新型窄带隙聚合物太阳能电池材料的合成及性能研究

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英文题名：Synthesis and Properties of Novel Low Band Gap Polymers for Organic Solar Cells 作者：李永玺 论文级别：博士 学科专业名称：先进材料与制备技术 中文关键词：有机太阳能电池 ; 共轭聚合物材料 ; 窄带隙聚合物 ; 拉/推电子基 英文关键词：solar cells ; conjugated polymer ; narrow band gap polymers ; electron-withdrawing/donating groups 学位年度：2014 导师：陈彧 学科代码：0805 学位授予单位：华东理工大学 论文提交日期：2014-05-24 答辩委员会主席：郑思珣

摘要

有机光伏技术为太阳能的有效利用提供了一条重要途径。凭借着其制造成本低廉、材料质量轻、加工性能好,易于携带等优势而备受关注。提高有机太阳能电池的光电转换效率是目前乃至未来的研究重点。设计和合成适合的窄带隙(LBG)的共轭聚合物是提高有机太阳能电池光电转化效率的核心。本论文主要围绕给体(D)-受体(A)型共轭聚合物材料的设计、合成以及光电性能研究,以期望获得高效的LBG共轭聚合物。主要内容如下：
     1.设计合成了一系列基于环戊并二噻吩(CPDT)单元的D-A型LBG共轭聚合物,研究了氟原子及侧链对聚合物光电性质的影响。聚合物在可见及近红外区域内都表现出很强的吸光能力,能带间隙均小于1.5eV。由于氟原子的缺电子性质以及侧链的不同构型,导致聚合物在固体状态下表现出不同的排列方式,其中聚[2,6-(4,4-双(2-乙基己基)-4氢-环戊[2,1-b；3,4-b']二噻吩)-交-4,7(单氟-2,1,3-苯并噻二唑)]EH-FBT的π-π堆砌距离仅为是3.8A,空穴迁移率为0.014cm2V-1s"1。基于PCPDTFBT的异质结(BHJ)太阳能电池器件经过性能优化后光电转化率(PCE)达到6.6%,短路电流为14.3mA/cm2。将其进一步应用到双结(异质结)电池中,PCE高达8.2%。
     2.设计合成了一个十一元共轭稠环化合物(IDTCPDT),该化合物表现出强的给电子能力,高的摩尔消光系数,好的刚性平面结构以及低的重组能。基于该给体单元,合成了窄带隙聚合物PIDTCPDT-DFBT,该聚合物拥有很强的吸光能力,很好的平面性,荧光寿命为1.52ns。理论计算得到的重组能只有3.2kcal/mol。基于该聚合物的场效应晶体管空穴传输能力可达2.4×10-2cm2V-1s-1。将该聚合物作为给体材料制作成为太阳能电池器件时,PCE为6.5%,短路电流为14.6mA/cm2,是目前已报道的基于阶梯型聚合物太阳能电池的最高值。
     3.采用微波辅助Stille偶联设计并合成了三个基于吡咯并吡咯二酮(DPP)的二维共轭聚合物。由于DPP的较强缺电子特性,得到的三个聚合物都表现出极窄的能带间隙,分别为1.50,1.48和1.43eV,可以与太阳光谱很好的匹配。再者,DPP骨架出色的平面性以及其形成氢键的能力使得三个共聚物都具有较强的π-π堆砌能力。电化学测试表明,增加侧链的共轭长度可以有效的降低HOMO能级,提高聚合物电池的开路电压。值得注意的是活性层的形貌对激子的解离和扩散是非常重要的。当使用纯邻二氯苯作溶剂制备活性层时,聚合物与PC71BM之间形成尺寸较大的团聚体。当使用共混溶剂时(如氯仿和邻二氯苯)团聚现象消失,聚合物和(6,6)-苯基C71丁酸甲基酯(PC71BM)之间形成网状纳米纤维,极大的改善了聚合物与PC71BM之间的相互聚集,提高了激子的有效分离几率,器件的光电转化效率从原来的0.24%提高到4.47%。优化后的器件PCE最高达到了5.34%。
     4.设计合成了苯并硒二唑衍生物单氟-2,1,3-苯并硒二唑(FBSe),增加了苯并硒二唑的吸电子能力并且降低了HOMO能级。并以FBSe为受体单元共聚得到聚合物PBDT-FBSe和PIDT-FBSe。两个聚合物都拥有较小的能带间隙分别只有1.60和1.58eV。其次,这两个聚合物具有良好的堆积作用,其薄膜紫外吸收较溶液吸收,发生了明显的红移。此外,PBDT-FBSe和PIDT-FBSe表现出较低的HOMO能级,这有利于在光伏器件中获得较高的开路电压。这两个聚合物的场效应晶体管空穴传输能力分别为1.1×10-4和3.0×10-3cm2V-1s-1。作为给体材料,BHJ太阳能电池的PCE分别达到5.00%和4.65%。
     5.设计并合成了以氟代喹喔啉为受体单元的窄带隙聚合物PCPDT-DFPhQ, PCPDT-DFPhQ-M和PCPDT-DFPhQ-O,这些聚合物都具有很好的溶解性,能溶于大多数有机溶剂中。当引入烷氧基侧链时,聚合物PCPDT-DFPQ-O的HOMO能级显著升高。原子力显微镜结果表明较长的烷氧基侧链会引起聚合物空间位阻的增加,与PC71BM之间会形成尺寸较大的团聚体,影响激子的分离和扩散。因此,基于PCPDT-DFPhQ-O的太阳能电池器件,仅取得0.94%的PCE。其中短路电流只有2.52mA/cm2。相反,基于PCPDT-DFPhQ的电池获得最大PCE为5.30%,开路电压达到了0.83V,短路电流提升到12.05mA/cm2。
     6.设计并合成了两个D-A1-D-A2型共轭聚合物PCPDT-DFBT-TPD和PCPDT-DFBT-DPP。这两个聚合物都拥有较小的能带间隙,较低的HOMO能级。此外PCPDT-DFBT-TPD和PCPDT-DFBT-DPP都能溶于绝大多数有机溶剂。而基于PCPDT-DFBT-TPD和PCPDT-DFBT-DPP的BHJ太阳能电池器件的PCE分别达到了3.15%以及3.11%。
     7.系统总结了第二章至第七章的主要研究结果。
Organic photovoltaic technology provides an essential way for the effective utilization of solar energy. The advantages of polymer solar cells (PSCs) include low cost, light weight, easy fabrication, flexible and tunable properties. There is increasing interest in PSCs during the last few years, however, the power conversion efficiency (PCE) of PSCs far beyond practical requirements. Up on this, Design and synthesis of appropriately low band gap polymers and probing into the mechanisms become the extremely interesting topics in the field of high efficiency PSCs. Based on these, several novel low band gap polymeric materials were successfully prepared. Besides, the PCE of the materials were also studied and some positive and original results were obtained.
     This thesis was divided into eight parts, as follows:
     Chapter1:A serials of cyclopentadithiophene-based conjugated polymers with varied alkyl side-chain patterns and fluor-substitutions were successful designed and synthesized. All of the polymers exhibit strong absorptions and extremely narrow band gaps (Eg<1.5eV). The poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b']dithiophene)-alt-4,7-(monofluoro-2,1,3-benzo-thiadiazole)](PCPDTFBT) with short side chain and mono-fluro substitution had the shortest π-π stacking distances (3.8A) and the highest mobility (0.014cm2V-1s-1). The bulk heterojunction solar cell devices based on this polymer showed the highest PCE of6.6%in single junction solar cells and8.2%in double junction solar cells.
     Chapter2:Two fused-rings ladder-type conjugated polymers were designed and synthesized. The fully fused PIDTCPDT-DFBT possesses lower band-gap, better planarity and lower reorganizational energy, and one-order higher hole-mobility. The charge separated configuration of PIDTCPDT-DFBT was found to be lived up to1.52ns in the chlorobenzene. The solar cells made from PIDTCPDT-DFBT also showed higher power conversion efficiency of6.46%. The short circuit current (Jsc) also increased～40%from10.40mA/cm2for a partially fused reference polymer, PIDTT-T-DFBT to14.59mA/cm2. This is among one of the highest Jsc reported for the ladder-type polymers. These results showed the strategy of extending conjugation length in fused-ring ladder-type polymers was an effective way to reduce band-gap and improve charge transport for polymers to obtain higher photovoltaic efficiencies.
     Chapter3:Three two-dimension polymers PBDT-DPP, PBDTTT-DPP and PBDT-TTDPP were successfully designed and synthesized. The change from thiophene to thieno[3,2-b]thiophene (TT) in the side chain and bridge caused variety of absorption, electrochemical and hole transport property. The results from UV-Vis measurements showed that enhanced absorption coefficient could be obtained when TT unit introduced as a bridge, which in turn could absorb light efficiently. The photovoltaic properties of these polymers were investigated using the device configuration of ITO/PEDOT:PSS/polymer:PC71BM/Bis-C6o/Ag. The highest achievable PCE for PBDT-DPP, PBDTTT-DPP, and PBDT-TTDPP were4.06%,4.47%, and5.34%, respectively. It was worth noting that the quality of the blending films played an important role in exciton separation and diffusion. The morphology of these films improved significantly due to the co-solvent processing, which leading to the remarkably increased PCE of these devices.
     Chapter4:A new Se-containing electron deficient building block monofluro-2,1,3-benzoselenadiazol (FBSe) was developed. By using a microwave-assisted palladium-catalyzed Stille polymerization, two novel FBSe-based low band gap polymers, PBDT-T-FBSe and PIDT-T-FBSe were successfully synthesized. Both of these two polymers showed narrow band gap of1.60and1.58eV for PBDT-T-FBSe and PIDT-T-FBSe, respectively. The hole mobility of PBDT-FBSe and PIDT-FBSe were1.1×10-4and3.0×10-3cm2V-1s-1, the PCE of PBDT-T-FBSe and PIDT-T-FBSe were4.65%and5.00%.
     Chapter5:Three quinoxaline-based conjugated polymers were successfully synthesized. All of these polymers showed good solubility in common organic solvent. An increased HOMO levels were observed in the alkoxy functionalized polymers and believed to be the result of the strong donating nature of alkoxyphenyl side-chains. In addition, the longer alkoxyphenyl side-chains resulted in large steric hindrance, which prohibited the exiton separation and dissociation. Up on this, the PSCs devices based on poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta-[2,1-b;3,4-b']dithiophene)-alt-6,7-difluoro-2,3-bis-(3"-octyloxyphenyl) quinoxaline](PCPDT-DFPhQ-O) only exhibited low Jsc of2.52mA/cm2and PCE of0.94%. On the contrary, the polymer of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-6,7-diflu oro-2,3-bis-(3"-phenyl) quinoxaline](PCPDT-DFPhQ) with short side chains showed high Jsc of12.05mA/cm2and PCE of5.30%.
     Chapter6:Two novel region-regular alternating conjugated polymers with a D-A1-D-A2structure in which CPDT acts electron donor and DFBT/DPP or DFBT/TPD as electron acceptor were prepared. The incorporation of two electron-deficient units in the structure of terpolymer could enhance solubility, adjust band gap, optimize stack property, and change the electron distribution in the system. Both of these polymers had good solubility and abroad absorption. The PSCs devices based on these two polymers showed the PCE of3.15%and3.11%for PCPDT-DFBT-TPD and PCPDT-DFBT-DPP. These results provided new insight into designing new generation CPs for light-emitting diodes, field-effect transistors, solar cells and other optoelectronic devices.
     Chapter7:The results from chapter2to chapter7were summarized.

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