Understanding the Formation and Temperature Dependence of Thick-Film Ag Contacts on High-Sheet-Resistance Si Emitters for Solar Cells

Abstract

Physical and electrical properties of screen-printed Ag thick-film contacts were studied and correlated to understand and achieve good-quality ohmic contacts to high-sheet-resistance emitters for solar cells. Analytical microscopy and surface analysis techniques were used to study the Ag-Si contact interface of three different screen-printed Ag pastes ͑A, B, and PV168͒ subjected to high ͑ϳ835°C͒ and conventional ͑740-750°C͒ temperature firing conditions. At ϳ750°C firing, all three pastes failed on a 100 ⍀/ᮀ emitter because of incomplete etching of the silicon nitride film ͑PV168͒, an irregular small distribution of regrown Ag crystallites ͑paste A͒, or an excessive diffusion of Ag into the p-n junction ͑paste B͒. At a firing temperature of ϳ835°C, paste A gave a lower open-circuit voltage because of the diffusion of Al from the glass frit into the emitter region. Paste B failed because of the formation of very large ͑0.3-1 m͒ Ag crystallites that shunted the p-n junction. Of the three pastes, the PV168 paste from DuPont gave the best contact quality on a 100 ⍀/ᮀ emitter with a solar cell fill factor of 0.782 only after annealing in a hydrogen atmosphere.Screen-printed ͑SP͒ solar cells have been around for about three decades; however, little is understood about the physics and chemistry of the formation of thick-film silver contacts to the Si emitter, as well as carrier transport at and near the Ag-Si interface and its precise correlation with the quality of the ohmic contact. The main fill factor ͑FF͒ loss mechanisms associated with SP metallization are shown in Fig. 1. As illustrated in the figure, the n + P diffusion forms the emitter region for the p-base single-junction Si solar cell. The contact resistance and gridline resistivity both depend on the firing cycle and the properties of the inorganic constituents of the SP paste. For SP solar cells, the location of high series resistance is at the Ag thick film/Si contact interface. 2 The junction leakage and shunting characteristics depend mainly on the emitter profile, firing cycle, and the paste constituents.Conventional emitters are homogeneously doped with a sheet resistance normally in the range of 35-55 ⍀/ᮀ. Reasonably goodquality SP Ag contacts are routinely achieved on ϳ45 ⍀/ᮀ emitters, while making good ohmic contacts to high-sheet-resistance ͑80-100 ⍀/ᮀ͒ emitters is still a challenge. Nevertheless, low-sheetresistance emitters contribute to loss in cell performance because of heavy-doping effects and high surface-recombination velocity. For this reason selective-emitter processes have been developed. 3-5 The advantages of a selective-emitter cell include a lower contact resistance resulting from the heavier doping underneath the metal grid and higher short-circuit current and open-circuit voltage resulting from the improved short-wavelength response and front-surface passivation of the lightly doped region between the metal gridlines. An alternative route, which is investigated in this paper, is to directly optimize the SP contacts to a homogeneous...

Add‐on laser tailored selective emitter solar cells

“…The low maximum etch depth z max ¼ 45 nm of the un-irradiated shallow furnace diffused emitter is smaller than typical etch depths z paste % 60 nm of screen printing pastes [21], explaining the measured high macroscopic contact resistivities r c > 400 mV cm 2 without add-on laser doping. The significantly deeper doping profile of the laser irradiated samples increases z max to values high above the etch depth, thus widening the process window for the firing of screen printed contacts and avoiding metal spiking through the pn-junction [22]. Figure 5 illustrates the efficiency potential of SE compared to standard cells by reducing the finger width w f .…”

Effect of glass frit chemistry on the physical and electrical properties of thick-film Ag contacts for silicon solar cells

“…However, at 750°C firing, high FF was achieved only for medium-high and high T g glass Ag pastes (33-462 and 33-455). This demonstrates that, unlike the case with the PV168 Ag paste, 10,11 a high firing temperature of 835°C is not always necessary to achieve a good ohmic contact to the 100 V/sq emitter, provided high T g glass and fast belt speed are used. However, lower T g glass frits GF1 (33-460) and GF2 (33-452) gave significantly lower FFs for the 750°C firing.…”

Role of PbO-based glass frit in Ag thick-film contact formation for crystalline Si solar cells

“…When the screen-printed Ag paste is fired, the glass frit acts both as an etching agent of the SiN x antireflection coating (ARC) layer deposited on the n + -Si emitter and as a transport medium for Ag in the bulk paste to precipitate at the paste/emitter interface [1][2][3][4][5][6][7][8][9][10]. A dense population of fine Ag crystallites at the paste/Si interface is desirable for achieving good quality ohmic contact with the Si emitters [6][7][8]10].…”