Development and Performance Evaluation of Screen-Printable Conductive Pastes for Photovoltaics, Double-sided Printed Wiring Boards, and Field Emission Devices Applications

Abstract

The advantage of screen-printing, compared with evaporations and plating techniques includes high-volume productivity of extremely cost-effective, simple, continuous automation processes. For this reason, screen-printing techniques have been widely developed to integrate various conductive fillers in the industry of photovoltaics (PVs), printed wiring boards (PWBs), and field emission displays (FEDs). The purpose of this dissertation is to offer comparative performance evaluations between commercialized screen-printable products and newly developed environment-friendly conductive pastes, which are optimized via designs of their composition and formulation for PVs, double-sided (D/S) PWBs, and FEDs applications in mass-production environments. To date, there have been various researches regarding metallization of Pb-free pastes for PVs applications. Some researchers demonstrated that the optimal formulation of Pb-free frits in the pastes also can leads to the same conversion efficiency as for the leaded paste. Nevertheless, the Pb-free frit for formations of Ag front electrodes is not commercialized yet. Furthermore, when the conventional Pb-free paste is used, the reason for the increase of series resistance still remains obscure. There also remains the difficulty to apply the rapid temperature co-firing (RTC) process for the formation of Ag front and Al rear electrodes at the same time. Therefore, it is important to establish of the optimal components, physical and chemical characteristics for Pb-free glass frits. In addition, the formulation and quantification of the inorganic binder, modifier and another dopant to complement the deficient metallization quality and cell efficiency caused by Pb-free glass frits are needed. The paste for PVs application should be also considered the applicability to the RTC process. The silver though holes (STHs) paste for the via connecting application of D/S PWBs has been widely used due to its low electrical resistivity, short tact time, environmentally friendly property instead of the electrochemical plating process. The alternative Cu paste has been also researched to replace Ag pastes to reduce manufacturing costs. However, the conventional Cu paste, which has a particular shelf-life or low storage condition to prevent oxidation, is readily oxidized at humid air atmospheres. Besides, the Cu paste is also not applicable to low cost flame-retardant 1(FR-1) copper clad laminates (CCLs) containing incomplete out gassing reactants due to high defect rates. Therefore, the important point for the development of the alternative paste with a low cost could be summed up in the stability from the oxidation and severe reliability test condition such as high temperature and humidity, and the applicability to low cost FR-1 CCLs. With regard to the CNT-based field emitters, there are some issues associated with short device lifetimes and low emission densities. The highly concentrated growth of CNTs by chemical vapor deposition (CVD) methods is very difficult as well as too costly in the large area. The degradation has of the emission current density, related with the Joule heating at the interfacial mechanical stress between CNTs and substrates, has also emerged as a serious problem. Moreover, the emission current of the CNT emitter can be also degraded by gaseous species such as oxygen and water molecules generated by Joule heating. Hence, the most important point to realize the high emission density and long device lifetime is to reduce Joule heating between CNTs and substrates. As development results for the application of PVs, optimally formulated Pb-free silver pastes for better conductivity, higher density of the Ag matrix compact, and continuous front ohmic contact metallization could be obtained by the formulation of inorganic modifiers and Pb-free glass frits with small sizes. Conversion efficiencies and fill factor values of optimized cell are slightly higher when compared with commercial leaded Ag pastes, although cells are fabricated by metalizing the lead-free Ag pastes. Besides, boron doping improves the electrical performance of back surface fields at even small amount of Al/B deposits, where wafer bowings and manufacturing costs could be reduced. Bowings of cells fired with the Al/B paste containing barium sulphate powders could be also reduced by formulation adjustments of glass frits and Al/B pastes deposit amount. Thereby, obtained results exhibited the average cell conversion efficiencies of ~18.49% (singlecrystalline) and ~16.93% (polycrystalline) Si solar cells at high throughput industrial processes, respectively. On the other hand, the outstanding result for applications on D/S PWBs is a far lower through-hole resistivity without pure Ag powders from the combination of spherical, flake and dendrite Ag-coated Cu powders. The resulting through holes showed even hole-shapes without micro-cracks or voids and high thermal stabilities. Developed low-cost Ag-coated Cu pastes (Ag: 9 wt%) had a higher current-carrying density than that of conventional pure Ag pastes at the breakdown point. Other reliability evaluations at severe environmental conditions indicated that the organic vehicle with a carboxylic ion and modified phosphate ion from titanate additives played roles as antioxidants and corrosion inhibitors of CCLs or other metal substrates as well as conductive fillers. Titanate additive was disposed on surfaces of conductive fillers and extended the hydrophobic dioctyl chains, which may effectively prevent conductive fillers from being oxidized or corroded by water absorptions, thereby giving rise to the improved surface-insulation-resistance and long-term conductive reliability on accelerated temperature-humidity-bias. These developed low-cost Ag-coated Cu pastes can possibly compensate for the low productivity and frequent failure rate of conventional Ag/Cu pastes in Pb-free D/S PWBs industries. The applications of carbon nanotubes (CNTs) pastes by using screen-printing technology was limited to high pattern resolution and uniformed thickness at prototype diodes of gated structures. However, the fabrication of field-emission devices was available by adopting photoimagable hybrids pastes of as-grown CNTs onto carbide-derived carbons (CDCs) and the back-side exposure technique. The resulting CNT-CDC hybrid materials showed low turn-on voltage, high emission current density, and high β value in comparison with the CNTs. The field emission devices with the CNT-CDC emitters also achieved a significant improvement in terms of long-term emission stability, which could be attributed to the synergistic coupling effects of CNTs and CDCs.

The originality for the development of each part could be summarized as follows. For the PVs application, the inventive key point of the paste formulation as distinguished from the commercialized pastes is based on the optimal components, physical and chemical characteristics of the Pb-free glass frits under RTC manufacturing conditions. Also, the Bi2O3 and MnO2 to complement the deficient metallization quality of the Pb-free glass frits performed a key role as inorganic binders. Thereby, the Rs value of the optimized cell was slightly lower when compared with the commercial leaded Ag paste, although cells were fabricated by metalizing the lead-free silver pastes. This cell showed the relatively less voids and better metallization quality after firing. Moreover, the cells containing a mixture of Bi2O3 as inorganic binders and CaO or MgO as functional materials with low junction barriers in metal/n-type Si interface regions showed the low Rs values and high FFs. For the effective BSFs formation, chemically-treated nano-Al and boron doping provides the rich concentration of P+ diffusions. Therefore, the wafer bowing phenomena and manufacturing cost by using the small consumption of Al pastes depositions could be reduced. In addition, the bowings of cells fired with the Al/B paste containing barium sulphate powders could be dramatically alleviated. Based on the repetitive experiments at the laboratory RTC processes, the narrow-distributed cell conversion efficiencies at the high throughput industrial process have been successfully achieved. For the via connecting application of D/S PWBs, the electrical resistivities of developed Ag-coated Cu pastes with a low cost are slightly lower when compared with commercialized pure Ag pastes, although their silver contents are low. The reliable quality of via holes connecting could be obtained by paste formulations to prevent Cu oxidations, i.e., repetitive Ag coating on Cu powder, surface modification of combined Ag-coated Cu powder with a spherical, flake and dendrite morphology, and addition of noepentyl (diallyl) oxy, tri (dioctyl) pyrophosphato titanates in dially isophthalate epoxy acrylates with a carboxylic group and resole condensation polymers. The hydrophobic pyrophosphato titanate containing phosphates and carboxylic groups in the paste leads to decreased hole-resistivity because they act as fluxing agents on the surface of Cu or CuO. Moreover, the combination of three types of Ag-coated Cu powders increases the packing density of the conductive filler after curing, the even hole shape without micro-cracks or voids was observed. The developed Ag-coated Cu paste showed the property of the higher current-carrying capacity and thermostability than commercialized Ag pastes. Besides, unlike the conventional Cu paste, the hole resistivity and the adhesion were safely maintained during severe Pb-free solder processes. With regard to the application of CNT-based field emitters, in order to reduce Joule heating between CNTs and substrates, there are a lot of reports for the field emission of CNTs synthesized directly onto carbon templates such as a carbon cloth, carbon felts, carbon aerogels, etc. A carbon cloth and carbon felts, however, may not be suitable as a support material because it is difficult to integrate them directly into the conventional field emission devices which are in a complex and fine configuration. On the other hand, CDCs can be used as a field emitter itself and simultaneously used as a support material for growth of CNTs. In case of CNT-CDC hybrids, moreover, Joule heating is low between CNT-CDC hybrids and substrates due to the inherent as-grown morphology. Moreover, strong binding site in CDC may prevent the CNT from being contaminated by degassed elements, thereby giving rise to the improved emission stability.