Wookjin Choi

The excellent cryogenic tensile properties of the CrMnFeCoNi alloy are generally caused by deformation twinning, which is difficult to achieve at room temperature because of insufficient stress for twinning. Here, we induced twinning at room temperature to improve the cryogenic tensile properties of the CrMnFeCoNi alloy. Considering grain size effects on the critical stress for twinning, twins were readily formed in the coarse microstructure by cold rolling without grain refinement by hot rolling. These twins were retained by partial recrystallization and played an important role in improving strength, allowing yield strengths approaching 1 GPa. The persistent elongation up to 46% as well as the tensile strength of 1.3 GPa are attributed to additional twinning in both recrystallized and non-recrystallization regions. Our results demonstrate that non-recrystallized grains, which are generally avoided in conventional alloys because of their deleterious effect on ductility, can be useful in achieving high-strength high-entropy alloys.

We introduce a novel transformation-induced plasticity mechanism, i.e., a martensitic transformation from fcc phase to bcc phase, in medium-entropy alloys (MEAs). A VCrFeCoNi MEA system is designed by thermodynamic calculations in consideration of phase stability between bcc and fcc phases. The resultantly formed bcc martensite favorably contributes to the transformation-induced plasticity, thereby leading to a significant enhancement in both strength and ductility as well as strain hardening. We reveal the microstructural evolutions according to the Co-Ni balance and their contributions to a mechanical response. The Co-Ni balance plays a leading role in phase stability and consequently tunes the cryogenic-temperature strength-ductility balance. The main difference from recently-reported metastable high-entropy dual-phase alloys is the formation of bcc martensite as a daughter phase, which shows significant effects on strain hardening. The hcp phase in the present MEA mostly acts as a nucleation site for the bcc martensite. Our findings demonstrate that the fcc to bcc transformation can be an attractive route to a new MEA design strategy for improving cryogenic strength-ductility.

Abstract Low dimensional (LD) perovskite materials generally exhibit superior chemical stability against ambient moisture and thermal stress than that of 3D perovskites. Recently, LD perovskite has been used as a passivation layer on the surface of 3D perovskite grains. Although various LD perovskites have been developed focusing on their hydrophobicity, the impact of crystal structure of LD perovskite on the photovoltaic performance of perovskite solar cell (PSC) is still uncertain. In this work, the effects of the structural characteristics of LD perovskites on the crystal formation of formamidinium lead triiodide (α‐FAPbI 3 ) and on the optoelectrical properties of PSCs are elucidated. The phase‐transformation kinetics of FAPbI 3 mixed with LD perovskites is studied using the Johnson–Mehl–Avrami–Kolmogorov model. It is found that the arrangement of PbI 6 octahedra in the LD perovskite changes the rate of α‐FAPbI 3 formation. Facilitated nucleation of α‐FAPbI 3 at the LD/FAPbI 3 interface results in minimal structural disorder and prolonged charge‐carrier lifetimes. As a result, the PSC with the optimized LD perovskite structure exhibits a power conversion efficiency of 21.25% from a reverse current–voltage scan, and stabilized efficiency of 19.95% with excellent ambient stability without being encapsulated.

Abstract A novel photomultiplication (PM)‐type organic photodiode (OPD) that responds much faster (109 kHz bandwidth) than conventional PM‐type OPDs is demonstrated. This fast response is achieved by introducing quantum dots (QDs) as a PM‐inducing interlayer at the interface between the electrode and the photoactive layer. When the device is illuminated, the photogenerated electrons within the photoactive layer are rapidly transferred and trapped in the trap states of the QD interlayer. The electron trapping subsequently leads to charging of the QD and a consequent shift of the QD energy levels, thereby inducing hole injection from the electrode. This PM mechanism is distinct from that of conventional PM‐type OPDs, whose PM usually requires a long time to induce hole (or electron) injection because of the slow transport and accumulation of electrons (or holes) within the photoactive layer. Because of its PM mechanism, the proposed QD‐interlayer PM‐type OPD achieves high bandwidth and high specific detectivity. In addition, it is demonstrated that the response speed of the proposed device is closely related to the charge trapping/detrapping dynamics of the QDs. This work not only offers a new concept in the design of fast‐responding PM‐type OPDs but also provides comprehensive understanding of the underlying device physics.

Abstract A photomultiplication (PM)‐type organic photodetector (OPD) that exploits the ionic motion in CsPbI 3 perovskite quantum dots (QDs) is demonstrated. The device uses a QD monolayer as a PM‐inducing interlayer and a donor–acceptor bulk heterojunction (BHJ) layer as a photoactive layer. When the device is illuminated, negative ions in the CsPbI 3 QD migrate and accumulate near the interface between the QDs and the electrode; these processes induce hole injection from the electrode and yield the PM phenomenon with an external quantum efficiency (EQE) >2000% at a 3 V applied bias. It is confirmed that the ionic motion of the CsPbI 3 QDs can induce a shift in the work function of the QD/electrode interface and that the dynamics of ionic motion determines the response speed of the device. The PM OPD showed a large EQE‐bandwidth product >10 6 Hz with a −3 dB frequency of 125 kHz at 3 V, which is one of the highest response speeds reported for a PM OPD. The PM‐inducing strategy that exploits ionic motion of the interlayer is a potential approach to achieving high‐efficiency PM OPDs.