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In the recent past, organometal halide perovskites have gained huge attention for application in various optoelectronic devices such as solar cells, light-emitting diodes (LEDs), photo-detectors, Lasers, etc.1,2 Such a wide range of applications are attributed to the extraordinary properties of these materials such as large range energy bandgap tunability from 2.43 eV to 1.50 eV, large absorption coefficients (~21.026 mm-1), large charge carrier lifetimes (~260 μs), mobilities (μe=73 cm2/V-sec, μh=43 cm2/V-sec), and diffusion lengths (~7.7 µm).3–6 However, the instability of bulk perovskite materials in ambient air conditions limits their usage at the device level. Contrary to bulk counterparts, the low-dimensional, as well as quasi dimensional perovskites, show improved stability which is promising for the development of these materials for optoelectronic devices. We present, the Ruddlesden-Popper phase 2D perovskites of type (CH)2(MA)n-1PbnI3n+1, [CH=2-cyclohexylethylamine; MA=methylamine] where, n is an integer and represents the number of inorganic layers of PbI6 octahedron in the perovskite crystal structure (Figure 1a). The spin-coating of these materials results into self-assembled highly crystalline films. The energy bandgap (Eg) of these films decreased with an increase in n-value and upon illumination with λex~470 nm LED strong and tunable photoluminescence is observed (Figure 1b). Photocurrent density (λex~470 nm) for the photo-detectors is found to increase with increase in n-value from 16.67 nA/cm2 (n=1) to 67.78 nA/cm2 (n=4) at 0V bias as well as with increase in bias.

AIP Publishing Horizons - Energy Storage and Conversion

Investigation of 2D Ruddlesden-Popper Perovskites Optoelectronic Devices

large grain size

good photoluminescence

vertical two electrode

good photo response

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MATERIALS FOR HUMANITY (MH 21)

MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

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Energy conversion and storage is a critical part of modern society. Applications continue to develop at a fast pace, from the development of new generation battery materials, to environmental sensors, catalytic materials for sustainable energy and solar cells and photodetectors. This conference will cover the latest advances in energy related fields and debate the needs and priorities currently existing or emerging in this field over the next few decades.

Conference Topics:

* Batteries and supercapacitors
* Catalytic materials and water splitting
* Light emitting materials and devices
* Solar cells and photodetectors
* Thermoelectric materials
* Sensing and environmental monitoring systems


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Energy conversion and storage is a critical part of modern society. Applications continue to develop at a fast pace, from the development of new generation battery materials, to environmental sensors, catalytic materials for sustainable energy and solar cells and photodetectors. This conference will cover the latest advances in energy related fields and debate the needs and priorities currently existing or emerging in this field over the next few decades.

Thermally chargeable supercapacitor is a new module in the field of thermoelectric conversion, which can simultaneously produce and store electric energy from heat source. In this paper, we use biomass derived carbon materials as the electrode of TCS, and mainly explore the effect of different electrolyte concentrations on the thermal charging performance of TCS. The application of biomass electrode with ultra-high specific surface area provides a guarantee for ion intercalation and adsorption, so that the system can produce a larger energy density at a lower temperature difference. The open circuit voltage of TCS system with different electrolyte concentration under the same temperature difference is experimentally measured. The results show that the electrolyte concentration has a significant effect on the thermal charging behavior of TCS. By adjusting the electrolyte concentration, the system can get the best temperature coefficient under the same temperature difference. This research can provide a new idea for the design of TCS.

Effect of electrolyte concentrations on electrochemical performance of thermally chargeable supercapacitor composed of biomass-derived electrodes

Bismuth-based metal oxides are an intriguing class of photoelectrode materials that can potentially enable large-scale solar hydrogen production via photoelectrochemical (PEC) water splitting. Due to their so-called defect tolerance features including other favorable photophysical and photoelectrochemical properties, Bismuth vanadate (BiVO4) and kusachiite (copper bismuth oxide) (CuBi2O4) are perhaps the most promising photoanode and photocathode materials for attaining high solar-to-hydrogen efficiency (STH). BiVO4 (BVO) has three different polymorphs; namely pucherite, dreyerite, and clinobisvanite. Here we attempt to synthesize phase pure BiVO4 with monoclinic scheelite (ms) crystal structure using a facile solid-state reaction technique aided with a two-step sintering process at ~ 400 – 500 0C for BVO and at ~750 0C for 4 h in the air for CBO. Both BiVO4 and CuBi2O4 powders have been synthesized utilizing a facile solid-state reaction technique for subsequent sol-gel derived fabrication of photoelectrodes atop the transparent conducting substrates. The structural, optical, and Raman studies suggest that phase pure ms BiVO4 and tetragonal CuBi2O4 could be grown from inorganic oxide powder as raw materials. We estimate the crystallographic lattice parameters and crystal domain size from X-Ray diffraction measurements. The crystallographic and vibrational structures are further analyzed with room temperature Raman spectroscopy. We observe clear evidence of ms crystal structure in our synthesized sample based on X-ray and Raman peak analyses. The optical bandgap measured in the range of ~2.38 – 2.58 eV for BiVO4 and ~1.70 – 1.85 eV for CuBi2O4 from the UV-VIS-NIR Diffuse reflection spectroscopy further corroborates the ms BiVO4 structure. The ms BiVO4 photoanode and CuBi2O4 photocathode with variable structural and optical properties could be a promising candidate for self-sustained PEC generation of solar fuels.

Facile Synthesis and Characterization of BiVO4 and CuBi2O4 for self-sustained photoelectrochemical water splitting devices

Organic solar cells (OSCs) based on nonfullerene acceptors (NFAs) have achieved rapid developments benefiting from the near-zero donor/acceptor (D/A) energy offset. However, a specific mechanism underlying the high-efficient charge generation remains an open question. Up to now, most of studies only focused on intra-molecular charge transfer (Intra-CT) character and inter-molecular aggregation structure of NFAs, but ignored their inter-molecular charge transfer (Inter-CT) character. In this study, by emphasizing the intra-molecular A-D-A electronic structure and inter-molecular aggregation mode of typical NFA molecules, we propose a universal quantum model incorporating both Intra- and Inter-CT characters. We confirm that the Inter-CT character is intrinsically presented in NFA molecular aggregates. Importantly, owing to the Intra- and Inter-CT synergistic effect, the excited state binding energy in NFA molecular aggregates can be significantly reduced (lowered to ~0.05 eV). These results clearly demonstrate that the excited state in NFA molecular aggregates is a CT state with both Intra- and Inter-CT characters, by which the charge generation dependence upon the D/A energy offset can be eliminated through rational designs of NFA molecules. 


Synergistic Effect Incorporating Intra- and Inter-Molecular Charge Transfer in Nonfullerene Acceptor Molecules for High-Efficient Organic Solar Cells

As a promising strategy, strain engineering has been applied to different low-dimensional or flexible semiconductor materials, aiming to modulate their photoelectric properties. An interesting experimental result is the directional migration of the photogenerated excitons induced by a local nonuniform strain in these systems, such as “exciton funneling”. Herein, to clarify the related microcosmic quantum dynamics and the impacting factors, we theoretically demonstrate the migration dynamics of an exciton/biexciton induced by a funnel-like nonuniform compression strain over organic polymers, where a quantum model combined with the nonadiabatic dynamical method is employed. Firstly, the migration dynamics of an exciton and a biexciton are separately demonstrated right after a funnel-like nonuniform compression strain created. It is found that both of them will migrate towards the strain center with the speeds two orders of magnitude higher than those determined by the traditional Förster or Dexter mechanism. Furthermore, we clarify the mechanism for the directional and ultrafast migrations of the exciton/biexciton, and compare the difference between their migration dynamics. In addition, some typical factors impacting the exciton migration dynamics are considered, such as the non-uniform degree of the strain, the initial interchain distance of organic polymers, and the proportion of the exciton initially lying in the strain region.

Migration dynamics of excitons/biexcitons induced by a funnel-like nonuniform compression strain over organic polymers

By employing a femtosecond electric pump pulse, we theoretically investigate the re-excitation dynamics of a “cold” charge transfer (CCT) state at organic donor/acceptor (D/A) interfaces. It is demonstrated that a relaxed CCT state can be pushed to different “hot” CT (HCT) states via experiencing electron (HCT1 state) and/or hole (HCT2 state) higher-energy transitions, where the transition modes and probabilities are primarily determined by the pulse energy. Without the assistance of a charge driving field, both the two HCT states relax to the initial CCT state through different internal conversion processes, which dynamics are clearly clarified in this work. However, after a driving field applied, we find that both of the two HCT states can be dissociated into free charges before their relaxations. Especially, HCT2 state is much easier dissociated than HCT1 state, as well as CCT state, due to the more delocalized hole charge distribution along donor. In addition, by enhancing the pulse intensity, we can further improve the hole delocalization along donor, so that the pulsed HCT2 state is more favorable to be dissociated. This work underlines the importance of charge delocalization for the interfacial charge dynamics, including both the internal conversion and charge separation, mediated by different intermediate HCT states in organic solar cells.

Re-excitation Dynamics of a “Cold” Charge Transfer State at Organic Donor/Acceptor Interfaces

Organic-inorganic metal halide perovskite solar cells (PvSCs) are an emerging photovoltaic technology with excellent power conversion efficiency (PCE) and good quantum efficiency. Due to the long response time, frequency-dependent variations in the external quantum efficiency (EQE) spectra of the PvSCs are observed. These variations are a cause of uncertainty in EQE measurements and cast doubts over the legitimacy of the measured EQE and the corresponding short circuit current density. Due to this, in most of the reported cases, there exists a mismatch between the current density measured directly from the J-V data and the EQE spectra, leading to confusion. In this work, we have analyzed and quantified the mismatch between the current density measured from the J-V measurements (JSC, JV) and the one measured from EQE (Jsc, EQE). The experiments have been performed for two different perovskite compositions, RbCsMAFA and MAPbI3, at different optical chopper frequencies between 10 Hz to 210 Hz with and without light biasing. The influence of bias light illumination on the current mismatch has also been discussed. In our case, the maximum value of the current density mismatch between J-V and EQE has been found to be 7.15 mA/cm2

J-V and EQE derived current mismatch studies of RbCsMAFA and MAPbI3 perovskite solar cells

In this work, we have performed a comparative study between spin-coated and sputter-deposited compact TiO2 electron transport layer for planar perovskite solar cells (PSCs). It has been observed that spin coated TiO2 leads to non uniform films
resulting in poor contact with superjacent layers, and hence an overall poor device performance. Solution processed techniques such as spin coating also limit the device area, as the films deposited are uniform only over a limited area. Here, we have reported the fabrication of PSCs with planar device architecture FTO/TiO2 /CH3NH3PbI3/Spiro-OMeTAD/Cu, where TiO 2 layer was deposited by spin coating and radio frequency (RF)magnetron sputtering. From this comparative study, it has been observed that performance of devices with sputter deposited TiO2 is better than those with spin coated TiO2.

A Comparison between Solution-Processed and Vapor Deposited TiO2 Layer for Perovskite Solar Cells

The effect of metallic nanoparticles on photovoltaic solar cells of various technologies have been extensively reported in recent years1. On the other hand, the use of green chemistry for the synthesis of nanoparticles is a matter of considerable interest due to its eco-friendly process and low carbon footprint2. In that sense, the use of different reducing agents of biological origin to synthesize metal nanoparticles has been discussed for antibacterial and energy conversion applications3,4.
In this study, it is presented the effect of a silver nanoparticles coating, biosynthesized using a solution of silver nitrate and the extract of a fungal agent (Ag-NPs), on the photovoltaic parameters of a chemically deposited CdS/PbS thin film solar cell, grown on flexible substrates of ITO/PET. The nanocrystalline CdS and PbS thin-films has an average thicknesses of 140 nm and 330 nm, respectively. X-ray diffraction studies shows that CdS and PbS thin films have a hexagonal and cubic structure, respectively. This solar cell also has electron- and hole-transport layers made of CdS:Al and PbS:Sn, respectively (Fig. 1). A coating of Ag-NPs was deposited on the PbS layer by dipping the solar cell in a solution with dispersed nanoparticles. A drop in the short-circuit voltage (from 115.7 mV to 5.8 mV) and a decrease in the short-circuit current (from 104.78 mA/cm2 to 11.45 mA/cm2) were observed after the treatment with Ag-NPs. The effect on the conversion efficiency of Ag-NPs concentration in the coating solution, and the optimal thicknesses of the electron and hole transport layers, were discussed.


Effect of a coating based on biosynthesized silver nanoparticles on the photovoltaic parameters of a CdS/PbS thin film solar cell

Conversion efficiencies of photovoltaic cells have been measured based on a calorimetric method or electrical substitution method. To automatize the electrical substitution, a constant temperature control is adopted. For a silicon photodiode as a photovoltaic cell, its internal conversion efficiency and internal spectral responsivity have been directly evaluated by using a Peltier device as a temperature difference sensor and use a thermistor as a heater. Its external spectral responsivity obtained by the calorimetric method agrees to an optically calibrated value within 11%.

Direct Measurement for Internal Conversion Efficiency of Photovoltaic Cells Based on Calorimetric Method

X-ray Absorption Spectroscopy (XAS) is being used for capturing the reaction mechanisms at the molecular level via the determination of local electronic configurations inside the Li-S batteries. In this work, a comprehensive investigation of the S K-edge X-ray Absorption Near-Edge Structure (XANES) of a series of stoichiometric lithium polysulfide solids1,2,3 was performed assuming that such polysulfides are being formed during the battery cycling process. The results based on density functional theory show that the pre-edge peak in the S K-edge XANES spectra does not occur in the solid phase in contrast to the case of polysulfide molecules where the pre-edge peak constitutes a unique signature in liquid electrolytes. A general redshift in XANES S K-edge peak is found when the Li-vacancy is introduced for these lithium polysulfide solids. Furthermore, the first peak positions of the S K-edge XANES spectra of the solid lithium polysulfides appear within a small range (2471-2473 eV) overlapping with the α-S8 bulk’s white line (~ 2472 eV). The results, therefore, suggest the use of additional characterization methods might be necessary in accurately identifying the elusive solid phase lithium polysulfides in Li-S battery instead of relying only on the S K-edge XANES spectra.

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