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The strong spin–orbit coupling (SOC) in lead halide perovskites, when inversion symmetry is lifted, has provided opportunities for investigating the Rashba effect in these systems. Moreover, the strong orbital moment, which, in turn, impacts the spin-pair in singlet and triplet electronic states, plays a significant role in enhancing the optoelectronic properties in the presence of external magnetic fields in lead halide perovskites. Here, we investigate the effect of weak magnetic fields (<1 T) on the photoluminescence (PL) properties of CsPbBr 3 nanocrystals with and without Ruddlesden–Popper (RP) faults and single crystals of CH 3 NH 3 PbBr 3. Along with an enhancement in the PL intensity as a function of an external magnetic field, which is observed in both lead bromide perovskites, the PL emission red-shifts in CsPbBr 3 nanocrystals. Density-functional theory calculations of the electronic band-edge in CsPbBr 3 show almost no change in the energy gap as a function of the external magnetic field. The experimental results, thus, suggest the role of mixing of the triplet and singlet excitonic states under weak magnetic fields. This is further deduced from an enhancement in PL lifetimes as a function of the field in CsPbBr 3. In CH 3 NH 3 PbBr 3 , an increase in PL intensity is observed under weak magnetic fields; however, no changes in the peak energy or PL lifetimes are observed. The internal magnetic fields due to SOC are characterized for all three samples and found to be the highest for CsPbBr 3 nanocrystals with RP faults. [ABSTRACT FROM AUTHOR]

Abstract: A series of narrow bandgap electron acceptors is designed and synthesized for efficient near‐infrared (NIR) organic solar cells. Extending π‐conjugation of donor frameworks leads to an intense intramolecular charge transfer, resulting in broad absorption profiles with band edge reaching 950 nm. When blended with an electron donor polymer PTB7‐Th, IOTIC‐2F exhibits efficient charge transfer even with a small energetic offset, so as to achieve a large photogenerated current over 22 mA cm−2 with small energy losses (≈0.49 eV) in solar cell devices. With an intense NIR absorbance, PTB7‐Th:IOTIC‐2F‐based cells achieve a power conversion efficiency of 12.1% with good visible transparency (52% transmittance from 370 to 740 nm). Analysis of film morphology reveals that processing with solvent additives enhances crystalline features of acceptor components, while keeping an appropriate level of donor:acceptor intermixing in the binary blends. The incorporation of the third component, ITIC‐2F, into the PTB7‐Th:IOTIC‐2F blends increases the device efficiency up to 12.9%. The improvement is assigned to the cascaded energy‐level structure and desirable nanoscale phase separation of the ternary blends, which is beneficial to the photocurrent generation. This work provides an efficient molecular design strategy to optimize nonfullerene acceptor properties for efficient NIR organic photovoltaics. [ABSTRACT FROM AUTHOR]

Abstract: Two narrow bandgap non‐fullerene acceptors (NBG‐NFAs), namely, COTIC‐4F and SiOTIC‐4F, are designed and synthesized for the fabrication of efficient near‐infrared organic solar cells (OSCs). The chemical structures of the NBG‐NFAs contain a D′‐D‐D′ electron‐rich internal core based on a cyclopentadithiophene (or dithienosilole) (D) and alkoxythienyl (D′) core, end‐capped with the highly electron‐deficient unit 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile (A), ultimately providing a A‐D′‐D‐D′‐A molecular configuration that enhances the intramolecular charge transfer characteristics of the excited states. One can thereby reduce the optical bandgap (Egopt) to as low as ≈1.10 eV, one of the smallest values for NFAs reported to date. In bulk‐heterojunction (BHJ) OSCs, NBG‐NFA blends with the polymer donor PTB7‐Th yield power conversion efficiencies (PCE) of up to 9.0%, which is particularly high when compared against a range of NBG BHJ blends. Most significantly, it is found that, despite the small energy loss (Egopt − eVOC) of 0.52 eV, the PTB7‐Th/NBG‐NFA bulk heterojunction blends can yield short‐circuit current densities of up to 22.8 mA cm−2, suggesting that the design and application of NBG‐NFA materials have substantial potential to further improve the PCE of OSCs. [ABSTRACT FROM AUTHOR]