Radovan Kopecek

Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing

We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of...

Factors Limiting the Formation of Uniform and Thick Aluminum–Back-Surface Field and Its Potential

Theoretical calculations reveal that the quality of an aluminum-back-surface field (BSF) in a silicon solar cell can be improved by either increasing the thickness of the deposited aluminum (Al), peak alloying temperature, or both. However, this study shows that there is a critical temperature for a given screen-printed Al thickness, above which the BSF quality begins to degrade because of nonuniformity triggered by the agglomeration of Al-Si melt in combination with the bandgap narrowing resulting from the high doping effect in the agglomerated regions. It is found that this critical temperature decreases with the increase in the thickness of the deposited Al layer and, therefore, limits the quality and thickness of the Al-BSF that can be achieved before degradation sets in. This nonuniformity of Al-BSF is observed in the form of scattered Al bumps with thick and thin BSF regions. A combination of experimental results and model calculations is used to provide improved understanding and guidelines for choosing the optimal combination of Al thickness and alloying temperature.

Effect of compensation and of metallic impurities on the electrical properties of Cz-grown solar grade silicon

In this work we present a study of a p-type Czochralski-grown Si ingot which was grown using 10% solar grade silicon (SoG-Si). As the SoG-Si contains a relatively high concentration of impurities including phosphorus, the electrical properties of the as-grown wafers from this ingot are affected by both the compensating dopants and other impurities. Measurements of the minority charge carrier lifetime in the as-grown material reveal very low values (4–8μs). The Hall mobilities at room temperature correspond to normal values for Czochralski silicon in the upper part of the ingot (which solidifies first) and decrease significantly toward the bottom of the ingot. Segregation leads to an accumulation of impurities toward the lower parts of the ingot as well as to a stronger increase in phosphorus than of boron, the latter of which results in a high compensation level (i.e., an increasing resistivity). A priori, both effects could be responsible for the degradation of the electrical properties in the lower part...

Experimental evidence of direct contact formation for the current transport in silver thick film metallized silicon emitters

Great advances have been achieved in the development of silver pastes. The use of smaller silver particles, higher silver content, and, thus, less glass frit allow modern silver pastes to contact high resistive emitters without the necessity of a selective emitter or subsequent plating. To identify the microscopic key reasons behind the improvement of silver paste, it is essential to understand the current transport mechanism from the silicon emitter into the bulk of the silver finger. Two current transport theories predominate: i) The current flows through the Ag crystallites grown into the Si emitter, which are separated by a thin glass layer or possibly in direct contact with the silver finger. ii) The current is transported by means of multistep tunneling into the silver finger across nano-Ag colloids in the glass layer, which are formed at optimal firing conditions; the formation of Ag crystallites into the Si surface is synonymous with over-firing. In this study, we contact Si solar cell emitters wi...

Analysis of the Annual Performance of Bifacial Modules and Optimization Methods

Bifacial modules have the advantage of capturing sunlight from front as well as from rear surfaces, and therefore, they are able to produce larger amounts of energy, compared with standard (monofacial) modules. However, their performance depends on the spatial distribution of the irradiance incident on the rear module surface, which is strongly affected by several site-specific conditions, such as albedo, reflective surface size, module elevation, and tilt angle. In this study, we elaborate upon the individual and combined effects of these factors on the annual energy yield of stand-alone south-facing bifacial modules through simulations at two site locations with contrary climatic conditions. Following the optimization of the tilt angle of bifacial modules dependent on the site, albedo, and module elevation, we demonstrate that the annual energy yield of a bifacial module increases linearly with albedo, which shows a monotonically increasing but, in addition, saturating behavior versus reflective surface size, and increases up to a certain module elevation. Through the simultaneous consideration of these dependences, we suggest an optimal positioning of bifacial modules. Finally, we show that under these optimal conditions, bifacial modules can supply up to 25% more energy compared with standard modules.

Large-Area Back-Contact Back-Junction Solar Cell With Efficiency Exceeding 21%

In this study, high-efficiency solar cells are presented with the use of low-cost industrially available technologies. This results in the so-called ZEBRA concept: a litho-free process in which standard 156 × 156 mm2 monocrystalline n-type Cz-Si wafers are processed into high-efficiency interdigitated back-contact solar cells. In our first attempt, we obtained energy conversion efficiencies of more than 20% on 239 cm2 area. With the help of a 3-D simulation of the device, a further improvement to more than 21.5% was determined. We indentify the open-circuit voltage and the large serial resistance as the main losses impeding from reaching the simulated efficiency. The cell results after an extensive optimization are presented in this study, focusing on the improvement of the fill factor of the cells. The optimized solar cells show an improvement in the energy conversion efficiency up to 21%. Furthermore, the simple metallization and the module interconnection design of the ZEBRA cells allow for a bifacial application. Indoor and outdoor I - V measurements on a bifacial one-cell-module show an enhancement in the total generated power of more than 12% as compared with a monofacial module.

Influence of Surface Topography on the Glass Coverage in the Contact Formation of Silver Screen-Printed Si Solar Cells

For screen-printed Ag-paste metallization, the growth of Ag crystallites into Si is essential for the current conduction from the Si emitter to the silver finger. There are strong indications that for low contact resistances, some of these Ag crystallites need to be in direct contact with the silver finger without a separating glass layer in between. In this paper, we concentrate on the origin of Ag crystallites grown into Si in direct contact with the silver finger. On textured surfaces, we vary the Si pyramid sizes, round the pyramid tips to varying degrees, and fabricate flat smooth surfaces. We observe that the size of the pyramids does not play an important role in the achievement of low specific contact resistivity unless the pyramid heights become smaller than the thickness of the glass layer, but rounding of the pyramid tips with standard heights increases specific contact resistivity significantly. From our microscopic investigations, we conclude that the largest influence on the topography-dependent contact resistance comes from the glass coverage governing the amount of Ag crystallites directly connected with the silver finger bulk. Furthermore, Ag crystallites in direct contact with the silver finger are also observed on c-Si without n+ emitter doping.

Large Area Screen Printed N-Type Silicon Solar Cells with Rear Aluminium Emitter: Efficiencies Exceeding 16%

We present low cost, screen printed industrial solar cells with a rear Al-emitter for n-type mono- and mc-Si large area thin wafers showing efficiencies of 16.4% and 14.4% respectively. The gap of 2% absolute between the cell efficiencies for mono- and mc-Si is not material related but is largely due to the non-textured surface of the mc-Si substrate. Applying a surface texture by acidic wet chemical etching could lead to efficiencies above 15% on the mc-Si material. The process is based on the standard industrial p-type firing through solar cell concept with slight modifications in individual processing steps similar to the Phostop cell presented by Ebara Solar. There is still need for further optimisation before industrialisation for instance in the fields of wafer bowing reduction and the implementation of AgAl pads for wafer interconnection. We propose two different ways of pad printing-one directly to the Al-emitter, the other on top of the Al rear contact

Bifacial Solar Cells on Multi-Crystalline Silicon with Boron BSF and Open Rear Contact

The standard industrial multi-crystalline silicon (mc-Si) solar cell is monofacial and includes screen printed aluminum back surface field (BSF). A simple approach to increase performance and reduce costs per Wpeak is to collect the albedo on the rear side. In this work a bifacial, screen printed mc-Si solar cell with boron BSF is demonstrated. Rear to front efficiency ratios of up to 0.83 have been reached on 100times100mm2 mc-Si wafers with a thickness of about 200mum. The best solar cell processed so far with a boron BSF had an efficiency under front side illumination of eta=16.1% and a back to front efficiency ratio of 0.77. The possible gain in performance in later operation was estimated using PC1D simulation and depends on the albedo that is the amount of light that penetrates into the solar cell from the rear side. The simulation was confirmed by outside module tests, leading to an average gain of 19.5% over one day

Multifunction metrology platform for photovoltaics

Advanced characterization for PV is a complex process that must address bulk defects, interfaces, passivation, and degradation phenomena. It requires not only appropriate measurement techniques, but also a coupling of measurements with treatments altering defect/interface activity. Preferably, the metrology should be noncontact and cost effective. The purpose of this work was to provide such multifunction wafer scale characterization capability for silicon PV. In this paper we describe a multifunction metrology platform. Example applications are given that illustrate the importance of sequenced measurements for 1 — monitoring of the light induced degradation in PV wafers and solar cells; 2 — correlation between interface trap density and surface recombination and the role of surface barrier, and 3 — monitoring of the field-effect potential emitter passivation.