Chalcogenide-based solar cells provide a critical pathway to cost parity between photovoltaic (PV) and conventional energy sources. Currently, only Cu(In,Ga)(S,Se) 2 (CIGS) and CdTe technologies have reached commercial module production with stable power conversion efficiencies of over 9 percent. [1,2] Despite the promise of these technologies, restrictions on heavy metal usage for Cd and limitations in supply for In and Te are projected to restrict the production capacity of the existing chalcogen-based technologies to <100 GWp per year, a small fraction of our growing energy needs, which are expected to double to 27 TW by 2050.[3-5] Earth-abundant copper-zinc-tin-chalcogenide kesterites, Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 , have been examined as potential alternatives for the two leading technologies, [6][7][8][9] reaching promising but not yet marketable efficiencies of 6.7% and 3.2%, respectively, by multilayer vacuum deposition. [7,8] Here we show a non-vacuum, slurry-based coating method that combines advantages of both solution processing [10][11][12][13] and particlebased deposition, [14][15][16][17] enabling fabrication of Cu 2 ZnSn(Se,S) 4 devices with over 9.6% efficiency-a factor of five performance improvement relative to previous attempts to use highthroughput ink-based approaches [16][17][18] and >40% higher than previous record devices prepared using vacuum-based methods. [7]

High‐performance Cu2ZnSnS4 photovoltaic devices are demonstrated using electrodeposition of metal stacks and annealing of a CuZnSn precursor in a sulfur atmosphere. A champion electroplated Cu2ZnSnS4 solar cell achieves a power conversion efficiency of 7.3%, which is a record efficiency for electrodeposited Cu2ZnSnS4 solar devices. The device performance points to electrodeposition and annealing as a low‐cost and viable approach to earth‐abundant solar cell fabrication.

There are numerous studies on the growth of planar films on sp 2 -bonded two-dimensional (2D) layered materials. However, it has been challenging to grow single-crystalline films on 2D materials due to the extremely low surface energy. Recently, buffer-assisted growth of crystalline films on 2D layered materials has been introduced, but the crystalline quality is not comparable with the films grown on sp 3 -bonded three-dimensional materials. Here we demonstrate direct van der Waals epitaxy of high-quality single-crystalline GaN films on epitaxial graphene with low defectivity and surface roughness comparable with that grown on conventional SiC or sapphire substrates. The GaN film is released and transferred onto arbitrary substrates. The post-released graphene/SiC substrate is reused for multiple growth and transfer cycles of GaN films. We demonstrate fully functional blue light-emitting diodes (LEDs) by growing LED stacks on reused graphene/SiC substrates followed by transfer onto plastic tapes.

We have carried out detailed microstructural studies of phase separation and grain boundary composition in Cu2ZnSnS4 based solar cells. The absorber layer was fabricated by thermal evaporation followed by post high temperature annealing on hot plate. We show that inter-reactions between the bottom molybdenum and the Cu2ZnSnS4, besides triggering the formation of interfacial MoSx, results in the out-diffusion of Cu from the Cu2ZnSnS4 layer. Phase separation of Cu2ZnSnS4 into ZnS and a Cu–Sn–S compound is observed at the molybdenum-Cu2ZnSnS4 interface, perhaps as a result of the compositional out-diffusion. Additionally, grain boundaries within the thermally evaporated absorber layer are found to be either Cu-rich or at the expected bulk composition. Such interfacial compound formation and grain boundary chemistry likely contributes to the lower than expected open circuit voltages observed for the Cu2ZnSnS4 devices.

optical absorption as a direct band-gap semiconductor [ 3 ] , 2) electrically benign grain boundaries due in part to large atomic relaxation, [ 4 ] and 3) reduced usage of the toxic element cadmium (i.e., compared with CdTe solar cells), [ 5 ] is an attractive material candidate for the absorber layer in PV devices. Record laboratory-scale power conversion effi ciency is above 20% for CIGS PV devices, [ 6 ] the highest among all thin-fi lm type solar cells. Although CIGS fi lms in record devices are deposited by a vacuum-based co-evaporation process, non-vacuum-based deposition techniques, such as chalcogenide-hydrazine complex precursors, [7][8][9] oxide/chalcogenide nanoparticle precursors, [10][11][12][13][14] and electrodeposition, [ 15 ] are attracting increasing attention due to their potential to achieve lower cost and larger volume manufacturing, with advantages including 1) lower equipment capital costs, 2) higher throughput, 3) more convenient scale-up potential for large area processing, and 4) higher materials utilization rate, compared with vacuum based deposition techniques. [ 16 , 17 ] Despite the great progress made on optimizing the deposition process and PV performance of CIGS devices, the development of this material and associated PV technology has been accomplished mainly on an empirical basis, where improvements have been found by trial and error. Knowledge about defects in CIGS, especially their correlation with process conditions and material properties as well as their infl uences on device performance, is still incomplete. [ 18 , 19 ] Controversies have surrounded the interpretation of defect levels observed in admittance spectroscopy and phenomena such as the partial saturation of the device forward current (rollover) and crossing of dark and illuminated current density-voltage ( J -V ) curves (crossover) observed in currentvoltage ( I -V ) characteristics of CIGS solar cells, whose origins have been attributed to Fermi-level pining at the interface, [ 20 , 21 ] bulk defects in CIGS, [ 22 , 23 ] non-Ohmic contact between CIGS and Mo, [24][25][26] deep acceptor defects in the CdS buffer layer, [27][28][29][30] and the existence of a defective chalcopyrite layer, sometimes referred to as an order defect compound (ODC) phase, close to the heterojunction. [31][32][33][34][35][36][37][38] Understanding defects in Cu(In,Ga)(Se,S) 2 (CIGS), especially correlating changes in the fi lm formation process with differences in material properties, photovoltaic (PV) device performance, and defect levels extracted from admittance spectroscopy, is a critical but challenging undertaking due to the complex nature of this polycrystalline compound semiconductor. Here we present a systematic comparative study wherein varying defect density levels in CIGS fi lms were intentionally induced by growing CIGS grains using different selenium activity levels. Material characterization results by techniques including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, secondary ...