The Role of Charge Transfer States in Free Charge Generation in Organic Donor-Acceptor Solar Cells

本提案的目的是利用时间延迟收集场（TDCF）实验来量化几种聚合物：富勒烯共混物作为激发波长的场和温度依赖关系，并阐明电荷转移（CT）状态在聚合物基有机太阳能电池中的作用。目前的申请者和其他小组的工作表明，有机供体-受体体异质结太阳能电池中的电荷分离涉及电子松弛和振动热化的ct状态。因此，太阳能电池的性能受到这些状态的分裂效率的显著影响。我们期望热化ct激子的确切能量位置与自由电荷能量的关系对有机太阳能电池的光伏功能至关重要。因此，本提案的中心观点是测量发电效率作为几个参数的函数，重点是只激发低能量的ct态。这些实验将在聚合物：富勒烯共混物上进行，分别具有可变的、可调的电离能位置和供体和受体的电子亲和力。通过将TDCF实验作为电场和温度的函数，我们想要检查经常观察到的场相关产生是否主要是热激活ct状态分离或场辅助隧道的结果。除了这些测量之外，我们还将研究从室温实验中得出的自由电荷产生主要由热化CT状态介导的假设是否普遍有效，即使在低热能的情况下也是如此。将在三元聚合物：富勒烯1：富勒烯2共混物上进行选择性ct激发的TDCF实验，以显示ct态是否在几个富勒烯上离域。最后，对低聚合物浓度的二元聚合物-富勒烯共混物进行了详细的实验研究。在这种共混物中，我们期望与聚合物形成两相结构：富勒烯共混结构域和纯富勒烯结构域。我们将研究CT态的无活化分离是否以及在何种情况下也会发生在这种两相体系中，就像现在在几种已建立的聚合物：富勒烯共混物中看到的那样。这项工作的发现对于全面了解有机太阳能电池的电荷产生具有重要意义，并将为进一步提高太阳能电池的效率奠定基础。

The aim of this proposal is to use time delayed collection field (TDCF) experiments to quantify the field and temperature dependence of several polymer:fullerene blends as a function of excitation wavelength, and to clarify the role of charge transfer- (CT-) states in polymer-based organic solar cells.Current applicants and other groups work indicated that charge separation in organic donor-acceptor bulk heterojunction solar cells involves electronically relaxed and vibronically thermalized CT-states. Thus, the solar cells behavior is significantly affected by the efficiency of split-up of these states. We expect that the exact energetic position of thermalized CT-excitons in relation to the energy of free charges is of vital importance for the photovoltaic function of organic solar cells.Therefore, the central point of this proposal is to measure generation efficiency as function of several parameters with emphasis on exciting only low energy CT-states. These experiments will be conducted on polymer:fullerene blends with variable, tunable position of the ionization energy and electron affinity of the donor and acceptor, respectively.With TDCF experiments as a function of the electric field and temperature we like to examine whether the often observed field dependent generation is a result of mainly thermally activated separation of CT-states or of field-assisted tunneling. In addition to these measurements, we will investigate whether the presumption, drawn from experiments at room temperature, that free charge generation is mainly mediated by thermalized CT states is generally valid, even in the case at low thermal energies. TDCF experiments with selective CT-excitation on ternary polymer:fullerene1:fullerene2 blends will be conducted to show whether CT-states are delocalized over several fullerenes or not. Finally, detailed experiments on binary polymer:fullerene blends with low polymer concentrations are planned. In such blends, we expect the formation of a two-phase structure with polymer:fullerene blend domains and pure fullerene domains. We will investigate if and under which circumstances activationless separation of CT states, as now seen in several established polymer:fullerene blends, does also occur in such two-phase systems. Findings of this work will be important for a comprehensive understanding of charge generation in organic solar cells and will form the base for further improvement of solar cell efficiency.