We characterized exciton diffusion and dissociation behaviour in two ITIC derivatives non-fullerene acceptors (NFA) by time-resolved spectroscopic methods. The exciton diffusion length was determined to be ∼26 and ∼34 nm in ITIC and IT4F. We further examined the dissociation of excitons in those NFA at the acceptor/donor planar heterojunction interfaces by transient absorption measurements, in which efficient charge generation was observed. Finally, we fabricated planar heterojunction solar cells using PM6/NFA bilayer planar heterojunctions; a power conversion efficiency (PCE) of over 7 % for PM6/IT4F bilayers was determined. More importantly, the spin-coating of top layer NFA has negligible influence on the morphology of the PM6 layer, suggesting a clear bilayer interface, rather than a quasi-bilayer structure with a portion of bulk heterojunction. The results suggest that enhanced exciton diffusion length and efficient exciton dissociation and charge generation are elemental characters to realize high PCE planar heterojunction organic solar cells. We established direct linking between the exciton diffusion length and the photocurrent generation in NFA layer by transfer matrix simulation. The large exciton diffusion length in NFAs makes the realization of high efficiency and stable bilayer organic solar cells feasible.

中文翻译：

---

长激子扩散距离和高效的激子解离使高效平面异质结非富勒烯有机太阳能电池成为可能


我们通过时间分辨光谱方法表征了两种 ITIC 衍生物非富勒烯受体 (NFA) 中的激子扩散和解离行为。 ITIC 和 IT4F 中的激子扩散长度确定为~26 和~34 nm。我们通过瞬态吸收测量进一步检查了 NFA 中受主/施主平面异质结界面处激子的解离，其中观察到有效的电荷生成。最后，我们利用PM6/NFA双层平面异质结制作了平面异质结太阳能电池； PM6/IT4F 双层的功率转换效率 (PCE) 超过 7%。更重要的是，顶层NFA的旋涂对PM6层的形貌影响可以忽略不计，表明存在清晰的双层界面，而不是具有部分本体异质结的准双层结构。结果表明，增强的激子扩散长度以及有效的激子解离和电荷产生是实现高PCE平面异质结有机太阳能电池的基本特征。我们通过传输矩阵模拟在 NFA 层中建立了激子扩散长度和光电流产生之间的直接联系。 NFA中的大激子扩散长度使得实现高效且稳定的双层有机太阳能电池成为可能。