Yamakata Akira, Kato Kosaku, Urakami Takumi, Tsujimura Sota, Murayama Kasumi, Higashi Masahiro, Sato Hirofumi, Kobori Yasuhiro, Umeyama Tomokazu, Imahori Hiroshi
Graduate School of Natural Science and Technology, Okayama University 3-1-1, Tsushima-naka, Kita-ku Okayama 700-8530 Japan
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University Nishikyo-ku Kyoto 615-8510 Japan
Chem Sci. 2024 Jul 10;15(32):12686-12694. doi: 10.1039/d4sc00917g. eCollection 2024 Aug 14.
The power conversion efficiency (PCE) of organic photovoltaics (OPVs) has reached more than 19% due to the rapid development of non-fullerene acceptors (NFAs). To compete with the PCEs (26%) of commercialized silicon-based inorganic photovoltaics, the drawback of OPVs should be minimized. This drawback is the intrinsic large loss of open-circuit voltage; however, a general approach to this issue remains elusive. Here, we report a discovery regarding highly efficient NFAs, specifically ITIC. We found that charge-transfer (CT) and charge dissociation (CD) can occur even in a neat ITIC film without the donor layer. This is surprising, as these processes were previously believed to take place exclusively at donor/acceptor heterojunctions. Femtosecond time-resolved visible to mid-infrared measurements revealed that in the neat ITIC layers, the intermolecular CT immediately proceeds after photoirradiation (<0.1 ps) to form weakly-bound excitons with a binding energy of 0.3 eV, which are further dissociated into free electrons and holes with a time-constant of 56 ps. Theoretical calculations indicate that stacking faults in ITIC (, V-type molecular stacking) induce instantaneous intermolecular CT and CD in the neat ITIC layer. In contrast, J-type stacking does not support such CT and CD. This previously unknown pathway is triggered by the larger dipole moment change on the excited state generated at the lower symmetric V-type molecular stacking of ITIC. This is in sharp contrast with the need of sufficient energy offset for CT and CD at the donor-acceptor heterojunction, leading to the significant voltage loss in conventional OPVs. These results demonstrate that the rational molecular design of NFAs can increase the local dipole moment change on the excited state within the NFA layer. This finding paves the way for a groundbreaking route toward the commercialization of OPVs.
由于非富勒烯受体（NFAs）的快速发展，有机光伏（OPVs）的功率转换效率（PCE）已达到19%以上。为了与商业化的硅基无机光伏的PCE（26%）竞争，OPVs的缺点应降至最低。这一缺点是开路电压的固有巨大损失；然而，解决这个问题的通用方法仍然难以捉摸。在这里，我们报告了一项关于高效NFAs，特别是ITIC的发现。我们发现，即使在没有供体层的纯ITIC薄膜中，电荷转移（CT）和电荷解离（CD）也会发生。这令人惊讶，因为这些过程以前被认为只发生在供体/受体异质结处。飞秒时间分辨可见到中红外测量表明，在纯ITIC层中，分子间CT在光照射后立即进行（<0.1 ps），形成结合能为0.3 eV的弱束缚激子，这些激子进一步以56 ps的时间常数解离为自由电子和空穴。理论计算表明，ITIC中的堆垛层错（V型分子堆积）在纯ITIC层中诱导瞬时分子间CT和CD。相比之下，J型堆积不支持这种CT和CD。这条以前未知的途径是由ITIC较低对称V型分子堆积产生的激发态上较大的偶极矩变化触发的。这与供体-受体异质结处CT和CD需要足够的能量偏移形成鲜明对比，导致传统OPV中显著的电压损失。这些结果表明，NFAs的合理分子设计可以增加NFA层内激发态上的局部偶极矩变化。这一发现为OPVs商业化的开创性路线铺平了道路。
