基于多元聚合物太阳能电池的效率优化研究

充分利用太阳能这类绿色能源是解决能源危机的一种有效途径。与传统的无机太阳能电池相比，聚合物太阳能电池（PSCs）因其轻质、柔性、易于溶液加工等优点，受到国内外研究人员的广泛关注。目前，通过改进分子结构、器件结构和器件制备工艺，聚合物太阳能电池的发展已经取得了巨大的进步。但是，如何有效提高聚合物太阳能电池的能量转换效率（PCE）和稳定性仍然是该研究领域面临的最大挑战。为了实现高效的聚合物太阳能电池，研究人员已经合成了大量的给受体分子。而相比之下，有关器件工程及其工作机理的研究却明显落后。因此，本论文致力于解决器件工程中优化调控聚合物太阳能电池活性层组分的问题，构建出了一系列的高效多元体系，进一步改善了聚合物太阳能电池光电性能，同时对各多元体系的工作机理进行了探讨。以改善本体异质结结构聚合物太阳能电池的重要性能参数（开路电压Voc、短路电流密度Jsc、填充因子FF和PCE）为出发点，本论文通过优化调控多组分活性层，调节聚合物太阳能电池的能级排布、光谱响应和分子聚集形貌，从而达到提高聚合物太阳能电池光电转换效率的目的。具体的主要研究内容及成果如下：

首先，利用两个不同的取代基，羟基（-OH）和乙酰氧基（-OCOMe），对BTIC核的末端β位点进行端基取代，合成了两种新型非富勒烯受体分子BTIC-OH-β和BTIC-OCOMe-β，从而获得具有更优分子聚集形貌的二元聚合物太阳能电池，将其作为下一步三元策略研究的主体。以改善聚合物太阳能电池性能参数中的Voc为出发点，引入PBDB-TF给体作为第三组分，实现了三元体系间级联结构的能级排布，从而将基于二元主体器件的Voc从0.84 V有效提高至0.89 V。结合稳态荧光光谱（PL），结果表明，三元聚合物太阳能电池的工作模型应为电荷转移。基于电荷转移模型，Voc和Jsc在最佳比例下达到平衡，PBDB-T:BTIC-OCOMe-β二元器件的PCE从10.80%有效提高至三元（PBDB-T:PBDB-TF:BTIC-OCOMe-β）器件的12.45%。另外，通过电致发光外量子效率（EQEEL）和瞬态光电技术，进一步研究了Voc提升的来源以及Jsc提升的原因，从而对多给体三元策略提升聚合物太阳能电池性能的作用机制有了更深入的理解。

其次，以改善聚合物太阳能电池性能参数中的Jsc为出发点，选用本课题组开发的氯代聚合物太阳能电池作为本身具有高Voc的二元主体。通过实现活性层组分间的吸收互补，尽可能多地覆盖太阳光谱。本论文基于相同的二元主体，分别衍生出两个三元体系（富勒烯三元体系和非富勒烯三元体系）。具体的，分别将富勒烯受体PC71BM和非富勒烯受体BT6IC-BO-4Cl引入PBT4Cl-Bz:IT-4F中作为第三组分，将器件PCE从9%左右有效提高至约11.30%。两个体系的光学性质、光谱响应和载流子传输性能的结果表明，基于合金结构的工作模型，PC71BM的各向同性载流子传输特性更有利于提升三元器件性能参数中的FF；而基于平行结构工作模型，BT6IC-BO-4Cl的近红外强吸收则更有利于促进了Jsc的提高。并且，基于PBT4Cl-Bz:IT-4F:PC71BM:BT6IC-BO-4Cl的四元器件进一步将Jsc从16.44 mA cm-2提升到更高水平（19.66 mA cm-2），使得优化后的器件最佳效率达到11.69%。

最后，以构建高效的四元体系，改善性能参数中的FF为出发点，本论文选用了本身具有高Jsc的PTB7-Th:IEICO-4F作为二元主体。引入与PTB7-Th混溶性良好的富勒烯衍生物受体作为第三和第四组分，构建了PTB7-Th:IEICO-4F:PC71BM:PC61BM四元体系。掠入射广角X射线散射（GIWAXS）结果表明，富勒烯PC71BM和PC61BM共受体有效调节了四元共混膜的分子堆积取向，增加了共混膜的结晶性；瞬态光电技术表征则进一步表明，四元器件的载流子传输能力也同时增强。因此，四元策略不仅促进了FF从58.24%上涨到69.30%，还将Jsc从22.07 mA cm-2提高至24.37 mA cm-2，使得太阳能电池器件获得了12.52%的最高转换效率（远高于二元器件9.67%的最佳效率）。另外，该四元体系对多组分调节比例还具有高耐受性。同时，该四元策略在其它与IEICO-4F相似的非富勒烯体系中也发挥了优化聚合物太阳能电池效率的作用，验证了其通用性。总而言之，本研究将为多元策略的实现提供更多的可能性，同时为改善聚合物太阳能电池性能提供一条简单高效的路径。

Fully exploiting the environmental-friendly solar energy is one of the promising ways to counter the energy crisis. In contrast to inorganic photovoltaics, polymer solar cells (PSCs) have attracted attention from academia at home and abroad due to its light-weight, flexible and solution-processing features. Till now, PSCs have gained great progress by modifying the molecular structures, devices structures and devices fabrications. But how to improve the power conversion efficiency (PCE) and stability of PSCs is still the hugest challenge for this field. To achieve high-efficiency PSCs, there existed lots of research on molecular synthesis, while the research on devices engineering and its mechanism obviously lagged. Therefore, this work mainly researched on how to optimize and control the multi components of active layer, constructing efficient multi-component system for further enhancing the quality of PSCs. Meanwhile, the mechanism of the multi-component strategy was deeply investigated as well. Taking the key performances which include open circuit (V_oc), short-circuit current density (J_sc), fill factor (FF) and PCE of Bulk Heterojunctions (BHJ) as the starting point, this work enhanced the PCE of PSCs by optimizing multi-components of active layer, efficiently modulating the energy levels alignment, spectrum response, and blend morphology. Detailed main research contents and results are as follow:

Firstly, two new non-fullerene acceptors, BTIC-OH-β and BTIC-OCOMe-β, were synthesized by substituting hydroxyl (-OH) and acetoxy (-OCOMe) group at the terminal β-position of the BTIC core, and then between them, the binary system which posessed the better film morphology was selected as the host for the tenary research. To improve V_oc, an analogue donor, PBDB-TF, was introduced as the third component, making the cascade energy level alignment among the ternary system, improving the V_oc from 0.84 V to 0.89 V. The charge transfer mechanism was deduced by steady-state photoluminescence (PL). Based on charge transfer, the J_sc and V_oc of ternary devices was banlanced so that the efficiency of devices based on PBDB-T:BTIC-OCOMe-β was pumped from 10.80% to a champion PCE of 12.45% based on the PBDB-T:PBDB- TF:BTIC-OCOMe-β devices. Moreover, external quantum efficiency for electroluminescence (EQE_EL) and transient photovoltaic techniques were utilized to further study the origin of V_oc and J_sc promotion, thus the better understandings on how bi-donor ternary strategy worked were obtained.

Secondly, before improving the J_sc, the chlorinated polymer solar cells published by our group were selected as the binary host to because of its high V_oc. Through realizing the complementary absorption among the active layer components, leading to the fully overlap of the solar spectrum, this work derivated two ternary systems (one was fullerene ternary system and another is non-fullerene ternary system) based on the same binary sytem. Detailedly, the fullerene acceptor PC₇₁BM and non-fullerene acceptor BT₆IC-BO-4Cl were added as the third component, respectively. As a result, the efficiency of binary devices was elevated from about 9% to approximately 11.30% of ternary devices. With the estimation of absorption property, photovoltaic response and carrier transport, it was suggested that the both ternary systems worked as different mechanism, so PC₇₁BM with isotropic electron-transporting property facilitated largely to the FF, while the BT₆IC-BO-4Cl with strong infrared absorption mostly benefited to the J_sc. Moreover, the PBT4Cl-Bz:IT-4F:PC₇₁BM:BT₆IC-BO-4Cl quaternary devices further promoted the J_sc from 16.44 mA cm^-2 to 19.66 mA cm^-2 which was the champion amongst these systems. As a result, the best efficiency of quaternary devices achieved 11.69%.

Lastly, to construct efficient quaternary system and before improving the FF, this work chose the PTB7-Th:IEICO-4F as the binary host due to its high J_sc. Introducing two fullerene derivations as the additives, the quaternary system based on PTB7-Th:IEICO-4F:PC₇₁BM:PC₆₁BM was built up. Confirmed by the grazing incidence wide-angle X-ray scattering (GIWAXS), it can be seen that the PC₇₁BM:PC₆₁BM co-acceptor efficiently tuned molecular stacking and crystalline of quaternary blend. With the transient techniques, it was demonstrated that the carrier transport quality was also strengthened. Therefore, the quaternary strategy not only increased the FF from 58.24% to 69.30%, but also raised the J_sc from 22.07 mA cm^-2 to 24.37 mA cm^-2. As a result, the quatenary devices obtained an optimal efficiency of 12.52% which was much higher than the binary devices with a best PCE of 9.67%. In addition, the quaternary system also showed a high-tolerance to the ratio of additives.At the same time, the quaternary strategy gained wide gernality in other classic non-fullerene systems resembling IEICO-4F-based sytem. In summary, this research may provide more possibilities for multi-component realization and may provide another simple and efficient way to enhance the performances of PSCs.

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