The reliability of lead-free Cu bonding technology is often limited by high bonding temperature and perpetual growth of intermetallic compounds between Sn solder and Cu substrate. Here, we report a low-bonding-temperature and highly reliable Cu bonding strategy with the use of graphene as an interlayer. By integrating a nanoscale graphene/Cu composite on the Cu substrate prior to thermocompression bonding, we observe a macroscale phenomenon where reliable Sn-Cu joints can be fabricated at a bonding temperature as low as 150 °C. During the bonding process, nanoscale features are replicated in the Sn solder by the Cu nanocone array morphology. Compared to microscale Sn, nanoscale Sn is mechanically weaker and thus can distribute on the Cu substrate at a much lower temperature. Furthermore, insertion of a graphene interlayer, which is one atom thick, can successfully retard the intermetallic compounds' growth and preserve a high bonding yield, following 96 h of aging, as confirmed through SEM and shear strength analyses. Our graphene-based Cu bonding strategy demonstrated in this work is highly reliable, cost-effective, and environmentally friendly, representing a much closer step toward industrial applications.

Silicon wafer with a nanostructured porous layer on top surface was obtained with Cu(NO 3 ) 2 -HF-H 2 O 2 aqueous solution treatment in a low Cu 2+ concentration region (0.001 M-0.02 M). The influences of different recipes concentrations on silicon surface morphology and the average etching rate were investigated. Craters and pores structures are successfully formed on the silicon surface layer. No copper particles are observed from the SEM images of as-prepared silicon surface with pores. The mechanism of forming nanostructured porous layer on silicon surface without metal nanoparticles was discussed. The morphology evolution of Si surface and the transition from craters to pores in the low Cu 2+ concentration region were investigated.

The unique one-dimensional crystal structure and low-temperature growth techniques make antimony selenide (Sb2Se3) a promising potential material for flexible and lightweight photovoltaic applications. The buried Sb2Se3/molybdenum back-contact interface is the main obstacle to high-efficiency flexible Sb2Se3 solar cells in a substrate configuration. To improve the crystalline quality of Sb2Se3 and enhance hole extraction, we introduce a new lead selenide (PbSe) transition layer, fabricated at room temperature, at the back-contact interface. The concomitant incorporation of tiny amounts of Pb into the Sb2Se3 readily reduces the formation of undesired deep-level traps. The champion device on a flexible polyimide (PI) foil yields a power-conversion-efficiency of 8.43%, which is a record efficiency in flexible Sb2Se3 photovoltaics. This work highlights the synergistic effect of the PbSe interlayer at the buried back-contact interface and its effect on the bulk absorber. This method provides a complete low-temperature vacuum-vapor-fabrication process for high-efficiency flexible Sb2Se3 solar cells in the substrate configuration.

This work reports upon the dilution effect of Ar + H2 on the microstructures, optical, and photovoltaic properties of the hydrogenated nanocrystalline silicon (nc-Si:H) thin films. High crystallinity (up to 82.6%) nc-Si:H thin films were fabricated from silane diluted by Ar + H2 in a low-frequency inductively coupled plasma (LFICP) facility at a low temperature of 300 °C. The substitution of H2 by Ar in the diluent gas leads to an increase of the deposition rate, grain size, and crystallinity, and a decrease of the optical bandgap. Varying the Ar content caused a fluctuation of the H concentration and a change of the preferential orientation from (111) to (220) in the synthesized thin films. These effects physically originated from changes of the Ar + H2 + SiH4 plasma environment in the LFICP system. The enhancement of the dissociation of SiH4/H2 molecules by ion Ar+ and the metastable state Ar* were discussed in terms of related chemical reactions between the diluent gases and silane. Furthermore, it was found that a heterojunction solar cell prototype based on the as-deposited nc-Si:H thin films exhibits an excellent photovoltaic response.

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Low-frequency inductively coupled plasma (ICP) has been widely used to deposit amorphous or microcrystalline Si thin films, but the intrinsic drawback namely ion bombardment effect limits its application in Si heterojunction solar cells. In this letter, we redesigned typical ICP and realized a remote plasma deposition with suppressed ion bombardment effect. This remote ICP system enables the synthesis of high quality amorphous Si layers with a compact network and a high hydrogen content (10.5%). By using this remote ICP system, we achieved amorphous/crystalline silicon heterojunction solar cells with an efficiency of 14.1% without any back surface field or textures.

Amorphous and microcrystal hydrogenated intrinsic silicon (a-Si:H/μc-Si:H) thin films with good silicon surface passivation effect were deposited using a precursor gases of silane and hydrogen, which were discharged by low frequency inductively coupled high density plasma source. With regard to silicon surface passivation, the effect of discharge power on thin films properties, including the optical band gap, the crystal fraction, and bond configuration, as well as the deposition rate were thoroughly investigated. It was found that the best passivation effect was obtained at the region near the transition regime from a-Si:H to μc-Si:H with a minimized incubation layer between the passivation layer and substrate. Cz-silicon wafer passivated by as-deposited μc-Si:H thin films without any post-deposition thermal annealing possesses minority carrier lifetime of about 234 μs. This is attributed to the chemical annealing from the high-density hydrogen plasma during the deposition process. Subsequent thermal annealing in hydrogen flow increased the lifetime to 524 μs with a suppressed maximum surface recombination velocity of as low as 60 cm/s. Throughout the process flow covering the pre-deposition H plasma treatment, the film deposition from H2 diluted feedstock gases and the post-deposition annealing, hydrogen plays a vital role to enhance the minority carrier lifetime by improving the interface properties. The injection level dependent surface recombination velocity was also extracted from the lifetime measurement. The effectivity of the a-Si:H/μc-Si:H for silicon surface passivation in a practical heterojunction solar cell was further validated by the excellent photovoltaic performance.

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