Isomer Effects of Fullerene Derivatives on Organic Photovoltaics and Perovskite Solar Cells

Solar energy conversion is one of the most important issues for creating and maintaining a future sustainable society. In this regard, photovoltaic technologies have attracted much attention because of their potential to solve energy and environmental issues. In particular, thin-film solar cells, such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs), are highly promising owing to their flexibility, light weight, and low-cost production. One of the most important factors used to evaluate solar-cell performance is the power conversion efficiency (PCE), which is the ratio of the output electric power divided by the input light power. The PCEs of PSCs have become comparable to those of multicrystalline silicon solar cells in a laboratory level, but the PCEs of OPVs have yet to catch up with them and still need to be improved. The insufficient durability of PSCs and OPVs is also a challenge that needs to be addressed. Fullerene derivatives have been utilized as electron acceptors and electron-transport materials in OPVs and PSCs. However, the use of fullerene derivatives requires attention to their isomers if they are multiadducts or even monoadducts produced from fullerenes with low symmetry. Their nonuniform structures and electronic properties may exert a negative effect on photovoltaic properties. However, most researchers in the field of OPVs and PSCs have been unaware of the importance of the isomerism. Even the most prevalent, high-performance fullerene acceptor, [6,6]-phenyl-C₇₁-butyric acid methyl ester ([70]PCBM), has been used as an isomer mixture. In this Account, we summarize recent studies on the effects of isomer separation of fullerene derivatives on the device performances of OPVs and PSCs. Largely, fullerene derivatives containing various isomers are categorized into [60]fullerene bisadducts, [70]fullerene bisadducts, and [70]fullerene monoadducts. In all cases, the difference in isomerism was found to have a large impact on PCEs. The miscibility with polymer donors and film-forming property of fullerene derivatives were affected by the isomer separations, which exert the most potent influence on device performances. Although the disorders in energy levels among isomers are not definitely influencing on photovoltaic properties of isomer mixtures, the molecular packing structures of fullerene derivatives make a significant effect on their photovoltaic properties. Notably, isomerically pure fullerene derivatives often-but not always-exhibit higher PCEs than the isomer mixture. The search for the best isomers of fullerene derivatives and their optimal compositional ratios, which extensively depend on their roles and the combined materials, will be an indispensable step to achieving consistently higher device performances for OPVs and PSCs.

Regioselectivity enhancement in synthesis of [70]fullerene derivatives by introduction of a branched structure

Regio-purity in [70]fullerene derivatives is of great importance to improve the power conversion efficiencies of organic photovoltaics. We found that the introduction of a branched structure to [70]PCBM enhanced the yield of α-isomers. The effect of the steric group in the reaction mechanism was theoretically investigated and a difference in the activation energies within specific pathways was revealed.

Electron transfer and exciplex chemistry of functionalized nanocarbons: effects of electronic coupling and donor dimerization

In the past few decades, research on the construction of donor-bridge-acceptor linked systems capable of efficient photoinduced charge separation has fundamentally contributed to the fields of artificial photosynthesis and solar energy conversion. Specifically, the above systems are often fabricated by using carbon-based nanomaterials such as fullerenes, carbon nanotubes, and graphenes, offering limitless possibilities of tuning their optical and electronic properties. Accordingly, since understanding the structure-photodynamics relationships of π-aromatic donor-bridge-nanocarbon linked systems is crucial for extracting the full potential of nanocarbon materials, this review summarizes recent research on their photophysical properties featuring nanocarbon materials as electron acceptors. In particular, we highlight the electronic coupling effects on the photodynamics of donor-bridge-nanocarbon acceptor linked systems, together with the effects of donor dimerization. On a basis of their time-resolved spectroscopic data, the photodynamics of donor-bridge-nanocarbon acceptor linked systems is shown to be substantially influenced by the formation and decay of an exciplex state, i.e., an excited-state consisting of a π-molecular donor and a nanocarbon acceptor with partial charge-transfer character. Such basic information is essential for realizing future application of carbon-based nanomaterials in optoelectronic and energy conversion devices.

cis-1 Isomers of tethered bismethano[70]fullerene as electron acceptors in organic photovoltaics

Isomer-controlled [70]fullerene bis-adducts can achieve high performance as electron-acceptors in organic photovoltaics (OPVs) because of their stronger absorption intensities than [60]fullerene derivatives, higher LUMO energy levels than mono-adducts, and less structural and energetic disorder than random isomer mixtures. Especially, attractive are cis-1 isomers that have the closest proximity of addends owing to their plausible more regular close packed structure. In this study, propylene-tethered cis-1 bismethano[70]fullerene with two methyl, ethyl, phenyl, or thienyl groups were rationally designed and prepared for the first time to investigate the OPV performances with an amorphous conjugated polymer donor (PCDTBT). The cis-1 products were found to be a mixture of two regioisomers, α-1-α and α-1-β as major and minor components, respectively. Among them, the cis-1 product with two ethyl groups (Et2-cis-1-[70]PBC) showed the highest OPV performance, encouraging us to isolate its α-1-α isomer (Et2-α-1-α-[70]PBC) by high-performance liquid chromatography. OPV devices based on Et2-cis-1-[70]PBC and Et2-α-1-α-[70]PBC with PCDTBT showed open-circuit voltages of 0.844 V and 0.864 V, respectively, which were higher than that of a device with typical [70]fullerene mono-adduct, [70]PCBM (0.831 V) with a lower LUMO level. However, the short-circuit current densities and resultant power conversion efficiencies of the devices with Et2-cis-1-[70]PBC (9.24 mA cm-2, 4.60%) and Et2-α-1-α-[70]PBC (6.35 mA cm-2, 3.25%) were lower than those of the device with [70]PCBM (10.8 mA cm-2, 5.8%) due to their inferior charge collection efficiencies. The results obtained here reveal that cis-1 [70]fullerene bis-adducts do not guarantee better OPV performance and that further optimization of the substituent structures is necessary.

Unique cohesive nature of the β1-isomer of [70]PCBM fullerene on structures and photovoltaic performances of bulk heterojunction films with PffBT4T-2OD polymers

Decreasing the amount of a diastereomer of β-[70]PCBM with high aggregation tendency improved the performances of OPV devices with PffBT4T-2OD:[70]PCBM films.