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The impact of the Ag particle ͑metal powder in the screen printed paste͒ size on the quality of Ag thick-film ohmic contacts to high-sheet-resistance emitters of Si solar cells is investigated. Spherical particle size was varied in the range of 0.10-10 m ͑ultrafine to large͒. Even though ultrathin glass regions are achieved for the large particle paste, giving low specific contact resistance ͑ c ͒, secondary ion mass spectroscopy measurements showed a higher Ag concentration ͑Ͼ10 15 cm −3 ͒ at the p-n junction that increased the junction leakage current ͑J o2 ͒ and decreased the V oc by ϳ7 mV and fill factor ͑FF͒ by ϳ0.02. Pastes with ultrafine Ag particles generally produced a thick glass layer at the Ag-Si contact interface, which increased c and series resistance ͑R s ͒ ͑ജ1 ⍀ cm 2 ͒, and lowered the FF by ϳ0.03. 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 ͑Ͻ1 ⍀ cm 2 ͒, high shunt resistance ͑60,558 ⍀ cm 2 ͒, low J o2 ͑ϳ20 nA/cm 2 ͒, and high FF ͑0.781͒. Cell efficiencies of ϳ17.4% were achieved on untextured float zone Si with 100 ⍀/ᮀ emitter by rapid firing of screen printed contacts in a lamp-heated belt furnace.Screen printed contacts are widely used for low-cost Si solar cells. Most photovoltaic ͑PV͒ manufacturers use low-sheetresistance emitters ͑Ͻ60 ⍀/ᮀ͒ because of the difficulty and challenge in achieving good screen printed contacts using high-sheetresistance emitters. Unfortunately, low-sheet-resistance emitters contribute to loss in cell performance because of heavy doping effects and high surface recombination velocity. Therefore, an attempt is made in this paper to achieve good ohmic contacts on high-sheetresistance emitters ͑100 ⍀/ᮀ͒ through improved understanding of the contact interface structure and its influence on contact quality and solar cell performance. Optimizing the inorganic constituents of the Ag paste can help achieve good-quality thick-film ohmic contacts. 1 This is particularly important when dealing with high-sheetresistance emitters. There are two possible routes to achieve goodquality ohmic contacts on lightly doped emitters. 2 The first is through self-doping techniques 3,4 and the second is via optimization of the Ag paste composition and firing cycle. 5-7 Because the diffusivity of Ag 8 is faster than that of P, 9 self-doping techniques using self-doping Ag pastes ͑Ag/P or Ag/Sb͒ probably do not prevent shunting. In this paper, the effects of Ag particle size are investigated and exploited to help in the development of Ag paste composition for high-sheet-resistance emitters using rapid firing conditions. ExperimentalIn this study, screen printed n + -p-p + solar cells ͑4 cm 2 ͒ were fabricated on single-crystal Si using carefully controlled Ag pastes and rapid firing. p-Type, 0.6 ⍀ cm, 300-m-thick ͑100͒ float-zone ͑FZ͒ substrates as well as high-sheet-resistance emitters were used for all the experiments to amplify and analyze the impact of paste ch...

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The hydrogenation of crystalline Si by methods used to passivate defects in Si solar cells has been studied by infrared spectroscopy. For these experiments, floating-zone Si that contained Pt impurities that act as traps for H was used as a model system in which H could be directly detected. In this model system, the concentration and indiffusion depth of H were determined for different hydrogenation treatments so that their effectiveness could be compared. The postdeposition annealing of a hydrogen-rich SiN x surface layer was found to introduce H into the Si bulk with a concentration of ϳ10 15 cm −3 under the best conditions investigated here.

Theoretical calculations reveal that the quality of an aluminum-back-surface field ͑BSF͒ in a silicon solar cell can be improved by either increasing the thickness of the deposited aluminum ͑Al͒, peak alloying temperature, or both. However, this study shows that there is a critical temperature for a given screen-printed Al thickness, above which the BSF quality begins to degrade because of nonuniformity triggered by the agglomeration of Al-Si melt in combination with the bandgap narrowing resulting from the high doping effect in the agglomerated regions. It is found that this critical temperature decreases with the increase in the thickness of the deposited Al layer and, therefore, limits the quality and thickness of the Al-BSF that can be achieved before degradation sets in. This nonuniformity of Al-BSF is observed in the form of scattered Al bumps with thick and thin BSF regions. A combination of experimental results and model calculations is used to provide improved understanding and guidelines for choosing the optimal combination of Al thickness and alloying temperature.

Solar cell efficiencies of 18.2 and 17.8% were achieved on edge-defined film-fed grown and string ribbon multicrystalline silicon, respectively. Improved understanding and hydrogenation of defects in ribbon materials contributed to the significant increase in bulk lifetime from 1–5 μs to as high as 90–100 μs during cell processing. It was found that SiNx-induced defect hydrogenation in these ribbon materials takes place within one second at 740–750 °C. The bulk lifetime decreases at annealing temperatures above 750 °C or annealing times above one second due to the enhanced dissociation of the hydrogenated defects coupled with the decrease in hydrogen supply from the SiNx film deposited by plasma enhanced chemical vapor deposition.

High‐efficiency 4 cm2 screen‐printed (SP) textured cells were fabricated on 100 Ω/sq emitters using a rapid single‐step belt furnace firing process. The high contact quality resulted in a low series resistance of 0·79 Ωcm2, high shunt resistance of 48 836 Ωcm2, a low junction leakage current of 18·5 nA/cm2 (n2 = 2) yielding a high fill factor (FF) of 0·784 on 100 Ω/sq emitter. A low resistivity (0·6 Ωcm) FZ Si was used for the base to enhance the contribution of the high sheet‐resistance emitter without appreciably sacrificing the bulk lifetime. This resulted in a 19% efficient (confirmed at NREL) SP 4 cm2 cell on textured FZ silicon with SP contacts and single‐layer antireflection coating. This is apparently higher in performance than any other previously reported cell using standard screen‐printing approaches (i.e., single‐step firing and grid metallization). Detailed cell characterization and device modeling were performed to extract all the important device parameters of this 19% SP Si cell and provide guidelines for achieving 20% SP Si cells. Copyright © 2005 John Wiley & Sons, Ltd.

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 screenprinting 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.