A.F. Carroll

Low resistance screen-printed Ag contacts to POCl 3 emitters with low saturation current density for high efficiency Si solar cells

The silicon (Si) PV industry recognizes the value of phosphorus (P) emitters with low saturation current density (J_0e) for ability to produce high final cell open circuit voltage (V_OC). However, emitters of such quality, which usually display low surface phosphorus concentration ([P_surface]) are notoriously difficult to contact using traditional screen-printed silver (Ag) thick film pastes. Here, we tailored P emitter profiles via POCl₃ diffusion to create solar cell emitters displaying low J_0e values of 67 - 148 fA/cm² and variable electrically active [P^surface] of 0.5E20 - 2.0E20 atoms/cm³ in order to study the conditions necessary for low resistance contact to such emitters without appreciably deviating from industrial process conditions. Using a screen-printable Ag conductor paste tailored to contact low [P_surface] emitters, we show fill factor (FF) as high as 80% while maintaining V_OC as high as 637 mV on tailored emitters with low J_0e. This results in average solar cell efficiencies of 18.6% with a best efficiency of 18.8%. Series resistance (R_SERIES) analysis revealed that contact resistance was the major resistive component dictating final R_SERIES and FF. Finally, microstructural SEM analysis of the Ag/Si contact interface suggested that thin interfacial glass films and extensive Ag precipitate/crystallite surface coverage may explain how such high FF can be attained on emitters with low J_0e and low [P_surface].

Investigation of the Mechanism Resulting in low Resistance Ag Thick-Film Contact to Si Solar Cells in the Context of Emitter Doping Density and Contact Firing for Current-Generation Ag Paste

Screen-printed thick-film Ag metallization has become highly successful in crystalline Si (c-Si) photovoltaics. However, a complete understanding of the mechanism resulting in low resistance contact is still lacking. In order to shed light on this mechanism for current-generation Ag paste, Si solar cells were fabricated using a range of emitter doping densities and contact firing conditions. Low resistance contact was found to vary as a function of emitter surface P concentration ( [Psurface]) and peak firing temperature. Scanning electron microscope (SEM) analysis revealed thin interfacial glass films (IGF) under the bulk Ag gridline. SEM analysis also showed increasing Ag crystallite density as both emitter [Psurface] and peak firing temperature increased. Two mechanisms are proposed in forming low resistance contact to highly doped emitters: 1) formation of ultrathin IGF and/or nano-Ag colloids at low firing temperature, and 2) formation of Ag crystallites at high firing temperature. However, on lightly doped emitters, low resistance contact was achieved only at higher firing temperatures, concomitant with increasing Ag crystallite density, and suggests that thin IGF decorated with nano-Ag colloids may not be sufficient for low resistance contact to lightly doped emitters.

Optimization of self-doping Ag paste firing to achieve high fill factors on screen-printed silicon solar cells with a 100 /spl Omega//sq. emitter

Self-aligned selective-emitter cells have been fabricated using a self-doping paste by co-firing the front and back contacts. Good ohmic contacts with /spl sim/0.774 fill factor were obtained on 100 /spl Omega//sq. emitters after alloying the self-doping Ag grid by a 900/spl deg/C spike firing in a belt furnace. Screen-printed selective emitter Fz Si cells gave an efficiency of 16.4%. Selective-emitter cells with effective front-surface passivation produced almost 0.4% higher absolute efficiency than the conventional 45 /spl Omega//sq. homogeneous-emitter cell co-fired at 850/spl deg/C. IQE data showed a 23% higher spectral response at 400 /spl mu/m wavelength for the passivated selective-emitter cell over the conventional 40-45 /spl Omega//sq. emitter cell. This is due to lower front-surface recombination velocity and reduced heavy doping effects. Long-wavelength response of the selective-emitter cell was also slightly superior due to the improved back-surface field. As a result, the selective-emitter cell shows a much higher J/sub sc/ and V/sub oc/ than a cofired conventional-emitter cell. Rapid firing of the self-doping paste was found to be more effective than the slow firing process.

Formation of high quality screen-printed contacts to homogeneous high sheet resistance emitters (HHSE)

In this paper we report on the formation of high quality contacts to HHSE with a sheet resistance variation of ≥30 Ω/sq using a commercial front-side Ag paste, PV16A from DuPont. We fabricated and characterized solar cells with emitter sheet resistances of 65, 75, 85, 95 and 105 Ω/sq. We found that emitter sheet resistances in the range of 65–95 Ω/sq can be contacted with low average series resistance of 0.63–0.77 Ω-cm² and high fill factor (FF) of 77.4–78.8%. The 75 Ω/sq emitter gave the best average efficiency of 18.3%, followed by 18.2% for the 65 Ω/sq, 18.0% for the 85 Ω/sq, and finally 17.7% for the 95 Ω/sq. The 105 Ω/sq emitter gave a low FF due to high series resistance, but the shunt resistance and ideality factor were excellent, which suggests the paste was not encroaching the shallow emitter junction.

Current Conduction Mechanism of Front-Side Contact of N-Type Crystalline Si Solar Cells With Ag/Al Pastes

Recently, n-type crystalline Si (c-Si) cells with front-side (FS) metallization Ag/Al paste have attracted considerable attention. However, a clear understanding of current conduction mechanism is still lacking. We report here the results of our microstructural investigation of the interfacial contact region using electron microscopy techniques. In optimally fired cells, we did not find any Al-Si eutectic layer on the emitter surface that would support a regrowth mechanism as found during the back surface field formation process commonly practiced to create the full plane Al back contact of p-type industrial solar cells. The presence of SiN x antireflection coating has possibly altered significantly the chemistry between Si and Al. The observed microstructures suggest that the current conduction is predominantly tunneling through ultrathin interfacial glass, assisted by the presence of nano-Ag colloids. We believe this mechanism is similar to the current conduction model we have proposed previously for FS Ag-contact of p-type c-Si solar cells with Ag paste.

Front-side Ag contacts enabling superior recombination and fine-line performance

The standard silicon solar cell process continues on an evolutionary improvement path. High quality monocrystalline cells are now able to reach 19.2 % conversion efficiencies in industrial production. A key enabler for these high efficiencies has been the front-side Ag contact. This paper will discuss recent developments in this technology on two parallel fronts: reduced recombination and fine line printing. Front-side Ag can reduce solar cell recombination currents directly through reduced metal contact saturation current. In addition front-side Ag can indirectly lower recombination through improved contact formation to low saturation current emitters (lightly doped emitters, or LDE). Through improvements in the frit chemistry a superior recombination performance was enabled, yielding a 3 mV Voc gain and 0.1 % efficiency gain over the control. Improvements in the Ag particle dimensions and paste rheology reduced the optimum finger width approximately 10 μm, increasing Jsc by 0.3 mA/cm2 improving the efficiency gain another 0.1 % over the incumbent technology. In net we are able to demonstrate a next generation front-side Ag paste that can improve efficiency 0.2 %, from 18.8 % to 19.0 %.

Microstructural characterization of front-side Ag contact of crystalline Si solar cells with lightly doped emitter

Crystalline Si (c-Si) solar cell production has reached an annual scale of ~20 GW globally. Development of this leading technology has been boosted by continuous innovation in material science and reduced material and processing costs. An example of such innovation is the step-wise progression to more lightly doped emitters (LDE) that reduces recombination in the solar cell. Continuous improvement in front-side (FS) metallization pastes has enabled this progression to lower series resistance and higher cell efficiency. We report here homogeneous emitter LDE cells with efficiencies as high as 18.9%, printed with advanced FS Ag paste. A clear understanding of the microstructure of the interfacial region between Ag contact and Si emitter, and the associated electrical conduction mechanism of LDE cells can provide the guidance needed to drive overall efficiency higher and end-user cost lower. We report our latest investigation of the microstructure of the interface between FS Ag contact and lightly-doped emitter using scanning electron microscopy techniques. The microstructural features such as nano-Ag colloids, interfacial glass, and Ag crystallites are studied in detail. The relationship between microstructure, cell performance, and current conduction mechanism for LDE cells are discussed.

Screen printed metal contacts to Si solar cells — Formation and synergistic improvements

Silicon solar cell technology has advanced by improvements to all aspects of cell design including metallization. Improvements to metal contacts have been synergistic with other cell improvements such as passivation, lightly doped emitters, and boron doped emitters. This paper briefly recounts the history of c-Si PV metallization technology and its synergy with other cell technologies. It concludes with a perspective on various metal contact mechanisms.

Effect of Al content on the performance of Ag/Al screen printed N-type Si solar cells

In this paper, the role of Al content in Ag/Al paste used for making screen printed contacts to B emitter is investigated. Five different Ag/Al pastes with varying Al content below <;5% Al were formulated. It is found that contact resistance decreased monotonically with the increase in Al content. In addition, a slight decrease in effective sheet resistance of boron emitter was observed for higher Al content paste, possibly due to some Al incorporation underneath the grid during contact formation. These two effects resulted in an increase in fill factor for higher Al content paste. However, increased Al content also resulted in wider grid line and lower aspect ratio, resulting in a slight decrease in Jsc and increase in grid line resistance. In addition, higher Al tends to increase emitter saturation current density (J0e) and degrade junction quality. The competition between the fill factor and Jsc led to an optimum Al content, which produced basic n-type front junction cells with an average cell efficiency of 19.8% in this study.