Charge Transfer States in D-A Excitonic Solar Cells: Photophysical Characterization and Loss Mechanisms for Charge Generation

替代能源的需求很大，以支持我们社会的经济增长。在过去的几十年里，世界一直受到化石燃料供应可能在相对较短的时间内耗尽的想法的冲击，随着能源变得越来越稀缺，能源成本急剧增加。面对这个问题的方法显然是寻找环境友好的能源，满足i）便宜，ii）环境友好和iii）取之不尽用之不竭的标准。其中满足标准的是利用太阳光通过光伏效应产生能量。最近，有机、聚合物或混合体系的光伏操作受到了广泛关注。这种太阳能电池需要便宜且易于生产以用于实际应用。此外，还需要解决能源转换效率和长期稳定性问题。出现了两种不同但互补的方法;介观染料敏化（DSC）和体有机（或聚合物）异质结（BHJ）太阳能电池。在BHJ配置中，共轭聚合物（供体）和电荷受体（通常是富勒烯衍生物）的混合物夹在两个金属电极之间，其中一个是透明的。这种结构的优点是可以从溶液中一步完成活性层的加工和组装，并且可以在不同类型的衬底上使用经典的印刷技术，避免了高温和更昂贵的沉积方法。有机太阳能电池与无机太阳能电池的不同之处在于产生束缚电子-空穴对（激子）在光吸收时;作为活性介质的低介电常数的结果，这些激子显示出相当大的电子-空穴结合能，约为0.4eV，因此，激子解离仅发生在两种具有不同电子亲合性的材料之间的界面处，作为电子供体（D）和电子受体（A），理解D和A材料之间的基本电子相互作用以及复合膜形态、器件结构和加工条件的作用对于实现高效率至关重要。在过去的几年里，主要通过理解有源层形态的重要性来实现进展，特别是所使用的溶剂类型、聚合物区域规整性和膜退火条件对器件最终性能的重要性。几种电子供体和受体材料将被研究，以解开控制有机材料中自由电荷载流子的形成和复合的过程。光伏器件研究的一个特别重点将是研究含有重原子络合物的共轭材料，这会产生一种促进三重态激子形成的内在机制。这非常快速和有效地将所有聚合物单线态转化为聚合物三线态，其可以用作光伏操作中的电子供体。这些材料作为BHJ太阳能电池中的供体材料的详细研究仍然缺乏，并且在电荷产生中使用长寿命的三重态激子的概念值得进一步关注，特别是为了澄清单重态和三重态的增加的混合对CT态和成对反向电子转移的能量的影响。

Alternative energy sources are in great demand to support the economic growth of our society. During the past decades the world has been assaulted by the idea of a possible exhaustion of our fossil fuel supply in a relative short period of time, dramatically increasing energy costs as it becomes more and more scarce. The way to face this problem is obviously to look for environment friendly energy sources that satisfy the criteria of being i) cheap, ii) environmentally benign and iii) inexhaustible. Among the ones that satisfy the criteria is the use of sunlight to produce energy by the photovoltaic effect.Recently much attention has been focused on organic, polymeric or hybrid systems for photovoltaic operation. Such solar cells need to be cheap and easy to produce for practical applications. In addition, questions of energy conversion efficiency and long-term stability need to be addressed. Two distinct, but complementary, methodologies have emerged; mesoscopic dye sensitized (DSC) and bulk organic (or polymeric) heterojunction (BHJ) solar cells.In the BHJ configuration, a mixture of a conjugated polymer (donor) and charge acceptor, usually a fullerene derivative, are sandwiched between two metallic electrodes, one of which is transparent. This architecture has the advantage of giving the possibility of process and assemble the active layer using a single step from solution, and make use of classical printing techniques on different types of substrates, avoiding high temperatures and more expensive deposition methods.Organic solar cells differ from their inorganic counterparts by producing bound electron-hole pairs (excitons) upon light absorption; these excitons, as a result of the low dielectric constants of the active medium, show a considerable electron-hole binding energy, around 0.4 eV, and as a consequence, exciton dissociation occurs only at the interface between two materials of different electron affinities, working as electron donor (D) and the electron acceptor (A), which yield a driving force for charge separation.Understanding the fundamental electronic interactions between the D and A materials as well as the role of the composite film morphology, device architecture and processing conditions, is crucial to achieve high efficiencies. Over the last few years progress has been mostly achieved through the understanding of the importance of the active layer morphology, especially the type of solvent used, polymer regioregularity and film annealing conditions to the device final performance.In the proposed research, several electron donor and acceptor materials will be investigated in order to unravel the processes that control the formation and recombination of free charge carriers in organic photovoltaic devices. A particular focus of the research will be the investigation of conjugated materials containing heavy atom complexes, which give rise to an intrinsic mechanism that promotes the formation of triplet excitons. This very rapidly and efficiently converts all polymer singlets into polymer triplets that can be used as electron donors in photovoltaic operation. A detailed investigation of these materials as donor materials in BHJ solar cells is still lacking, and the concept of using long lived triplet excitons in charge generation deserves further attention, particularly in order to clarify the effect of the increasing mixing of singlet and triplet states on the energy of the CT state and geminate back electron transfer.