Industrial Solar Cell Production Research Articles

Electrical Performance, Loss Analysis, and Efficiency Potential of Industrial‐Type PERC, TOPCon, and SHJ Solar Cells: A Comparative Study

ABSTRACTCurrently, the efficiency of p‐type passivated emitter and rear contact (PERC) cells has been growing at an absolute efficiency of 0.5% per year and has reached 23%–23.5% in mass production while getting closer to its theoretical efficiency limit. n‐Type tunnel oxide passivated contact (TOPCon) and silicon heterojunction (SHJ) cells with their superior “passivating selective contacts” technology were the most interesting photovoltaics (PV) technology in the industry. The effect of different passivated contact layers with respect to their influence on the J0, J0,metal, ρc, and the carrier selectivity (S10) and the loss analysis and efficiency potential of industrial‐type PERC, TOPCon, and SHJ solar cells were studied and compared. The results showed that TOPCon structure with a high passivation performance and good optical performance is more suitable for bifacial solar cell and the highest theoretical limiting efficiency with metal shading on the n‐type Si wafer (ηb,e,h,m,max) can be achieved to 27.62%. Although SHJ structure with the highest passivation performance but the worst optical performance owing to the parasitic absorption of a‐Si:H layer and high contact resistivity, the value of ηb,e,h,m,max is 0.7% lower than that of TOPCon solar cells. PERC structure has superior optical performance than SHJ structure, but due to poor passivation performance, the ηb,e,h,m,max is only 26.42%. The next‐generation products may be heterojunction back‐contact (HBC) and TOPCon back‐contact (TBC) cells with high ηb,e,h,m,max of 28.12% and 27.99%, respectively. Exploiting a perfect passivation of the noncontact area, the wide process window and low cost are required and transferring these new concepts to industrial solar cell production will be the next major challenge.

Charge pumping in solar cell structure

Modern understanding of the material science of semiconductor silicon allowed the authors to propose a new concept of the so-called “charge pumping” in the structures of photovoltaic converters or solar cells. This paper presents theoretical estimates of the rate of separation and collection of light-generated charge carriers in the structures of conventional silicon solar cells and charge-pumped solar cells. Relatively cheaper so-called “solar silicon” of p-type conductivity is typically used in the industrial production of solar cells. This type of silicon is particularly prone to the formation of thermodonor centers. Partial or, at higher temperatures (about 400 °C), even complete overcompensation of the hole type of conductivity in the base region may occur as a result of prolonged heating. This paper presents an original model describing the local formation of n+ regions in the solar cell structure by the so-called “local photon annealing”. These regions were named “charge pumps”. Experimental data on the formation of n+ regions as a result of Li diffusion are reported as an experimental confirmation of the theoretical estimations made in this work. Comparative volt-ampere characteristics of experimental charge-pumped photovoltaic converters and conventional solar cells are presented, showing an up to 30% increase in the short-circuit current Js.c for the experimental structures under standard illumination (AM1.5). The proposed technological aspects of charge-pumped photovoltaic converter fabrication deliver a cheap process and can be implemented in the industrial production of solar cells with little effort.

Effect of nickel doping on the spectral sensitivity of silicon solar cells

In the modern industrial production of solar cells (SC), there is a growing trend to utilize “solar silicon” as the base material due to its cost-effectiveness. However, solar silicon possesses a drawback - it has a shorter lifetime of minority charge carriers (MCC), making it challenging to achieve high efficiency in solar cells. To address this limitation and improve the efficiency of solar cells based on “solar silicon,” two key objectives need to be met. Firstly, it is essential to increase the lifetime of photogenerated charge carriers. Secondly, there is a need to minimize both optical and electrical energy losses. To achieve an increase in the lifetime of minority charge carriers in SCs, a process called gettering can be employed. This process involves utilizing clusters of nickel atoms to trap uncontrolled impurity atoms. The paper presents the results of a study on the additional doping of silicon solar cells with nickel atoms, for factors affecting the long-wave and short-wave efficiency. Nickel doping has been shown to increase the efficiency of solar cells. It is determined that in the visible region of the spectrum the spectral sensitivity of a silicon solar cell doped with nickel is higher up to 25÷35% due to a decrease in surface recombination. Technological recommendations for nickel doping of single-crystal silicon solar cells are proposed to be combined without significant changes with the standard technological process for manufacturing solar cells.

Electrochemical oxidation pre-treatment for wet texturing of monocrystalline silicon solar cells

To improve the textured structure uniformity of monocrystalline silicon solar cells, an electrochemical anodic oxidation pretreatment technology was implemented to the silicon surface before the alkali texturing process. The effects of pretreatment on the textured structure were investigated at different oxidation conditions (voltage and duration) in three typical aqueous electrolytes with different pH values. The structure and thickness of the silicon oxide layer on the silicon wafer surface were analyzed by X-ray photoelectron spectroscopy. It was found that the uniform oxide layer of 1.5–2 nm formed on the Si surface can promote the uniform growth of pyramidal nuclei in the initial texturing stage, while the excessive thicker oxide layer will reduce their density and uniformity. The average pyramid sizes were 1.4 ± 0.7 μm and 1.6 ± 1.7 μm with and without electrochemical oxidation pretreatment, respectively. The distribution of pyramid size became more uniform because the fraction of extremely large pyramids was greatly reduced. Compared with the traditional texturing process, the average reflectivity of the textured surface decreased from 13.1 % to 10.4 %. This technique is expected to be a suitable substitute for the H2O2 pre-cleaning process in the Si texturization for the industrial production of solar cells.

Over 30% efficiency bifacial 4-terminal perovskite-heterojunction silicon tandem solar cells with spectral albedo

We developed and designed a bifacial four-terminal perovskite (PVK)/crystalline silicon (c-Si) heterojunction (HJ) tandem solar cell configuration albedo reflection in which the c-Si HJ bottom sub-cell absorbs the solar spectrum from both the front and rear sides (reflected light from the background such as green grass, white sand, red brick, roofing shingle, snow, etc.). Using the albedo reflection and the subsequent short-circuit current density, the conversion efficiency of the PVK-filtered c-Si HJ bottom sub-cell was improved regardless of the PVK top sub-cell properties. This approach achieved a conversion efficiency exceeding 30%, which is higher than those of both the top and bottom sub-cells. Notably, this efficiency is also greater than the Schockley–Quiesser limit of the c-Si solar cell (approximately 29.43%). The proposed approach has the potential to lower industrial solar cell production costs in the near future.

Revealing the perovskite formation kinetics during chemical vapour deposition

High control chemical vapour deposition of (Cs,FA)PbI3 perovskite provides insights into CVD kinetics and perovskite phase evolution.

Analysis of different conductive pastes to form the contact with the boron back surface field in PERT silicon solar cells

The industrial production of solar cells of the PERC family is growing, because the potential to increase the efficiency due to the passivation of the rear face. The PERT solar cell is a cost-effective structure of the PERC configurations. The goal of this paper is to analyze the influence in the electrical parameters of different metal pastes used to form the contact with the boron back surface field of PERT solar cells passivated with silicon dioxide on both sides and developed with a cost-effective process. The boron doped BSF and phosphorus emitter were carried out with reduction of steps. Solar cells were processed with three different conductive pastes: (1) an aluminum paste (PV381), (2) a silver/aluminum paste (PV3N1) and (3) a silver paste (PV51G), with different viscosity and solids content. The pastes were produced by DuPont. The PV381 and PV3N1 pastes produced solar cells with the efficiency of 16.2% and 15.9%, respectively. The higher open circuit voltage was achieved with the aluminum paste, indicating that this paste is more effective to produce the selective back surface field. The PV51G paste is not suitable to form the rear contact.

Flame-made ultra-porous TiO2 layers for perovskite solar cells

We report methyl ammonium lead iodide (MAPbI3) solar cells with an ultra-porous TiO2 electron transport layer fabricated using sequential flame aerosol and atomic layer depositions of porous and compact TiO2 layers. Flame aerosol pyrolysis allows rapid deposition of nanostructured and ultra-porous TiO2 layers that could be easily scaled-up for high-throughput low-cost industrial solar cell production. An efficiency of 13.7% was achieved with a flame-made nanostructured and ultra-porous TiO2 electrode that was coated with a compact 2 nm TiO2 layer. This demonstrates that MAPbI3 solar cells with a flame-made porous TiO2 layer can have a comparable efficiency to that of the control MAPbI3 solar cell with the well-established spin-coated porous TiO2 layer. The combination of flame aerosol and atomic layer deposition provides precise control of the TiO2 porosity. Notably, the porosity of the as-deposited flame-made TiO2 layers was 97% which was then fine-tuned down to 87%, 56% and 35% by varying the thickness of the subsequent compact TiO2 coating step. The effects of the decrease in porosity on the device performance are discussed. It is also shown that MAPbI3 easily infiltrates into the flame-made porous TiO2 nanostructure thanks to their high porosity and large pore size.

Fast Regeneration Processes to Avoid Light-Induced Degradation in Multicrystalline Silicon Solar Cells

Light-induced degradation (LID) of multicrystalline silicon (mc-Si) solar cell performance has been reported to be surprisingly strong for conditions relevant under field operation. In paticular, solar cells with dielectrically passivated rear sides such as passivated emitter and rear cells are affected by this LID effect that can cause a loss of more than 10%rel in cell efficiency. With the root cause for the observed degradation being unknown to date, the underlying defect, however, has also been reported to be permanently deactivated under the same conditions at even longer time scales. However, a severe power loss due to the mc-Si specific LID is observed before the cells recover due to the long time scales of the regeneration under these conditions. Hence, regeneration on short time scales similar to the fast regeneration processes being reported for the boron–oxygen defect within p-type Czochralski-grown silicon is highly desirable also for p-type mc-Si, especially since mc-Si currently dominates industrial solar cell production. Within this work, partial regeneration of the defect causing the mc-Si specific LID is shown to be possible within less than 30 s, reducing the impact of LID by up to 60%, which leads to a significantly increased performance of the regenerated mc-Si solar cells.

Impact of the firing temperature profile on light induced degradation of multicrystalline silicon

Light‐ and elevated temperature‐induced degradation in multicrystalline silicon can reduce the efficiency of solar cells significantly. In this work, the influence of the firing process and its temperature profile on the degradation behaviour of neighbouring mc‐Si wafers is analysed. Five profiles with measured high peak temperatures ≥800 °C and varying heating and cooling ramps are examined. With spatially resolved and lifetime calibrated photoluminescence images, normalized defect concentrations N*t are calculated to determine the degradation intensity. Wafers that underwent a fast firing process typical for industrial solar cell production show a significantly stronger degradation than samples that were subjected to the same peak temperature but with slower heating and cooling rates. A spatially resolved analysis of the carrier lifetime in the whole wafer shows that the degradation begins in low lifetime areas around dislocation clusters, spreading into good grains after several hours. By the use of optimized ramp‐up and/or ramp‐down rates during the firing even at very high peak temperatures, light and elevated temperature induced degradation can be suppressed. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Understanding the Light-induced Lifetime Degradation and Regeneration in Multicrystalline Silicon

In this contribution, we focus on improving the fundamental understanding of the carrier lifetime degradation and regeneration observed in block-cast multicrystalline silicon (mc-Si) wafers under illumination at elevated temperature. We observe a pronounced degradation in lifetime at 1 sun light intensity and 75̊C after rapid thermal annealing (RTA) in a belt-firing furnace at a set peak temperature of 900̊C. However, almost no lifetime instability is detected in mc-Si wafers which are fired at a peak temperature of only 650̊C, clearly showing that the firing step is triggering the degradation effect. Lifetime spectroscopy reveals that the light-induced recombination centre is a deep-level centre with an asymmetric electron-to-hole capture cross section ratio of 20±7. After completion of the degradation, the lifetime is observed to recover and finally reaches even higher carrier lifetimes compared to the initial state. While the lifetime degradation is found to be homogeneous, the regeneration shows an inhomogeneous behaviour, which starts locally and spreads later laterally throughout the sample. Furthermore, the regeneration process is extremely slow with time constants of several hundred hours. We demonstrate, however, that by increasing the regeneration temperature, it is possible to significantly speed up the regeneration process so that it might become compatible with industrial solar cell production. To explain the observed lifetime evolution, we propose a defect model, where metal precipitates in the mc-Si bulk dissolve during the RTA treatment and the mobile metal atoms bind to a homogeneously distributed impurity. Restructuring and subsequent dissociation of this defect complex is assumed to cause the lifetime degradation, whereas a subsequent diffusion of the mobile species to the sample surfaces and crystallographic defects explains the regeneration.

Lifetime degradation and regeneration in multicrystalline silicon under illumination at elevated temperature

We examine the carrier lifetime evolution of block-cast multicrystalline silicon (mc-Si) wafers under illumination (100 mW/cm2) at elevated temperature (75°C). Samples are treated with different process steps typically applied in industrial solar cell production. We observe a pronounced degradation in lifetime after rapid thermal annealing (RTA) at 900°C. However, we detect only a weak lifetime instability in mc-Si wafers which are RTA-treated at 650°C. After completion of the degradation, the lifetime is observed to recover and finally reaches carrier lifetimes comparable to the initial state. To explain the observed lifetime evolution, we suggest a defect model, where metal precipitates in the mc-Si bulk dissolve during the RTA treatment.

Potential Gain in Multicrystalline Silicon Solar Cell Efficiency by n-Type Doping

This study aims for a quantitative investigation of the material limitations and the efficiency potential of an entire multicrystalline (mc) n-type silicon block in comparison with an mc p-type block of the same purity level in order to predict the potential of mc n-type silicon for the industrial production of solar cells. Therefore, two standard mc silicon blocks were crystallized under identical conditions (same high purity feedstock, crucible system, and temperature profiles), only differing in their type of doping. The material quality of wafers along the whole block height is analyzed after different solar cell process steps by photoluminescence imaging of the diffusion length. The bulk recombination related efficiency losses are assessed by an “efficiency limiting bulk recombination analysis (ELBA),” combining injection dependent lifetime images with PC1D cell simulations. The influence of the base resistivity variation along the block is considered in the PC1D cell simulations and backed up by Sentaurus Device simulations. This analysis predicts a significantly higher material-related efficiency potential after typical solar cell processes along the whole block height for mc n-type silicon compared with mc p-type silicon. In addition, the efficiency potential for mc n-type silicon depends less on block position.

The Research on Solar Cell of a Novel Ribbon Silicon Material

Ribbon silicon material is specially designed for solar cell wafers. In this paper, a novel ribbon silicon material “dipping method” has been designed in order to lower the cost of solar cell. The principle and procedure of dipping method were described. In addition, the diffusion of impurities in the silicon wafer and its influence on the efficiency of solar cells were investigated. The photovoltaic performance of polycrystalline silicon solar cells which were based on the metallurgical grade silicon substrate with the thickness of 600μm, was simulated by AMPS1-D software. And some import parameters were obtained including I-V characteristic, 17.004% conversion efficiency. This artic is provided theoretical support to the industrial production of solar cells by dipping method, and it will open a new road to production low cost solar cell.

Sulfurization of near‐stoichiometric CuIn precursor and the influence of low‐temperature slopes on morphology and structural quality

AbstractPolycrystalline CuInS2 (CIS) films are prepared by sulfurization of CuIn metallic precursors with Cu/In = 1.2 on Mo substrates. Variation of the rapid thermal processing (RTP) parameters adjusted to the low copper surplus yielded near stoichiometric CIS films with considerably less CuSx precipitation at the surface. A start temperature and low temperature slopes were introduced to guarantee crystal quality and large grain size. XPS/UPS, X‐ray diffraction measurement, and scanning electron microscopy data show the distinct properties of the chalcopyrite phase and its morphological structure in comparison to the standard sulfurization process. A considerable improvement has been achieved by variation of the temperature profiles. The proposed process modules obtain near stoichiometric CIS films for industrial solar cell production.

Spectrometric analysis of process etching solutions of the photovoltaic industry—Determination of HNO3, HF, and H2SiF6 using high-resolution continuum source absorption spectrometry of diatomic molecules and atoms

The surface of raw multicrystalline silicon wafers is treated with HF–HNO3 mixtures in order to remove the saw damage and to obtain a well-like structured surface of low reflectivity, the so-called texture. The industrial production of solar cells requires a consistent level of texturization for tens of thousands of wafers. Therefore, knowing the actual composition of the etch bath is a key element in process control in order to maintain a certain etch rate through replenishment of the consumed acids.The present paper describes a novel approach to quantify nitric acid (HNO3), hydrofluoric acid (HF), and hexafluosilicic acid (H2SiF6) using a high-resolution continuum source graphite furnace absorption spectrometer. The concentrations of Si (via Si atom absorption at the wavelength 251.611nm, m0,Si=130pg), of nitrate (via molecular absorption of NO at the wavelength 214.803nm, m0,NO3−=20 ng), and of total fluoride (via molecular absorption of AlF at the wavelength 227.46nm, m0,F=13 pg) were measured against aqueous standard solutions. The concentrations of H2SiF6 and HNO3 are directly obtained from the measurements. The HF concentration is calculated from the difference between the total fluoride content, and the amount of fluoride bound as H2SiF6. H2SiF6 and HNO3 can be determined with a relative uncertainty of less than 5% and recoveries of 97–103% and 96–105%, respectively. With regards to HF, acceptable results in terms of recovery and uncertainty are obtained for HF concentrations that are typical for the photovoltaic industry. The presented procedure has the unique advantage that the concentration of both, acids and metal impurities in etch solutions, can be routinely determined by a single analytical instrument.

Advances in the Surface Passivation of Silicon Solar Cells

The surface passivation properties of aluminium oxide (Al2O3) on crystalline Si are compared with the traditional passivation system of silicon nitride (SiNx). It is shown that Al2O3 has fundamental advantages over SiNx when applied to the rear of p-type silicon solar cells as well as to the p+ emitter of n-type silicon solar cells. Special emphasis is paid to the transfer of Al2O3 into industrial solar cell production. We compare different Al2O3 deposition techniques suitable for mass production such as ultrafast spatial atomic layer deposition, inline plasma-enhanced chemical vapour deposition and reactive sputtering. Finally, we review the most recent cell results with Al2O3 passivation and give a brief outlook on the future prospects of Al2O3 in silicon solar cell production.

Low-cost texturization of large-area crystalline silicon solar cells using hydrazine mono-hydrate for industrial use

Reduction in optical losses in mono-crystalline silicon solar cells by surface texturing is one of the important issues of modern silicon photovoltaics. In order to achieve good uniformity in pyramidal structures on the silicon surface, a mixture of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and isopropyl alcohol (IPA) is generally used during texturization of mono-crystalline silicon solar cell. However, due to the high cost of IPA, there is always a search for alternate chemical which plays the same role as IPA during texturization for industrial solar cell production. For a better texturization, the interfacial energy between silicon and ionized electrolyte of chemical solution should be reduced to achieve sufficient wettability of the silicon surface, which will enhance the pyramid nucleation. In this work, we have investigated the role of hydrazine mono-hydrate as a surface-active additive, which supplies OH − ions after its dissociation. Our process cuts down the IPA consumption during texturing without any loss in uniformity of textured pyramids. We are the first to report the novel idea to add hydrazine mono-hydrate in NaOH solution for texturing mono-crystalline silicon surface to fabricate solar cells with more than 85% yield in the efficiency range of 14.5–15.3%.

Low-cost multicrystalline back-contact silicon solar cells with screen printed metallization

Abstract Adaptation to market requirements is a permanent challenge in industrial solar-cell production. Both increase of cell efficiency as well as lowering costs is demanded. Back-contacted solar cells offer multiple advantages in terms of reducing module assembling costs and enhanced cell efficiency. The investigated emitter-wrap-through (EWT) design [1] has a collecting emitter on front and rear side. These emitter areas are electrically connected by small holes. Due to the double-sided collecting junction, this cell design is favourable for materials with a low-minority charge carrier diffusion length leading to a higher short circuit current density. Untill now most investigations on EWT solar cells were performed on Cz or even FZ silicon. This was justified as long as different processing techniques had to be developed and compared. But as an industrially applicable process sequence has recently been developed [2] , the advantages of the EWT concept compared to conventionally processed cells have to be shown on multicrystalline material. In the following, a manufacturing process of EWT solar cells is presented which is especially adapted to the requirements of multicrystalline silicon. Effective surface texturization was reached by mechanical V-texturization and bulk passivation by a hydrogen plasma treatment. The efficiency of the best solar cells within this process reached 14.2% which is the highest efficiency reported so far for mc-Si 10×10 cm 2 EWT solar cells [3] .