反式钙钛矿太阳能电池的空穴传输层及光伏性能研究

反式钙钛矿太阳能电池(PSCs)具有可低温溶液加工、迟滞效应小和便于与单晶硅等光伏技术进行串联等多方面优势。目前基于反式结构的单节PSCs和串联器件已经获得的PCE分别超过24%和29%。作为反式PSCs器件的关键组成部分，空穴传输材料(HTMs)不仅起到空穴传输和提取的作用，还在很大程度上影响沉积于其上面的钙钛矿活性层的薄膜质量以及器件长期稳定性。因此，开发高性能、稳定的HTMs以及改善HTMs的加工工艺是提高反式PSCs性能和长期稳定性的有效策略。为此，本论文从HTMs的分子设计策略及其制备工艺的角度出发，通过开发含羧基的新型给体-受体(D-A) HTMs和优化其加工工艺以提高反式PSC器件的效率和稳定性为目的，制备了高性能的反式PSC器件。

通过紫外可见光谱、傅里叶红外光谱、X射线光电子能谱等多种表征发现，聚[3-(4-羧丁基)噻吩](P3CT)直接作为反式PSC器件的空穴传输材料具有多方面优势。与P3CT的衍生物P3CT-Na相比，P3CT的分子侧链羧基不仅可以牢固地锚定在氧化铟锡(ITO)电极表面上并提升其表面功函数，而且还对钙钛矿活性层的界面缺陷具有良好的钝化效果。P3CT的上述双重功能使得沉积其表面的钙钛矿薄膜具有更好的成膜质量。基于P3CT的反式PSC器件具有更低的非辐射复合几率和空穴陷阱态密度，有利于提高开路电压(V_OC) (1.12 V)。最终，基于P3CT的反式PSC器件实现了较高的PCE (21.33%)。此外，随着器件面积增加到0.80 cm²，基于P3CT的反式PSC器件的PCE仍然可高达19.65%。这为后续开发和制备高性能空穴传输界面材料提供了重要的指导。

受染料敏化太阳能电池中高性能染料分子以及含羧基的P3CT的启发，采用具有锚定基团(2-氰基丙烯酸，CA)且低成本的新型D-A小分子MPA-BT-CA 作为反式钙钛矿太阳能电池的空穴传输材料。多种表征发现CA的引入使得MPA-BT-CA分子材料具有如下四个优点：首先，有效调节前沿分子轨道能级，其中最高占据分子轨道(HOMO)能级低至-5.29 eV；其次，提升ITO电极的表面功函；再次，有效钝化钙钛矿活性层的界面缺陷，提升钙钛矿的成膜质量；最后，增加材料在低毒绿色溶剂中的溶解性，便于绿色加工工艺的实施。因此，基于MPA-BA-CA的反式PSCs取得了出色的器件性能，其PCE高达21.24% 且长期稳定性优良。另外，当采用绿色溶剂乙醇作为MPA-BT-CA的加工溶剂时，相应的反式PSCs器件实现了较高的PCE，达20.52%，且迟滞效应可忽略不计。该部分的研究结果表明，这种基于锚定基团材料的新颖分子设计策略很好地结合了低成本、可绿色溶剂加工和高性能的优点，为探索适用于PSC商业化的高性能HTM奠定了基础。

三种具有不同锚定基团(2-氰基丙烯酸，CA；苯甲酸BA；罗丹宁-3-丙酸，RA)的新型D-A自组装分子(SAMs) (MPA-BT-CA、MPA-BT-BA和 MPA-BT-RA)表现出显著差异的理化性质以及对反式PSCs器件的光伏性能和器件稳定性产生不同的影响。其中，MPA-BT-CA和MPA-BT-RA表现出更强的分子偶极矩，均超过12.6 Debye。MPA-BT-CA和MPA-BT-BA在ITO表面通过采用直立的自组装模式形成均匀且近似单分子层的空穴传输层。然而，RA基团上sp³杂化碳原子(-CH₂-)的存在，使得羧基具有高的旋转自由度；MPA-BT-RA分子在固态时有很强的聚集倾向，不利于实现长程有序自组装层。因此，得益于良好的热稳定性、高偶极矩以及致密且均匀的自组装膜，基于自组装分子MPA-BT-CA的反式钙钛矿器件实现优异的光伏性能，其PCE高达21.86%。此外，当器件的有效面积增加到 0.80 cm²时，基于MPA-BT-CA的反式PSC器件仍然可以获得接近20%的PCE。该研究为开发高性能空穴传输型SAM和实现高效、柔性、大面积以及串联PSC提供了重要的理论依据和指导意义。

在MPA-BT-CA的研究基础上，利用含氟的新型D-A小分子材料FMPA-BT-CA作为反式钙钛矿太阳能电池的空穴传输界面材料。FMPA-BT-CA具有低至-5.45 eV的HOMO能级，与钙钛矿的价带能级更匹配，有利于提高PSC器件的V_OC。另外，FMPA-BT-CA具有强偶极矩和钙钛矿缺陷钝化功能，有利于减少缺陷，提高电荷提取效率。此外，CA锚定基团增加了FMPA-BT-CA在醇类绿色溶剂(如乙醇和异丙醇)中的溶解度，有利于绿色加工工艺的实施。当采用异丙醇作为FMPA-BT-CA的加工溶剂，基于FMPA-BT-CA的绿色加工的反式PSC器件获得优异的光伏性能（PCE = 22.35%)。此外，基于可绿色加工FMPA-BT-CA的反式PSC器件经历1000小时的持续光照之后仍可保持90%的PCE，其光稳定性明显优于基于MPA-BT-CA的PSC器件。

Inverted perovskite solar cells (PSCs) have several distinctive advantages, including low-temperature solution processability, neglectable hysteresis, and the easy combination with other advanced photovoltaic technologies such as those based on monocrystalline silicon. To date, single and tandem devices based on the inverted structures have obtained PCEs above 24% and 29%, respectively. As a key component of inverted PSCs, hole-transporting materials (HTMs) not only play the critical role for hole transport and extraction, but also greatly affect the film quality of perovskite active layer deposited onto the HTM surface as well as the long-term stability of devices. Therefore, developing high-performance HTMs has served as one of the most effective strategies to improve the performance and long-term stability of inverted PSCs. By combining the efforts from the molecular design of HTMs and the optimization of device fabrication process, we aimed at developing novel high-performance carboxyl-containing donor-acceptor (D-A) type HTMs and optimizing their processing condition to improve the PCE and stability of the corresponding inverted PSC devices. The main research works have been carried out as the following five aspects:

The polymer poly[3-(4-carboxybutyl)thiophene] (P3CT) was directly applied as the hole-transporting layer in inverted PSCs. At the same time, P3CT-Na, a derivative of P3CT, was used as a controlled hole-transporting layer. By carrying out a series of characterization techniques such as ultraviolet-visible spectroscopy, Fourier infrared spectroscopy, and X-ray photoelectron spectroscopy, it was found that the carboxyl (-COOH) group in the side chains of P3CT molecule can not only firmly anchor onto the indium tin oxide (ITO) electrode surface and optimize its work function, but also show effective passivation function to the defects of perovskite active layer. Benefiting from these dual-functionalities, a better perovskite film with good film quality was obtained on P3CT. Therefore, the inverted PSC devices incorporating P3CT show lower non-radiative recombination, which is beneficial to increasing the open-circuit voltage (V_OC) to 1.12 V. Consequently, the inverted PSC devices based on P3CT exhibited a high PCE of 21.33%. In addition, when the device area increases to 0.80 cm², the PCE of P3CT-based devices maintians as high as 19.65%. This work provides an important strategy for the preparation and development of high-performance HTMs for inverted PSCs.

Inspired by the application of high-performance dye molecules in dye-sensitized solar cells and the carboxyl-containing polymer P3CT, a new kind of low-cost D-A type small molecule MPA-BT-CA with an anchor group (2-cyanoacrylic acid, CA) was used as the hole-transporting material for inverted perovskite solar cells. By carrying out multiple characterizations, it was found that the introduction of CA endows MPA-BT-CA with the following advantages: firstly, its energy levels can be effectively tuned, showing a supressed highest occupied molecular orbital (HOMO) energy level of -5.29 eV; secondly, the surface work function of the ITO electrode surface  can be optimized; thirdly, MPA-BT-CA can effectively passivate surface defects of perovskite active layer; finally, the alcohol solubility of HTM is increased. As a result, the inverted PSCs based on MPA-BA-CA HTM achieved a high PCE of up to 21.24% with excellent long-term stability. In addition, when processing the MPA-BT-CA HTM film with a green solvent, i.e. ethanol, the corresponding inverted PSC devices delivered a substantial PCE up to 20.52% with a negligible hysteresis. This work clearly shows that the HTM based on the molecular design of anchoring group combines the advantages of low cost, environmentally friendly processability and high performance, which will pave a way for the exploration of highly efficient HTMs applicable to the PSC commercialization.

A series of new self-assembled donor-acceptor (D-A) type SAMs (MPA-BT-CA, MPA-BT-BA, and MPA-BT-RA) with distinct anchoring groups (2-cyanoacrylic acid, CA; benzoic acid, BA; rhodanine-3-propionic acid, RA) were developed. As hole-stransporting interface materials, their effects on on the peroformance of inverted perovskite solar cells were systematically studied. Among them, MPA-BT-CA and MPA-BT-RA show stronger molecular dipole moments. By adopting an upstanding self-assembly mode, MPA-BT-CA and MPA-BT-BA can form a uniform, almost monolayer HTM on the ITO surface. However, the MPA-BT-RA molecules tend to aggregate severely in solid state due to the sp³ hybridization of the carbon atom (-CH₂-) on the RA group, which is not favorable for achieving a long-range ordered self-assembled layer. As a result, benefiting from its good thermal stability, high dipole moment, as well as dense and uniform self-assembled film, the devices based on MPA-BT-CA yielded a remarkable PCE of 21.86%, which is the highest value reported to date for inverted PSC devices based on self-assembled hole contacts. Encouragingly, an impressive PCE approaching 20% can still be obtained for the MPA-BT-CA-based PSCs while the device area is increased to 0.80 cm². Our work sheds light on the design principles and processing method for developing hole selecting SAMs, which should pave a new avenue for realizing highly efficient, flexible, large-area, and/or tandem PSCs.

A new kind of fluorine-containing D-A type small molecule FMPA-BT-CA was used as the hole-transporting material for high-performance inverted perovskite solar cells. FMPA-BT-CA shows low-lying HOMO energy levels of as low as -5.45 eV, which can better match the valence band of perovskite active layer and should be beneficial to increasing the V_OC of PSC devices. In addition, CA anchoring group has maintained the solubility of FMPA-BT-CA in alcoholic solvents (e.g., ethanol and isopropanol), which is beneficial to the implementation of the green alcohol solution process. When isopropanol is used as the processing solvent for FMPA-BT-CA, the inverted PSC devices based on FMPA-BT-CA obtain an excellent PCE up to 22.35%. Moreover, the FMPA-BT-CA-based inverted PSC devices present good light stability with 90% PCE retained after 1000 hour light soaking, which is better than FMPA-BT-CA-based inverted PSC devices under the  same condition.

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