Tomokazu Umeyama

Few-layered antimonene (FLSb) nanosheets were noncovalently functionalized with fullerene C60 clusters by quick addition of a poor solvent (i.e., acetonitrile) into a mixed dispersion of FLSb and C60 in a good solvent (i.e., toluene). In a flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurement, the FLSb-C60 composite, (FLSb+C60 )m , showed a rapid rise in transient conductivity, whereas no conductivity signal was observed in the single components, FLSb and C60 . This demonstrated the occurrence of photoinduced charge separation between FLSb and C60 in (FLSb+C60 )m . Furthermore, a photoelectrochemical device with an electrophoretically deposited (FLSb+C60 )m film exhibited an enhanced efficiency of photocurrent generation, compared to those of the single-components, FLSb and C60 , due to the photoinduced charge separation between FLSb and C60 . This work provides a promising approach for fabrication of antimonene-organic molecule composites and paves the way for their application in optoelectronics.

Electron-acceptor small-molecules possessing a long exciton lifetime and a narrow energy band gap, opposing the energy gap law, are highly desirable for high-performance organic photovoltaics (OPVs) by realizing their efficient light-harvesting ability (LH), exciton diffusion (ED), and charge transfer (CT). Toward this goal, we designed an acceptor-donor-acceptor (A-D-A) type nonfullerene acceptor (NFA), TACIC, having an electron-donating, self-assembling two-dimensional (2D) nanographene unit, thienoazacoronene, at the center with electron-withdrawing groups at both ends. The TACIC film exhibited a narrow band gap (1.59 eV) with excellent LH. Surprisingly, the TACIC film showed an extremely long exciton lifetime (1.59 ns), suppressing undesirable nonradiative decay by its unique self-assembling behavior. When combined with a conjugated polymer donor, PBDB-T, slow ED and CT were observed (60 ps) with the excitation of TACIC owing to the large TACIC domain sizes. Nevertheless, the unusually high efficiencies of ED and CT (96% in total) were achieved by the long TACIC exciton lifetime. Additionally, unusual energy transfer (EnT) from the excited PBDB-T to TACIC was seen, demonstrating its dual LH role. The OPV device with PBDB-T and TACIC showed a high incident photon-to-current efficiency (IPCE) exceeding 70% at up to 710 nm and a power conversion efficiency of ∼10%. This result will open up avenues for a rational strategy of OPVs where LH, ED, and CT from the acceptor side as well as LH, EnT, ED, and CT from the donor side can be better designed by using 2D nanographene as a promising building block for high-performance A-D-A type NFAs.

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.

As structure defined cutouts of the graphene lattice, nanographene molecules have gained plenty of attention because of their high potential for versatile applications in organic electronics and energy conversion devices and as ideal model systems for the better understanding of intrinsic structure-property correlations of graphenes. In this study, well-defined nanographenes with sp2 carbon networks of different sizes, hexa-peri-hexabenzocoronene (HBC) and its rectangularly π-extended version, a short graphene nanoribbon (GNR), have been covalently functionalized with photoactive porphyrin molecules. On the basis of their spectroscopic studies, the photodynamics of the porphyrin-linked nanographenes was found to be influenced substantially by the size of the nanographenes. Photoexcitation of the porphyrin-HBC linked system led to exclusive energy transfer (EnT) from the first singlet excited state (S1) of the nanographene to the porphyrin, whereas opposite selective EnT occurred from the first and second singlet excited states (S1 and S2) of the porphyrin to the nanographene in the porphyrin-GNR linked system. In particular, ultrafast efficient EnTs from both the S2 and S1 states of the porphyrin to GNR mimic the corresponding ultrafast EnTs from the S2 and S1 states of carotenoids to chlorophylls in light-harvesting systems of natural photosynthesis. Such unique photophysical properties will be useful for the rational design of carbon-based photofunctional nanomaterials for optoelectronics and solar energy conversion devices.

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.

Composite films that consisted of C60 and well-exfoliated nanosheets of transition metal dichalcogenides (TMDs), such as MoS2 or WS2, with a bulk heterojunction structure were easily fabricated onto a semiconducting SnO2 electrode via a two-step methodology: self-assembly into their composite aggregates by injection of a poor solvent into a good solvent with the dispersion, and subsequent electrophoretic deposition. Upon photoexcitation, the composites on SnO2 exhibited enhanced transient conductivity in comparison with single components of TMDs or C60, which demonstrates that the bulk heterojunction nanostructure of TMD and C60 promoted the charge separation (CS). In addition, the decoration of the TMD nanosheets with C60 hindered the undesirable charge recombination (CR) between an electron in SnO2 and a hole in the TMD nanosheets. Owing to the accelerated CS and suppressed CR, photoelectrochemical devices based on the MoS2–C60 and WS2–C60 composites achieved remarkably improved incident photon-to-current efficiencies (IPCEs) as compared with the single-component films. Despite more suppressed CR in WS2–C60 than MoS2–C60, the IPCE value of the device with WS2–C60 was smaller than that with MoS2–C60 owing to its inhomogeneous film structure.

The effects of regioisomer and diastereomer separations of [70]PCBM on structures and photovoltaic properties of PffBT4T-2OD:[70]PCBM blend films have systematically been investigated for the first time. Decreasing the amount of a diastereomer of β-[70]PCBM with high aggregation tendency (β1-[70]PCBM) improved the photovoltaic performances.

Over the past several years, organometal halide perovskite solar cells (PSCs) have attracted considerable interest from the scientific research community because of their potential as promising photovoltaic devices for use in renewable energy production. To date, high power conversion efficiencies (PCEs) of more than 20% have been primarily achieved with mesoscopic-structured PSCs, where a mesoporous TiO2 (mTiO(2)) layer is incorporated as an electron-transporting mesoporous scaffold into the perovskite crystal, in addition to a compact TiO2 (cTiO(2)) as an electron-transporting layer (ETL). In this Perspective, we first summarize recent research on the preparation strategies of the mTiO(2) layer with a high electron transport capability by facile sol-gel methods instead of the conventional nanoparticle approach. The importance of the control of the pore size and grain boundaries of the mTiO(2) in achieving high PCEs for PSCs is discussed. In addition, an alternative method to improve the electron transport in the mTiO(2) layer via the incorporation of highly conductive nanocarbon materials, such as two-dimensional (2D) graphene and one-dimensional (1D) carbon nanotubes, is also summarized. Finally, we highlight the utilization of zero-dimensional (0D) nanocarbon, i.e., fullerenes, as an n-type semiconducting material in mesostructure-free planar PSCs, which avoids high-temperature sintering during the fabrication of an ETL.

Despite numerous organic semiconductors being developed during the past decade, C-70 derivatives are predominantly used as electron acceptors in efficient polymer solar cells (PSCs). However, as-prepared C-70 mono-adducts intrinsically comprise regioisomers that would mask individual device performances depending on the substituent position on C-70. Herein, we separate the regioisomers of C-70 monoadducts for PSC applications for the first time. Systematic investigations of the substituent position effect using a novel symmetric C-70 mono-adduct ([70] NCMA) and a prevalent, high-performance one ([70] PCBM) reveals that we can control the structures of the blend films with conjugated polymers and thereby improve the PSC performances by regioisomer separation. Our approach demonstrates the significance of exploring the best-matching regioisomer of C-70 mono-adducts with high-performance conjugated polymers, which would achieve a remarkable progress in PSC devices.

The photoexcitation of the pyrene dimer on graphene resulted in the final formation of a charge-separated state following an exciplex formation, while that of the pyrene monomer on graphene generated the corresponding exciplex solely due to the difference in the electronic coupling between the pyrene and the graphene.

To shed a light on fundamental molecular functions of photoinduced charge conductions by organic photovoltaic materials, it is important to directly observe molecular geometries of the intermediate charges just after the photoinduced electron-transfer reactions. However, highly inhomogeneous molecular environments at the bulk heteojunction interfaces in the photoactive layers have prevented us from understanding the mechanism of the charge conductions. We have herein investigated orbital geometries, electronic couplings, and hole-dissociation dynamics of photo induced charge-separated (CS) states in a series of poly(3-hexylthiophene) fullerene linked dyads bridged by rigid oligo-p-phenylene spacers by using time-resolved EPR spectroscopy. It has been revealed that one-dimensional intramolecular hole-dissociations exothermically take place from localized holes in initial CS states, following bridge-mediated, photoinduced charge separations via triplet exciton diffusions in the conjugated polymer-backbones. This molecular wire property of the photoinduced charges in solution at room temperature demonstrates the potential utility of the covalently bridged polymer molecules applied for the molecular devices.

Although the utilization of [70]fullerene bis-adducts can enhance the power conversion efficiencies of polymer solar cells (PSCs) owing to their strong absorption intensities and high-lying lowest unoccupied molecular orbital energy levels, this synthetic strategy typically yields a mixture of regioisomers that would mask the intrinsic device performances depending on the substituent pattern on the [70]fullerene derivatives. In this study, a single cis-2 regioisomer of C-70 bis-adduct (cis-2-[70]BIEC) has been prepared for the first time by the same strategy that had been applied to [60]fullerene to obtain a regioisomerically pure C-60 bis-adduct (cis-2-[60]BIEC). DielsAlder reaction was conducted between a rationally designed ethylene-tethered indene dimer and [70]fullerene, followed by isolation using high-performance liquid chromatography suitable for the separation of fullerene derivatives. A series of structural analysis techniques including NMR spectroscopies and X-ray crystallography were used to identify the absolute configuration of the bis-adduct. A systematic study on the optical, electrochemical, and photovoltaic properties of cis-2-[70]BIEC as well as the corresponding regioisomer mixture (bis-[70]BIEC) and the monoadduct (a-mono-[70]BIEC) has been performed to examine the effect of the pure cis-2 regioisomer. More importantly, their properties are compared with those of cis-2-[60]BIEC to address the effect of fullerene cage structures, that is, C-60 versus C-70. The PSC based on cis-2-[70]BIEC and poly(3-hexylthiophene) showed a remarkable power conversion efficiency of 4.2%, which is higher than those with bis-[70]BIEC (2.2%), alpha-mono-[70]BIEC (2.2%), cis-2-[60]BIEC (2.8%), and even a prevalent high-performance C-70 monoadduct ([70]PCBM, 3.8%). Our synthetic strategy will pave the way for further development on the rational design and isolation of single fullerene bis-adduct regioisomers exhibiting high device performances.

The close solid-state structure-property relationships of organic pi - aromatic molecules have attracted interest due to their implications for the design of organic functional materials. In particular, a dimeric structure, that is, a unit consisting of two molecules, is required for precisely evaluating intermolecular interactions. Here, we show that the sidewall of a singlewalled carbon nanotube (SWNT) represents a unique molecular dimer platform that can be directly visualized using high-resolution transmission electron microscopy. Pyrene is chosen as the pi - aromatic molecule; its dimer is covalently linked to the SWNT sidewalls by aryl addition. Reflecting the orientation and separation of the two molecules, the pyrene dimer on the SWNT exhibits characteristic optical and photophysical properties. The methodology discussed here-form and probe molecular dimers-is highly promising for the creation of unique models and provides indispensable and fundamental information regarding molecular interactions.

A pure cis-2 isomer of indene dimer-based fullerene bis-adduct has been synthesized, isolated and applied to bulk heterojunction solar cells for the first time. The device based on the cis-2 isomer and poly(3-hexylthiophene) exhibited a power conversion efficiency of 2.8%, which is higher than those of the reference devices.