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Kesterite solar cells often suffer from low open‐circuit voltage due to interface recombination. To address this, a passivation approach is explored using a dielectric layer between the absorber and buffer layer, coupled with a point contact structure. In this method, carrier transport is enabled through point contacts while effectively passivating regions covered by oxide. In this study, although forming point contacts on the front surface poses processing challenges, a novel solution is presented. Herein, in situ formed ZnS nanoparticles (NPs) during Cu2ZnSn(S, Se)4 (CZTSSe) absorber fabrication is utilized as a nanomask for front surface point contacts. A thin TaOx layer, acting as the passivation layer, is deposited over the CZTSSe surface with ZnS NPs by using atomic layer deposition (ALD). The selective removal of these NPs by deionized water soaking creates size‐controlled point contacts without damaging the absorber surface. In this approach, the first successful implementation of TaOx in enhancing CZTSSe solar cell efficiency, which jumps from 6.17% to 9.02% due to this strategy, is marked. The passivation mechanism involves sodium attraction during ALD, reduced interface defects, and the generation of positive fixed oxide charges. In this innovative method, not only solar cell efficiency is improved but new insights are also provided into using secondary phases as nanomasks in interface engineering. [ABSTRACT FROM AUTHOR]

Antiferromagnet (AFM) has currently participated in the spin‐orbit torque (SOT) technology due to its great potential to be applied to the field‐free SOT switching and to promote the thermal stability of MRAM. However, the effect of varying AFM ordering on the SOT switching and the associated properties is still not comprehensively understood. This work reports how an AFM ordering modifies the strength of Dzyaloshinskii–Moriya‐interaction (DMI) in a heavy metal (Pt)/FM (Co)/AFM (IrMn) trilayer and its effects on SOT switching. Increasing the AFM ordering reflects the enhanced exchange bias through increasing IrMn thickness appears to significantly reduce the DMI strength of the trilayer. Controlling the IrMn thickness appears to serve as a unique switch to activate memristivity/stochasticity in the devices via tailoring AFM ordering on exchange bias: The strong AFM ordering via increasing IrMn thickness enables to increase the stability of multi‐levels for SOT switching, which promotes the memristivity for neuromorphic application. On the contrary, the weak AFM ordering via reducing IrMn thickness will lead to significant stochasticity for the physically unclonable functionality. This work demonstrates an intrinsic tuning over the AFM ordering will serve as a switch to turn the SOT device into a stochastic/memristive cell to bridge probabilistic and neuromorphic computing. [ABSTRACT FROM AUTHOR]

Post‐selenization‐fabricated (using elemental Se vapor) Cu(In,Ga)Se2 solar cell efficiency is limited by a low open‐circuit voltage, which is attributable to the Ga‐deficient surface and insufficient grain growth. In this study, a band‐grading structure is demonstrated by combining Ag‐front and Ga‐back grading in selenized (Ag,Cu)(In,Ga)Se2 (ACIGSe) absorbers with a properly designed precursor structure (Mo/CuGa/In/AgGa) and high Ag content ([Ag]/([Ag]+[Cu]) = 0.5). The phase evolution during post‐selenization reveals that the precursor structure suppresses Ag2Se formation and promotes the ACIGSe phase formed at a low temperature with enhanced grain growth. A widened surface bandgap by Ag‐front grading substantially increases the open‐circuit voltage. Furthermore, Ag addition promotes ordered vacancy compound (OVC) formation on the front surface to enlarge the valence band offset, which in turn reduces interface recombination. Furthermore, the OVC phase also assists interface passivation. Promoting surface OVC phase by Ag addition is also validated by first‐principles calculations. Furthermore, the K‐doped CuGa precursor is used for a ACIGSe absorber to address the significantly reduced carrier density by the Ag addition. With a band‐grading structure and surface OVC phase, the superior device achieves an efficiency of > 19%, the highest efficiency by post‐selenization with an elemental Se source. [ABSTRACT FROM AUTHOR]

The antiferromagnet (AFM) and ferromagnet (FM) interface is a unique branch of magnetics of broad scientific interest. AFMs play an important role in spin‐orbit torque devices based on their ability to generate spin‐polarized current and exchange bias when combined with FMs. In this study, an interesting spin‐orbit torque (SOT) ratchet involving the exchange spring effect in an IrMn/CoFeB bilayer device with perpendicular anisotropy and exchange bias is developed. The combined use of electrical and spectroscopic analysis reveals that the exchange spring in IrMn/CoFeB bilayer yields unidirectional anisotropy, resulting in a collinear/orthogonal AFM/FM spin configuration at the interface upon switching CoFeB magnetization upward/downward. The ratcheting characteristics resulting from unidirectional anisotropy manifest in SOT switching. In this process, magnetization against the exchange spring features digital‐like switching with a sharp transition, whereas the reverse function is characteristic of analog switching with a gradual transition tail. The dual digital‐analog characteristics of the IrMn/CoFeB bilayer may be of benefit in neuromorphic and memory applications. [ABSTRACT FROM AUTHOR]

Severe Ga accumulation at the back contact is regarded as one of the major limiting factors for high efficiency Cu(In,Ga)Se2 (CIGSe) solar cells fabricated by the sequential process. The Ga deficiency near the front surface of absorber leads to the reduction of open‐circuit voltage; however, controlling the Ga grading is quite challenging during the selenization process. Herein, the tuning of Ga profile is demonstrated by forming the ordered vacancy compound (OVC) phase at the beginning of selenization process under high Se partial pressure (PSe) with the precursor composed of a relatively high In content. The OVC phase formed with a columnar structure promotes Ga diffusion through Cu vacancies, resulting in Ga increase at the front side of CIGSe. However, high PSe also increases the thickness of MoSe2, significantly raising the series resistance. Thereby, a 5 nm TiN layer is deposited on top of Mo to suppress the formation of MoSe2. In addition, the extra Na‐contained precursor is added to increase the carrier concentration because the TiN layer also blocks the Na outdiffusion from the substrate. By controlling PSe, interlayer TiN thickness, and carrier concentration, a 16.04% record high efficiency of CIGSe solar cells (sulfur‐free) via elemental Se source is achieved. [ABSTRACT FROM AUTHOR]

Single‐layered MoS2 is a naturally stable material. Integrating spin, valley, and circularly polarized photons is an interesting endeavor to achieve advanced spin‐valleytronics. In this study, room‐temperature ferromagnetism in MoS2 induced by the magnetic proximity effect (MPE) of yttrium iron garnet (YIG) and the antiferromagnetic coupling at the interface is demonstrated. Insulating YIG without charge carriers is an excellent magnetic candidate featuring a long spin diffusion length and remarkable surface flatness, enabling long‐range magnetic interactions with MoS2. Spin‐resolved photoluminescence spectroscopy and magnetic circular dichroism (MCD) reveal that the spin‐polarized valleys of MoS2 can achieve sustained ferromagnetism even at room temperature. The bandgap‐sensitivity of MCD further demonstrates the extent of antiferromagnetic coupling between the MPE‐induced moments of MoS2 and YIG. This work provides a layer‐selected approach to study magnetic interactions/configurations in the YIG/MoS2 bilayer and highlights the role of MoS2 in achieving the MPE toward high temperature. [ABSTRACT FROM AUTHOR]

Direct sputtering of a single quaternary Cu(In,Ga)Se2 (CIGS) target without postselenization is a promising approach to fabricating CIGS absorbers. However, the device efficiency of the quaternary-sputtered CIGS is limited to 10%-11% due to the low and uncontrollable Se supply during the quaternary sputtering process. Here, an enhanced efficiency of 14.1% is reported by directly sputtering from a CIGS target without extra Se supply followed by sequential postdeposition treatments (PDT) of NaF and KF. The effects of different post-treatments of alkali metals on quaternary-sputtered CIGS thin films are discussed in detail. A Cu-depleted surface is not observed in the quaternary-sputtered CIGS thin films after KF-PDT, different from the observation in the coevaporated CIGS, in which the Cu-depleted surface layer induced by KF-PDT enhances the efficiency. On the other hand, it is found that KF-PDT reduces Se vacancies more effectively than NaF-PDT, which could be another electrically benign behavior of KF-PDT. The effective passivation of Se vacancies after KF-PDT overcomes the Se-poor nature of the quaternary sputtering process without postselenization. Therefore, KF-PDT combined with Na doping, which is known to annihilate InCu defects, significantly improves minority carrier lifetime and cell performance. [ABSTRACT FROM AUTHOR]

Self-assembled vertical heterostructure with a high interface-to-volume ratio offers tremendous opportunities to realize intriguing properties and advanced modulation of functionalities. Here, a heterostructure composed of two visible-light photocatalysts, BiFeO3 (BFO) and ε-Fe2O3 ( ε-FO), is designed to investigate its photoelectrochemical performance. The structural characterization of the BFO-FO heterostructures confirms the phase separation with BFO nanopillars embedded in the ε-FO matrix. The investigation of band structure of the heterojunction suggests the assistance of photoexcited carrier separation, leading to an enhanced photoelectrochemical performance. The insights into the charge separation are further revealed by means of ultrafast dynamics and electrochemical impedance spectroscopies. This work shows a delicate design of the self-assembled vertical heteroepitaxy by taking advantage of the intimate contact between two phases that can lead to a tunable charge interaction, providing a new configuration for the optimization of photoelectrochemical cell. [ABSTRACT FROM AUTHOR]

Among different process routes for Cu(In,Ga)Se2 (CIGS) solar cells, sufficient Se supply is commonly required to obtain high-quality CIGS films. However, supplying extra Se increases the cost and the complexity. In this work, we demonstrate that extra Na incorporation can substantially increase efficiency of Se-deficient CIGS solar cells, fabricated by sputtering from a quaternary CIGS target without extra Se supply, from 1.5% to 11.0%. The Se-deficient CIGS device without extra NaF reveals a roll-over I-V curve at room temperature as well as significantly reduced Jsc and fill factor at low temperatures. The electrical characteristics of Se-deficient CIGS films are well explained and modeled by the low p-type doping due to high density of compensating donors and the presence of deep defects possibly originating from the anti-bonding levels of Se vacancies. The significant improvement after extra Na incorporation is attributable to the Na-induced passivation of Se vacancies and the increased p-type doping. Our result suggests that extra Na addition can effectively compensate the Se deficiency in CIGS films, which provides a valuable tuning knob for compositional tolerance of absorbers, especially for the Se-deficient CIGS films. We believe that our findings can shine light on the development of novel CIGS processes, distinct from previous ones fabricated in Se-rich atmosphere. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]