United States

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. 


AIP Publishing Horizons - Energy Storage and Conversion

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

molecular aggregation

organic solar cells

charge transfer

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.

poster

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.

First-Principles Study of XANES Study on Lithium Polysulfide Solids

We have focused on the fabrication of low-cost Cu-based large-area perovskite solar cells and their stability studies in our present work. The large area devices with the structure FTO/c-TiO2/mTiO2/MAPbI3/Spiro-OMeTAD/Cu were fabricated by the two-step spin coating method. The X-ray diffraction analysis confirmed the formation of a tetragonal phase of the perovskite material. The small area device (0.25 cm2) showed a photoconversion efficiency (PCE) of 8.91%, whereas the scaled-up device with an area of 2.5 cm2 showed a PCE of 4.36%. Eight such large area devices were fabricated.The statistical distribution of fabricated devices showed an average value of 4.06 %, with a standard deviation of 0.31%. The stability of the fabricated devices under ambient environment conditions for two weeks showed that the significant changes in the device efficiency occurred due to a decrease in photocurrent. It happened primarily because of the active layer degradation, resulting in a loss of light absorption.

Fabrication of Cu Electrode Based Large-Area Perovskite Solar Cells and Their Stability Analysis

Over past half-century, the cardiac pacing technology always maintains one basic system paradigm, particularly the power supply and the leads, which brings out many pacemaker-related complications. Implantable piezoelectric energy harvesting has been demonstrated as one of the promising batteryless pacing technologies. A heart-worn pacemaker strategy, which allows batteryless powering and leadless pacing, has been firstly presented by our research group recently. The key challenge is to obtain sufficient power supply in vivo. Our buckled piezoelectric energy harvesting strategy performs great potential. The d31 and d33 coupled mode has been presented for this conversion process. The behavior of d31 and d33 coupled piezoelectric dynamics is discussed.

Mechanism on Buckled Piezoelectric Energy Harvesting for Batteryless Heart-worn Pacemaker

Hydrogen has considerable potential as an alternative fuel because it is carbon-free, facilitates the use of more efficient power generation systems, and can reduce carbon oxides to chemical fuels chemically. Solar-to-chemical energy conversion of water to H2 is especially attractive given the abundance of water and the free energy available in sunlight. While TiO2 absorbs little of the solar spectrum, doping may redshift the TiO2 absorption spectrum into the visible. In the case of noble metals, if the size is reduced below its electron mean path, it induces the property of visible and near UV light absorption1. The incorporation of gold nanostructures to modify the surfaces leads to a remarkable improvement in photonic activity owing to the formation of Schottky barriers that prevent the recombination of photogenerated electron-hole pairs. Gold nanoclusters (Au NCs) are smaller in size than Au NPs and are used to modify surface properties. In contrast to nanoparticles, it is not the crystal structure but the geometric fluxionality and the size dependence of the electronic structure important for surface modification by clusters. For this reason, the number of atoms forming an Au NC strongly influences the properties of NCs modified surfaces. The catalytic properties of cluster-modified surfaces change with the size of Au NCs, and the size of the NCs needs to be preserved to retain specific surface properties. Agglomeration of NCs leads to an increase in NC size, eventually forming NPs, and should be avoided; otherwise, the agglomeration strongly affects the photocatalytic properties of the catalyst2. Application of mesoporous TiO2 with remarkable surface defects, high surface area, and extensive 3D pore network can be beneficial to prevent agglomeration of gold clusters after post heat treatment. Also, to have stronger interaction between Au NCs and semiconductor surface leading to decreased agglomeration and increasing the loading of the Au NCs, the functionalizing reagent is used to make a system with a high number of active sites to obtain highly active photocatalytic systems.

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

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

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