Highest-quality surface passivation of low-resistivity p-type silicon using stoichiometric PECVD silicon nitride

The surface passivation properties of silicon nitride (SiN) films fabricated by high-frequency direct plasma-enhanced chemical vapour deposition (PECVD) on low-resistivity (1 Ω cm) p-type silicon solar cell substrates have been investigated. The process gases used were ammonia and a mixture of silane and nitrogen. In order to find the optimum set of SiN deposition parameters, a large number of carrier lifetime test structures were prepared under different deposition conditions. The optimised deposition parameters resulted in outstandingly low surface recombination velocities (SRVs) below 10 cm/s. Interestingly, we find the lowest SRVs for stoichiometric SiN films, as indicated by a refractive index of 1.9. In former studies similarly low SRVs had only been obtained for silicon-rich SiN films. The fundamentally different passivation behaviour of our SiN films is attributed to the addition of nitrogen to the process gases.     

High-density inductively coupled plasma chemical vapor deposition of silicon nitride for solar cell application

The silicon nitride films were deposited by means of high-density inductively coupled plasma chemical vapor deposition in a planar coil reactor. The process gases used were pure nitrogen and a mixture of silane and helium. Passivated by silicon nitride, solar cells show efficiency above 13%. Strong H-atom release from the growing SiN film and Si–N bond healing are responsible for the improved electrical and passivation properties of SiN film. This paper presents the optimal refractive index of SiN for single layer antireflection coating as well as double layer antireflection coating in solar cell applications.     

SiO2 passivation layer grown by liquid phase deposition for silicon solar cell application

Surface passivation is one of the primary requirements for high efficient silicon solar cells. Though the current existed passivation techniques are effective, expensive equipments are required. In this paper, a comprehensive understanding of the SiO₂ passivation layer grown by liquid phase deposition (LPD) was presented, which was cost-effective and very simple. It was found that the post-annealing process could significantly enhance the passivation effect of the LPD SiO₂ film. Besides, it was revealed that both chemical passivation and field-effect passivation mechanisms played important roles in outstanding passivation effect of the LPD SiO₂ film through analyzing the minority carrier lifetime and the surface recombination velocity of n-type and p-type silicon wafers. Although the deposition parameters had little influence on the passivation effect, they affected the deposition rate. Therefore, appropriate deposition parameters should be carefully chosen based on the compromise of the deposition rate and fabrication cost. By utilizing the LPD SiO₂ film as surface passivation layer, a 19.5%-efficient silicon solar cell on a large-scale wafer (156 mm × 156 mm) was fabricated.     

Effect of hydrogen radical annealing on SiN passivated solar cells

Remote plasma was used for PE-CVD of SiN films and it was found that hydrogen radical (H^* ) annealing of c-Si cells with SiN films improved the efficiency of the cells. Cell efficiency of 21.8% was obtained by applying a SiN/SiO₂ double-layer structure on the emitter of a PERL-type solar cell. It was found that the H^* annealing has two effects: it reduces surface recombination velocity (SRV); and it degrades bulk-lifetime of p-type c-Si. To apply SiN practically, it is effective to use a rear n-floating or a triode structure. Reducing the exposed area of the p-type substrate by using n-type diffused layer increases the efficiency of solar cells.     

CdSe thin films as solar control coatings

CdSe thin films deposited by a physical vapour deposition method were investigated as solar control coatings on architectural glazings. The optical transmittance and the near-normal specular reflectance in the range 0.40−2.40 μm and spectral distribution of reflected and transmitted intesities in the same range showed that CdSe thin films have solar control characteristics comparable to commercially available metallic coatings and other materials such as PbS and Cu_ξS films. The solar control characteristics of CdSe films were found to be dependent on film parameters, including deposition rate and deposition temperature.     

Progress in electrodeposited absorber layer for CuIn(1−x)GaxSe2 (CIGS) solar cells

Thin film solar cells with chalcopyrite CuInSe₂/Cu(InGa)Se₂ (CIS/CIGS) absorber layers have attracted significant research interest as an important light-to-electricity converter with widespread commercialization prospects. When compared to the ternary CIS, the quaternary CIGS has more desirable optical band gap and has been found to be the most efficient among all the CIS-based derivatives. Amid various fabrication methods available for the absorber layer, electrodeposition may be the most effective alternative to the expensive vacuum based techniques. This paper reviewed the developments in the area of electrodeposition for the fabrication of the CIGS absorber layer. The difficulties in incorporating the optimum amount of Ga in the film and the likely mechanism behind the deposition were highlighted. The role of deposition parameters was discussed along with the phase and microstructure variation of an as-electrodeposited CIGS layer from a typical acid bath. Related novel strategies such as individual In, Ga and their binary alloy deposition for applications in CIGS solar cells were briefed.     

Photovoltaic cells based on chemically deposited p-type SnS

The aim of this work is to attract the attention of the scientific workers in the field of PV conversion of solar energy to SnS polycrystalline thin film as a candidate for construction of cheap solar cells, since it posseses similar photoelectric properties as polycrystalline silicon, but it can be produced on any kind of substrate, by simple, economic and environmentally approved technique. By the use of the method of chemical deposition from two separate solutions, complete preparation of three types of cells was done. All of them use SnS as base absorbing layer, with a difference in the window layer electrode. The first one has CdO, the second one has Cd₂SnO₄ thin film window electrode, both prepared by the chemical deposition method. The third cell was purely Schottky barrier cell in which the window electrode was SnO₂:F, prepared by spray pyrolysis. The I–V, C–V and spectral characteristics were registered and the conclusion was drawn that the best performances has shown the cells with Cd₂SnO₄ film as a window electrode.     

Widegap a-Si:H films prepared at low substrate temperature

Wide bandgap hydrogenated amorphous silicon (a-Si:H) films have been prepared by the PECVD method at a low substrate temperature (80°C) controlling the incorporation of hydrogen (bonded with silicon) into the film. Optimizing the deposition parameters viz. hydrogen dilution, rf power, a-Si:H film with E_g ∼ 1.90 eV and σ_ph ≥ 10⁻⁴ Scm⁻¹ has been developed. This film exhibited better optoelectronic properties compared to a-SiC:H of similar optical gap. The quantum efficiency measurement on the Schottky barrier solar cell structure showed a definite enhancement of blue response. Surface reaction as well as structural relaxation under suitable deposition condition have been claimed to be responsible for the development of such material.     

Influence of In2S3 film properties on the behavior of CuInS2/In2S3/ZnO type solar cells

Solar cells of CuInS₂/In₂S₃/ZnO type are studied as a function of the In₂S₃ buffer deposition conditions. In₂S₃ is deposited from an aqueous solution containing thioacetamide (TA), as sulfur precursor and In³⁺. In parallel, variable amounts of In₂O₃ are deposited that have an important influence on the buffer layer behavior. Starting from deposition conditions determined in a preliminary study, a set of parameters is chosen to be most determining for the buffer layer behavior, namely the solution temperature, the concentration of thioacetamide [TA], and the buffer thickness. The solar cell results are discussed in relation with these parameters. Higher efficiency is attained with buffer deposited at high temperature (70 °C) and [TA] (0.3 M). These conditions are characterized by short induction time, high deposition rate and low In₂O₃ content in the buffer. On the other hand, the film deposited at lower temperature has higher In₂O₃ content, and gives solar cell efficiency sharply decreasing with buffer thickness. This buffer type may attain higher conversion efficiencies if deposited on full covering very thin film.     

Study of CuInS2/ZnS/ZnO solar cells, with chemically deposited ZnS buffer layers from acidic solutions

Thin film solar cells based on CuInS₂/ZnS/ZnO have been prepared with ZnS buffer film of different thickness. ZnS films are grown by chemical bath deposition (CBD) from acidic solutions of ZnSO₄ and thioacetamide (TA). The change of the growth rate with time is studied by means of the quartz crystal microbalance. Films with different thickness show variable physical, chemical and morphological properties. The structure is studied with X-ray diffraction, showing different crystallinity with deposition time. The absorption coefficient depends also on the CBD deposition time, and shows absorption edges between 2.70 and 3.65 eV. The compositional analysis carried out with XPS (surface) and EDAX (bulk). Bulk composition reflects highly stoichiometric films, with Zn/S ratios close to unit. Preliminary results with CuInS₂-based solar cell show efficiencies around 5%, lower than usually found with standard CdS buffer films (around 9%).     

Angle-dependent XPS analysis of silicon nitride film deposited on screen-printed crystalline silicon solar cell

In this work silicon nitride (Si₃N₄) film was deposited as an antireflection coating (ARC) on crystalline silicon solar cell (cell?A) using plasma-enhanced chemical vapor deposition (PECVD). Two solar cells XA and XB of approximately equal area were diced from cell#A and characterized by angle-dependent X-ray photoelectron spectroscopy (XPS). The XPS profiling shows the presence of silicon (Si), nitrogen (N), carbon (C) and oxygen (O) in the Si₃N₄ film. The presence of C and O indicates that organic substances, involved in processing steps were not released completely from the surface and may diffuse in Si₃N₄ ARC during deposition. The XPS spectra corresponding to Si2p, N1s, C1s and O1s were recorded at angles 0° (normal to the surface), 30° and 45°, as angle increases spectra becomes more surface sensitive. Peak positions in Si2p and N1s spectra explain the oxygen contamination in the Si₃N₄ film. The shift in the peak positions of C1s and O1s as angle increases from 0° to 45° explains the surface contamination of carbon and oxygen. The atomic composition of elements Si, N, C and O show more carbon, oxygen concentration and smaller N/Si ratio than stoichiometry, i.e. Si₃N₄ in cell XB. However, cell XA not only show better photovoltaic performance in terms of parameters open-circuit voltage (V_oc), short-circuit current density (J_sc), fill factor (FF) and efficiency (η) but also have more uniform texturization and regular pyramids on the surface as revealed by scanning electron microscopy (SEM). The presence of higher concentration of impurities (carbon and oxygen), non-uniformity in texturization and in the Si₃N₄ film as well could be responsible for less satisfactory photovoltaic performance of cell XB.     

The effect of transient depletion of source gases on the properties of microcrystalline silicon solar cells

In is paper, the transient behavior of silane molecules in the initial plasma ignition stage on the properties of microcrystalline silicon films is studied using tailored initial SiH₄ density method, and the results are analyzed by Raman spectroscopy and spectroscopic ellipsometry. Compared with standard plasma ignition conditions, tailored initial SiH₄ density conditions result higher crystallinity in the interface between substrate and bulk film. Finally, tailored and standard conditions are used in i-layer deposition processes of p-i-n and n-i-p solar cells. It is demonstrated that tailored initial SiH₄ density conditions is helpful for the efficiency improvement of n-i-p solar cells and standard plasma ignition conditions for p-i-n solar cells.     

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.     

Structural, optical and photoelectrochemical studies of the cadmium sulphide films grown by chemical bath deposition

Cadmium sulphide films have been grown by chemical bath deposition using an aqueous medium. Growth rate has been studied on the basis of the film thickness variation with deposition period. Structural investigation using XRD has shown the presence of polycrystalline deposits of wurtzite structure. The optical absorbance has revealed that the band gap of the CdS layers has been 2.39 eV. The CdS films have been employed in electrochemical photovoltaic cells of configuration glass/ITO/CdS//S₂S²⁻₂//Ni to test their photoactivity. The solar cell parameters have been correlated with the growth parameters.     

Polycrystalline silicon film deposited by ICP-CVD

We studied the deposition of polycrystalline silicon (poly-Si) using SiH₄/SiH₂Cl₂/H₂ mixtures by inductively coupled plasma chemical vapor deposition. The deposition rate and crystalline quality were improved by increasing RF power. The poly-Si film deposited with the [SiH₂Cl₂]/[SiH₄] ratio of 2 and the RF power of 1500 W exhibited the deposition rate of 4.2 Å/s, the polycrystalline volume fraction of 88%, the Raman FWHM of 7 cm⁻¹, and the TEM grain size of 1200 Å. The solar cell made of this material exhibited a conversion efficiency of 3.14%.     

Characterization and optimization of indium tin oxide films for heterojunction solar cells

Well suited and reliable values of the optical and electrical properties of thin indium tin oxide (ITO) films are needed in order to choose the optimal deposition parameters and to perform reliable modeling for solar cells design. In this work, a new method will be presented to evaluate the ITO transparency directly on silicon substrates.The effects of each deposition parameter that influences the ITO transparency and conductivity force a trade-off in the frame of values useful for SHJ solar cells. The deposition of our optimized ITO film on a textured wafer yields a weighted average reflectance as low as 4.4±0.2%.The deposition of an MgF₂/ITO double-layer anti-reflection coating (DL-ARC) on textured cells increases the efficiency from 17.9%, measured immediately after contacts have been added to the ITO, to 18.4% after the MgF₂ deposition. An annealing step at 200 °C for 10 min proved to further increase the efficiency up to 18.9%, for a total gain of 1%.     

The novel usage of spectroscopic ellipsometry for the development of amorphous Si solar cells

We present the novel use of spectroscopic ellipsometry (SE) for the development of a-Si:H solar cell. SE is a very fast and useful tool to measure various optical properties of thin film. In the case of a-Si:H thin film analysis, generally, SE is used to estimate the film thickness, roughness, void fraction, optical constants such as (n,k), and so forth. In this study, optical parameters from SE measurements were analyzed with relation to structural and electrical properties of a-Si:H thin film for solar cell. By analyzing IR absorption spectra and conductivity measurements, it was affirmed that <ε₂> and parameter A by Tauc-Lorentz model fitting of SE data are representative parameters qualifying a-Si:H thin film, and that they have close relationships with FF and light induced degradation property of solar cells. Based on the analysis, solar cells that have i-layers with various E_g were optimized. By this research, easier and faster methodology to develop a-Si:H thin film for thin film Si solar cells using SE measurements was established.     

Atomic layer deposited thin film metal oxides for fuel production in a solar cavity reactor

Alumina thin film structures were produced by coating high surface area polymer particles via atomic layer deposition (ALD), using the polymer as a sacrificial template. Burnout of the polymer material left high surface area, high pore volume structures, with 15 nm wall thickness. Further deposition of up to 27 mol% Co and Fe was performed via ALD to produce high surface area CoFe₂O₄ particles for thermochemical water splitting. The ALD particles were thermally cycled in electrically heated lab reactors and on-sun using a concentrated solar, reflective cavity reactor. Surface area measurements of cycled ALD particles showed improved surface area retention as compared to bulk Fe₂O₃ nanopowders. Reaction rates as high as 15.2 and 9.8 μmol/s/g were observed, on-sun, for H₂O and CO₂ splitting respectively. Thermochemical cycling in a concentrated solar cavity reactor showed an order of magnitude increase in solar utilization efficiency between ALD particles and bulk Fe₂O₃ nanopowders.     

Optimization of PECVD SiN:H films for silicon solar cells

We have grown silicon nitride (SiN:H) thin films on silicon and glass by the Plasma Enhanced Chemical Vapor Deposition (PECVD) Method at low temperature in order to study their electro-optical properties and correlate these properties to the chemical composition of the layers, so that optimum films may be achieved for silicon solar cells. By varying the silane to ammonia ratio in the plasma gas we have been able to modify the index of refraction, the optical band gap and the silicon surface state passivation properties of the films. From this information we have determined that the optimum silane to ammonia ratio, with other constant parameters in our system, should be 20/65. Our results indicate that the mid-gap surface state density in silicon can be reduced down to 10¹⁰ cm⁻² eV⁻¹ when this optimum (silane to ammonia) ratio is used for depositing SiN:H layers. We have confirmed this optimal ratio by making quantum efficiency measurements on silicon solar cells having their emitter passivated with SiN:H layers deposited with different silane to ammonia ratios. A great reduction of the surface recombination velocity was achieved, as observed from the internal quantum efficiency measurements, for cells with optimal SiN:H layers as compared to those with non-optimum SiN:H layers.     

Optimization of process parameters in a large-area hot-wire CVD reactor for the deposition of amorphous silicon (a-Si:H) for solar cell application with highly uniform material quality

Scale-up of a-Si:H-based thin film applications such as solar cells, entirely or partly prepared by hot-wire chemical vapor deposition (HWCVD), requires research on the deposition process in a large-area HWCVD system. The influence of gas supply and filament geometry on thickness uniformity has already been reported, but their influence on material quality is systematically studied for the first time. The optimization of deposition parameters for obtaining best material quality in our large-area HWCVD system resulted in an optimum filament temperature, T_fil≈1600°C, pressure, p=8 mTorr and silane flow, F(SiH₄)=100 sccm, keeping the substrate temperature at T_S=200°C. A special gas supply (gas shower with tiny holes of uniform size) and a filament grid, consisting of six filaments with an interfilament distance, d_fil=4 cm were used. The optimum filament-to-substrate distance was found to be d_fil–S=8.4 cm. While studying the influence of different d_fil and gas supply configurations on the material quality, the above-mentioned setup and parameters yield best results for both uniformity and material quality. With the setup mentioned, we could achieve device quality a-Si:H films with a thickness uniformity of ±2.5% on a circular area of 20 cm in diameter. The material, grown at a deposition rate of r_d≈4 Å/s, was characterized on nine positions of the 30 cm×30 cm substrate area, and revealed reasonable uniformity of the opto-electronic properties, e.g photosensitivity, σ_Ph/σ_D=(2.46±0.7)×10⁵, microstructure factor, R=0.17±0.05, defect densities, N_d(PDS)=(2.06±0.6)×10¹⁷ cm⁻³ and N_d(CPM)=(2.05±0.5)×10¹⁶ cm⁻³ (film properties are given as mean values and standard deviations). Finally, we fabricated pin solar cells, with the i-layer deposited on small-area p-substrates distributed over an area of 20 cm×20 cm in this large-area deposition system, and achieved high uniformity of the cell parameters with initial efficiencies of η=(6.1±0.2)% on the 20 cm×20 cm area.