摘要
有机太阳能电池因为其成本低,可采用大规模的卷对卷柔性制备等优势而受到了广泛关注,但是其较低的性能制约了其应用前景。为了提高有机太阳能电池的性能,一方面是设计新型光敏材料和电池结构,另一方面就是从器件中的界面处进行改进,这也是影响器件性能参数的重要环节。在活性层中的电子给体与受体之间界面,影响活性层薄膜的微观形貌和载流子的产生,传输与复合；而另外一类界面通常是指器件和活性层之间的界面,它决定了电极的功函数,两者之间的能级是否形成欧姆接触,电极和活性层之间界面的电荷选择和收集能力,器件对水和氧的稳定性等。本论文围绕有机太阳能电池中的各个界面进行研究,包括合成新型活性层材料来调控给受体的异质结界面,设计新型界面材料来修饰活性层和电极之间的缓冲层界面,最终达到提高器件效率和稳定性的目的。
活性层中我们通过合成新型具有自组装性能的活性层材料,包括液晶自组装和氢键自组装两类,来调控活性层中给受体材料的接触界面。设计合成了一种以芴和苯并噻二唑共聚物为主连,以氰基联苯基团为液晶侧链的新型液晶共聚物PFcbpDTBTo在相应的液晶态对聚合物薄膜PFcbpDTBT进行热处理,可以使得聚合物主链和侧链都采取有序排列的方式。而取向有序的结构给聚合物带来了很多新的特性,包括其光学吸收能力增强,边带红移,荧光强度增强,和LUMO(?)级的降低。液晶聚合物的取向排列不但可以使得整个活性层薄膜中形成良好的纳米相分离,还能够诱导富勒烯受体采取较为有序的堆积方式分布在活性层中。液晶聚合物有序的自组装排列为有机光伏提供了一种控制活性层形貌的简单易处理优化方法。器件在液晶态(200℃)下退火后,光电转换效率提高了两倍左右达到了1.1%。另外一类为合成聚噻吩烷基末端带咪唑基团的共聚物P3HTM,可以与含羧基的富勒烯衍生物PCBA形成分子间的O-H…N氢键。通过溶液极性调控和热退火,不但可以调控自组装排列,还可以诱导富勒烯也随之取向堆砌,并影响聚噻吩和P3HTM:PCBA活性层的光吸收范围和强度。活性层薄膜形成双分子的自组装体系,该纳米有序形貌为激子的分离提供了更多界面并且为载流子的传输提供了传输通道。因此,P3HTM:PCBA为活性层器件的光电转换效率提升至3.2%。此外P3HTM中多余的Br基团还可以用于紫外交联而提高器件的稳定性。
在缓冲层中我们首先研究了通过界面材料来调控界面的电学性能,通过分子设计分别合成了以聚噻吩为主链的电子传输层和空穴传输层。新型带铵基的离子聚噻吩P3HTN可作为阴极缓冲材料,P3HNT的引入可以降低金属Al的功函数,保证阴极和活性层之间形成良好的欧姆接触,器件在空气中热退火后以P3HTN为缓冲层的器件的PCE从1.8%提高到了3.3%。通过比较不同主链的离子聚合物缓冲材料,我们发现P3HNT离子聚合物不仅可以修饰阴极金属的能级,还能与活性层相互作用从而提高活性层和阴极金属的接触,减少载流子在界面的损失并提高电子的收集能力。同时设计并合成一类可以作为空穴传输材料的新型嵌段共聚物P3HT-b-P3FAT,该嵌段聚噻吩合物由烷基段和氟取代烷基段组成。由于F烷基链低的表面能,其可以在旋涂成膜时在界面自组装形成一层单分子。对于不同比例的嵌段聚噻吩,我们观察到形成单分子层所需的嵌段聚合物浓度也不同。嵌段聚合物中氟烷基段可以修饰界面的费米能级,烷基段能增强活性层和电极之间的接触,提高空穴的收集效果,以饱和浓度的PFT-3HT(1.5mg mL-1)为缓冲层时,器件的PCE提高到了4.6%。要优于采用PEDOT:PSS的器件性能(2.9%),此外,相对于吸湿性和酸性的PEDOT:PSS,嵌段聚噻吩具有良好的稳定性,是一类新型有效并且简易的空穴传输材料。
此外,研究了反向器件中双层缓冲材料的杂化组合对器件界面的优化。在电子传输层ZnO中,在能级调控方面,以PEIE/ZnO为双层缓冲层组合时能得到最低的功函数,ZnO的功函数减小了0.7eV,适用于高LUMO能级的非富勒烯受体材料PIDSe-DFBT,可增加电子的传输和收集。然而以富勒烯衍生物PCBM-COOH修饰ZnO得到了双层缓冲层,不但可以修饰去除ZnO表面缺陷,还可以增加其与活性的作用,可能更适合以PCBM为受体的有机太阳能电池。在空穴传输层方面,本文设计了新型的组合双层空穴传输材料GO/MoO3, GO的引入可以使得整个空穴传输层更加的连续均匀分布,而且GO可以通过表面掺杂的形式来提高界面的电导率。通过各层缓冲层界面的优化,我们证明了器件在反向结构中可以取得和正向结构相当的器件效率,而且不同的缓冲层组合可以适用不同的体系,对有机太阳能电池结构的多样化发展提供了更多可行路线。
最后,除了电学方面的作用,界面缓冲层的修饰同样可以改善有机太阳能电池的光学性能。本文中设计采用高效的等离子体效应把两种尺寸不同的三角银纳米柱双掺杂入不同的缓冲层中。分析双掺杂纳米粒子后的等离子体器件的吸收光谱增加和外量子效率光谱的增加,发现选用两类消光吸收不同的三角银纳米可以取得相互补充的效果。更重要的是,缓冲层中掺杂三角银纳米柱的方法具有很好的普遍性,使用于多种不同结构的聚合物有机光伏器件,性能都取得了16-18%的增加。其中以PIDTT-DFBT:PC71BM为活性层的器件的最高效率从原本的7.7%提高到9.0%。本文的研究表明通过优化纳米粒子的形状和尺寸大小,可以使得缓冲层中掺杂金属纳米粒子的等离子体方法可以适用于多种活性层材料体系。
Organic photovoltaic (OPV) cells have attracted extensive research and development due to their low cost and compatibility with large-scale, flexible, and high throughput roll-to-roll production. To achieve high efficiencies and long lifetime, advances in the design of new absorber materials and device structures are required. Moreover, interface engineering also plays an important role in determining the efficiency of an organic photovoltaic cell and determines different device parameters. Interface, between the electron donor and electron acceptor in activelayer, are responsible for the film morphology and the carrier generation, transporting and recombination; while the interface layer, between the activelayer and the electrodes are responsible for forming ohmic contact, the internal electric field, the film morphology, the carrier recombination rate and device stability. In this dissertation, new approaches for controlling donor/acceptor interface are presented and the development of new interfacial materials in charge transporting layers is presented, for high efficiency and air stability OPVs.
In the activelayer interface, two self-organization approaches, liquid-crystalline and intermolecular hydrogen bonds, are used to control the molecular packing and develop easy-to-process bulk heteroj unction films with nanoscale morphologies for organic photovoltaics. First, we report a novel donor-acceptor type liquid-crystalline copolymer, PFcbpDTBT, which contains both electron-donating fluorene and electron-accepting benzothiadiazole units. The films with structural anisotropy can endow the PFcbpDTBT with special features, including absorption band red-shift, fluorescence enhancement and lower lying LUMO level. When blended with fullerene PCBM, the polymer enables PCBM to adopt the preferential well-oriented arrangement in the bulk. From the device annealed at200℃, the power conversion efficiency values reaches1.1%without extensive optimization. Then, a regioregular polythiophene (P3HTM) containing imidazole rings and acceptor including carboxylic acids is synthesized for the intermolecular interaction. The results from red-shifted absorption and enhanced quenching photoluminescence of the P3HTM:PCBA in solvents with polar additives indicate the building blocks through hydrogen bonding interactions. Processing of P3HTM and PCBA complexes with heat-annealing, constructed from cooperative self-assembly, show optimized photovoltaic performance, with PCE reached3.2%. Besides, the achieved optimum nanomorphology after annealing can be freeze using photocrosslinking method to preserve long term performance.
For the interface materials between activelayer and electrodes, we developed and synthesized a new thiophene polymer for charge transporting layer. Water-soluble HT-poly[3-(6'-N,N,N-trimethylammonium)-hexyl thiophene](P3HTN) is simply inserted between activelayer and cathode as an interfacial dipole layer by spin-coating, the power conversion efficiency (PCE) of the devices annealed in air is enhanced from1.8%to3.3%, resulting from a reduction of the metal work-function and improved electron extraction efficiency. In particularly, the analogue of active layer as buffer layer could improve interchain interactions between the P3HT and the P3HTN to modify interfacial contact, consequently obtaining an unattainable enhancement Jsc, with respect to the interlayer polymer replaced with unanalogous conjugated polymer. In comparison with P3HTN, fluoroalkyl side-chain diblock copolymers,(P3HT-b-P3FAT) were used as hole transporting layer in inverted device. Driven by the low surface energy of fluoro alkyl side chains, the fluorinated polymers can spontaneously segregate on the surface of activelayer (P3HT:PCBM) during spin-coating processes. Required concentrations of fluoro-polymers to form the self-assembled monolayer on surface are related with block ratios. The fluorinated part forms an interfacial dipole that shifts the work function of the anode metal while the P3HT block can interact with the P3HT donor for hole transport. Overall, devices prepared with copolymer PFT-3HT (3:1ratio of P3HT to P3FAT block) in the activelayer solution displayed PCE values of4.6%(50%increase over a PEDOT: PSS control device) and showed a significant long-term stability in air.
Moreover, to meet the requirement of different activelayer, we represent interface optimization work by using hybrid materials. For the acceptor system with high position LUMO level (non-fullerene), the electron transporting layer ZnO modified with PEIE can cause a vacuum level shift up to0.7eV. The reduced work function of ZnO can facilitate the electron transfer from PIDSe-DFBT and achieve higher Voc. However, for inverted solar cells using fullerene as electron acceptor, fullerene-SAM modified ZnO showed the best device performance compared with other polyelectrolyte. Because the fullerene based materials not only passivate the inorganic surface traps but also enhance the electronic coupling at the ZnO-organic interface. Meanwhile, we developed a new GO/MoO3bilayer film as the hole transporting layer, which provides a more continuous and high conductive surface. The device therefore showed significantly improved Jsc, Voc, and FF with PCE up to7.3%in optimized devices. These results indicate that hybrid bilayer interfacial material can take the full advantage of each compound to get the best device performance for various activelayer systems.
Lastly, interface modification can also enhance light harvesting in devices. We provide a powerful, general, and tunable means to increased light absorption at desired wavelength bands in existing and emerging OPV materials by embedding various types of colloidal silver nanoprisms into both interfacial layers. By permitting incorporation of the plasmonic particles into layers on both sides of the bulk heterojunction, our method enables universal cooperative optical enhancement up to16-18%in the best cases, with maximum power conversion efficiency from7.7%to9.0%in devices based on PIDTT-DFBT:PC71BM. We show that the method can be applied to various bulk heteroj unction systems with various types of nanoparticles (size and shape), all without compromising the active layer morphology.
引文
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