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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.

This study describes the fabrication of silicon nanocrystals (Si NCs) in silicon oxide layers, which led to high-performance passivation and enhanced carrier transport in crystalline silicon (c-Si) solar cells. These Si NCs comprised nanocrystalline transport pathways in ultrathin dielectrics for reinforced passivating contact structures (NAnocrystalline Transport path in Ultrathin dielectrics for Reinforcing (NATURE) contacts). Si NCs were formed in silicon oxide layers by depositing hydrogenated amorphous silicon oxide (a-SiOx:H) with different oxygen concentrations, followed by postdeposition annealing (PDA). Based on microscopic images, the silicon oxide layer was maintained after PDA, and the Si NCs were formed in the silicon oxide matrix, leading to a relatively low recombination current (178.8 fA/cm2) compared with simple a-SiOx:H layer structures. Furthermore, the contact resistivity for the NATURE contact was 13.1 mΩ·cm2, which was comparable to that of a single a-SiOx:H layer with a low oxygen concentration. The developed NATURE contact structure expands the design flexibility scope for various functional devices containing a passivation contact layer. It allows for the production of c-Si solar cells with passivating contacts using thicker dielectric layers for improved reliability and long-term stability.

The unique samples fabricated using seed crystals with artificial grain boundaries (GBs) allowed a systematic investigation of the effect of the macroscopic GB structure (orientation, asymmetric angle, Σ values) on the GB growth direction.

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 photo conductivity (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.

We investigated the effect of the B2H6 plasma treatment on p-type hydrogenated amorphous silicon (p-a-Si:H) surfaces for high-performance silicon heterojunction (SHJ) solar cells. Secondary ion mass spectroscopy measurements revealed that the boron concentration at the p-a-Si:H surface is increased by employing the B2H6 plasma treatment. Furthermore, specific contact resistance is decreased by about one-third after the B2H6 plasma treatment. No degradation of passivation performance is induced by the B2H6 plasma treatment. The power conversion efficiency of the SHJ solar cells with the B2H6 plasma treatment is improved by the increase in fill factor (FF) due to decreased series resistance and increased shunt resistance. From numerical simulations, the upward band bending is enhanced at the heterointerface between transparent conductive oxide (TCO) and p-a-Si:H by the B2H6 plasma treatment, which is responsible for the improved FF owing to facilitated tunneling holes from c-Si to p-a-Si:H layers and the TCO/p-a-Si:H heterointerface.

The degradation of surface passivation performance by metallization is a challenge in realizing highly efficient crystalline Si solar cells that use novel carrier-selective contacts. Here, we report on a simple method to study the effect of metallization on passivation of titanium oxide (TiO x )/Si heterostructures. We investigated the relationship between the implied open-circuit voltage (iV OC) and the photoluminescence (PL) intensity imaging of solar cell precursors before metallization. Based on the relationship obtained, the change of the iV OC before and after metallization on the TiO x was evaluated quantitatively. The results showed that the iV OC predicted by the PL measurement decreases by 23–104 mV after metal deposition and shows a good agreement with the measured V OC in the finished solar cells. These results demonstrate that the iV OC evaluation by PL measurement provides a good prediction of the V OC after metallization, which is useful in analyzing the passivation degradation induced by metallization.

A different aspect of PEDOT:PSS as a passivation material of c-Si known as solute PEDOT:PSS is reported, where the carrier lifetimes of passivated c-Si are recorded to be more than 1 millisecond under an injection level of 1 × 1014 cm−3. Optimizing its fabrication with different solvents opens a universal route to produce excellent passivation materials which are facile, reproducible, and environmentally friendly.

The impact of hydrogen desorption on the electrical properties of TiOx on crystalline silicon (c-Si) with SiOy interlayers is studied for the development of high-performance TiOx carrier-selective contacts. Compared with the TiOx/c-Si heterocontacts, a lower surface recombination velocity of 9.6 cm/s and lower contact resistivity of 7.1 mΩ cm2 are obtained by using SiOy interlayers formed by mixture (often called SC2). The hydrogen desorption peaks arising from silicon dihydride (α1) and silicon monohydride (α2) on the c-Si surface of the as-deposited samples are observed. The α1 peak pressure of as-deposited heterocontacts with SiOx interlayers is lower than that of heterocontacts without a SiOy interlayer. Furthermore, the hydrogen desorption energies are found to be 1.76 and 2.13 eV for the TiOx/c-Si and TiOx/SC2-SiOy/c-Si heterocontacts, respectively. Therefore, the excellent passivation of the TiOx/SC2-SiOy/c-Si heterocontacts is ascribed to the relatively high rupture energy of bonding between Si and H atoms.

Strain engineering of group IV semiconductors has been extensively investigated with an aim to produce high mobility platform for high performance electronic devices. In particular, combination of strain and 3D structures requires elaborate engineering of strain to obtain optimum effect. Effects of utilization of non-(001)-oriented channel and anisotropic strain are major factors to be considered. The lattice structure of a strained material is generally anisotropic. Symmetry of the lattice structure is expected to have significant influence on electrical properties of crystals, which should be taken into consideration for advanced application of the lattice deformation. In addition, anisotropy in the lattice structure significantly affects the generation of defects during heterostructure formations, which can result in even larger asymmetry in the lattice structure. As a matter of fact, it is found that strain components and morphology of defects are highly asymmetric in the strained Si/SiGe heterostructures formed on (110) substrates. Recently, significant enhancement of the hole mobility was demonstrated in the topmost strained Si film. In this presentation, formation mechanism of the highly asymmetric strain in this heterostructure system and its influence on the properties of strained Si pMOSFET are discussed. Through a computational study of strain effect on the valence band structure, we found that the lowering of the hole effective mass due to strain is expected to be larger on (110) surface than on (001) surface. According to the calculation, the cyclotron effective mass of a hole on (110)-oriented strained Si can be lowered to ~ 0.2 m0 by strain. On the other hand, it was found that the minimum hole effective mass is ~ 0.3 m0 in the case of the (001)-oriented strained Si film. In addition, the result of the calculation shows that the hole effective mass on the (001)-oriented strained Si does not decrease with the increase of strain. These results motivated us to carry out experimental investigation on the growth and electrical properties of (110)-oriented strained Si. Strain-relaxed SiGe was employed as a buffer layer for strained Si layer. The growths were carried out using solid-source molecular beam epitaxy. On a Si(110) substrate, SiGe layer is subjected to biaxial compressive stress which induces glides of partial dislocations on (111) and (11-1) planes. Since the Burgers vectors of these partial dislocations are perpendicular to the in-plane axis [-110], the resultant strain relaxation is highly anisotropic. Accordingly, the surface morphology is also highly anisotropic. It was confirmed that strain in the topmost Si layer is also highly anisotropic in accordance with the lattice parameters of the SiGe layer. These anisotropic features play significant roles in electronic properties of the strained-Si MOSFET formed on this structure in the following manners. First, both the surface and the crystalline morphologies are almost homogeneous in the [-110] direction, for which the relaxation time is expected to be long for the motion in the [-110] direction. Secondly, the anisotropic strain shifts the valence band edges of the layers so that the holes are effectively confined to the high-mobility strained Si layer. A pMOSFET was fabricated on a (110)-oriented strained Si/SiGe structure and hole effective mobility was evaluated by IV and CV measurements. A significantly high hole effective mobility (480 cm2/Vs) was obtained in a pMOSFET with the [-110] channel. High hole mobility in the (110)-oriented strained Si with [-110] channel was also confirmed by gated Hall measurements.

BaSi

Boron-doped silicon nanocrystals (Si-NCs)/amorphous silicon oxide (a-SiO y ) multilayers were prepared by plasma-enhanced chemical vapor deposition and post-annealing of boron-doped Si-rich amorphous silicon oxide (a-SiO x ) and a-SiO y multilayers. The diameter of Si-NCs was changed by varying the thickness of the a-SiO x layer (t a-SiOx ) from 3 to 50 nm. The electrical conductivity (σ) was increased in the t a-SiOx range of 3 to 13 nm and saturated around 5.7 kS·m−1. This tendency corresponds to crystal volume fraction in the Si-NCs multilayers. Seebeck coefficient (S) was almost constant at 230 μV·K−1 and showed no dependence on t a-SiOx . Thermal conductivity (κ) was in the range of 1.4–1.5 W·m−1·K−1 and almost independent of t a-SiOx , which is much lower than that of bulk Si. A maximum power factor of 0.33 mW·m−1·K−2 was obtained at t a-SiOx = 13 nm.

Effective strategies for improving the performance of a droplet-based electricity generator (DEG) remain a challenge. Herein, we propose to introduce an intermediate layer of cyclic transparent optical polymer (CYTOP) by adjusting the thickness, injecting ionized ions into the surface, and increasing the surface area. We observed the positive effects of the introduction of a CYTOP layer on outputs, especially with a greater thickness, surface ionized-air modification and larger surface area, which could promote the practical application of DEG in energy harvesting.

Multicrystalline Mg2Si crystal with a diameter of 15 mm was grown via vertical Bridgman method. To clarify the growth mechanism of the multicrystalline structure, the grain growth behavior of the crystal was analyzed. This was carried out through segmenting grains by mean shift clustering using the light intensity profile obtained from multiple optical reflection images of the wafers and stacking the segmented images through the growth direction. Further crystal orientation measurement revealed that a grain with a higher surface energy competitively expanded to the lateral direction during crystal growth. We speculated that the growth behavior occurred because the supercooling was high enough to show difference in each grain’s growth rate. This idea was supported by crystal growth simulation to show a tendency for the crystallization rate to increase toward the latter half growth stage, which is consistent with the assumption for crystal growth with high supercooling.

We have investigated the chemical and electrical properties of a thin SiO2/TiO2 stacking layer deposited on n-Si and heavily phosphorus-doped n++ Si substrates to elucidate effects of phosphorus doping of Si absorbers on the band alignment and electrical performance of a SiO2/TiO2 stack-based electron-selective contact deposited on the differently doped Si substrates. From our XPS study, we show a shift of the TiO2 energy levels up to ~0.13 eV with respect to those of Si as the doping level of Si substrates changes. We also show that the conduction band offset of the SiO2/TiO2 stacking layer at the interface with the n++ Si substrate seems to smaller than that of the SiO2/TiO2 stacking layer at the interface with n-Si substrate. Finally, from our electrical transport measurements, we could conclude that the thinner tunneling barrier, the increased electron density in front of the SiO2 layer in the n++ Si surface, and/or the reduced barrier height by heavy doping, seem to enhance the majority electron transport property of the SiO2/TiO2/n++ Si samples compared to that of the SiO2/TiO2/n-Si samples.

The impact of the Al and Ge ratio in the Al-Ge pastes are investigated for fabricating the single-crystalline Si1-xGex thick layers on large area Si substrates by screen-printing metallization process. From X-ray reciprocal space maps, Ge fraction in the fabricated Si1-xGex thick layers are found to increase up to 40% with increasing the Ge ratio in the Al-Ge pastes. On the other hand, the interface of the Si and Si1-xGex layers are getting winding with increasing the Ge ratio in the Al-Ge pastes. The Al-Si-Ge phase diagram indicated that uniform SiGe layer can be fabricated by adjusting the Al-Ge ratio in the pastes within the liquid phase region.

We report on our attempt to perform the three-dimensional (3D) visualization of dislocation clusters in multicrystalline silicon (mc-Si) ingot by processing photoluminescence (PL) images and analysis of dislocation clusters in mc-Si. As-sliced wafers prepared using a high-performance (HP) mc-Si ingot were sequentially measured by PL imaging with intentional superposition of reflection. Then, various image processing techniques were applied to all the PL images to extract dark regions, which most likely correspond to dislocation clusters, as well as microstructures. By 3D reconstruction using a large quantity of 2D images, we could successfully visualize the generation, propagation and annihilation of dislocation clusters in HP mc-Si ingot. In addition, relationship between source region of dislocation clusters and crystal orientation were investigated by combining data scientific and experimental approaches. As a result, it was suggested that small angle grain boundaries with angular deviation of less than 10 degrees cause the generation of dislocation clusters.

A new aspect of solute PEDOT:PSS was explored, its function as an excellent passivation material. Solute PEDOT:PSS with a thickness of 9.4 nm exhibited an effective carrier lifetime of 940 μs at an injection level of 1 × 1014 cm−3. A new architecture of hybrid crystalline silicon solar cells with highly conductive PEDOT:PSS on the front side and solute PEDOT:PSS on the backside was demonstrated. The reverse saturation current density Jo of 9.4 nm-thick solute PEDOT:PSS was evaluated as 2.2 × 10−14 A cm−2, which is ten times lower than that without solute PEDOT:PSS on the backside, 1.45 × 10−13 A cm−2. With the introduction of 7.5 nm-thick solute PEDOT:PSS on the backside, the performance drastically improved with an open-circuit voltage VOC of 618 mV, short-circuit current density JSC of 28.33 mA cm−2 and power conversion efficiency Eff of 11.45% owing to efficient passivation. Employment of solute PEDOT:PSS as an efficient passivation material opens a new route for the fabrication of hybrid c-Si solar cells, which is facile, reproducible and environmentally friendly.

We report on the effects of grain boundary (GB) structures on the carrier recombination velocity at GB ( vGB) in multicrystalline Si (mc-Si). The fabricated artificial GBs and an originally developed machine learning model allowed an investigation of the effect of three macroscopic parameters, misorientation angle α for Σ values, asymmetric angle β, and deviation angle θ from the ingot growth direction. Totally, 13 GBs were formed by directional solidification using multi-seeds with controlled crystal orientations. vGB was evaluated directly from photoluminescence intensity profiles across GBs using a pre-trained machine learning model, which allowed a quantitative and continuous evaluation along GBs. The evaluation results indicated that the impact of θ on vGB would be relatively large among the three macroscopic parameters. In addition, the results for the Σ5 and Σ13 GBs suggested that the minimum vGB would be related to the GB energy. These results were discussed in terms of the complexity of the local reconstruction of GB structures. The deviation would make a more complex reconstructed GB structure with local distortion, resulting in an increase in the electrical activity of GBs. The obtained knowledge will contribute to improving various polycrystalline materials through a comprehensive understanding of the relationship between GB structures and their properties.

We apply NAnocrystalline Transport path in Ultra-thin dielectrics for Reinforced passivating contact (NATURE contact) to crystalline silicon solar cells for simultaneous achievement of passivation and carrier extraction. The Si nanocrystals are formed in silicon oxide by deposition of hydrogenated amorphous silicon oxide with different oxygen concentration and subsequent annealing. NATURE contact with total layer thickness of 8 nm exhibits reasonably good passivation performance. Furthermore, we demonstrate that NATURE contact can be successfully implemented to the both sides of crystalline silicon solar cells. NATURE contact, which permits flexible structural and chemical design of a passivation contact layer, will open a possibility for various functional devices.