Emitter Of Silicon Solar Cells Research Articles

An Evaluation of Constituents in Paste for Silicon Solar Cells with Floating Contact Method: A Case Study of Tellurium Oxide Effects

Metallization contacts on emitter of silicon solar cells cause shunting of p-n junction and induce carrier recombination, resulting in a loss in open-circuit voltage Voc of the cells, which must severely limit cell efficiency. But, the mechanism of the shunting and the recombination has not been understood enough yet, especially on the contacts with conductive paste, because the shunting and the recombination caused by the paste come of multiple effects of each constituent in the paste such as silver metal, glass frit, and some additives. Thus, the effects of the each constituent in the paste should be clarified to elucidate the mechanism and to achieve an advanced paste for high-efficiency silicon solar cells. In our previous study, effects of aluminum in silver/aluminum paste and effects of glass frit itself on the shunting and the recombination in n-type solar cells were clearly shown by using “floating contact method”, and its effectiveness on the paste evaluation were demonstrated in the solar cells. In this study, the floating contact method was applied to conventional p-type solar cells with silver paste to clarify effects of tellurium oxide on the silver paste, because the detailed effects of the tellurium oxide on the shunting and the recombination have not been clear yet even though the oxide is widely used for commercial paste. Our study clearly shows that the tellurium oxide reduces the shunting and the recombination caused by the silver paste in the solar cells, and also demonstrates the effectiveness of the floating contact method on the paste evaluation.

Characterization of Glass Frit in Conductive Paste for N-Type Crystalline Silicon Solar Cells

Contacting silver paste for an emitter of silicon solar cells has been pointed out to create shunting and to increase carrier recombination due to silver-crystallites at the emitter. However, since these observed electrical losses come of not only the crystallites but also multiple effects of constituents in the paste, whether the crystallites mainly cause these losses has still remained unknown. In this study, how glass frit “itself,” which is most important constituent in the paste, affects the electrical losses is investigated by applying the “floating contact method.” Furthermore, the contact interface between the frit and silicon surface is also investigated to elucidate the mechanism of the losses. The metal oxide glass frit itself is found to drastically cause the shunting and the recombination in n-type solar cells, resulting in loss in open-circuit voltage, and creates metal lead crystallites through redox reaction at silicon surface during firing. The shunting originates from the metallic lead, and the recombination is induced by diffused impurities or defects into the silicon emitter through the reaction. The electrical losses in the solar cells are led to not only by the silver-crystallites, but also by the glass frit itself.

Challenges for lowly-doped phosphorus emitters in silicon solar cells with screen-printed silver contacts

This work points out that the application of phosphorus emitters with low surface concentration Nsurf of a few 1019 cm-3 in combination with state-of-the-art screen-printed and fired silver contacts is no more limited by the specific contact resistance ρC but by the dark saturation current densities underneath the metal contacts j0,met. Eight emitter doping profiles have been designed with different Nsurf ranging between 3.3·1019 cm-3 and 1.2·1020 cm-3 which feature very similar junction depths of about 350 nm. The measured ρC values for these emitters are determined to be between 2 mΩcm2 and 5 mΩcm2 for about 50 µm-wide contact fingers. Their emitter dark saturation current density (alkaline textured and passivated surface) range from 40 fA/cm² to 90 fA/cm². From these parameters and the current-voltage data of large-area Czochralski-grown silicon solar cells featuring an aluminum back surface field, we estimate j0,met to be between 1200 fA/cm² and 3900 fA/cm². Our results show that for decreasing Nsurf, the recombination underneath the metal contacts increases significantly which mainly limits the cell’s open-circuit voltage. We thus conclude that the major challenge for emitters with low Nsurf consists in achieving reasonable low j0,met values in order to benefit on the cell level from the lower recombination activity in the passivated, non-metallized surface area.

Optimization of oxidation processes to improve crystalline silicon solar cell emitters

Control of the oxidation process is one key issue in producing high-quality emitters for crystalline silicon solar cells. In this paper, the oxidation parameters of pre-oxidation time, oxygen concentration during pre-oxidation and pre-deposition and drive-in time were optimized by using orthogonal experiments. By analyzing experimental measurements of short-circuit current, open circuit voltage, series resistance and solar cell efficiency in solar cells with different sheet resistances which were produced by using different diffusion processes, we inferred that an emitter with a sheet resistance of approximately 70 Ω/□ performed best under the existing standard solar cell process. Further investigations were conducted on emitters with sheet resistances of approximately 70 Ω/□ that were obtained from different preparation processes. The results indicate that emitters with surface phosphorus concentrations between 4.96 × 1020 cm−3 and 7.78 × 1020 cm−3 and with junction depths between 0.46 μm and 0.55 μm possessed the best quality. With no extra processing, the final preparation of the crystalline silicon solar cell efficiency can reach 18.41%, which is an increase of 0.4%abs compared to conventional emitters with 50 Ω/□ sheet resistance.

Analytical models for the series resistance of selective emitters in silicon solar cells including the effect of busbars

AbstractA theoretical analysis of the power loss and series resistance of the front side emitter in silicon solar cells is presented. Existing 1D models (infinitely long finger) and 2D models (including the effect of busbars) of emitter series resistance contribution are extended to the case of selective emitters. The general case of different current densities for both emitters in the selective emitter scheme is considered in these extensions. The resulting models depend on the individual sheet resistances and current densities in both emitters and the device's overall grid geometry. The models are corroborated by finite element simulation of the potential in the emitter. An excellent agreement is found between the analytical models, and the simulations for a wide range of sheet resistances typically encountered in silicon solar cells. Grid simulations using the 2D model are applied to solar cells with selective emitters, where the width of the low‐resistive emitter was varied. The simulations demonstrate that the 2D model can explain the absolute change in fill factor observed in these cells. Copyright © 2013 John Wiley & Sons, Ltd.

Formation of nanostructured emitter for silicon solar cells using catalytic silver nanoparticles

A simple process for nanotexturing on the emitter of silicon solar cells using catalyzed wet chemical etching by size-controlled silver nanoparticles was reported. A fine textured black surface was achieved to realize the low light reflectivity less than 5%. After screen printing and firing by the industrial standard fabrication protocol, we obtained the nanotextured Si solar cells with 15.7%-efficiency without any additional antireflection (AR) coating. This result suggests that the inexpensive metal-assisted wet chemical nanotexture method is prospective to be used in photovoltaic industry.

Printed contact on emitter with low dopant surface concentration

Pastes and inks to contact silicon solar cell emitters have been further developed since decades and considerable improvements have been achieved. According to different R&D roadmaps for silicon solar cells emitter sheet resistances will further increase and the surface dopant concentration will decrease as narrower contact lines support such a development. This leads to new requirements of contact pastes. In this paper an improved contact ink based on silver will be presented which is able to contact phosphorous emitters with low surface dopant concentrations of ND = 8á1018 cm-3.

Dopant profile control of epitaxial emitter for silicon solar cells by low temperature epitaxy

We report an alternative approach to grow phosphorus-doped epitaxial silicon emitter by rapid thermal chemical vapor deposition at low temperature (T ≥ 700 °C). A power conversion efficiency (PCE) of (6.6 ± 0.3)% and a pseudo PCE of (10.2 ± 0.2)% has been achieved for the solar cell with epi-emitter grown at 700 °C, in the absence of surface texturization, antireflective coating, and back surface field enhancement, without considering front contact shading. Secondary ion mass spectroscopy revealed that lower temperature silicon epitaxy yields a more abrupt p-n junction, suggesting potential applications for radial p-n junction wire array solar cells.

Nanometer‐thin sputtered phosphorus layers for laser‐doped solar cells

AbstractSputtered phosphorus precursor layers for laser‐doped silicon solar cell emitters lead to a new record energy conversion efficiency η = 18.9% in full‐area laser‐doped solar cells. Layer characterization with XPS and SIMS enables us to understand the influence of ambient air and native oxide on pure phosphorus precursor layers. Removing the native oxide from the silicon surface serves for less formation of H3PO4 increasing the quality of laser‐doped emitters. The native oxide layer favors the formation of radical OH− groups during the sputtering process. These OH− groups undergo a reaction with the phosphorus layer, resulting in a larger part of H3PO4. Copyright © 2010 John Wiley & Sons, Ltd.

Selective emitter using porous silicon for crystalline silicon solar cells

This study is devoted to the formation of high–low-level-doped selective emitter for crystalline silicon solar cells for photovoltaic application. We report here the formation of porous silicon under chemical reaction condition. The chemical mixture containing hydrofluoric and nitric acid, with de-ionized water, was used to make porous on the half of the silicon surface of size 125×125 cm. Porous and non-porous areas each share half of the whole silicon surface. H 3PO 4:methanol gives the best deposited layer with acceptable adherence and uniformity on the non-porous and porous areas of the silicon surface to get high- and low-level-doped regions. The volume concentration of H 3PO 4 does not exceed 10% of the total volume emulsion. Phosphoric acid was used as an n-type doping source to make emitter for silicon solar cells. The measured emitter sheet resistances at the high- and low-level-doped regions were 30–35 and 97–474 Ω/□ respectively. A simple process for low- and high-level doping has been achieved by forming porous and porous-free silicon surface, in this study, which could be applied for solar cells selective emitter doping.

Analytical modelling and minority current measurements for the determination of the emitter surface recombination velocity in silicon solar cells

A new analytical model is used to describe the emitter of silicon solar cells in order to gain information on the surface recombination velocity S. The procedure takes advantage of the combined use of experimental measurements, done to determine the emitter saturation current J oe, and analytical modelling to relate J oe to S. Several experiments have been carried out on silicon solar cells having different emitter profiles subjected to various surface treatments. The influence of the surface on significant cell parameters has been analysed.

Development of a Phosphorus Spray Diffusion System for Low-Cost Silicon Solar Cells

Phosphoric acid was used an n-type doping source to make an emitter for silicon solar cells. This paper reports on a cold spray method to coat phosphoric acid on the silicon wafer without any additional complicated and corrosive heating system. The key spray parameters such as belt speed, flow rate of the carrier gas, and concentration of phosphoric acid are optimized to get uniform and reproducible sheet resistance for the emitter. The diffusion process has been studied by firing silicon wafers that are spray-coated with phosphoric acid. screen printed solar cells have been fabricated using the emitter formed by the spray coating. The effects of the emitter on cell performance have been investigated and compared with those of the conventional -diffused emitter. screen printed/spray-diffused cells give impressive cell efficiencies of on float zone wafers, which is nearly equal to those of emitter cells . Compared with the emitter cell, the spray-diffused cells show slightly lower quantum efficiency at the short-wavelength response and slightly higher emitter saturation current density ( compared with ). Further optimization can eliminate this small difference in efficiency.

The quantum collection efficiency of heavily doped emitters in silicon solar cells

Abstract The importance of the minority carrier degeneracy on the collection efficiency (air mass (AM) 1) of heavily doped emitters in silicon solar cells is shown by means of a simple analytical model. In our calculations we include explicitly the effects of space-dependent mobility, lifetime, band gap narrowing and the Fermi-Dirac statistics for donor concentrations higher than N d = 1 × 10 20 cm −3 . We show that for passivated in emitters the collection efficiency is high and almost independent of the surface dopant concentration. However, for thick emitters ( x j > 2 μm ) it is convenient to have small surface impurity concentrations ( N s 20 cm −3 ) in order to achieve high collection efficiencies. In addition, for non-passivated solar cells the collection efficiency should be higher when the surface impurity concentration is greater, in conjunction with a smaller thickness. All of the above confirm experimental results that both thin and thick emitter solar cells can be made that attain high conversion efficiencies, using an appropriate design.