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Transient absorption spectroscopy (TAS) is a time‐resolved spectroscopic technique that can quantitatively observe temporal changes in the absorption spectrum associated with transient species, such as excitons and charges. Therefore, it is extensively used in photophysics, photochemistry, and photobiology. TAS plays a crucial role in unveiling charge generation and recombination dynamics in organic solar cells (OSCs). However, the interpretation of TA data is sometimes difficult for those who are not familiar with TAS. In this Perspective, therefore, the utility of TAS is briefly explained in studying OSCs and highlight key considerations for interpreting TA data. Measuring TA data at appropriate excitation fluence and appropriate spectral assignment are essential to correctly interpret photophysics of OSCs. With this Perspective, it aims to offer readers sufficient advice to enable them to perform TA measurements appropriately and interpret the TA data correctly. [ABSTRACT FROM AUTHOR]

The large voltage loss is a significant disadvantage of organic solar cells (OSCs) compared with inorganic and perovskite solar cells and should be reduced to further improve the power conversion efficiency of OSCs. Herein, the voltage loss in OSCs with a systematically controlled energy difference between the excited and charge transfer states is discussed. The voltage loss is compared between two types of OSCs: narrow‐bandgap donors paired with wide‐bandgap acceptors and narrow‐bandgap fused‐ring nonfullerene acceptors (NFAs) paired with wide‐bandgap donors, to elucidate whether there are any essential advantages in the latter. The first advantage of narrow‐bandgap‐NFA‐based OSCs is their higher photoluminescence quantum yield, owing to their rigid fused‐ring architecture. The second advantage is the ability to achieve efficient charge separation with a small voltage loss. [ABSTRACT FROM AUTHOR]

Suppressing charge recombination is pivotal for further improving the power conversion efficiency of organic solar cells (OSCs). The bimolecular recombination of free charge carriers leads to the formation of both singlet and triplet charge transfer (CT) states at a ratio of 1:3 when considering simple spin statistics, meaning that charge recombination via the triplet excited state is the main deactivation channel for OSCs. Although the formation of local triplet excitons through back charge transfer from triplet CT states is thought to be a terminal loss process, it is shown that charge separation from triplet excitons can occur in nonfullerene acceptor (NFA)‐based OSCs. To reveal the triplet exciton dynamics, a triplet sensitizer PtTPBP is doped into an OSC consisting of PM6 and Y6 as an electron donor and acceptor, respectively. Upon photoexcitation of PtTPBP, triplet excitons are formed, which subsequently transfer their energy to Y6, resulting in Y6 triplet excitons formation. Based on transient absorption and external quantum efficiency measurements, clear experimental evidence of charge photogeneration from Y6 triplet excitons at the PM6:Y6 interface is provided. This study highlights the importance of minimizing the energy difference between the singlet and triplet excited states of NFAs to suppress charge recombination. [ABSTRACT FROM AUTHOR]

The voltage loss incurred by nonradiative charge recombination should be reduced to further improve the power conversion efficiency of organic solar cells (OSCs). This work discusses the nonradiative voltage loss in OSCs with systematically controlled energy offset between optical bandgap and charge transfer (CT) states. It is demonstrated that the nonradiative voltage loss is a function of the energy offset; it drops sharply with decreasing energy offset. By measuring the quantum yields of electroluminescence from OSCs and decay kinetics of CT states, it is found that the radiative decay rate of CT states becomes larger when the energy offset is negligible compared with those in conventional OSCs with sufficient energy offset. This behavior is rationalized by hybridization between CT and local excited states, resulting in a considerable enhancement of the oscillator strength of CT states. Based on a trend observed in this study, the precise mechanism by which the energy offset affects the nonradiative voltage loss is discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The mechanism of light-induced degradation in organic solar cells based on regioregular poly(3-hexylthiophene) and indene-C60 bisadduct is studied by transient absorption (TA) and electron spin resonance (ESR) measurements. After 45 h light exposure under simulated solar illumination at 100 mW cm−2, the short-circuit current density, open-circuit voltage, and fill factor are all degraded by about 20%-30% relative to the initial photovoltaic parameters. For the assignment of limiting conversion processes in the degraded solar cells, exciton diffusion into a donor/acceptor interface, charge transfer at the interface, charge dissociation into free charge carriers, and charge collection to each electrode are observed before and after the light exposure by the TA measurement. As a result, it is found that the charge collection deteriorates after the light exposure because of light-induced charge trap formation in the bulk of the active layer. The origin of charge traps is further discussed on the basis of ESR measurements and density functional theory calculation. [ABSTRACT FROM AUTHOR]