Industrial Multicrystalline Silicon Solar Cells Research Articles

Conductive Hole‐Selective Passivating Contacts for Crystalline Silicon Solar Cells

AbstractDefect state passivation and conductivity of materials are always in opposition; thus, it is unlikely for one material to possess both excellent carrier transport and defect state passivation simultaneously. As a result, the use of partial passivation and local contact strategies are required for silicon solar cells, which leads to fabrication processes with technical complexities. Thus, one material that possesses both a good passivation and conductivity is highly desirable in silicon photovoltaic (PV) cells. In this work, a passivation‐conductivity phase‐like diagram is presented and a conductive‐passivating‐carrier‐selective contact is achieved using PEDOT:Nafion composite thin films. A power conversion efficiency of 18.8% is reported for an industrial multicrystalline silicon solar cell with a back PEDOT:Nafion contact, demonstrating a solution‐processed organic passivating contact concept. This concept has the potential advantages of omitting the use of conventional dielectric passivation materials deposited by costly high‐vacuum equipment, energy‐intensive high‐temperature processes, and complex laser opening steps. This work also contributes an effective back‐surface field scheme and a new hole‐selective contact for p‐type and n‐type silicon solar cells, respectively, both for research purposes and as a low‐cost surface engineering strategy for future Si‐based PV technologies.

3-D Modeling of Multicrystalline Silicon Materials and Solar Cells

We present a method to model large-area multicrystalline materials using Quokka3, based on artificially generated lifetime images created by combining injection-dependent intragrain lifetimes and defect recombination velocity maps extracted from photoluminescence-based lifetime images. It is found that simulations based only on measured lifetime images underestimate the detrimental impacts of crystal defects and, hence, overestimate the overall cell performance of multicrystalline silicon solar cells. As demonstration, we applied Quokka3 simulations to resolve various bulk recombination losses in industrial multicrystalline silicon solar cells, quantify the effects of phosphorus diffusion and hydrogenation on the final cell performance, and evaluate the detrimental impact of crystal defects on individual cell parameters. Through numerical simulations, this paper has also demonstrated the potential errors of using a single average value to predict the cell performance of inhomogeneous multicrystalline silicon materials. It is observed that the commonly used harmonic mean can provide a good indication for V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> , but is less effective for predicting J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> , and is almost entirely ineffective for predicting the fill factor.

Enhancing power conversion efficiency of multicrystalline silicon solar cells by plasmonic effect of Ag nanoparticles embedded in SiNx layer

In this paper, we demonstrate that the performance of the industrial multicrystalline silicon solar cells can be improved by embedding the silver nanoparticles (Ag-NPs) into the SiNx layer. On the one hand, the cells have a certain optical loss in short wavelengths near the plasmonic resonance frequency of Ag-NPs, but their open circuit voltages and filling factors are increased due to depressed surface recombination as those short wavelength photons are mainly absorbed by Ag-NPs instead of the surface; on the other hand, the cells show strong absorption in long wavelengths, which can be attributed to the forward-scattering effect of Ag-NPs. Taking together, UV-absorbing Ag-NPs may act as a “sunscreen” to shield the UV damage, while improve the cell efficiency from 18.05 % to 18.25 % by embedding proper Ag-NPs. The techniques presented in this work can be easily incorporated into the current mc-Si solar cell production line, thus have great potential for the mass practical application.

A continuous, single-face wet texturing process for industrial multicrystalline silicon solar cells using a surfactant treated photoresist mask

In this paper, a high-throughput continuous, single-face texturing method was introduced into the fabrication process for industrial multicrystalline silicon solar cell production. With a patterned photoresist film as the mask for the wet etching, wafers without back protection were transported by rollers and only the masked face could contact the etching solution and be etched through the mask openings. The first etching results indicated that serious heat aggregation and poor wettability of the acid solution on the mask surface resulted in a seriously uneven and uncontrolled etching reaction. However, after applying a surfactant to the photoresist mask to improve the mask's hydrophility, a stable and uniform etching reaction could be achieved. A honeycomb-like textured surface with a pitch of 18 µm was fabricated successfully. The etched pits had a nearly smooth spherical segment surface and a surface occupancy of 70%. This regular textured surface had a low in-plane average reflectivity of approximately 20% and greatly increased the carrier lifetime. Compared with multicrystalline silicon solar cells textured by conventional acid etching, the average efficiency increased from 18.31% to 18.57%. In addition, this texturing technique is expected to promote the application of diamond-wire cut multicrystalline silicon wafers in the future.

Effect of Surface Structure on Electrical Performance of Industrial Diamond Wire Sawing Multicrystalline Si Solar Cells

We report industrial fabrication of different kinds of nanostructured multicrystalline silicon solar cells via normal acid texturing, reactive ion etching (RIE), and metal-assisted chemical etching (MACE) processes on diamond wire sawing wafer. The effect of different surface structure on reflectivity, lifetime, and electrical performance was systematically studied in this paper. The difference between industrial acid, RIE, and MACE textured multicrystalline silicon solar cells to our knowledge has not been investigated previously. The resulting efficiency indicates that low reflectivity surface structure with the size of 0.2–0.8 μm via RIE and MACE process do not always lead to low lifetime compared with acid texturing process. Both RIE and MACE process is promising candidate for high efficiency processes for future industrial diamond wire sawing multicrystalline silicon solar cells.

A high efficiency industrial polysilicon solar cell with a honeycomb-like surface fabricated by wet etching using a photoresist mask

In this paper, an effective and low cost method of texturization was introduced into the fabrication process for industrial multicrystalline silicon solar cell production. The purpose of the method was to reduce reflectance by creating a honeycomb-like textured surface using a masked wet etching process. A negative photoresist film was selected as an etching mask. Although large surface roughness of wafer was considered to affect the adhesion and acid resistance of etching mask, a honeycomb-like textured surface with a pitch of 18μm was fabricated successfully. The etched pits had a nearly smooth spherical segment surface, an average aperture of 15.1μm, and a depth of 6.5μm. This regular textured surface had a low light reflectivity of approximately 20.5% and greatly increased the carrier lifetime. Compared with multicrystalline silicon solar cells textured by conventional acid etching, the average short circuit current increased by 2.2% and the average efficiency increased from 17.41% to 17.75%, a net gain of 0.34%. And a high throughput above 2400 pieces per hour was obtained. This texturing technique is expected to promote the application of diamond-wire cut multicrystalline silicon wafers with the low saw-damage in the future.

Photoluminescence image evaluation of solar cells based on implied voltage distribution

All previous methods for quantitatively evaluating photoluminescence (PL) images of solar cells assumed a laterally constant short circuit current density Jsc. Moreover, they had to subtract a Jsc PL image from all other PL images for considering the diffusion-limited carriers. Here a more realistic PL evaluation method is introduced, which is based on a recently published alternative model of the illuminated solar cell. In this model an analytic expression is derived by considering the illuminated current as a diffusion process between bulk and the pn-junction and linking the implied voltage in the bulk with the local pn-junction voltage under illumination. This model does not assume a laterally constant Jsc but a constant light absorption rate, and it leads to a prediction of the Jsc distribution solely based on PL imaging results. Moreover, it regards the shadowing of the cell by the busbars and grid lines. This model is applied to the quantitative evaluation of PL images of an industrial multicrystalline silicon solar cell. The resulting series resistance and saturation current density images are compared with that of an established PL evaluation method, and the resulting distribution of Jsc is compared with LBIC results. The results of the new method appear slightly more realistic than that of the old one, since they consider the inhomogeneity of Jsc.

A simplified method to modulate colors on industrial multicrystalline silicon solar cells with reduced current losses

This paper investigates the impact of SiOxNy/SiNx:H double layer antireflection (DLAR) coatings on color modulation and cell efficiencies of multicrystalline silicon (mc-Si) solar cells. We presented a three dimensional ellipsoid surface model to simulate the concave-like morphology formed by acidic texturization. Afterwards, reflectivities of these acidic textures coated by DLAR coatings over 300–1100nm are calculated by Monte Carlo ray tracing method. Simulated results show that DLAR coatings can flexibly modulate the color with reduced current losses compared to single SiNx:H layer. SiOxNy was deposited by electron beam evaporation onto fabricated industrial solar cells to vary their colors. The busbars were sheltered by a mask to prevent the deposition of SiOxNy on them. This simplified method avoids adjustment of the standard fabricating process. High agreement between simulated and measured reflectance is achieved. The IV test results of colored solar cells are in good accord with the calculated results, which indicates the effectiveness of DLAR coatings in reducing the current losses.

High-Resolution Lock-in Thermography Investigation on Industrial Multicrystalline Silicon Solar Cells

Industrial multicrystalline silicon (mc-Si) solar cells with different types of shunts have been analyzed in detail by dark lock-in thermography (DLIT). Several types of nonlinear shunts were found in our samples and most of them could only be detected in low forward-bias images of DLIT. However, we also observed nonlinear shunts that are only visible or have much stronger signal under the low reverse-bias than equivalent forward-bias condition, which is a new finding compared with the common observations. The edge of an mc-Si solar cell was found vulnerable for shunting. A weak leakage current around edges was frequently observed under 0.5 V forward bias on both shunted and normal cells. It reveals that the edge is one of the major recombination paths under the cell operation condition, which is due to the imperfect edge passivation and material quality limitation of mc-Si. Light-beam-induced current (LBIC) was also applied on one material-induced shunt. LBIC mapping with long wavelength revealed the degraded current response due to poor wafer quality. However, an LBIC image of short wavelength did not show the defect structure because the current was dominated by the Auger recombination, while not influenced by the bulk lifetime. Some pre-breakdown sites were found in the material-induced shunt sample and were only visible under the reverse-bias condition of DLIT.

Improved multicrystalline Si solar cells by light trapping from Al nanoparticle enhanced antireflection coating

Significant photocurrent enhancement of 0.4 mA/cm2 has been achieved for industrial textured multicrystalline silicon solar cells, due to the light trapping provided by aluminium nanoparticle enhanced antireflection coating. Aluminium nanoparticles support surface plasmon resonances, which can effectively scatter the light into the solar cells. By blue shifting the detrimental Fano resonances away from the important silicon absorption spectrum, aluminium nanoparticles can provide a broadband light absorption enhancement without a reduction at the short wavelengths. Combining the discovery with 75 nm silicon nitride antireflection coating, which can significantly enhance the absorption at the peak solar spectrum, we have achieved the strong broadband light absorption enhancement.

Electron microscope verification of prebreakdown-inducing α-FeSi2 needles in multicrystalline silicon solar cells

It had been shown already earlier by X-ray microanalysis that, in positions of defect-induced junction breakdown in industrial multicrystalline (mc) silicon solar cells, iron-containing precipitates may exist. However, the nature of these precipitates was unknown so far. Here, in such positions, scanning transmission electron microscopy was performed after defect-controlled focused ion beam preparation. First of all, the defect site was localized by microscopic reverse-bias electroluminescence imaging. The high accuracy of following FIB target preparation (&amp;lt;0.1 μm necessary) was obtained by both, electron beam-induced current imaging and secondary electron material contrast observation during the slice-by-slice milling of the TEM specimen. By nano-beam electron diffraction (NBED) and energy dispersive spectroscopy, the iron-containing precipitates were identified as α-type FeSi2 needles, about 30 nm in diameter and several μm in length. The FeSi2 needles show preferential orientation relationships to the silicon matrix and are located in terraced large-angle grain boundaries. Elaborate nano-beam electron diffraction investigation of the FeSi2 revealed orientation relationships of the precipitate to the silicon, which confirm earlier investigations on monocrystalline material. A model explaining the defect-induced breakdown mechanism due to rod-like α-FeSi2 precipitates is presented.

Lock-in and Heterodyne Carrierographic Imaging Characterization of Industrial Multicrystalline Silicon Solar Cells

A novel non-destructive, non-contact laser-induced near-infrared imaging technique using an InGaAs camera and based on dynamic carrierography (a spectrally gated modulated photoluminescence modality) was used in direct lock-in (LICG) and heterodyne (HDCG) modes to characterize industrial multicrystalline silicon solar cells. The image amplitude depends on the free-photocarrier diffusion-wave density, the spatial resolution is a function of modulation frequency, and the contrast is due to the spatial distribution of important parameters influencing the efficiency of the solar cell, such as base recombination lifetime. High-spatial-resolution and high-frequency carrierographic images have been obtained using a HDCG method. The relationship between the LICG amplitude and the solar cell terminal photovoltage under an external load was investigated. The influence of surface recombination velocities and damage of the solar cell p–n junction was studied. A correlation between the solar cell efficiency and the surface-integrated LICG amplitude from the entire solar cell is presented.

Characterization of PIII textured industrial multicrystalline silicon solar cells

Optimized textured structure is one of the most important elements for high efficiency multicrystalline silicon solar cells. In this paper, in order to incorporate low reflectance nanostructures into conventional industrial solar cells, structures with aspect ratios of about 1:1 and average reflectance of 8.0% have been generated using plasma immersion ion implantation. A sheet resistance of 56.9Ω/sq has been obtained by adjusting the phosphorous diffusion conditions, while the thickness of the silicon nitride vary in 70–80nm by extending the deposition time by 100s. Under the conventional co-firing conditions, a solar cell with efficiency of 16.3% and short-circuit current density 34.23mA/cm2 has been fabricated.

Silicon lifetime enhancement by SiNx:H anti-reflective coating deposed by PECVD using SiH4 and N2 reactive gas

Hydrogenated films of silicon nitride SiNx:H are largely used as antireflective coating as well as passivation layer for industrial crystalline and multicrystalline silicon solar cells. In this work, we present a low cost plasma enhanced chemical vapor deposition (PECVD) of this thin layer by using SiH4 and N2 as a reactive gases. A study was carried out on the variation effect of the ratio silane (SiH4) to nitrogen (N2) and time deposition on chemical composition, morphologies, reflectivity and carrier lifetime. The thickness was varied, in order to obtain a homogeneous antireflective layer. The Fourier transmission infrared spectroscopy (FTIR) shows the existence of Si–N and Si–H bonds. The morphologies of the sample were studied by Atomic Force Microscopy (AFM). The resulting surface of the SiNx:H shows low-reflectivity less than 5% in wavelength range 400–1200nm. As a result, an improvement in minority carrier lifetime has been achieved to about 15μs.

Understanding junction breakdown in multicrystalline solar cells

Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 Ω cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between −4 and −9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between −9 and −13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond −13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade material these classes can be distinguished. However, due to the higher net doping concentration of this material, their onset voltage is considerably reduced here.

Non-destructive infrared optoelectronic lock-in carrierography of mc-Si solar cells

A novel non-destructive, non-contact laser-induced infrared lock-in carrierographic imaging technique based on photocarrier radiometry (PCR) was introduced to characterize industrial multi-crystalline silicon solar cells. The modulated photovoltage (MPV) was measured simultaneously with the PCR signal in point-by-point imaging using focused laser scanning. The PCR and MPV signals were also investigated as functions of modulation frequency and load resistance at fixed coordinate points in order to better understand and interpret the physical optoelectronic origins of the carrierographic imaging contrast. A InGaAs-camera based carrierograhic lock-in imaging technique with a spread laser-beam illumination of solar cells was also introduced and was shown to exhibit contrast based on p-n junction RC time constants and on quasi-neutral bulk minority carrier recombination lifetimes.

Luminescence emission from forward- and reverse-biased multicrystalline silicon solar cells

We study the emission of light from industrial multicrystalline silicon solar cells under forward and reverse biases. Camera-based luminescence imaging techniques and dark lock-in thermography are used to gain information about the spatial distribution and the energy dissipation at pre-breakdown sites frequently found in multicrystalline silicon solar cells. The pre-breakdown occurs at specific sites and is associated with an increase in temperature and the emission of visible light under reverse bias. Moreover, additional light emission is found in some regions in the subband-gap range between 1400 and 1700 nm under forward bias. Investigations of multicrystalline silicon solar cells with different interstitial oxygen concentrations and with an electron microscopic analysis suggest that the local light emission in these areas is directly related to clusters of oxygen.

A new energy efficient, environment friendly and high productive texturization process of industrial multicrystalline silicon solar cells

A new texturization process based on a uniform, isotropic and slow removal of silicon, using a composition of sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) solution at an elevated temperature is developed recently for multicrystalline silicon solar cells. This process is applied in optimized condition in regular industrial production line and it immediately replaces the old popular industrial process of texturization using a combination of NaOH solution, alcoholic NaOH solution and hydrochloric acid solution in different steps at a higher temperature. Also the gain in solar cell efficiency at global AM1.5 spectrum, 1 SUN intensity condition is nearly 10% in final value. In addition, it has become finally an energy efficient and environment friendly texturization process for large area multicrystalline silicon solar cells for commercial use. In this paper the cost effectiveness and environment friendly aspects of the proposed process have been studied in detail along with the surface texture analysis of wafers with SEM and AFM micrographs to substantiate the reasons behind the above facts.

Correlation of optical and photoluminescence properties in amorphous SiN x:H thin films deposited by PECVD or UVCVD

Hydrogenated silicon nitride SiN x :H films are largely used as antireflective coating as well as passivation layer for industrial crystalline and multicrystalline silicon solar cells. This work is focused on the optical and photoluminescence (PL) properties of SiN x :H deposited by either Plasma-Enhanced Chemical Vapour Deposition (PECVD) or UltraViolet photo-assisted CVD (UVCVD). Photoluminescence phenomena were investigated in SiN x :H having different stoechiometries. On the other hand, spectroscopic ellipsometry was carried out in order to obtain the optical properties of the films, from which the optical gap could be determined. The evolution of the photoluminescence with stoechiometry was correlated to the evolution of the optical properties, and especially the absorption within the SiN x :H layer. A good agreement was found considering the confinement of excitons in strongly absorbing silicon nanostructures (ns-Si) formed in the SiN x matrix, with different sizes according to the NH 3 / SiH 4 gas flow ratio and the deposition technique. The main PL peak showed an increase of the emission intensity along with a blueshift as silicon concentration decreases. These observations indicate a radiative recombination mechanism dominated by confined excitons within ns-Si rather than emission related to defects. Furthermore, these ns-Si are supposed to be responsible of the global higher absorption, and hence lower optical gap, of the near-stoechiometric SiN x :H films in comparison with the stoechiometric Si 3N 4 ones. These assumptions were confirmed thanks to transmission electron microscopy (TEM) images performed on one of the samples, showing crystalline silicon quantum dots (c-Si QDs) embedded in the SiN x matrix.

Novel low cost chemical texturing for very large area industrial multi-crystalline silicon solar cells

Multi-crystalline silicon surface etching without grain-boundary delineation is a challenging task for the fabrication of high efficiency solar cells. The use of sodium hydroxide–sodium hypochlorite (NaOH–NaOCl) solution for texturing a multi-crystalline silicon wafer surface in a solar cell fabrication line is reported in this paper. The optimized etching solution of NaOH–NaOCl does not have any effect on multi-crystalline silicon grain boundaries and it also has excellent isotropic etch characteristics, which ultimately helps to achieve higher values of performance parameters, especially the open circuit voltage (Voc) and fill factor (FF), than those in the case of conventional silicon texturing. Easy control over the reaction of the NaOH–NaOCl solution is also one of the major advantages due to which sophistication in controlling the temperature of the etching bath is not required for the industrial batch process. The FTIR analysis of the silicon surface after etching with the current approach shows the formation of Si–Cl bonds, which improves the quality of the diffused junction due to chlorine gettering during diffusion. We are the first to report 14–14.5% efficiency of very large area (150 mm × 150 mm) multi-crystalline silicon solar cells using a NaOH–NaOCl texturing approach in an industrial production line with a yield greater than 95%.