Specific contact resistance measurements of the screen-printed Ag thick film contacts in the silicon solar cells by three-point probe methodology and TLM method

The specific contact resistance of the screen-printed Ag contacts in the silicon solar cells has been investigated by applying two independent test methodologies such as three-point probe (TPP) and well-known transfer length model (TLM) test structure respectively. This paper presents some comparative results obtained with these two measurement techniques for the screen-printed Ag contacts formed on the porous silicon antireflection coating (ARC) in the crystalline silicon solar cells. The contact structure consists of thick-film Ag metal contacts patterned on the top of the etched porous silicon surface. Five different contact formation temperatures ranging from 725 to 825 °C for few minutes in air ambient followed by a short time annealing step at about 450 °C in nitrogen ambient was applied to the test samples in order to study the specific contact resistance of the screen-printed Ag metal contact structure. The specific contact resistance of the Ag metal contacts extracted based on the TPP as well as TLM test methodologies has been compared and verified. It shows that the extraction procedure based on the TPP method results in specific contact resistance, ρ _c  = 2.15 × 10⁻⁶ Ω-cm² indicating that screen-printed Ag contacts has excellent ohmic properties whereas in the case of TLM method, the best value of the specific contact resistance was found to be about ρ _c  = 8.34 × 10⁻⁵ Ω-cm². These results indicate that the ρ _c value extracted for the screen-printed Ag contacts by TPP method is one order of magnitude lower than that of the corresponding value of the ρ _c extracted by TLM method. The advantages and limitations of each of these techniques for quantitatively evaluating the specific contact resistance of the screen-printed Ag contacts are also discussed.     

Application of power loss calculation to estimate the specific contact resistance of the screen-printed silver ohmic contacts of the large area silicon solar cells

The establishment of a suitable contact formation methodology is a critical part of the technological development of any metal-to-semiconductor contact structure. Many test structures and methodologies have been proposed to estimate the specific contact resistance (ρ_c) of the planar ohmic contacts formed on the heavily doped semiconductor surface. These test structures are usually processed on the same wafer to monitor a particular process. In this study, new experimental procedure has been evolved to assess the value of ρ_c of the screen-printed front silver (Ag) thick-film metal contact to the silicon surface. The essential feature of this methodology is that it is an iteration technique based on the calculation of power loss associated with various resistive components of the solar cell normalized to the unit cell area. Therefore, this method avoids the complexity of making the design of any lay out of a standard contact resistance test structure like transmission line model (TLM) or Kelvin resistor, etc. It was shown that value of specific contact resistance of the order of 1.0 × 10⁻⁵ Ω−cm ² is measured for the Ag metal contacts formed on the n⁺ silicon surface. This value is much lower than the ρ_c data previously reported for the screen-printed Ag contacts. The sintering process of the front metal contact structure at different furnace setting is carried out to understand the possible wet interaction and metal contact formation as a function of the firing. Therefore, the study is further extended to study the peak firing temperature dependence of the ρ_c of screen-printed Ag metal contacts. It will help to assess the specific contact resistance of the ohmic contacts as a function of firing temperature of sintering process.

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Specific contact resistance and metallurgical process of the silver-based paste for making ohmic contact structure on the porous silicon/p-Si surface of the silicon solar cell

Porous silicon has been considered as a promising optoelectronic material for developing a variety of optoelectronic devices and sensors. In the present study, the electrical properties and metallurgical process of the screen-printed Ag metallization formed on the porous silicon surface of the silicon solar cell have been investigated. The contact structure consists of thick-film Ag metal contact patterned on the top of the porous silicon surface. The sintering process consists of a rapid firing step at 750–825 °C in air ambient. It results in the formation of a nearly perfect contact structure between the Ag metal and porous silicon/p-Si structure that forms the top metalization for the screen-printed silicon solar cells. The SEM picture shows that Ag metal firmly coalesces with the silicon surface with a relatively smooth interfacial morphology. This implies that high temperature fire-through step has not introduced any signs of adverse effect of junction puncture or excessive Ag indiffusion, etc. The three-point probe (TPP) method was applied to estimate the specific contact resistance, ρ _c (Ω-cm²) of the contact structure. The TPP measurement shows that contact structure has excellent ohmic properties with ρ _c = 1.2 × 10⁻⁶ Ω-cm² when the metal contact sintered at 825 °C. This value of the specific contact resistance is almost three orders of magnitude lower than the corresponding value of the ρ _c = 7.35 × 10⁻³ Ω-cm² obtained for the contact structure sintered at 750 °C. This improvement in the specific contact resistance indicates that with increase in the sintering temperature, the barrier properties of the contact structure at the interface of the Ag metal and porous silicon structure improved which in turn results a lower specific contact resistance of the contact structure.     

Nanopatterned silicon emitters of a solar cell fabricated by anodic aluminum oxide masks

We investigated the nanopattern transferring process by a template of anodic aluminum oxide and the formation of a nanoporous aluminum oxide layer on a Si solar cell by the anodization process of Al thin films. The anodization process provided a template to transfer the nanopattern onto the Si surface. The small-sized nanoporous alumina template was attached to be covered on the textured surface and played the role of etching mask in the F-based dry etching process. Furthermore, we deposited an Al thin film onto the Si surface and the subsequent anodization process was performed. The alumina formulated on the deposited Al thin film did not show the array of nanoporous structure and no nanopatterns were transferred onto the surface. The large-areal alumina deposited on the Si surface showed enhanced photo-absorption in the ultraviolet spectral region of 243 nm, but increased the photo-reflectance in the visible and infrared spectral regions when compared to the Si-bare sample.     

Effect of dispersibility of silver powders in conductive paste on microstructure of screen-printed front contacts and electrical performance of crystalline silicon solar cells

Effect of dispersibility of silver powders in conductive paste on microstructure of screen-printed front contacts and electrical performance of crystalline silicon solar cells was investigated. Two different dispersed degree silver powders were experimented. It is found that the dispersibility of the silver powders strongly affects the microstructure of thick film and electrical performance of solar cells and highly dispersed silver powders can form even compact thick film and exhibit higher open-circuit voltage (V _oc ), short-circuit current (I _sc ), shunt resistance (R _sh ), maximum power (P _max ), optimum operating voltage (V _mp ), optimum operating current (I _mp ), fill factor (FF) and photoelectric conversion efficiency (Eff) than seriously agglomerated silver powders under otherwise identical conditions.     

SEM and SIMS study of the buried SixNy layer formed in silicon

Standard 20 Ωcm boron doped Cz Si wafers were subjected to 100 keV hydrogen ion implantation at room temperature to fluences of 1 × 10¹⁶ or 4 × 10¹⁶ at/cm². Subsequently, nitrogen was incorporated in silicon from a DC plasma source at a temperature of 300 °C. Finally, all samples were annealed at 700 °C for 2 h in vacuum. Structural properties of samples were studied by SIMS and SEM. The SEM study was carried out both in the secondary electrons (SE) and the Surface-Electron-Beam-Induced-Voltage (SEBIV) modes. The experiments have demonstrated that incorporated from plasma nitrogen atoms were accumulated in the buried damage layer and the formed nitrogen-containing layer has an island-like structure.     

Nano-scaled Pt/Ag/Ni/Au contacts on p-type GaN for low contact resistance and high reflectivity

We synthesized the vertical-structured LED (VLED) using nano-scaled Pt between p-type GaN and Ag-based reflector. The metallization scheme on p-type GaN for high reflectance and low was the nano-scaled Pt/Ag/Ni/Au. Nano-scaled Pt (5 A) on Ag/Ni/Au exhibited reasonably high reflectance of 86.2% at the wavelength of 460 nm due to high transmittance of light through nano-scaled Pt (5 A) onto Ag layer. Ohmic behavior of contact metal, Pt/Ag/Ni/Au, to p-type GaN was achieved using surface treatments of p-type GaN prior to the deposition of contact metals and the specific contact resistance was observed with decreasing Pt thickness of 5 A, resulting in 1.5 x 10(-4) ohms cm2. Forward voltages of Pt (5 A)/Ag/Ni contact to p-type GaN showed 4.19 V with the current injection of 350 mA. Output voltages with various thickness of Pt showed the highest value at the smallest thickness of Pt due to its high transmittance of light onto Ag, leading to high reflectance. Our results propose that nano-scaled Pt/Ag/Ni could act as a promising contact metal to p-type GaN for improving the performance of VLEDs.     

溅射法低温生长纳米硅薄膜及异质结电池

用射频溅射法在P型硅衬底上生长了纳米硅薄膜,衬底温度控制在100℃左右,工作气体选用H2 Ar,氢气的分压控制在31%到73%,同时改变薄膜的沉积时间.用Raman、XRD、AFM、SEM 及椭偏仪对薄膜的特性进行了测定.XRD的测试结果表明,样品中存在一种微结构,不同于用PECVD方法生长的薄膜.椭偏仪的测试结果给出这种薄膜具有宽带隙.在室温条件下对异质结薄膜电池的I-V特性进行了测量.     

Investigation of contact resistance between silicon and silver paste made by screen-printing

In this paper, the front contact resistance of screen-printing crystalline silicon solar cells was investigated. By establishing a fine model for silicon and silver paste contact made by screen-printing and fire-through, the three kinds of conduction mechanism were analyzed quantitatively. According to the model, the comprehensive calculation of the contact resistance between silicon and silver paste sintering was made for the first time. The results calculated in this study suggest that the contact resistance of silicon and silver paste is about 1.20 mΩ for 125 mm × 125 mm single crystalline silicon solar cells, this approximately agrees with the measured values. By optimizing composition of silver paste according to our results, the silver consumption per watt can be reduced, and the efficiency of crystalline silicon solar cells can be further improved. The results laid the foundation for studying the screen-printed crystalline silicon solar cell front contact metallization system.     

The effects of Ge content on the microstructure and specific contact resistance of solid-state NiAuGe/ZrB2/Au ohmic contacts to n-InGaAs

The Ge thickness, x, of NiAuGe(5 nm/45 nm/xnm)/ZrB₂(50 nm)/Au(20 nm) ohmic contacts to n-lnGaAs was varied between 0 and 20 nm. The microstructures of these contacts, after annealing at 270°C, were investigated using transmission electron microscopy (TEM) and correlated with the respective specific contact resistances. In the absence of Ge, a Ni-Ga-As phase was formed at the metal-semiconductor interface and the specific contact resistance was high (0.63 mm). When thicknesses of x = 10 nm or x = 15 nm of Ge were added, Ni-Ge-As phases were observed, but they were replaced by AuGeAs and NiGe when x = 20 nm. The specific contact resistance was a minimum (0.11 mm) for this composition. Ge was clearly beneficial for ohmic-contact formation. The low-temperature I–V characteristics of the contact containing the largest amount of Ge (that is, x = 20 nm) indicated that electron tunnelling through degenerately-Ge-doped regions was not the dominant ohmic mechanism in these contacts.     

Effect of edge shading on the performance of silicon solar cell

Non-uniform solar radiation incident on solar cells is of significant concern in cell performance. It may result in non-uniform cell local heating and borders de-activation. Edge shading is a problem occurring in photovoltaic concentrator systems, due to tracking low accuracy and misalignment. An increase in both open circuit voltage and fill factor resulted from shading the edges of the cell by opaque frames of different widths. On the contrary, short circuit current density showed a drop by introducing such frames, illustrating that cell shading is responsible for the formation of series and parallel sub-cells operating at different operating conditions.     

Incremental compliance and resistance of contacts and contact clusters: Implications of the cross-property connection

Cross-property connection for a rough interface - that relates its normal incremental compliance to its resistance - allows one to transfer knowledge available in one of the two fields to the other one. This yields results that do not seem to have been discussed earlier. We consider (1) clusters of microcontacts, and implications of available results for resistances of clusters to their elastic compliances, and (2) implications of results for rigid indenters of various shapes pressed against elastic half-space for resistances of these shapes. We also extend the cross-property connection to the shear compliance of contact clusters.     

The energy levels in quantum wells formed at the contacts between cubic and hexagonal polytypes of silicon carbide

The energies of the ground (ε₀) and the first excited state (ε₁) in a quantum well (QW) are evaluated within the framework of the triangular QW model. It is shown that the ε₀ level in QWs at the contacts between cubic (3C) and hexagonal (NH, N = 2, 4, 6, 8) polytypes of silicon carbide (SiC) can be effectively controlled only by means of doping a wide-bandgap n-NH-SiC polytype with shallow donors.     

Highly conductive microcrystalline silicon carbide films deposited by the hot wire cell method and its application to amorphous silicon solar cells

Microcrystalline silicon carbide (μc-Si_1−xC_x) films were successfully deposited by the hot wire cell method using a gas mixture of SiH₄, H₂ and C₂H₂. It was confirmed by Fourier transform infrared and X-ray diffraction analyses that the films consisted of μc-Si grains embedded in a-Si_1−xC_x tissue. The p-type μc-Si_1−xC_x films were deposited using B₂H₆ as a doping gas. A dark conductivity of 0.2 S/cm and an activation energy of 0.067 eV were obtained. The p-type μc-Si_1−xC_x was used as a window layer of a-Si solar cells, in which the intrinsic layer was deposited by photo-chemical vapor deposition, and an initial conversion efficiency of 10.2% was obtained.     

Numerical simulation analysis of the in-cavity residual stress distribution of lignocellulosic (wood) polymer composites used in shallow thin-walled parts formed by the injection moulding process

In this paper, a numerical analysis of in-cavity residual stress formation in the thin-walled parts of injection-moulded parts is presented by considering the residual stresses produced during the post-filling stage. Injection moulding of shallow thin-walled parts with a thickness of 0.7 mm was performed using lignocellulosic polymer composites (PP + 50 wt% wood), and the parts have been systematically investigated using simulation results from Autodesk MoldFlow Insight® software. In-cavity residual stresses constitute the primary stage for analysis because of the need to control the quality of moulded parts to prevent problems with shrinkage and warpage. The analysis showed that the cooling times and packing times had a less significant effect; nevertheless, the optimal levels that are required to be used in the moulding process for thin-walled parts yielded better results. The in-cavity residual stress results show that the stress variation across the thickness exhibits a high tensile stress at the part surface, which changes to a low tensile stress peak value close to the surface, with the core region experiencing a parabolic tensile stress peak. The optimum parameter ranges for obtaining the minimum in-cavity residual stresses are as follows: a mould temperature of 40–45 °C, a cooling time of 20–30 s, a packing pressure of 0.85P_inject, and a packing time of 15–20 s.     

The materials characteristic and the efficiency degradation of solar cells from solar grade silicon from a metallurgical process route

The rising conventional energy prices have opened up the market for photovoltaic, but the lack of polycrystalline silicon from the chemical route restricts the growth of crystalline silicon solar cells. Recently there is a trend that produces solar cells by using the newly developed solar grade silicon feedstock from a metallurgical process route. In this article, the chemical components of solar grade silicon feedstock are analyzed. The single crystalline silicon solar cells from 100% solar grade silicon feedstock from a metallurgical process route are investigated. The outdoor performance of solar modules encapsulated by such cells is reported. The experimental evidence suggests that such solar cells can achieve the average efficiency higher than 14% on single crystalline silicon wafers. However, the efficiency degradation of solar cells under natural sunlight is significant, and the electrical uniformity of small cells diced from the whole cell is too bad. The metal impurities, oxygen, carbon, and their complexes influence the performance stabilization. The article proves that the module made by such cells has a big cell mismatch loss than normal cells made by electronic grade silicon, even if these cells come from the same sort. And the operating temperature of the cells of the modules is 15–22 °C higher than normal modules under the same conditions. The solar grade silicon feedstock from a metallurgical process route has to be improved farther in order to be used in photovoltaic industry.     

1.8 Influence of defects in the silicon wafer on the properties of silicon oxide films formed by reactive sputtering

It is well known that the properties of SiO₂ films, formed by reactive sputtering in oxygen, sensitively depend upon the details of chemical pretreatment of the Si surface and on the conditions of oxide formation. The influence of bulk defects in the silicon wafer has, however, not been adequately studied. Bombardment of the silicon surface with electrons and ions during the initial stages of the oxide formation is accompanied by extensive interaction with the surface defects, leading to changes in the charge of these defects and properties of the deposited film in general. If vacancies are the dominant defects, then a positive charge is created in the oxide film. If both vacancies and stacking faults are present, then both positive and negative charge can appear in the SiO₂ film.     

Effect of spontaneous polarization on the energy levels in quantum wells formed at the contact between cubic and noncubic silicon carbide polytypes

We have studied the effect of spontaneous polarization, which is inherent in noncubic silicon carbide (SiC) polytypes, on the characteristics of quantum wells (QWs) formed in the cubic region at the heterojunction. The analysis is performed for various QW models, including triangular, parabolic, and exponential. Conditions for the appearance of a two-dimensional electron gas at the interface are briefly discussed.