The recent development of nonfullerene acceptors (NFAs) has resulted in an abrupt increase in the organic photovoltaic (OPV) efficiency. It remains unclear why charge separation (CS) can occur at a high yield in NFA OPVs despite the small energy-level offset at the donor-acceptor (D-A) interface. $\mathrm{In}$ order to resolve this issue, zinc phtholacyanine (ZnPc) and its fluorinated derivative (${\mathrm{F}}_8\mathrm{Zn}\mathrm{Pc}$) are used to build a model D-A interface for understanding the mechanism underlying the efficient CS in NFA OPVs. At this interface, we find that bound charge-transfer (CT) excitons can undergo a heating process in which the enthalpy increases at a surprisingly fast rate. Specifically, the CT exciton gains 0.3 eV from the environment in merely 10 ps. This fast exciton heating, which eventually leads to CS, resembles the CS behavior observed in NFA bulk heterojunctions (BHJs). Interestingly, when the pair of D-A molecules switches from a face-on to an edge-on orientation, the CT-exciton heating process is reversed to the typical hot CT-exciton cooling process in which the enthalpy decreases with time. By using sub-band-gap excitation, we further find that the exciton heating can occur directly from cold CT excitons without the need of excess energy provided by the energy-level offset. The competition between the exciton heating and cooling is explained by a model that incorporates the effect of entropy, electron delocalization, and structural anisotropy. It is found that for the face-on orientation, the small contact area between the delocalized electron and hole within the CT exciton significantly reduces the number of cold CT states, which increases the entropic driving force and drives the reaction into the heating direction. This finding provides insight on why the entropy-and-delocalization-driven CS would preferentially occur in NFA BHJs. The relative orientation of the NFA molecular stack and the polymer chain, together with bulky side groups, make the delocalized electron and hole in contact with each other via isolated pointlike junctions, which is the key for activating the exciton heating and subsequent CS.

中文翻译：

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激子加热与冷却：由与平面分子的界面处的熵和离域驱动的电荷分离

非富勒烯受体（NFA）的最新发展已导致有机光伏（OPV）效率的突然提高。为什么电荷分离（CS）可以在NFA OPV中以高收率，尽管小的能量级在施主-受主（偏移发生率仍不清楚d -甲接口）。$在$ 为了解决这个问题，酞菁锌（ZnPc）及其氟化衍生物（${\mathrm{F}}_8锌\mathrm{个人电脑}$）被用于建立一个模型d -甲用于理解NFA OPV中的高效CS的基础的机制的接口。在这个界面上，我们发现束缚电荷转移（CT）激子可以经历加热过程，其中焓以令人惊讶的快速速率增加。具体而言，CT激子仅在10 ps的时间内从环境中获得0.3 eV的电压。这种快速的激子加热最终导致CS，类似于NFA本体异质结（BHJ）中观察到的CS行为。有趣的是，当双DA当分子从面朝上转换为边沿朝上时，CT激子加热过程与典型的热CT激子冷却过程相反，在该过程中，焓随时间降低。通过使用子带隙激励，我们进一步发现，激子加热可以直接从冷CT激子发生，而不需要能级偏移提供的多余能量。激子加热和冷却之间的竞争是通过一个模型来解释的，该模型结合了熵，电子离域和结构各向异性的影响。已经发现，对于面对面取向，CT激子内离域电子与空穴之间的较小接触面积会显着降低冷CT状态的数量，从而增加熵驱动力并将反应驱动到加热方向。该发现提供了关于为什么熵和离域驱动的CS会优先在NFA BHJ中发生的见解。NFA分子堆栈和聚合物链以及庞大的侧基的相对取向，使离域电子和空穴通过孤立的点状结点相互接触，这是激活激子加热和后续CS的关键。