Bis- and Tris-norbornadienes with High Energy Densities for Efficient Molecular Solar Thermal Energy Storage

Molecular solar thermal energy storage (MOST) systems can convert, store and release solar energy in chemical bonds, i.e., as chemical energy. In this work, phenyl- and naphthyl-linked bis- and tris-norbornadienes are presented as promising MOST systems with very high energy densities. The substrates were synthesized by Suzuki–Miyaura coupling reactions and their absorption properties and characteristic parameters for MOST applications were investigated. The norbornadiene derivatives showed absorption onsets of up to 386 nm and photoisomerization quantum yields of 56 % per photoisomerization event. The resulting quadricyclane products have half-lifes up to 14 d and very high energy densities of up to 734 kJ/kg. Overall, these norbornadienes fulfill necessary criteria for an optimal MOST system and are, therefore, a highly promising basis for the development of materials for efficient solar energy conversion and storage.     

Fused Bis(hemi-indigo): Broad-Range Wavelength-Independent Photoswitches

Wavelength-independent conversion of organic photoswitches in the photostationary state is a rare phenomenon that opens up a way for many practical applications. In this work, three fused bis(hemi-indigo) derivatives with different substitution patterns were synthesized and their photoswitching was investigated by optical spectroscopy, real-time NMR spectroscopy and TD-DFT calculations. We disclosed that the Z-E photoisomerization of the meta-bis(hemi-indigo) derivative was remarkably independent of the irradiation wavelength from UV up to yellow light. The wavelength-independent forward photoswitching together with the inhibited backward photoisomerization, high thermal stability of the photoinduced isomers as well as significant overlap between the photoswitch absorption and the solar spectrum allows to suggest bis(hemi-indigo) derivatives as promising candidates for molecular solar thermal energy storage (MOST) systems.     

Synthesis,characterization and computational evaluation of bicyclooctadienes towards molecular solar thermal energy storage

Molecular solar-thermal energy storage (MOST) systems are based on photoswitches that reversibly convert solar energy into chemical energy. In this context, bicyclooctadienes (BODs) undergo a photoinduced transformation to the corresponding higher energy tetracyclooctanes (TCOs), but the photoswitch system has not until now been evaluated for MOST application, due to the short half-life of the TCO form and limited available synthetic methods. The BOD system degrades at higher temperature via a retro-Diels–Alder reaction, which complicates the synthesis of the compounds. We here report a cross-coupling reaction strategy that enables an efficient synthesis of a series of 4 new BOD compounds. We show that the BODs were able to switch to the corresponding tetracyclooctanes (TCOs) in a reversible way and can be cycled 645 times with only 0.01% degradation. Half-lives of the TCOs were measured, and we illustrate how the half-life could be engineered from seconds to minutes by molecular structure design. A density functional theory (DFT) based modelling framework was developed to access absorption spectra, thermal half-lives, and storage energies which were calculated to be 143–153 kJ mol⁻¹ (0.47–0.51 MJ kg⁻¹), up to 76% higher than for the corresponding norbornadiene. The combined computational and experimental findings provide a reliable way of designing future BOD/TCO systems with tailored properties.

Molecular solar-thermal energy storage (MOST) systems are based on photoswitches that reversibly convert solar energy into chemical energy.     

Monoaryl-Substituted Norbornadiene Photoswitches as Molecular Solar Thermal Energy Storage Compounds: Synthesis and Investigation

In the search for improved photochromic systems that may be used for the efficient reversible conversion of light into chemical energy, eight monoaryl-substituted norbornadienes are presented, which carry naphthyl, anthracenyl, or donor-acceptor-phenyl substituents to establish an extended π system. The substrates were synthesized by Suzuki-Miyaura coupling reactions, and their absorption properties, the key parameters of the photochromic equilibrium, and the energy storage density of the quadricyclane products were examined. It was observed that these compounds have favorable properties for chemical energy storage. Specifically, 2-(1-naphthyl)norbornadiene showed a pronounced red shift of the absorption, a long half-life (35 d) and a relatively high energy storage density (361 KJ/Kg) of the respective quadricyclane. Therefore, the so far neglected monoaryl-norbornadienes represent a useful and complementary class of compounds that should be considered in the development of efficient molecular solar thermal energy storage (MOST) compounds, which can reversibly convert sunlight into chemical energy.     

用于染料敏化太阳能电池的D5同类物分子设计(英文)

使用密度泛函理论(DFT)和含时密度泛函理论(TDDFT)以及自然键轨道(NBO)分析,设计比有机染料D5更优秀的用于染料敏化太阳能电池(DSSC)的D5同类物分子.在D5骨架的给电子基团上对称地引入给电子基(—OH,—NH2,—OCH3),既可以使分子的最低未占据分子轨道(LUMO)能级提高,又可以使吸收光谱红移,从而既提高染料分子捕获太阳辐射光子的能力,又提高由染料分子的激发态向TiO2电极注入电子的驱动力.在D5分子的骨架上,对称地引入受电子基(—CF3,—F,—CN),可以使染料分子的吸收光谱强烈地红移,从而更有效地利用太阳能.由LUMO能级的提高和吸收光谱的红移来考虑,所设计的D516,D536,D537分子是比D5优秀的同类物分子,其中D516是最好的.单从吸收光谱红移来考虑,所设计的D565,D567,D568分子是比D5优秀的同类物分子,其中D565的吸收光谱有望与太阳辐射光谱更好地匹配.挑选出来的这6种D5同类物分子都是D-π-A(电子给体-共轭π桥-电子受体)结构.这几种分子的光激发引起的最高占据分子轨道(HOMOs)到LUMOs的跃迁是π-π*跃迁,是分子内电荷转移,吸收光谱是电子吸收光谱,位于近紫外-可见光区.D516和D565有望成为比D5更优秀的用于DSSC的非金属有机染料分子.     

Molecular Solar Thermal Batteries through Combination of Magnetic Nanoparticle Catalysts and Tailored Norbornadiene Photoswitches

Cobalt catalysts are immobilized on the surface of iron oxide nanoparticles for the preparation of highly active quasi-homogeneous catalysts toward an efficient release of photochemically stored energy in norbornadiene-based photoswitches. The facile separation of the iron oxide nanoparticles through exploitation of the intrinsic magnetic properties of this material enables efficient cyclization of energy storage and release. Through the transition from cobalt (II) salphen to cobalt porphyrins, a 22.6-fold increase in the catalytic efficiency of the QC-NBD back-conversion is achieved, with an initial TOF of up to 3.64 s⁻¹ and excellent TON of over 3305. In addition, a series of novel “push–pull” functionalized norbornadiene derivatives is prepared, featuring excellent absorption properties with maxima up to 366 nm, quantum yields around 70 %, high energy storage capacities of up to 98.0 kJ mol⁻¹, and outstanding thermal stability with t_1/2 (25 °C) over 100 days. Finally, the energy storage potential of these molecular solar thermal (MOST) systems is harnessed in a heat release experiment. This demonstrates the potential of norbornadiene-based photoswitches in combination with efficient magnetic catalysts for the generation of environmentally benign process heat.     

Study of the influence of substituents on spectroscopic and photoelectric properties of zinc phthalocyanines

In this paper we describe conversion of light energy into electric energy in a photoelectrochemical cell containing zinc phthalocyanine (ZnPc) dyes. For all dyes investigated in liquid polyvinyl alcohol with dimethyl sulfoxide solution and located in the photoelectrochemical cell the following measurements have been done: absorption, fluorescence, photoacoustic spectra, photovoltaic spectra, kinetics of photocurrent and current–voltage characteristics. It has been shown that all dyes located in the photoelectrochemical cell are able to convert light into electric energy but with different effectiveness. The influence of substituted different peripheral groups to ZnPc core and the correlation between the molecular structure and effectiveness of solar to electric energy conversion were observed and described. The unique behavior of ZnPc substituted with fluorines was indicated.     

Polydopamine-coated metal-organic framework-based composite phase change materials for photothermal conversion and storage

The liquid leakage and weak solar absorption capacity of organic phase change materials (PCMs) seriously hinder the efficient utilization of solar energy and thermal energy storage. To address these issues, we prepared nanoporous metal organic framework (Ni-MOF) for the vacuum infiltration of paraffin wax (PW), followed by the coating of solar-absorbing functional polydopamine (PDA) on the surface of PW@MOF for photothermal conversion and storage. As an efficient photon harvester, PDA coating endows PW@MOF/PDA composite PCMs with excellent photothermal conversion and storage properties due to the robust broadband solar absorption capability in the UV–vis region. Resultantly, our prepared PW@MOF/PDA composite PCMs exhibit a high photothermal conversion and storage efficiency of 91.2%, while that of PW@MOF composite PCMs is only zero. In addition, PW@MOF/PDA composite PCMs also exhibit excellent thermal stability, shape stability, energy storage stability, and photothermal conversion stability. More importantly, this coating strategy is universal by integrating different MOFs and solar absorbers, showing the potential to accelerate the major breakthroughs of high-efficiency MOF-based photothermal composite PCMs in solar energy utilization.     

Enhanced Conversion Efficiency Enabled by Species Migration in Direct Solar Energy Storage

Solar energy can be stored via either an indirect route in which electricity is involved as an intermediate step, or a direct route that utilizes photogenerated charge carriers for direct solar energy conversion. In this study, we investigate the fundamental difference between the direct and indirect routes in solar energy conversion using a new photoelectrochemical energy storage cell (PESC) as a model device. This PESC centers on a liquid junction that utilizes CH₃NH₃PbI₃ perovskite to drive photoelectrochemical reactions of Benzoquinone (BQ) and Ferrocene (Fc) redox species. The experimental studies show that the equilibrium redox potentials are 0.1 V and −0.78 V (vs Ag/AgNO₃) for Fc⁺/Fc and BQ/BQ^.−, respectively, which would produce a theoretical open-circuit voltage of 0.88 V for the storage device. The physics-based computational analysis shows a relatively flat reaction rate distribution in the electrode for the indirect route; however, in the direct route the photoelectrochemical reaction rate is critically affected by electron concentration due to strong light absorption of the perovskite material, which has been shown to vary by at least 10-fold in the transverse direction across the photoelectrode. The drastic variation of reaction rate in the photoelectrode creates an electric field that is 7.5 times stronger than the bulk electrolyte, which causes the photo-converted reaction product (i. e., BQ^.−) to drift away from the photoelectrode thereby creating a constant reaction driving force. As a result, it has been shown that the intrinsic solar to chemical conversion (ISTC) efficiency improves by ∼40 % for the direct route compared to the indirect route at 0.05 mA/cm².     

Towards Solar Energy Storage in the Photochromic Dihydroazulene–Vinylheptafulvene System

One key challenge in the field of exploitation of solar energy is to store the energy and make it available on demand. One possibility is to use photochromic molecules that undergo light‐induced isomerization to metastable isomers. Here we present efforts to develop solar thermal energy storage systems based on the dihydroazulene (DHA)/vinylheptafulvene (VHF) photo/thermoswitch. New DHA derivatives with one electron‐withdrawing cyano group at position 1 and one or two phenyl substituents in the five‐membered ring were prepared by using different synthetic routes. In particular, a diastereoselective reductive removal of one cyano group from DHAs incorporating two cyano groups at position 1 turned out to be most effective. Quantum chemical calculations reveal that the structural modifications provide two benefits relative to DHAs with two cyano groups at position 1: 1) The DHA–VHF energy difference is increased (i.e., higher energy capacity of metastable VHF isomer); 2) the Gibbs free energy of activation is increased for the energy‐releasing VHF to DHA back‐reaction. In fact, experimentally, these new derivatives were so reluctant to undergo the back‐reaction at room temperature that they practically behaved as DHA to VHF one‐way switches. Although lifetimes of years are at first attractive, which offers the ultimate control of energy release, for a real device it must of course be possible to trigger the back‐reaction, which calls for further iterations in the future.     

二氢吲哚类染料用于染料敏化太阳能电池光敏剂的比较

采用密度泛函理论(DFT)和含时密度泛函理论(TD-DFT)对四种二氢吲哚染料进行研究, 从中筛选出相对优秀的染料敏化太阳能电池光敏剂. 对前线分子轨道的计算表明, 二氢吲哚染料的前线分子轨道结构非常有利于染料激发态向TiO2电极的电子注入. 对真空中的紫外和可见光吸收光谱的计算表明, 二氢吲哚染料的吸收光谱与太阳辐射光谱匹配较好. 对染料分子的能级计算表明, 二氢吲哚染料的能级结构比较适合于I-/I-3作电解液的TiO2纳米晶太阳能电池的光敏剂. 二氢吲哚染料最低未占据分子轨道(LUMO) 能级均比TiO2晶体导带边能级高, 能够保证激发态染料分子高效地向TiO2电极转移电子. 二氢吲哚染料最高占据分子轨道(HOMO)的能级比I-/I-3能级低, 保证了失去电子的染料分子能够顺利地从电解液中得到电子. 与实验数据比较, 得出在提高染料敏化太阳能电池转换效率方面, 对染料的关键要求是LUMO能级的位置. 染料分子的稳定性是染料敏化太阳能电池使用寿命的关键因素. 通过对化学键键长的比较表明, 二氢吲哚染料的分子稳定性基本相同. 对计算结果的分析表明, 二氢吲哚染料1(ID1)的LUMO能级最高, 分子稳定性最好, 在酒精溶液中的吸收光谱与太阳辐射光谱匹配很好, 在同类染料中是较好的染料敏化太阳能电池光敏剂.     

Density functional theory as a guide for the design of pyran dyes for dye-sensitized solar cells

Abstract  

Using density functional theory and hybrids, we examined several derivatives of the dye 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran, with the objective of identifying modifications which would improve the properties of dyes for dye-sensitized solar cells. We calculated the electronic structure of numerous derivatives at the HOMO and LUMO energy levels, with the hypothesis that directing the flow of excited electrons to the point of the dye at which the molecule attaches to TiO₂ would increase the energy conversion efficiency of the cell. We also examined the UV–visible absorption spectra of the dyes, with the objective of capturing the maximum amount of solar light. By use of the derivatives we compared the use of two electron-donating groups instead of one, extension of the conjugated chain leading to the attachment point of the dye, use of oxygen versus sulfur or selenium in the dye, and the use of different electron-donating groups. We identified several promising donating groups and determined that the other modifications to the dye are likely to increase solar cell efficiency.     

吲哚染料的电荷转移和能量转移:应用于无金属的染料敏感太阳能电池

Metal-free indoline dyes for dye-sensitized solar cells were studied by employing quantum chemistry methods.Comparative study of the properties of both ground and excited states of metal-free indoline dyes for dye-sensitized solar cells revealed: (i) as the number of rhodanine rings increases, the energy di?erence betweenHOMO and LUMO decreases and there is a red shift in the absorption spectrum with the binding energyincreased, and the transition dipole moment decreased; (ii) Based on an analysis of charge di?erential density,we observed that the charge and energy are transfered from the phenylethenyl to the indoline and rhodaninerings; (iii) The electron-hole coherences are mainly on the indoline and rhodanine rings, and the exciton sizesare 30 and 40 atoms for indoline dyes with one and two rhodanline rings, respectively. These results serveas a good example of computer-aided design in metal-free indoline dyes for dye-sensitized solar cells.     

萘酰亚胺-卟啉星型电子受体分子的构筑及其在非富勒烯太阳能电池中的应用

非富勒烯太阳能电池目前已经成为有机太阳能电池的研究热点,大量的共轭电子受体分子被开发,并成功应用到高性能光伏器件中。共轭分子作为非富勒烯电子受体,需要综合考虑吸收、能级、电子传输以及结晶性等,其中宽吸收光谱可以提高对太阳光谱的利用,是分子设计中重要因素之一。本工作中,我们设计一种新型电子受体分子,以卟啉为核、萘酰亚胺为端基以及炔为桥连基团。这种新型分子具有近红外的吸收光谱以及合适的能级。将一种具有吸收互补的共轭聚合物为电子给体,星型分子为电子受体应用到电池的活性层中,我们获得了1.8%的能量转换效率,电池的光谱响应为300–900 nm。实验结果证明了这种以卟啉为核的分子设计在实现近红外吸收的电子受体方面具有重要应用前景。     

甲胺基-甲脒基混合钙钛矿的第一性原理研究

有机铅卤钙钛矿APbX_3的稳定性是制约其应用的重要因素,对APbX_3钙钛矿中A和X采用不同种类离子混合的化学组分调控是改进其稳定性最有效的方式之一。其中,A位点采用不同比例的甲脒离子(FA)和甲胺离子(MA)是当前研究的热点方向。本文通过第一性原理计算,系统研究了FA_(1-x)MA_xPbI_3体系的结构和光电性质。研究发现FA与MA的混合增加体系的稳定性,其中FA_(0.5)MA_(0.5)PbI_3最稳定。通过分析不同混合比例的结构,揭示了晶格常数随x的增加线性减小;以及带隙随x减小而线性增加。此外,计算结果发现MA所占比例增加时吸收光谱蓝移。研究表明通过FA和MA离子的混合能有效调控钙钛矿的光电性质,从而获得更有效的钙钛矿太阳能电池。     

Superabsorbing fullerenes: spectral and kinetic characterization of photoinduced interactions in perylenediimide-fullerene-C60 dyads

Two n-type molecular materials are covalently combined into a new photovoltaic component for polymer solar cells. Light harvesting by the perylenediimide results in very fast energy transfer to the fullerene unit, as shown with femtosecond transient absorption spectroscopy in toluene solution. Two energy transfer rates are observed of 2.5 x 10(12) s-1 (53%) and 2 x 10(11) s-1 (47%), attributed to two conformations. The final excited state that is populated is a perylenediimide-based triplet state that is formed on the nanosecond time scale with a high yield.     

Potential of Nonfullerene Small Molecules with High Photovoltaic Performance

Over the past decades, fullerene derivatives have become the most successful electron acceptors in organic solar cells (OSCs) and have achieved great progress, with power conversion efficiencies (PCEs) of over 11 %. However, fullerenes have some drawbacks, such as weak absorption, limited energy‐level tunability, and morphological instability. In addition, fullerene‐based OSCs usually suffer from large energy losses of over 0.7 eV, which limits further improvements in the PCE. Recently, nonfullerene small molecules have emerged as promising electron acceptors in OSCs. Their highly tunable absorption spectra and molecular energy levels have enabled fine optimization of the resulting devices, and the highest PCE has surpassed 12 %. Furthermore, several studies have shown that OSCs based on small‐molecule acceptors (SMA) have very efficient charge generation and transport efficiency at relatively low energy losses of below 0.6 eV, which suggests great potential for the further improvement of OSCs. In this focus review, we analyze the challenges and potential of SMA‐based OSCs and discuss molecular design strategies for highly efficient SMAs.     

Monodisperse Dual‐Functional Upconversion Nanoparticles Enabled Near‐Infrared Organolead Halide Perovskite Solar Cells

Extending the spectral absorption of organolead halide perovskite solar cells from visible into near‐infrared (NIR) range renders the minimization of non‐absorption loss of solar photons with improved energy alignment. Herein, we report on, for the first time, a viable strategy of capitalizing on judiciously synthesized monodisperse NaYF₄:Yb/Er upconversion nanoparticles (UCNPs) as the mesoporous electrode for CH₃NH₃PbI₃ perovskite solar cells and more importantly confer perovskite solar cells to be operative under NIR light. Uniform NaYF₄:Yb/Er UCNPs are first crafted by employing rationally designed double hydrophilic star‐like poly(acrylic acid)‐block‐poly(ethylene oxide) (PAA‐b‐PEO) diblock copolymer as nanoreactor, imparting the solubility of UCNPs and the tunability of film porosity during the manufacturing process. The subsequent incorporation of NaYF₄:Yb/Er UCNPs as the mesoporous electrode led to a high efficiency of 17.8 %, which was further increased to 18.1 % upon NIR irradiation. The in situ integration of upconversion materials as functional components of perovskite solar cells offers the expanded flexibility for engineering the device architecture and broadening the solar spectral use.     

Energy alignment and surface dipoles of rylene dyes adsorbed to TiO2 nanoparticles

The energy loss in dye-sensitized solar cells calculated from the energy difference between the lowest electronic transition of the dye and the obtained open-circuit voltage is often 1 eV or even more. To minimize this loss, it is important to accurately determine the energy alignment at the TiO(2)/dye/redox-mediator interface. In this study, we compared the results from electrochemistry and photoelectron spectroscopy for determining the energy alignment of three rylene dyes, two of which absorb relatively far in the red. The trends observed with the methods were different, as in the former, the energy alignment is measured relative to an external reference and includes contributions from solvent reorganization energies, while in the latter, it is measured relative to the energetics of the TiO(2) and is lacking such contributions. The influence of the dyes' dipole moments on the energetics of the TiO(2) was also measured and explained some of the differences in trends. Finally, we compared the injection efficiencies of the two red-absorbing dyes and found that the differences in injection efficiencies can be better explained using the energy alignment determined from photoelectron spectroscopy. This shows that the method for measuring the energetics of a DSC should be chosen according to what process one intends to study.     

Recent developments of di-amide/imide-containing small molecular non-fullerene acceptors for organic solar cells

Non-fullerene organic solar cells have received increasing attentions in these years, and great progresses have been made since 2013. Among them, aromatic di-amide/imide-containing frameworks have shown promising applications. The outstanding properties of them are highly associated with their unique electronic and structural features, such as strong electron-withdrawing nature, broad absorption in UV-visible region, tunable HOMO/LUMO energy levels, easy modifications, and excellent chemical, thermal and photochemical stabilities. In this review, we give an overview of recent developments of aromatic diamide/imide-containing small molecules used as electron acceptors for organic solar cells.