Fabrication Of Silicon Solar Cells Research Articles

Low surface damage dry etched black silicon

Black silicon (bSi) is promising for integration into silicon solar cell fabrication flow due to its excellent light trapping and low reflectance, and a continuously improving passivation. However, intensive ion bombardment during the reactive ion etching used to fabricate bSi induces surface damage that causes significant recombination. Here, we present a process optimization strategy for bSi, where surface damage is reduced and surface passivation is improved while excellent light trapping and low reflectance are maintained. We demonstrate that reduction of the capacitively coupled plasma power, during reactive ion etching at non-cryogenic temperature (−20 °C), preserves the reflectivity below 1% and improves the effective minority carrier lifetime due to reduced ion energy. We investigate the effect of the etching process on the surface morphology, light trapping, reflectance, transmittance, and effective lifetime of bSi. Additional surface passivation using atomic layer deposition of Al2O3 significantly improves the effective lifetime. For n-type wafers, the lifetime reaches 12 ms for polished and 7.5 ms for bSi surfaces. For p-type wafers, the lifetime reaches 800 μs for both polished and bSi surfaces.

Fabrication and Modeling of High-Efficiency Front Junction N-Type Silicon Solar Cells With Tunnel Oxide Passivating Back Contact

This paper reports on in-depth understanding, modeling, and fabrication of 23.8% efficient 4 cm2 n-type Float Zone (FZ) silicon cells with a selective boron emitter and photolithography contact on front and tunnel oxide passivating contact on the back. Tunnel oxide passivating contact composed of a very thin chemically grown silicon oxide (∼15 A) capped with plasma-enhanced chemical vapor deposition (PECVD) grown 20 nm n+ poly Si gave excellent surface passivation and carrier selectivity with very low saturation current density (∼5 fA/cm2). A high-quality boron selective emitter was formed using ion implantation and solid source diffusion to minimize metal recombination and emitter saturation current density. Process optimization resulted in a cell $V_{{\rm{oc}}}$ of 712 mV, $J_{{\rm{sc}}}$ of 41.2 mA/cm2, and FF of 0.811. A simple methodology is used to model these cells which replaces tunnel oxide passivating contact region by electron and hole recombination velocities extracted from measured saturation current density of tunnel oxide passivating contact region and analysis. Using this approach and two-dimensional device modeling gave an excellent match between the measured and simulated cell parameters and efficiency, supporting excellent passivation and carrier selectivity of these contacts. Extended simulations showed that 26% cell efficiency can be achieved with this cell structure by further optimization of wafer quality, emitter profile, and contact design.

TCO contacts for high efficiency c-Si solar cells: Influence of different annealing steps on the Si substrates and TCO layers properties

Different Transparent Conductive Oxide (TCO) layers properties are evaluated after annealing steps at temperatures above 200°C, in order to study their potential use in crystalline silicon (c-Si) solar cells fabrication processes. While the conductivity of Indium Tin Oxide (ITO) layers obtained by magnetron sputtering (MS) is almost stable after annealing in air, Aluminum doped Zinc Oxide (AZO) layers deposited by Atomic Layer Deposition (ALD) need a controlled atmosphere to maintain high carrier densities and mobilities. During the annealing processes, contaminating atoms (such as Zn) diffuse into the c-Si bulk and may potentially decrease its quality. Thus, both the contamination of the c-Si bulk and the properties of the AZO layer have been analyzed.

Preparation of axial-type wire-structure crystalline silicon solar cells

The fabrication of an axial-type microwire-structure silicon (Si-MW) solar cell has been studied, and it is compared with the radial-type Si-MW solar cell. In this study, a number of microwires were formed on a p-type Si wafer and an n-layer were fabricated on the top region of microwires to form p–n junctions. After that, Si-MWs were filled with an Al1−xOx film, deposited by thermal atomic layer deposition, and silica particles, respectively. Then, the Al1−xOx film and silica particles on the n-layer were removed by chemical mechanical polishing to form an electrical contact with the electrode. As a result, we obtained a conversion efficiency of 8.2% from the axial-type Si-MW solar cell. In addition, compared with a radial-type Si-MW solar cell, a higher quantum efficiency was obtained in the short-wavelength region (300–600 nm) for the axial-type Si-MW solar cell.

FORMATION OF ANTIREFLECTIVE SILICON SURFACES BY ELECTROCHEMICAL AND CHEMICAL METHODS

Background. Application of the silicon-based porous textures as an efficient and commercially viable coating has to be maximally adapted to processes of the silicon solar cell manufacturing. To improve the antireflective property of Si frontal surface it is desirable to use methods allowing simultaneous changing of the value of refraction coefficient and the fabrication process parameters. Therefore, it is necessary to seek other, more perspective methods of the antireflective coating fabrication to improve the solar cell efficiency.Objective. The aim of the paper is the fabricaion of antireflective coatings based on porous silicon by electrochemical and chemical methods for photovoltaic converters with improved parameters.Methods. Electrochemical etching and metal-assisted chemical etching were used to form the textures on the Si wafer surface. The surface morphology of Si samples was examined using scanning electron microscopy, the elemental content was investigated using Time of Flight Secondary Ion Mass Spectrometer. The investigation of optical properties of obtained textures was performed using a Specord Plus spectrophotometer.Results. The micro- and nanotextured Si surfaces with an average diameter 1 μm and 200 nm were obtained by electrochemical and metal-assisted chemical etching methods, respectively. In addition, the nanotextured Si samples had a lowest reflective coefficient in comparison with other textures.Conclusions. Electrochemical and chemical methods are promising to fabricate the frontal antireflective Si surfaces of solar cells. It is possible to form electrochemically a microtextured porous Si surface with low reflectivity with a proper selected anodic charge density. The metal-assisted chemical etching method allowed forming a nanoporous surface on Si wafer with improved antireflective properties of Si surface in optical spectral range.

Low‐refractive‐index nanoparticle interlayers to reduce parasitic absorption in metallic rear reflectors of solar cells

Metal reflectors or electrodes in contact with optoelectronic devices can induce parasitic light absorption. A low‐refractive‐index (low‐n) layer inserted between the metal reflector and the optically active layer(s) reduces this absorption. We investigate the use of porous, nanoparticulate films as low‐n layers, and fabricate silicon solar cells with nanoparticle/silver rear reflectors. We vary the porosity and thus n (between 1.1 and 1.5) of the nanoparticle films, which are deposited by a controllable aerosol spray process, and investigate their effectiveness in reducing infrared parasitic absorption in the solar cells. Optical test structures incorporating films with the highest n exhibit an internal reflectance of over 99%, matching best‐in‐class structures; lower‐n layers should in theory perform better still but their rougher surfaces appear to induce plasmonic absorption in the overlying silver layer. No loss in open‐circuit voltage or fill factor is observed when applying the best nanoparticle films in silicon heterojunction solar cells, enabling efficiencies similar to those achieved with reference cells that employ a thick indium tin oxide layer between the wafer and the rear silver electrode.

Simple processing of back-contacted silicon heterojunction solar cells using selective-area crystalline growth

For crystalline-silicon solar cells, voltages close to the theoretical limit are nowadays readily achievable when using passivating contacts. Conversely, maximal current generation requires the integration of the electron and hole contacts at the back of the solar cell to liberate its front from any shadowing loss. Recently, the world-record efficiency for crystalline-silicon single-junction solar cells was achieved by merging these two approaches in a single device; however, the complexity of fabricating this class of devices raises concerns about their commercial potential. Here we show a contacting method that substantially simplifies the architecture and fabrication of back-contacted silicon solar cells. We exploit the surface-dependent growth of silicon thin films, deposited by plasma processes, to eliminate the patterning of one of the doped carrier-collecting layers. Then, using only one alignment step for electrode definition, we fabricate a proof-of-concept 9-cm2 tunnel-interdigitated back-contact solar cell with a certified conversion efficiency >22.5%. The most efficient silicon solar cells use interdigitated back-contact silicon heterojunction architectures. Here, the authors fabricate this type of cell via a simpler process, using an interband silicon tunnel junction for the electron contact and reaching a certified efficiency higher than 22%.

Efficiency Optimization of Pyramid-Textured Crystalline Solar Cells with Metal-Assisted Chemical Etching

1. Introduction Crystalline silicon solar cells are the mainstream technology for the photovoltaic market. Due to the great prospect of next-generation green energy, their module efficiency requires further improvement. Moreover, the continuous cost reduction is another big issue for crystalline silicon solar cells. Recently, the one-step metal-assisted etching (MAE) method for improving the efficiency of crystalline silicon solar cells has been drawn much attention because it increases both of the light ravel distance and the number of scattering events. The MAE method also meets the requirement of industrial-scale fabrication regarding the large-scale and large-area production in a controlled manner. However, the systematic study on the relationship between the photovoltaic performance and the corresponding pyramid-textured surface under various MAE duration have been less reported. In this paper, the efficiency optimization for pyramid-textured crystalline silicon solar cells with MAE is reported. In addition, we have also incorporated the MAE method as a batch process into a standard crystalline silicon solar cell fabrication line so as to verify the accuracy of experimental results. 2. Fabrication Process The fabrication process of pyramid-textured crystalline silicon solar cells with MAE is illustrated in Fig. 1. At first, the standard clean process was applied on the Czochralski-grown 200-µm-thick P-type crystalline silicon wafers with dimensions of 156mm × 156mm of which resistivity is 1-5 Ω∙cm. After that, the pyramid-textured silicon surface was performed in hot sodium hydroxide (NaOH) solution, followed by the MAE method. In brief, HF and silver nitrate (AgNO3) for 5, 15, 25, 35, 45, and 55 seconds. At the end of the process, the associated wafers were cleaned using nitric acid (HNO3) to remove residual Ag nanoparticles (NPs). Next, the n-type emitter layer and a silicon nitride layer as an anti-reflection (AR) coating was deposited via plasma-enhanced chemical vapor deposition (PECVD). Finally, the front and rear electrodes were printed and co-fired, accompanying with the formation of back-surface field (BSF). 3. Experimental Results and Discussion The surface morphology of the hierarchical structures as a function of the etching duration without removal of Ag NPs as shown in Fig. 2(a)-(b) with the instrument of FEI Dual-Beam NOVA 600i Focused Ion Beam. As the etching duration increases, the SEM images show that the size and density of Ag NPs keep increasing until the larger Ag NPs start sinking into pyramid-textured surface. The pyramid-textured silicon structures tend to collapse while the etching time exceeds 45 sec. To investigate the effect of various MAE-based etching durations on the performance of pyramid-textured crystalline silicon solar cells, Fig. 3 shows the efficiency and the associated fill factor in relation to various etching duration. As shown in the figure, efficiency and fill factor increase up to a specific etching duration after which it slightly decreases, and then drop significantly with the etching time longer than 45 sec. Fig. 4 demonstrates the corresponding Isc and Voc versus etching duration. As confirmed in the figure, Isc and Voc also shows enhancement with an increase in etching time by up to 35 seconds, and then decrease dramatically. This could be reasoned by the initial destruction of pyramid-textured surface from the longer sinking track of Ag NPs. In addition, the higher series resistance can be another evidence regarding the surface destruction as seen in the inlet of Fig. 4. The series resistance apparently increases after the etching duration of 35 seconds, which discoveries the collapsed pyramid-textured silicon surface as well. 4. Discussion From the analysis of efficiency, fill factor, Isc, Voc, and series resistance, the hook-liked trends in relation to the etching duration can be summarized as follows. The density and size of Ag NPs increase at the early increment of etching duration, which leads to stronger light scattering and higher light absorbance. Nevertheless, with continuously increasing the etching duration, the surface area of hierarchical structures increase rapidly due to the longer sinking track of Ag NPs. This results too many broken / dangling Si bonds generated on the pyramid-textured surface, given that the reduced efficiency of devices is acquired. In the respect of suppressing surface destruction, the reduction of AgNO3 concentration experiment is indicated in the Fig. 5. It is expected that the maximum of efficiency and fill factor for the crystalline silicon solar cells occurs at higher etching duration due to the lower AgNO3 concentration. However, the further optimization for device performance during the etching time between 45 and 55 seconds is needed. The proposed results report the successful implementation of one-step MAE method on the standard industrial fabrication process for pyramid-textured crystalline silicon solar cells. Figure 1

Solar cell efficiency improvement employing down-shifting silicon quantum dots

We report the synthesis and characterization of silicon quantum dots (Si-QDs) that exhibit down-shifting, photo-luminescent characteristics, and the dependence of luminescent characteristics on sodium l-ascorbate, APTES and the reaction time. The synthesized silicon nanostructures were observed to absorb UV photons and subsequently reemit in a broad visible region extending from 450 to 650 nm. The incorporation of these nanostructures as a photon down-shifting layer on solar cells triggered improvements in the performance of the fabricated photovoltaic devices, especially in the open circuit voltage (Voc) and short circuit current density (Jsc). Specifically, the experimental results showed increments in the Voc from 532.6 to 536.2 mV and in the Jsc from 33.4 to 38.3 mA/cm2. The combined effect of those improved Voc and Jsc values led to an increment in the power conversion efficiency (PCE) from 11.9 to 13.4%. This increment represents an improvement of the order of 12.4% on the power conversion efficiency on the silicon solar cells employed. The observed results could be conducive to promoting the proliferation of photovoltaic structures. Additionally, we discuss the fabrication and characterization of the single crystal Silicon (c-Si) Solar cells employed in this exercise.

A numerical model for soldering process in silicon solar cells

Solar or Photovoltaic (PV) cells are utilized to convert solar energy into electricity through the photovoltaic effect. Although, Silicon solar cell is one of the most prevalent type of solar cells; manufacturing of this type of solar cells especially soldering copper electrodes to the silicon wafer is very challenging. If the soldering parameters are not selected properly, small cracks will be created in silicon layer, and finally, fracture of the solar cell will happen. The goal of the present study is to develop, for the first time, a finite element model to simulate the soldering process in the fabrication of silicon solar cell in which the soldering system is passing over the solar cell to connect the copper electrode to the silicon wafer. The temperature and thermomechanical stress distributions induced in different layers of the cell are extracted. The effects of several parameters such as soldering power, soldering speed, and the thickness of silicon wafer on the temperature and stress distributions are probed. The results show that the silicon layer is the most critical layer in a silicon solar cell during the soldering process which is susceptible to high thermomechanical stresses. It is shown that increasing the soldering system power and decreasing the soldering system speed lead to increase of thermomechanical stresses induced in the silicon wafer. In addition, using thicker silicon wafer can decrease the thermomechanical stresses generated in this layer.

Rheological effect on screen-printed morphology of thick film silver paste metallization

Up to now the topography of thick film silver paste metallization is still a great challenge to the crystalline silicon solar cell manufacturers due to lack of overall understanding about the dependence of screen-printed morphology on paste rheology. In this article, we demonstrate that both the shear thixotropy and wall slip are primary rheological behaviors of pastes affecting screen-printed morphology. By comparison among three different commercial pastes, the association of screen-printed morphology with apparent viscosity and slip velocity of pastes were investigated. The results from printing finger lines with designed width of 45 μm show clearly that the widening degree of lines is associated with the paste viscosity, and the height and cross-sectional profile of lines mainly rely on the wall slip velocity of paste passing through screen meshes. Briefly, the lower the apparent viscosity, the wider the lines; and the greater the wall slip velocity, the less the line height and more likely the formation of trapezoidal cross-sectional profile with larger area. Our study shows for the first time that the wall slip of pastes may produce significant influence on cell efficiency, since its influence on screen-printed morphology is considerably remarkable and vital.

Si太陽電池製作とエネルギー変換効率シミュレーションを利用した半導体教育の実践

In higher education institutions such as universities and colleges of technology, practical and theoretical education of semiconductor devices is required. Therefore, we introduce a silicon (Si) solar cell fabrication into student experiment to make them understand the pn junction as a basis of semiconductor devices. Furthermore, spectral response characteristic measurement using light emitting diodes (LED) and energy conversion efficiency simulation of the solar cell are introduced into the student experiment to make them understand the relation between light energy and energy band gap, and the function of minority carriers. We verify the effectiveness of introducing these experiments by conducting examination and questionnaire before and after the experiment.

6-105 シリコン太陽電池作製実験教材の地域貢献事業及び国際交流事業への展開((20)地域貢献・地場産業との連携-I)

6-105 シリコン太陽電池作製実験教材の地域貢献事業及び国際交流事業への展開((20)地域貢献・地場産業との連携-I)

Realizing a facile and environmental-friendly fabrication of high-performance multi-crystalline silicon solar cells by employing ZnO nanostructures and an Al2O3 passivation layer

Nowadays, the multi-crystalline silicon (mc-Si) solar cells dominate the photovoltaic industry. However, the current acid etching method on mc-Si surface used by firms can hardly suppress the average reflectance value below 25% in the visible light spectrum. Meanwhile, the nitric acid and the hydrofluoric contained in the etching solution is both environmental unfriendly and highly toxic to human. Here, a mc-Si solar cell based on ZnO nanostructures and an Al2O3 spacer layer is demonstrated. The eco-friendly fabrication is realized by low temperature atomic layer deposition of Al2O3 layer as well as ZnO seed layer. Moreover, the ZnO nanostructures are prepared by nontoxic and low cost hydro-thermal growth process. Results show that the best passivation quality of the n+ -type mc-Si surface can be achieved by balancing the Si dangling bond saturation level and the negative charge concentration in the Al2O3 film. Moreover, the average reflectance on cell surface can be suppressed to 8.2% in 400–900 nm range by controlling the thickness of ZnO seed layer. With these two combined refinements, a maximum solar cell efficiency of 15.8% is obtained eventually. This work offer a facile way to realize the environmental friendly fabrication of high performance mc-Si solar cells.

Fast Co-Diffusion Process for Bifacial n-Type Solar Cells

We present a new simplified and fast process sequence for the fabrication of large area bifacial n-type silicon solar cells. A co-diffusion process simultaneously forms the boron-doped emitter and the phosphorus-doped back surface field in one single high temperature step. We use borosilicate glasses (BSG) deposited by atmospheric pressure chemical vapor deposition (APCVD) and a POCl3 gas ambient as doping sources. Tailoring of the diffusion process yields a reduced co-diffusion process time of 114 min, well suited for production. The presented co-diffusion process allows a lean fabrication process. A proof of concept experiment leads to 20.1% bifacial n-type solar cells.

Impact of thermal processes on multi-crystalline silicon

Fabrication of modern multi-crystalline silicon solar cells involves multiple processes that are thermally intensive. These include emitter diffusion, thermal oxidation and firing of the metal contacts. This paper illustrates the variation and potential effects upon recombination in the wafers due to these thermal processes. The use of light emitter diffusions more compatible with selective emitter designs had a more detrimental effect on the bulk lifetime of the silicon than that of heavier diffusions compatible with a homogenous emitter design and screen-printed contacts. This was primarily due to a reduced effectiveness of gettering for the light emitter. This reduction in lifetime could be mitigated through the use of a dedicated gettering process applied before emitter diffusion. Thermal oxidations could greatly improve surface passivation in the intragrain regions, with the higher temperatures yielding the highest quality surface passivation. However, the higher temperatures also led to an increase in bulk recombination either due to a reduced effectiveness of gettering, or due to the presence of a thicker oxide layer, which may interrupt hydrogen passivation. The effects of fast firing were separated into thermal effects and hydrogenation effects. While hydrogen can passivate defects hence improving the performance, thermal effects during fast firing can dissolve precipitating impurities such as iron or de-getter impurities hence lower the performance, leading to a poisoning of the intra-grain regions.

Enhancing the power conversion efficiency of solar cells employing down-shifting silicon quantum dots

We report the synthesis and characterization of silicon quantum dots that exhibit down-shifting, photo luminescent characteristics. We also discuss the fabrication and characterization of single crystal Silicon (c-Si) Solar cells with and without the influence of the previously mentioned QDs. The incorporation of these nanostructures triggers improvements in the performance of the fabricated photovoltaic devices, especially in the open circuit voltage (Voc) and short circuit current density (Jsc). Specifically, the experimental results showed increments in the Voc from 532.6 to 536.2 mV and in the Jsc from 33.4 to 38.3 mA/cm2. The combined effect of those improved Voc and Jsc values led to an increment in the power conversion efficiency (PCE) from 11.90 to 13.37%. This increment represents an improvement of the order of 12.4% on the power conversion efficiency of this type of solar cells. The observed results could be conducive to promoting the proliferation of photovoltaic structures.

Influence of photo-luminescent CdSe/CdS core shell quantum dots in solar cell efficiency

We report the synthesis and characterization of CdSe/CdS core-shell quantum dots (CdSe/CdS-QDs) that exhibit absorption in the UV range of the solar spectrum and emit photons with wavelengths centered around 625 nm, a wavelength that is well suited for silicon absorption and electron-hole pair generation. We also report the fabrication and characterization of single crystal silicon (c-Si) solar cells with and without the aforementioned photo luminescent, down-shifting CdSe/CdS- QDs. The incorporation of these nanostructures triggered improvements in the performance of the devices, particularly in the open circuit voltage (Voc) and short circuit current density (Jsc) for which the measured values showed an increase from 543 to 546 mV and from 32.5 to 37.0 mA/cm2, respectively. The combined effect of the improved values led to an increment in the power conversion efficiency (PCE) from 12.01 to 13.54%. This increase represents a 12.7% improvement in the PCE of the fabricated devices. The effort described herein is considered a good fit to the generalized trend to improve the efficiency of solar cells with mass-compatible techniques that could serve to promote their widespread utilization.

Design of a Multitask Reactive Distillation with Intermediate Heat Exchangers for the Production of Silane and Chlorosilane Derivates

Silane and chlorosilanes are essential materials for manufacturing silicon solar cells, glass microscope slides, oxidation masks, and corrosion-resistant films among other products. Monochlorosilane (SiH3Cl) and dichlorosilane (SiH2Cl2) are produced on a large scale as intermediates in the synthesis of silane (SiH4) by disproportionation of trichlorosilane (SiHCl3). However, seldom are they isolated due to the highly integrated nature of silane production, and the high commercial demand for silane relative to monochlorosilane and dichlorosilane. This study proposes a multitask reactive distillation (MTRD) column with intermediate heat exchangers that has the flexibility to switch between the production of SiH4, SiH3Cl, and SiH2Cl2 from the SiHCl3 disproportionation. Because the reactive distillation that separates SiH4 uses an expensive refrigerant, intermediate heat exchangers are installed to reduce the cost and energy consumption of expensive refrigerants in the optimized MTRD. Process simulation and m...

Non-Vacuum Processed Polymer Composite Antireflection Coating Films for Silicon Solar Cells

A non-vacuum processing method for preparing polymer-based ZrO2/TiO2 multilayer structure antireflection coating (ARC) films for crystalline silicon solar cells by spin coating is introduced. Initially, ZrO2, TiO2 and surface deactivated-TiO2 (SD-TiO2) based films were examined separately and the effect of photocatalytic properties of TiO2 film on the reflectivity on silicon surface was investigated. Degradation of the reflectance performance with increasing reflectivity of up to 2% in the ultraviolet region was confirmed. No significant change of the reflectance was observed when utilizing SD-TiO2 and ZrO2 films. Average reflectance (between 300 nm–1100 nm) of the silicon surface coated with optimized polymer-based ZrO2 single or ZrO2/SD-TiO2 multilayer composite films was decreased down to 6.5% and 5.5%, respectively. Improvement of photocurrent density (Jsc) and conversion efficiency (η) of fabricated silicon solar cells owing to the ZrO2/SD-TiO2 multilayer ARC could be confirmed. The photovoltaic properties of Jsc, the open-circuit photo voltage (VOC), the fill factor (FF), and the η were 31.42 mA cm−2, 575 mV, 71.5% and 12.91%. Efficiency of the solar cells was improved by the ZrO2-polymer/SD-TiO2 polymer ARC composite layer by a factor of 0.8% with an increase of Jsc (2.07 mA cm−2) compared to those of fabricated without the ARC.