Aziz S. Shaikh

Effect of Ag Particle Size in Thick-Film Ag Paste on the Electrical and Physical Properties of Screen Printed Contacts and Silicon Solar Cells

The impact of the Ag particle (metal powder in the screen printed paste) size on the quality of Ag thick-film ohmic contacts to high-sheet-resistance emitters of Si solar cells is investigated. Spherical particle size wasvaried in the range of 0.10-10 μm (ultrafine to large). Even though ultrathin glass regions are achieved for the large particle paste, giving low specific contact resistance (ρ c ), secondary ion mass spectroscopy measurements showed a higher Ag concentration (>10 1 5 cm - 3 ) at the p-n junction that increased the junction leakage current (J o 2 ) and decreased the V o c by ∼7 mV and fill factor (FF) by ∼0.02. Pastes with ultrafine Ag particles generally produced a thick glass layer at the Ag-Si contact interface, which increased ρ c and series resistance (R s ) (≥ 1 Ω cm 2 ), and lowered the FF by ∼0.03. Small to medium size Ag particles in the paste produced thin glass regions and many regularly distributed Ag crystallites at the contact interface. This resulted in low R s (< 1 Ω cm 2 ), high shunt resistance (60,558 Ω cm 2 ), low J o 2 (∼20 nA/cm 2 ), and high FF (0.781). Cell efficiencies of ∼17.4% were achieved on untextured float zone Si with 100 Ω/□ emitter by rapid firing of screen printed contacts in a lamp-heated belt furnace.

Dielectric properties of ultrafine grained BaTiO/sub 3/

The dielectric constant and spontaneous polarization of fine-grained BaTiO/sub 3/ prepared from powder produced by metalloorganic decomposition technology were studied. The room-temperature dielectric constant of BaTiO/sub 3/ was found to increase sharply with increase in grain size, reach a maximum at about 0.4 mu m, and decrease with further increase in grain size. Spontaneous polarization continuously decreased with decrease in grain size. A model is proposed to explain the grain size dependence of the dielectric constant.<<ETX>>

Investigation of Modified Screen-Printing Al Pastes for Local Back Surface Field Formation

This paper reports on a low-cost screen-printing process to form a self-aligned local back surface field (LBSF) through dielectric rear surface passivation. The process involved formation of local openings through a dielectric (SiNx or stacked SiO2/SiNx) prior to full area Al screen-printing and a rapid firing. Conventional Al paste with glass frit degraded the SiNx surface passivation quality because of glass frit induced pinholes and etching of SiNx layer, and led to very thin LBSF regions. The same process with a fritless Al paste maintained the passivation quality of the SiNx, but did not provide an acceptably thick and uniform LBSF. Al pastes containing appropriate additives gave better LBSF because of the formation of a thicker and more uniform Al-BSF region. However, they exhibited somewhat lower internal back surface reflectance (<90%) compared to conventional Al paste on SiNx. More insight on these competing effects is provided by fabrication and analysis of complete solar cells

Wire bonding characteristics of gold conductors for low temperature co-fired ceramic applications

Abstract Low temperature co-fired ceramic (LTCC) with gold conductors has been used for high reliability applications, such as satellite communications. When the gold metallization is co-fired with the LTCC tape, inorganic adhesion additives in the gold conductor interact with the LTCC glass. This interaction is essential to provide adequate adhesion for the gold metallization. However, this interaction can also reduce the softening point of the LTCC glass. The reaction product can migrate to the surface of the gold conductor and affect the wire bondability of the gold conductor. Effort has been made to develop surface gold conductors with optimized interaction with LTCC. The new gold conductor shows significant reduction of inorganic particles on the surface and improvement of wire bondability. A systematic wire bonding study has been performed on gold conductors under various wire bonding conditions. It is demonstrated that the wire bonding window can be improved significantly by reducing the interaction between LTCC and gold metallization. This paper reports the results of the wire bonding study.

Diffusion Paste Development for Printable IBC and Bifacial Silicon Solar Cells

Increasing cell efficiency and lowering the cost of solar cell manufacturing is a continuous challenge in solar cell industries. Higher efficiency concepts like interdigitated back contact (IBC), and bifacial cells, with using thinner silicon solar cells are ways to reduce the total solar cell manufacturing cost. Typically the high efficiency concepts require costly extra processing steps. Developing the proper paste for printing application of high efficiency concepts could make these designs cost effective and easily manufactured. These pastes can be printable in a continuous manner using processes like screen printing or ink jet printing. This paper describes newly developed low cost phosphorous, boron, and diffusion barrier pastes. It shows the characterization of the newly developed phosphorous paste (99-038), boron paste (99-033), and diffusion barrier paste (99-001). These low cost pastes can easily be printed and make high efficiency concept cell designs a low cost reality. Characteristic features of sheet resistance, junction depth and minority carrier lifetime is discussed

Hot-melt screen-printing of front contacts on crystalline silicon solar cells

This work describes a study of hot-melt (HM) screen-printing of front contacts as a function of print speed, print table temperature, squeegee temperature, paste composition and firing profiles. The results presented here will show that it is possible to print lines with higher aspect ratio compared with standard screen-printing. Optimum parameters for printing seems to be high print speed in combination high squeegee temperature. There is a trade off between the height and width of the finger lines when adjusting the table temperature. Firing with fingers facing down at high belt speeds results in shrinking of line width between 15% and 20%. At present, the best efficiency results from HM screen-printing are at par with standard printed solar cells.

SEM microstructure, composition, property correlation-ships in aluminum conductors

Al paste applied on the back surface of silicon solar cells, plays a critical role in the overall performance of solar cells. The quality of the BSF (back surface field) impacts the Voc values and thus the efficiency obtained in the solar cell [1–3]. A Taguchi screening experiment was conducted to study effects of processing and formulation on paste viscosity, uniformity of BSF, bowing and dusting. The thickness and uniformity of BSF (back surface field) formed was found to be a direct function of various factors including type of Al powder, chemistry of glass, other oxide and metal additives as well as the firing conditions used. The electrical performance, bowing and dusting are correlated with microstructure and paste compositions, and are discussed below.

Advanced screen printed polymer Ag paste for HIT cells

The Photovoltaic market has grown tremendously during the past decade. HIT (Heterojuction with Intrinsic Thin layer) cells have drawn special attention because of their potential ability for further cost reduction to achieve grid parity. HIT cells have achieved higher conversion efficiency than traditional crystalline silicon solar cells, and demonstrated better performance stability at high temperature. Similar to thin film solar cell, the pastes for HIT cell are printed on TCO (transparent conductive oxide) substrate, and are required to be curable at low temperature (≤200°C) to minimize the possibility of cell degradation. In this paper we will discuss the paste requirements for HIT cell application and the effect of paste chemistry on cell performance. This paper will also present the various attributes of these pastes such as excellent electrical conductivity, strong adhesion to ITO layer, high aspect ratio and good printability.

Screen-printed high performance aluminum and polymer pastes for n-type solar cells

This paper presents high performance Al pastes for n-type solar cells with p-type backside. These pastes were screen printed and subsequently fired in a belt furnace to form rear-Al emitters. Internal Quantum Efficiency (IQE), and full I-V curve characterization and curve fitting to investigate leakage currents were carried out to study the effect of glass frits, additives and firing profiles. The paste performance including bow and adhesion are also presented. This paper also presents a screen printable, low temperature curable conductive polymer paste and its application in n-type solar cell fabrication. The solderable polymer paste enables full Al coverage of the back surface. Its excellent adhesion and low contact resistivity to Al allow it to be used as rear Ag busbar.

Screen-printed reflector pastes for back passivated crystalline silicon solar cells

Solar cell manufacturers across the world are working to find innovative ways to reduce cost per installed watt of photovoltaic energy generation in order to achieve grid parity. Some of these approaches involve the use of revolutionary cell architectures such as metal wrap-through (MWT) [1], PERC cells, local back surface field (BSF) [2] and selective emitter among others. Many of these new approaches call for development of special back Al pastes that can either fire-through passivation stacks such as Al2O3/SiNx [3] or form local BSF in laser opened vias. In other designs, the Al paste needs to act as a reflective surface below the SiNx layer, without firing through the dielectric layer (reflector paste). In this paper we discuss the requirements for reflector paste in back-passivated cell applications. The paper will also present the various attributes of these pastes such as non firethrough properties, adhesion to SiO2, Al2O3 or SiNx passivation layers, excellent reflectance and low electrical resistance.