Free-Standing Electrode and Fixed Surface Tiny Electrode Implemented Triboelectric Nanogenerator with High Instantaneous Current.

Conventional triboelectric nanogenerators (TENGs) face challenges pertaining to low output current density at low working frequencies and high internal impedance. While strategies, such as surface modification to enhance surface charge density, permittivity regulation of materials, and circuit management, have partially mitigated these issues. However, they have also resulted in increased complexity in the fabrication process. Therefore, there is an urgent demand for a universal and simplified approach to address these challenges. To fulfill this need, this work presents a free-standing electrode and fixed surface tiny electrode implemented triboelectric nanogenerator (FFI-TENG). It is fabricated by a straightforward yet effective method: introducing a tiny electrode onto the surface of the tribo-negative material. This approach yields substantial enhancements in performance, notably a more than tenfold increase in output current density, a reduction in effective working frequencies, and a decrease in matching resistance as compared to vertical contact-separation TENGs (CS-TENGs) or single-electrode TENGs (SE-TENGs). Simultaneously, a comprehensive examination and proposition regarding the operational mechanism of FFI-TENG, highlighting its extensive applicability are also offered. Significantly, FFI-TENG excels in mechanical energy harvesting even under ultra-low working frequencies (0.1 Hz), outperforming similar contact-separation models. This innovation positions it as a practical and efficient solution for the development of low-entropy energy harvesters.

Hydrogenation of silicon-nanocrystals-embedded silicon oxide passivating contacts.

We investigate the effect of hydrogen passivation of dangling bonds in silicon oxide passivating contacts with embedded silicon nanocrystals (NAnocrystalline Transport path in Ultra-thin dielectrics for REinforced passivation contact, NATURE contact). We first investigated the differences in electrical properties of the samples after hydrogen gas annealing and hydrogen plasma treatment (HPT). The results show that the NATURE contact was efficiently passivated by hydrogen after HPT owing to the introduction of hydrogen radicals into the structure. Furthermore, we examined the dependence of process parameters such as HPT temperature, duration, and H2pressure, on the electrical properties and hydrogen depth profiles. As a result, HPT at 500 °C, 15 min, and 0.5 Torr resulted in a large amount of hydrogen inside the NATURE contact and the highest implied open-circuit voltage of 724 mV. Contact resistivity and surface roughness hardly increased when HPT was performed under the optimized condition, which only improved the passivation performance without deteriorating the electron transport properties of the NATURE contact.

Fabrication of light trapping structures specialized for near-infrared light by nanoimprinting for the application to thin crystalline silicon solar cells.

Vehicle-integrated photovoltaics (VIPV) are gaining attention to realize a decarbonized society in the future, and the specifications for solar cells used in VIPV are predicated on a low cost, high efficiency, and the ability to be applied to curved surfaces. One way to meet these requirements is to make the silicon substrate thinner. However, thinner substrates result in lower near-infrared light absorption and lower efficiency. To increase light absorption, light trapping structures (LTSs) can be implemented. However, conventional alkali etched pyramid textures are not specialized for near-infrared light and are insufficient to improve near-infrared light absorption. Therefore, in this study, as an alternative to alkaline etching, we employed a nanoimprinting method that can easily fabricate submicron-sized LTSs on solar cells over a large area. In addition, as a master mold fabrication method with submicron-sized patterns, silica colloidal lithography was adopted. As a result, by controlling silica coverage, diameter of silica particles (D), and etching time (tet), the density, height, and size of LTSs could be controlled. At the silica coverage of 40%, D = 800 nm, and tet = 5 min, the reduction of reflectance below 65% at 1100 nm and the theoretical short-circuit current gain of 1.55 mA/cm2 was achieved.

Bayesian optimization of hydrogen plasma treatment in silicon quantum dot multilayer and application to solar cells.

Silicon quantum dot multilayer (Si-QDML) is a promising material for a light absorber of all silicon tandem solar cells due to tunable bandgap energy in a wide range depending on the silicon quantum dot (Si-QD) size, which is possible to overcome the Shockley-Queisser limit. Since solar cell performance is degenerated by carrier recombination through dangling bonds (DBs) in Si-QDML, hydrogen termination of DBs is crucial. Hydrogen plasma treatment (HPT) is one of the methods to introduce hydrogen into Si-QDML. However, HPT has a large number of process parameters. In this study, we employed Bayesian optimization (BO) for the efficient survey of HPT process parameters. Photosensitivity (PS) was adopted as the indicator to be maximized in BO. PS (σp/σd) was calculated as the ratio of photoconductivity (σp) and dark conductivity (σd) of Si-QDML, which allowed the evaluation of important electrical characteristics in solar cells easily without fabricating process-intensive devices. 40-period layers for Si-QDML were prepared by plasma-enhanced chemical vapor deposition method and post-annealing onto quartz substrates. Ten samples were prepared by HPT under random conditions as initial data for BO. By repeating calculations and experiments, the PS was successfully improved from 22.7 to 347.2 with a small number of experiments. In addition, Si-QD solar cells were fabricated with optimized HPT process parameters; open-circuit voltage (VOC) and fill factor (FF) values of 689 mV and 0.67, respectively, were achieved. These values are the highest for this type of device, which were achieved through an unprecedented attempt to combine HPT and BO. These results prove that BO is effective in accelerating the optimization of practical process parameters in a multidimensional parameter space, even for novel indicators such as PS.

Effect of the Niobium-Doped Titanium Oxide Thickness and Thermal Oxide Layer for Silicon Quantum Dot Solar Cells as a Dopant-Blocking Layer.

Silicon quantum dot (Si-QD) embedded in amorphous silicon oxide is used for p-i-n solar cell on quartz substrate as a photogeneration layer. To suppress diffusion of phosphorus from an n-type layer to a Si-QD photogeneration layer, niobium-doped titanium oxide (TiOx:Nb) is adopted. Hydrofluoric acid treatment is carried out for a part of the samples to remove the thermal oxide layer in the interface of TiOx:Nb/n-type layer. The thermal oxide acts as a photo-generated carrier-blocking layer. Solar cell properties using 10-nm-thick TiOx:Nb without the thermal oxide are better than those with the thermal oxide, notably short circuit current density is improved up to 1.89 mA/cm2. The photo-generated carrier occurs in Si-QD with quantum confinement effect. The 10-nm-thick TiOx:Nb with the thermal oxide layer effectively blocks P; however, P-diffusion is not completely suppressed by the 10-nm-thick TiOx:Nb without the thermal oxide. These results indicate that the total thickness of TiOx:Nb and thermal oxide layer influence the P-blocking effect. To achieve the further improvement of Si-QD solar cell, over 10-nm-thick TiOx:Nb is needed.

Silicon Nanowire Heterojunction Solar Cells with an Al2O3 Passivation Film Fabricated by Atomic Layer Deposition.

Silicon nanowires (SiNWs) show a great potential for energy applications because of the optical confinement effect, which enables the fabrication of highly efficient and thin crystalline silicon (c-Si) solar cells. Since a 10-µm-long SiNW array can absorb sufficient solar light less than 1200 nm, the 10-µm-long SiNW was fabricated on Si wafer to eliminate the influence of the Si wafer. On the other hand, Surface passivation of the SiNWs is a crucial problem that needs to be solved to reduce surface recombination and enable the application of SiNWs to c-Si solar cells. In this study, aluminum oxide (Al2O3) was fabricated by atomic layer deposition for the passivation of dangling bonds. However, owing to a complete covering of the SiNWs with Al2O3, the carriers could not move to the external circuit. Therefore, chemical-mechanical polishing was performed to uniformly remove the oxide from the top of the SiNWs. A heterojunction solar cell with an efficiency of 1.6% was successfully fabricated using amorphous silicon (a-Si). The internal quantum efficiencies (IQE) of the SiNW and c-Si solar cells were discussed. In the wavelength region below 340 nm, the IQE of the SiNW solar cell is higher than that of the c-Si device, which results in an increase of the absorption of the SiNW cells, suggesting that SiNWs are promising for crystalline-silicon thinning.

Influence of Fabrication Processes and Annealing Treatment on the Minority Carrier Lifetime of Silicon Nanowire Films.

Surface passivation and bulk carrier lifetime of silicon nanowires (SiNWs) are essential for their application in solar cell devices. The effective minority carrier lifetime of a semiconductor material is influenced by both its surface passivation and bulk carrier lifetime. We found that the effective carrier lifetime of SiNWs passivated with aluminum oxide (Al2O3) was significantly influenced by the fabrication process of SiNWs. We could not measure the effective lifetime of SiNWs fabricated by thermal annealing of amorphous silicon nanowires. Nevertheless, the SiNWs fabricated by metal-assisted chemical etching of polycrystalline silicon displayed an effective lifetime of 2.86 µs. Thermal annealing of SiNWs at 400 °C in a forming gas improved the effective carrier lifetime from 2.86 to 15.9 µs because of the improvement in surface passivation at the interface between the SiNWs and Al2O3 layers.

Investigation of hydrogen plasma treatment for reducing defects in silicon quantum dot superlattice structure with amorphous silicon carbide matrix.

We investigate the effects of hydrogen plasma treatment (HPT) on the properties of silicon quantum dot superlattice films. Hydrogen introduced in the films efficiently passivates silicon and carbon dangling bonds at a treatment temperature of approximately 400°C. The total dangling bond density decreases from 1.1 × 1019 cm-3 to 3.7 × 1017 cm-3, which is comparable to the defect density of typical hydrogenated amorphous silicon carbide films. A damaged layer is found to form on the surface by HPT; this layer can be easily removed by reactive ion etching.

Silicon quantum dot superlattice solar cell structure including silicon nanocrystals in a photogeneration layer.

The solar cell structure of n-type poly-silicon/5-nm-diameter silicon nanocrystals embedded in an amorphous silicon oxycarbide matrix (30 layers)/p-type hydrogenated amorphous silicon/Al electrode was fabricated on a quartz substrate. An open-circuit voltage and a fill factor of 518 mV and 0.51 in the solar cell were obtained, respectively. The absorption edge of the solar cell was 1.49 eV, which corresponds to the optical bandgap of the silicon nanocrystal materials, suggesting that it is possible to fabricate the solar cells with silicon nanocrystal materials, whose bandgaps are wider than that of crystalline silicon. PACS: 85.35.Be; 84.60.Jt; 78.67.Bf.

Radical-assisted chemical doping for chemically derived graphene.

Carrier doping of graphene is one of the most challenging issues that needs to be solved to enable its use in various applications. We developed a carrier doping method using radical-assisted conjugated organic molecules in the liquid phase and demonstrated all-wet fabrication process of doped graphene films without any vacuum process. Charge transfer interaction between graphene and dopant molecules was directly investigated by spectroscopic studies. The resistivity of the doped graphene films was drastically decreased by two orders of magnitude. The resistivity was improved by not only carrier doping but the improvement in adhesion of doped graphene flakes. First-principles calculation supported the model of our doping mechanism.

Optical assessment of silicon nanowire arrays fabricated by metal-assisted chemical etching.

Silicon nanowire (SiNW) arrays were prepared on silicon substrates by metal-assisted chemical etching and peeled from the substrates, and their optical properties were measured. The absorption coefficient of the SiNW arrays was higher than that for the bulk silicon over the entire region. The absorption coefficient of a SiNW array composed of 10-µm-long nanowires was much higher than the theoretical absorptance of a 10-µm-thick flat Si wafer, suggesting that SiNW arrays exhibit strong optical confinement. To reveal the reason for this strong optical confinement demonstrated by SiNW arrays, angular distribution functions of their transmittance were experimentally determined. The results suggest that Mie-related scattering plays a significant role in the strong optical confinement of SiNW arrays.

Improvement of carrier diffusion length in silicon nanowire arrays using atomic layer deposition.

To achieve a high-efficiency silicon nanowire (SiNW) solar cell, surface passivation technique is very important because a SiNW array has a large surface area. We successfully prepared by atomic layer deposition (ALD) high-quality aluminum oxide (Al2O3) film for passivation on the whole surface of the SiNW arrays. The minority carrier lifetime of the Al2O3-depositedSiNW arrays with bulk silicon substrate was improved to 27 µs at the optimum annealing condition. To remove the effect of bulk silicon, the effective diffusion length of minority carriers in the SiNW array was estimated by simple equations and a device simulator. As a result, it was revealed that the effective diffusion length in the SiNW arrays increased from 3.25 to 13.5 µm by depositing Al2O3 and post-annealing at 400°C. This improvement of the diffusion length is very important for application to solar cells, and Al2O3 deposited by ALD is a promising passivation material for a structure with high aspect ratio such as SiNW arrays.