Molecular approaches to solar energy conversion with coordination compounds anchored to semiconductor surfaces

Strategies toward the realization of molecular control of interfacial charge transfer at nanocrystalline semiconductor interfaces are described. Light excitation of coordination compounds, based on (dpi)6 transition metals, anchored to wide band-gap semiconductors, such as TiO2, can initiate electron-transfer processes that ultimately reduce the semiconductor. Such photoinduced charge-separation processes are a key step for solar energy conversion. The thermodynamics and kinetic rate constants for three different interfacial charge separation mechanisms are discussed. Tuning the energetic position of the semiconductor conduction band relative to the molecular sensitizer has provided new insights into interfacial charge transfer. Supramolecular compounds that efficiently absorb light, promote interfacial electron transfer, and feature additional functions such as intramolecular electron transfer when bound to semiconductor surfaces have also been studied. New approaches for enhancing charge-separation lifetimes for solar energy conversion are presented.     

Deposition of a thin film of TiOx from a titanium metal target as novel blocking layers at conducting glass/TiO2 interfaces in ionic liquid mesoscopic TiO2 dye-sensitized solar cells

In dye-sensitized TiO2 solar cells, charge recombination processes at interfaces between fluorine-doped tin oxide (FTO), TiO2, dye, and electrolyte play an important role in limiting the photon-to-electron conversion efficiency. From this point of view, a high work function material such as titanium deposited by sputtering on FTO has been investigated as an effective blocking layer for preventing electron leakage from FTO without influencing electron injection. X-ray photoelectron spectroscopy analysis indicates that different species of Ti (Ti4+, Ti3+, Ti2+, and a small amount of Ti0) exist on FTO. Electrochemical and photoelectrochemical measurements reveal that thin films of titanium species, expressed as TiOx, work as a compact blocking layer between FTO and TiO2 nanocrystaline film, improving Voc and the fill factor, finally giving a better conversion efficiency for dye-sensitized TiO2 solar cells with ionic liquid electrolytes.     

Co-sensitization of organic dyes for efficient ionic liquid electrolyte-based dye-sensitized solar cells

The co-sensitization of two organic dyes (SQ1 and JK2), which are complementary in their spectral responses, shows enhanced photovoltaic performance compared with that of an individual organic dye-sensitized solar cell. The power conversion efficiency of the co-sensitized organic dye solar cell based on the newly developed binary ionic liquid (solvent-free) electrolyte gives 6.4% under AM 1.5 sunlight at 100 mW/cm2 irradiation, which is higher than that of individual dye-sensitized solar cells. The incident monochromatic photon-to-current conversion efficiency (IPCE) of the co-sensitized solar cell shows typical absorption peaks at 530 and 650 nm corresponding to the two dyes and displays a broad spectral response over the entire visible spectrum with IPCE of >40% in the 400-700 nm wavelength domain.     

从液态阳光人工光合成淀粉

人工光合成是利用太阳能等可再生能源通过连续催化反应将水和二氧化碳转化为液态燃料的过程,是减少二氧化碳排放、实现绿色低碳发展的一种重要方法.人工光合成的目标产物不仅包括二氧化碳转化与利用得到的能源小分子,还包括淀粉和蛋白质等生物基大分子.在自然光合作用中,高等植物、绿藻和蓝细菌首先利用太阳能将水氧化放出氧气并产生还原型辅...     

Chemical control of charge transfer and recombination at semiconductor photoelectrode surfaces

Semiconductor/liquid contacts provide very efficient systems for converting sunlight into electrical and/or chemical energy. Until recently, relatively little was understood about the factors that control the rates of interfacial charge transfer in such systems. This Forum Article summarizes recent results that have elucidated the key factors that control such charge-transfer rates, including verification of the Marcus inverted region, identification of the maximum charge-transfer rate constant for outer-sphere, nonadsorbing redox couples at optimal exoergicity, the role of nuclear reorganization on the value of the interfacial charge-transfer rate constant at semiconductor electrodes, and the effects of pH-induced changes in the driving force on the rates of such systems. In addition, we discuss methods for using main group inorganic chemistry to control the electrical properties of surfaces of important semiconductors for solar energy conversion, with specific emphasis on alkylation of the (111)-oriented surface of Si. Control of the rates at which carriers cross such interfaces, along with control of the rates at which carriers recombine at such interfaces, forms the basis for exerting chemical control over the key solar energy conversion properties of semiconductor photoelectrode-based devices.     

Novel two-step CdS deposition strategy to improve the performance of Cu_2ZnSn(S,Se)_4 solar cell

Kesterite Cu_2ZnSn(S,Se)_4(CZTSSe)solar cells have drawn worldwide attention for their promising photovoltaics performance and earth-abundant element composition,yet the record efficiency of this type of device is still far lower than its theoretical conversion efficiency.Undesirable band alignment and severe non-radiative recombination at CZTSSe/CdS heterojunction interfaces are the major causes limiting the current/voltage output and overall device performance.Herein,we propose a novel two-step CdS deposition strategy to improve the quality of CZTSSe/CdS heterojunction interface and thereby improve the performance of CZTSSe solar cell.The two-step strategy includes firstly pre-deposits CdS thin layer on CZTSSe absorber layer by chemical bath deposition(CBD),followed with a mild heat treatment to facilitate element inter-diffusion,and secondly deposits an appropriate thickness of CdS layer by CBD to cover the whole surface of pre-deposited CdS and CZTSSe layers.The solar energy conversion efficiency of CZTSSe solar cells with two-step deposited CdS layer approaches to 8.76%(with an active area of about 0.19 cm~2),which shows an encouraging improvement of over 87.98% or 30.16% compared to the devices with traditional CBD-deposited CdS layer without and with the mild annealing process,respectively.The performance enhancement by the two-step CdS deposition is attributed to the formation of more favorable band alignment at CZTSSe/CdS interface as well as the effective decrease in interfacial recombination paths on the basis of material and device characterizations.The two-step CdS deposition strategy is simple but effective,and should have large room to improve the quality of CZTSSe/CdS heterojunction interface and further lift up the conversion efficiency of CZTSSe solar cells.     

Excited-state dynamics of organic dyes at liquid/liquid interfaces

Liquid/liquid interfaces play a crucial role in numerous areas of science. However, direct spectroscopic access to this thin (～1 nm) region is not possible with conventional optical methods. After a brief review of the most used techniques to perform interfacial optical spectroscopy, we will focus on time-resolved surface second harmonic generation, which allows the measurement of the excited-state dynamics of probe molecules at interfaces. By comparing these dynamics with those measured in bulk solutions, precious information on the properties of the interfacial region can be obtained. To illustrate this, several studies performed in our group will be presented.     

Potential-modulation spectroscopy at solid/liquid and liquid/liquid interfaces.

Potential-modulation spectroelectrochemical methods at solid/liquid and liquid/liquid interfaces are reviewed. After a brief summary of the basic features and advantages of the methods, practical applications of potential-modulation spectroscopy are demonstrated using our recent studies of solid/liquid and liquid/liquid interfaces, including reflection measurements for a redox protein on a modified gold electrode and fluorescence measurements for various dyes at a polarized water/1,2-dichloroethane interface. For both interfaces, the use of linearly polarized incident light enabled an estimation of the molecular orientation. The use of a potential-modulated transmission-absorption measurement for an optically transparent electrode with immobilized metal nanoparticles is also described. The ability of potential-modulated fluorescence spectroscopy to clearly elucidate the charge transfer and adsorption mechanisms at liquid/liquid interfaces is highlighted.     

The boundary element method: the simulation of voltammetry at immiscible liquid/liquid interfaces

The boundary element method is presented as an efficient and powerful method for the analysis of time-dependent electrochemical processes occurring at immiscible liquid/liquid interfaces. This paper outlines the theory and numerical details required for the development and application of two-dimensional transient diffusion models for the simulation of cyclic voltammetry behaviour at a range externally polarised immiscible liquid/liquid interfaces of differing topography. The benefits of the BEM approach are discussed, including the reduction in dimensionality brought about by the formulation procedure and complete elimination of the need for domain discretisation with the time-domain convolution approach. The versatility and efficiency of the numerical procedures are examined with respect to a number of liquid/liquid interface geometries and a series of working curves established to quantify the influence of interface topography on the observed voltammetric behaviour.     

钙钛矿介导强化的铁基氧载体脱合金-脱溶循环及其太阳能热化学CO2裂解性能

大气中CO2含量的增加已对气候和环境造成巨大影响,要实现碳中和的目标,目前迫切需要开发CO2高效利用技术.太阳能热化学循环CO2裂解可充分利用太阳全光谱能量将CO2转化为CO,从而实现太阳能到化学能的存储.进一步引入CH4作为氧载体的还原气体,不仅能有效降低反应温度、提高氧载体供氧能力,还能联产高质量合成气,为生产甲醇和乙酸及费托合成提供原料,达到一举多得的效果.铁基材料因其成本低、环境友好等优点受到广泛关注,但普通铁氧化物(如Fe3O4,FeO)催化甲烷活化性能差,且受热力学限制,CO2分解转化率较低.本文制备了一种FeNi合金修饰的钙钛矿复合材料为氧载体(FeNi-LFA),其在两步法太阳能热化学CO2裂解反应中展现出较好的反应活性和循环稳定性.在反应温度为850℃时,CO2分解速率达到381 mL g?1 min?1(STP),转化率达到99％,氧化后材料可在恒温条件下经甲烷还原再生,合成气收率达96％以上,30次循环性能无明显下降.本文还结合高分辨透射电子显微镜(HRTEM),原位X射线衍射(XRD),原位扫描透射电镜(STEM)和57Fe穆斯堡尔谱等表征深入研究了热化学循环反应中氧载体的结构演变,并借助密度泛函理论(DFT)计算,研究其构效关系.HRTEM及EDS结果表明,FeNi-LFA中FeNi合金颗粒尺寸为20～50 nm,且合金颗粒部分嵌入到钙钛矿基体中,从而显著增强了金属-载体间相互作用.为了研究FeNi-LFA氧载体动态构造演化过程,采用XRD对氧载体反应中衍射峰变化进行研究.当FeNi-LFA暴露于CO2中,FeNi合金的特征衍射峰向高角度偏移,同时,La2O3衍射峰减弱.钙钛矿衍射峰增强,说明氧化气氛中,FeNi合金发生Fe脱合金过程,而氧化后的铁离子能与La2O3快速反应生成钙钛矿氧化物.当反应气氛切换成CH4后,钙钛矿衍射峰强度降低,La2O3信号相应增强,说明钙钛矿中铁离子脱溶析出,使材料在恒温条件下完成再生.进一步利用STEM对FeNi-LFA的结构演变中的元素迁移进行研究发现,新鲜样品中,FeNi合金与钙钛矿载体接触密切,FeNi合金周围仅有少量铁分布,在FeNi合金与氧化物载体的界面处,Fe信号强度有所下降,表明FeNi合金嵌入在富含La2O3的钙钛矿载体中,这与HRTEM表征结果一致.在CO2裂解过程中,FeNi合金中的Fe信号明显降低,并出现金属Ni颗粒,说明Fe原子从合金中脱出.此外,在Ni颗粒表面没有出现FeOx等钝化层,说明氧化后的Fe可与载体快速反应,抑制了钝化层的形成,从而有利于提高CO2裂解转化率.相应57Fe穆斯堡尔谱结果表明,FeNi合金中的Fe与La2O3反应转化为LaFeO3,与XRD结果一致.对反应过程进行DFT计算,发现FeNi/La2O3界面很容易发生CO2吸附和活化,其反应活性远高于FeNi合金.同样地,氧化后形成的Ni/LaFeO3(110)界面有利于CH4吸附和C?H键解离,有利于Fe离子还原和FeNi合金再生.两步法太阳能热化学CO2裂解工艺经济可行性的一个关键指标是太阳能利用效率,热力学分析结果表明,即使在没有热回收的情况下,该过程的理论太阳能利用效率可达58％,当显热回收效率为90％时,太阳能利用效率可达78％.综上,本文发展了一种新型高效CO2热化学裂解氧载体,实现铁在钙钛矿-合金中的定向可逆迁移,通过勒沙特列原理显著提高了CO2分解转化率及反应活性,同时可产生高质量合成气.通过构建复合氧载体,实现Fe离子的原位稳定作为一种新型的氧载体设计策略,可推广应用至其他氧载体的设计开发,从而有效提高太阳能燃料生产效率.     

Electrochemistry at micro- and nanoscopic liquid/liquid interfaces

In this tutorial review, we will briefly introduce the history and basic concepts of micro- and nanoscopic liquid/liquid interfaces (size from nm to μm) in electrochemical studies of charge (electron and ion) transfer reactions at soft molecular interfaces. Their advantages and problems are usually compared with those of conventional liquid/liquid interfaces (size from mm to cm); and with solid/electrolyte interfaces. Three methods of fabrication of micro-liquid/liquid interfaces and one approach to support a nano-liquid/liquid interface are surveyed. The experimental and theoretical aspects are discussed along with possible approaches to characterize these micro- and nanoscopic liquid/liquid interfaces, and the methods to modify them with new functionality. Unique examples of applications of electrochemistry at micro- and nanoscopic liquid/liquid interfaces are provided. Some novel and potential research interests in the future in this field are discussed.     

Organische Solarzellen. Energie der Zukunft

Photovoltaic solar cells are of increasing importance in the use of regenerative energies due to the high supply of solar radiation. Therefore beside the established inorganic solar cells more low costs solar cells are developed which contain organic materials as active compounds for energy conversion. The article describes construction and function of dye‐sensitized solar cells and organic solid state solar cells. For comparison and understanding of these cells it is necessary to mention also some aspects of photosynthesis and inorganic solar cells. Altogether an insight in solar energy conversion systems under consideration of current developments is presented.     

Artificial Photosynthesis at Dynamic Self‐Assembled Interfaces in Water

Artificial photosynthesis is one of the big scientific challenges of today. Self‐assembled dynamic interfaces, such as vesicles or micelles, have been used as microreactors to mimic biological photosynthesis. These aggregates can help to overcome typical problems of homogeneous photocatalytic water splitting. Microheterogeneous environments organize catalyst–photosensitizer assemblies at the interface in close proximity and thus enhance intermolecular interactions. Thereby vesicles and micelles may promote photoinitiated charge separation and suppress back electron transfer. The dynamic self‐assembled interfaces solubilize non‐polar compounds and protect sensitive catalytic units and intermediates against degradation. In addition, vesicles provide compartmentation that was used to separate different redox environments needed for an overall water splitting system. This Minireview provides an overview of the applications of micellar and vesicular microheterogeneous systems for solar energy conversion by photosensitized water oxidation and hydrogen generation.     

Towards a bright future: polymer solar cells with power conversion efficiencies over 10%

Remarkable progress in high-performance polymer solar cells demonstrates their great potential for practical applications in the near future. Indeed, the power conversion efficiencies over 10% have been reported by many research groups, which are achieved through rational optimization of light-harvesting materials, interfaces and device processing technologies. In this mini review, we summarized the recent progress of highly efficient polymer solar cells, with specifically concern on successful strategies of rational molecular design of electron-donating and electron-accepting materials, elaborative interfacial engineering, and reasonable device architectures.     

Toward the Realization of A Practical Diketopyrrolopyrrole‐Based Small Molecule for Improved Efficiency in Ternary BHJ Solar Cells

An easily accessible DPP‐based small molecule ( DMPA‐DTDPP ) has been synthesized by a simple and efficient route. The resulting molecule, when incorporated into a P3HT:PCBM‐based BHJ solar cell, is found to significantly improve the efficiency. The utility of DMPA‐DTDPP as an additive yields an increase in the short circuit current density (J_sc) because DMPA‐DTDPP serves as an energy funnel for P3HT excitons at the P3HT:PCBM interfaces, resulting in an improved overall power conversion efficiency, compared to the P3HT:PCBM control device. Considering the trouble‐free and cost effective synthesis of DMPA‐DTDPP , it may prove very useful in high‐performance solar cells.     

A Solar Cell That Is Triggered by Sun and Rain

All‐weather solar cells are promising in solving the energy crisis. A flexible solar cell is presented that is triggered by combining an electron‐enriched graphene electrode with a dye‐sensitized solar cell. The new solar cell can be excited by incident light on sunny days and raindrops on rainy days, yielding an optimal solar‐to‐electric conversion efficiency of 6.53 % under AM 1.5 irradiation and current over microamps as well as a voltage of hundreds of microvolts by simulated raindrops. The formation of π‐electron|cation electrical double‐layer pseudocapacitors at graphene/raindrop interface is contributable to current and voltage outputs at switchable charging–discharging process. The new concept can guide the design of advanced all‐weather solar cells.     

Electrochemical recognition of synthetic heparin mimetic at liquid/liquid microinterfaces

Electrochemically controlled molecular recognition of a synthetic heparin mimetic, Arixtra, at nitrobenzene/water microinterfaces was investigated to obtain a greater understanding of interfacial recognition and sensing of heparin and its analogues with biomedical importance. In contrast to unfractionated heparin, this synthetic pentasaccharide that mimics the unique Antithrombin III binding domain of heparin possesses well-defined structure and ionic charge to enable quantitative interpretation of cyclic voltammetric/chronoamperometric responses based on the interfacial recognition at micropipet electrodes. Arixtra is electrochemically extracted from the water phase into the bulk nitrobenzene phase containing highly lipophilic ionophores, methyltridodecylammonium or dimethyldioctadecylammonium. Numerical analysis of the kinetically controlled cyclic voltammograms demonstrates for the first time that formal potentials and standard rate constants of polyion transfer at liquid/liquid interfaces are ionophore dependent. Moreover, octadecylammonium and octadecylguanidinium are introduced as new, simple ionophores to model recognition sites of heparin-binding proteins at liquid/liquid interfaces. In comparison to octadecyltrimethylammonium, the best ionophore for heparin recognition at liquid/liquid interfaces reported so far, these new ionophores dramatically facilitate Arixtra adsorption at the interfaces. With a saline solution at physiological pH, an Arixtra molecule is selectively and cooperatively bound to 5 molecules of the guanidinium ionophore, suggesting hydrogen-bond-directed interactions of each guanidinium with a few of 10 negatively charged sulfo or carboxyl groups of Arixtra at the interfaces.     

A sulfide/polysulfide-based ionic liquid electrolyte for quantum dot-sensitized solar cells

Further development of quantum dot-sensitized solar cells (QDSCs) will require long-term stability in addition to the continuous increase of photovoltaic (PV) conversion efficiency achieved in the last years. We report a robust S(2-)/S(n)(2-) electrolyte that has been specifically designed for compatibility with CdSe quantum dots in sensitized solar cells. The new pyrrolidinium ionic liquid reaches 1.86% efficiency and a short-circuit current close to 14 mA·cm(-2) under air-mass 1.5 global illumination and improves the device lifetime with good photoanode stability over 240 h. PV characterization showed that the solar cell limitations relate to poor catalysis of regeneration at the counter electrode and high recombination. Further improvement of these factors in the robust electrolyte configuration may thus have a significant impact for advancing the state-of-the-art in QDSCs.     

Electroviscoelasticity of liquid/liquid interfaces: fractional-order model

A number of theories that describe the behavior of liquid-liquid interfaces have been developed and applied to various dispersed systems, e.g., Stokes, Reiner-Rivelin, Ericksen, Einstein, Smoluchowski, and Kinch. A new theory of electroviscoelasticity describes the behavior of electrified liquid-liquid interfaces in fine dispersed systems and is based on a new constitutive model of liquids. According to this model liquid-liquid droplet or droplet-film structure (collective of particles) is considered as a macroscopic system with internal structure determined by the way the molecules (ions) are tuned (structured) into the primary components of a cluster configuration. How the tuning/structuring occurs depends on the physical fields involved, both potential (elastic forces) and nonpotential (resistance forces). All these microelements of the primary structure can be considered as electromechanical oscillators assembled into groups, so that excitation by an external physical field may cause oscillations at the resonant/characteristic frequency of the system itself (coupling at the characteristic frequency). Up to now, three possible mathematical formalisms have been discussed related to the theory of electroviscoelasticity. The first is the tension tensor model, where the normal and tangential forces are considered, only in mathematical formalism, regardless of their origin (mechanical and/or electrical). The second is the Van der Pol derivative model, presented by linear and nonlinear differential equations. Finally, the third model presents an effort to generalize the previous Van der Pol equation: the ordinary time derivative and integral are now replaced with the corresponding fractional-order time derivative and integral of order p<1.