mc-Si Solar Cells Research Articles

Towards the efficiency limits of multicrystalline silicon solar cells

In this contribution, we present our recent results for high efficiency multicrystalline silicon solar cells. Based on n-type high-performance multicrystalline silicon substrates in combination with the TOPCon solar cell concept featuring a full area passivating back contact and a boron-diffused emitter as well as a plasma-etched black-silicon texture at the front side, a certified conversion efficiency of 22.3% has been achieved, which is currently the world record efficiency for multicrystalline silicon solar cells. A detailed loss analysis of the record solar cell batch discloses the nature of the remaining loss mechanisms, revealing the route for further improvements. We observe an efficiency gap between the multicrystalline and the FZ reference solar cells of ~1%abs. Compared to the FZ reference cells, the mc-Si cells also feature a significantly larger scattering in Voc and Jsc as well as a fill factor loss of ~1.5%abs. We show that the scattering in Jsc correlates with the area fraction of recombination-active structural crystal defects and the scattering in Voc additionally with lateral emitter-induced inhomogeneities. The fill factor loss is attributed to the general presence of strongly recombination-active grain boundaries. A detailed loss analysis of the record mc-Si solar cell shows that the major electrical losses are due to recombination at grain boundaries (0.7%abs) and recombination in the emitter (0.6%abs). By reducing these electrical loss channels, e.g. by an improved crystallization process together with a hydrogenation of the bulk and application of an adapted emitter, we expect to reach efficiencies for mc-Si solar cells in the range of 23%.

Photoluminescence Imaging and LBIC Characterization of Defects in mc-Si Solar Cells

Today’s photovoltaic market is dominated by multicrystalline silicon (mc-Si) based solar cells with around 70% of worldwide production. In order to improve the quality of the Si material, a proper characterization of the electrical activity in mc-Si solar cells is essential. A full-wafer characterization technique such as photoluminescence imaging (PLi) provides a fast inspection of the wafer defects, though at the expense of the spatial resolution. On the other hand, a study of the defects at a microscopic scale can be achieved through the light-beam induced current technique. The combination of these macroscopic and microscopic resolution techniques allows a detailed study of the electrical activity of defects in mc-Si solar cells. In this work, upgraded metallurgical-grade Si solar cells are studied using these two techniques.

The study of the defect removal etching of black silicon for diamond wire sawn multi-crystalline silicon solar cells

We produced low-reflectivity nanostructured black silicon by metal-assisted chemical etching (MACE) technology on the 15.6 cm × 15.6 cm diamond wire sawn (DWS) multi-crystalline silicon (mc-Si) wafers. Subsequently, defect removal etching (DRE) processes were carried out for various times to form nanohills with different sizes on the wafer surface. The experimental results indicated that the DRE process could influence the size of the nanohills, reflectivity and the recombination velocities of the silicon wafers. It was found that for the black silicon wafer, with the increasing of DRE time, the reflectivity became larger and the passivation effect became better. The reflectivity and the passivation effect can be adjusted by suitable DRE processes. By this way, we successfully fabricated the DWS mc-Si solar cell (15.6 cm × 15.6 cm) with the highest conversion efficiency of 19.07%, showing an improvement in efficiency of 0.6% compared to a DWS mc-Si solar cell (15.6 cm × 15.6 cm) prepared by conventional acidic texturization.

18.88%-efficient multi-crystalline silicon solar cells by combining Cu-catalyzed chemical etching and post-treatment process

Mass production of diamond-wire-sawn (DWS) multi-crystalline silicon (mc-Si) solar cells reached a significant point of maturity through utilization of metal-catalyzed chemical etching (MCCE). However, the reported studies always focus on how to optimize the MCCE process but there are few studies concentrating on the post-treatment techniques to improve the cell efficiency. In this paper, we use a combination of Cu-MCCE method and a HF/HNO3/H3PO4 post-processing treatment process to decorate the surface of Si textures for the first time. The submicron polygonal chamfered cone structure produced by the post-processing treatment is demonstrated to be helpful in reducing surface recombination and improving the cell performance in terms of surface morphology, reflectivity, internal quantum efficiency (IQE) and external quantum efficiency (EQE) measurements as well as Electroluminescence (EL) spectra characterizations. The highest efficiency of Cu-MCCE mc-Si solar cells subjected to such post-processing treatment process is 18.88% with short circuit current (Isc), open circuit voltage (Voc) and fill factor (FF) of 36.67 mA/cm2, 638.6 mV and 80.64%, respectively, in great contrast to that (16.81%) of Cu-MCCE mc-Si solar cells without any post treatment. The post-treatment process is, therefore, of great potential for the Si photovoltaic industry, especially for the Cu-MCCE mc-Si solar cells.

Defect characterization of UMG mc-Si solar cells using LBIC and luminescence imaging techniques

ABSTRACTUpgraded metallurgical-grade silicon (UMG Si) solar cells with different ranges of efficiencies were characterized through electroluminescence imaging (ELi) and light-beam induced current (LBIC) measurements. The results showed a good correlation between the EL intensity and the efficiency of the solar cells. ELi images gave a bright contrast at the defects, grain boundaries and intragrain defects, and dark contrast inside the grain bodies. Metallic impurities are much more present in some cells due to the directional solidification of the Si ingot. Local short-circuit current mapping with LBIC measurements revealed a bright zone in the neighborhoods of the defects due to the depletion of impurities. Internal quantum efficiencies (IQE) and effective diffusion lengths (Leff) were calculated using different excitation wavelengths. High resolution LBIC measurements revealed micrometric clusters of impurities around intragrain defects.

Carrier-Induced Degradation in Multicrystalline Silicon: Dependence on the Silicon Nitride Passivation Layer and Hydrogen Released During Firing

Carrier-induced degradation (CID) of multicrystalline silicon (mc-Si) solar cells has been receiving significant attention; however, despite this increasing interest, the defect (or defects) responsible for this degradation has not been determined yet. Previous studies have shown that the surface passivation layer and the firing temperature have a significant impact on the rate and extent of this degradation. In this paper, we further study this impact through an investigation of the CID behavior of the mc-Si wafers passivated with six different silicon nitride layers, each fired at four different peak temperatures. At low firing temperatures, no significant difference in the CID was identified between the samples with different passivation layers; however, a large range of degradation extents was observed at higher firing temperatures. Using Fourier transform infrared spectroscopy, a correlation was found between the degradation extent and the amount of hydrogen released from the dielectric during firing. We verified that no degradation of the surface passivation quality occurred, indicating that the degradation is primarily associated with a bulk defect.

Interior and Surface Degradation Analysis of Multicrystalline Si Solar Cell Module Using Laser-Beam-Induced Current Technique

The interior and surface degradation of a multicrystalline silicon (mc-Si) solar cell module was investigated using the laser beam-induced current (LBIC) technique. Reverse voltage was applied to avoid the influence of the solar cells not irradiated with light and to evaluate the LBIC intensity only of the illuminated solar cell. In addition, we found that the mc-Si, but not the solar cell surface, is comprised of homogeneous crystals and the degraded layer is estimated to be $\sim 0.6~\mu\text{m}$ . The LBIC technique was applied at several wavelengths under reverse bias conditions in order to analyze degraded solar cells.

Solubility and Diffusivity: Important Metrics in the Search for the Root Cause of Light- and Elevated Temperature-Induced Degradation

Light- and elevated temperature-induced degradation (LeTID) is a detrimental effect observed under operating conditions in p-type multicrystalline silicon (mc-Si) solar cells. In this contribution, we employ synchrotron-based techniques to study the dissolution of precipitates due to different firing processes at grain boundaries in LeTID-affected mc-Si. The synchrotron measurements show clear dissolution of collocated metal precipitates during firing. We compare our observations with degradation behavior in the same wafers. The experimental results are complemented with process simulations to provide insight into the change in bulk point defect concentration due to firing. Several studies have proposed that LeTID is caused by metal-rich precipitate dissolution during contact firing, and we find that the solubility and diffusivity are promising screening metrics to identify metals that are compatible with this hypothesis. While slower and less soluble elements (e.g., Fe and Cr) are not compatible according to our simulations, the point defect concentrations of faster and more soluble elements (e.g., Cu and Ni) increase after a high-temperature firing process, primarily due to emitter segregation rather than precipitate dissolution. These results are a useful complement to lifetime spectroscopy techniques, and can be used to evaluate additional candidates in the search for the root cause of LeTID.

Passivation properties of alumina for multicrystalline silicon nanostructure prepared by spin-coating method

In this paper, we report passivation properties of inverted pyramidal nanostructure based multi-crystalline silicon (mc-Si) by Al2O3 films with spin-coating method. Precursors AlCl3 and Al(acac)3 for Al2O3 films were chosen for comparison. Al2O3/SiOx stacks were found to be able to passivate the nanostructured surface well. With the number of spin-coating up to five, the Al2O3 films could conformally attach the nanostructure. The weighted average reflectance values (ranging from 400–900 nm) of the passivated silicon surface could be reduced to 10.74% (AlCl3) and 11.12% (Al(acac)3), and the effective carrier lifetime could reach 7.84 and 16.98 μs, respectively. This work presented a potential process to fabricate low cost high efficiency mc-Si solar cells.

Investigated performance improvement of the micro-pressure sandblast-treated multi-crystalline Si wafer sliced using diamond wire sawing

In this work, the textured surface of the multi-crystalline silicon (mc-Si) wafers sliced by the diamond wire sawing (DWS) technique was formed using the X-Y axis micro-pressure sandblast treatment and the acidic texturing process. From the scanning electron microscopy (SEM) images, the X-Y axis micro-pressure sandblast treated DWS-sliced mc-Si wafers could effectively remove the saw marks caused by the fixed diamond abrasives. Furthermore, the textured surface of the acidic-textured DWS-sliced mc-Si wafers was also observed in the SEM images. Besides, using the X-Y axis micro-pressure sandblast treatment, the average reflectivity of the DWS-sliced mc-Si wafers was reduced from 28.94% to 22.28%. Consequently, the short-circuit current of 8.70 A and the power conversion efficiency of 17.92% for the DWS-sliced mc-Si solar cells with micro-pressure sandblast treatment were respectively better than 8.59 A and 17.35% in comparison with the DWS-sliced mc-Si solar cells without micro-pressure sandblast treatment.

Electro-analytical investigation of potential induced degradation in mc-silicon solar cells: case of sodium ion induced inductive loop.

Potential induced degradation of the shunt type (PID-s) in multi-crystalline silicon (mc-Si) solar cells is becoming critical for performance reduction of solar panels in large scale photovoltaic (PV) power plants. In this article PID-s has been investigated by applying high voltage stress on mc-Si solar cells for their degradation and recovery and results have been explained on the basis of DC and AC characterization. The efficiency decreases drastically from 15.7% to 2.9% due to a high voltage stress of -800 V at 85 °C for 48 hours, which is attributed to a reduction in shunt resistance and an increase in depletion and diffusion capacitances. The reduction in electrical performance due to PID-s has been further explained by morphological, structural and elemental analysis. Observed negative capacitance behaviour in impedance spectra of mc-Si solar cells after PID-s has been attributed to structural deformation caused by potential induced migration of sodium ions (Na+) into mc-Si. The structural deformation induced by potential induced migration of Na+ ions has been confirmed by using non-destructive and lattice strain sensitive micro-Raman spectroscopy. The obtained experimental results have been correlated with existing theoretical understanding of p-n junction solar cells to explain the consequences of PID-s.

Broadband spectral response of diamond wire sawn mc-Si solar cell with omnidirectional performance and improved appearance

Highly efficient diamond wire sawn (DWS) multi-crystalline Si (mc-Si) solar cells with a satisfactory visual appearance are expecting to dominate the photovoltaic industry soon. Here, we report the realization of broadband spectral response of DWS mc-Si solar cells with omnidirectional performance and improved appearance. The success lies in the effective surface texturization based on MACE technique followed by post acid modification and the introduction of SiO2/SiNx stack layers on the rear side. Bowl-like pits with an open size of about 1µm are uniformly formed on the Si surface regardless of the crystallographic directions, which significantly enhances the antireflection ability in the short wavelength and makes the grain boundaries less noticeable. We have also shown that the bowl-like textured cells possess exceptional optical absorption over wide angles of incidence from 0° to 70°. Moreover, the SiO2/SiNx stack layers enhance the rear internal reflection and passivation, effectively increasing the long wavelength absorption and suppressing the electrical losses. We have successfully mass-produced DWS mc-Si solar cells with an average efficiency of 19.1%, which is 1.2% absolutely higher than that of the conventional micro-textured counterparts.

Rear passivated mc-Si solar cells textured by atmospheric pressure dry etching

In this contribution, atmospheric pressure dry etching (ADE) texture is integrated into a passivated emitter and rear cell (PERC) process. In order to make the texture suitable for the solar cell processing, two post-etching processes (inverted pyramid, spherical cap) are implemented and compared. The passivation of the front surface could be improved substantially by the introduction of an Al2O3 layer deposited via atomic layer deposition under the classical antireflection coating SiNX. The impact of the addition of the Al2O3 layer on the contact resistance cannot be neglected and becomes significant for thicknesses higher than 4 nm. Multicrystalline silicon (mc-Si) PERC solar cells are fabricated and measured, leading to a maximum efficiency of about 18.6% for the acidic texture, and for both posts-processed dry chemical etching (ADE) textures. No current gain for the ADE textured wafer compared to the acidic texture could be observed on cell level due to an inhomogeneous etching. However, a gain of 0.4 mA/cm2 for the ADE texture was calculated from local EQE measurements. ADE texture presents therefore 2 advantages: compatibility to diamond wire sawing and a potential current gain for PERC solar cell structure.

Pilot production of highly efficient metal catalyzed textured diamond wire sawn mc-Si solar cells combined with nickel-copper plated front contact processing

This paper presents first results of combining two promising future technologies: i) metal catalyzed textured diamond-wire sawn (DWS) mc-Si wafers and ii) nickel-copper-silver (Ni/CuAg) plated front contact processing. Results of first optimizations of laser patterning, annealing and module string soldering are presented. A 60-cell module with a power output of 266 W was successfully manufactured. The pseudo fill-factor (pFF) has been identified as the parameter with the highest potential for further improvements. A very promising approach is moving from an emitter profile that is mainly adapted to silver paste properties to an optimized emitter profile for Ni/Cu/Ag-plated contacts. The idea is to reduce the surface doping concentration (reduces Auger recombination) and increase the junction depth (increased pFF and FF).

Rapid mitigation of carrier-induced degradation in commercial silicon solar cells

We report on the progress for the understanding of carrier-induced degradation (CID) in p-type mono and multi-crystalline silicon (mc-Si) solar cells, and methods of mitigation. Defect formation is a key aspect to mitigating CID. Illuminated annealing can be used for both mono and mc-Si solar cells to reduce CID. The latest results of an 8-s UNSW advanced hydrogenation process applied to industrial p-type Czochralski PERC solar cells are shown with average efficiency enhancements of 1.1% absolute from eight different solar cell manufacturers. Results from three new industrial CID mitigation tools are presented, reducing CID to 0.8–1.1% relative, compared to 4.2% relative on control cells. Similar advanced hydrogenation processes can also be applied to multi-crystalline silicon passivated emitter with rear local contact (PERC) cells, however to date, the processes take longer and are less effective. Modifications to the firing processes can also suppress CID in multi-crystalline cells during subsequent illumination. The most stable results are achieved with a multi-stage process consisting of a second firing process at a reduced firing temperature, followed by extended illuminated annealing.

Optimized multicrystalline silicon for solar cells enabling conversion efficiencies of 22%

Multicrystalline (mc) n-type silicon has proven to be a suitable substrate for the fabrication of highly efficient mc-Si solar cells. In this paper, we elaborate the impact of base material parameters on the efficiency potential of n-type mc-Si solar cells featuring a boron-diffused front side emitter and a full-area passivating rear contact (TOPCon). The electrical material quality can be significantly improved by replacing the standard crystallization process with a seed-assisted growth for crystallization of high-performance (HP) mc silicon. Using high-purity quartz crucibles or larger crucibles in combination with an optimization of the grain boundary area fraction with an adapted seed structure leads to further improvements of the material quality in terms of charge carrier lifetimes. However, not only the charge carrier lifetime, but also the base resistivity is of crucial importance for the efficiency potential depending on the cell concept. Based on experimental data and simulations, we assess the optimal range for the base resistivity and the wafer thickness for n-type mc-Si TOPCon solar cells. With the optimal material parameters, an “efficiency limiting bulk recombination analysis” (ELBA) reveals an efficiency potential in the range of 22.5% for n-type mc-Si TOPCon solar cells. Finally, we fabricated TOPCon solar cells based on optimized n-type HP mc-Si substrate and demonstrate a certified efficiency of 21.9%, which is the highest efficiency reported for multicrystalline silicon solar cells so far.

Microcrystalline-Silicon Solar Cells With Photonic Crystals on the Top Surface

We investigate microcrystalline-silicon (μc-Si) solar cells with photonic crystals on the top surface, which exploit the large-area resonant effect in photonic crystals to enhance light absorption, for the first time. Guidelines for designing the surface photonic crystals are discussed, showing the importance of oblique leakage loss suppression. The influence of direct surface etching of the intrinsic μc-Si layer (or photovoltaic layer) is studied, and it is shown that unbalanced etching of the amorphous and crystalline components of the μc-Si layer deteriorates the electronic performance, in particular, the open-circuit voltage and fill factor. We then introduce photonic crystals under an adequate etching condition, and confirm that the light absorption (or the external quantum efficiency) is successfully enhanced by improving the incident light coupling to the resonant modes. Finally, we fabricate a few-micrometer-thick μc-Si solar cell with a photonic crystal on the top surface and demonstrate an active-area efficiency of 11%, resulting from enhancement of the short-circuit current density, while simultaneously suppressing the deterioration of open-circuit voltage and fill factor. The obtained efficiency is the highest reported for a category of μc-Si solar-cell in which all the n/i/p layers are composed of Si, excluding wide-gap materials such as silicon oxide.

Constructing submicron textures on mc-Si solar cells via copper-catalyzed chemical etching

Mass production of diamond-wire-sawn (DWS) multicrystalline silicon (mc-Si) solar cells reached a significant point of maturity through utilization of metal-catalyzed chemical etching (MCCE). However, nanotextured DWS mc-Si solar cells usually produced with Ag-MCCE still suffer from certain drawbacks, such as remaining saw marks, color differences among grains, and slight decreases in the open-circuit voltage (Voc). In this work, we show that unoriented Cu-based MCCE (Cu-MCCE) not only depresses the saw marks and color differences but also introduces random shallow pits, which act as artificial defects that can be easily converted into a submicron texture using conventional HNO3/HF etching. Moreover, we demonstrate that the efficiency of DWS mc-Si solar cells produced with the Cu-MCCE process is greater than 19%, with improved Voc resulting from better surface passivation. This cost-effective Cu-MCCE method is, therefore, of significant potential for the photovoltaic industry.

High-Efficient Solar Cells by the Ag/Cu-Assisted Chemical Etching Process on Diamond-Wire-Sawn Multicrystalline Silicon

In this paper, we presented a novel low-cost method for diamond-wire-sawn (DWS) multicrystalline silicon (mc-Si) wafer texturation based on the metal-assisted chemical etching process with Ag/Cu dual elements and nanostructure rebuilding (NSR) treatment to remove the saw marks and to realize uniform invert pyramid textured structures. Benefiting from both the increased optical absorption and better passivation, an efficiency of 18.71% for invert pyramid mc-Si solar cells from a DWS wafer with a standard size of 156 × 156 mm2 was obtained, which was 0.58% and 2.33% absolutely higher than that (18.13%) of the traditional mc-Si solar cell and than that (16.38%) of the black mc-Si solar cell without NSR treatment, respectively.

Multicrystalline Silicon Texturing By Novel Bi-Component Etching Solution

Texturization of surface of a solar cell plays a important role in reducing reflection of incident light. Performance of silicon solar cell depends both on reflectance and light trapping. In the past decades a large number of techniques for texturing of mono-crystalline or multi-crystalline silicon have already investigated by researcher. In spite of mono-crystalline silicon being more efficient, industry turned to multi-crystalline silicon (mc-Si) due to high cost of mono-crystalline silicon. Multi-crystalline silicon (mc-Si) is today the dominant technology for solar cells fabrication sharing more than 56% of market share.Texturing of multicrystalline silicon surface using various types of mixed etching solutions of Sodium Hypochlorite (NaOCl) with (i) Sodium hydroxide (NaOH) (ii) Tri-sodium phosphate(Na3PO4) .The reaction (etching) rate during the texturing process is easier to control due to the presence of NaOCl as an oxidizing agent in the solutions. Moreover, the reaction rate is more controllable because less alkalinity of tri-sodium phosphate compared with NaOH solution. Reflectivities of all the textured samples are measured by UV VIS NIR Spectrophotometer. Among all cases texturing with Na3PO4-NaOCl solution shows lowest reflectivity.Moreover, Na3PO4-NaOCl texturing solution can generate mc-Si polishing and texturization without grain boundaries delineation and thus has an excellent isotropic etch characteristics to enhance the performance parameter of solar cell. For the industrial production of mc-Si solar cells, the Na3PO4-NaOCl texturing can provide a convenient and low-cost alternative.