements. The films with long photoluminescence lifetime and favorable energy level alignment resulted in superior device efficiency when evaluated in standard single-junction solar cells. The best power conversion efficiency of 7.74% was obtained when the composition was FA0.75MA0.25SnI2.25Br0.75

Composition–Property Mapping in Bromide-Containing Tin Perovskite Using High-Purity Starting Materials

The wide band gaps of bromide-containing tin perovskites, ASnI3–xBrx, make them attractive materials for use as the top-layer absorber in tandem solar cells, as well as in single-junction solar cells for indoor applications. In the present work, a series of ASnI3–xBrx films was systematically fabricated by varying the A-site (FA+, MA+, and Cs+) and X-site (I– and Br–) ions. The use of a solvent-coordinated SnBr2 complex as a high-purity source of bromide combined with Sn(IV) scavenging treatment helps to ensure that the optimal film quality across the compositional space is realized. The energy levels and electronic properties of the films were characterized by photoemission yield spectroscopy and photoluminescence measurements. The films with long photoluminescence lifetime and favorable energy level alignment resulted in superior device efficiency when evaluated in standard single-junction solar cells. The best power conversion efficiency of 7.74% was obtained when the composition was FA0.75MA0.25SnI2.25Br0.75

Composition–Property Mapping in Bromide-Containing Tin Perovskite Using High-Purity Starting Materials

The wide band gaps of bromide-containing tin perovskites, ASnI3–xBrx, make them attractive materials for use as the top-layer absorber in tandem solar cells, as well as in single-junction solar cells for indoor applications. In the present work, a series of ASnI3–xBrx films was systematically fabricated by varying the A-site (FA+, MA+, and Cs+) and X-site (I– and Br–) ions. The use of a solvent-coordinated SnBr2 complex as a high-purity source of bromide combined with Sn(IV) scavenging treatment helps to ensure that the optimal film quality across the compositional space is realized. The energy levels and electronic properties of the films were characterized by photoemission yield spectroscopy and photoluminescence measurements. The films with long photoluminescence lifetime and favorable energy level alignment resulted in superior device efficiency when evaluated in standard single-junction solar cells. The best power conversion efficiency of 7.74% was obtained when the composition was FA0.75MA0.25SnI2.25Br0.75