Screen printable copper pastes for silicon solar cells

High-throughput front and rear side metallization of silicon solar cells using rotary screen printing

Developing a better method for the metallization of silicon solar cells is integral part of realizing superior efficiency. Currently, contact realization using screen printing is the leading technology in the silicon based photovoltaic industry, as it is simple and fast. However, the problem with metallization of this kind is that it has a lower aspect ratio and higher contact resistance, which limits solar cell efficiency. The mounting cost of silver pastes and decreasing silicon wafer thicknesses encourages silicon solar cell manufacturers to develop fresh metallization techniques involving a lower quantity of silver usage and not relying pressing process of screen printing. In recent times nickel/copper (Ni/Cu) based metal plating has emerged as a metallization method that may solve these issues. This paper offers a detailed review and understanding of a Ni/Cu based plating technique for silicon solar cells. The formation of a Ni seed layer by adopting various deposition techniques and a Cu conducting layer using a light induced plating (LIP) process are appraised. Unlike screen-printed metallization, a step involving patterning is crucial for opening the masking layer. Consequently, experimental procedures involving patterning methods are also explicated. Lastly, the issues of adhesion, back ground plating, process complexity and reliability for industrial applications are also addressed.

Ni:Si as Barrier Material for a Solderable PVD Metallization of Silicon Solar Cells

Within this work, we provide a comprehensive overview about research activities and current status with respect to rotary printed metallization for silicon solar cells. We will present the major results of the research project »Rock-Star« and previous activities which focused on the metallization of Si solar cells using flexographic printing and rotary screen printing. We demonstrate that the rear side metallization of passivated emitter and rear cells (PERC) can be realized with rotary screen printing on the same quality level as state-of-the-art flatbed screen printing. Furthermore, it is shown that both rotary screen printing and flexographic printing are able to realize the fine line front side metallization. Fine line front side contacts down to approx. 40 µm for rotary screen printing and around 30 µm for flexographic printing are demonstrated. However, further optimization is required to reduce finger width, increase finger height and thus decrease the mean lateral finger resistance. A major result of project »Rock-Star« is the development of an innovative demonstrator platform to enable rotary printed solar cell metallization with high printing speed and low cycle time per wafer. Newly developed transport, alignment and printing concept enables a cycle time of down to 0.45 s/wafer. The concept and features of the demonstrator machine are presented within this paper.

Sol–gel derived nano‐sized glass frits were incorporated into the Ag conductive ink for silicon solar cell metallization. This mixture was specifically formulated for inkjet printing on textured Si wafers with 80 nm thick SiNx anti reflection coating layers. The correlation between the contact resistance and interface microstructures were studied using scanning electron microscopy and transmission electron microscopy. In addition, the specific contact resistance between the front contact and emitter was measured at various firing conditions using the transfer length model. On an emitter with the sheet resistance of 60 Ω/sq, a specific contact resistance below 5 mΩ cm2 could be achieved at a peak firing temperature around 800 °C. We found that the incorporated nano‐glass frit act as a very effective fire through agent, and an abundant amount of Ag crystallites was observed along the interface glass layer. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

In solar cell production, metallization is the manufacturing of metal contacts at the surfaces of solar cells in order to collect the photo-generated current for use. Being one of the most expensive steps in solar cell fabrication, it plays both an electrical and an optical role, because the contacts contribute to shading, and to the series resistance of solar cells. In addition, metal contacts may reduce the solar cells voltage due to charge carrier recombination at the metal / silicon interface. Addressing these challenges could increase solar cell conversion efficiency while cutting their production costs. This work presents state of the art methods for the metallization of crystalline Si solar cells for industrial production as well as for research and development. Different metallization technologies are compared, and ongoing RD activities for the most relevant silicon solar cell metallization technologies are described in detail. Chapters cover fundamentals of metallization and metallization approaches, evaporated, plated and screen-printed contacts, alternative printing technologies, metallization of specific solar cell types, module interconnection technologies, and also address module technology. Written by a selection of world-renowned experts, the book provides researchers in academia and industry, solar cell manufacturing experts and advanced students with a thorough and systematic guide to advanced metallization of solar cells.

Studies were conducted on several fundamental aspects of electroless nickel/solder metallization for silicon solar cells. A process proposed by Motorola, which precedes the electroless nickel plating with several steps of palladium plating and heat treatment, was compared directly with single step electroless nickel plating. Work has directed toward answering specific questions concerning the effect of silicon surface oxide on nickel plating, effects of thermal stresses on the metallization, sintering of nickel plated on silicon, and effects of exposure to the plating solution on solar cell characteristics. The Motorola process was compared with simple electroless nickel plating in a series of parallel experiments. Results are presented. (WHK)

This paper characterizes the adhesion quality of evaporated aluminum rear metallization on rear passivation layers of silicon solar cells. A peel-test is introduced for adhesion rating and the adhesion quality between various Al and passivation layers on test structures is compared and discussed. The results show that the adhesion strongly varies with passivation type and evaporation parameters. Strong adherence is attributed to the formation of a strongly bonded, extended interphase layer. We derive factors that influence adhesion quality on our sample structures and find that an intimate contact between the layers and elevated temperatures are very important parameters. Additionally, a sufficiently high adhesion quality for an evaporated rear metallization of passivated emitter and rear cells (PERC) is achieved for all investigated passivation layers.

<p>The fabrication of low cost, high efficiency solar cells is imperative in competing with existing energy technologies. Many research groups have explored using III-V materials and thin-film technologies to create high efficiency cells; however, the materials and manufacturing processes are very costly as compared to monocrystalline silicon (Si) solar cells. Since commercial Si solar cells typically have efficiencies in the range of 17-19%, techniques such as surface texturing, depositing a surface-passivating film, and creating multi-junction Si cells are used to improve the efficiency without significantly increasing the manufacturing costs. This research focused on two of these techniques: (1) a tandem junction solar cell comprised of a thin-film perovskite top cell and a wafer-based Si bottom cell, and (2) Si solar cells with single- and double-layer silicon nitride (SiNx) anti-reflection coatings (ARC).</p><p>The perovskite/Si tandem junction cell was modeled using a Matlab analytical program. The model took in material properties such as doping concentrations, diffusion coefficients, and band gap energy and calculated the photocurrents, voltages, and efficiencies of the cells individually and in the tandem configuration. A planar Si bottom cell, a cell with a SiNx coating, or a nanostructured black silicon (bSi) cell can be modeled in either an n-terminal or series-connected configuration with the perovskite top cell. By optimizing the bottom and top cell parameters, a tandem cell with an efficiency of 31.78% was reached.</p><p>Next, planar Si solar cells were fabricated, and the effects of single- and double-layer SiNx films deposited on the cells were explored. Silicon nitride was sputtered onto planar Si samples, and the refractive index and thicknesses of the films were measured using ellipsometry. A range of refractive indices can be reached by adjusting the gas flow rate ratios of nitrogen (N2) and argon (Ar) in the system. The refractive index and thickness of the film affect where the minimum of the reflection curve is located. For Si, the optimum refractive index of a single-layer passivation film is 1.85 with a thickness of 80nm so that the minimum reflection is at 600nm, which is where the photon flux is maximized. However, using a double-layer film of SiNx, the Si solar cell performance is further improved due to surface passivation and lowered surface reflectivity. A bottom layer film with a higher refractive index passivates the Si cell and reduces surface reflectivity, while the top layer film with a smaller refractive index further reduces the surface reflectivity. The refractive indices and thicknesses of the double-layer films were varied, and current-voltage (IV) and external quantum efficiency (EQE) measurements were taken. The double-layer films resulted in an absolute value increase in efficiency of up to 1.8%.</p>

Bifacial solar cells and modules are a promising approach to increase the energy output of photovoltaic systems, and therefore decrease levelized cost of electricity (LCOE). This work discusses the bifacial silicon solar cell concepts PERT (passivated emitter, rear totally diffused) and BOSCO (both sides collecting and contacted) in terms of expected module cost and LCOE based on in‐depth numerical device simulation and advanced cost modelling. As references, Al‐BSF (aluminium back‐surface field) and PERC (passivated emitter and rear) cells with local rear‐side contacts are considered. In order to exploit their bifacial potential, PERT structures (representing cells with single‐sided emitter) are shown to require bulk diffusion lengths of more than three times the cell thickness. For the BOSCO concept (representing cells with double‐sided emitter), diffusion lengths of half the cell thickness are sufficient to leverage its bifacial potential. In terms of nominal LCOE, BOSCO cells are shown to be cost‐competitive under monofacial operation compared with an 18% efficient (≙pMPP = 18 mW/cm2) multicrystalline silicon (mc‐Si) Al‐BSF cell and a 19% mc‐Si PERC cell for maximum output power densities ofpMPP ≥ 17.3 mW/cm2andpMPP ≥ 18.1 mW/cm2, respectively. These values assume the use of $10/kg silicon feedstock for the BOSCO and $20/kg for the Al‐BSF and PERC cells. For the PERT cell, corresponding values arepMPP ≥ 21.7 mW/cm2andpMPP ≥ 22.7 mW/cm2, respectively, assuming the current price offset (≈50%, at the time of October 2014) ofn‐type Czochralski‐grown silicon (Cz‐Si) compared with mc‐Si wafers. The material price offset ofn‐type top‐type Cz‐Si wafers (≈15%, October 2014) currently accounts for approximately 1 mW/cm2, which correlates to a conversion efficiency difference of 1%absfor monofacial illumination with 1 sun. Fromp‐type mc‐Si top‐type Cz‐Si (≈30% wafer price offset, October 2014), this offset is approximately 2.5 mW/cm2for a PERT cell. When utilizing bifacial operation, these required maximum output power densities can be transformed into required minimum rear‐side illumination intensities for arbitrary front‐side efficienciesηfrontby means of the performed numerical simulations. For a BOSCO cell withηfront = 18%, minimum rear‐side illumination intensities of ≤ 0.02 suns are required to match a 19% PERC cell in terms of nominal LCOE. For ann‐type Cz‐Si PERT cell withηfront = 21%, corresponding values are ≤ 0.11 suns with 0.05 suns being then‐type top‐type material price offset. This work strongly motivates the use of bifacial concepts to generate lowest LCOE. Copyright © 2016 John Wiley & Sons, Ltd.

Photovoltaic (PV) cells can absorb up to 80% of the incident solar radiation of the solar spectrum, however, only certain percentage of the absorbed incident energy is converted into electricity depending on the conversion efficiency of the PV cell technology used, while the remainder energy is dissipated as heat accumulating on the surface of the cells causing elevated temperatures. Temperature rise at the PV cell level is addressed as one of the most critical issues influencing the performance of the cells causing serious degradations and shortens the life-time of the PV cells, hence cooling of the PV module during operation is essential. Hybrid PV designs which are able to simultaneously generate electrical energy and utilize the waste heat have been proven to be the most promising solution. In this study, analytical investigation of a hybrid system comprising of a Heat Pipe-based Photovoltaic-Thermoelectric Generator (HP-PV/TEG) for further enhanced performance is presented. The system presented incorporates a PV panel for direct electricity generation, a heat pipe to absorb excessive heat from the PV cells and assist uniform temperature distribution on the surface of the panel, and a thermoelectric generator (TEG) to perform direct heat-to-electricity conversion. A mathematical model based on the heat transfer process within the system is developed to evaluate the cooling capability and predict the overall thermal and electrical performances of the hybrid system. Results are presented in terms electrical efficiencies of the system. It was observed that the integration of TEG modules with PV cells aid improving the performance of the PV cells through utilizing the waste-heat available, leading to higher output power. The system presented can be applied in regions with hot desert climates where electricity demand is higher than thermal energy.

PERC cells are projected to comprise 35% of the p-type silicon cells produced in 2018, due to their 10% relative improvement in power conversion efficiency over AlBSF with only a marginal increase in manufacturing costs. Known risks of PERC cells, including light-induced degradation and passivation layer stability, are of key concern in this fastgrowing market. Here we measure performance and mechanistic parameters of bare PERC and Al-BSF cells stepwise through UV + heat exposure by ${Suns} - V_{OC}$, external quantum efficiency, and microwave photoconductive decay to obtain timeseries of mechanistic and performance variables to evaluate in our analytic framework. With the creation of statistical models relating these variables, individual degradation modes are determined and ranked for PERC and Al-BSF cells. By comparing PERC and Al-BSF cells, we can identify PERC-specific degradation modes and inform future mitigation strategies. In this study, PERC cells showed two significant pathways related to power loss: one via degradation of the minority carrier lifetime and diffusion/rear side recombination, and another via bulk/emitter recombination and voltage losses.

The objective of this work was to investigate how cobalt oxide doped coatings in the form of thin films could affect the electrical response of silicon (Si) solar cells. Spin coated thin films consisting of a polymeric solution of cobalt oxide-antimony doped tin oxide (CoO-ATO) were used as coatings on 3 × 6 Si solar cells. Based on the spectral response of CoO-ATO, it is hypothesized that coatings of this material onto a solar cell can help improve the performance of the cell. CoO-ATO thin films are coated onto a Si solar cell to synthesize a potential mat for the solar cell to filter out infrared light, thus improving the effectiveness and performance of the solar cell since it would primarily be absorbing visible light rather than infrared. CoO-ATO has been shown to be effective in the range of 3000 nm to 5000 nm. Obtained results show that 16% by wt of cobalt oxide and 94% by wt of antimony doped tin oxide produced maximum wattage.

This work presents an approach to lower the silver consumption of screen printed TOPCon (Tunnel Oxide Passivated Contact) solar cells by reducing and partially replacing the silver by low cost copper metallization paste. On the solar cells front side an adapted process sequence enables the use of a pure Ag paste, which compared to the industrially established AgAl paste offers a better conductivity and respectively requires less laydown to enable similarly low grid resistance. On the solar cells rear side the conductivity of the metal grid is accomplished by a copper paste which is realized within a print-on-print step on top of the Ag contact layer with minimized laydown. A reduction in Ag paste consumption by 40% to less than 9 mg/ Wp without sacrificing conversion efficiency seems achievable and can enable a significant cost reduction for TOPCon solar cells.

PERC solar cells based on Gallium p-type doping were treated at elevated temperatures in the range of 80…160°C and current injection. Current injection is conducted in a current-induced degradation (CID) setup in the dark, measuring voltage while applying a current at 0.06, 0.53 and 1.0 times the short-circuit current – ranging from maximum power point to open-circuit injection conditions. The results of the CID treatment show no significant light and elevated temperature induced degradation (LeTID) behaviour for these Ga-doped PERC cells. PERC cells treated at temperatures above 110°C show an increase in relative voltage during CID starting the earliest at a measurement time of 50 hours. It was found that the voltage increase could be related to increased contact resistance between the front silver metallization and the phosphourus emitter as indicated by comparing electroluminescence imaging before and after CID treatment and confirmed by transfer length method measurements. These measurements on a cut-out stripe showed an increase in contact resistivity and a lateral intensity variation as visible in the post electroluminescence image.