Emitter-wrap-through Research Articles

A novel strategy for preparing a selective back surface field of n-type emitter wrap through solar cells

In the present study, we have developed a novel mixed co-diffusion (MCD) process by which to prepare a selective back surface field (BSF) of n-type emitter wrap through (EWT) solar cells, which combines a plasma enhanced chemical vapor deposition (PECVD) phosphorus-doped n-type microcrystalline silicon as the dopant source, with a low temperature thermal oxidation (LTO) process. Through comparison between our MCD process and a standard co-diffusion (SCD) process, a BSF with a shallow doping depth of 0.56µm and high doping concentration of 1.9×1020at/cm3 is easily obtained by the MCD process under the low temperature of 750℃. Therefore, the MCD process is shown to reduce the number of high temperature processes, which cannot produce dopant redistribution, and can accurately control the doping concentrations and depths of the BSF and emitter. In addition, the novel method also eliminates the boron-rich layer, which induces misfit dislocations and bulk lifetime degradation, without extra chemical treatment. Therefore, the MCD process' open circuit voltage, short circuit current density, conversion efficiency and fill factor of the solar cells are respectively increased by 7mV, 6mA/cm2, 2% and 2%. These results indicate that the MCD process is a novel and potential agent for the SCD process.

Efficiency enhanced emitter wrap-through (EWT) screen-printed solar cells with non-uniform thickness of silicon nitride passivation layer in via-holes

Emitter wrap-through (EWT) solar cells with an 18.6% cell efficiency were fabricated by etch-back with etch resist, a selective emitter (SE) with reactive ion etching (RIE) and screen printing technologies. The short-circuit current densities (Jsc) were more than 40mA/cm2, but the cell efficiency was limited by the fill factor (FF), which was as low as 73.4%. An electroluminescence (EL) analysis showed that this low value was linked to shunts occurring in the via-holes. The doping profile difference between the wafer surface and the inside of the via-holes was statistically insignificant. However, the thickness of the deposited SiNx passivation layer in the via-holes decreased from the surface to the inner side, allowing the paste to penetrate the junction through the passivation layer and causing shunts during the firing process. By changing to Ag paste for the shallow emitter, changing the firing conditions and performing a process optimization, an efficiency of 19.5% with a FF of 77.6% was achieved using industrial 6-inch. Cz wafers under AM 1.5G.

2-D SPICE Simulation and Analytical Calculation of Spreading Resistance Effects in Emitter Wrap-Through Cells With Nonsquare Via-Hole Pattern

Cell concepts that feature via-holes, like the emitter wrap-through concept, suffer from higher series resistance losses than conventional solar cells. A part of these resistance losses is due to the fact that a small size element, in this case a via-hole, has to collect the current of a wide area on the front side, which leads to the so-called spreading resistance. An analytical equation already exists which describes the spreading resistance effects that are present in these cells. This equation is an accurate description for a quadratic or hexagonal pattern of via-holes. For cells that feature an interdigitated metallization grid on the rear side, like emitter wrap-through solar cells, the arrangement is often nonsquare because the distance between two via-holes is different in the x- and y-directions. In this paper, it is shown that the existing formula is in excellent correlation with the results from simulation program with integrated circuit emphasis (SPICE) simulation if a quadratic symmetry element is chosen. The SPICE simulation is then adjusted for nonsquare symmetry elements, showing that the existing formula is not valid for these arrangements and has to be modified. The derived analytical modification is adapted to yield a better description of the spreading resistance in the emitter of such a cell structure for variable via-hole distances in the x- and y-directions. The relative deviation between the simulation results and the analytic approach is less than 15%. This enables an accurate calculation of the resistance losses of relevant technological configurations.

EWT 태양전지 제작을 위한 레이저 미세 관통홀 가공 기술

Laser drilling of vias is the one of key technologies in developing Emitter-Wrap Through(EWT) solar cell which is particularly attractive due to the use of industrial processing and common solar grade p-type silicon materials. While alternative economically feasible drilling process is not available to date, the processing time and laser induced damage should be as small as possible in this process. This paper provides an overview on various factors that should be considered in using the laser via drilling technology for developing highly efficient and industrially applicable EWT solar cells.

High Efficiency N-Type Emitter-Wrap-Through Silicon Solar Cells

In the ALBA-II project, Q-Cells SE, Bitterfeld-Wolfen, Germany, and the Institute for Solar Energy Research Hamelin, Emmerthal, Germany, are developing high-efficiency emitter-wrap-through (EWT) solar cells on n-type silicon wafers. N-type silicon grown by the Czochralski (Cz) method forms the basis of this high-efficiency solar cell development as it offers high bulk carrier lifetimes. The EWT device structure allows us to employ a simplified process sequence compared with interdigitated back-contact back-junction solar cells. High open-circuit voltages of our solar cells are achieved by different passivation layers for base and emitter surfaces and picosecond laser ablated contact openings. An optimization of the resistances along the current paths in base and emitter leads to an improvement in fill factor (FF) over former EWT solar cells. Together with the inherently high current densities of EWT solar cells, we achieve on our small-area (4-cm2, designated area without busbars) cells a short-circuit current density JSC of 40.4 mA/cm2, an open-circuit voltage VOC of 661 mV, FFs well above 80%, and, thus, cell efficiencies of up to 21.6%.

Features of photovoltaic production technology through-emitter converters

The concept of Emitter Wrap-Through (EWT) solar cells comprises a carrier collecting p-n junction covering the whole front surface which is wrapped through a large number of via-holes to the rear side where it is then contacted by the cell metallization. The technology of production EWT solar cells was adapted to the existing equipment of production conversional solar cells. The EWT solar cell workability visualization was made by electroluminescence

Development of industrial high‐efficiency back‐contact czochralski‐silicon solar cells

AbstractBack‐contact silicon solar cells feature high efficiencies, simpler module assembly, and improved aesthetics. The emitter wrap through (EWT) cell structure is particularly attractive for use with industrial processing (e.g., screen‐printed metallization) and common solar‐grade p‐type Si materials. We report development of a high‐efficiency EWT back‐contact cell using p‐type Cz silicon and only industrial processing techniques such as screen‐printed metallizations, diffusions, and PECVD. An efficiency of 19.0% has been achieved with large area (156‐mm pseudo‐square) cells. Detailed characterization and modeling have been performed to understand the optical and electrical losses, and these results were used to generate a roadmap to improve the efficiency to over 20%. Copyright © 2011 John Wiley & Sons, Ltd.

Laser Processing in Industrial Solar Module Manufacturing

The use of lasers in the processing of solar cell structures has been known for many years both for c-Si and thin-film solar technologies. The maturity of the laser technology, the increase in scale of solar module production and the pressures to drive down cost of ownership and increase cell efficiencies have all contributed to the adoption of laser processes in industrial manufacturing. Today laser systems are the tool of choice in thin-film module manufacturing both for scribing the cell interconnects and for the module edge isolation. For c-Si solar cells the primary laser application today is edge isolation and this is well-established in industrial production of most types of waferbased cells. Other laser processes are used in the production of advanced high-efficiency c-Si cell designs such as laser grooved buried contacts, emitter wrap-through or metal wrap-through interconnects, selective emitters and laser fired contacts. In the mission of the solar industry to reduce the cost of electricity generation there are increasing opportunities for laser processing to contribute to the goal of low cost of ownership in industrial manufacturing through improved module efficiencies, higher throughput and reduced process costs.

Loss analysis of emitter-wrap-through silicon solar cells by means of experiment and three-dimensional device modeling

Currently, the emitter-wrap-through (EWT) design of Si solar cells is being intensively investigated as a potential candidate for cheap, low-quality Si materials. So far, experimentally achieved energy conversion efficiencies have stayed unexpectedly far below the expectations of common device theory. Therefore, we analyze fabricated EWT cells in detail and refine device theory to account for the limiting loss mechanisms present only in EWT cells. By means of rigorous three-dimensional numerical device modeling, we show that the fill factor (FF) is significantly reduced, primarily due to a effect we call the via-resistance induced recombination enhancement effect. The FF is only secondarily reduced by the resistive losses in the vias where the emitter is wrapped through the cell. This implies that lowering the base resistivity will improve cell efficiency more effectively than lowering the resistance in the vias. Our simulations predict that the EWT design with a nonpassivated rear emitter and a homogeneous emitter diffusion leads to an efficiency improvement of about 1% absolute, as compared to the common front-junction design. This is so for excess carrier lifetimes in the bulk between 1 and 100 μs, which means also for multicrystalline cells.

The origin of reduced fill factors of emitter‐wrap‐through‐solar cells

AbstractLow fill factors generally limit the efficiency of emitter‐wrap‐through (EWT) solar cells. Until now, a conventional series resistance limitation along the laser‐drilled EWT vias has usually been assumed to be responsible for this effect. We demonstrate that the characteristic fill factor loss is caused by a crucial change in the diffusion currents inside the base, which are influenced by the conductivity along the laser‐drilled EWT vias. In addition, we show that the EWT via conductivity influences the fill factor loss caused by an iron contaminated base. This result affects the proposition that the EWT design is suitable for multicrystalline silicon in which interstitial iron is known to be the main contaminant. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Bow in screen-printed back-contact industrial silicon solar cells

In this paper, we present a model of the bow in thin back-contact silicon solar cells with screen-printed (SP) silver grid metallization. A modification of the bimetallic strip model is used to model the bow for the interdigitated back-contact, emitter-wrap-through (EWT) solar cell. It is proposed that the contact area fraction of the thick regions (>100 nm)of the binder glass at the Ag–Si contact interface responsible for metallization adhesion is an important parameter necessary for modeling the bow for SP back-contact solar cells with better accuracy. Techniques for reducing the bow are also proposed.

Back‐contact solar cells: a review

AbstractEver since the first publications by R.J. Schwartz in 1975, research into back‐contact cells as an alternative to cells with a front and rear contact has remained a research topic. In the last decade, interest in back‐contact cells has been growing and a gradual introduction to industrial applications is emerging. The goal of this review is to present a comprehensive summary of results obtained throughout the years. Back‐contact cells are divided into three main classes: back‐junction (BJ), emitter wrap‐through (EWT) and metallisation wrap‐through (MWT), each introduced as logical descendents from conventional solar cells. This deviation from the chronology of the developments is maintained during the discussion of technological results. In addition to progress on manufacturing these cells, aspects of cell modelling and module manufacturing are discussed and an outlook towards industrial implementation is presented. Copyright © 2005 John Wiley & Sons, Ltd.

Low-cost multicrystalline back-contact silicon solar cells with screen printed metallization

Abstract Adaptation to market requirements is a permanent challenge in industrial solar-cell production. Both increase of cell efficiency as well as lowering costs is demanded. Back-contacted solar cells offer multiple advantages in terms of reducing module assembling costs and enhanced cell efficiency. The investigated emitter-wrap-through (EWT) design [1] has a collecting emitter on front and rear side. These emitter areas are electrically connected by small holes. Due to the double-sided collecting junction, this cell design is favourable for materials with a low-minority charge carrier diffusion length leading to a higher short circuit current density. Untill now most investigations on EWT solar cells were performed on Cz or even FZ silicon. This was justified as long as different processing techniques had to be developed and compared. But as an industrially applicable process sequence has recently been developed [2] , the advantages of the EWT concept compared to conventionally processed cells have to be shown on multicrystalline material. In the following, a manufacturing process of EWT solar cells is presented which is especially adapted to the requirements of multicrystalline silicon. Effective surface texturization was reached by mechanical V-texturization and bulk passivation by a hydrogen plasma treatment. The efficiency of the best solar cells within this process reached 14.2% which is the highest efficiency reported so far for mc-Si 10×10 cm 2 EWT solar cells [3] .

Investigations on low-cost back-contact silicon solar cells

Back-contacted cells offer multiple advantages in regard of reducing module assembling costs and avoiding grid shadowing losses. The investigated emitter-wrap-through (EWT) design is equipped with a front and a rear emitter (Gee et al., Proceedings of the 23th IEEE PVSC, 1993, p. 265). Both emitter areas are electrically connected by small holes drilled into the crystalline silicon wafer. Renouncing on expensive photolithographic steps several techniques appropriate for a low-cost process have been investigated. An increase in J sc of 15% was measured when compared to a conventionally processed reference cell. However, the open-circuit voltage V oc and the fill factor did not improve. Possible reasons for these losses are presented in this paper. The efficiency of one of the best low-cost EWT cells within this work reached 13.6% on Cz-silicon, which is the highest efficiency reported so far for this cell type (Schönecker et al., Proceedings of the Second WCPSEC, 1998, p. 1677).

Circuit modeling of the emitter-wrap-through solar cell

Back-contact solar cells have the potential to reduce module assembly costs and give a higher conversion efficiency. Such a device must be simple to fabricate on an industrial scale and be tolerant of low minority-carrier diffusion lengths. The emitter-wrap-through (EWT) cell is a device design that can meet these goals. In this device, the diffused junction is present on both sides and is connected by laser-drilled holes through the silicon. EWT cells were frequently found to have poor fill factors (FFs) due to shunt-like behavior. The holes were found to possess no defects that adversely affect device performance. However, detailed equivalent circuit modeling of the EWT cell was able to explain the shunt-like behavior. Experiments were performed to confirm the physical mechanisms described by the equivalent circuit model. Device optimization guided by the equivalent circuit model has led to the demonstration of a large area EWT cell with a FF of 77.64% and efficiency of 18.2%.

Low-cost back contact silicon solar cells

Back-contacted solar cells offer multiple advantages in regard of reducing module assembling costs and avoiding grid shadowing losses. The investigated emitter-wrap-through (EWT) device design has an electrical connection of the front emitter and the rear emitter grid in form of small holes drilled into the crystalline silicon wafer. The obtained cell structure is especially suitable for low-cost base material with small minority carrier diffusion lengths. Different industrially applicable solar cell manufacturing processes for EWT devices are described and compared. The latest experimental results are presented and interpreted; the photocurrent is found to be distinctly increased. The relation between open circuit voltage and rear side passivation is discussed based on two-dimensional (2-D) computer simulations.