The thin-fi lm photovoltaic material Cu 2 ZnSnS x Se 4-x (CZTSSe) has drawn world-wide attention due to its outstanding performance and earth-abundant composition. Until recently, [ 1 ] stateof-the-art CZTSSe thin-fi lm solar cells were limited to 11.1% power conversion effi ciency (PCE), with these performance levels being achieved via a hydrazine slurry approach. [ 2 ] Other vacuum-and non-vacuum-based deposition techniques have also been successful in fabricating CZTSSe solar cells with PCE above 8%. [ 3,4 ] However, even record devices with PCE of 11% are still far below the physical limit, known as the ShockleyQueisser (SQ) limit, of about 31% effi ciency under terrestrial conditions. [ 5 ] For a solar cell with 1.13 eV bandgap such as the previous 11.1% champion, [ 2 ] the SQ limits for open circuit voltage ( V oc ) and short-circuit current density ( J sc ) are 820 mV and 43.4 mA cm −2 , respectively. The previous 11.1% champion only achieved a V oc of 460 mV and a J sc of 34.5 mA cm −2 , corresponding to about 56% and 79% of the SQ limit values. In order to boost J sc , an optical architecture with optimized transparent conductive oxide (TCO) and CdS thicknesses has recently been reported, leading to a new CZTSSe record PCE of 12.0% and a J sc that reaches 83% of the SQ limit. [ 1 ] Despite improvements in shortcircuit current, the V oc defi cit, equal to the difference between the bandgap and V oc , is currently the biggest hurdle preventing CZTSSe devices from achieving higher effi ciency. [ 6 ] Enhancement of V oc also directly improves device fi ll factor. [ 7 ] Although many factors can infl uence V oc in a solar cell, carrier generation and recombination near the charge-separating junction play a dominant role. Thus, in order to decrease the V oc defi cit and increase effi ciency beyond 12%, it is critical to understand junction characteristics, current collection, and recombination mechanisms in the current generation of devices.Here, an independently certifi ed world-record 12.6% PCE CZTSSe thin-fi lm solar cell is presented. The new champion device was fabricated using a recently described hydrazine pure-solution approach, targeting a Cu-poor and Zn-rich condition. [ 8 ] Secondary ion mass spectrometry (SIMS) shows that the obtained CZTSSe fi lms exhibit very low carbon and oxygen concentrations, comparable to fi lms fabricated by the more traditional hydrazine-slurry method. [ 9 ] The rheological properties of the particle-free solution, relative to the slurry process, signifi cantly improves the coating uniformity and fi lm structure and, consequently, the performance of the solar cells. [ 4,8 ] By simultaneously optimizing the TCO and CdS thicknesses to maximize photon transmission to the absorber and improving the bulk qualities of CZTSSe with the hydrazine pure-solution approach, both J sc and V oc are boosted in the 12.6% champion device. Device characteristics of the new champion cell, as deduced from current-voltage, quantum effi ciency, capacitance, and electron-beam-induced current (EBI...

Using vacuum process, we fabricated Cu 2 ZnSnS 4 solar cells with 8.4% efficiency, a number independently certified by an external, accredited laboratory. This is the highest efficiency reported for pure sulfide Cu 2 ZnSnS 4 prepared by any method. Consistent with literature, the optimal composition is Cu-poor and Zn-rich despite the precipitation of secondary phases (e.g., ZnS). Despite a very thin absorber thickness (~600 nm), a reasonably good short-circuit current was obtained. Time-resolved photoluminescence measurements suggest a minority carrier-diffusion length on the order of several hundreds of nanometers and relatively good collection of photo-carriers across the entire absorber thickness.

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Owing to its high carrier mobility and saturation velocity, graphene has attracted enormous attention in recent years. In particular, high-performance graphene transistors for radio-frequency (r.f.) applications are of great interest. Synthesis of large-scale graphene sheets of high quality and at low cost has been demonstrated using chemical vapour deposition (CVD) methods. However, very few studies have been performed on the scaling behaviour of transistors made from CVD graphene for r.f. applications, which hold great potential for commercialization. Here we report the systematic study of top-gated CVD-graphene r.f. transistors with gate lengths scaled down to 40 nm, the shortest gate length demonstrated on graphene r.f. devices. The CVD graphene was grown on copper film and transferred to a wafer of diamond-like carbon. Cut-off frequencies as high as 155 GHz have been obtained for the 40-nm transistors, and the cut-off frequency was found to scale as 1/(gate length). Furthermore, we studied graphene r.f. transistors at cryogenic temperatures. Unlike conventional semiconductor devices where low-temperature performance is hampered by carrier freeze-out effects, the r.f. performance of our graphene devices exhibits little temperature dependence down to 4.3 K, providing a much larger operation window than is available for conventional devices.

Wrinkling is a ubiquitous phenomenon in two-dimensional membranes. In particular, in the large-scale growth of graphene on metallic substrates, high densities of wrinkles are commonly observed. Despite their prevalence and potential impact on large-scale graphene electronics, relatively little is known about their structural morphology and electronic properties. Surveying the graphene landscape using atomic force microscopy, we found that wrinkles reach a certain maximum height before folding over. Calculations of the energetics explain the morphological transition and indicate that the tall ripples are collapsed into narrow standing wrinkles by van der Waals forces, analogous to large-diameter nanotubes. Quantum transport calculations show that conductance through these "collapsed wrinkle" structures is limited mainly by a density-of-states bottleneck and by interlayer tunneling across the collapsed bilayer region. Also through systematic measurements across large numbers of devices with wide "folded wrinkles", we find a distinct anisotropy in their electrical resistivity, consistent with our transport simulations. These results highlight the coupling between morphology and electronic properties, which has important practical implications for large-scale high-speed graphene electronics.