Screen-printed Aluminum Pastes Research Articles

Quantitative theoretical and experimental analysis of alloying from screen-printed aluminum pastes on silicon surfaces

In this study, we present detailed theoretical and experimental investigations on full-area alloying from screen-printed aluminum pastes on silicon surfaces for solar cell applications. We introduce a simple analytical model for the description of the alloying process derived from existing models for evaporated Al layers, which we adapt to printed Al pastes. Thereby, we particularly account for the recrystallization of Si within the paste particles, which we refer to as parasitic Si recrystallization. Applying our model, we demonstrate good accordance of calculated with measured eutectic layer thicknesses. We show that the model can be versatilely used to investigate screen-printed Al-alloyed contacts in detail: We demonstrate that the latent heat of the Al paste significantly influences the alloying process. Thus, the effective peak temperature of the alloying process can be several 100°C below the set peak temperature of the firing furnace. By combining calculations of the effective peak temperature with measurements of the Al doping concentration, we determine a parameterization of the solid solubility of Al in Si down to the eutectic temperature of 577°C. Our investigations therefore provide improved understanding of alloying from printed Al pastes and enable the specific optimization of Al-alloyed contacts.

Investigation of Rear Localized Back Surface Field Formed from Boron Laser Doping and Screen-printed Aluminum Paste in High-Efficiency Solar Cells

A localized back surface field (LBSF) cell was developed using boron laser doping (LD) and screen-printed aluminum paste firing to produce p-type passivated emitter, rear locally diffused cells with the objective of further enhancing the efficiency of the cell. After LD, the B dopant depth was over 6.0μm and the B surface concentration was 1 × 1019 atoms/cm3. The Al paste was screen-printed and fired to form electrodes, and the LBSF composed of Al and B was observed using scanning capacitance microscopy. Secondary ion mass spectrometry was used to detect the B dopant in the LBSF area after firing the Al paste to clarify the reason for the improvement in the performance of the cell, and the ratio of B/Al was determined to be more than 0.9 wt%. The LBSF cell had an efficiency of 19.5%, which exceeded that of the reference cell by ∼1.1%, and the open circuit voltage of the cell improved remarkably. This indicated that the B content resulting from LD influenced Al diffusion and enhanced the LBSF effect.

Bilayer–metal assisted chemical etching of silicon microwire arrays for photovoltaic applications

Silicon microwires with lateral dimension from 5 μm to 20 μm and depth as long as 20 μm are prepared by bilayer metal assisted chemical etching (MaCE). A bilayer metal configuration (Metal 1 / Metal 2) was applied to assist etching of Si where metal 1 acts as direct catalyst and metal 2 provides mechanical support. Different metal types were investigated to figure out the influence of metal catalyst on morphology of etched silicon. We find that silicon microwires with vertical side wall are produced when we use Ag/Au bilayer, while cone–like and porous microwires formed when Pt/Au is applied. The different micro-/nano-structures in as-etched silicon are demonstrated to be due to the discrepancy of work function of metal catalyst relative to Si. Further, we constructed a silicon microwire arrays solar cells in a radial p–n junction configurations in a screen printed aluminum paste p–doping process.

Theoretical and experimental investigation of aluminum-boron codoping of silicon

We present a detailed study on aluminum-boron codoping of silicon by alloying from screen-printed aluminum pastes containing boron additives (Al–B pastes). We derive an analytical model for the formation of the Al–B acceptor profiles by quantitatively describing (i) the composition of the Al–B–Si melt and (ii) the incorporation of Al and B acceptor atoms into the recrystallizing Si lattice. We show that measured Al–B dopant profiles can be excellently described by this model, which therefore offers a straightforward method for the comprehensive investigation of alloying from Al–B pastes. The formation of a characteristic kink in the Al–B dopant profile curve can thus be ascribed to the exhaustion of the B additive dissolution during alloying. By intentionally adding elemental B powder to an Al paste, we demonstrate that only a low percentage of the B powder actually dissolves into the melt. We show that this incomplete dissolution of the B additive strongly affects the recombination characteristics of Al–B–p+ regions and, thus, is an important element of alloying from Al–B pastes. This study therefore provides improved understanding of aluminum-boron codoping of silicon. Copyright © 2015 John Wiley & Sons, Ltd.

Liquid-phase-deposited siloxane-based capping layers for silicon solar cells

We apply non-vacuum processing to deposit dielectric capping layers on top of ultrathin atomic-layer-deposited aluminum oxide (AlOx) films, used for the rear surface passivation of high-efficiency crystalline silicon solar cells. We examine various siloxane-based liquid-phase-deposited (LPD) materials. Our optimized AlOx/LPD stacks show an excellent thermal and chemical stability against aluminum metal paste, as demonstrated by measured surface recombination velocities below 10 cm/s on 1.3 Ωcm p-type silicon wafers after firing in a belt-line furnace with screen-printed aluminum paste on top. Implementation of the optimized LPD layers into an industrial-type screen-printing solar cell process results in energy conversion efficiencies of up to 19.8% on p-type Czochralski silicon.

Direct Metallization Local Al-Back Surface Field for High Efficiency Screen Printed Crystalline Silicon Solar Cells

In this paper, we present a detailed study on the local back contact (LBC) formation of rear-surface-passivated silicon solar cells, where both the LBC opening and metallization are realized by one-step alloying of a dot of fine pattern screen-printed aluminum paste with the silicon substrate. Based on energy dispersive spectrometer (EDS) and scanning electron microscopy (SEM) characterizations, we suggest that the aluminum distribution and the silicon concentration determine the local-back-surface-field (Al-p+) layer thickness, resistivity of the Al-p+ and hence the quality of the Al-p+ formation. The highest penetration of silicon concentration of 78.17% in aluminum resulted in the formation of a 5 microm-deep Al-p+ layer, and the minimum LBC resistivity of 0.92 x 10-6 omega cm2. The degradation of the rear-surface passivation due to high temperature of the LBC formation process can be fully recovered by forming gas annealing (FGA) at temperature and hydrogen content of 450 degrees C and 15%, respectively. The application of the optimized LBC of rear-surface-passivated by a dot of fine pattern screen(-) printed aluminum paste resulted in efficiency of up to 19.98% for the p-type czochralski (CZ) silicon wafers with 10.24 cm2 cell size at 649 mV open circuit voltage. By FGA for rear-surface passivation recovery, efficiencies up to 20.35% with a V(OC) of 662 mV, FF of 82%, and J(SC) of 37.5 mA/cm2 were demonstrated.

Alloying From Screen-Printed Aluminum Pastes Containing Boron Additives

We present a detailed study on alloying from screen-printed aluminum pastes containing boron additives (Al-B pastes) to further enhance the efficiency of p- and n-type silicon solar cells with an Al-alloyed back-surface field and rear emitter, respectively. Due to the high B solubility in Si, the additional incorporation of B atoms as acceptors into the Al-alloyed p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> region-referred to as Al-B codoping of Si-provides improved shielding of electrons from the recombination-active surface. Thus, alloying from Al-B pastes allows for significantly thinner p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> regions and leads to a considerable reduction of the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> saturation current densities. By comparing surface-passivated p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> regions alloyed from Al-B pastes or conventional Al pastes with each other, we show that a highly recombination-active defect limits the minority carrier lifetime in these p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> regions. We demonstrate that the acceptor concentration profiles of the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> regions can easily be modified by adding different amounts of aluminum diboride or boron trioxide as B sources to the Al pastes.

Investigation of Aluminum-boron Doping Profiles Formed by Coalloying from Screen-printed Pastes

We present a detailed study on aluminum-boron doping profiles formed in silicon by alloying from screen-printed aluminum pastes containing boron additives. We show that an increase in the effective peak temperature Tpeak,eff (determined from phase diagram calculations) of the alloying process leads to higher concentrations of both Al and B atoms within the alloyed p+ region, resulting in (i) higher potential barriers for electrons, but also (ii) increased densities of recombination-active Al defects. While the improved potential barrier predominates for Tpeak,eff ≲ 770°C, the increased defect density prevails for Tpeak,eff ≲ 800°C, thus defining an optimal effective peak temperature. Furthermore, we show that, by increasing the amount of elemental B added to the paste, the acceptor concentration can be increased without affecting the defect density. Therefore, the optimal printing and firing conditions comprise high B amounts and low, adapted effective peak temperatures. For a B content of 0.9 wt% and Tpeak,eff = 765°C, we have achieved a saturation current density of 253 fA/cm2, corresponding to an implied open-circuit voltage of 665mV, which demonstrates the high potential of B additives within Al pastes to further improve the efficiency of Si solar cells with Al-alloyed p+ rear.

Microstructural and electrical properties of different-sized aluminum-alloyed contacts and their layer system on silicon surfaces

The firing of screen-printed aluminum pastes is well established for the formation of a back surface field (BSF) and back contacts since many years in silicon solar cell fabrication. In this paper we investigate the electrical and microstructural properties of Al-alloyed contacts and their layer system, consisting of (i) the Al-doped p +-layer (2–14 μm), the eutectic layer (1–15 μm) and the layer of paste residuals (20–100 μm). We show the influence of process parameters like the amount of printed paste, the alloying time and the peak temperature. Special emphasis is devoted to the properties of small alloyed screen-printed aluminum structures for the formation of local aluminum back contacts. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), electrochemical capacitance voltage (ECV) and conductivity measurements have been applied to characterize the samples. A simple model is qualitatively augmented to describe all effects occurring in a technical alloying process. For example, for increasing aluminum amounts, a saturation of the p +-layer thickness was found in the range of 10 mg/cm 2. For small screen-printed structures, the p +-layer is formed very homogenously and with a greater thickness compared to samples with a full-area Al metallization, which have been processed with similar alloying conditions. For the eutectic layer a high electrical conductivity of about 16×10 6 S/m, only 2–3 times below that of pure aluminum has been determined. This is advantageous for the lateral conductivity especially for high paste amounts and small structures, which feature a relatively thick eutectic layer.

Evaluating the Aluminum-Alloyed $\hbox{p}^{+}$-Layer of Silicon Solar Cells by Emitter Saturation Current Density and Optical Microspectroscopy Measurements

Surface-passivated and surface-unpassivated aluminum-alloyed p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -layers are characterized. By varying the firing conditions and the thickness of the screen-printed aluminum paste, different sheet resistances <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> of the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -layer were fabricated. The emitter saturation current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> plotted versus <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> follows distinctly different trends for the passivated and unpassivated samples. An aluminum paste with a boron additive achieves a much higher doping concentration and a lower sheet resistance but nevertheless follows the same J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> curves as the pure Al paste. The aluminum p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -layer was quantitatively analyzed with microphotoluminescence and Fano-Raman measurements. The latter shows an increased defect recombination at the interface between the p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -layer and the moderately doped Si bulk. The lower Shockley-Read-Hall lifetime in this region can be attributed to a high defect concentration in the most highly doped layer, represents an impediment to the reduction of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> for Al-doped emitter regions, and needs to be optimized in future investigations.

Distribution of Silicon in the Aluminum Matrix for Rear Passivated Solar Cells

Abstract 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.

Screen-printed Emitter-Wrap-Through solar cell with single step side selective emitter with 18.8% efficiency

A fabrication process for Emitter-Wrap-Through solar cells on monocrystalline material with high quality gap passivation by wet thermal silicon dioxide is investigated. Masking and structuring steps are performed by screen-printing technology. Via-holes are created by an industrially applicable high-speed laser drilling process. The cell structure features a selective emitter structure fabricated in a single high temperature step: a highly doped emitter at the via-holes and the rear side, allowing for a low via-hole resistivity as well as a low resistivity contact to screen-printed pastes, and a moderately doped front side emitter exhibiting high quantum efficiency in the low wavelength range. Therefore a novel approach is applied depositing either doped or undoped PECVD silicon dioxide layers on the front side. It is shown that doping profiles advantageous for the EWT-cell structure can be achieved. The screen-printed aluminum paste is found to penetrate the underlying thermal dioxide layer at appropriate contact firing conditions leading to a zone of high recombination in the overlap region of aluminum and silicon dioxide. It is shown that conventional PECVD-anti-reflection silicon nitride acts as effective protection layer reducing the recombination in this region. Designated area conversion efficiencies up to 18.8% on FZ material are obtained applying the single step side selective emitter fabrication technique. Copyright © 2010 John Wiley & Sons, Ltd.

Optical and Electrical Characterization of Silver Microflake Colloid Films

A conductive, diffuse reflector fabricated from a silver microflake colloid is introduced. When sintered at relatively low temperatures of , the reflectance of the colloid film is 99.6% diffuse over wavelengths ranging from . The diffuse reflectance characteristic of the sintered colloid arises from its porous structure, which closely mimics the structure of the nearly ideal Lambertian reflectors that are fabricated using packed/sintered polytetrafluoroethylene particles and are widely used in optical instruments. Because diffuse rear reflectors are useful for enhancing the optical absorption of solar cells, the characteristics of silver colloid reflectors on silicon substrates are also examined. With either sintering or room-temperature drying, the colloid reflector exhibits a reflectance of with an higher diffuse component than a conventional silver paste of similar reflectance. In addition, the electrical resistivities of both the sintered and room-temperature dried silver colloid films are comparable to that of high temperature fired screen-printed aluminum and silver pastes.