Investigation of texturization for crystalline silicon solar cells with sodium carbonate solutions

We investigate a new texturization technique for crystalline silicon solar cells with sodium carbonate (Na₂CO₃) solutions. We show the dependence of the hemispherical surface reflectance on solution temperature, the etching time and the Na₂CO₃ concentration. Furthermore, we investigate what element in Na₂CO₃ solution influences the texturing for reducing the texturing time. As a result of experiments, we find it possible to get low reflectance in a shorter texturing time by the addition of NaHCO₃. The size of texture becomes smaller by the addition of NaHCO₃ but the etching rate does not change. We conclude carbonic ion and/or its compound seems to play an important role as the initiator of pyramidal structure. This texturing method is cost effective because there is no need of expensive IPA, and the surface reflectance is reduced sufficiently in a short time. This method is promising for a large-scale production of crystalline silicon solar cells.     

Thin crystalline silicon solar cells

A 50 μm thin layer of high quality crystalline silicon together with efficient light trapping and well passivated surfaces is in principle all that is required to achieve stable solar cell efficiencies in the 20% range. In the present work, we propose to obtain these layers by directly cutting 50 μm thin wafers from an ingot with novel cutting techniques. This development is discussed in the frame of a defect tolerant mass production scenario and aims at obtaining twice the amount of wafers as compared to present wire/slurry technology. The ability to process such mechanically flexible wafers into solar cells with standard laboratory equipment is experimentally verified.     

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%.     

Role of hydrazine monohydrate during texturization of large-area crystalline silicon solar cell fabrication

Reduction of optical losses in monocrystalline silicon solar cells by surface texturing is one of the important issues of modern silicon photovoltaic. For texturization during commercial monocrystalline silicon solar cell fabrication, a mixture of NaOH or KOH and isopropyl alcohol (IPA) is generally used in order to achieve good uniformity of pyramidal structure on the silicon surface. The interfacial energy between silicon and electrolyte should be reduced in order to achieve sufficient wettability for the silicon surface which in turn will enhance the pyramid nucleation. In this work, we have investigated the role of hydrazine monohydrate as a surface-active additive, which supplies OH⁻ ions after dissociation. This cuts down the IPA consumption during texturing without any loss of uniformity of textured pyramid. We are probably the first group to report such a novel idea of using hydrazine monohydrate addition in NaOH solution for texturization of solar cell. We were able to fabricate monocrystalline silicon solar cells with more than 85% yield in the range of 14–15% efficiency.     

Industrial manufacturing of semitransparent crystalline silicon POWER solar cells

This paper gives an extract of the state of the art of the manufacturing of semitransparent crystalline silicon POWER solar cells in an industrial environment. A short introduction of the POWER devices concept (see Fig. 1) will be given followed by an insight in the applied production process. Finally, examples effecting the efficiency distribution in the cell production and their solutions are given. It is believed that the lessons we learned in optimising the manufacturing process and production line of transparent POWER solar cells can be helpful for the increasing activities in the direction of thin wafers as well as novel solar cell devices.     

Experimental and theoretical radiation damage studies on crystalline silicon solar cells

An experimental facility was developed to asses in situ the degradation of crystalline silicon solar cells, fabricated by the Solar Energy Group of the National Atomic Energy Commission (CNEA), by measuring the current–voltage characteristic curve. The cells were irradiated with 10 MeV protons and fluences between 10⁸ and 10¹³ p/cm², using an external beam of the linear tandem accelerator TANDAR, at CAC-CNEA. Furthermore, theoretical simulations were performed to establish the relation between the variation of the electrical parameters and the degradation of the lifetime of minority carriers in the base. The damage constant for 10 MeV proton irradiated silicon solar cells of n⁺–p–p⁺ structure and 1 Ω cm base resistivity was determined. Finally, a proposal of a new model of radiation damage for silicon solar cells is discussed.     

Investigation of texturization for monocrystalline silicon solar cells with different kinds of alkaline

In this paper, monocrystalline silicon was textured with different kind of etchants for solar cells, respectively. It was found that, only with sodium hydroxide (NaOH) or sodium acetate anhydrous (CH₃COONa) solution, the textural results were very weak, resulting in high reflectance of silicon surface. However, if using the mixture solution of NaOH and CH₃COONa, the reflectance was noticeably decreased. Moreover, the dependence of reflectance on the etching time showed that longer etching time was necessary for texturization in the NaOH+CH₃COONa+H₂O system. And it was also found that the addition of isopropyl alcohol (IPA) to this mixture solution had a detrimental effect on the texturization. All these results suggested that acetate (CH₃COO⁻) plays a similar role as IPA for alkaline texturization, but they cannot coexist. Finally, the mechanisms of texturization with different kinds of etchant were discussed in detail.     

晶体硅太阳电池制备工艺进展

晶体硅太阳电池是目前应用最广、技术最为成熟的太阳电池,以晶体硅太阳电池生产流程为基础．主要从降低生产成本和提高电池转换效率方面出发,介绍了太阳电池制备工艺的最新进展,并对各种制备工艺作出了评价和展望。     

Rapid thermal technologies for high-efficiency silicon solar cells

This paper shows that rapidly formed emitters in less than 6 min in the hot zone of a conveyor belt furnace or in 3 min in an rapid thermal processing (RTP) system, in conjunction with a screen-printed (SP) RTP Al-BSF and passivating oxide formed simultaneously in 2 min can produce very simple high-efficiency n⁺-p-p⁺ cells with no surface texturing, point contacts, or selective emitter. It is shown for the first time that an 80 Ω/□ emitter and SP Al-back surface field (BSF) formed in a high throughput belt furnace produced 19% FZ cells and greater than 17% CZ cells with photolithography (PL) contacts. Using PL contacts, we also achieved 19% efficient cells on FZ, >18% on MCZ, and 17% boron-doped CZ by emitter and SP Al-BSF formation in <10 min in a single wafer RTP system. Finally, manufacturable cells with 45 Ω/□ emitter and SP Al-BSF and Ag contacts formed in the conveyor belt furnace gave 17% efficient cells on FZ silicon. Compared to the PL cells, the SP cell gave 2% lower efficiency along with a decrease in J_sc and fill factor. This loss in performance is attributed to a combination of the poor blue response, higher series resistance and higher contact shading in the SP devices     

Optimized rapid thermal process for high efficiency silicon solar cells

Rapid thermal processing is opening new possibilities for a low-cost and environmentally safe silicon solar cell production, keeping the process time at high temperature in the order of 1 min, due to enhanced diffusion and oxidation mechanisms. Controlling the surface concentration of the junction is one of the major parameters, in order to obtain suitable front surface recombination velocities. Simultaneous diffusion of phosphorus and aluminum is used to realize emitter and back surface field in a single high-temperature step, with optimized gettering effect. Controlling the mentioned parameters on industrial 1 Ω cm Cz material lead in 17.5% efficient solar cells on a surface of 25 cm². All results are discussed in terms of process temperature, dopant source concentration and effective process time, below 1 min including high heating and cooling rates.     

On the texturization of monocrystalline silicon with sodium carbonate solutions

The texturization of monocrystalline silicon wafers using sodium carbonate solution has been investigated. This etching process has been evaluated in terms of the surface morphology and the reflectance value. The results show that for low concentration of sodium carbonate the increase of texturing time decreases the reflectance value because of the change in morphology from hillocks to pyramidal; on the contrary for intermediate and high concentrations the increase of time has a detrimental effect on texturization because it increases both the pyramid sizes and their non-uniform distribution. However, a good cell performance could be obtained by etching at high concentrations and short times.     

Development of CuInS2-based solar cells and modules

Starting from a small area cell published in 1993, CuInS₂ technology has been continuously improved with respect to performance and manufacturability. Major milestones include successful preparation by rapid thermal processing, a monolithically integrated module test structure on a 5×5 cm² substrate, implementation of an industrial pilot line, incorporation of gallium for higher open circuit voltages and better performance and demonstration of Cd-free devices. Phase formation, reaction pathways and interdiffusion mechanisms have been investigated and modelled as have been electronic and device properties such as current transport. This review summarizes the most significant aspects of development and our current understanding of the technology.     

Texturisation of multicrystalline silicon solar cells by RIE and plasma etching

Texturing by reactive ion etching (RIE) is demonstrated as an attractive technical solution for lowering of reflectance of multicrystalline silicon solar cells. A suitable sequence of processes is developed to combine the advantage of reactive ion etching with “natural lithography” based on colloidal masks. The RIE single-wafer texturisation is driven to an industrial applicable batch process by plasma etching with a gain in efficiency of 0.3% absolute.     

Optimization of interface structures in crystalline silicon heterojunction solar cells

In heterojunction solar cells consisting of hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si), suppression of epitaxial growth at the heterointerface is found to be crucial to achieve high solar cell efficiencies. In order to avoid the epitaxial growth, wide-gap hydrogenated amorphous silicon oxide (a-SiO:H) has been applied to the heterojunction solar cells. We have fabricated a-SiO:H/c-Si solar cells using n-type and p-type c-Si substrates and demonstrated that incorporation of the a-SiO:H i layer prevents the harmful epitaxial growth at the heterointerface completely.     

Production technology of large-area multicrystalline silicon solar cells

In 1996 a conversion efficiency of 17.1% had been obtained on 15 cm×15 cm mc-Si solar cell. In this paper, large-scale production technology of the high-efficiency processing will be discussed. Enlarging reactive ion etching (RIE) equipment size, technology of passivation, and fine contact grid with low resistance by screenprinted metallization, which is firing through PECVD SiN, have been investigated.     

In-line measurement of “trench structures” caused by the texturization of mc-silicon solar cells

Acidic texturing of multicrystalline silicon (mc-Si) wafers often leads to rough surfaces with strong etch attacks (trench structures), especially at sites with crystal defects. The appearance of trench structures on the wafer surface has been cited in earlier publications and has been often recognised as being harmful to solar cell performance [Mathijssen et al., 2009]. In this work, an in-line measurement method for these structures is presented, using a line camera system with diffuse illumination. The presented method can be used for the in-line quality control of the acidic texturization. The number of trench structures is quantified in the images via a newly developed algorithm by an adaptive threshold method. With the help of AFM images the measurement method could be regarded in detail. It has been shown that the true area fraction of trench structures only lies between 0.3% and 1.8% instead of 2–12%, which is estimated from measurements. As expected, the number of trench structures strongly depends on the texture strength and the number of crystal defects in as-cut material. Therefore, if the texturization method remains constant, it is possible to easily measure material quality by measuring the fraction of trench structures and the presented measurement method could be used as cheap alternative to photoluminescence measurements. The reliability of the algorithm is demonstrated by the correlation between material quality, texture strength and the resulting number of trench structures.     

Easy-to-fabricate 20% efficient large-area silicon solar cells

Recently, an innovative silicon solar cell structure has been developed at ISFH which is capable of achieving very high cell efficiencies on industrial-size wafers with a simple photolithography-free processing sequence. As the corresponding solar cells essentially rely on the application of obliquely evaporated contacts they are denoted as OECO cells. In this paper the successful up-scaling of the novel OECO process from 21% efficient 4 cm² laboratory devices to the fabrication of large-area (100 cm²) silicon solar cells is described, and independently confirmed total area efficiencies of 20% are reported for 10×10 cm² OECO-type solar cells fabricated on p-type float-zone silicon.     

Efficiency improved by acid texturization for multi-crystalline silicon solar cells

In this paper, we will show that efficiency of multi-crystalline silicon (mc-Si) solar cells may be improved by acid texturization. In order to enhance overall efficiency of mc-Si for solar-cell applications, the surface treatment of texturization with wet etching using appropriate solutions can improve incident light into the cell. Alkali etchant cannot produce uniformly textured surface to generate enough open circuit voltage (V_OC) and high efficiency of the mc-Si due to the unavoidable grain randomly oriented with higher steps formed during etching process. Optimized acid etching conditions can be obtained by decreasing the reflectance (R) for mc-Si substrate below levels generated by alkali etching. Short-circuit current (I_SC) measurements on acid textured cells reveal that current gain can be significantly enhanced by reducing reflection. The optimal acid etching ratio HF:HNO₃:H₂O = 15:1:2.5 with etching time of 60 s and lowering 42.7% of the R value can improve 112.4% of the conversion efficiency (η) of the developed solar cell. In order to obtain more detailed information of different defect region, high-resolution light beam induced current (LBIC) is applied to measure the internal quantum efficiency (IQE) and the lifetime of minority carriers. Thus, the acid texturing approach is instrumental to achieve high efficiency in mass production using relatively low-cost mc-Si as starting material with proper optimization of the fabrication steps.     

Laser structuring of crystalline silicon thin-film solar cells on opaque foreign substrates

Crystalline silicon thin-film solar cells were fabricated on graphite substrates. A laser ablation process was developed for edge isolation of the thin-film cells. The shunt resistance was comparable to otherwise identical cells isolated by plasma etching, while the reproducibility of the laser isolation process was higher. The solar cells were characterized by current-voltage and light beam induced current measurements (LBiC). No interference was detected along the ablated edges. Spatial variations of the minority carrier lifetime are attributed to the grain structure of the seeding layer obtained by the zone melting recrystallization (ZMR).     

The influence of porous silicon coating on silicon solar cells with different emitter thicknesses

The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n⁺p silicon junction whose emitter depth was varied between 0.5 and 2.2 μm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n⁺ emitter. The electrical properties of the samples with PS were improved with decrease of the n⁺p junction depth. Our results demonstrate short-circuit current values of about 35–37 mA/cm² using n⁺ region with 0.5 μm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modeling. The spectral response measurements were performed at a wavelength range of 0.4–1.1 μm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400–500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19–20% efficiency by the proposed simple technology.