Emitter Saturation Current Density J0e Research Articles

High-efficiency selective boron emitter formed by wet chemical etch-back for n-type screen-printed Si solar cells

Front metal contact induced recombination and resistance are major efficiency limiting factors of large-area screen-printed n-type front junction Si solar cells with homogeneous emitter and tunnel oxide passivated back contact (TOPCON). This paper shows the development of a selective boron emitter (p+/p++) formed by a screen-printed resist masking and wet chemical etch-back process, which first grows a porous Si layer and subsequently removes it. Various wet-chemical solutions for forming porous Si layer are investigated. An industrial compatible process with sodium nitrite (NaNO2) catalyst is developed to uniformly etch-back the ∼47 Ω/◻ atmospheric pressure chemical vapor deposited heavily doped boron emitter to ∼135 Ω/◻ by growing a 320 nm porous Si layer within 3 min and subsequently removing it. After etching back, the boron emitter was subjected to a thermal oxidation to lower the surface concentration and the emitter saturation current density J0e. Various etched-back emitters were evaluated by measuring J0e on symmetric test structures with atomic layer deposited aluminum oxide (Al2O3) passivation. Very low J0e of 21, 14, and 9 fA/cm2 were obtained for the 120, 150, and 180 Ω/◻ etched-back emitters, respectively. A solar cell with a selective emitter (65/180 Ω/◻) formed by this etch-back technology and with an Al/Ag contact on the front and TOPCON on the back gave an open-circuit voltage (Voc) of 682.8 mV and efficiency of 21.04% on n-type Czochralski Si wafer. This demonstrates the potential of this technology for next generation high-efficiency industrial n-type Si solar cells.

Dissolution of Electrically Inactive Phosphorus by Low Temperature Annealing

In this study we investigate the dissolution of electrically inactive phosphorus complexes by low temperature annealing after the POCl3 diffusion process. This has the immediate consequence that the existing near-surface emitter volume SRH recombination can be reduced. Thereby, a significant reduction of emitter saturation current density j0E is achieved without driving the emitter further into the silicon substrate. For short-term temperature treatments well below the POCl3 diffusion temperature, a reduction of up to -60 fA/cm2 has been achieved. This study increases our understanding of the formation and dissolution of electrically inactive phosphorus complexes during post-annealing processes.

Boron Emitters from Doped PECVD Layers for n-type Crystalline Silicon Solar Cells with LCO

The intensified research into n-type silicon solar cells over the last few years let the application of boron doped emitters in suitable cell concepts become the preferred method to form the necessary p-n-junction. In this study an alternative process to fabricate a boron doped emitter via diffusion from a PECV-deposited doping source is presented and optimized for n-type crystalline silicon solar cell concepts. Doping profiles with a high surface concentration in combination with low emitter saturation current density values are achieved for improved contact and passivation characteristics. The boron emitter profile is compatible with various contacting techniques i.e. screen printing and vapour deposited Al, allowing for low-resistant contacting with Ag/Al pastes or sputtered Al. The comparably low emitter saturation current density j0E of 44 fA/cm2 allows for a VOC of 666mV, and thereby a cell efficiency of 19.7% is demonstrated on a large area (156.25 cm2) solar cell.

Numerical Analysis of p+ Emitters Passivated by a PECVD AlOx/SiNx Stack

Evaluation of the level of surface passivation at highly doped crystalline silicon (c-Si) surfaces is not trivial, particularly when the surfaces are textured. In this work we present an advanced numerical analysis that can be used to evaluate the level of surface passivation at both planar and textured samples. First, using Sentaurus TCAD, we compare two widely used extraction methods of the emitter saturation current density J0e, the general definition and Kane & Swanson's method. Experimentally determined doping profiles on planar wafers are used to calibrate two- dimensional process simulations. Process simulations are subsequently used to calculate p+ emitter doping profiles for textured wafers, which are required to simulate J0e. Additionally, while matching simulated and experimentally measured J0e values, we find that a high density of negative fixed charge in a plasma enhanced chemical vapour deposited AlOx/SiNx stack has a significant impact on surface recombination at the surface. Furthermore, we compare J0e values from textured and planar wafers and confirm that the difference from the expected geometrical factor can be attributed to surface recombination. Finally, by considering surface charges, we find that the electron surface recombination velocity parameter Sn0 is around 1X104cm/s for all p+ emitters studied in this work.

Numerical Modelling of Silicon p+ Emitters Passivated by a PECVD AlOx/SiNx Stack

Extraction of the exact surface recombination velocity at highly doped crystalline silicon (c-Si) surfaces is not straightforward and typically involves advanced computer modelling. In this work, two theoretical methods (the Kane-Swanson slope method and the general definition) for the extraction of the emitter saturation current density J0e are compared in SENTAURUS TCAD. We find good agreement between the J0e values obtained by the two methods. Experimental p+ emitter doping profiles on planar {100} samples are used to calibrate the process simulation, followed by a calculation of the doping profiles for textured {111} samples featuring upright pyramids. The experimentally measured J0e values of both textured and planar samples passivated by PECVD AlOx/SiNx stacks are reproduced by adjusting the surface recombination velocity. The electron surface recombination velocity parameter Sn0 at the p+ c-Si/AlOx interface is determined to be ∼1x104cm/s for all investigated p+ emitters on planar wafers, comparable to values reported for lightly doped c-Si. A high density of fixed charge is found to reduce J0e by up to 90%, due to the suppression of Shockley-Read-Hall recombination at the surface.

A Method for Imaging the Emitter Saturation Current With Lateral Resolution

A method is developed to measure the emitter saturation current density <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> with lateral resolution. The method uses photoluminescence lifetime imaging at several different injection densities to evaluate the injection-dependent effective excess carrier lifetime, from which <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> is derived at each pixel using the method of Kane and Swanson. The applicability of the method is successfully demonstrated on laser-doped selective emitters for crystalline Si solar cells. The derived <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> images of selective emitter test structures are discussed, including measurement uncertainties. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> images may, for example, be used to optimize the laser parameters of selective emitters and to quantify laser damage, and may also provide input parameters for device simulation.

Chemical etching of boron-rich layer and its impact on high efficiency n-type silicon solar cells

This paper reports on an effective chemical etching treatment to remove a boron-rich layer which has a significant negative impact on n-type silicon (Si) solar cells with boron emitter. A nitric acid-grown oxide/silicon nitride stack passivation on the boron-rich layer-etched boron emitter markedly decreases the emitter saturation current density J0e from 430 to 100 fA/cm2. This led to 1.6% increase in absolute cell efficiency including 22 mV increase in open-circuit voltage Voc and 1.9 mA/cm2 increase in short-circuit current density Jsc. This resulted in screen-printed large area (239 cm2) n-type Si solar cells with efficiency of 19.0%.

Evaluating The Quality of Selective Emitter Structures by Imaging the Emitter Saturation Current Density

A method to derive the emitter saturation current density J0e with lateral resolution is applied to investigate selective emitter structures. The method uses PL lifetime imaging at several injection densities to laterally evaluate J0e by applying the method of Kane and Swanson [1] pixel by pixel. Samples with two-sided diffused emitters on lowly-doped Cz wafers were used to produce selective emitter structures by laser doping of the phosphorus-rich glass (LD-SE). By comparison of experimental and numerical simulation results of J0e linescans, a limited resolution of a feature size of an inhomogeneous emitter is determined to be theoretically between 0.5-1.0mm and experimentally about 2mm. The method was successfully applied to investigate the dependence of J0e on the laser power of a selective emitter structure. The expected behaviour of a maximum J0e for medium laser intensities is observed. The method is suitable to evaluate the selective emitter process and its optimization.

Comparison of emitter saturation current densities determined by injection‐dependent lifetime spectroscopy in high and low injection regimes

ABSTRACTThe determination of the emitter saturation current density J0e of symmetrical test structures, analyzed by injection‐dependent lifetime spectroscopy (IDLS), in high as well as in low injection regimes is compared. A detailed investigation of the influence of different models for the Auger recombination on the evaluation of the measured minority carrier lifetime is performed. It can be concluded that a good agreement for the extraction of J0e under high and low injection conditions for lightly and highly doped emitters on textured and untextured surfaces was obtained if the Auger parameterization of Kerr et al. is applied. For deep‐diffused emitter profiles on textured surfaces a significant increase of J0e well above the geometrical surface area ratio of untextured and textured surfaces of 1.7 was observed. A verification of the experimentally determined values for J0e is performed by numerical device simulations. The simulated surface recombination velocity for the investigated emitter doping profiles is in good agreement to analytical calculations. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Evaluating the Aluminum-Alloyed $\hbox{p}^{+}$-Layer of Silicon Solar Cells by Emitter Saturation Current Density and Optical Microspectroscopy Measurements

Surface-passivated and surface-unpassivated aluminum-alloyed p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -layers are characterized. By varying the firing conditions and the thickness of the screen-printed aluminum paste, different sheet resistances <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> of the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -layer were fabricated. The emitter saturation current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> plotted versus <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> follows distinctly different trends for the passivated and unpassivated samples. An aluminum paste with a boron additive achieves a much higher doping concentration and a lower sheet resistance but nevertheless follows the same J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> curves as the pure Al paste. The aluminum p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -layer was quantitatively analyzed with microphotoluminescence and Fano-Raman measurements. The latter shows an increased defect recombination at the interface between the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -layer and the moderately doped Si bulk. The lower Shockley-Read-Hall lifetime in this region can be attributed to a high defect concentration in the most highly doped layer, represents an impediment to the reduction of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> for Al-doped emitter regions, and needs to be optimized in future investigations.

The ${\bf ALU}^{+}$ Concept: $N$-Type Silicon Solar Cells With Surface-Passivated Screen-Printed Aluminum-Alloyed Rear Emitter

Aluminum-doped p-type (Al-p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) silicon emitters fabricated by means of screen-printing and firing are effectively passivated by plasma-enhanced chemical-vapor deposited (PECVD) amorphous silicon (a-Si) and atomic-layer-deposited (ALD) aluminum oxide (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) as well as Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> stacks, where the silicon nitride (SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) layer is deposited by PECVD. While the a-Si passivation of the Al-p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> emitter results in an emitter saturation current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> e of 246 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiN <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> double layers result in emitter saturation current densities as low as 160 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is the lowest J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> e reported so far for screen-printed Al-doped p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> emitters. Moreover, the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> as well as the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiN <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> stacks show an excellent stability during firing in a conveyor belt furnace at 900 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. We implement our newly developed passivated Al-p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> emitter into an n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> np <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> solar cell structure, the so-called ALU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> cell. An independently confirmed conversion efficiency of 20% is achieved on an aperture cell area of 4 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , clearly demonstrating the high-efficiency potential of our ALU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> cell concept.

High efficiency n-type Si solar cells on Al2O3-passivated boron emitters

In order to utilize the full potential of solar cells fabricated on n-type silicon, it is necessary to achieve an excellent passivation on B-doped emitters. Experimental studies on test structures and theoretical considerations have shown that a negatively charged dielectric layer would be ideally suited for this purpose. Thus, in this work the negative-charge dielectric Al2O3 was applied as surface passivation layer on high-efficiency n-type silicon solar cells. With this front surface passivation layer, a confirmed conversion efficiency of 23.2% was achieved. For the open-circuit voltage Voc of 703.6mV, the upper limit for the emitter saturation current density J0e, including the metalized area, has been evaluated to be 29fA∕cm2. This clearly shows that an excellent passivation of highly doped p-type c-Si can be obtained at the device level by applying Al2O3.