Theoretical characterisation and design of D–π–A star-shaped molecules with triphenylamine as core and diketopyrrolopyrroles as arms for organic solar cells

A series of novel D–π–A star-shaped molecules with triphenylamine (TPA) fragments as cores, diketopyrrolopyrrole (DPP) fragments as arms, and different conjugate π-bridges those connect TPA and DPP fragments have been designed for small molecules based organic solar cells (OSCs) applications. The optical, electronic, and charge transporting properties have been systematically investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The parameters such as energetic driving force ΔE_L-L, adiabatic ionisation potentials (AIP), and adiabatic electron affinities (AEA) were also calculated at the same level to preliminarily evaluate the performance of the OSCs. The results of frontier molecular orbitals (FMOs) show that the designed molecules provide the best match matched energy levels with typical acceptors PCBM, bisPCBM, and PC70BM except for molecule containing benzo[c][1,2,5]thiadiazole unit because of its low energetic driving force ΔE_L-L. It turned out that the FMO energy levels, the band gaps, AIP, AEA, and absorption spectrum can be tuned effectively by the introduction of different groups. Additionally, our results suggest that the designed molecules can act as promising candidates for donor materials and ambipolar charge transport materials for OSCs.     

有机太阳能电池的研究进展

能量转化效率低是有机太阳能电池实现商业化生产的一个瓶颈,因此,制备高性能太阳能电池的关键之一是提高能量转化效率(ηp),文章介绍了有机太阳能电池的工作原理,论述了目前有机太阳能电池的研究现状,重点从提高有机太阳能电池的开路电压(Voc)、短路电流(Isc)、光电转换效率(ηEOE)和填充因子(FF)等几方面分析总结了提高有机太阳能电池能量转化效率的几种有效途径,并简要阐述了有机太阳能电池稳定性的研究进展.     

石墨烯衍生物作为有机太阳能电池界面材料的研究进展

本文综述了石墨烯及其衍生物作为界面材料在有机太阳能电池中的应用, 包括作为阳极界面层、阴极界面层和叠层电池中间层等方面. 氧化石墨烯由于较好的透光性, 易于分散在水溶液中与溶液加工等优点已被应用在有机太阳能电池中. 对氧化石墨烯作为阳极界面层的研究包括通过部分还原或掺杂提高其导电性、通过引入高负电性原子提高其表面功函数, 以及通过与其他材料复合提高性能等. 同时, 本文综述了石墨烯衍生物及复合材料作为有机太阳能电池阴极界面层和叠层电池中间层的研究. 最后本文展望了石墨烯及其衍生物在有机太阳能电池与有机无机复合钙钛矿太阳能电池中的应用前景.     

Photoconductivity as loss mechanism in organic solar cells

We observe that the reverse current under illumination in solar cells containing C₆₀ and ZnPc is dominated by a photoshunt. This shunt, not present in the dark, causes a linear current–voltage relation under illumination showing no saturation. Although observable in bulk heterojunctions, this effect is more pronounced in the presence of a pristine C₆₀ layer. An internal quantum efficiency larger than unity under an applied negative voltage and in the spectral range where C₆₀ absorbs identifies charges which are injected in addition to those photogenerated. The photoshunt is also present in the power‐generating region and represents a loss mechanism limiting the fill factor in particular for flat heterojunction devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theory study on the properties of thiadiazole polymer donors for organic solar cells

In this work, the properties of [1,2,5] thiadiazolo [3,4‐c] pyridine ‐alt‐cyclopenta [2,1‐b:3,4‐b′] dithiophene (PT‐CDT) and [1,2,5] thiadiazolo [3,4‐c] pyridine‐6‐carbonitrile‐alt‐cyclopenta [2,1‐b:3,4‐b′] dithiophene (PCNT‐CDT) as donors were investigated by means of Density Functional Theory. The electronic properties and optical absorption properties were discussed, and hole‐transfer properties of donors were studied by Marcus electron transfer theory. The results indicate that the linear structure of PCNT‐CDT and PT‐CDT is more stable than the spiral structure of PCNT‐CDT and PT‐CDT; the absorption peak in visible region of PCNT‐CDT is stronger and wider, and the absorption spectrum is more matchable to solar spectrum than PT‐CDT, while the maximum absorption wavelength of PCNT‐CDT has an obvious red shift; the two designed materials show strong intramolecular and intermolecular charge transfer properties; PCNT‐CDT owns a large open‐circuit voltage and low reorganization energy, as well as high hole mobility. Therefore, the newly designed PCNT‐CDT can be a potential donor material of organic solar cell. Copyright © 2013 John Wiley & Sons, Ltd.     

Thermally evaporated Ag nanoparticle films for plasmonic enhancement in organic solar cells: effects of particle geometry

We report on the simple fabrication of Ag NP films via thermal evaporation and subsequent annealing. The NPs are formed on indium tin oxide electrodes, coated with PEDOT:PSS and implemented into PCPDTBT:PC70BM solar cells. Scanning electron microscopy and atomic force microscopy are used to determine the size distributions and surface coverage of the NP film. We apply finite‐difference time‐domain techniques to model the optical properties of different nanoparticle films and compare this with the absorption properties of the organic active layer. The simulations demonstrate that the absorption and scattering efficiency of the particles are very sensitive to particle geometry. Solar cells prepared with window electrodes containing NP layers with less surface coverage, show a 14.8% improvement in efficiency. We discuss variations in the external quantum efficiency of the devices in terms of forward scattering and parasitic absorption losses induced by the NP layer. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Photoinduced degradation of organic solar cells with different microstructures

An in situ measurement setup is established to investigate the photoinduced degradation effects in a controllable inert gas ambient environment for the two different microstructures of poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl-C61-butyricacid methyl ester(PCBM) bulk-heterojunction organic solar cells. The two devices are fabricated with the solvent vapor drying process followed by a thermal annealing(vapor drying device) and only a normal thermal annealing process(control device), respectively. Their power conversion efficiencies(PCEs) and aging features are compared. Their different degradation behaviors in light absorption are confirmed. In addition, irradiation-induced changes in both nanostructure and surface morphology of the P3HT:PCBM blend films treated with two different fabrication processes are observed through scanning electron microscopy and atomic force microscopy. Aggregated bulbs are observed at the surfaces for control devices after light irradiation for 50 h, while the vapor drying devices exhibit smooth film surfaces, and the corresponding device features are not easy to degrade under the aging measurement. Thus the devices having solvent vapor drying and thermal annealing show better device stabilities than those having only the thermal annealing process.     

给体-受体型有机太阳电池光敏层的优化设计

针对有机给体-受体型异质结薄膜太阳电池，基于薄膜中的光学干涉效应和激子扩散理论建立模型，定量分析了有机层厚度对太阳电池性能的影响，通过限制有机光敏层厚度来控制由光学干涉效应引起的局部光子密度(光强度)的分布，进而优化有机层的吸收效率和激子扩散效率，以此达到提高器件光伏效率的目的.并通过实验进行了验证. 关键词： 有机太阳电池 给体-受体 优化设计 激子     

有机共混结构叠层太阳电池的研究进展

有机太阳电池由于质轻、价廉、柔性,受到人们的广泛关注.单个有机材料只能吸收部分太阳光,叠层结构的太阳电池将不同吸收带隙的有机材料通过中间层连接起来,既能充分吸收太阳光,又能提高太阳电池的开路电压或短路电流.本文综述了近年来有机共混结构叠层太阳电池的研究进展,介绍了各种叠层有机太阳电池的结构、原理及性能,阐述了国内外有机叠层太阳电池研究的现状及存在问题,为高性能有机太阳电池的研究提供有价值的参考.     

Influence of optical interference and carrier lifetime on the short circuit current density of organic bulk heterojunction solar cells

Based on simple analytical equations, short circuit current density (J_SC) of the organic bulk heterojunction solar cells has been calculated. It is found that the optical interference effect plays a very important role in the determination of J_SC; and obvious oscillatory behaviour of J_SC was observed as a function of thickness. At the same time, the influence of the carrier lifetime on J_SC also cannot be neglected. When the carrier lifetime is relatively short, J_SC only increases at the initial stage and then decreases rapidly with the increase of active layer thickness. However, for a relatively long carrier lifetime, the exciton dissociation probability must be considered, and J_SC behaves wave-like with the increase of active layer thickness. The validity of this model is confirmed by the experimental results.     

Effective absorption enhancement in small molecule organic solar cells using trapezoid gratings

We demonstrate that the optical absorption is enhanced in small molecule organic solar cells by using a trapezoid grating structure. The enhanced absorption is mainly attributed to both waveguide modes and surface plasmon modes, which is simulated by using finite-difference time-domain method. The simulated results show that the surface plasmon along the semitransparent metallic Ag anode is excited by introducing the periodical trapezoid gratings, which induce the increase of high intensity field in the donor layer. Meanwhile, the waveguide modes result in a high intensity field in acceptor layer. The increase of field improves the absorption of organic solar cells significantly, which is demonstrated by simulating the electrical properties. The simulated results also show that the short-circuit current is increased by 31% in an optimized device, which is supported by the experimental measurement. Experimental result shows that the power conversion efficiency of the grating sample is increased by 7.7%.     

有机半导体薄膜生长原位实时测量方法的研究

针对原位实时监测有机半导体薄膜生长情况的需求, 提出了差分反射光谱法与场效应晶体管法结合的光电联合测量方法, 设计研制了测量系统. 以并五苯有机分子为例, 通过自制底栅底接触式场效应管微结构, 实验测试了热蒸发法生长导电膜层过程中光电信号的演变与相互关联. 光谱信号显示, 并五苯以薄膜态结构进行生长, 光谱随生长进程变化显著. 实验数据与四相结构模型仿真结果的良好吻合, 表明因薄膜增厚引起干涉条件的改变是光谱变化的主因, 由此推算出薄膜生长速率为0.23 nm/min. 当薄膜等效厚度达到28 nm时, 场效应管的导电性显著增强, 标志着并五苯有效传输层的形成. 此后, 薄膜厚度持续增加, 但测试电流增长缓慢, 说明该结构进入电学特性饱和区. 光电联合法不仅有助于研究有机半导体薄膜的光谱信息、电学特性和薄膜结构之间的相互对应关系, 也为发展原位监测有机半导体薄膜制备过程, 探索最佳工艺提供了新的研究手段.     

Rubrene作电子传输层的异质结有机太阳能电池

以Rubrene为电子传输层(ETL),制备了结构为ITO/MoO3(5nm)/Rubrene(50nm)/C60(45nm)/Rubrene(0,3,5.5,9.5nm)/Al(130nm)的有机太阳能电池.与没有ETL的器件相比,含5.5nmRubrene的电池的开路电压、填充因子、功率转换效率分别从0.68V,0.488,0.315%增加到0.86V,0.574,0.490%.实验结果分析表明:热的Al原子直接沉积在C60上,破坏了C60层,形成高功函数的C60/Al阴极,弱化内建电场,降低电池性能;当插入ETL后,C60层得到保护,热的Al原子沉积破坏了Rubrene层,形成了缺陷态能级,提高电池的内建电场,促进了电子的传输.进一步的单电子电池实验表明,缺陷态能级低于C60的最低未占据分子轨道.     

Interfacial processes in small molecule organic solar cells

This paper presents an overview of the recent progress of small molecule organic solar cells mainly based on the previous worksof our group. We will mainly focus on the interfacial processes in the cells. The dissociation of excitons at electrode/organic andorganic/organic interfaces can be directly observed by transient photovoltage measurements. A simple model including dissociationof excitons at the interface and drift of free carriers in the built-in field is proposed to explain the observed signals of ...     

Highly conductive and transparent carbon nanotube-based electrodes for ultrathin and stretchable organic solar cells

In this work, we have presented a freestanding and flexible CNT-based film with sheet resistance of 60 ?/ and transmittance of 82% treated by nitric acid and chloroauric acid in sequence. Based on modified CNT film as a transparent electrode, we have demonstrated an ultrathin, flexible organic solar cell(OSC) fabricated on 2.5-μm PET substrate. The efficiency of OSC, combined with a composite film of poly(3-hexylthiophene)(P3HT) and phenyl-C61 butyric acid methyl ester(PCBM) as an active layer and with a thin layer of methanol soluble biuret inserted between the photoactive layer and the cathode, can be up to 2.74% which is approximate to that of the reference solar cell fabricated with ITO-coated glass(2.93%). Incorporating the as-fabricated ITO-free OSC with pre-stretched elastomer, 50% compressive deformation can apply to the solar cells. The results show that the as-prepared CNT-based hybrid film with outstanding electrical and optical properties could serve as a promising transparent electrode for low cost, flexible and stretchable OSCs, which will broaden the applications of OSC and generate more solar power than it now does.     

Computational study of new azo dyes with different anchoring groups for dye-sensitised solar cells

Some of new azo dyes with different anchoring groups, such as biscarbodithiolic acid, hydroxamic acid, phosphonic acid, carboxcylic acid and sulfonic acid have been investigated theoretically to evaluate the effects of various anchoring groups on the optical and electronic properties of the dyes in dye-sensitised solar cells. Optical and electronic properties, UV–Vis absorption spectra, light-harvesting efficiency, lifetime of the excited state, chemical hardness and lowest unoccupied molecular orbital (LUMO) orbital weight of the dyes on the anchoring groups, have been studied to shed light on how the various anchoring groups influence the properties of the dyes. The biscarbodithiolic acid-based dye shows the longest maximum absorption wavelength and the widest absorption spectra together with the highest light-harvesting efficiency, the longest lifetime of the excited state and the highest the LUMO orbital weight of the dye on the atoms of the anchoring group, suggesting the good ability in electron injection. Theoretical calculations have been also performed on the adsorption of these dyes on the TiO₂ anatase (101) surface. These results show that the biscarbodithiolic acid-based dye has the highest adsorption energy and the largest negative shift of the conduction band of TiO₂ due to the adsorption of the dye onto the TiO₂.     

Reduced exciton binding energy in organic semiconductors: Tailoring the Coulomb interaction

For organic photovoltaics (OPV) the maximum in obtainable power conversion efficiency is limited by a low semiconductor permittivity and the resulting enhanced Coulomb interaction (CI). This, however, is an aspect rarely addressed in the OPV development. Here, a concept is introduced which allows a reduced CI in organic semiconductors. This is the result of a device structure, which upon exciton formation forces part of the electric field between complementary charges through a high‐k material, resulting in partial field screening and as such a reduced CI. The feasibility of this concept is substantiated by an investigation on the exciton separation efficiency in pentacene devices. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stability issues of the next generation solar cells

Polymer, perovskite, and dye‐sensitized solar cells (DSSCs) are promising technologies for next generation low cost photovoltaic cells. Among these, perovskite solar cells are the newest technology and have the highest efficiency, while DSSCs are closest to commercialization with several companies producing the DSSC materials and modules and existing DSSC installations. However, all three types of solar cells share a concern about lifetime and stability. For each type of devices, there are specific concerns and degradation mechanisms, and all of the devices require encapsulation and exhibit varying degrees of sensitivity to moisture, oxygen, elevated temperature and UV illumination depending on the device structure and materials used. We are discussing the stability and lifetime for each type of cells and future outlook of these technologies. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Single-layer antireflection coatings for GaAs solar cells

Optimal parameters of antireflection coatings for GaAs solar cells are simulated and calculated by the optical matrix approach method. It is shown that by using single-layer Ta₂O₅ and ITO antireflection coatings it is possible to reduce essentially the reflectance from the surface of GaAs solar cells.     

PEDOT:PSS薄膜的山梨醇掺杂对光电池性能的影响

采用共混-旋涂技术在ITO导电玻璃上制备出经山梨醇掺杂的PEDOT:PSS导电膜,将所制得的薄膜作为空穴传输层用于有机太阳能电池研究.通过对比分析掺杂前后光电池暗电流曲线与光电流曲线的变化,考察了山梨醇掺杂对器件光伏性能的影响,并就其中的影响机理进行了讨论分析.结果表明,山梨醇的加入,可以明显提高光电池的短路电流,填充因子以及能量转换效率.较未掺杂器件,8wt %山梨醇掺杂条件下,器件短路电流由8.82 mA/cm²增加至11.27 mA/cm²,FF由0.43 关键词： PEDOT:PSS薄膜 山梨醇 有机太阳能电池 性能