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The importance of efficient carrier selective transport at the backside contact significantly increases with thickness reduction of c-Si solar cells. Here, MoOx backside hole-transporting layer is fabricated on TiOx electron-transporting layer-based ultrathin c-Si solar cell with a final configuration of Ag/ITO/Mg/TiOx/45 μm p-type c-Si/MoOx/Ag by reactive magnetron sputtering method at room temperature. The effects of oxygen ratio and sputtering power on the film phase, bandgap, and surface roughness are investigated. Moreover, the contact performance between Ag and p-type c-Si is systematically studied and optimized by MoOx insertion. Based on the optimized MoOx thin film, the obtained totally dopant-free cell shows an enhancement of all cell parameters with a resultant high efficiency of 12.81%, which is about 12.8% relatively higher than that of conventional backside p+-based one (11.36%). In the combination of experiment and simulation processes, better performance of MoOx-based cell can be ascribed to the improvement of both electrical and optical performances of the device. The realization of MoOx-based contact at room temperature enables the solar cell fabrication under planar state possible, which can greatly avoid the bowing effect and reduce the yield losses and energy consumption during the fabrication of ultrathin c-Si solar cells.

NaY(WO4)2 phosphors with different Er3+ doping and Er3+/Li+ co-doping concentrations were prepared via high temperature solid state reaction. X-ray diffraction measurement demonstrated the presence of tetragonal-phase NaY(WO4)2, and no impurity phase was found in Er3+/Li+ co-doped samples. Well-crystallized NaY(WO4)2 phosphors showed a fine morphology with particle sizes of 1–6 μm determined by scanning electron microscope. Under excitation at 980 nm, the origins of three emission peaks located at 527 nm (green emission 1), 549 nm (green emission 2) and 665 nm (red emission), respectively, were identified. By introducing Li+, the up-converted (UC) emission intensity was enhanced by 0.5 times and 2 times in green and red emission region, respectively. The enhanced luminescence was attributed to the distortion of the local symmetry around Er3+ due to Li+ incorporation in the lattices. The 2H11/2 → 4I15/2 (green emission 1) and 4S3/2 → 4I15/2 (green emission 2) transitions of the Er3+ ion presented a temperature dependent behavior from 300 to 30 K and were proposed for temperature sensing (optical thermometry) by using the fluorescence intensity ratio (FIR) method. The FIR data obtained by experiment was well fitted with a theoretical function. A higher maximum value of sensitivity (0.0061 K-1) was obtained in 5% Er3+, 0% Li+ doped sample at 300 K. However, due to the distortion of local crystal field around Er3+ ions caused by the introduction of Li+, the sensitivity value of 5% Er3+ and 1% Li+ co-doped sample was higher than that of Li+ free sample below 270 K, which indicated that appropriate Li+ co-doping could optimize the temperature sensing behaviors of Er3+ doped NaY(WO4)2 phosphors.

Realizing flexible Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cell with high efficiency and excellent mechanical durability has always been a challenging task in green demanded era. In this work, Cu2CdxZn1-xSn(S,Se)4 (CCZTSSe) absorber was synthesized by selenizing Cd2+ treated CZTS precursor. Due to the powerful ability of Cd2+ soaking in boosting grain growth, the grain size of CCZTSSe was remarkably increased from nano to micron level. Cd atom actually act entering into the lattice and replaces Zn location in the post-selenization process. The band gap of CCZTSSe exhibits a decreasing trend (1.37 eV–1.29 eV) as x increases from 0 to 0.13. In comparison to CZTSSe absorber without Cd2+ soaking treatment, Cu2CdxZn1-xSn(S,Se)4 (x = 0.03, 0.07 and 0.13) can establish the heterojunction with better matching effect with CdS buffer layer, demonstrating an enhanced conduction band offset (CBO). The improved heterojunction characteristic substantially prevents carrier recombination caused by interface state defects, which can be the crucial reason of the enhanced device performance. Finally, for x = 0.07, the device efficiency was optimized to 4.38%. This systematic work offers a novel solution for vital problems encountered in flexible CZTSSe thin film solar cells, which can lead to prospective applications in wearable devices.

The fluorescence-based temperature sensor using temperature-dependent fluorescence properties is a great candidate to replace traditional temperature-measurement detector like thermocouple, because the latter is inaccessible to conquer some special circumstances such as a corrosive environment. In this study, the nano-sized Er3+ doped NaY(WO4)2 phosphors were synthesized by solvothermal method. The origins of three emission peaks located at 527 nm, 549 nm and 665 nm were identified. The 2H11/2 → 4I15/2 (green emission 1) and 4S3/2 → 4I15/2 (green emission 2) transitions of the Er3+ ion possessed a temperature dependent behavior over the range of 300–30 K and it was proposed for temperature sensing (optical thermometry) using the fluorescence intensity ratio (FIR) method. The FIR value of nano-sized phosphors decreased with the decrease of temperature, and the experimental data were well fitted with a theoretical function. A large energy difference (1048.5) was obtained from the fitting curve, which indicate that the sensitivity of nano-sized NaY(WO4)2: Er is higher than that of micro-sized one prepared by high temperature solid state reaction. Particularly, at temperatures below 150 K, the sensitivity of nano-sized UC phosphors was enhanced obviously. The results suggested that this nano-sized phosphor could be a splendid option for next generation luminescence-based temperature sensing devices in cryogenic region.

Since challenges still exist in size control fabrication of inverted pyramids (IPs) on c-Si substrate, size difference of IPs among reported literatures still can not be explained reasonably. Here, formation mechanism of IPs from sub-micro scale to micro scale for light trapping on c-Si substrate is reported based on metal assisted chemical etching (MACE) temperature control for the first time. The formation of the IPs is realized through a mask-less Ag assisted wet chemical etching method followed by a post nanostructure rebuilding (NSR) process. It is found that the etching directions on (1 0 0) Si can be influenced by the MACE temperature due to the shrink of Ag nanoparticles at high MACE temperature, leaving behind few pore channels in the deepest region of black silicon layer as nucleation sites. Thus large IPs can be formed during the following NSR process. It is believed that the elucidation of the fundamental formation will speed up the fabrication of wafer-scale c-Si IPs for application in bulk and ultrathin c-Si solar cells.

Effects of sulfurization pressure on structural and electrical property of Cu2ZnSnS4 (CZTS) thin film prepared by sulfurizing co-sputtered Cu–Zn–Sn–S precursor were investigated. X-ray diffraction patterns and Raman spectra confirmed the shrinkage of lattice and compressive stress forming in low pressure sulfurized CZTS thin film, which prevented the overgrowing of grains along vertical direction. In addition, low pressure sulfurization could reduce the number of small grains in film. Thus CZTS solar cell based on low pressure sulfurized CZTS thin film obtained a 124 % enhancement in conversion efficiency. Temperature-dependent conductivity measurement revealed the mechanism of the improvement in CZTS solar cell by low pressure sulfurization, which was due to the promotion of VCu forming and the removal of localized states in defect band.

Flexible ultrathin c-Si (U-Si) solar cell is widely researched for its potential usage in places with various shapes and sizes. However, light incident angle (θ) varies with positions on the same cell accompanied with the easy crack of cell due to surface textures when the U-Si cell is under bending. Therefore, the fabrication of efficient antireflectance structures with both quasi-omnidirectional performance and superior mechanical performance is greatly required. Here, size control of inverted pyramid (IP) from nano to micro scale as antireflectance structures is realized by altering ρ ([HF]/[HF] + [H2O2]) values at room temperature. The IP is obtained through the nanostructure rebuilding (NSR) treatment of Ag assisted chemical etched nanostructures. In comprehensive consideration of optical and mechanical performances, IP-60 (ρ = 60%) textured 40 µm U-Si cell is successfully fabricated with a short-current density of 34.8 mA/cm2 and energy-conversion efficiency of 17.44%, showing an 0.2 mA/cm2 and 0.13% improvement respectively compared with that of the micro pyramids (MP) based one. Moreover, the quasi-omnidirectional characteristics of IP-60 based 40 μm cell over 0–60° is demonstrated by experiment and simulation. The above finding makes it possible to broaden the application scope of U-Si cells into some new areas like stratospheric airships and wearable electronics.

In this paper, Cu2ZnSnS4 (CZTS) nanoparticles were synthesized by applying ultrasound and microwave irradiation simultaneously. The effects of solution temperature on properties of CZTS nanoparticles were studied by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and UV–vis spectroscopy. The results showed that as-synthesized CZTS nanoparticles had a single phase and stoichiometric composition, when solution temperature was kept at 180 °C. However, when solution temperature was low, the as-prepared nanoparticles contained Cu2−xS, Cu2SnS3 and ZnS secondary phases according to the results of XRD and Raman spectroscopy. Furthermore, TEM showed that the rod-like and hexagonal plate-like nanoparticles formed at low solution temperature. The crystallinity of as-prepared nanoparticles increased with the increasing solution temperature. Moreover, the band gap of CZTS nanoparticles obtained at 180 °C was 1.58 eV, and that obtained at low temperature was large due to secondary phases.

Cu2FeSnS4 nanocrystals were rapidly fabricated by ultrasound-assisted microwave irradiation using thioacetamide as a sulfur source. The phase structure, shape and composition of Cu2FeSnS4 nanocrystals were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS), respectively. The results showed that the phase of as-obtained Cu2FeSnS4 nanocrystals was stannite and the average size was about 5 nm. UV–vis absorption spectrum showed that the band gap of Cu2FeSnS4 nanocrystals was about 1.35 eV.