【作者】 Shamsa Bibi；

【导师】 张景萍；

【作者基本信息】 东北师范大学 ， 物理化学， 2014， 博士

【摘要】 在过去十年里，有机聚合物太阳能电池的研究突飞猛进，因其原料电池成本低廉，最大光电转换效率(PCE)已经突破10%。然而，有机小分子太阳能电池的PCE因吸收光谱和相应的能量损耗不匹配，其PCE也比有机聚合物太阳能电池低.使得有机小分子研究受限。因此，本文采用DFT方法，选用苯并二噻吩和齐聚噻吩作为给体片段，选择不同的受体片段，通过分子构建模式、拓扑结构、给受体片段组成与比例等因素，设计和研究系列枝杈型及星型给体分子。本研究旨在设计出具有与受体分子能级匹配、强的光吸收和高电荷载流子迁移率等性质的含硫有机小分子给体材料，从而提高有机小分器件的PCE。首先筛选合适的太阳能电池（OSC）给体材料构筑模块。设计系列共轭给体分子（DmAn，m=1-4，n=1-7，D=苯并二噻吩，A=苯并恶二唑），通过改变D/A片段比例、拓扑结构，来调节材料的光电子性质。发现增加受体片段的比例可以降低LUMO能级从而设计窄带系（Eg）分子。与线型构型相比，互相垂直的给体和受体片段构型可以观测到设计分子的光谱发生明显的红移。然而线型结构分子吸收增强。D-A-D分子相对于D-D拓扑结构，具有优异的光电性能。此外，D-A-D拓扑结构比多垂直连接的受体片段的拓扑结构具有更佳的光电性质。D-A-D型给体分子（m=2-4）的空穴重组能小于单给体组成（DAn）的分子。尽管如此，D-D型分子比DAn分子的电子重组能小，主要是由于第二个给体片段的存在。并且，电荷转移也依赖于分子的形状，枝杈型或各向异性X，H，π，n及正方形因高维度，显示出比线型异构体高的电荷迁移率.相对于系列PDI基受体分子，建议相应匹配的给体分子，为构造太阳能电池器件提供理论依据。利用DFT方法采用双波段重叠策略，为了提高材料的溶解性，设计了五个X-型各向异性低能隙给体分子（D1-D5)。通过调节PDI1的P1和P2位置的取代基，构建一系列新的PDI受体分子是为了与设计的给体分子相匹配。设计的给体包含给体片段苯并二噻吩（DF）作为核，电子受体片段（A1到A5）及三呋喃环和乙炔基三呋喃环作为桥（分别为B1和B2）。TD-DFT计算的给体分子吸收带覆盖了可见光和近红外区。给体片段DF和多枝杈B1和B2贡献短波吸收和中等波长区域，光谱的中间和长波长区域归因于受体片段和给受体片段间电荷转移吸收。所设计给体中，D1有理想的最低能隙、前线分子轨道能级和强的宽吸收是由于其具有强的吸电子片段。与空穴重组能相比，电子重组能较低，表明五个受体分子有利于电子转移。采用P21/c空间群预测结晶态D1的迁移率。D1显示出高迁移率（μe=2.00cm2/V.s和μh=1.7×10-2cm2/V.s）。计算所得D1的开路电压Voc为1.02V。所设计的给体分子和PDI衍生物受体相匹配，可以应用于高性能的太阳能电池件。为了进一步探讨分子各向异性对3D多枝化合物的吸收性质的影响，我们以并噻吩为给体(DF)单元，通过乙炔作为π桥(Ps)连接吡啶-噻二唑受体(AF)单元，再与不同的中心原子结合，设计并研究了不同的共轭的三臂及四臂星形分子。三臂及四臂星形分子的中心原子分别为N、B和C、Si。结合密度泛函理论(DFT)和时间依赖密度泛函理论(TD-DFT)两种方法来探究中心原子对两种不同拓扑结构(即“core-D-π-A”和“core-A-π-D”)类型分子的光学，电子及电荷转移性质的影响。“core-D-π-A”型分子(N3-Mol，B3-Mol，Si4-Mol和C4-Mol)的HOMO能级比“core-A-π-D”型分子(N3-RMol，B3-RMol，Si4-RMol和C4-RMol)更理想。“core-A-π-D”型分子的λmax值与“core-D-π-A”型分子相比发生了显著红移。三臂型分子N3-Mol和N3-Rmol在所设计的不同分子中λmax值最大。有趣的是，B3-RMol和C4-RMol分子的λmax波长几乎相同。然而，以B和Si为中心原子的三臂及四臂型分子的吸收带强度与以N和C为中心原子的分子相比更强。C4-RMol，Si4-RMol，B3-RMol和N3-RMol分子与C4-Mol，Si4-Mol，B3-Mol和N3-Mol分子相比分别发生了59，14，28和39nm的红移。重组能和迁移率的研究结果都表明四臂型分子与三臂型分子相比电荷转移性质更加优异，这是由于前者具有更好的维度。因此，光电子和电荷转移性质的分析结果证实本文设计的三臂及四臂星形分子是一种十分具有应用前景的有机太阳能电池给体材料。本文得到的结果可以为今后高效有机光伏小分子给体的研究提供更多的思路。更多还原

【Abstract】 Harvesting sunlight energy is regarded as being one of the most important ways to dealgrowing global energy demands using photovoltaic technology. Organic photovoltaics (OPV)is a field of applied research which has been growing speedily in the last decade leading to acurrent record value of power-conversion efciency (PCE) of10%. One major motive forthis boom is a potentially low-cost production of solar modules on flexible polymer substrate.However, the PCE of OPVS based on the small molecular donors is comparably smaller thanthose based on polymer. The lower PCE is confined because of the poor mismatch betweenthe absorption spectra of the small organic molecules and that of sun which in turn causesenergy lose. To this end, the research activities include the designing and investigation ofsmall branched or star shape donor molecules based on the different molecular strategies,configurations, topologies, donor-acceptor ratios and compositions by using theoreticaldensity functional theory (DFT) approaches. The aim of the research was to design optimalorganic donor small molecules with improved parameters like appropriate energy levels withrespect to the acceptor molecules, strong visible light absorption and sufficient high chargecarrier mobility, which can be used as potential candidates for small molecular OPV withhigher PCE.The first part of thesis focuses to screen out proper building blocks for the design oforganic solar cell (OSC) donor materials. Hence, Varying ratio of D/A fragments, topologiesand their effects on the optical and electronic properties of a series of conjugated donormolecules (DmAnwhere m=1-4and n=1-7while D=benzodithiophene and A=benzooxadiazole) are explored for OPV applications. An increase in the ratio of acceptorfragments lowers the LUMO energy level and narrows the Eg for the designed molecules. Inthe case of molecules containing more vertically bonded acceptor fragments with donorfragment, a significant red shift in the absorption wavelength of the final donor molecules isobserved as compared to linearly bonded ones. While, the linear binding side of donorfragments assisted considerably intensify the absorption bands. D-A-D topology moleculesexhibit more significant optical and electronic characteristics than those of D-D topology. Inaddition, D-A-D topology shows prominent affect over the factor of having more verticalbonded fragments in terms of opto-electro properties. All donor molecules (m=2-4) of D-A-Dtype exhibit lower λhthan those of1donor containing (DAn) molecules. Nevertheless, D-Dtype molecules show only lower λethan DAnmolecules due to the presence of second donorfragment. Furthermore, charge transfer phenomenon is also shape dependent, branched or anisotropic X, H, π, n, and square shapes display higher charge transfer rate thancorresponding linear isomers because of having better dimensionality. Finally, the designeddonor molecules and matched acceptor molecules hold immense potential to construct solarcells devices.Secondly, employing a double overlapping wave band strategy based on DFT, five X-shape anisotropic low energy gap donor compounds (D1-D5) have been designed for solarcells applications possessing good solubility. A series of new PDIs acceptor molecules arebuilt to match each designed donor in terms of frontier molecular orbital energy levels bytuning substituents at P1and P2positions of PDI1. The designed donors consist of centralelectron donor fragment benzodithiophene (DF), electron accepting fragments (A1to A5) andterfuran and ethynyl-terfuran bridges (B1and B2respectively). The absorption bands of thedesigned donors based on TD-DFT not only cover the visible region but also extend toinfrared region of spectrum. The multibranched π-conjugated B12-DF-B22donor fragmentprovides the strong and broad short wavelength π–π*absorption while the anisotropicmultibranched intramolecular charge transfer (ICT) between the B12-DF-B22donor and Afavors fragment the strong and broad middle and long wavelength absorption. In addition,PDIs also exhibit complementary absorptions in the visible range of solar spectrum. Amongdesigned donors, D1exhibits ideal lowest band gap, FMO energy levels and exclusivebroadest absorption because of the strongest electron withdrawing fragment. The lower λevalues as compared to λhillustrate that these five donors would be favorable for electrontransfer. The carrier mobility of D1in the crystalline state has been predicted using P21/cspace group. D1displays higher carrier mobilities for μe=2.00cm2/V.s and μh=1.7×10-2cm2/V.s. The calculated Voc of D1is1.02V. The designed donors and PDIs acceptors aresuitable and recommended for high performance solar cell devices.Thirdly, with the aim to investigate further the effect of molecular anisotropy on theabsorption properties of3D multi-branched compounds, we have designed and investigatedconjugated three and four arms star shape molecules with bithiophene as donor (DF)fragment connected to pyridine-thiadiazole as acceptor fragment (AF) via ethyne as π-spacer(Ps) in the arms linking with different central core atoms. The central cores in three and fourarms molecules are N, B and C, Si respectively. A combination of density functional theory(DFT) and Time-dependent-DFT (TD-DFT) approaches is applied to understand the effect ofdifferent central cores on the optical, electronic and charge transport properties in twodifferent topologies i.e.“core-D-π-A” and “core-A-π-D”. HOMO energy levels of the “core- D-π-A” type molecules (N3-Mol, B3-Mol, Si4-Mol and C4-Mol) are more ideal related tothose of “core-A-π-D” type molecules (N3-RMol, B3-RMol, Si4-RMol and C4-RMol). Theλmaxvalues of “core-A-π-D” type molecules are significantly red shifted than those of “core-D-π-A” type molecules. Three arms N3-Mol and N3-RMol display the largest λmaxamong therespective designed molecules. Interestingly, B3-RMol and C4-RMol show more a less sameλmaxwavelength. However, in case of three and four arms molecules absorption bands of Band Si cores containing molecules are of strong intensity as compared to those containing Nand C cores respectively. The molecules C4-RMol, Si4-RMol, B3-RMol and N3-RMol showa red shift of59,14,28and39nm than λmaxof C4-Mol, Si4-Mol, B3-Mol and N3-Mol. Bothreorganization energy and mobility results reveal that four arms molecules are better chargetransport materials than three arms molecules because of better dimensionality. Thus, opticalelectronic and charge transport properties analysis confirms that these designed three and fourarms molecules can act as promising donor materials for organic solar cells. We believe thatthe present results can also provide a potential information to develop small molecular donorsfor highly efficient organic photovoltaics.更多还原