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The trade-off between short-circuit current density (JSC) and open-circuit voltage (VOC) has been one of the largest challenges in improving the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Although the energy offset between the excited and charge transfer (CT) states should remain minimal to achieve a high VOC, a very small energy offset typically leads to degradation of JSC, even when novel nonfullerene acceptors (NFAs), such as Y6, are used. Therefore, understanding the limit to what extent the energy offset can be minimized and the physics underlying the trade-off relationship is important to optimize the design of new materials and further improve the PCEs. This study provides a threshold energy that can ensure high charge photogeneration quantum efficiencies for Y-series NFA-based OSCs and discusses the role of the energy offset in device performances. We found that an insufficient energy offset led to not only slow hole transfer at the donor:acceptor interfaces, but also inefficient long-range spatial dissociation of the CT states and degradation of the fill factor (FF). This study also discusses the interplay of the energy levels of the two NFAs that constitute ternary blend OSCs. We found that, by introducing a low-efficiency NFA into a high-efficiency donor:acceptor blend, the voltage loss can be reduced while maintaining a high charge photogeneration quantum efficiency. Our findings highlight the importance of overcoming the trade-off between FF and VOC for further improving the PCE.

Realizing efficient near-infrared to visible photon upconversion in the solid state is pivotal for commercial applications in various fields. We previously reported a solid-state upconversion device which imitated the photovoltaic conversion mechanisms of organic solar cells. This leads to a significant improvement of up to 2.3% in the external quantum efficiency, which is two orders of magnitude higher than that of conventional devices. Here, we investigate the upconversion mechanism of this device. We examine exciton and charge dynamics using transient absorption spectroscopy and find that approximately 67% of incident photons are utilized owing to fast singlet exciton diffusion in the nonfullerene acceptor layer. Strikingly, triplet excitons are accumulated near the donor/acceptor interface, enabling accelerated triplet–triplet annihilation by a factor of more than 10.

To harvest the full potential of polymer-based solid-state photon upconversion (UC) devices, we examined the effect of the molecular weight of a fluorescent polymer on the UC efficiency. With a high-molecular-weight polymer, a long triplet lifetime of 11.2 ms was achieved, which led to a characteristic threshold intensity of 67 mW cm⁻², considerably lower than those of previously reported polymer-based UC devices. Furthermore, the external quantum efficiency of our UC device was as high as ∼0.35%. Consequently, fluorescent conjugated polymers with long triplet lifetimes can serve as attractive candidates for efficient solid-state UC devices.

The power conversion efficiencies of organic solar cells (OSCs) have routinely lagged far behind those of their inorganic counterparts. However, owing to the enormous contributions of many researchers, the power conversion efficiencies of OSCs have rapidly improved and now exceed 19%. The charge generation mechanisms in OSCs have been heavily debated during this period while acquiring valuable knowledge. This review highlights fundamental and cutting-edge research that rationalizes why OSCs can generate photocurrent so efficiently. In particular, a photophysicist's views on exciton diffusion to donor:acceptor interfaces, charge transfer at the donor:acceptor interface, and long-range spatial dissociation of charge transfer states are discussed. Although a general consensus in this area has not been reached yet, recent time-resolved spectroscopic measurements provide important photophysical insights that can help achieving a better understanding of the charge generation mechanism in OSCs. Based on these observations, future research directions for realizing further improvements in OSC performance are discussed.

Recent studies have shown that efficient free carrier (FC) generation with a small voltage loss can be achieved in organic solar cells (OSCs); however, the photophysical insights underpinning this remain unclear. Herein, we examined the mechanisms underlying the FC generation in a state-of-the-art OSC consisting of PM6 and Y6 as electron donor and acceptor, respectively, wherein the energy offset between the lowest excited singlet state and the charge transfer state is as small as ∼0.12 eV. We used transient absorption spectroscopy to track the time evolution of electroabsorption caused by electron–hole pairs generated at donor/acceptor interfaces. After hole transfer from Y6 to PM6, we observed slow yet efficient spatial charge dissociation on a time scale of picoseconds. Based on temperature-dependence measurements, we found that this slow yet efficient FC generation is driven by downhill energy relaxation of charges through the energy cascade generated near the interfaces. We provide here direct experimental evidence for the FC generation mechanism in the very topical PM6/Y6 blend system.

Herein, we study the origin of differences in open-circuit voltage (VOC) for polymer:fullerene solar cells employing highly crystalline conjugated polymers (PTzBT) based on the same thiophene–thiazolothiazole backbone with different side chains. By analyzing the temperature dependence of VOC and cyclic voltammogram, we find that the difference in VOC originates in the different cascaded energy structures for the highest occupied molecular orbital (HOMO) levels in the interfacial mixed phase. Furthermore, we find that this is due to the stabilization of HOMO caused by the different branching of side chains on the basis of density functional theory calculation. Finally, we discuss the molecular design strategy based on side-chain engineering for ideal interfacial cascaded energy structures leading to higher VOC and photocurrent simultaneously.

Understanding the excited-state dynamics of nonfullerene electron acceptors is essential for further improvement of organic solar cells as they are responsible for near-IR light absorption. Herein, we investigated the singlet and triplet excited-state dynamics in Y6, a novel nonfullerene acceptor, using transient absorption spectroscopy. We found that, even at low excitation fluences, pristine Y6 films show biphasic singlet exciton decay kinetics with decay constants of ∼220 ps and ∼1200 ps. The majority of the Y6 singlet excitons decayed with the faster (∼220 ps) component, whereas a clear photoluminescence with the slower (∼1200 ps) component was observed, which is the origin of the large discrepancies in the previously reported exciton lifetimes of Y6 in the solid state. At high excitation fluences, on the other hand, Y6 singlet excitons are more likely to decay via singlet–singlet exciton annihilation due to fast exciton diffusion in crystalline domains, after which we observed ultrafast triplet formation, assigned to singlet fission from higher excited singlet states.

Increasing photon harvest is an essential issue in improving the efficiency of organic photovoltaics. Here, we study ternary blend polymer solar cells that are composed of a thiazolothiazole-based crystalline semiconducting polymer and a fullerene derivative as the host binary system and a very small content of a series of narrow-band gap non-fullerene acceptors as the third-component sensitizer that is selectively located at the interface between the host donor and acceptor. Surprisingly, the cells give an external quantum efficiency in the sensitizer absorption range that is as high as that in the polymer absorption range despite the fact that the optimal sensitizer content is only 6 wt %, which is far smaller than the host polymer contents. This leads to significantly improved photocurrents and, in turn, high power conversion efficiencies relative to the binary blend cell. Such pronounced sensitization is found to originate in the markedly amplified sensitizer absorption owing to the optical interference effect of more than 300 nm-thick photoactive layers. In parallel, the ternary blend system realizes markedly reduced photon energy loss, which is also important for the power conversion efficiency improvement, and high thermal stability. With such excellent features, we believe that the sensitized ternary blend cells have exceptional possibilities and that exploring more well-matched material combinations would improve the performance further.

Charge recombination is a major loss process in state-of-the-art organic photovoltaics (OPVs). Nongeminate recombination (NGR) is typically in competition with collection of photogenerated charge carriers to each electrode not only at the short-circuit but also at the open-circuit conditions, and therefore impacts on both external quantum efficiency (EQE) and fill factor (FF). In most cases, therefore, the optimised active layers are kept as thin as possible (typically ∼100 nm) to avoid NGR loss although it is not thick enough to absorb the solar light effectively. As such, a key to efficient EQE and FF is suppressing NGR beyond limitations arising from the Langevin recombination. In addition, NGR is closely related to open-circuit voltage (VOC) as well. In particular, nonradiative NGR is a critical loss process in OPVs. In this review, we aim to summarise our current understanding of underlying photophysics that governs NGR in OPVs. Recent experimental results and proposed mechanisms relevant to NGR are summarised where we highlight the significant importance of morphology and energetics for suppressing NGR. These insights will provide a comprehensive understanding of the recombination and future materials/devices design principles.

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.

Controlling the energetics and backbone order of semiconducting polymers is essential for the performance improvement of polymer‐based solar cells. The use of fluorine as the substituent for the backbone is known to effectively deepen the molecular orbital energy levels and coplanarize the backbone by noncovalent interactions with sulfur of the thiophene ring. In this work, novel semiconducting polymers are designed and synthesized based on difluoronaphthobisthiadiazole (FNTz) as a new family of naphthobisthiadiazole (NTz)–quaterthiophene copolymer systems, which are one of the highest performing polymers in solar cells. The effect of the fluorination position on the energetics and backbone order is systematically studied. It is found that the dependence of the solar cell fill factor on the active layer thickness is very sensitive to the fluorination position. It is thus further investigated and discussed how the structural features of the polymers influence the photovoltaic parameters as well as the diode characteristics and bimolecular recombination. Further, the polymer with fluorine on both the naphthobisthiadiazole and quaterthiophene moieties exhibits a quite high power conversion efficiency of 10.8% in solar cells in combination with a fullerene. It is believed that the results would offer new insights into the development of semiconducting polymers.

In this paper, we introduce a new strategy for improving the efficiency of upconversion emissions based on triplet–triplet exciton annihilation (TTA-UC) in the solid state. We designed a ternary blend system consisting of a triplet sensitizer (TS), an exciton-transporting host polymer, and a small amount of an annihilator in which the triplet-state energies of the TS, host, and annihilator decrease in this order. The key idea underpinning this concept involves first transferring the triplet excitons generated by the TS to the host and then to the annihilator, driven by the cascaded triplet energy landscape. Because of the small annihilator blend ratio, the local density of triplet excitons in the annihilator domain is higher than those in conventional binary TS/annihilator systems, which is advantageous for TTA-UC because TTA is a density-dependent bimolecular reaction. We tracked the triplet exciton dynamics in the ternary blend film by transient absorption spectroscopy. Host triplet excitons are generated through triplet energy transfer from the TS following intersystem crossing in the TS. These triplet excitons then diffuse in the host domain and accumulate in the annihilator domain. The accumulated triplet excitons undergo TTA to generate singlet excitons that are higher in energy than the excitation source, resulting in UC emission. Based on the excitation-intensity and blend-ratio dependences of TTA-UC, we found that our concept has a positive impact on accelerating TTA.