Screen-printed Silicon Solar Cells Research Articles

Using hydrofluoric acid to reduce the contact resistance of screen-printed silicon solar cells – Its recombination impact and a method to eliminate it

Using hydrofluoric acid (HF) to improve the contact resistance of a screen-printed silicon solar cell with a high sheet resistance emitter was found to concurrently reduce its pseudo-fill factor. By treating the cell in phosphoric acid, this impact was found to be significantly reduced or eliminated. In the solar cell presented in this work, the pseudo-fill factor reduced from 82.1% to 79.9% after treating the cell in HF, but increased to 81.8% after a subsequent phosphoric acid treatment. Similar effects were found in three other screen-printed solar cells that were fired at different peak furnace temperatures and belt speeds. It was also shown that the phosphoric acid treatment alone does not affect the pseudo-fill factor of a cell. HF treatment can now be used to reduce the contact resistance of a screen-printed silicon solar cell with a high sheet resistance emitter without suffering from a lower pseudo-fill factor so that the full benefit of a HF treatment on a screen-printed silicon solar cell can be realized. This combination of HF and phosphoric acid treatments can potentially be a useful failure analysis method for screen-printed silicon solar cells in the production line and the laboratory.

Exceeding the limit of plasmonic light trapping in textured screen-printed solar cells using Al nanoparticles and wrinkle-like graphene sheets

AbstractThe solar cell market is predominantly based on textured screen-printed solar cells. Due to parasitic absorption in nanostructures, using plasmonic processes to obtain an enhancement that exceeds 2.5% of the short-circuit photocurrent density is challenging. In this paper, a 7.2% enhancement in the photocurrent density can be achieved through the integration of plasmonic Al nanoparticles and wrinkle-like graphene sheets. For the first time, we experimentally achieve Al nanoparticle-enhanced solar cells. An innovative thermal evaporation method is proposed to fabricate low-coverage Al nanoparticle arrays on solar cells. Due to the ultraviolet (UV) plasmon resonance of Al nanoparticles, the performance enhancement of the solar cells is significantly greater than that from Ag nanoparticles. Subsequently, we deposit wrinkle-like graphene sheets over the Al nanoparticle-enhanced solar cells. Compared with planar graphene sheets, the bend carbon layer also exhibits a broadband light-trapping effect. Our results exceed the limit of plasmonic light trapping in textured screen-printed silicon solar cells.

The capacitance–voltage measurement of the screen-printed silicon solar cells with electrochemically etched nanostructured porous silicon antireflection coating

In the present study, capacitance–voltage measurement of screen-printed silicon solar cells with electrochemically etched nanostructured porous silicon structure has been investigated. The structural and optical properties of the freshly etched as well as thermally oxidized porous silicon surfaces was characterized by FTIR, Raman spectroscopy, photoluminescence and atomic force spectroscopy measurements. The values of the doping level, NA ≅ 8.4 × 1016 atoms cm−3 in the bulk Si substrate and the built-in potential, Vbi = 0.91 V was estimated by extrapolating the plot of the inverse-square capacitance–reverse bias voltage (I/C2–VR plot). The impedance spectra indicate a significant change in the frequency response of thermally oxidized porous silicon surface which is more distinct in the lower frequency range. The Raman spectra of the etched porous silicon has a sharp peak with a broadening and downshift of phonon energy which indicates that porous silicon is characterized by a nanoscale distribution of the pores and crystallites network and, thereby giving a detailed description on the ‘quantum confinement model’ for the light emission in the visible range from electrochemically etched porous silicon structure.

Inductively coupled plasma chemical vapour deposited AlOx/SiNy layer stacks for applications in high-efficiency industrial-type silicon solar cells

Passivated emitter and rear cells (PERC) are considered to be the next generation of industrial-type screen-printed silicon solar cells. Deposition methods for rear passivation layers have to meet both the high-throughput and low-cost requirements of the PV industry in combination with high-quality surface passivation properties. In this paper, we evaluate and optimise a novel deposition technique for AlOx passivation layers by applying an inductively coupled plasma (ICP) plasma-enhanced chemical vapour deposition (PECVD) process. The ICP AlOx deposition process enables high deposition rates up to 5nm/s as well as excellent surface recombination velocities below 10cm/s after firing. A fixed negative charge of −4×1012cm−2 is measured for ICP AlOx single layers with an interface state density of 11.0×1011eV−1cm−2 at midgap position. When applied to PERC solar cells the ICP AlOx layer is capped with a PECVD SiNy layer. We achieve independently confirmed conversion efficiencies of up to 20.1% for large-area (15.6×15.6cm2) PERC solar cells with screen-printed metal contacts and ICP AlOx/SiNy rear side passivation on standard boron-doped Czochralski-grown silicon wafers. The internal quantum efficiency reveals an effective rear surface recombination velocity Srear of (90±30)cm/s and an internal rear reflectance Rb of (91±1)% which demonstrates the excellent rear surface passivation of the ICP AlOx/SiNy layer stack.

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.

Development and Understanding of High-Efficiency Screen-Printed Concentrator Silicon Solar Cells

Concentrator cells have the potential to reduce the usage of semiconductor material while producing high efficiency and more power density in a cell. Silicon solar cells now provide a unique opportunity for low-cost concentrator systems that are suitable for low to medium (2-20X) concentration because cell technology and screen-printed contacts have improved considerably. In this paper, we report on the understanding and development of low-cost manufacturable screen-printed concentrator solar cells. Computer modeling was performed first to show that, under 10% metal coverage, it is possible for screen-printed cells to have series resistance that is low enough (<;0.29 Ω·cm2) to maintain high efficiency at low to medium concentrations. This was validated by design and fabrication of 40.56-cm2 screen-printed cells using an industrial feasible process that achieved 18.8% peak efficiency at ~6 suns and 17.2% efficiency at 20 suns. Dicing a 9.9-cm2 cell, which reduces the line resistance, raised the peak efficiency to 18.9% at 10 suns and 18.5% at 20 suns. Model calculations are performed to quantitatively establish the requirements for ~20% screen-printed 2-20X concentrator cells.

Effects of the Contact Resistivity Variations of the Screen-Printed Silicon Solar Cell

Problem statement: Unmatched combination of emitter and base contact resistance will influence the total performance of the solar cell. To optimize this combination, single crystalline silicon solar cell was analyzed using quasi-one-dimensional transport of electrons and holes in crystalline semiconductor, PC1D. Approach: Effects of the resistance of the emitter and base contact have been investigated with a view to find the best resistive combinations. A range of contact resistance of emitter and base were applied to PC1D software for evaluation. Results: The short circuit current Isc, the open circuit voltage Voc, the maximum power Pm and the fill factor are the observed parameters due to the variations of the resistance on the emitter and base contact. As the two variable factors that take into the account, while evaluating one factor, the other was set to constant value. It is found that as the contact resistance goes higher, the values of the parameters deceased. Conclusion/Recommendations: From the evaluation, the lowest emitter resistance that will give highest value of parameter in the selected ranged is 1 m
while for the base contact will be 15 m
. The overall investigation on single crystalline silicon solar cell base and emitter contact were done, gives potential parametric suggestion that may assist in the fabrication of high efficiency single crystalline silicon solar cells. A different range of resistance variation are suggested for future related investigation.

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, ρc (Ω-cm2) of the contact structure. The TPP measurement shows that contact structure has excellent ohmic properties with ρc = 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 ρc = 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.

Effect of the Thermal Properties of Frits on the ElectricalProperties of Screen-Printed Silicon Solar Cells

Effect of the Thermal Properties of Frits on the ElectricalProperties of Screen-Printed Silicon Solar Cells

Electrical Properties of Screen Printed Silicon Solar Cell Dependent upon Thermophysical Behavior of Glass Frits in Ag Pastes

Ag pastes consist of Ag powder, vehicles, glass frits and additives, such as bismuth, aluminum and zinc. Pb–Bi oxide glass frits have been used in Ag pastes of silicon solar cells. The glass frit plays important roles in Ag paste. However, there are few studies of glass frit in Ag paste for silicon solar cell. In this study, the screen-printed silicon solar cell was investigated with Pb–Bi oxide glass frits in Ag pastes. The electrical properties of fabricated silicon solar cell were correlated to the viscous flow of glass frits in the Ag pastes during the fast firing. The glass particles were spread perfectly over the Ag and SiNx substrates with decreasing contact angle at approximately 700 °C. Based on our results, the thermophysical properties of Pb–Bi oxide glass frits can be helpful for understanding the contact formation of silicon solar cell and improving the solar cell performance.

The ohmic properties and current–voltage characteristics of the screen-printed silicon solar cells with porous silicon surface

The ohmic properties and current–voltage characteristics of the silicon solar cells with fire-through screen-printed Ag metal contacts on the etched porous silicon surface have been investigated. The porous silicon layer of approximately 85 nm thickness was electrochemically etched on the n + -Si surface prior to the deposition of the front Ag grid contacts. Then, Ag contacts are formed by suitably applying a fire-through step at a temperature typically between 700–825 ∘C for 2 min in air ambient followed by annealing step at a lower temperature of 450 ∘C for 5 min in nitrogen ambient. The specific contact resistance ( ρ c ) is one of the important parameters in studying the interfacial properties of the contact metalization system. The value of ρ c of the contact structure is determined using the extrapolation technique based on the three-point probe (TPP) method. It shows that the best value of ρ c is about 8.54×10 −4 Ω cm 2 is obtained for the contact structure which is an order less than the typical value of the ρ c of 3 mΩ cm 2 reported for the sintered Ag metal contacts. The sintering of the Ag contacts on the etched porous silicon at temperature above 800 ∘C in air ambient has resulted in considerable reduction in the value of ρ c of the contact structure. This improvement in ρ c is attributed to the improvement in the barrier properties of the contact structure upon sintering at high temperature. The value of ρ c follows a linear relationship with the surface doping level, N S > 10 19 atoms/cm 3 and improves with an increase in the sintering temperature. The ohmic losses of the fire through Ag contacts are assessed using the power loss algorithm and have shown that relative ohmic losses of the contact structure have improved with higher firing temperatures. These results suggest that fire-through Ag metal contacts on the porous silicon have good ohmic quality and can be expected to provide low ohmic losses.

Electron microscopy study of front-side Ag contact in crystalline Si solar cells

Efficiency of industrial screen-printed silicon solar cells depends critically on their firing conditions. We investigated the microstructure of the Ag/emitter contact of solar cells fired at temperatures from below to above optimal by electron microscopy. We found that the interfacial microstructure evolved from one richly decorated with nanometer-size Ag colloids into one with many Ag crystallites grown onto the emitter surface. Selective etching revealed a dearth of Ag crystallites in optimally fired cells. Our microstructural observations suggest that a tunneling mechanism (likely assisted by nano-Ag colloids) is responsible for current extraction in these cells fired with optimal conditions.

SEM and specific contact resistance analysis of screen-printed Ag contacts formed by fire-through process on the shallow emitters of silicon solar cell

The SEM and specific contact resistance measurements of the Ag metal contact formed by applying a fire-through process on the shallow emitter region of the silicon solar cell have been investigated. The metal contact consists of screen-printed Ag paste patterned on the silicon nitride (Si3N4) deposited over the n+-Si emitter region of the solar cell. The sintering step consists of a rapid firing step at 800 °C or above in air ambient. This is followed by an annealing step at 450 °C in nitrogen ambient. It enables to drive the Ag metal paste onto the Si3N4 layer and facilitates the formation of an Ag metal/p-Si contact structure. It serves as the top metallization for the screen-printed silicon solar cell. The SEM measurement shows that sintering of the Ag metal paste at 800 °C or above causes the Ag metal to firmly coalesce with the underlying n+-Si surface. A thin layer of conductive glassy layer is also presents at the interface of the Ag metal and n+-Si surface. The electrical quality of the contact structure was characterized by measuring the specific contact resistance, ρc (in Ω-cm2) using the iteration technique based on the power loss calculation for the solar cell. It shows that best value of ρc = 2.53 × 10−5 Ω-cm2 is estimated for the Ag metal contact sintered at temperature above 800 °C. This value of ρc is two orders of magnitude lower than the typical value of ρc = 3 × 10−3 Ω-cm2 reported previously for the Ag contacts of the solar cell. Such low value of ρc for the Ag metal contacts indicates that fire-through process results in excellent ohmic properties. The plot of the ρc versus impurity doping level (Ns) shows that measured value of the ρc follows a linear relationship with the Ns as predicted by the theory for the heavily doped semiconductor surface. Hence, carrier injection across the Schottky barrier height is quite appropriate to explain the observed ohmic properties of the Ag metal contacts on the n+-Si surface of the silicon solar cell.

Spatially resolved series resistance of silicon solar cells obtained from luminescence imaging

The fast determination of the spatially resolved series resistance of silicon solar cells from luminescence images is demonstrated. Strong lateral variation of the series resistance determined from luminescence images taken on an industrial screen printed silicon solar cell is confirmed qualitatively by a Corescan measurement and quantitatively by comparison with the total series resistance obtained from the terminal characteristics of the cell. Compared to existing techniques that measure the spatially resolved series resistance, luminescence imaging has the advantage that it is nondestructive and orders of magnitude faster.

Metal Surface Treatment Effects on Screen Printed Silicon Solar Cells

High series resistance due to the presence of glass frit is one of the major problems for screen printed silicon solar cells. Cells having electrical parameters below the prescribed values are usually rejected during solar module fabrication. Therefore, it is highly desirable to improve the electrical parameters of the silicon solar cells and thereby to increase the overall production yield. It was observed that, the performance of low quality mono-crystalline silicon solar cells made by standard screen printing technology could be improved remarkably by novel surface treatment. We have chemically treated the surface using sodium hydroxide (NaOH) and silver nitrate (<TEX>$AgNO_3$</TEX>) solutions. NaOH treatment helps to reduce the series resistance by decreasing the presence of excess glass frit on the top silver grid contact. The <TEX>$AgNO_3$</TEX> treatment is used to reduce the series resistance comes from the deposition of silver on the grids by filling the holes present (if any) within the grid pattern.

Double porous silicon layer on multi-crystalline Si for photovoltaic application

Double porous silicon (d-PS) layers formed by acid chemical etching on a top surface of n +/p multi-crystalline silicon solar cells were investigated with the aim to improve the performance of standard screen-printed silicon solar cells. First a macro-porous layer is formed on mc-Si. The role of this layer is texturization of surface. Next, the cells have been manufactured using standard technology based on screen-printing metallization. Finally, a second mezo-porous layer in n + emitter of cell has been produced. The role of this PS layer is to serve as an antireflection coating. In this way, we have obtained d-PS layers on these solar cells. The paper present observation of d-PS microstructure with SEM as well as measurements of its effective reflectance at the level of 2.5% in the 400–1000 nm length wave range. The efficiency of the solar cells with this structure is about 12%.

Optimization of front metal contact firing scheme to achieve high fill factors on screen printed silicon solar cells

For widespread implementation of silicon PV, the module cost must be reduced by a factor of 2–4. This can be accomplished by lowering the cost of solar cell materials and processing without sacrificing cell efficiency. A combination of high throughput belt line processing, SP contacts and mc-Si material offers an opportunity for significant cost reduction. However, most cell manufacturers who use the above combination are only able to achieve fill factors in the range of 0.68–0.75 with cell efficiencies in the range of 10–14%. Thus throughput gains are attained at the expense of device performance. In addition, there is considerable scatter in the fill factor of the SP cells in the literature with no clear guidelines for achieving high fill factors. This paper shows that proper understanding of loss mechanisms and optimization of SP paste and firing cycle can lead to fill factors approaching 0.77 and 0.79 on mc-Si and single crystal silicon, respectively, on a 45 Ω/□ rapidly formed belt line emitter with a shallow junction depth of ∼0.27 μm. It was observed that deep and shallow emitters on mc-Si could lead to the same values of fill factors ∼0.77 when the proper combination of paste and firing cycle is applied. The peak firing temperature for deep emitter is higher than the shallow ones with superior value of junction leakage current.

A novel method for the determination of the front contact resistance in large area screen printed silicon solar cells

The contact resistance at the interface of the semiconductor (Si)/metal (Ag) contact in a solar cell is generally small because it is made on a heavily doped silicon surface. In the case of large area terrestrial silicon solar cells contact is made by screen printing silver paste which has pores even after baking and sintering. Therefore the entire area is not active and the contact resistance is not entirely negligible. This paper describes the use of a simple method for the measurement of contact resistance of the front grid in a large area silicon solar cell based on the application of three-point probes using the existing fixed equidistant grid fingers of the solar cell. In addition, the value of the sheet resistivity is obtained from the measurement and is compared with that measured by the standard four-point probe method on different samples. The power losses due to current travelling through different resistive elements in the solar cell were obtained by using the standard formulae. The calculations show that power loss due to contact resistance is a small part of the total power loss. The total power loss agrees with that determined from the illuminated I-V characteristics of the solar cell.

The Effect of Low and High Temperature Anneals on the Hydrogen Content and Passivation of Si Surface Coated with SiO2 and SiN Films

A detailed comparison of the passivation quality and its dependence on the low and high temperature anneals is presented for various promising Si surface passivation schemes. The passivation schemes investigated in this study include: conventional furnace oxide (CFO), rapid thermal oxide (RTO), belt line oxide (BLO), plasma deposited oxide (PDO), SiN deposited by plasma enhanced chemical vapor deposition (PECVD), CFO/SiN, RTO/SiN, BLO/SiN, PDO/SiN, and RTO/PDO. Passivated low resistivity (1 Ω cm) p‐type silicon samples were subjected to three annealing treatments: (a) 400°C in forming gas (FGA), (b) 730°C in air, and (c) 850°C in air, to simulate heat‐treatments, which are typically used for contact anneal, front ohmic contacts, and back surface field formation, repectively, for screen printed silicon solar cells. It is found that the passivation quality of PDO, SiN, RTO, and CFO single layers improves significantly after the 400°C FGA and 730°C thermal cycles with RTO resulting in the lowest surface recombination velocities (Seff) of 154 and 405 cm/s, respectively. Silicon wafers coated with belt oxide (BLO and BLO/SiN) did not show any improvement in Seff, which remained at 5000 cm/s due to the inferior quality of BLO formed in compressed air. The oxide/nitride stack passivation is found to be far superior to single‐layer passivation resulting in Seff of 70 cm/s for the RTO/SiN scheme after the two high temperature anneals (850 and 730°C). The hydrogen concentration measurements by Fourier transform infrared spectroscopy show a greater decrease in the hydrogen content in the annealed RTO/SiN stack compared to the as‐deposited SiN single layer after the 730 and 850°C anneals. A combination of reduced hydrogen content and very low Seff in the RTO/SiN stack suggest that the release of hydrogen from SiN during the anneal further passivates the RTO/Si interface underneath. © 1999 The Electrochemical Society. All rights reserved.

Self-aligned selective-emitter plasma-etchback and passivation process for screen-printed silicon solar cells

We studied whether plasma-etching techniques can use standard screen-printed gridlines as etch masks to form self-aligned, patterned-emitter profiles on multicrystalline-silicon (mc-Si) cells from Solarex. We conducted an investigation of plasma deposition and etching processes on full-size mc-Si cells processed in commercial production lines, so that any improvements obtained would be immediately relevant to the PV industry. This investigation determined that reactive ion etching (RIE) is compatible with using standard, commercial, screen-printed gridlines as etch masks to form self-aligned, selectively doped emitter profiles. This process results in reduced gridline contact resistance when followed by plasma-enhanced chemical vapor deposition (PECVD) treatments, an undamaged emitter surface easily passivated by plasma-nitride, and a less heavily doped emitter between gridlines for reduced emitter recombination. This allows for heavier doping beneath the gridlines for even lower contact resistance, reduced contact recombination, and better bulk defect gettering. Our initial results found a statistically significant improvement of about half an absolute percentage point in cell efficiency when the self-aligned emitter etchback was combined with a PECVD-nitride surface passivation treatment. Some additional improvement in bulk diffusion length was observed when a hydrogen passivation treatment was used in the process. We attempted to gain additional benefits from using an extra-heavy phosphorus emitter diffusion before the gridlines were deposited. However, this required a higher plasma-etch power to etch back the deeper diffusion and keep the etch time reasonably short. The higher power etch may have damaged the surface and the gridlines so that improvement due to surface passivation and reduced gridline contact resistance was inhibited.