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Good quality contacts between metal and silicon emitter are crucial for high crystalline solar cell efficiencies. We investigate the impact of defects originating from electrically inactive phosphorus on contact formation within silver thick film metallized silicon solar cells. For this purpose, emitters with varying sheet resistance, depth, and dead layer were metallized with silver pastes from different generations. Macroscopic contact resistivity measurements were compared with the microscopic contact configurations studied by scanning electron microscopy. The density of direct contacts between Ag crystallites grown into Si and the Ag finger bulk is essential for low contact resistivity. The presence of glass-free regions needed for such direct contacts depends on the paste composition and on the surface texture, and does not vary with the Si emitter properties. Indeed, the decrease in contact resistivity correlates with increasing density of Ag crystallites embedded in the Si surface. Furthermore, the density of Si surface-embedded Ag crystallites scales proportional to the electrically inactive P and is independent of the sheet resistance. Using the newest silver paste, the Ag crystallite density is independent of the emitter doping, but the Ag crystallite size increases as a function of the thickness of the dead layer. Transmission electron microscopy characterization of the excess P-doped Si crystal lattice shows that significant strain and Si bond weakening may play a major role for both Ag crystallite nucleation and growth. Finally, we studied Si crystal defects by metallizing nanocracks, dislocations, and grain boundaries and found that Ag crystallite nucleation is defect-property dependent. Copyright © 2013 John Wiley & Sons, Ltd.

For screen-printed Ag-paste metallization, the growth of Ag crystallites into Si is essential for the current conduction from the Si emitter to the silver finger. There are strong indications that for low contact resistances, some of these Ag crystallites need to be in direct contact with the silver finger without a separating glass layer in between. In this paper, we concentrate on the origin of Ag crystallites grown into Si in direct contact with the silver finger. On textured surfaces, we vary the Si pyramid sizes, round the pyramid tips to varying degrees, and fabricate flat smooth surfaces. We observe that the size of the pyramids does not play an important role in the achievement of low specific contact resistivity unless the pyramid heights become smaller than the thickness of the glass layer, but rounding of the pyramid tips with standard heights increases specific contact resistivity significantly. From our microscopic investigations, we conclude that the largest influence on the topography-dependent contact resistance comes from the glass coverage governing the amount of Ag crystallites directly connected with the silver finger bulk. Furthermore, Ag crystallites in direct contact with the silver finger are also observed on c-Si without n+ emitter doping.

We present work on the impact of surface contamination before phosphorus diffusion on solar cells and life-time samples. Metal surface contamination on KOH/IPA textured Cz wafers was measured by the Sandwich-Etch ICP-MS technique. High surface contamination, especially of Cu, was found directly after texturing, which is attributed to low solubility of metals in diluted, non-oxidizing alkaline solution. Different contamination levels were reached by applying standard cleaning procedures such as HCl/HF dip sequences and the piranha etch. The emitter profiles that were tested ranged from heavy diffusions of 45 Ω/sq to shallow diffusions of 120 Ω/sq. It was found that threshold values which obviously impact life-times and solar cell performance (Voc) were much higher than expected.

We study recombination properties and the formation of base contacts, which are realized by local laser ablation of a dielectric stack and the subsequent full-area screen printing of an Al paste. Based on charge-carrier lifetime measurements using the camera-based calibration-free dynamic infrared lifetime mapping technique, we determine contact reverse saturation current densities as low as J0, cont = 9 X102 fA/cm2 on 1.5-Ωcmp-type float-zone silicon (FZ-Si) and J0, cont = 2X104 fA/cm2 on 200- Ωcmp-type FZ-Si. Scanning electron microscopy images reveal that the thickness of the highly Al-doped (Al-p+) layer considerably depends on the contact size and the contact layout (e.g., point or line contacts). Based on this finding, we show that Al-p+ layer thickness WAl-p+ significantly affects the contact recombination. As a result, we show which local contact geometry is most appropriate for the lowest contact recombination employing local Al-alloyed contacts.

Solar cell concepts with passivated, locally contacted rear side have gained much interest over the last years due to the development of advanced passivation techniques. However, there are still open questions regarding the understanding of the local Al-Si contact formation and its influence on the local back-surface-field formation for industrial screen-printed aluminum pastes. In this work we show that the geometry of the rear pattern (contact size and contact spacing) strongly influences the Al-Si interaction in the local contact openings: Si diffuses in Al through narrow dielectric barrier openings and spreads laterally to a determined limit determined by the firing temperature. We found that the contact spacing can affect the tendency for formation of Kirkendall voids below the Al-Si contacts instead of an eutectic layer. When decreasing the contact spacing and therefore the overlap of Al on each side of the local opening, the melt saturates faster and the formation of a deep (high-quality) local back-surface-field is increased. This may minimize the presence of voids. This physical observation helps on the design of the rear pattern for rear passivated solar cells and gives further understanding of the Al-Si interaction during the firing process.

We have investigated the influence of diffusion temperature during phosphorus emitter diffusion from a spray-on source on the performance of screen-printed multicrystalline silicon solar cells. Because of the dual diffusion mechanism present at high concentration in-diffusion of phosphorus in silicon, applying lower diffusion temperatures for a longer duration results in significantly enhanced penetration of the low concentration tail relative to the high concentration region. Moreover, we show that the sheet resistance of in-diffused emitters from a high concentration source depends primarily on the extension of the high concentration region, thus significantly different emitter profiles can be manufactured without altering the sheet resistance considerably. Because of the enhanced tail penetration, emitters of a specified sheet resistance diffused at reduced temperatures can result in higher fill factors of screen-printed solar cells due to diminution of Schottky type shunts. Furthermore, emitters diffused at lower temperatures for longer durations can yield a higher gettering efficiency, resulting in increased bulk recombination lifetime, and thus a higher internal quantum efficiency at long wavelengths. The deeper tail extension of low temperature emitters, however, causes increased absorption within the highly recombinative emitter, resulting in current losses due to a lower internal quantum efficiency at short wavelengths. Copyright © 2006 John Wiley & Sons, Ltd.

Existing models of the silver thick-film contact to an emitter are summarised and recent developments focusing on contact formation and current transport mechanisms are presented. As a glass layer exists at the silicon-thick-film interface the current transport mechanisms are not obvious. The main hypotheses are: current transport via spike-like direct silver–silicon interconnections or via tunnelling through the chemically modified glass layer. Recent investigations showed that silver crystallites grow into the emitter from the glass frit containing dissolved silver [G. Schubert, B. Fischer, P. Fath, in: Proceedings of photovoltaics PV in Europe Conference, Rome, 2002, pp. 343–346 [1] ]. These silver islands are covered by a thin glass layer [C. Ballif, D.M. Huljic, A. Hessler-Wyser, G. Willeke, in: Proceedings of the 29th IEEE PVSC, Glasgow, 2002, pp. 360–363 [2] ]. Further investigations are necessary to study the crystallite-growth mechanism as well as the current-transport mechanism from the crystallites to the finger.

For the current transport mechanism between the n+ Si emitter and the screen printed silver contact, there is strong experimental evidence that the largest current contribution flows through Ag crystallites directly connected with the silver finger, which are preferably concentrated in some tips of the Si pyramid surface. For this purpose, we focused on the origin of these Ag crystallites and we analyzed the contact formation on different surface topographies such as pyramid height and rounding degree variations, and flat smooth surfaces, with and without phosphorus doped emitter and not only on mono- but also on multicrystalline Si material with its dislocations, grain boundaries and impurities. Combining contact resistance measurements with SEM investigations, we discovered that smaller pyramids are more capable of creating better contact than rounded pyramids, even though in our case they cover only a few percent of the area on the Si surfaces. This can be explained with the wetting behavior of the glass, which leads to some pyramid tips being glass-free and thus in direct contact with the Ag-bulk, as long as the pyramid heights exceed the thickness of the glass layer. In the case of surfaces which are smoothly polished or with strongly rounded pyramidal surfaces, a continuous glass-layer separates the Ag crystallites from the Ag-finger preventing good contact. Moreover, without a P doped emitter it is possible not only to create Ag crystallites underneath the glass, but also Ag crystallites in direct contact with the silver finger.

The application of local aluminum (Al)-alloyed contacts to the p-type base of silicon solar cells reduces minority charge carrier recombination due to the formation of a local back surface field (LBSF). We study the recombination properties and formation of base contacts, which are realized by local laser ablation of a dielectric stack (laser contact opening - LCO) and subsequent full area screen printing of Al paste. Based on charge carrier lifetime measurements using the camera-based and calibration-free dynamic infrared lifetime mapping (ILM) technique, we determine contact recombination velocities at the contacts as low as Scont = 65 cm/s on 200 Ωcm float-zone silicon (FZ-Si) and corresponding reverse saturation current densities of J0,cont = 900 fA/cm² on 1.5 Ωcm FZ-Si. As a result we show that local contact geometries with point contact radii r > 100 µm and line contact widths a > 80 µm are appropriate for lowest contact recombination employing local Al alloyed contacts. Furthermore, complete and high quality laser ablation of the dielectric stack is necessary for the formation of a sufficiently thick LBSF. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011. local back surface field; carrier lifetime; laser ablation; silicon solar cells

Boron and oxygen contamination in Czochralski-grown (Cz) silicon leads to a degradation of the minority charge carrier lifetime within short times due to the formation of recombination active complexes. The formation of these complexes is investigated for longer times showing a further development of the defect. This development called ‘regeneration’ is triggered by illumination or applied forward voltages and leads to a new state of the defect. This new state of the defect is proven to be less recombination active allowing higher stable minority carrier lifetimes and conversion efficiencies of solar cells. The influences of temperature and light intensity are discussed. Copyright © 2007 John Wiley & Sons, Ltd.

A new reaction model concerning the boron-oxygen related degradation is presented, introducing a third recombination inactive state, that stabilizes the electrical parameters of Cz-Si solar cells, and the transition to this new inactive state is proven by experimental data. Furthermore, the stability under solar cell working conditions and the formation kinetics of this additional state are discussed.

Great advances have been achieved in the development of silver pastes. The use of smaller silver particles, higher silver content, and, thus, less glass frit allow modern silver pastes to contact high resistive emitters without the necessity of a selective emitter or subsequent plating. To identify the microscopic key reasons behind the improvement of silver paste, it is essential to understand the current transport mechanism from the silicon emitter into the bulk of the silver finger. Two current transport theories predominate: i) The current flows through the Ag crystallites grown into the Si emitter, which are separated by a thin glass layer or possibly in direct contact with the silver finger. ii) The current is transported by means of multistep tunneling into the silver finger across nano-Ag colloids in the glass layer, which are formed at optimal firing conditions; the formation of Ag crystallites into the Si surface is synonymous with over-firing. In this study, we contact Si solar cell emitters wi...

We study the influence of the gravity field orientation on the microstructure of Al–Si forming alloy. Due to the difference between the diffusivity of aluminum and silicon, Kirkendall void formations are normally found at the back of rear-passivated solar cells instead of an eutectic layer. We show that the voids may partially be avoided by sintering the samples with the solid/liquid interface oriented opposite to the direction of the gravity field. A local back-surface-field is found underneath the voids using this approach. This phenomenon strongly applies to rear-passivated solar cells, which exhibited strong fill factor losses.

We show that the lateral spread of silicon in a screen-printed aluminum layer increases by (1.50±0.06) μm/°C, when increasing the peak firing temperature within an industrially applicable range. In this way, the maximum spread limit of diffused silicon in aluminum is predictable and does not depend on the contact area size but on the firing temperature. Therefore, the geometry of the rear side pattern can influence not only series resistance losses within the solar cell but the process of contact formation itself. In addition, too fast cooling lead to Kirkendall void formations instead of an eutectic layer.

For high efficiency silicon solar cells, the rear surface passivation by a dielectric layer has significant advantages compared to the standard fully covered Al back-contact structure. In this work the rear contact formation of the passivated emitter and rear cell device structure is analyzed. Contrary to expected views, we found that the contact resistivity of fine screen printed Al fingers alloyed on narrow p-type Si areas depends on the geometry of the Al–Si alloy formation below the contacts, and decreases by reducing the contact area, while the contact resistance remains constant. At the solar cell level, the reduction in the contact resistivity leads to a minimization of the fill factor losses. At the same time, narrow Al–Si alloy formations increased the passivated area below the contacts, improving the optical properties of the rear side, reducing the short-circuit current and open-circuit voltage losses. Scanning electron microscopy analysis of the Al–Si alloy geometry is performed, in order to u...

Efficiencies of large area n-type silicon solar cells with a screen printed rear side aluminum-alloyed emitter are mainly limited by their front surface recombination velocity. The front surface therefore has to be passivated by an effective passivation layer combined with a front surface field (FSF). In this work we investigate the influence of the front surface passivation quality and the base resistivity for a selective FSF n-type solar cell. The potential of this solar cell concept is assessed by PC1D simulations and QSSPC measurements. Furthermore we present solar cell results of all screen printed large area n-type Cz-Si solar cells with an aluminum rear emitter and a selective etch-back FSF passivated by a PECVD-SiNx or a SiO2/SiNx stack. The applied processing sequence is based on industrially available processing equipment and results in an independently confirmed cell efficiency of 19.4% on a 6” solar cell.

Good progress in the front silver metallization paste development has lead to significant efficiency improvements for standard crystalline silicon solar cells in the past years. In order to gain more insight into the contact mechanism, recent silver pastes capable of contacting higher resistive emitters – without selective emitter and without subsequent plating – were studied in this work. Within solar cell silver thick film contact investigations, two different theories predominate to explain the current transport from the silicon into the silver finger's bulk. The first one establishes that the current mainly flows through the Ag-crystallites grown into the silicon emitter that are either directly connected to the silver bulk or separated from the silver finger by a thin glass layer. The second one suggests that at optimum firing conditions nano-Ag-colloids are formed inside the glass layer allowing the current to flow into the silver finger through multi-step tunnelling. By sequential selective silver and glass etch-back and reconnection via liquid conductive silver, different combinations of conductivity path systems were investigated. On pyramidal textured Si, contacts from recent silver pastes reveal Ag-crystallites at the pyramid tops which are likely directly connected to the body of the Ag-finger. Even on flat Si we observe these likely direct contacts with advanced silver pastes. Our liquid conductive silver experiments indicate that the largest current contribution is flowing through these Ag-crystallites.