A study of electrical uniformity for monolithic polycrystalline silicon solar cells

The aim of this work is to investigate the electrical uniformity of monolithic polycrystalline silicon solar cells prepared by various process techniques. By a series of experiments such as P and Al impurity gettering and silicon nitride passivation, a new conclusion is that the application of P and Al gettering as well as silicon nitride passivation enhances the electrical uniformity of small area solar cells diced from the same polycrystalline silicon solar cells, even if impurity gettering is not effective when the dislocation density is above a threshold value of about 10⁶ cm⁻². The experiments give us some hints that when we cut large area polycrystalline silicon solar cells into small pieces needed for application, we should modify production process slightly.     

Solar cells prepared on multicrystalline silicon subjected to new gettering and passivation treatments

New combined gettering and passivating procedures for solar cells prepared from multicrystalline silicon (mc-Si) have been considered. Passivation has been performed by (i) diamond-like carbon films deposition onto front or rear side of the wafers with following annealing, or (ii) hydrogen plasma treatments. Gettering region has been formed by deposition of Al film on specially prepared Si with developed surface. The advantages of such a gettering process in comparison with traditional gettering with Al are demonstrated. The improving influence of the treatments on diffusion length in mc-Si and efficiency of prepared solar cells have been found out. Physical mechanisms responsible for the observed effects of gettering and passivation are discussed.     

Application of phosphorus diffusion gettering process on upgraded metallurgical grade Si wafers and solar cells

Although phosphorus (P) diffusion gettering process has been wildly used to improve the performance of Si solar cells in photovoltaic technology, it is a new attempt to apply P diffusion gettering process to upgraded metallurgical grade silicon (UMG-Si) wafers with the purity of 99.999%. In this paper, improvements on the electrical properties of UMG-Si wafers and solar cells were investigated with the application of P diffusion gettering process. To enhance the improvements, the gettering parameters were optimized on the aspects of gettering temperature, gettering duration and POCl₃ flow rate, respectively. As we expected, the electrical properties of both multicrystalline Si (multi-Si) and monocrystalline Si (mono-Si) wafers were significantly improved. The average minority carrier lifetime increased from 0.35 μs to nearly about 2.7 μs for multi-Si wafers and from 4.21 μs to 5.75 μs for mono-Si wafers, respectively. Accordingly, the average conversion efficiency of the UMG-Si solar cells increased from 5.69% to 7.03% for multi-Si solar cells (without surface texturization) and from 13.55% to 14.55% for mono-Si solar cells, respectively. The impurity concentrations of as-grown and P-gettered UMG-Si wafers were determined quantitively so that the mechanism of P diffusion gettering process on UMG-Si wafers and solar cells could be further understood. The results show that application of P diffusion gettering process has a great potential to improve the electrical properties of UMG-Si wafers and thus the conversion efficiencies of UMG-Si solar cells.     

Passivation and gettering of defective crystalline silicon solar cell

Different polycrystalline silicon and single-crystalline silicon with dislocations were used for passivation and gettering processes. These materials have defects and more impurity in the crystal. The dominant increase of electronic performance was found for wafers with more defects by using a different casting method. The wafers of single crystalline silicon with dislocations also have higher increase of efficiency of cell in comparison with that wafer without dislocations during oxide passivation processes used. POCl₃ was used for gettering processes. Single-crystal wafer with or without dislocations was used for comparison of gettering.     

SOLAR CELLS WITH 15.6% EFFICIENCY ON MULTICRYSTALLINE SILICON,USING IMPURITY GETTERING,BACK SURFACE FIELD AND EMITTER PASSIVATION

Three features have been combined to raise the efficiency of solar cells made on industrial multicrystalline silicon wafers: 1) reduction of bulk recombination by a special gettering process, 2) reduction of back recombination by using a p/p ⁺ junction, 3) reduction of front recombination by emitter back-etching and passivation.

A conversion efficiency of 15.6% has been achieved on 2 × 2 cm² solar cells. Spectral response measurements are used to identify the role of each processing parameter.     

P/A1 co-gettering effectiveness in various polycrystalline silicon

The various polycrystalline silicon materials (cast ingots, ribbons) which are commercially available for the solar cells manufacturing differ very much among themselves due to the different growth processes. The resulting microstructure and impurity content will influence differently the material characteristics during thermal treatments inherent to the device manufacturing. As the gettering efficiency depends on the kind of polycrystalline material, the variations observed in the optimal gettering conditions or passivation will be discussed. In this paper, we compare the performances of various types of polycrystalline silicon upon classical and rapid thermal-process-induced co-diffusion of phosphorus and aluminium. We show that a large bulk minority carrier diffusion length enhancement occurs in the case of co-diffusion when compared to the separate diffusion of phosphorus and aluminium.     

多晶硅电池片双面扩散的仿真研究

结合PC1D模拟软件,对减薄至200μm的多晶硅电池片进行双面扩散与背靠背扩散的对比研究。试验表明:双面扩散工艺和背靠背扩散工艺均具有良好的吸杂效果,少子寿命有很大提高,少子扩散长度已大于电池片厚度。双面扩散比背靠背扩散具有更好的吸杂和钝化效果,少子寿命更长。但PC1D软件仿真及试验结果显示,扩散后电池片的转换效率、短路电流、开路电压等电学性能没有显著改善。     

Two-dimensional LBIC and internal quantum efficiency investigations of porous silicon-based gettering procedure in multicrystalline silicon

In this work, a porous silicon-based gettering technique was applied to multicrystalline silicon (mc-Si) wafers. Porous silicon (PS) was formed by the stain-etching technique and was used as a sacrificial layer for efficient external purification technique. The gettering procedure consists of achieving a PS/mc-Si/PS structure that undergoes a heat treatment at 900 °C for 90 min in an infrared furnace under a N₂ ambient. After removing the PS layers, mc-Si solar cells were realized. The effect of the gettering procedure was evaluated by means of the laser beam-induced current (LBIC) mapping, the internal quantum efficiency (IQE) mapping and the dark current-voltage (I-V) characteristic. Consequently, LBIC and IQE images show an enhancement of the gettered sample as compared to a reference untreated one. The serial resistance and the shunt resistance carried out from the dark I-V curves confirm this gettering-related solar cell improvement.     

Recent results obtained at IMEC on multicrystalline silicon solar cells

In order to optimize the efficiency of multicrystalline silicon solar cells, the influence of specific process steps and sequences were studied. Therefore clean-room high efficiency as well as industrial screen-printed cells were fabricated. Benefits are found in choosing a substrate with lower base resistivity, using front and rear oxide passivation, using hydrogen passivation for bulk and surfaces, the use of Si₃N₄ with a double function i.e. as an anti-reflection and passivation layer and the use of mechanical V-grooving. Efficiencies of 17% are found on 4 cm² clean-room fabricated cells and 15.2% has been obtained on 100 cm² V-grooved screenprinted industrial 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.     

Defect passivation of industrial multicrystalline solar cells based on PECVD silicon nitride

Low surface recombination velocity and significant improvements in bulk quality are key issues for efficiency improvements of solar cells based on a large variety of multicrystalline silicon materials. It has been proven that PECVD silicon nitride layers provide excellent surface and bulk passivation and their deposition processes can be executed with a high throughput as required by the PV industry. The paper discusses the various deposition techniques of PECVD silicon nitride layers and also gives results on material and device properties characterisation. Furthermore the paper focuses on the benefits achieved from the passivation properties of PECVD SiN_x layers on the multi-Si solar cells performance. This paper takes a closer look at the interaction between bulk passivation of multi-Si by PECVD SiN_x and the alloying process when forming an Al-BSF layer. Experiments on state-of-the-art multicrystalline silicon solar cells have shown an enhanced passivation effect if the creation of the alloy and the sintering of a silicon nitride layer (to free hydrogen from its bonds) happen simultaneously. The enhanced passivation is very beneficial for multicrystalline silicon, especially if the defect density is high, but it poses processing problems when considering thin (<200 μm) cells.     

Improving the performance of textured silicon solar cells through the field-effect passivation of aluminum oxide layers and up-conversion via multiple coatings with Er/Yb-doped phosphors

This paper examines the influence of field-effect passivation (from a coating of aluminum oxide) in conjunction with up-conversion (from multiple coatings containing Er/Yb-doped phosphors) on the performance of silicon solar cells. Note that the phosphors were applied to the rear surface of the cells. The surface morphology of the coatings was characterized by scanning electron microscopy and the chemical composition of Er/Yb-doped phosphors coating was examined using energy-dispersive X-ray spectroscopy. The fluorescence emissions of the coatings were examined using photoluminescence and optical image measurements. We examined the influence of field-effect passivation on dark current-voltage as well as photo-current density and external quantum efficiency (EQE). Improvements in photovoltaic performance after applying coatings containing Er/Yb-doped phosphors were estimated in terms of EQE and conversion efficiency. The field-effect passivation of Al₂O₃ and up-conversion provided by Er/Yb-doped phosphors resulted in EQE enhancements over a wavelength range of 600 to 1050 nm. Field-effect passivation was shown to enhance the conversion efficiency by 1.77% (from 16.91% to 17.21%), up-conversion enhanced conversion efficiency by 2.9% (from 17.21% to 17.71%), and a combination of field-effect passivation and up-conversion enhanced conversion efficiency by 4.73% (from 16.91% to 17.71%).     

Laser enhanced gettering of silicon substrates

One challenge to the use of lightly-doped, high efficiency emitters on multicrystalline silicon wafers is the poor gettering efficiency of the diffusion processes used to fabricate them. With the photovoltaic industry highly reliant on heavily doped phosphorus diffusions as a source of gettering, the transition to selective emitter structures would require new alternative methods of impurity extraction. In this paper, a novel laser based method for gettering is investigated for its impact on commercially available silicon wafers used in the manufacturing of solar cells. Direct comparisons between laser enhanced gettering (LasEG) and lightly-doped emitter diffusion gettering demonstrate a 45% absolute improvement in bulk minority carrier lifetime when using the laser process. Although grain boundaries can be effective gettering sites in multicrystalline wafers, laser processing can substantially improve the performance of both grain boundary sites and intra-grain regions. This improvement is correlated with a factor of 6 further decrease in interstitial iron concentrations. The removal of such impurities from multicrystalline wafers using the laser process can result in intra-grain enhancements in implied open-circuit voltage of up to 40 mV. In instances where specific dopant profiles are required for a diffusion on one surface of a solar cell, and the diffusion process does not enable effective gettering, LasEG may enable improved gettering during the diffusion process.     

Microsystems enabled photovoltaics: 14.9% efficient 14 μm thick crystalline silicon solar cell

Crystalline silicon solar cells 10-15 times thinner than traditional commercial c-Si cells with 14.9% efficiency are presented with modeling, fabrication, and testing details. These cells are 14 μm thick, 250 μm wide, and have achieved 14.9% solar conversion efficiency under AM 1.5 spectrum. First, modeling results illustrate the importance of high-quality passivation to achieve high efficiency in thin silicon, back contacted solar cells. Then, the methodology used to fabricate these ultra thin devices by means of established microsystems processing technologies is presented. Finally, the optimization procedure to achieve high efficiency as well as the results of the experiments carried out with alumina and nitride layers as passivation coatings are discussed.     

R & D activities of crystalline silicon solar cells in Japan

Research and development of crystalline silicon solar cells in Japan have greatly advanced for the past 10 years. Fundamental research has been conducted on the recombination and passivation of minority carriers at Si/SiO₂ interfaces and in bulk regions including grain boundaries. Qualities of Si feedstock and substrates have been improved. A small-area cell efficiency using monocrystalline silicon substrates has reached 21 % and that for large-area, multicrystalline solar cells up to 17% by using low-cost cell fabrication processes. Such high efficiency values are realized by tenacious improvement of substrate quality and the development of new processes for fabricating solar cells.     

Characteristics of vapor-liquid-solid grown silicon nanowire solar cells

We report fabrication and characterization of solar cells based on vapor-liquid-solid (VLS) grown silicon nanowires (NWs) that form core-shell radial p-n junction structures. We observe efficiency enhancement due to the presence of the NWs that increase the light trapping within the device, while the use of gold as VLS catalyst results in increased carrier recombination within the wires. From the spectral efficiency data, we identify that the surface recombination effect becomes more significant in the large surface area NW cells. To remedy this issue we demonstrate the efficacy of a highly conformal Al₂O₃ film grown by atomic layer deposition to serve as surface passivation layer. This work highlights the key issues confronted by NW-based solar cells grown by VLS technique.     

A detailed study of H2 plasma passivation effects on GaAs/Si solar cell

The hydrogen plasma passivation effects of MOCVD-grown GaAs solar cell on Si substrate have been studied in detail. To get a more reproducible increase of conversion efficiency and test the thermal stability of the plasma-exposed GaAs/Si solar cell, both the plasma exposure and post-passivation annealing conditions were optimized. Annealing the H₂ plasma passivated GaAs/Si solar cell at 450°C in AsH₃/H₂ ambient seems a very essential parameter to restore the carrier concentration, especially, without losing the beneficial effects of H incorporation into GaAs on Si. For the H₂ plasma passivated GaAs/Si solar cell, a highest conversion efficiency of 18.3% was obtained compared with that of the as-grown cell (16.6%) due to the H passivation effects on nonradiative recombination centers, which increased the minority carrier lifetime.     

The electrical properties and hydrogen passivation effect in mono crystalline silicon solar cell with various pre-deposition times in doping process

The doping process in diffusion furnace consisting of pre-deposition and drive-in steps is essential to create p–n junction in crystalline silicon solar cell fabrication, and its optimization is necessary to obtain the high conversion efficiency. In this work, pre-deposition time was varied to study the electrical properties of solar cells and its effect on the hydrogen passivation with various phosphorous doping profiles. As a result, solar cell conversion efficiency of 17.8% with 7 min pre-deposition was achieved. Dopant (phosphorous) concentration in the emitter measured by SIMS indicated that the surface with shorter pre-deposition time had lower dopant concentration. High concentration of phosphorous on the surface appears to be the source for the electron consumed by the stored hydrogen in making the neutral H₂ gas during firing. The formation of neutral hydrogen gas is thermodynamically and stochastically more favorable than the reaction between Si with dangling bond and H. This means that the passivation by the stored H during firing is strongly controlled by the dopant on the surface. This result obtained herein lays the foundations to understand the relationship between the doping profile of diverse dopant species and its passivation effect.     

SiO2 passivation layer grown by liquid phase deposition for silicon solar cell application

Surface passivation is one of the primary requirements for high efficient silicon solar cells. Though the current existed passivation techniques are effective, expensive equipments are required. In this paper, a comprehensive understanding of the SiO₂ passivation layer grown by liquid phase deposition (LPD) was presented, which was cost-effective and very simple. It was found that the post-annealing process could significantly enhance the passivation effect of the LPD SiO₂ film. Besides, it was revealed that both chemical passivation and field-effect passivation mechanisms played important roles in outstanding passivation effect of the LPD SiO₂ film through analyzing the minority carrier lifetime and the surface recombination velocity of n-type and p-type silicon wafers. Although the deposition parameters had little influence on the passivation effect, they affected the deposition rate. Therefore, appropriate deposition parameters should be carefully chosen based on the compromise of the deposition rate and fabrication cost. By utilizing the LPD SiO₂ film as surface passivation layer, a 19.5%-efficient silicon solar cell on a large-scale wafer (156 mm × 156 mm) was fabricated.     

The influence of annealing conditions on the efficiency of phosphorous gettering in CZ-Si for solar cell applications

The effect of the annealing ambient on the efficiency of the phosphorous gettering process for Czochralski (CZ) silicon wafers is investigated in this paper. Phosphorous is diffused from a POCl₃ source at different temperatures into single-crystal p-type silicon wafers having a resistivity of around 1 ohm/cm. This is followed by an additional heat treatment in either oxidizing (wet and dry oxide) or in inert (argon) ambient. The laser microwave photoconductivity decay method is used to monitor the changes in the minority carrier lifetime after the phosphorous diffusion and the subsequent annealing. Furthermore, solar cells are fabricated on the treated samples in order to correlate the lifetime measurements with the illuminated I-V characteristics of the cells.