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1. Ultrafast dynamics of exciton formation and decay in two-dimensional tungsten disulfide (2D-WS 2 ) monolayers

   https://doi.org/10.1039/d0cp03220d  

   Eroglu, Zeynep Ezgi ; Comegys, Olivia ; Quintanar, Leo S. ; Azam, Nurul ; Elafandi, Salah ; Mahjouri-Samani, Masoud ; Boulesbaa, Abdelaziz ( August 2020 , Physical Chemistry Chemical Physics)

   null (Ed.)

   Excitons in two-dimensional transition metal dichalcogenide monolayers (2D-TMDs) are of essential importance due to their key involvement in 2D-TMD-based applications. For instance, exciton dissociation and exciton radiative recombination are indispensible processes in photovoltaic and light-emitting devices, respectively. These two processes depend drastically on the photogeneration efficiency and lifetime of excitons. Here, we incorporate femtosecond pump–probe spectroscopy to investigate the ultrafast dynamics of exciton formation and decay in a single crystal of monolayer 2D tungsten disulfide (WS 2 ). Investigation of the formation dynamics of the lowest exciton (X A ) indicated that the formation time linearly increases from ∼150 fs upon resonant excitation, to ∼500 fs following excitation that is ∼1.1 eV above the band-gap. This dependence is attributed to the time it takes highly excited electrons in the conduction band (CB) to relax to the CB minimum (CBM) and contribute to the formation of X A . This is confirmed by infrared measurements of electron intraband relaxation dynamics. Furthermore, pump–probe experiments suggested that the X A ground state depletion recovery dynamics depend on the excitation energy as well. The average recovery time increased from ∼10 ps in the case of resonant excitation to ∼50 ps following excitation well above the band-gap. Having the ability to control whether generating short-lived or long-lived electron–hole pairs in 2D-TMD monolayers opens a new horizon for the application of these materials. For instance, long-lived electron–hole pairs are appropriate for photovoltaic devices, but short-lived excitons are more beneficial for lasers with ultrashort pulses. 

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2. Probing the Spatial Heterogeneity of Carrier Relaxation Dynamics in CH 3 NH 3 PbI 3 Perovskite Thin Films with Femtosecond Time‐Resolved Nonlinear Optical Microscopy

   https://doi.org/10.1002/adom.201901185  

   Yu, Jin ; Li, Zhongguo ; Liao, Yuheng ; Kolodziej, Charles ; Kuyuldar, Seher ; Warren, Warren S. ; Burda, Clemens ; Fischer, Martin C. ( August 2019 , Advanced Optical Materials)

   The spatial heterogeneity of carrier dynamics in polycrystalline metal halide perovskite (MHP) thin films has a strong influence on photovoltaic device performance; however, the underlying cause is not yet clearly understood. Here, the sub‐micrometer scale mapping of charge carrier dynamics in CH₃NH₃PbI₃thin films using time‐resolved nonlinear optical microscopy, specifically transient absorption microscopy (TAM) with sub‐picosecond (ps) and time‐resolved photoluminescence (PL) microscopy with nanosecond temporal resolution is reported. To study the influence of physical morphology on charge carrier dynamics, MHP thin films having granular‐ and fibrous structures are investigated. On both types of films, spatial regions with short‐lived transient gain signals (fast nonradiative relaxation within ≈1 ps) typically show slower charge recombination via radiative relaxation, which is attributed to the presence of additional energy states near the band edge. In addition, fibrous films show longer PL lifetimes. Interestingly, the functional contrast shown in TAM images exhibits fundamental differences from the structural contrast shown in scanning electron microscopy images, implying that the variation of trap density in the bulk contributes to the observed spatial heterogeneity in carrier dynamics.

    

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3. Nanoscale Local Contacts Enable Inverted Inorganic Perovskite Solar Cells with 20.8 % Efficiency

   https://doi.org/10.1002/anie.202400018  

   Wang, Sanlong ; Qi, Shanshan ; Sun, Hongrui ; Wang, Pengyang ; Zhao, Ying ; Zhang, Xiaodan ( March 2024 , Angewandte Chemie International Edition)

   Inorganic perovskite solar cells (IPSCs) have gained significant attention due to their excellent thermal stability and suitable band gap (~1.7 eV) for tandem solar cell applications. However, the defect‐induced non‐radiative recombination losses, low charge extraction efficiency, energy level mismatches, and so on render the fabrication of high‐efficiency inverted IPSCs remains challenging. Here, the use of 3‐amino‐5‐bromopyridine‐2‐formamide (ABF) in methanol was dynamically spin‐coated on the surface of CsPbI_2.85Br_0.15film, which facilitates the limited etching of defect‐rich subsurface layer, resulting in the formation of vertical PbI₂nanosheet structures. This enabled localized contacts between the perovskite film and the electron transport layer, suppress the recombination of electron‐hole and beneficial to electron extraction. Additionally, the C=O and C=N groups in ABF effectively passivated the undercoordinated Pb²⁺at grain boundaries and on the surface of CsPbI_2.85Br_0.15film. Eventually, we achieved a champion efficiency of 20.80 % (certified efficiency of 20.02 %) for inverted IPSCs with enhanced stability, which is the highest value ever reported to date. Furthermore, we successfully prepared p‐i‐n type monolithic inorganic perovskite/silicon tandem solar cells (IPSTSCs) with an efficiency of 26.26 %. This strategy provided both fast extraction and efficient passivation at the electron‐selective interface.

    

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4. One‐Photon Excitation Followed by a Three‐Step Sequential Energy–Energy–Electron Transfer Leading to a Charge‐Separated State in a Supramolecular Tetrad Featuring Benzothiazole–Boron‐Dipyrromethene–Zinc Porphyrin–C 60

   https://doi.org/10.1002/chem.202004262  

   Badgurjar, Deepak ; Seetharaman, Sairaman ; D'Souza, Francis ; Chitta, Raghu ( December 2020 , Chemistry – A European Journal)

   A panchromatic triad, consisting of benzothiazole (BTZ) and BF₂‐chelated boron‐dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using¹H NMR spectroscopy, ESI‐MS, UV‐visible, steady‐state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ‐BODIPY‐ZnP, and dyads, BTZ‐BODIPY and BODIPY‐ZnP, along with the reference compounds BTZ‐OMe, BODIPY‐OMe, and ZnP‐OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ‐BODIPY‐ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ‐BODIPY‐ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from¹BTZ^*to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the¹BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two‐step singlet–singlet energy transfer. Further, a supramolecular tetrad, BTZ‐BODIPY‐ZnP:ImC₆₀, was formed by axially coordinating the triad with imidazole‐appended fulleropyrrolidine (ImC₆₀), and parallel steady‐state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from¹ZnP* to ImC₆₀. Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely,¹BTZ^*‐BODIPY‐ZnP:ImC₆₀→BTZ‐¹BODIPY^*‐ZnP:ImC₆₀→BTZ‐BODIPY‐¹ZnP^*:ImC₆₀→BTZ‐BODIPY‐ZnP^.+:ImC₆₀^.−in the supramolecular tetrad. The evaluated rate of energy transfer,k_EnT, was found to be 3–5×10¹⁰ s⁻¹, which was slightly faster than that observed in the case of BODIPY‐ZnP and BTZ‐BODIPY‐ZnP, lacking the coordinated ImC₆₀. The rate constants for charge separation and recombination,k_CSandk_CR, respectively, calculated by monitoring the rise and decay of C₆₀^.−were found to be 5.5×10¹⁰and 4.4×10⁸ s⁻¹, respectively, for the BODIPY‐ZnP:ImC₆₀triad, and 3.1×10¹⁰and 4.9×10⁸ s⁻¹, respectively, for the BTZ‐BODIPY‐ZnP:ImC₆₀tetrad. Initial excitation of the tetrad, promoting two‐step energy transfer and a final electron‐transfer event, has been successfully demonstrated in the present study.

    

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5. Passivating the interface between halide perovskite and SnO 2 by capsaicin to accelerate charge transfer and retard recombination

   https://doi.org/10.1063/5.0082785  

   Lin, Siyuan ; Xia, Pufeihong ; Wu, Shuyue ; Zhang, Wenhao ; Hu, Yue ; Liu, Biao ; Kong, Deming ; Huang, Han ; Gao, Yongli ; Zhou, Conghua ( March 2022 , Applied Physics Letters)

   Capsaicin is used to modify SnO 2 quantum dots and then used as an electron-transfer material for perovskite solar cells. After capsaicin modification, the power conversion efficiency of the devices increases from 19.90 (± 0.47)% to 21.87 (± 0.28)% with a champion device of 22.24% (AM 1.5G, 100 mW/cm 2 ). Transient photovoltage and photocurrent decay show that, after the capsaicin doping, the lifetime increases from 21.55 (± 1.54) to 27.63 (± 1.45)  μs, while the charge extraction time reduces from 1.90 (± 0.09) to 1.67 (± 0.06)  μs. Time-resolved photoluminescence and impedance spectrum studies show similar results. The accelerated charge transfer and retarded recombination are due to defect passivation. Space charge limited current study shows that, after modification, the trap density of devices is reduced from 2.24 × 10 15 to 1.28 × 10 15  cm −3 . X-ray photoelectron spectroscopy and theoretical calculation indicate that the reduced trap density is due to the chemical interaction between carbonyl group (from capsaicin) and Sn atom, and that between carbonyl group and Pb atom. 

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