Effect of ultrasonic frequency on electrochemical Si etching in porous Si layer transfer process for thin film solar cell fabrication

A porous Si (PS) layer transfer process that monocrystalline Si film grown on a Si substrate wafer is separated with the substrate and transferred to a non-Si device realizes to get monocrystalline Si film on low-cost substrates such as glass. PS film is fabricated by electrochemical etching in a chemical mixture of HF and ethanol. Effect of ultrasonic frequency on surface morphology of PS film is studied. By applying ultrasonic waves during etching, the pores on PS film with uniform size can be fabricated.     

Advances in monocrystalline Si thin film solar cells by layer transfer

The transfer of monocrystalline Si films enables the fabrication of efficient thin film solar cells on glass or plastic foils. Chemical vapor deposition serves to epitaxially deposit Si on quasi-monocrystalline Si films obtained from thermal crystallization of a double-layer porous Si film on a Si wafer. A separation layer that forms during this crystallization process allows one to separate the epitaxial layer on top of the quasi-monocrystalline film from the starting Si wafer after solar cell processing. Independently confirmed thin film solar cell efficiencies are 15.4% and 16.6% for thin film solar cells transferred to a glass superstrate with a total Si film thickness of 24.5 and 46.5 μm, respectively, and a cell area of 4 cm². Device simulations indicate an efficiency potential above 20%.     

Crystalline silicon thin-film solar cells on ZrSiO4 ceramic substrates

The development of a low-cost substrate is one of the major technological challenges for crystalline Si thin-film solar cells. Zirconium silicate (ZrSiO₄) ceramics is a material which can meet the demanding physical requirements as well as the cost goals. Thin microcrystalline Si films were deposited by atmospheric pressure CVD on ZrSiO₄-based ceramic substrates coated with barrier layers. The Si film was transferred into a multicrystalline grain structure by zone-melting recrystallization (ZMR). Film growth was analyzed in situ and correlated with substrate and barrier layer properties. Thin-film solar cells were fabricated from selected coarse-grained films. The best solar cell achieved an efficiency of 8.3% with a short circuit current density of 26.7 mA/cm². The effective diffusion length obtained from internal quantum efficiency measurements was about 25 μm.     

Thin film solar cells on glass based on the transfer of monocrystalline Si films

Thin film solar cells based on monocrystalline Si films are transferred onto a glass superstrate. Chemical vapor deposition serves to epitaxially deposit Si on quasi-monocrystalline Si films obtained from thermal crystallization of a double-layer porous Si film on a Si wafer. A separation layer that forms during this crystallization process allows one to separate the epitaxial layer on top of the quasi-monocrystalline film from the starting Si wafer. At present, we achieve an independently confirmed efficiency of 10.6% with a thin film solar cell of an area of 1.92 cm² that consists of a 24.5 μm thick Si film transferred to glass. Device simulation indicates an efficiency potential of around 17%.     

Si-film growth using liquid phase epitaxy method and its application to thin-film crystalline Si solar cell

We have developed a new apparatus for the growth of liquid-phase epitaxy (LPE)-Si films on 5 in Si wafers. We have obtained high growth rates of 0.1–1.0 μm/min and minority-carrier lifetime of average value of 10 μs over the whole of wafer, whereas the thickness uniformity was degraded when rotating the wafers in the solvent. We also demonstrated to growth of LPE-Si films on porous Si layers and to separate the Si films from the porous layers. A 9.5% cell was obtained using a LPE-Si film after separation.     

Structural and electrical studies of plasma-deposited polycrystalline silicon thin-films for photovoltaic application

Plasma-deposited polycrystalline Si films [or microcrystalline Si (μc-Si) films] produced by plasma enhanced chemical vapor deposition (PECVD) have attracted considerable attention as novel, low-cost and stable materials for the photovoltaic i-layer in p–i–n junction thin-film solar cells. The μc-Si films prepared under various deposition conditions show widely various microstructures, from a crystalline–amorphous mixed state to an almost perfect crystalline state, with different crystallographic orientations. These structural changes directly influence the carrier transport properties that play a dominant role in determining photovoltaic performance. Furthermore, obtaining a uniform built-in electric field throughout the i-layer is a crucial issue for achieving excellent photovoltaic performance. To obtain a uniform electric field, the following terms should be required for the i-layer: ‘truly’ intrinsic characteristic (or not n-type characteristics) as well as structural uniformity in the growth direction without an incubation layer. Here, structural properties of μc-Si for achieving truly intrinsic characteristics are reviewed with an emphasis on collations with the crystalline volume function and the degree of (2 2 0) crystallographic preferential orientation in the crystalline phase. In addition, we reviewed a growth mechanism for the μc-Si film that is actually used in the photovoltaic i-layer in highly efficient solar cells: hybrid-phase growth consisting of conventional vapor-phase growth at the surface and the solid-phase crystallization that simultaneously occurs in the film. That growth is very effective in producing structural uniformity along the growth direction and for formation of crystallites directly on the underlying doped layer.     

Simple and fast fabrication of a-Si:H/c-Si hetero-junction solar cells by dual-chamber hot wire chemical vapor deposition

One of the fabrication issues in hetero-junction crystalline Si solar cells is the overhead time between the deposition steps of the top and bottom surfaces, because flipping of the progressing wafer is necessary to process the both sides of the wafer. To reduce the overall processing time by reducing the overhead time, we propose a dual-chamber deposition system, where thin films on the top and bottom surfaces of the Si wafer are simultaneously deposited. We have evaluated the proposed deposition system by demonstrating fabricated hetero-junction crystalline Si solar cells, which were compared with solar cells fabricated by a conventional plasma-enhanced chemical deposition system. We have obtained the power conversion efficiency of 15.5% from solar cells fabricated by our dual-chamber system; and additional analyses confirmed that the proposed dual-chamber system is, in principle, competitive with conventional systems in terms of the fabricated solar cell performance. This novel concept for the fabrication of a hetero-junction crystalline Si solar cell is expected to lay an important foundation in the future thin film crystalline Si based photovoltaic industry.     

The role of the substrate on the mechanical and thermal stability of Pd thin films during hydrogen (de)sorption

In this work, we studied the mechanical and thermal stability of ~100 nm Pd thin films magnetron sputter deposited on a bare oxidized Si(100) wafer, a sputtered Titanium (Ti) intermediate layer, and a spin-coated Polyimide (PI) intermediate layer. The dependence of the film stability on the film morphology and the film-substrate interaction was investigated. It was shown that a columnar morphology with elongated voids at part of the grain boundaries is resistant to embrittlement induced by the hydride formation (α?β phase transitions). For compact film morphology, depending on the rigidity of the intermediate layer and the adherence to the substrate, complete transformation (Pd-PI-SiO₂/Si) or partly suppression (Pd-Ti-SiO₂/Si) of the α to β-phase was observed. In the case of Pd without intermediate layer (Pd-SiO₂/Si), buckling delamination occurred. The damage and deformation mechanisms could be understood by the analysis of the stresses and dislocation (defects) behavior near grain boundaries and the film-substrate interface. From diffraction line-broadening combined with microscopy analysis, we showed that in Pd thin films, stresses relax at critical stress values via different relaxation pathways depending on film-microstructure and film-substrate interaction. On the basis of the in-situ hydriding experiments, it was concluded that a Pd film on a flexible PI intermediate layer exhibits free-standing film-like behavior besides being strongly clamped on a stiff SiO₂/Si substrate.     

A high efficiency thin film silicon solar cell and module

A photoelectric conversion efficiency of over 10% has been achieved in thin-film microcrystalline silicon solar cells which consist of a 2 μm thick layer of polycrystalline silicon. It was found that an adequate current can be extracted even from a thin film due to the very effective light trapping effect of silicon with a low absorption coefficient. As a result, this technology may eventually lead to the development of low-cost solar cells. Also, an initial aperture efficiency as high as 13.5% has been achieved with a large area (91 cm × 45 cm) tandem solar cell module of microcrystalline silicon and amorphous silicon (thin film Si hybrid solar cell). An even greater initial efficiency of 14.7% has been achieved in devices with a small size (area of 1 cm²), and further increases of efficiency can be expected.     

Dislocations,texture and stress development in hydrogen-cycled Pd thin films: An in-situ X-ray diffraction study

For Pd thin films, microstructural changes involved during hydrogen cycling provide the information needed to predict and optimize the film's mechanical strength. In this paper, a systematic study of the morphology, microstructure, texture, and stress has been performed on Pd thin films during hydrogen loading and deloading cycles at room temperature. Pd thin films of similar morphology were prepared by magnetron sputtering on substrates of different compliances, i.e., Si-oxide, Titanium (Ti) and Polyimide (PI). The evolution of the morphology, grain-orientation distribution (texture), state of stress, and dislocation densities are analyzed for each of the film substrate types for 20 hydrogen loading/deloading cycles. The lattice expansion and contraction caused by the transition from Pd to Pd-hydride and back result in a strong stress increase. This stress increase stabilizes after a few cycles by grain boundary motion that leads to a gradual enhancement of the (111) texture and changes in the dislocation density for Pd films that are strongly clamped on to an oxidized Si(100) wafer substrate with an intermediate layer (Ti or PI). For Pd on PI, the stress is also partly released by a crack-based (crack widening/growth/propagation) pathway. Pd films on Ti and PI do not buckle or blister after 20 hydrogen cycles. By providing a sufficiently compliant substrate the traditional problems of buckle-delamination of a film on a stiff substrate are mitigated.     

Porous silicon layer transfer processes for solar cells

A promising cost-effective way of converting sun light into electricity could be a solar cell realized in a thin monocrystalline silicon film, due to its potential to achieve cell efficiencies of more than 20% in a 20 μm thick film. A porous silicon layer transfer technique provides an opportunity to get monocrystalline films on low-cost substrates such as glass. This paper reviews various processes, which are being developed for the layer transfer using porous silicon as a sacrificial layer while reusing of starting silicon substrate. The four basic steps—porous silicon formation, active layer deposition, layer separation and transfer, and device fabrication—have been identified in layer transfer process. The processes have been categorized and compared on the basis of the sequence of steps used in individual processes.     

Low temperature μc-Si film growth using a CaF2 seed layer

This paper describes low temperature thin film Si growth by remote plasma chemical vapor deposition system for photovoltaic device applications. Using CaF₂/glass substrate, we were able to achieve an improved μc-Si film at a low process temperature of 300°C. The μc-Si film on CaF₂/glass substrate shows that a crystalline volume fraction of 65% and dark conductivity of 1.65×10⁻⁸ S/cm with the growth conditions of 50 W, 300°C, 88 mTorr, and SiH₄/H₂=1.2%. XRD analysis on μc-Si/CaF₂/glass showed crystalline film growth in (1 1 1) and (2 2 0) planes. Grain size was enlarged as large as 700 Å for a μc-Si/CaF₂/glass structure. Activation energy of μc-Si film was given as 0.49 eV. The μc-Si films exhibited dark- and photo-conductivity ratio of 124.     

Reduction of amorphous incubation layer by HCl addition during deposition of microcrystalline silicon by hot-wire chemical vapor deposition

The amorphous incubation layer, which is formed in the initial growth stage of hydrogenated microcrystalline silicon (μc-Si:H) thin film deposited at low temperature, is harmful to the electric properties of film. In this study, the effect of the addition of HCl gas on the reduction of such an amorphous incubation layer was investigated during the silicon deposition on a glass substrate at 220 °C by hot-wire chemical vapor deposition process using the Raman spectroscopy, the X-ray diffraction and the field-emission scanning electron microscopy. In the initial stage of deposition where the silicon film deposited without HCl addition consisted almost entirely of the amorphous incubation layer; highly crystalline silicon films could be deposited with HCl addition. As the flow rate of HCl increased, the crystallinity of silicon films increased but the film growth rate decreased. The surface morphology of films prepared with HCl addition became smoother with smaller grain size than that prepared without HCl.     

Solid-phase crystallized Si films on glass substrates for thin film solar cells

We investigate the potential of solid-phase crystallized Si films on glass for use in polycrystalline Si thin film solar cells. Low-pressure chemical vapour deposition serves to form amorphous Si films on borosilicate, SiO₂-coated borosilicate, aluminosilicate glass and fused silica substrates. The films are crystallized at temperatures of around 600°C. Using transmission electron microscopy we determine the grain size in the crystallized films. The average grain size strongly depends on the substrate type, increases with the deposition rate of the amorphous film and is independent of the film thickness. The grain size distribution in our films is log-normal. Films crystallized on SiO₂-coated borosilicate glass have an average grain size up to 2.3 μm, while the area weighted average grain size peaks at 4 μm. Since thin crystalline Si solar cells only require a film thickness of several micron, our films seem to be suitable for application to such devices.     

CIS film growth by metallic ink coating and selenization

A novel technique was demonstrated for the growth of CuInSe₂ (CIS) thin films. The technique used an ink formulation containing sub-micron size particles of Cu–In alloys. A metallic precursor layer was first formed by coating this ink onto the substrate by spraying. The precursor film was then made to react with Se to form the CIS compound. The morphology of the CIS layers depended on the initial composition of the Cu–In particles as well as the post-deposition treatments. Solar cells were fabricated on CIS absorber layers prepared by this low-cost ink-coating approach and devices with a conversion efficiency of over 10.5% were demonstrated.     

Growth of crystalline WSe2 and WS2 films on amorphous substrate by reactive (Van der Waals) rheotaxy

Thin films of metal dichalcogenide compounds with a layered structure, such as MoS₂ (WSe₂), play an important role in a number of technologies, like solid lubrication, experimental photovoltaic cells, etc. Such films usually adopt a type-I texture, in which case the c-axis of the crystallites is parallel to the substrate plane. However, for the aforementioned applications, type-II texture, where the c-axis of the crystallite is perpendicular to the substrate, is required.We have recently demonstrated a novel growth technique (Van der Waals rheotaxy, VdWR) which yields a crystalline film having exclusively type-II texture on amorphous (quartz) substrate. In the present work superior crystalline, optical and electronic properties of the overlying WSe₂ (WS₂) film together with an improved adhesion of the film to the quartz substrate are obtained by replacing the ultra thin Ni film with a Ni/Cr film.     

Heteroepitaxial technologies of III–V on Si

Many kinds of heteroepitaxial technologies to improve the quality of III–V on Si are discussed. Surface cleaning and preparation for epitaxial growth, initial interfacial buffer layers between the substrate and the film, many kinds of intermediate layers such as strained layer super lattice (SLS) inserted in the films, low substrate temperature growth and thermal treatments during and after film growth are reviewed. Defect passivation and growth on the patterned substrates are also discussed. Among these methods, essential processes are proposed to evaluate their effectiveness for the defect reduction.     

Present status and future of crystalline silicon solar cells in Japan

Crystalline silicon solar cells show promise for further improvement of cell efficiency and cost reduction by developing process technologies for large-area, thin and high-efficiency cells and manufacturing technologies for cells and modules with high yield and high productivity.In this paper, Japanese activities on crystalline Si wafers and solar cells are presented. Based on our research results from crystalline Si materials and solar cells, key issues for further development of crystalline Si materials and solar cells will be discussed together with recent progress in the field. According to the Japanese PV2030 road map, by the year 2030 we will have to realize efficiencies of 22% for module and 25% for cell technologies into industrial mass production, to reduce the wafer thickness to 50–100 μm, and to reduce electricity cost from 50 Japanese Yen/kWh to 7 Yen/kWh in order to increase the market size by another 100–1000 times.     

Silicon sheet from silane: first results

A process to obtain self-supported thin silicon films is being developed. Films are grown by optical chemical vapour deposition (CVD) (using halogen lamps as heating system) from silane, at low temperature and relatively high growth rates, on silicon substrates with a sacrificial layer of porous Si (PS), which allows film detachment. The PS layer was formed by anodisation of the Si-substrate surface in a HF:ethanol solution. Film deposition was carried out in an optical cold wall horizontal CVD reactor, operating at atmospheric pressure, and specially designed for this study. Deposition rates and film morphology were studied as a function of substrate nominal temperature, gas concentration and flux. In the final chosen conditions, deposition occurs at a nominal temperature of 840°C, with a silicon growth rate between 2 and 4 μm/min, which is relatively high for the low temperature used. A good usage of silane gas was already achieved, with 80–85% of the silicon in the silane gas being deposited on a 40×40 mm² substrate, with very low deposition rate on the reactor wall. Films of thicknesses from 10 to 150 μm were deposited. The films were found to be continuous with surfaces coated with whiskers. Film detachment from multicrystalline substrates has proved unsuccessful so far, while it readily occurs when monocrystalline substrates are used. The reason for this is macropore collapse and film rupture, usually occurring in the smaller grain regions of the multicrystalline substrates.     

Porous silicon as an intermediate layer for thin-film solar cell

The potential of porous silicon (PS) with dual porosity structure as an intermediate layer for ultra-thin film solar cells is described. It is shown that a double-layered PS with a porosity of % allows to grow epitaxial Si film at medium temperature (725°–800°C) and at the same time serves as a gettering/diffusion barrier for impurities from potentially contaminated low-cost substrate. A 3.5 μm thin-film cell with reasonable efficiency is realized using such a PS intermediate layer.