Hole transporting materials (HTMs) play a crucial role in achieving highly efficient and stable perovskite solar cells (PSCs). Spirotyped materials being the most widely used HTMs are commonly utilized with dopants, such as Li-TFSI, to improve their carrier mobility significantly. However, dopants could affect the morphology of hole transporting layer negatively by forming defects and pinholes which restrict the performance of devices. Here, we adopt the extended πconjugated structures N-ethylcarbazole and dibenzothiophene to substitute the donor group 4-methoxyphenyl of spiro-OMeTAD, devising two novel HTMs, SC and ST, respectively. Notably, SC possesses low crystallinity and good solubility due to the existence of ethyl in side groups, leading to decent miscibility with Li-TFSI to prevent unfavorable phase-separation. The SC-based device delivers the best power conversion efficiency (PCE) of 21.76% which is higher than that of spiro-OMeTAD (20.73%), attributed to the formation of smooth and pinhole-free morphology. Moreover, it exhibits long-term stability and retains over 90% of initial PCE value for more than 30 days without encapsulation in ambient air. In contrast, the STbased device suffers from dense pinholes induced by its relatively high crystallinity and poor solubility, resulting in a low PCE of 18.18% and inferior stability. Thus, it is effective to modify the side groups in spiro-typed HTMs with specific structures to obtain predictable properties, fabricating PSCs with high efficiency and stability facilely.

Entropy stabilization is a novel materials-design paradigm to realize new compounds with widely tunable properties. However, almost all entropystabilized materials so far are either conducting metals or insulating ceramics, with a clear dearth in the semiconducting regime. Here, a new class of the multicationic and -anionic entropy-stabilized chalcogenide alloys based on the (Ge,Sn,Pb)(S,Se,Te) formula are synthesized and characterized experimentally. The configurational entropy from the disorder of both the anion and the cation sublattices reaches a record value of ∼2.2 R mol −1 for the equimolar composition and stabilizes the singlephase solid solution. Theoretical calculations and experiments both show that the synthesized alloys are thermodynamically stable at the growth temperature and kinetically metastable at room temperature, segregating by spinodal decomposition at moderate temperatures. Doping and electronic transport measurements verify that the synthesized materials are ambipolarly dopable semiconductors, which pave the way for the wider adoption of entropy-stabilized chalcogenide alloys in functional applications.

The two-dimensional (2D)/three-dimensional (3D) heterojunction perovskite solar cell (PSC) has recently been recognized as a promising photovoltaic structure for achieving high efficiency and long-term stability. Rational design of the 2D spacer cation is important to achieve a win–win situation for defects’ passivation and photogenerated carrier extraction. Herein, we carry out first-principles calculation to analyze the dipole moment of phenethylamine-type molecules and their resulting 2D/3D perovskites. Based on the results of theoretical calculation, the dipole moment of 2D cations can be well tuned by varying the number of fluorine atoms on the para-position of the benzene ring, which further determines the interfacial dipole across the 2D/3D heterojunction interface. A high dipole 2D perovskite layer at the interface between the 3D perovskite and hole-transporting material is found to promote charge transport and suppress charge trapping efficiently. As a result, our 2D/3D PSCs exhibit a champion power conversion efficiency over 22% and a fill factor over 83%. Moreover, our solar cells also show a remarkable stability, maintaining 80% of its initial efficiency for more than 1400 h without encapsulation under a 30 ± 5% relative humidity.

The stacking of 2D perovskites on the top of 3D perovskites has been recognized as a promising interfacial treatment approach to improve the stability and efficiency of planar perovskite solar...

We present evidence, from theory and experiment, that ZnSnN 2 and MgSnN 2 can be used to match the band gap of InGaN without alloying-by exploiting cation disorder in a controlled fashion. We base this on the determination of S, the long-range order parameter of the cation sublattice, for a series of epitaxial thin films of ZnSnN 2 and MgSnN 2 using three different techniques: x-ray diffraction, Raman spectroscopy, and in situ electron diffraction. We observe a linear relationship between S 2 and the optical band gap of both ZnSnN 2 (1.12-1.98 eV) and MgSnN 2 (1.87-3.43 eV). The results clearly demonstrate the correlation between controlled heterovalent cation ordering and the optical band gap, which applies to a broad group of emerging ternary heterovalent compounds and has implications for similar trends in other material properties besides the band gap.

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We present experimental results confirming extreme quantum confinement in GaN/Al x Ga1–x N (x = 0.65 and 1.0) nanowire and planar heterostructures, where the GaN layer thickness is of the order of a monolayer. The results were obtained from temperature- and excitation-dependent and time-resolved photoluminescence measurements. In the GaN/AlN nanowire heterostructure array sample, the measured emission peak at 300 K is ∼5.18–5.28 eV. This is in excellent agreement with the calculated optical gap of 5.23 eV and 160–260 meV below the calculated electronic gap of 5.44 eV, suggesting that the observed emission is excitonic in nature with an exciton binding energy of ∼160–260 meV. Similarly, in the monolayer GaN/Al0.65Ga0.35N planar heterostructure, the measured emission peak at 300 K is 4.785 eV and in good agreement with the calculated optical gap of 4.68 eV and 95 meV below the calculated electronic gap of 4.88 eV. The estimated exciton binding energy is 95 meV and in close agreement with our theoretical calculations. Excitation-dependent and time-resolved photoluminescence data support the presence of excitonic transitions. Our results indicate that deep-ultraviolet excitonic light sources and microcavity devices can be realized with heterostructures incorporating monolayer-thick GaN.

Ultrawide-band-gap (UWBG) semiconductors are promising for fast, compact, and energyefficient power-electronics devices. Their wider band gaps result in higher breakdown electric fields that enable high-power switching with a lower energy loss. Yet, the leading UWBG semiconductors suffer from intrinsic materials limitations with regards to their doping asymmetry that impedes their adoption in CMOS technology. Improvements in the ambipolar doping of UWBG materials will enable a wider range of applications in power electronics as well as deep-UV optoelectronics. These advances can be accomplished through theoretical insights on the limitations of current UWBG materials coupled with the computational prediction and 2 experimental demonstration of alternative UWBG semiconductor materials with improved doping and transport properties. As an example, we discuss the case of rutile GeO2 (r-GeO2), a waterinsoluble GeO2 polytype which is theoretically predicted to combine an ultra-wide gap with ambipolar dopability, high carrier mobilities, and a higher thermal conductivity than β-Ga2O3. The subsequent realization of single-crystalline r-GeO2 thin films by molecular beam epitaxy provides the opportunity to realize r-GeO2 for electronic applications. Future efforts towards the predictive discovery and design of new UWBG semiconductors include advances in first-principles theory and high-performance computing software, as well as the demonstration of controlled doping in high-quality thin films with lower dislocation densities and optimized film properties.