| 摘要 | 通过光伏技术从太阳光中直接获取能量是应对能源危机的最重要途径之一。有机太阳能电池(OSCs)具有低成本、轻薄、易于加工、可制备大面积柔性器件等优点，在过去的几十年中，在学术界和工业界均获得了广泛关注。开路电压(VOC)、短路电流(JSC)和填充因子(FF)是决定有机太阳能电池能量转换效率(PCE)的主要参数。虽然OSCs具有诸多优势，但目前较低的效率仍是限制OSCs工业化的主要障碍。在过去的十年中，各国科学家通过优化器件结构、开发新的光伏材料等手段使OSCs取得了长足进步，并获得了超过10%的PCE。尽管如此，PCE仍然需要进一步提高以满足工业化和大规模实际应用的需求。设计并合成新的窄带隙给体材料是提高OSCs能量转换效率的最常用且有效的策略之一。
环戊并[2,1-b:3,4-b′]二噻吩(CPDT)具有良好的溶解性、较强的给电子能力并且其桥碳位置可方便地进行修饰，本论文的研究工作主要围绕CPDT及其衍生物展开，通过在CPDT桥碳位置修饰、聚合物骨架中引入烷基噻吩及调节取代基等手段来调控聚合物的性质，最终达到提高电池器件光电转换效率的目的。
在第二章中，我们将二硫富瓦烯（DTF）作为共轭侧链引入到CPDT或芴的桥碳位置，并合成了三种聚合物(P1-3)。三种聚合物都具有良好的热稳定性，出现5%失重率的温度均高于300 °C。电化学氧化和紫外-可见吸收光谱的实验表明DTF基团可以被氧化为DTF•+，并且该氧化过程独立于聚合物的共轭骨架。随着氧化程度的逐渐增加，三种聚合物的π-π*跃迁吸收峰逐渐减弱，氧化产生的DTF•+会在750-1100 nm处产生一个弱的吸收峰，这一吸收峰是该阳离子自由基的特征吸收峰。我们还考察了聚合物的光伏性质，其中P2的光电转换效率最高，为1.05%，这一数值高于文献报道的所有含四硫富瓦烯(TTF)基团的聚合物的效率。
第三章中，我们将己基噻吩作为π桥引入氟代聚[2,6-(4,4-双-(2-乙基己基)- 环戊并[2,1-b:3,4-b']二噻吩)-交替-4,7-(2,1,3-苯并噻二唑)]（PCPDTBT）中来改善其溶解性和活性层形貌。得到的聚合物P-F、P-DF具有良好的溶解性和热稳定性，两种聚合物在氮气气氛下失重5%的分解温度均高于380 °C，满足电池器件加工工艺要求。经过器件工艺的优化，在氯苯溶液中给-受比5:4、总质量浓度25 mg mL-1、添加2% DIO、转速1250 rpm的工艺条件下，基于P-DF的电池器件我们获得了5.85%的PCE，VOC为0.70 V，JSC为13.58 mA cm-2，FF为61.6%。在引入己基噻吩作为π桥后，P-DF的JSC 和FF均得到提高，最终效率由不含π桥聚合物的3.37%提高到5.85%。效率的提高应归功于作为π桥引入的己基噻吩，这一改变极大地提高了聚合物的溶解性，活性层形貌得到改善。
第四章中，我们设计并合成了三种基于二噻吩并[3,2-b:2',3'-d]硅咯(DTS)和2,1,3-苯并噻二唑(BT)的聚合物，三种聚合物的氟原子数目和侧链不同。三种聚合物P-hF, P-hDF和P-ehDF均具有良好的溶解性和热稳定性。我们通过调节聚合物/PC71BM质量比、添加剂比例等条件来优化基于这三种聚合物的太阳能电池器件的性能。与P-hF相比，得益于较深的HOMO能级和较好的平面性，基于P-hDF的电池器件获得了较高的VOC，为0.593 V，JSC为15.98 mA cm-2，FF为64.8%，PCE为6.14%。聚合物P-ehDF中π桥上的烷基链为2-乙基己基，位阻相比于己基要大，导致聚合物骨架扭曲，HOMO能级比P-hDF低了0.14 eV，因此获得了较高的VOC，达0.805 V。这在以DTS为给体单元、以BT衍生物为受体单元的所有聚合物中是最高数值之一。我们的工作说明了聚合物的光伏性质受吸电子取代基和侧链的位阻影响较大，对设计光伏材料具有一定的参考意义。
第五章中，我们经过大量的尝试，摸索出一条可以获得在大气条件下稳定存在的二烷基二噻吩并[3,2-b:2',3'-d]锡咯和二烷基锡芴衍生物的合成路线。我们通过这条合成路线合成了三种含锡化合物，即DTSn-1、SnF-1和SnF-3。该合成路线的特点在于先通过炔键将端基连接至共轭骨架，而后生成锡咯环，避免了通常做法中Stille反应时锡咯开环。通过这条合成路线可以合成一类新的含二噻吩锡咯和锡芴的有机共轭材料。利用密度泛函理论进行的构象分析结果显示，由于锡原子较大的半径，锡-碳键长较长，烷基基团进一步远离共轭骨架，有利于π-π堆积和电荷传输。我们考察了三种含锡化合物的光学性质，结果显示其荧光均可被Ru3+淬灭，而目前可用于Ru3+检测的探针很少，含锡化合物的这一特性具有较大的潜在应用价值。 |
| 其他摘要 | Harvesting energy directly from sunlight using photovoltaic technology is considered as being one of the most important ways to address growing global energy needs. Organic solar cells (OSCs) have attracted considerable interests from both the academic and industrial communities over the past decade due to advantages such as easy processability over large-area size via printing or roll-to-roll technologies, low-cost manufacturing, and compatibility with flexible substrates. Three key parameters determine the power conversion efficiency (PCE) of a solar cell are open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF). Despite the many advantages of OSCs, low PCE is still a major impediment to real commercialization. Although remarkable progress has been achieved by optimization of the device architectures and developing ideal photovoltaic polymers over the past decade, which led to a higher than 10% PCE, further improvements are needed for mass production and practical applications. One of the most successful strategy to improve the PCE is developing new low band gap donor materials.
Cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) have attracted considerable research interest due to its fully coplanar structure, many intrinsic properties, stronger intermolecular interactions. Furthermore, the option of functionalization at the bridging carbon allows greater structural variations for fine-tuning both the electronic and steric properties. In this dissertation, we designed and synthesized novel CPDT derivatives-based polymers. In order to improve the photovoltaic performance, the properties of the polymers were fine tuning by functionalization at the bridging carbon, introduction of alkylthiophene as π bridge and variation of substitutes.
In the second chapter, dithiafulvalene (DTF) was grafted on CPDT and fluroene, three DTF-fused polymers, P1-3, were synthesized and characterized. These polymers have good thermal stability with decomposition temperatures higher than 300 °C. The electrochemical characteristics and UV-vis absorption spectroscopy indicated that the DTF moiety could be oxidized to DTF•+, and the process was independent of the conjugated main chain of the polymer. Upon increasing oxidation, the strong π-π* transition absorbing band of the three polymers decreases gradually, and the resulting DTF•+ species give rise to an additional band at 750-1100 nm, which can be assigned to a distinguishing feature of the cation radical species. The photovoltaic properties of the polymers were investigated, and P2 exhibited the best PCE of 1.05%, higher than all the reported tetrathiafulvalene (TTF)-fused and ensembles.
In the third chapter, 4-substituted n-hexylthiophene as a π-bridge was introduced to fluorinated poly[2,6-(4,4-bis(2-ethylhexyl)-cyclopenta[2,1-b:3,4-b'] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]s (PCPDTBTs) in order to improve their solubility and nanoscale phase separation in the active layer. The resulting polymers P-F and P-DF exhibit excellent solubility and good thermal stability with decomposition temperatures higher than 380 °C, demonstrating their sufficiently high thermal stability for the applications of OSCs. Finally, the highest PCE of 5.85% for P-DF is obtained with a VOC of 0.70 V, a JSC of 13.58 mA cm-2, and an FF of 61.6%, when the device was fabricated at a donor-acceptor weight ratio of 5:4 in chlorobenzene with a total concentration of 25 mg mL-1 containing 2% DIO as an additive, spin-cast at 1250 rpm. After the introduction of n-hexylthiophene, the JSC and FF of P-DF have been increased, as a result the PCE increased from 3.37% to 5.85%. The improvement of photovoltaic performance should be attributed to the introduction of the n-hexyl substituted thiophene π-bridge, which also improves the solubility obviously.
In the fourth chapter, three low band gap polymers based on dithieno [3,2-b:2',3'-d]silole (DTS) and 2,1,3-benzothiadiazole (BT) derivatives with different number of F atoms and side chains were designed and synthesized. All the polymers, P-hF, P-hDF and P-ehDF, exhibited excellent solubility and good thermal stability with decomposition temperatures higher than 420 °C. The photovoltaic properties of the polymers were carefully optimized with different polymer/PC71BM weight ratios and additive volume ratios. When compared with P-hF, contributed by deeper HOMO energy level and better coplanarity structure, the optimized P-hDF-based device exhibited higher VOC of 0.593 V, JSC of 15.98 mA cm-2, FF of 64.8% and a higher PCE of 6.14%. The HOMO level of P-ehDF was 0.14 eV lower than that of P-hDF due to the bigger dihedral angle between DTS and thiophene which was caused by the increased steric hindrance of 2-ethylhexyl, resulting a higher VOC of 0.805 V. This was one of the highest VOC in devices based on polymers with DTS as the electron-rich unit and BT derivatives as electron-deficient unit. Our work shows that the photovoltaic performance of polymer can be tuned by the electron withdrawing groups and steric hindrance of side chains.
In the fifth chapter, after many attempts, three novel Sn-containing heteroaromatic conjugated oligomers, DTSn-1, SnF-1 and SnF-3, were designed and prepared by a new synthetic route. The distinguishing feature of this synthetic route is that terminal groups are linked to the backbone before the stannole formed, which can avoid the cleavage of stannole under Stille conditions. It would be feasible to construct a new class of conjugated materials containing dithieno[3,2-b:2',3'-d] stannole (DTSn) derivatives and stannafluorene (SnF) derivatives as crucial building blocks with our synthetic route. The conformational analyses were performed by density functional theory (DFT), and the results showed that the bond lengths between Sn and the nearest aromatic/alkyl carbons were longer than those of Si or Ge analogues due to the larger atomic radius of Sn. The alkyl groups are further displaced from the conjugated backbone, allowing a stronger π-stacking interaction to occur, which will be favorable for the charge transporting. The fluorescence of the three Sn-containing small molecules can be quenched by Ru3+, which shows potential great value since fluorescent probes for Ru3+ are rare at present. |
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