近年来，有机太阳能电池（OSC）因其重量轻、成本低、易合成、可大面积加工和制备柔性器件等优点受到广泛关注。典型的体异质结OSC由作为给体的 p 型共轭聚合物和作为受体的 n 型有机半导体组成。目前来说，高性能的受体材料以稠环电子受体为主导，其能级可调制、吸收可扩展至近红外区，并且种类非常丰富。相比之下，高性能的聚合物给体材料种类则相对较少。迫切需要开发新型的聚合物给体，尤其是宽带隙聚合物给体材料，从而更好地匹配窄带隙受体材料。此外，可溶液加工的聚合物通常由刚性的共轭结构骨架和柔性的烷基侧链构成，高性能的有机太阳能器件需要给受体共混膜形成合适相分离尺度的互穿网络结构。然而，具备良好形貌的前提是聚合物具有合适的溶解性和薄膜状态下的有序堆叠。因此，通过侧链工程，改变侧链的长度、数量、位置等可以调节聚合物的溶解性，同时会对活性层形貌、分子堆叠以及电荷传输产生影响，进而影响器件的光伏性能。我的硕士学位论文聚焦于侧链工程对聚合物材料光伏性能研究，主要研究内容如下：
一、设计并合成了一系列含烷基、烷氧基和烷硫基侧链的宽带隙（WBG）共轭聚合物PTFBDT-C，PTFBDT-O和PTFBDT-S，以苯并二噻吩衍生物作为给体单元和苯并噻二唑衍生物作为受体单元，通过改变侧基，可以很好地调节能级、聚集行为以及相应的共混膜形貌。结果表明，PTFBDT-C，PTFBDT-O和PTFBDT-S都具有宽的光学带隙。此外，与烷氧基侧链的PTFBDT-O、烷硫基侧链的PTFBDT-S相比，烷基侧链的聚合物PTFBDT-C具有更宽的光学带隙（1.83 eV），较深的HOMO能级（5.50 eV）和更高的空穴迁移率（2.31 × 10^-4 cm² V⁻¹ s⁻¹），基于PTFBDT-C:L8-BO-4F的共混膜显示出适当的相分离。因此，基于PTFBDT-C:L8-BO-4F的OSC的能量转换效率（PCE）为14.02 %，远高于基于PTFBDT-O: L8-BO-4F的器件（9.06 %）和基于PTFBDT-S: L8-BO-4F的器件（10.45 %）。这些结果表明，侧链工程是提高宽带隙聚合物光伏性能的有效策略。 
二、基于侧链工程，设计合成了不同长度烷氧基侧链的宽带隙聚合物给体材料BDTDCS-68，BDTDCS-46和BDTDCS-24，三种聚合物具有相同的骨架结构，相似但长度不同的烷氧基侧链，通过引入氰基吸电子基，可有效降低其HOMO能级，提高电池的开路电压，进而提高电池的能量转换效率。同时，利用碳-碳双键中的氢原子和烷氧基侧链中的氧原子之间形成氢键，通过氢键的作用，实现对聚合物主链平面性的调控，有利于电荷传输。与BDTDCS-68和BDTDCS-24相比，BDTDCS-46的具有更高的空穴迁移率为4.17×10^-4 cm² V⁻¹ s⁻¹，有望实现更高效的电荷传输和收集。此外，基于BDTDCS-46的光伏器件能够形成更好的活性层形貌。在没有添加任何添加剂的条件下，基于BDTDCS-46的器件光电转换效率可达12.26 %，其中Voc为0.84 V，Jsc为24.94 mA/cm²，FF为58.54 %，远高于BDTDCS-68 （8.23 %）和BDTDCS-24（2.92 %）的光伏器件。结果表明，不同长度烷基侧链的聚合物可以有效调节溶解性和形貌，这是提高聚合物光电性能的有效策略。

In recent years, organic solar cells (OSCs) have attracted a wide attention due to their advantages of light weight, low cost, easy synthesis, large area processing and preparation of flexible devices. The most typical bulk heterojunction OSCs consist of P-type conjugated polymers as donors and N-type organic semiconductors as acceptors. At present, high-performance acceptor materials are dominated by fused-ring electron acceptors, whose energy levels can be modulated, and their absorption can be extended to the near-infrared region, and the types are very rich . In contrast, there are relatively few types of high-performance polymer donor materials. There is an urgent need to develop novel polymeric donors, especially wide-bandgap polymeric donor materials, to better match narrow-bandgap acceptor materials. In addition, the solution processable conjugated polymers are usually composed of rigid conjugate molecular backbone and flexible alkyl side chain. The active layer of high-performance OSCs should form an interpenetrating network structure with suitable phase separation in nanoscale. However, the premise of good morphology is that the polymer should have a good solubility in solution and an ordered molecular stacking in the film state. Therefore, through side chain engineering, the solubility of polymer can be adjusted by changing the length, number and orientation of side chain, which will also affect the morphology of active layer, molecular stacking and charge transfer, thus affecting the photovoltaic performance of devices. My master's thesis focuses on the photovoltaic performance of polymer materials by side chain engineering. The main research contents are as follows:
1. A series of wide-band gap (WBG) conjugate polymers PTFBDT-C, PTFBDT-O and PTFBDT-S, which bearing alkyl, alkoxy and alkylthio side chains are designed and synthesized. Benzodithiophene derivatives are used as donor units and benzothiadiazole derivatives as acceptor units. The energy level, aggregation behavior and the morphology of the blend film can be well adjusted. The results show that PTFBDT-C, PTFBDT-O and PTFBDT-S are all wide band gap polymers. In addition, compared with PTFBDT-O with alkoxy side chain and PTFBDT-S with alkylthio side chain, PTFBDT-C with alkyl side chain has even wider optical band gap (1.83 eV), deeper HOMO energy level (5.50 eV) and higher hole mobility (2.31×10^-4 cm² V⁻¹ s⁻¹ ). The blend films based on PTFBDT-C:L8-BO-4F show appropriate phase separation. Therefore, the power conversion efficiency (PCE) of the OSC based on PTFBDT-C: L8-BO-4F is 14.02%, much higher than that of the device based on PTFBDT-O:L8-BO-4F (9.06%) and the device based on PTFBDT-S:L8-BO-4F (10.45%). These results indicate that side chain engineering is an effective strategy to improve the photovoltaic performance of wide-band gap polymers.
2. Based on side chain engineering, wide bandgap polymer donor materials BDTDCS-68, BDTDCS-46 and BDTDCS-24 bearing alkoxy side chains in different lengths are designed and synthesized. The three polymers have the same skeleton structure and similar alkoxy side chains in different lengths. Improving the open circuit voltage of the OSC can improve the energy conversion efficiency of OSC. Hydrogen bonds are formed between hydrogen atoms in the carbon-carbon double bond and oxygen atoms in the alkoxy side chain. Through the role of hydrogen bonds, the planarity of the polymer main chain can be adjusted, which is beneficial to charge transport. Compared with BDTDCS-68 and BDTDCS-24, BDTDCS-46 has a higher hole mobility of 4.17×10^-4 cm² V⁻¹ s⁻¹, which is expected to achieve more efficient charge transport. In addition, photovoltaic devices based on BDTDCS-46 can form better active layer morphologies. Without any additives, the photoelectric conversion efficiency of the device based on BDTDCS-46: eC9-4F can reach 12.26% , in which Voc is 0.84V, Jsc is 24.94mA/cm², and FF is 58.54%, much higher than BDTDCS-68 (8.23%) and BDTDCS-24 (2.92%) photovoltaic devices.The results show that polymers bearing alkyl side chains in different lengths can effectively adjust the solubility of polymer and the morphology of active layer, demonstrating that side chain engineering is an effective strategy to improve the power conversion efficiency of polymers.