Submitted to Advanced Materials: Molecular Crystallinity and Twin Carrier Transport in Non-Fullerene Organic Photovoltaic Cells by Liu Feng, Gao Ke of Shandong University, and Others
Release Date:
2022-03-21 13:34
Source:
Professor Feng Liu and Professor Yongming Zhang of Shanghai Jiao Tong University, Professor Ke Gao of Shandong University, and Professor Alex Jen of City University of Hong Kong Through collaborative research, we conducted a detailed investigation into the structural details and arrangement patterns of classic ITIC-type non-fullerene acceptor molecules across the “single crystal–pure film–blended film” hierarchy. We discovered that non-fullerene acceptors (NFAs) exhibit a strong intrinsic carrier-generation phenomenon and identified two primary pathways for charge generation in organic thin-film photovoltaic cells: (1) intrinsic carrier generation within the NFA phase, and (2) charge generation via exciton dissociation at the interface CT state. This dual-channel mechanism represents another significant advantage of NFAs, in addition to their superior light absorption and favorable energy-level alignment; together, these features have underpinned the success of NFA materials in organic photovoltaic devices.
Figure 1. (a) Morphology and (b) photophysical pathway diagram of the blend film in non-fullerene organic solar cells.
This study begins with a comparative investigation of the crystal structures of three NFA molecules: ITIC, 4TIC, and 6TIC. It reveals that π–π interactions and side-chain interactions are the two primary factors governing the crystal structure of NFAs. The volume ratio of the side chains to the backbone is a critical determinant of spatial confinement. For ITIC, this ratio is 1.059, resulting in a two-dimensional brickwork packing motif in which the backbone forms layered structures via terminal π–π interactions, while the side chains aggregate within the interlayer voids. As the backbone length decreases, the side-chain-to-backbone volume ratio for 4TIC increases to 1.146, leading the molecule to adopt a three-dimensional, interwoven web-like structure. In 4TIC, the side chains assemble in a tightly packed arrangement, filling the spaces around the π–π-stacked backbone to achieve a densely packed configuration that balances multiple intermolecular forces. When the backbone is further elongated, the side-chain-to-backbone volume ratio for 6TIC drops to 1.030, giving rise to a hierarchical assembly architecture. Under the combined influence of side-chain interactions and π–π stacking, 6TIC molecules first form Z-shaped self-assembled units; these units then organize into two-dimensional assembly layers via hydrogen bonding, with interlayer insertion of blue- and yellow-colored conformational molecules establishing a three-dimensional structure.
Figure 2. Crystal structures of three molecules.
Next, the authors employed grazing-incidence wide-angle X-ray scattering (GIWAXS) to investigate the crystalline structure of NFA molecules in both pure and blend films. By comparing pure films prepared under different processing conditions—specifically, with or without the DIO additive—and integrating GIWAXS measurements with computational simulations, they explored the molecular assembly pathways of NFA. The study revealed that molecules with higher energetic stability, such as ITIC and 4TIC, directly adopt a crystalline morphology, with variations in film-forming conditions leading to differences in crystallinity and structural order. In contrast, the hierarchically assembled acceptor 6TIC preferentially forms two-dimensional layered structures, which further organize into three-dimensional crystalline architectures in the presence of the DIO additive. These differences in intermolecular interactions ultimately determine the extent to which the crystalline phase of NFA molecules is preserved in blend films. In blend films, the interplay between donor polymers and acceptor molecules generally reduces the crystallinity of NFA. However, the use of the DIO additive can enhance NFA crystallinity, as strong intermolecular interactions help maintain the crystalline phase. Notably, due to its hierarchical assembly, the 6TIC molecule retains a two-dimensional layered packing even in films fabricated under DIO-containing conditions.
Figure 3. GIWAXS of the pure film and the assembly process of molecular crystallization within the pure film.
Furthermore, ultrafast transient absorption spectroscopy (TA) was employed to investigate the photophysical properties of the blend films, with long-wavelength laser excitation selectively targeting the NFA acceptor. In the TA spectra of pure ITIC films, an excited-state signal at 955 nm and a polaron signal at 1335 nm can be observed. In contrast, pure films prepared under DIO conditions exhibit a faster conversion of excitons into polarons. This suggests that the crystalline phase of the NFA provides an intrinsic pathway for spontaneous exciton dissociation, a process that does not rely on exciton dissociation at the donor–acceptor interface, thereby representing a novel mechanism for charge-carrier formation. The TA spectra of pure 4TIC and 6TIC films also reveal results similar to those of ITIC. Moreover, the decay curve of the polaron signal in the pure 6TIC film displays a broad plateau (with a lifetime exceeding 1000 ps), indicating that the two-dimensional structure of 6TIC is conducive to stabilizing and transporting dissociated polarons.
In the TA spectra of the blend films, polaron formation can be observed as early as 0 fs, along with the hole-transfer process from the NFA acceptor to the donor material. In blend films fabricated via the DIO method, the polaron yield increases and the hole-transfer rate accelerates. Quantitative comparison reveals that at 0 fs, the interfacial polaron yield in the blend film is comparable to the intrinsic polaron yield, indicating that both charge-generation pathways are equally important in non-fullerene donor–acceptor blend films. Device data further show that broadening the absorption range of the NFA material enhances the device current, while increasing the CT-state energy and reducing the CT-state density helps minimize non-radiative losses and thereby boosts the open-circuit voltage. Together with the intrinsic charge-generation pathway, this mechanism provides a more comprehensive explanation of the photovoltaic conversion process in organic solar cells.
Figure 4. Ultrafast spectroscopy of blend films, schematic diagram of polaron lifetime, polaron yield, and photophysical processes.
Figure 5. CT-state fitting of blend films, and EL spectra of pure and blend films.
In summary, this work provides an in-depth investigation of the crystalline structure and self-assembly processes of NFA molecules, elucidates the influence of intermolecular interactions on the orderly assembly of these molecules, establishes a relatively systematic methodology for studying the crystal and thin-film morphologies of NFAs, and reveals the dual-channel charge-carrier formation mechanism in non-fullerene organic photovoltaic cells. These findings offer new insights into the structure of organic photovoltaic materials and their photoelectric conversion processes, thereby facilitating the development of novel materials.
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