Francesco Zimbardi

N-Type, Ion-Implanted Silicon Solar Cells and Modules

Ion-implanted, screen-printed, high-efficiency, stable, n-base silicon solar cells fabricated from readily available 156-mm pseudosquare Czochralski wafers are described, along with prototype modules assembled from such cells. Two approaches are presented. The first approach, which involves a single phosphorus implant, has been used to produce cells (239 cm2) having a tight distribution of Jsc, Voc, and fill factor over a wide range of wafer resistivity (factor of 10), with Fraunhofer-certified efficiencies up to 18.5%. In spite of the full screen-printed and alloyed Al back, a method has been developed to solder such cells in a module. The second approach, which involves implanting both phosphorus for back-surface field (BSF) and boron for front emitter, has been used to produce n-base cells having local back contacts and dielectric (SiNx/SiO2) surface passivation. Efficiencies up to 19.1%, certified by Fraunhofer, have been realized on 239-cm2 cells. A method is also presented to express recombination activity in the cell base as a component of total reverse saturation current density. This allows recombination activity in all three regions of the cell (n+ region and its surface, n-base, and p+ region and its surface) to be compared as components of the total cell J0 to aid in maximizing Voc.

Fully Ion-Implanted and Screen-Printed 20.2% Efficient Front Junction Silicon Cells on 239 cm

In this study, we present fully ion-implanted screen-printed high-efficiency 239 cm2 n-type silicon solar cells that are fabricated on pseudosquare Czochralski wafers. Implanted boron emitter and phosphorous back-surface field (BSF) were optimized to produce n-type front junction cells with front and back SiO2 /SiNx surface passivation and rear point contacts. Average efficiency of 19.8%, with the best efficiency of 20.2%, certified by Fraunhofer ISE, Freiburg, Germany, was achieved. In addition, the planarized rear side gave better surface passivation, in combination with optimized BSF profile, raised the average efficiency to ~20% for the fully implanted and screen-printed n-type passivated emitter, rear totally diffused cells.

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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.

n-Type CZ Substrate

In this paper we report on the evaluation of the feasibility of jetting full gridline contacts to fabricate solar cells without additional plating step. We have demonstrated, for the first time, fully ink jetted front Ag gridlines with average line width of only 56.6 μm and height of 30 μm. A high series resistance of 1.1 Ω-cm2 resulted in average fill factor of 0.767 and led to average efficiency of 18.0% on 239 cm2 commercial CZ wafers with sheet resistance of 65-Ω/sq. This result is very promising and leaves room for improvement, especially with optimized finger spacing, improved ink and co-firing process.

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

Concentrator cells have the potential to reduce the usage of semiconductor material while producing high efficiency and more power density in a cell. Silicon solar cells now provide a unique opportunity for low-cost concentrator systems that are suitable for low to medium (2-20X) concentration because cell technology and screen-printed contacts have improved considerably. In this paper, we report on the understanding and development of low-cost manufacturable screen-printed concentrator solar cells. Computer modeling was performed first to show that, under 10% metal coverage, it is possible for screen-printed cells to have series resistance that is low enough (<;0.29 Ω·cm2) to maintain high efficiency at low to medium concentrations. This was validated by design and fabrication of 40.56-cm2 screen-printed cells using an industrial feasible process that achieved 18.8% peak efficiency at ~6 suns and 17.2% efficiency at 20 suns. Dicing a 9.9-cm2 cell, which reduces the line resistance, raised the peak efficiency to 18.9% at 10 suns and 18.5% at 20 suns. Model calculations are performed to quantitatively establish the requirements for ~20% screen-printed 2-20X concentrator cells.

Implementing narrow front silver gridlines through ink jet machine for high quality contacts to silicon 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.

Development and Understanding of High-Efficiency Screen-Printed Concentrator Silicon Solar Cells

This paper demonstrates an epitaxial silicon based technology from wafer to module that can greatly reduce Kerf loss and give high efficiency. Porous Si layer was formed on a reusable Si substrate to grow and transfer thin epi layers. 17.2% cell efficiency on thin, 156cm2 large epitaxially grown Si (Epi-Si) wafers was achieved with tabs under glass/EVA. This is equivalent to ~18.0% uncapsulated cell tested in air, assuming 5% encapsulated loss. The built-in epi Back Surface Field (BSF) gave a Back Surface Recombination Velocity (BSRV) value of 500 cm/s. Bulk lifetime of 120us was achieved in ~3ohm-cm epi-wafers and no light induced degradation was observed in the cells. Further improvement of back passivation can lead to even higher efficiency.

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

Al₂O₃ film with SiNx capping layer is widely used for rear side passivation of p-type PERC cells and passivation of p+ emitter in n-PERT cells because of very effective field-induced passivation by high density of negative charge in Al₂O₃ (5e12~1e13cm^-2). This paper reports on a promising field-effect passivation by charge injection in SiO₂/SiNx stack using a novel low-cost plasma charging method which can replace plasma ALD Al₂O₃. In addition, this tool injects either positive or negative charge in a controlled manner. It is demonstrated that emitter saturation current density(Joe) of a SiO₂/SiNx passivated boron emitter decreases from ~80fA/cm² to ~50fA/cm² after -7.9e12cm^-2 negative charge injection, which is equivalent to the Al₂O₃/SiNx passivated boron emitter. In addition, a 0.4% increase in absolute efficiency was observed after the injection of 1e13cm^-2 negative charge in the SiO₂/SiNx passivated boron emitter. Sentaurus device modeling was performed to estimate the impact of field-effect passivation by extracting Joe values as a function of injected charge in SiO₂/SiNx passivated boron and phosphorus emitters. It was found that charge injection is more effective for boron emitters. And the field-effect passivation quality saturated after ~1e13cm^-2 charge in both types of emitters. We expect negative and positive charging on both sides of the cell structure will further enhance field-effect passivation and achieve even higher cell efficiency.