Multicrystalline Silicon Solar Cells Research Articles

Dislocation density reduction in multicrystalline silicon via cyclic annealing

Manufacturing processes to reduce dislocation densities in multicrystalline silicon (mc‐Si) solar cells could reduce the cost of photovoltaics by increasing cell efficiency with minimum additional process complexity. The present work investigates how cyclically varying temperature (stress) in mc‐Si blocks can lead to dislocation‐density reduction. Cyclic annealing is found to be more effective at activating immobile dislocations than isothermal annealing. Once immobile dislocations are activated, however, they are hypothesized to impede density reduction of mobile dislocations by accelerating multiplication of overall dislocations. Thus, a combined process of thermal cycling and isothermal annealing is found to provide a maximum dislocation density reduction of ∼70%, with 12 h of static annealing followed by 12 h of cyclic annealing.Dislocation etch‐pit images of as‐grown control and mc‐Si sample after 12 h cyclic annealing followed by 12 h static annealing.

Impact of structural defect density on gettering of transition metal impurities during phosphorus emitter diffusion in multi-crystalline silicon solar cell processing

The impact of structural defect density on gettering of transition metal impurities during phosphorous emitter diffusion has been investigated using a pair of multi-crystalline silicon (mc-Si) wafers. Chromium (Cr) impurities incorporated during growth were identified by deep level transient spectroscopy (DLTS) and used to evaluate the gettering efficiency. The Cr impurity concentration in the low defect density region of mc-Si wafers was reduced from ~3.5 × 1013 cm−3 to ~1.7 × 1012 cm−3 after phosphorous diffusion gettering (PDG), while for the high defect density region, there is no appreciable variation in the Cr concentration which only changed from ~3.0 to ~2.2 × 1012 cm−3 following PDG. It was concluded that the gettering process is not effective for highly defective regions of mc-Si wafers due to the ineffective impurity release from structural defects during the PDG process. Open image in new window

Theoretical and experimental analysis on effect of porous silicon surface treatment in multicrystalline silicon solar cells

As widely known in silicon-based solar cells, the intrinsic parameters are decisive factors for internal quantum efficiency (IQE) and solar cells performances. Particularly, the front surface recombination velocity is one of the most important electrical parameters used to quantify the optoelectronic quality of the solar cells. In this study, the impact of the front surface recombination velocities (Sf) and anti-reflection coating are examined. Theoretical analysis shows that a decrease in Sf enhances the IQE for photons with shorter wavelengths (400–700nm). Cells with front porous silicon treatment (PS) was made and studied under ambient condition to improve performance of solar cells. In fact, internal quantum efficiency of mc-Si solar cells with PS was enhanced in shorter wavelengths, while the reflectivity was reduced to about 7% after PS treatment and conversion efficiency was improved by about 5% compared with conventional mc-Si solar cells. Furthermore, the laser-beam-induced current (LBIC) mapping shows that the PS treatment improves their quantum response.

Degradation of multicrystalline silicon solar cells and modules after illumination at elevated temperature

In this work the performance stability of rear side passivated multicrystalline silicon solar cells and modules under carrier injection at different temperatures is investigated. Compared to most other tests the degradation procedure were extended to significantly longer time periods and include relevant module temperatures in field. Severe power degradation levels of above 10% can be detected after several hundred to thousand hours (corresponding ~5–20 years depending on the location) which cannot be explained by B–O complex formation or FeB pair dissociation. After detection of this light induced degradation at temperatures above 50°C the common abbreviation LID was renamed and state more precisely Light and elevated Temperature Induced Degradation (LeTID). A high number of cells and modules degraded in laboratory and outdoor using material from different wafer suppliers confirm the relevance of this effect. LeTID is a multicrystalline silicon bulk phenomenon leading to a highly injection dependent lifetime characteristic after degradation and features a regeneration phase after degradation. The time constant of this degradation mechanism accelerates with increasing temperature, however, the time span for degradation and regeneration of thousands of hours at relevant temperatures between 60 and 85°C demands for a solution on wafer material or processing side. LeTID can be significantly reduced by adapting the cell process and processing sequence.

Next-generation multi-crystalline silicon solar cells: Diamond-wire sawing, nano-texture and high efficiency

The absence of an effective texturing technique for diamond-wire sawn multi-crystalline silicon (DWS mc-Si) solar cells has hindered commercial upgrading from traditional multi-wire slurry sawn silicon (MWSS mc-Si) solar cells. In this paper, a nano-texture technique has been developed to achieve 18.31% efficient DWS mc-Si solar cells on a pilot production line. Their unique pyramidal nanostructure, which has the most close-packed {111} surface of Si diamond crystal, not only benefits both light-trapping and electric properties but also can effectively remove the saw-marks and amorphous layer of the cells. Therefore, the short-circuit current Isc of a nano-textured DWS mc-Si solar cell is ~324mA higher than that of a micron-textured one, while its open-circuit voltage Voc does not show an evident decrease with the increase of its surface area. The technique has paved the way for the mass production of DWS mc-Si solar cells by satisfying the exact requirements of the PV industry for high efficiency and low cost.

Improving the Efficiency of Multicrystalline Silicon by Adding an ARC Layer in the Front Device

High efficiency multicrystalline solar cells must improve performance while replacing higher cost monocrystalline silicon with lower cost multicrystalline silicon. Composite silicon dioxide-titanium dioxide (SiO2−TiO2) films are deposited on a large area of 15.6×15.6 cm2 textured multicrystalline silicon solar cells to increase the incident light trapped within the device. This is being achieved through new cell device structures that improve light trapping and energy conversion capability. These new structures depend on passivated thick and thin layers of silicon dioxide and titanum dioxide grown via wet and dry thermal oxidation. By replacing dry oxidation with wet oxidation the temperatures process can be lowered from 1050∘C to 850∘C to reduce both cycle time and wafer damage.

Optimization of the high-performance multi-crystalline silicon solidification process by insulation partition design using transient global simulations

A transient global model was established to investigate the effect of the raise velocities of the partition block on the crystal growth rate, the crystal–melt (C–M) interface and the thermal stress distribution during the solidification process. The simulation results showed that among the different raise velocities of the partition block, initially slowly raising and then rapidly raising the partition block was the most favorable for the solidification process. A slightly convex C–M interface and low thermal stress distribution were obtained, and a fast crystal growth rate was also achieved. Thus, this design was implemented in casting experiments, and the experimental results indicated that this design was beneficial for optimizing the C–M interface in the solidification process. The average conversion efficiencies of high-performance multi-crystalline silicon solar cells was about 0.13% higher with this design (18.18%) than with a fixed partition block design (18.05%).

Study on the front contact mechanism of screen-printed multi-crystalline silicon solar cells

The two key issues of front contact of screen-printed multi-crystalline silicon have been studied. Firstly, the effects of doping profiles in the emitter on the contact-resistance have been systematically investigated. Constant source diffusion is used to explain the doped phosphorus profile in the textured emitter by simulation. Secondly, the penetration mechanism of silver into SiNx layer was studied by using SEM and EDX to reveal the microstructure of the Ag/Si contact. A model has been proposed to explain the formation of the ohmic contact. It is found that the effectiveness of the contact is determined by the direct contact area of the silver crystallites to both the emitter and the finger. For a commercial multi-crystalline silicon solar cell of 156×156mm2, the optimal contact-resistance of about 1.2mΩ is suggested, corresponding to the doping sheet resistance of around 80–90Ω/□. Multi-crystalline Si solar cells with excellent results, up to 18.51% in conversion efficiency, have been achieved. Moreover, to further understand the model, effects of the firing temperature and the thickness of SiNx layer on the contact-resistance have been investigated in the solar cells. It is proved that the formation of the silver crystallites is related to the existence of SiNx layer.

Influence of porous silicon formation on the performance of multi-crystalline silicon solar cells

The effect of formation of porous silicon on the performance of multi-crystalline silicon (mc-Si) solar cells is presented. Surface treatment of mc-Si solar cells was performed by electrochemical etching in HF-based solution. The effect of etching is viewed through scanning electron microscope (SEM) photographs that indicated the formation of a porous layer on the surface. Total reflection spectroscopy measurements on solar cells revealed reduced reflection after etching. In order to demonstrate the effect of this porous layer on the solar cell performance, illumination-dependent j–V characteristics and spectral response measurements were performed and analysed before and after etching. At all illumination intensities, short-circuit current density and open-circuit voltage values for the etched solar cell were higher than those before etching, whereas fill factor values were lower for the etched cell at high illumination intensities. An interpretation of these findings is presented.

Electric properties and carrier multiplication in breakdown sites in multi-crystalline silicon solar cells

This paper studies the effective electrical size and carrier multiplication of breakdown sites in multi-crystalline silicon solar cells. The local series resistance limits the current of each breakdown site and is thereby linearizing the current-voltage characteristic. This fact allows the estimation of the effective electrical diameters to be as low as 100 nm. Using a laser beam induced current (LBIC) measurement with a high spatial resolution, we find carrier multiplication factors on the order of 30 (Zener-type breakdown) and 100 (avalanche breakdown) as new lower limits. Hence, we prove that also the so-called Zener-type breakdown is followed by avalanche multiplication. We explain that previous measurements of the carrier multiplication using thermography yield results higher than unity, only if the spatial defect density is high enough, and the illumination intensity is lower than what was used for the LBIC method. The individual series resistances of the breakdown sites limit the current through these breakdown sites. Therefore, the measured multiplication factors depend on the applied voltage as well as on the injected photocurrent. Both dependencies are successfully simulated using a series-resistance-limited diode model.

Performance enhancement of multicrystalline silicon solar cells and modules using double‐layered SiNx:H antireflection coatings

AbstractSilicon nitride coating deposited by the plasma‐enhanced chemical vapor deposition method is the most widely used antireflection coating for crystalline silicon solar cells. In this work, we employed double‐layered silicon nitride coating consisting of a top layer with a lower refractive index and a bottom layer (contacting the silicon wafer) with a higher refractive index for multicrystalline silicon solar cells. An optimization procedure was presented for maximizing the photovoltaic performance of the encapsulated solar cells or modules. The dependence of their photovoltaic properties on the thickness of silicon nitride coatings was carefully analyzed. Desirable thicknesses of the individual silicon nitride layers for the double‐layered coatings were calculated. In order to get statistical conclusions, we fabricated a large number of multicrystalline silicon solar cells using the standard production line for both the double‐layered and single‐layered antireflection coating types. On the cell level, the double‐layered silicon nitride antireflection coating resulted in an increase of 0.21%, absolute for the average conversion efficiency, and 1.8 mV and 0.11 mA/cm2 for the average open‐circuit voltage and short‐circuit current density, respectively. On the module level, the cell to module power transfer factor was analyzed, and it was demonstrated that the double‐layered silicon nitride antireflection coating provided a consistent enhancement in the photovoltaic performance for multicrystalline silicon solar cell modules than the single‐layered silicon nitride coating. Copyright © 2015 John Wiley & Sons, Ltd.

Synchrotron-based analysis of chromium distributions in multicrystalline silicon for solar cells

Chromium (Cr) can degrade silicon wafer-based solar cell efficiencies at concentrations as low as 1010 cm−3. In this contribution, we employ synchrotron-based X-ray fluorescence microscopy to study chromium distributions in multicrystalline silicon in as-grown material and after phosphorous diffusion. We complement quantified precipitate size and spatial distribution with interstitial Cr concentration and minority carrier lifetime measurements to provide insight into chromium gettering kinetics and offer suggestions for minimizing the device impacts of chromium. We observe that Cr-rich precipitates in as-grown material are generally smaller than iron-rich precipitates and that Cri point defects account for only one-half of the total Cr in the as-grown material. This observation is consistent with previous hypotheses that Cr transport and CrSi2 growth are more strongly diffusion-limited during ingot cooling. We apply two phosphorous diffusion gettering profiles that both increase minority carrier lifetime by two orders of magnitude and reduce [Cri] by three orders of magnitude to ≈1010 cm−3. Some Cr-rich precipitates persist after both processes, and locally high [Cri] after the high-temperature process indicates that further optimization of the chromium gettering profile is possible.

Efficient passivation of solar cells by silicon nitride

In this study we investigated the deposition parameters of a PECVD silicon nitride (SiN) film leading to an efficient passivation of multicrystalline silicon solar cells. The parameters investigated were the temperature, the power and the refractive index through the gas process flow rate ratio SiH4/NH3.Using a symmetrical structure SiN/Si/SiN and Quasi-Steady-State Photo-Conductance (QSSPC) characterization we found that 380 °C and 4500 W are the optimal temperature and optimal power, respectively. The optimal refractive index was determined using a method which encompasses optical and electrical properties of SiN films deposited on multicrystalline silicon solar cells. This method, based on the calculation of the short circuit current densities, revealed that the optimal refractive index is 1.9.

Circularly polarised solar antenna for airborne communication nodes

A circularly polarised solar cell antenna consisting of four sequentially rotated printed inverted-F antennas is proposed. Four multicrystalline silicon solar cells act as the ground plane and the antenna is suitable for low-power airborne communication nodes and wireless sensor networks. The antenna design was developed to allow 100% insolation of the cells when directly facing a light source. The low-profile antenna minimises shadowing of the solar cell for oblique angle insolation.

The luminescent solar concentrators with the H-aggregate of perylene diimide dye imbedded into PMMA

A perylene diimide fluorescent dye, N,N′-bis-(2-tert-butylaniline)-3,4,9,10-perylenetetracarboxylic diimide (BTBPD) was synthesized and crystallized to H-aggregate. The dye aggregates were studied for their photophysical properties, such as absorption and emission behavior, fluorescence efficiency, and Stokes shift in comparison with synthesized raw powder-formed dyes. The fluorescent dyes were then imbedded into the PMMA sheet through the thermal free radical polymerization of a methyl methacrylate monomer. Various luminescent solar concentrators were fabricated using these dye-imbedded PMMA sheets with the aid of a multi-crystalline silicon solar cell, side mirror, and backside diffuse reflector. The solar cells were studied on their photovoltaic performance, such as Voc, Isc, photoelectric conversion efficiencies, IPCE, and external quantum efficiencies. The effect of the crystalline form of the dyes on solar cell performance was investigated.

Influence of Different Types of Recombination Active Defects on the Integral Electrical Properties of Multicrystalline Silicon Solar Cells

In this contribution the influence of different types of recombination-active defects on the integral electrical properties of multicrystalline Si solar cells is investigated. Based on a previous classification scheme related to the luminescence behavior of crystal defects, Type-A and Type-B defects are locally distinguished. It is shown that Type-A defects, correlated to iron contaminations, are dominating the efficiency by more than 20% relative through their impact on the short circuit current ISC and open circuit voltage VOC in standard Si material (only limited by recombination active crystal defects). Contrarily, Type-B defects show low influence on the efficiency of 3% relative. The impact of the detrimental Type-A defects on the electrical parameters is studied as a function of the block height. A clear correlation between the area fraction of Type-A defects and both the global Isc and the prebreakdown behavior (reverse current) in voltage regime-2 (−11 V) is observed. An outlier having an increased full-area recombination activity is traced back to dense inter- and intragrain nucleation of Fe precipitates. Based on these results it is concluded that Type-A defects are the most detrimental defects in Si solar cells (having efficiencies > 15%) and have to be prevented by optimized Si material quality and solar cell process conditions.

Realization of improved efficiency on nanostructured multicrystalline silicon solar cells for mass production

We report the realization of both excellent optical and electrical properties of nanostructured multicrystalline silicon solar cells by a simple and industrially compatible technique of surface morphology modification. The nanostructures are prepared by Ag-catalyzed chemical etching and subsequent NaOH treatment with controllable geometrical parameters and surface area enhancement ratio. We have examined in detail the influence of different surface area enhancement ratios on reflectance, carrier recombination characteristics and cell performance. By conducting a quantitative analysis of these factors, we have successfully demonstrated a higher-than-traditional output performance of nanostructured multicrystalline silicon solar cells with a low average reflectance of 4.93%, a low effective surface recombination velocity of 6.59 m s−1, and a certified conversion efficiency of 17.75% on large size (156 × 156 mm2) silicon cells, which is ∼0.3% higher than the acid textured counterparts. The present work opens a potential prospect for the mass production of nanostructured solar cells with improved efficiencies.

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

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

Photovoltaic performance and LCoE comparison at the coastal zone of the Atacama Desert, Chile

Two photovoltaic technologies are compared with regard to the energy yield, performance ratio and their levelized cost of energy. Plants based on amorphous/microcrystalline silicon tandem thin films and multicrystalline silicon solar cells installed at the coastal zone of the Atacama Desert, Chile, were monitored for 21months. This region can be one of the most suitable places for the use of solar energy due to the high solar radiation levels. However, the coastal desert climate may influence the performance of photovoltaic systems. The global tilted solar irradiation reached mean values of 8.6kWh/m2day in summer and 6kWh/m2day in winter demonstrating the high irradiation available. It came out that the performance ratio is influenced by the dust accumulation and the temperature associated to this place. The performance ratio of thin films decreased due to the dust accumulation at a rate from −4.2 to −3.7%/month for decreasing temperature and from −4.8 to −4.4%/month for increasing temperature. For multicrystalline silicon modules, the degradation rates were −2.4 to −1.8%/month for decreasing temperature, and −6.2 to −3.7%/month for increasing temperature. It was concluded that the electricity costs were 14.48cents€/kWh and 15.65cents€/kWh for thin film and mc-Si, respectively. Thus, the thin films had more benefit after cleaning than multicrystalline modules.

Characterization of deep level defects present in mono-like, quasi-mono and multicrystalline silicon solar substrates

Defects on mono-like (ml-Si), quasi-mono (qm-Si) and multicrystalline silicon solar cell substrates are studied in depth. Using the thermal admittance spectroscopy technique we found a single deep level with an activation energy between 213 and 224 meV and a capture cross section in the order of 10−15−10−14 cm2, in the case of ml-Si samples. The 271, 291 and 373 meV levels were found in qm-Si samples. The first one is associated with a capture cross section in the order of 10−16 cm2, the second one in the order of 10−14, while the third one is associated, for the same magnitude, with a value in the order of 10−12 cm2. Multicrystalline samples showed two tendencies in the Arrhenius plot fit associated with a deep level in each one. The activation energy of the first one ranges from 336 meV to 342 meV, and the capture cross sections are in the order of 10−13−10−11 cm2. The values obtained for the second one are 251 and 171 meV, with the capture cross section values in the order of 10−15 and 10−18 cm2, respectively. The nature of these defects is probably due to iron-based impurities in different complexes. Segregation into extended defects of Fei or Fei-V is the most probable cause of the deep levels with higher capture cross section value. Punctual complexes such as Fei or Fei-V2 are probably the reason for the deep levels with lower capture cross section value.