2 Molecular design of non-fullerene acceptors (NFAs) is of vital importance for highefficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach as which may introduce undesirable steric hindrance that reduces charge transport in NFAs. Here we show the design and synthesis of a highly efficient NFA family by substituting the beta position of thiophene unit on Y6-based A-DAD-A backbone with branched alkyl chains. It was found that such modification of different alkyl chain length could completely change the molecular packing behavior of NFAs, leading to improved structure order and charge transport in thin films. Unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of branched alkyl chain topology in tuning the molecular packing and blend morphology that leads to improved organic photovoltaic performance.

Modest exciton diffusion lengths dictate the need for nanostructured bulk heterojunctions in organic photovoltaic (OPV) cells; however, this morphology compromises charge collection. Here, we reveal rapid exciton diffusion in films of a fused-ring electron acceptor that, when blended with a donor, already outperforms fullerene-based OPV cells. Temperature-dependent ultrafast exciton annihilation measurements are used to resolve a quasi-activationless exciton diffusion coefficient of at least 2 × 10–2 cm2/s, substantially exceeding typical organic semiconductors and consistent with the 20–50 nm domain sizes in optimized blends. Enhanced three-dimensional diffusion is shown to arise from molecular and packing factors; the rigid planar molecular structure is associated with low reorganization energy, good transition dipole moment alignment, high chromophore density, and low disorder, all enhancing long-range resonant energy transfer. Relieving exciton diffusion constraints has important implications for OPVs; large, ordered, and pure domains enhance charge separation and transport, and suppress recombination, thereby boosting fill factors. Further enhancements to diffusion lengths may even obviate the need for the bulk heterojunction morphology.

Blue emissions in organic light-emitting devices (OLEDs) are of great significance for their application in full color flat-panel displays and white lightings. [1] However, high-performance blue emitters are still relatively rare. In OLEDs, the injected electrons and holes recombine to form singlet and triplet excitons in the ratio of 1:3, according to the spin statistics, whereas only singlet exciton can decay radiatively in fluorescent materials. [2] Approximately 75% of the triplet excitons are wasted in nonradiative processes, leading to an upper limit of the internal quantum efficiency (IQE) of only 25% in conventional fluorescent devices. One of the methods to enhance the efficiency of OLEDs is to make use of the nonemissive triplet excitons. [3] Phosphorescent OLEDs (PhOLEDs) based on Ir, Pt, and Os organic-metal complexes can approach 100% IQE, which is attributed to the heavy-atom effect. [4] Yet, pure-blue and deep-blue phosphors with Commission Internationale de l'Eclairage (CIE) y values smaller than 0.15 are particularly scarce due to the inherently great challenge in their molecular design; similarly, proper host materials with a large band gap that allows for the refinement of the triplet excitons in devices are also rare. Therefore, it is important to find a way to develop efficient, stable, pure-and deep-blue fluorescent materials. In principle, new-generation, purely organic fluorescent materials can also utilize the nonemissive triplet excitons and achieve high efficiency by converting triplet excitons into singlet excitons. The main mechanisms involve triplet-triplet annihilation (TTA), thermally activated delayed fluorescence (TADF) and the "hot exciton" channel. [5] Essentially, both the TTA and TADF processes can promote the external quantum efficiency (EQE) of the devices by converting excitons from the lowest triplet excited state (T 1 ) to the lowest singlet excited state (S 1 ). Experimental results have confirmed that devices based on TTA and TADF materials can realize a high EQE with a breakthrough of the spin statistical limitation. [6] Although a high EQE has been obtained in TTA and TADF materials, pure-and deep-blue emitters with high efficiency and stability are still exiguous. Unlike TTA and TADF materials, the "hot exciton" materials reported by our group highlight the reverse intersystem crossing from Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet-triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T 1 ) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high-lying excited state (T 2 ), referred to here as a "hot exciton" path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S 1 and T 2 and rather large T 2 -T 1 energy gap, which are expected to...

This paper reports the successful preparation of Mg-doped VO2 nanoparticles via hydrothermal synthesis. The metal-insulator transition temperature (T(c)) decreased by approximately 2 K per at% Mg. The Tc decreased to 54 °C with 7.0 at% dopant. The composite foils made from Mg-doped VO2 particles displayed excellent visible transmittance (up to 54.2%) and solar modulation ability (up to 10.6%). In addition, the absorption edge blue-shifted from 490 nm to 440 nm at a Mg content of 3.8 at%, representing a widened optical band gap from 2.0 eV for pure VO2 to 2.4 eV at 3.8 at% doping. As a result, the colour of the Mg-doped films was modified to increase their brightness and lighten the yellow colour over that of the undoped-VO2 film. A first principle calculation was conducted to understand how dopants affect the optical, Mott phase transition and structural properties of VO2.

Enhancing the luminescence property without sacrificing the charge collection is one key to high-performance organic solar cells (OSCs), while limited by the severe non-radiative charge recombination. Here, we demonstrate efficient OSCs with high luminescence via the design and synthesis of an asymmetric non-fullerene acceptor, BO-5Cl. Blending BO-5Cl with the PM6 donor leads to a record-high electroluminescence external quantum efficiency of 0.1%, which results in a low non-radiative voltage loss of 0.178 eV and a power conversion efficiency (PCE) over 15%. Importantly, incorporating BO-5Cl as the third component into a widely-studied donor:acceptor (D:A) blend, PM6:BO-4Cl, allows device displaying a high certified PCE of 18.2%. Our joint experimental and theoretical studies unveil that more diverse D:A interfacial conformations formed by asymmetric acceptor induce optimized blend interfacial energetics, which contributes to the improved device performance via balancing charge generation and recombination.