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A Direct Mapping Approach to Understand Carrier Relaxation Dynamics in Varied Regions of a Polycrystalline Perovskite Film
(1970-01-01)
Fathi, Amir
;
Jokar, Efat
;
Lee, Yuan-Pern
;
Diau, Eric Wei-Guang
;
交大名義發表
We developed a direct mapping approach to overlay the image of a polycrystalline perovskite film obtained from the transient absorption microscope (TAM) with that from the scanning electron microscope (SEM). By mapping these imaging data pixel by pixel, we are able to observe the relaxation dynamics of the photo-generated charge carriers on varied regions of the film. The carrier relaxation dynamics contain a dominated single-exponential decay component owing to the recombination of charge carriers. The lifetime distribution of charge recombination shows a bimodal feature, for which the rapid and slow distributions are assigned as free and trapped carriers, respectively. The charge recombination was slower in the grain boundary (GB) region than in the grain interior (GI) region. The small grains have longer lifetimes than the large grains for the crystal size smaller than 500 nm. Therefore, GB with retarded charge recombination might play a positive role in a perovskite solar cell.
Realizing a Cosolvent System for Stable Tin-Based Perovskite Solar Cells Using a Two-Step Deposition Approach
(2020-08-14)
Shahbazi, Saeed
;
Li, Meng-Yu
;
Fathi, Amir
;
Diau, Eric Wei-Guang
;
交大名義發表
Applying a two-step procedure and solvent engineering, we fabricated a stable tin-based perovskite, formamidinium tin triiodide (FASnI(3)), solar cell for lead-free photovoltaic applications. The first step was deposition of a SnI2 layer with the solvent dimethyl sulfoxide; the second step was application of a cosolvent system containing hexafluoro-2-propanol (HFP), isopropyl alcohol (IPA), and chlorobenzene (CB) in a 5:5:2 ratio to deposit the FAI layer on the SnI2 layer. The traditional IPA solvent prevented the formation of a stable FASnI(3) layer such that a stable device could not be fabricated. HFP was hence used to form hydrogen bonds with IPA and FM to retard the crystal growth of FASnI(3); CB served as the antisolvent. Ethylenediammonium dihypophosphite in the first step was an effective reducing agent to increase the efficiency of power conversion from similar to 5% to similar to 7% with great reproducibility and stability over 4000 h.
Application of a natural antioxidant as an efficient strategy to decrease the oxidation in Sn-based perovskites
(2020-12-15)
Mohammadian-Sarcheshmeh, Hamideh
;
Mazloum-Ardakani, Mohammad
;
Rameez, Mohammad
;
Shahbazi, Saeed
;
Diau, Eric Wei-Guang
This study aims at the application of uric acid (UA), as a natural antioxidant, an inexpensive and available additive, in the fabrication of Sn-perovskite solar cells (Sn-PSCs). First, we fabricate a carbon structure Sn-PSC without the HTM layer which is more economical and environmentally friendly. Then, we investigate the effect of a UA additive on their performance. An important problem in Sn-PSCs is tin oxidation, producing p-type doping and Sn vacancies in the cells. All of these lead to serious device deterioration and low performance. In this case, adding UA helps to improve the photovoltaic parameters and, in particular, the device performance as a result of an effective decrease in Sn2+ oxidation and carrier recombination. Also, the device stability improves, in comparison to devices not treated with UA. According to the findings of this study, the UA can replace some expensive additives to prevent tin oxidation and significantly improve photovoltaic parameters. (C) 2020 Elsevier B.V. All rights reserved.
Femtosecond Transient Absorption Spectra and Dynamics of Carrier Relaxation of Tin Perovskites in the Absence and Presence of Additives
(2020-07-16)
Narra, Sudhakar
;
Jokar, Efat
;
Pearce, Orion
;
Lin, Chia-Yi
;
Fathi, Amir
The effects of additives SnF2 (10%) and EDAI(2) (1%) on the dynamics of carrier relaxation of formamidinium tin triiodide (FASnI(3)) perovskite were studied using femtosecond transient absorption spectra (TAS) with excitation at 600 and 870 nm. The TAS were analyzed according to a parallel sequential kinetic model with a global fit through singular-value decomposition. For excitation at 600 nm, two relaxation paths were found: one involved hot and cold carriers in the bulk state undergoing shallow bulk-defect-mediated charge recombination; the other involved trap carriers in the surface state undergoing deep surface-defect-mediated charge recombination. For excitation at 870 nm, only cold carriers were subjected to the bulk-state relaxation channel. Our spectral results indicate significant effects of the additives on retarding charge recombination in both bulk and surface states as well as decreasing the bandgap renormalization energy, the bandwidth of the photobleaching (PB) band, and the Stokes shift between the PB and photoluminescence bands, explaining how the device performance of FASnI(3) solar cells became enhanced in the presence of SnF2 and EDAI(2).
Switching between spectral broadening and narrowing of the exciton absorption band of a CH3NH3PbI3 film on altering the polarity of an applied electric field
(2020-06-22)
Awasthi, Kamlesh
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Kala, Kannankutty
;
Rana, Shailesh
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
Electric field-induced broadening and narrowing of an exciton absorption band have been observed to depend on the polarity of the applied electric field for methylammonium lead triiodide perovskite (MAPbI(3)) as a thin film sandwiched between a conducting film of fluorine-doped tin oxide (FTO) and an insulating film of poly(methyl methacrylate) (PMMA). The width of the absorption band increased or decreased when the direction of the applied field was toward FTO or PMMA, respectively, indicating that the spectral broadening and narrowing become switchable on altering the direction of the applied electric field. When titanium oxide (TiO2) in a compact layer was introduced between the MAPbI(3) and FTO layers, the linear electric field effect that depended on the polarity was either not observed or decreased significantly although the quadratic field effects on the exciton absorption band were similarly observed with or without the TiO2 layer.
Inter- and Intragrain Inhomogeneity in 2D Perovskite Thin Films Revealed by Relative Grain Orientation Imaging Using Low-Frequency Polarized Raman Microspectroscopy
(2020-05-21)
Toda, Shogo
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Yanagita, Naoya
;
Jokar, Effat
;
Diau, Eric Wei-Guang
;
Shigeto, Shinsuke
Two-dimensional (2D) organic-inorganic hybrid lead halide perovskites make up an emerging class of semiconductor materials for optoelectronic applications such as solar cells. The grain structure of polycrystalline 2D perovskites is one of the key factors that dictate their functionality in the devices, but currently available methods for in situ, chemically specific characterization of 2D perovskite grains are scarce. Here we show that ultra-low-frequency polarized Raman microspectroscopy is a facile yet powerful tool for visualizing relative grain orientations within 2D perovskite thin films. We demonstrate this method on the simplest 2D perovskite, (CH3(CH2)(3)NH3)(2)PbI4. Hierarchical clustering and detailed band decomposition analysis of the low-frequency polarized Raman imaging data reveal not only relative grain orientations but also intragrain inhomogeneity. We envisage that with high chemical specificity, this method will find broad applications ranging from other 2D perovskites to perovskite-based optoelectronic devices.
Development of Hybrid Pseudohalide Tin Perovskites for Highly Stable Carbon-Electrode Solar Cells
(2020-05-13)
Rameez, Mohammad
;
Lin, Eric Yan-Ru
;
Raghunath, Putikam
;
Narra, Sudhakar
;
Song, Donghoon
Tin-based perovskites degrade rapidly upon interaction with water and oxygen in air because Sn-I bonds are weak. To address this issue, we developed novel tin perovskites, FASnI((3-x))(SCN)(x) (x = 0, 1, 2, or 3), by employing a pseudohalide, thiocyanate (SCN-), as a replacement for halides and as an inhibitor to suppress the Sn2+/Sn4+ oxidation. The structural and electronic properties of pseudohalide tin perovskites in this series were explored with quantum-chemical calculations by employing the plane-wave density functional theory (DFT) method; the corresponding results are consistent with the experimental results. Carbon-based perovskite devices fabricated with tin perovskite FASnI(SCN)(2) showed about a threefold enhancement of the device efficiency (2.4%) relative to that of the best FASnI(3)-based device (0.9%), which we attribute to the improved suppression of the formation of Sn4+, retarded charge recombination, enhanced hydrophobicity, and stronger interactions between Sn and thiocyanate for FASnI(SCN)(2) than those for FASnI(3). After the incorporation of phenylethyleneammonium iodide (PEAI, 10%) and ethylenediammonium diiodide (EDAI(2), 5%) as coadditives, the FASnI(SCN)(2) device gave the best photovoltaic performance with J(SC) = 20.17 mA cm(-2), V-OC = 322 mV, fill factor (FF) = 0.574, and overall efficiency of power conversion PCE = 3.7%. Moreover, these pseudohalide-containing devices display negligible photocurrent-voltage hysteresis and great stability in ambient air conditions.
Development of Novel Mixed Halide/Superhalide Tin-Based Perovskites for Mesoscopic Carbon-Based Solar Cells
(2020-04-02)
Rameez, Mohammad
;
Shahbazi, Saeed
;
Raghunath, Putikam
;
Lin, Ming Chang
;
Hung, Chen Hsiung
Tin perovskites suffer from poor stability and a self-doping effect. To solve this problem, we synthesized novel tin perovskites based on superhalide with varied ratios of tetrafluoroborate to iodide and implemented them into solar cells based on a mesoscopic carbon-electrode architecture because film formation was an issue in applying this material for a planar heterojunction device structure. We undertook quantum-chemical calculations based on plane-wave density functional theory (DFT) methods and explored the structural and electronic properties of tin perovskites FASnI(3-x)(BF4)(x) in the series x = 0, 1, 2, and 3. We found that only the x = 2 case, FASnI(BF4)(2), was successfully produced, beyond the standard FASnI(3). The electrochemical impedance and X-ray photoelectron spectra indicate that the addition of tin tetrafluoroborate instead of SnI2 suppressed trap-assisted recombination by decreasing the Sn4+ content. The power conversion efficiency of the FASnI(BF4)(2) device with FAI and Sn(BF4)(2) in an equimolar ratio improved 72% relative to that of a standard FASnI(3) solar cell, with satisfactory photostability under ambient air conditions.
Label-Free Optical Microscope Based on a Phase-Modulated Femtosecond Pump-Probe Approach with Subdiffraction Resolution
(2020-03-18)
Fathi, Amir
;
Chung, Chao-Yu
;
Lee, Yuan-Pern
;
Diau, Eric Wei-Guang
;
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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 degrees. 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.
Efficiency and Stability Enhancement of Fully Ambient Air Processed Perovskite Solar Cells Using TiO2 Paste with Tunable Pore Structure
(1970-01-01)
Seyed-Talebi, Seyedeh Mozhgan
;
Kazeminezhad, Iraj
;
Shahbazi, Saeed
;
Diau, Eric Wei-Guang
;
交大名義發表
Crystallization and nucleation of the perovskite layer in the mesoscopic perovskite solar cells (PSCs) depend on the nucleation sites of the electron transport layer (ETL). The porosity optimization of TiO2 film as an efficient ETL plays an important role in the performance improvement of PSCs. In the present study, nontoxic carbon spheres synthesized with uniform morphology and controllable size under hydrothermal conditions and used as a template to generate the tunable porous TiO2 films. Furthermore, the effect of porosity modification of TiO2 on the formation of perovskite films with large grain size is studied in an ambient atmosphere with humidity higher than 50%. The best TiO2 film is produced with carbon spheres 8 wt% (C8), which results in the formation of a pinhole-free, and compact-packed perovskite layer. The fully air processed PSC device with the ETL made of C8 film exhibits an efficiency of 16.66% with reduced hysteresis, which is much superior in performance compared to the standard cell (11.72%). It is believed that this porosity optimization of TiO2 layer is a simple practical strategy for improved stability of fully air processed efficient perovskite solar cells and usable for the fabrication of reproducible compact perovskite layers in uncontrolled laboratories.

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