Keith Tate

High-Efficiency Large-Area Rear Passivated Silicon Solar Cells With Local Al-BSF and Screen-Printed Contacts

This paper describes the cell design and technology on large-area (239 cm2) commercial grade Czochralski Si wafers using industrially feasible oxide/nitride rear passivation and screen-printed local back contacts. A combination of optimized front and back dielectrics, rear surface finish, oxide thickness, fixed oxide charge, and interface quality provided effective surface passivation without parasitic shunting. Increasing the rear oxide thickness from 40 to 90 Å in conjunction with reducing the surface roughness from 1.3 to 0.2 μm increased the Voc from 640 mV to 656 mV. Compared with 18.6% full aluminum back surface field (Al-BSF) reference cell, local back-surface field (LBSF) improved the back surface reflectance (BSR) from 65% to 93% and lowered the back surface recombination velocity (BSRV) from 310 to 130 cm/s. Two-dimensional computer simulations were performed to optimize the size, shape, and spacing of LBSF regions to obtain good fill factor (FF). Model calculations show that 20% efficiency cells can be achieved with further optimization of local Al-BSF cell structure and improved screen-printed contacts.

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.

High efficiency silicon solar cells with ink jetted seed and plated grid on high sheet resistance emitter

Fine and tall grid lines on high sheet resistance emitter can increase the efficiency of silicon solar cells by reducing the shadow loss and improving the blue response. However, current screen printing pastes are neither able to provide the adequate line geometry nor make good ohmic contact to high sheet resistance emitter. This paper uses a combination of inkjet printing and light induced plating of silver to solve both problems. The ink jetting of grid lines is used to write ∼38 µm wide and ∼4 µm tall seed lines. These grid lines grow to 65 µm wide and 20–25 µm tall after light induced plating of silver. The silver plating solution penetrates the entire contact region underneath the grid lines to reduce the PbO in the glass layer to Pb metal in addition to plating the silver crystallites underneath. This lowers the contact resistance. Prolonged plating builds the line height to reduce the line resistance. A combination of low contact and line resistances produced fill factor of 79.2% on 95-Ω/sq emitter. This resulted in 239-cm2 Cz silicon solar cell efficiencies up to 18.7% with an average of 18.4%.

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

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.

Ion-Implanted Screen-Printed n-Type Solar Cell With Tunnel Oxide Passivated Back Contact

This paper shows the results and the limitations of a 21% N-Cz 239-cm² screen-printed cell with blanket p⁺ emitter and n⁺ back surface field. In addition, we show the properties and impact of tunnel oxide capped with doped n⁺ polysilicon and metal on the back side, which can overcome those limitations. Since both the doped n⁺ layer and the metal contact are outside the bulk silicon wafer, the J_o is dramatically reduced, resulting in much higher V_oc. Process optimization has resulted in high iV_oc of 728 mV on symmetric structures. The unmetallized cell structure with Al₂O₃/SiN passivated lightly doped p⁺ emitter and a tunnel oxide/n⁺ poly back also gave high iV_oc of 734 mV. The finished screen-printed 132-cm² device gave a V_oc of 683 mV, J_sc of 39.4 mA/cm², FF of 77.6%, and an efficiency of 20.9%. Cell analysis show that implementation of a selective emitter can give higher efficiency.

Large area 19.4% efficient rear passivated silicon solar cells with local Al BSF and screen-printed contacts

This paper describes the cell design and technology for achieving 19.4% efficient cells on large-area (239 cm2) commercial grade Cz Si wafers using industrially feasible oxide/SiNx rear passivation and screen-printed local back contacts. A combination of optimized front and back dielectrics, rear surface finish, oxide thickness and fixed oxide charge and interface quality provided effective surface passivation without parasitic shunting. Increasing the rear oxide thickness from 40 Å to 90 Å in conjunction with reducing the surface roughness from 5 mm to 0.2 mm increased the Voc by 16 mV to 656 mV, Jsc was 38.1 mA/cm2 and FF was 0.778 for the 19.4% cell, which is the highest efficiency 239 cm2 fully screen-printed Cz cell. Compared to 18.6% full Al-BSF reference cell, LBSF improved the BSR from 71% to 95% and lowered the BSRV from 310 to 130 cm/s. 2D computer simulations were performed to optimize the size, shape and spacing of local BSF regions to obtain good FF. Model calculations show that 20% efficiency cells can be achieved with further optimization of local Al-BSF cell structure and improved screen-printed contacts.

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

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.

18% Large Area Screen-Printed Solar Cells on Textured MCZ Silicon with High Sheet Resistance Emitter

In this paper we report on high efficiency screen-printed 49 cm 2 solar cells fabricated on randomly textured float zone (1.2 Omega-cm) and magnetic Czochralski (MCZ) silicon with resistivities of 1.2 and 4.8 Omega-cm, respectively. A simple process involving POCl 3 diffused emitters, low frequency PECVD silicon nitride deposition, Al back contact print, Ag front grid print followed by co-firing of the contacts and forming gas anneal produced efficiencies of 17.6% on 1.2 Omega-cm textured float Zone Si, 17.9% on 1.2 Omega-cm MCZ Si and 18.0% on 4.8 Omega-cm MCZ Si. A combination of high sheet resistance emitter (~95 Omega-/square) and the surface texturing resulted in a short circuit current density of 37.8 mA/cm2 in the 4.8 Omega-cm MCZ cell, 37.0 mA/cm2 in the 1.2 Omega-cm2 MCZ cell and 36.5 mA/cm2 in the 1.2 Omega-cm2 float zone cell. The open circuit voltages were consistent with the base resistivities of the two materials. The fill factors were in the range of 0.760-0.770 indicating there is considerable room for improvement. Detailed modeling and analysis is performed to explain the cell performance and provide guidelines for achieving 20% efficient screen-printed cells on MCZ Si

Analysis of cast mono-crystalline ingot characteristics with applications to solar cells

In this paper we investigate the performance of commercial cast mono-crystalline (cast-mono) ingots provided by two industrial vendors (Vendors A and B). Samples were analyzed from the top, middle, and bottom regions of the central, side, and the corner bricks. Using a conventional Al-BSF cell structure we fabricated cells on wafers from each region. Cell efficiencies up to 18.2% and 18.3% were achieved on cells fabricated from the center bricks. Cells fabricated from other regions where limited by optical and bulk properties. In addition it is found that cast-mono cells show much less light induced degradation (LID) relative to the widely used mono-crystalline Czochralski (Cz) cells.