Phase-Field Study of Polycrystalline Growth and Texture Selection During Melt Pool Solidification

Grain growth competition during solidification determines microstructural features, such as dendritic arm spacings, segregation pattern, and grain texture, which have a key impact on the final mechanical properties. During metal additive manufacturing (AM), these features are highly sensitive to manufacturing conditions, such as laser power and scanning speed. The melt pool (MP) geometry is also expected to have a strong influence on microstructure selection. Here, taking advantage of a computationally efficient multi-GPU implementation of a quantitative phase-field model, we use two-dimensional cross-section simulations of a shrinking MP during metal AM, at the scale of the full MP, in order to explore the resulting mechanisms of grain growth competition and texture selection. We explore MPs of different aspect ratios, different initial (substrate) grain densities, and repeat each simulation several times with different random grain distributions and orientations along the fusion line in order to obtain a statistically relevant picture of grain texture selection mechanisms. Our results show a transition from a weak to a strong $\langle10\rangle$ texture when the aspect ratio of the melt pool deviates from unity. This is attributed to the shape and directions of thermal gradients during solidification, and seems more pronounced in the case of wide melt pools than in the case of a deeper one. The texture transition was not found to notably depend upon the initial grain density along the fusion line from which the melt pool solidifies epitaxially

A graphene/ZnO electron transfer layer together with perovskite passivation enables highly efficient and stable perovskite solar cells

Interface engineering in organometal halide perovskite solar cells (PSCs) has been an efficient tool to boost the performance and stability of photovoltaic (PV) devices. It is known that zinc oxide (ZnO) is one of the promising electron transporting layers for solar cells and is also applicable for flexible devices. However, the utilization of ZnO in PSCs is restricted due to its reactivity with the perovskite film during the annealing process. Here, we demonst rate improved photovoltaic performance and stability by introducing monolayer graphene (MLG) at the interface of the ZnO ETL and perovskite absorber, which results in a stable electric to power conversion efficiency (PCE) of 19.81%. The device based on this modified ETL maintains more than 80% of its initial PCE value after 300 h under continuous illumination. Interestingly, we find that the presence of MLGattheETL/perovskite interface not only improves the carrier extraction and photovoltaic properties but also protects the perovskite film from decomposition at elevated temperatures, which is beneficial for the stability of the device.To improve the stability even further, we have passivated the surface of the perovskite film by using a new modulator, i.e. ,3-(penta fl uorophenyl)-propionamide (PFPA) to abate the surface trap states of the perovskite. Based on our modification with MLG and PFPA, a stable PSC device with a PCE of 21% was achieved under AM 1.5G illumination with negligible hysteresis. The stability result indicates that the passivated device on MLG/ZnO maintains 93% of its initial PCE value after 300 h under continuous illumination.MIT Energy Initiative. Solar Frontier Cente

Interface Engineering of Perovskite Solar Cell Using a Reduced-Graphene Scaffold

Interface engineering of solar cell device is a prominent strategy to improve the device performance. Herein, we synthesize reduced-graphene scaffold (rGS) by using a new and simple chemical approach. In this regard, we synthesize 4 hollow structure of graphene and then fabricate a three-dimensional scaffold of graphene with a superior surface area using electrophoretic process. We employ this, scaffold as an interface layer between the electron transfer and absorber: layers in perovskite solar cell: The characterization tests and photovoltaic results show that rGS improves the carrier transportation, yielding a 27% improvement in device performance as compared to conventional device. Finally, a power conversion efficiency Of 17.2% is achieved :for the device based on the graphene scaffold. Besides, rGS amends the stability and hysteresis effect of the perovskite solar cell

Ambient Stable and Efficient Monolithic Tandem Perovskite/PbS Quantum Dots Solar Cells via Surface Passivation and Light Management Strategies

Here, highly efficient and stable monolithic (2-terminal (2T)) perovskite/PbS quantum dots (QDs) tandem solar cells are reported, where the perovskite solar cell (PSC) acts as the front cell and the PbS QDs device with a narrow bandgap acts as the back cell. Specifically, ZnO nanowires (NWs) passivated by SnO2 are employed as an electron transporting layer for PSC front cell, leading to a single cell PSC with maximum power conversion efficiency (PCE) of 22.15%, which is the most efficient NWs-based PSCs in the literature. By surface passivation of PbS QDs by CdCl2, QD devices with an improved open-circuit voltage and a PCE of 8.46% (bandgap of QDs: 0.92 eV) are achieved. After proper optimization, 2T and 4T tandem devices with stabilized PCEs of 17.1% and 21.1% are achieved, respectively, where the 2T tandem device shows the highest efficiency reported in the literature for this design. Interestingly, the 2T tandem cell shows excellent operational stability over 500 h under continuous illumination with only 6% PCE loss. More importantly, this device without any packaging depicts impressive ambient stability (almost no change) after 70 days in an environment with controlled 65% relative humidity, thanks to the superior air stability of the PbS QDs