Organic photovoltaics (OPVs) have achieved great progress in recent years due to delicately designed non-fullerene acceptors (NFAs). Compared with tailoring of the aromatic heterocycles on the NFA backbone, the incorporation of conjugated side-groups is a cost-effective way to improve the photoelectrical properties of NFAs. However, the modifications of side-groups also need to consider their effects on device stability since the molecular planarity changes induced by side-groups are related to the NFA aggregation and the evolution of the blend morphology under stresses. Herein, a new class of NFAs with local-isomerized conjugated side-groups are developed and the impact of local isomerization on their geometries and device performance/stability are systematically investigated. The device based on one of the isomers with balanced side- and terminal-group torsion angles can deliver an impressive power conversion efficiency (PCE) of 18.5%, with a low energy loss (0.528 V) and an excellent photo- and thermal stability. A similar approach can also be applied to another polymer donor to achieve an even higher PCE of 18.8%, which is among the highest efficiencies obtained for binary OPVs. This work demonstrates the effectiveness of applying local isomerization to fine-tune the side-group steric effect and non-covalent interactions between side-group and backbone, therefore improving both photovoltaic performance and stability of fused ring NFA-based OPVs.
Polymeric coatings with randomly distributed dielectric nanoparticles have attracted intensive attention in the passive daytime radiative cooling application. Here, we propose a modified Monte–Carlo method for investigating the spectral response and cooling performance of polymer coating with gradient‐dispersed nanoparticles. Using this method, we carry out a quantitative analysis on the solar reflectance, infrared emittance and cooling power of four categories of gradient structures. It is shown that the gradient profile of particle distribution at the near‐surface region has a significant influence on the overall performance of the coatings. Compared to a randomly distributed structure, the downward size‐gradient structure exhibits superiority in both solar reflectance and cooling power. The presented gradient design, also applicable to porous structures, provides an effective and universal strategy for significantly improving the cooling performance of radiative cooling coatings.
Perovskite‐based single‐junction and tandem solar cells have recently attracted considerable attention due to their remarkable advantages in power conversion efficiency (PCE) and fabrication cost; however, their commercialization remains challenging. One crucial limiting factor is the incompetent thermal management, which is inclined to degrade the PCE and stability of the device. Here, a rigorous opto–electro–thermal (OET) simulation is performed to disclose the internal energy conversion and heat mechanisms within devices. Taking a low‐bandgap PSC as an example, the microscopic energy conversion processes concerning the contributions from thermalization, Joule, Peltier, and bulk/interface recombination heats are quantitatively identified. Then various thermal manipulation strategies are proposed, including external (cooling effect) and internal (transport layer materials, photoluminescence colorants, and tandem strategy) methods with the purposes of reducing the heat generation and device temperature. Through the joint OET optimization, the predicted temperature of the considered single‐junction (tandem) PSC is reduced to 44.3 °C (33.5 °C) with the possible PCE up to 22.35% (29.08%). Based on the simulation, a tandem PSC (under two‐terminal configuration) is fabricated and a PCE of 25.03% is realized. This study offers an effective approach for energy analysis and manipulation to realize higher‐performance PSCs with lower operation temperatures.
The hysteresis effect is a critical factor affecting the widespread application of perovskite solar cells (PSCs). To eliminate this adverse effect, it is necessary to uncover the underlying physics, which characterize the microscopic behaviors of electrons, holes, and ions within PSCs. Herein, addressing the hysteresis effect of PSCs, the migration mechanisms of mobile ions (i.e., anions and cations) within the perovskite layer is explored, the simulation model is developed, and the corresponding experiments are performed. The electromagnetic response, the transport of electrons, holes, anions, and cations, and the electrostatic characteristics determined by the charges are considered in detail. The simulation verifies that the performance degradation is indeed originating from the mobile ions, especially under a high ion concentration. The physical reason of the unbalanced performance under forward and reverse electric scans is presented by optoelectronic simulation. The manipulation of the hysteresis effect increasing the built‐in electric field and reducing the hysteresis index (HI) of low ion concentration devices, but increased HI under a high ion concentration is further investigated. The simulation guides the fabrication of a normal‐bandgap PSC, which achieves the reverse (forward) power‐conversion efficiency up to 23.35% (22.22%) with a HI as low as 4.8%.
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