The escalating global energy predicament implores for a revolutionary resolution—one that converts sunlight into electricity—holding the key to supreme conversion efficiency. This comprehensive review embarks on the exploration of the principle of generating multiple excitons per absorbed photon, a captivating concept that possesses the potential to redefine the fundamental confines of conversion efficiency, albeit its application remains limited in photovoltaic devices. At the nucleus of this phenomenon are two principal processes: multiple exciton generation (MEG) within quantum-confined environments, and singlet fission (SF) inside molecular crystals. The process of SF, characterized by the cleavage of a single photogenerated singlet exciton into two triplet excitons, holds promise to potentially amplify photon-to-electron conversion efficiency twofold, thereby laying the groundwork to challenge the detailed balance limit of solar cell efficiency. Our discourse primarily dissects the complex nature of SF in crystalline organic semiconductors, laying special emphasis on the anisotropic behavior of SF and the diffusion of the subsequent triplet excitons in single-crystalline polyacene organic semiconductors. We initiate this journey of discovery by elucidating the principles of MEG and SF, tracing their historical genesis, and scrutinizing the anisotropy of SF and the impact of quantum decoherence within the purview of functional mode electron transfer theory. We present an overview of prominent techniques deployed in investigating anisotropic SF in organic semiconductors, including femtosecond transient absorption microscopy and imaging as well as stimulated Raman scattering microscopies, and highlight recent breakthroughs linked with the anisotropic dimensions of Davydov splitting, Herzberg–Teller effects, SF, and triplet transport operations in single-crystalline polyacenes. Through this comprehensive analysis, our objective is to interweave the fundamental principles of anisotropic SF and triplet transport with the current frontiers of scientific discovery, providing inspiration and facilitating future ventures to harness the anisotropic attributes of organic semiconductor crystals in the design of pioneering photovoltaic and photonic devices.

Singlet fission is a spin-conserving process for the multiplication conversion of one singlet exciton into two individual triplet excitons by absorbing one photon. Such a multiplication is believed to circumvent the Shockley–Queisser thermodynamic limit for improving efficiency of solar energy conversion. A mechanistic understanding of generation and yields of triplet excitons from singlet fission materials is essential for efficient exploitation of solar energy. Here we employ temperature-dependent transient absorption spectroscopy to examine the dynamical nature of singlet fission and triplet excitons in hexacene. The generation and dissociation rates of the intermediate correlated biexciton, 1(TT), are independent of temperature from 77 K to the room temperature. On the other hand, the triplet excitons in spatially separated biexcitons, 1(T···T), relax via geminate and nongeminate recombination. The former was found to be temperature-dependent, whereas the latter is temperature-independent. Quantitative analyses of the temperate-dependent rates for the two recombination processes yield the energy difference between the 1(T···T) and 1(TT), which were further confirmed by our density functional theory (DFT) calculations.

The development of active, durable, and nonprecious electrocatalysts for hydrogen electrochemistry is highly desirable but challenging. In this work, we design and fabricate a novel interface catalyst of Ni and Co₂N (Ni/Co₂N) for hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR). The Ni/Co₂N interfacial catalysts not only achieve a current density of −10.0 mA cm^–2 with an overpotential of 16.2 mV for HER but also provide a HOR current density of 2.35 mA cm^–2 at 0.1 V vs reversible hydrogen electrode (RHE). Furthermore, the electrode couple made of the Ni/Co₂N interfacial catalysts requires only a cell voltage of 1.57 V to gain a current density of 10 mA cm^–2 for overall water splitting. Hybridizations in the three elements of Ni-3d, N-2p, and Co-3d result in charge transfer in the interfacial junction of the Ni and Co₂N materials. Our density functional theory calculations show that both the interfacial N and Co sites of Ni/Co₂N prefer to hydrogen adsorption in the hydrogen catalytic activities. This study provides a new approach for the construction of multifunctional catalysts for hydrogen electrochemistry.

Singlet fission is believed to improve the efficiency of solar energy conversion by breaking up the Shockley–Queisser thermodynamic limit. Understanding of triplet excitons generated by singlet fission is essential for solar energy exploitation. Here we employed transient absorption microscopy to examine dynamical behaviors of triplet excitons. We observed anisotropic recombination of triplet excitons in hexacene single crystals. The triplet exciton relaxations from singlet fission proceed in both geminate and non-geminate recombination. For the geminate recombination, the different rates were attributed to the significant difference in their related energy change based on the Redfield quantum dissipation theory. The process is mainly governed by the electron–phonon interaction in hexacene. On the other hand, the non-geminate recombination is of bimolecular origin through energy transfer. In the triplet–triplet bimolecular process, the rates along the two different optical axes in the a–b crystalline plane differ by a factor of 4. This anisotropy in the triplet–triplet recombination rates was attributed to the interference in the coupling probability of dipole–dipole interactions in the different geometric configurations of hexacene single crystals. Our experimental findings provide new insight into future design of singlet fission materials with desirable triplet exciton exploitations.

Singlet fission is known to improve solar energy utilization by circumventing the Shockley-Queisser limit. The two essential steps of singlet fission are the formation of a correlated triplet pair and its subsequent quantum decoherence. However, the mechanisms of the triplet pair formation and decoherence still remain elusive. Here we examined both essential steps in single crystalline hexacene and discovered remarkable anisotropy of the overall singlet fission rate along different crystal axes. Since the triplet pair formation emerges on the same timescale along both crystal axes, the quantum decoherence is likely responsible for the directional anisotropy. The distinct quantum decoherence rates are ascribed to the notable difference on their associated energy loss according to the Redfield quantum dissipation theory. Our hybrid experimental/theoretical framework will not only further our understanding of singlet fission, but also shed light on the systematic design of new materials for the third-generation solar cells.