磁场下共轭聚合物纳米线调控小分子本体异质结形貌的机理及其光伏性能

It is the key to improve the photovoltaic properties of solar cells based on small molecules by optimizing the morphology of the active layer. The project will choose the two characteristic donor molecules showing of liquid crystalline properties and high power conversion efficiency (PCE) value, and the molecules contain the core of BDT and DTS units. The P3HT and P3BT as well as their nanowires will be incorporated into the small molecule heterojunction systems, serving as the template to induce the crystallization of donor molecules, improving the ability to form membranes. The nanowires will provide the three dimensional network pathways, facilitating the transporting of charge carriers vertical to the substrate. The magnetic field will be further applied during the film formation process, inducing the orientation of donor molecules and improving the quality of crystals. Based on the synchrotron radiation and other advanced characterization techniques, the morphology of the membranes in the presence of conjugated polymers and their nanowires will be deeply investigated in the nanometer scale, including the crystalline structure of donor molecules, crystal diameters, size and distribution of PCBM aggregates, domain size and purity, etc. The mechanism of morphology evolution of the small molecule bulk heterojunction optimized by conjugated polymers and their nanowires will be understood. The effect of the strength, direction and time of the magnetic field on the morphology change will be investigated, especially for the orientation of donor molecules and domain purity, in order to understand the mechanism of crystallization of donor molecules and morphology evolution of small molecule bulk heterojunction. The relationship between morphology of the active layer, charge transporting properties and photovoltaic properties of the devices will be analyzed, in order to fabricate the solar cells showing of high PCE values.

优化活性层薄膜的形貌是提高有机小分子太阳能电池性能的关键。本项目选择最具代表性的两类高效率的液晶给体小分子，即结构单元为BDT和DTS的小分子，将P3HT和P3BT及其纳米线引入小分子异质结体系中，作为模板，诱导给体小分子结晶，改善成膜性，并提供三维网络通道，提高垂直基底方向上的载流子传输。进一步在成膜过程中施加磁场作用，诱导给体小分子的取向，提高其结晶质量。通过同步辐射等先进表征技术，研究共轭聚合物及其纳米线存在下薄膜的微观形貌，包括给体小分子的结晶结构、结晶尺寸、PCBM聚集体的尺寸与分布、相尺度与纯度等，理解共轭聚合物及其纳米线调控小分子本体异质结形貌的机理。研究磁场强度、方向与作用时间对薄膜形貌，尤其是给体小分子取向与相纯度的影响，理解磁场调控给体小分子结晶以及小分子本体异质结形貌的机理。建立活性层薄膜的微观形貌与载流子迁移率和器件光伏性能之间的关联，制备高效太阳能电池。

有机太阳能电池近年来取得了较大突破，光电转换效率逐步达到了商业化应用的水平。然而，如何进一步优化活性层薄膜的形貌是提高其综合性能的关键。为了优化小分子本体异质结的形貌，将共轭聚合物及其纳米线引入p-DTS(FBTTh2)2:PC70BM和DR3TBDTT:PC71BM中。发现P3BT可作为异相成核剂诱导p-DTS(FBTTh2)2结晶，并形成纳米纤维贯穿整个活性层薄膜，器件效率从3.4%提高至5.0%。进一步将侧链长度不同的P3HT和P3OT引入活性层，发现烷基侧链更短的P3BT与p-DTS(FBTTh2)2及PC71BM之间具有更好的相容性，器件性能最高。采用甲醇-氯萘二元溶剂进行表面处理，促使p-DTS(FBTTh2)2形成了纳米纤维，器件效率提高至6.5%。采用溶剂退火处理，短路电流和填充因子均提高，但开路电压损失高达246 mV，与小分子结晶度提高有关，而后退火可以恢复电压损失，器件最高效率达7.2%。在DR3TBDTT:PC71BM中引入结晶性不同的PTB7-Th，PCDTBT和P3HT聚合物，发现10%的PTB7-Th可使器件效率从4.4%提高到5.7%，而结晶性更强的聚合物则使器件效率下降，与DR3TBDTT的结晶相干长度变大有关。磁场处理后，小分子会沿磁场方向取向，结晶尺寸下降，共轭聚合物P3HT会形成更多的纳米线结构。在PTB7-Th:PC70BM中加入1,4-丁二硫醇(BT)作为溶剂添加剂，可将游离的和限制在聚合物相中的富勒烯分子抽提出来，提高了活性层的相纯度，提高了薄膜的光稳定性和效率，克服了DIO降低光稳定性的缺点。同时，发现BT能使薄膜的热稳定性提高，归因于BT沸点低而易挥发，使PTB7-Th具有更高的玻璃化转变温度。相关研究为从分子尺度上的相互作用来优化有机太阳能电池活性层薄膜形貌从而提高器件效率以及光和热稳定性提供了理论指导。