The Molecular Ordering and Double‐Channel Carrier Generation of Nonfullerene Photovoltaics within Multi‐Length‐Scale Morphology

• Published in: Advanced materials (Weinheim) 2022-04, Vol.34 (16), p.e2108317-n/a
• Main Authors: Xu, Jinqiu, Jo, Sae Byeok, Chen, Xiankai, Zhou, Guanqing, Zhang, Ming, Shi, Xueliang, Lin, Francis, Zhu, Lei, Hao, Tianyu, Gao, Ke, Zou, Yecheng, Su, Xuan, Feng, Wei, Jen, Alex K.‐Y., Zhang, Yongming, Liu, Feng
• Format: Article
• Language: English
• Subjects: Assembly; Blending; Carrier lifetime; Charge transfer; Current carriers; Energy levels; energy loss; Excitons; MATERIALS SCIENCE; Molecular chains; Morphology; nonfullerene acceptors; Photovoltaic cells; Physical properties; Polarons; single crystals; Solar cells; Thin films; transient absorption spectroscopy
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202108317

The success of nonfullerene acceptor (NFA) solar cells lies in their unique physical properties beyond the extended absorption and suitable energy levels. The current study investigates the morphology and photophysical behavior of PBDB‐T donor blending with ITIC, 4TIC, and 6TIC acceptors. Single‐crystal study shows that the π–π stacking and side‐chain interaction dictate molecular assembly, which can be carried to blended films, forming a multi‐length‐scale morphology. Spontaneous carrier generation is seen in ITIC, 4TIC, and 6TIC neat films and their blended thin films using the PBDB‐T donor, providing a new avenue of zero‐energy‐loss carrier formation. The molecular packing associated with specific contacts and geometry is key in influencing the photophysics, as demonstrated by the charge transfer and carrier lifetime results. The 2D layer of 6TIC facilitates the exciton‐to‐polaron conversion, and the largest photogenerated polaron yield is obtained. The new mechanism, together with the highly efficient blending region carrier generation, has the prospect of the fundamental advantage for NFA solar cells, from molecular assembly to thin‐film morphology. Double carrier‐generation channels are observed: one is the direct photogeneration in the nonfullerene acceptor (NFA) crystal region and the other is the traditional charge‐transfer (CT) state channel in the mixing region. The channel from the NFA crystal provides a new avenue of zero energy loss carrier formation. The NFA molecular packing and geometry is key in influencing the photophysics of NFA solar cells.