Browsing by Author "Jokar, Efat"

Now showing 1 - 15 of 15

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.
Anomalous Charge-Extraction Behavior for Graphene-Oxide (GO) and Reduced Graphene-Oxide (rGO) Films as Efficient p-Contact Layers for High-Performance Perovskite Solar Cells
(2018-01-25)
Jokar, Efat
;
Huang, Zhong Yi
;
Narra, Sudhakar
;
Wang, Chi-Yung
;
Kattoor, Vidya
Reduced graphene oxides (rGO) are synthesized via reduction of GO with reducing agents as a hole-extraction layer for high-performance inverted planar heterojunction perovskite solar cells. The best efficiencies of power conversion (PCE) of these rGO cells exceed 16%, much greater than those made of GO and poly(3,4-ethenedioxythiophene):poly(styrenesulfonate) films. A flexible rGO device shows PCE 13.8% and maintains 70% of its initial performance over 150 bending cycles. It is found that the hole-extraction period is much smaller for the GO/methylammonium lead-iodide perovskite (PSK) film than for the other rGO/PSK films, which contradicts their device performances. Photoluminescence and transient photoelectric decays are measured and control experiments are performed to prove that the reduction of the oxygen-containing groups in GO significantly decreases the ability of hole extraction from PSK to rGO and also retards the charge recombination at the rGO/PSK interface. When the hole injection from PSK to GO occurs rapidly, hole propagation from GO to the indium-doped tin oxide (ITO) substrate becomes a bottleneck to overcome, which leads to a rapid charge recombination that decreases the performance of the GO device relative to the rGO device.
Control of Crystal Structures and Optical Properties with Hybrid Formamidinium and 2-Hydroxyethylammonium Cations for Mesoscopic Carbon-Electrode Tin-Based Perovskite Solar Cells
(2018-09-01)
Tsai, Cheng-Min
;
Lin, Yu-Pei
;
Pola, Murali Krishna
;
Narra, Sudhakar
;
Jokar, Efat
Alcohol-based bifunctional ammonium cations, 2-hydroxyethylammonium (HEA(+)), HO(CH2)(2)NH3+, were introduced into formamidinium (FA(+)) tin-based perovskites (HEA(x)FA(1-x)Snl(3); x = 0-1) to absorb light in carbon-based mesoscopic solar cells. We found that HEAP cations play a key role to control the crystal structures, the lattice structures altered from orthorhombic (x = 0) to rhombohedral (x = 0.2-0.4) with greater symmetry. When x was increased to 0.6-1.0, tin and iodide vacancies were formed to generate 3D-vacant perovskites (HEA(x)FA(1-x)Sn(0.6)I(2.33), x >= 0.6) with a tetragonal structure. Tin-based perovskites in this series were fabricated into mesoporous solar cells using one-step drop-cast (DC), two-step solvent-extraction (SE), and SE + 3% ethyl-enediammonium diiodide (EDAI(2)) as an additive. After optimization of device performance with the SE + 3% EDAI2 approach, the HEA(0.4)FA(0.6)SnI(3) (HEAT = 40%) device gave the best photovoltaic performance with J(sc) = 18.52 mA cm(-2)) V-OC 371 mV, FF = 0.562, and overall efficiency 17 = 3.9% after the device was stored for a period of 340 h.
Enhanced Dissociation of Hot Excitons with an Applied Electric Field under Low-Power Photoexcitation in Two-Dimensional Perovskite Quantum Wells
(2019-08-15)
Kattoor, Vidya
;
Awasthi, Kamlesh
;
Jokar, Efat
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
The dependence of photoluminescence (PL) on excitation power and the effect of an external electric field have been studied for a two-dimensional (2D) perovskite (C4H9NH3)(2)PbI4 thin-film sample. The efficiency of dissociation of hot excitons to produce free carriers was enhanced with a small excitation power because the relaxation of hot excitons to cold emissive excitons was slow, indicating that the thermal energies of hot carriers can be utilized in solar cells under weak photoirradiation. The dissociation was notably enhanced with an applied electric field, resulting in efficient field-induced quenching of the PL. The present results shed light on an application of 2D perovskite materials to photovoltaic (PV) devices with dim radiation, e.g., for indoor PV applications; the concept of electric field-assisted solar cells might be applicable to next-generation solar cells.
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).
Functionalization of Graphene Oxide Films with Au and MoOx Nanoparticles as Efficient p-Contact Electrodes for Inverted Planar Perovskite Solar Cells
(2018-09-12)
Bhosale, Sumit S.
;
Jokar, Efat
;
Fathi, Amir
;
Tsai, Cheng-Min
;
Wang, Chi-Yung
A graphene oxide (GO) film is functionalized with metal (Au) and metal-oxide (MoOx) nanoparticles (NPs) as a hole-extraction layer for high-performance inverted planar-heterojunction perovskite solar cells (PSCs). These NPs can increase the work function of GO, which is confirmed with X-ray photoelectron spectra, Kelvin probe force microscopy, and ultraviolet photoelectron spectra measurements. The down-shifts of work functions lead to a decreased level of potential energy and hence increased V-oc of the PSC devices. Although the GO-AuNP film shows rapid hole extraction and increased V-oc, a J(sc) improvement is not observed because of localization of the extracted holes inside the AuNP that leads to rapid charge recombination, which is confirmed with transient photoelectric measurements. The power conversion efficiency (PCE) of the GO-AuNP device attains 14.6%, which is comparable with that of the GO-based device (14.4%). In contrast, the rapid hole extraction from perovskite to the GO-MoOx layer does not cause trapping of holes and delocalization of holes in the GO film accelerates rapid charge transfer to the indium tin oxide substrate; charge recombination in the perovskite/GO-MoOx interface is hence significantly retarded. The GO-MoOx device consequently shows significantly enhanced V-oc and J(sc), for which its device performance attains PCE of 16.7% with great reproducibility and enduring stability.
Hole-extraction and photostability enhancement in highly efficient inverted perovskite solar cells through carbon dot-based hybrid material
(2019-08-01)
Benetti, Daniele
;
Jokar, Efat
;
Yu, Che-Hsun
;
Fathi, Amir
;
Zhao, Haiguang
We report the effect of the integration of carbon dots (Cdots) in high-performance inverted planar-heterojunction (PHJ) perovskite solar cells (PSCs). We used Cdots to modify the hole-transport layer in planar PSC devices. By introducing Cdots on graphene oxide (GO) as hole-transporting layer, the efficiency of the PSC improved significantly from 14.7% in the case of bare GO to 16.2% of the best device with optimized Cdots content. When applying Cdots with an engineered absorption in the UV range as downshifting layer, the device performance was further improved, attaining a maximum PCE of 16.8% (+ 14%); the stability of the device was also enhanced of more than 20%. Kelvin probe force microscopy (KPFM) and cyclic voltammetry (CV) were employed to analyze the electronic band alignment at the interface between GO/Cdots and the perovskite film. Holes were extracted and transferred to the conductive substrate more efficiently in the presence of Cdots, thus delaying charge recombination. Photoluminescence (PL), transient PL decays and transient photovoltage (TPV) decays investigated the charge-transfer kinetics and proved the retardation of charge recombination. This work reveals an effective enhancement of the performance of planar PSCs by using Cdots/GO as hole transport material.
Integral Method Analysis of Electroabsorption Spectra and Electrophotoluminescence Study of (C4H9NH3)(2)Pbl(4) Organic-Inorganic Quantum Well
(2018-11-22)
Kattoor, Vidya
;
Awasthi, Kamlesh
;
Jokar, Efat
;
Diau, Eric Wei-Guang
;
Ohta, Nobuhiro
Electric-field-induced changes in absorption and photoluminescence (PL) spectra and in PL decay profile have been measured for two-dimensional hybrid organic inorganic halide perovskite semiconductor, ((C4H9NH3)(2)Pbl(4)) (N1). Electroabsorption (E-A) spectra observed at room temperature and at a low temperature of 45 K were analyzed by assuming the Stark shift, and the magnitudes of the change in electric dipole moment and polarizability following photoexcitation were determined. The strong signal observed in the E-A spectra at 45 K was interpreted in terms of the weak absorption band which shows extremely large Stark shift resulting from the large change in polarizability following photoexcitation. Electrophotoluminescence spectra of this compound, that is, field-induced change in PL spectra, show that PL of N1 is quenched by the application of electric field. Field effects on PL decay profiles show that the quenching results both from the field-induced decrease of the population of the emitting state following photoexcitation and from the field-induced lifetime shortening caused by the enhancement of the nonradiative decay rate at the emitting state. At a low temperature of 45 K, two exciton emissions, each of which originates from different phase, appear, and both emissions are quenched by the applied electric field with different efficiencies from each other. It is also found that trap emissions observed at low temperature in the wavelength region longer than the sharp exciton bands show more efficient field-induced quenching than that of the exciton emissions, suggesting that energy transfer from the photoexcited state to trapped states is decelerated by the applied electric field.
Inverted planar solar cells based on perovskite/graphene oxide hybrid composites
(2017-07-21)
Chung, Chih-Chun
;
Narra, Sudhakar
;
Jokar, Efat
;
Wu, Hui-Ping
;
Diau, Eric Wei-Guang
Mixing graphene oxide (GO) with CH3NH3PbI3 (PSK) in varied proportions (0.025, 0.05 and 0.075 mg mL(-1)) and using GO nanosheets as a p-type electrode for devices with a layer-by-layer thin-film configuration ITO/GO/PSK: GO/PCBM/Ag produced hybrid composite perovskite solar cells. The efficiency of power conversion (PCE) of the device prepared with GO in PSK solution (0.05 mg mL(-1)) attained 15.2%, which is greater than that of a conventional planar heterojunction device (PCE 12.3%) fabricated using pristine PSK. In the homogeneous PSK: GO active layers, PSK acted as a hole donor and GO as a hole acceptor. The optimum GO concentration (0.05 mg mL(-1)) increased the charge mobility (investigated using Hall effect measurements), enhanced the charge separation (investigated using photoluminescence decays) and retarded the charge recombination (investigated using electrochemical impedance spectra), consistent with the trend of corresponding device performances. Our results thus indicate that the hybrid PSK: GO layer increased the interfacial contact between the donor and acceptor of holes to balance the charge mobility and improved the photovoltaic performance with excellent reproducibility and stability.
Mechanism of Photocatalytic CO2 Reduction by Bismuth-Based Perovskite Nanocrystals at the Gas-Solid Interface
(2019-12-25)
Bhosale, Sumit S.
;
Kharade, Aparna K.
;
Jokar, Efat
;
Fathi, Amir
;
Chang, Sue-min
We report here a series of nontoxic and stable bismuth-based perovskite nanocrystals (PeNCs) with applications for photocatalytic reduction of carbon dioxide to methane and carbon monoxide. Three bismuth-based PeNCs of general chemical formulas A(3)Bi(2)I(9), in which cation A(+) = Rb+ or Cs+ or CH3NH3+ (MK), were synthesized with a novel ultrasonication top-down method. PeNC of Cs3Bi2I9 had the best photocatalytic activity for the reduction of CO2 at the gas solid interface with formation yields 14.9 mu mol g(-1) of methane and 77.6 mu mol g(-1) of CO, representing a much more effective catalyst than TiO2 (P25) under the same experimental conditions. The products of the photocatalytic reactions were analyzed using a gas chromatograph coupled with a mass spectrometer. According to electron paramagnetic resonance and diffuse-reflectance infrared spectra, we propose a reaction mechanism for photoreduction of CO2 via Bi-based PeNC photocatalysts to form CO, CH4, and other possible side products.
Robust Tin-Based Perovskite Solar Cells with Hybrid Organic Cations to Attain Efficiency Approaching 10%
(2019-01-01)
Jokar, Efat
;
Chien, Cheng-Hsun
;
Tsai, Cheng-Min
;
Fathi, Amir
;
Diau, Eric Wei-Guang
The stability of a tin-based perovskite solar cell is a major challenge. Here, hybrid tin-based perovskite solar cells in a new series that incorporate a nonpolar organic cation, guanidinium (GA(+)), in varied proportions into the formamidinium (FA(+)) tin triiodide perovskite (FASnI(3)) crystal structure in the presence of 1% ethylenediammonium diiodide (EDAI(2)) as an additive, are reported. The device performance is optimized at a precursor ratio (GAI:FAI) of 20:80 to attain a power conversion efficiency (PCE) of 8.5% when prepared freshly; the efficiencies continuously increase to attain a record PCE of 9.6% after storage in a glove-box environment for 2000 h. The hybrid perovskite works stably under continuous 1 sun illumination for 1 h and storage in air for 6 days without encapsulation. Such a tin-based perovskite passes all harsh standard tests, and the efficiency of a fresh device, 8.3%, is certified. The great performance and stability of the device reported herein attains a new milestone for lead-free perovskite solar cells on a path toward commercial development.
Slow surface passivation and crystal relaxation with additives to improve device performance and durability for tin-based perovskite solar cells
(2018-09-01)
Jokar, Efat
;
Chien, Cheng-Hsun
;
Fathi, Amir
;
Rameez, Mohammad
;
Chang, Yu-Hao
We investigated the doping effect of bulky organic cations with ethylenediammonium diiodide (EDAI(2)) and butylammonium iodide (BAI) as additives to passivate surface defects, to control the film morphology and to improve the crystallinity for inverted planar FASnI(3) perovskite solar cells. The addition of BAI altered significantly the orientation of crystal growth and improved the connectivity of the crystal grains, but the existence of pinholes in the surface of the pristine FASnI(3) films was unavoidable; this effect impeded further improvement of the device performance (PCE 5.5%), which is nevertheless superior to that of a pristine FASnI(3) film (PCE 4.0%). The addition of EDAI(2) had the effect of curing the pinhole problem, passivating the surface defect states, preventing Sn2+/Sn4+ oxidation and inducing slow relaxation of the crystal structure. In the presence of the EDAI(2) additive (1%), the FASnI(3) device attained the best initial efficiency, 7.4%, and the device performance continuously increased as a function of duration of storage; the maximum PCE, 8.9%, was obtained for a device stored in a glove box for over 1400 h with only slight degradation for storage beyond 2000 h. The observed slow passivation of surface defects and relaxation of crystal strain were verified with X-ray diffraction, X-ray photoelectron spectroscopy and photoluminescence decay techniques.
Strategies To Improve Performance and Stability for Tin-Based Perovskite Solar Cells
(2019-08-01)
Diau, Eric Wei-Guang
;
Jokar, Efat
;
Rameez, Mohammad
;
交大名義發表
;
應用化學系
This Perspective reviews the developments of tin-based perovskite solar cells (PSCs) during the period 2014-2019 based on two organic cations (methylammonium vs formamidinium) and two device architectures (normal vs inverted) with the strategies of using cocations and coadditives. Unlike their lead-based analogues, tin-based PSCs suffer from the problems of Sn2+/Sn4+ oxidation and film formation. The current best cell is based on an inverted planar FASnI(3) device with guanidinium as cocation, SnF2 as additive, and ethyl-enediammonium iodide as coadditive, which attains record efficiency near 10% with great stability. The device stability and performance might be further improved upon introducing bulky organic cations with a suitable coadditive to form a hybrid 2D/3D crystal structure. Considering the theoretical limit on device efficiency for tin-based PSCs to be, similar to 33%, there is much room for further performance improvement if the problems can be resolved according to the approaches discussed in this Perspective.
Sulfur-Doped Graphene Oxide Quantum Dots as Photocatalysts for Hydrogen Generation in the Aqueous Phase
(2017-08-24)
Gliniak, Jacek
;
Lin, Jia-Hoa
;
Chen, Yi-Ting
;
Li, Chuen-Ru
;
Jokar, Efat
Sulfur-doped graphene oxide quantum dots (S-GOQDs) were synthesized and investigated for efficient photocatalytic hydrogen generation application. The UV/Vis, FTIR, and photoluminescence spectra of the synthesized S-GOQDs exhibit three absorption bands at 333, 395, and 524 nm, characteristic of C=S and C=S stretching vibration signals at 1075 and 690 cm(-1), and two excitation-wavelength-independent emission signals with maxima at 451 and 520 nm, respectively, confirming the successful doping of S atom into the GOQDs. Electronic structural analysis suggested that the S-GOQDs exhibit conduction band minimum (CBM) and valence band maximum (VBM) levels suitable for water splitting. Under direct sunlight irradiation, an initial rate of 18166 mu molh(-1) g(-1) in pure water and 30519 mu molh(-1) g(-1) in 80% ethanol aqueous solution were obtained. Therefore, metal-free and inexpensive S-GOQDs hold great potential in the development of sustainable and environmentally friendly photocatalysts for efficient hydrogen generation from water splitting.
Well-Organized Mesoporous TiO2 Photoanode by Using Amphiphilic Graft Copolymer for Efficient Perovskite Solar Cells
(2016-05-12)
Chung, Chih-Chun
;
Lee, Chang Soo
;
Jokar, Efat
;
Kim, Jong Hak
;
Diau, Eric Wei-Guang
The control of the thickness and porosity of a mesoporous TiO2 layer is important to improve the photovoltaic performance of perovskite solar cells. We produced organized mesoporous TiO2 (om-TiO2) layers using a low-cost amphiphilic graft copolymer, poly(vinyl chloride)-graft-poly(oxyethylene methacrylate) (PVC-g-POEM), as a sacrificial template. This simple but effective synthetic approach generates highly mesoporous and well-organized TiO2 nanostructures with interconnected and size-tunable features. Specifically, the average pore size increased with the amount of hydrophobic PVC main chain in the graft copolymer, which acted as the pore forming agent. Perovskite layers were prepared on top of an om-TiO2 layer according to a two-step sequential deposition: after coating the PbI2, solution in dimethylformamide (DMF) on an om-TiO2 substrate, the substrate was prewetted in isopropyl alcohol (IPA) solvent before immersing into a CH3NH3I/IPA solution. This prewetting treatment not only improves the yields of conversion from PbI2, to CH3NH3PbI3, but also increases the size of perovskite crystals with cuboid morphology. On varying the pore size and the film thickness of the om-TiO2 layer, the device performance attained 11.9% of power conversion efficiency (PCE) at pore size 70 nm and film thickness 300 nm. We measured extracted charge densities and decays of transient photovoltage to understand the kinetics of charge recombination in relation to the corresponding device performance.