Ian B. Cooper

Understanding and Use of IR Belt Furnace for Rapid Thermal Firing of Screen-Printed Contacts to Si Solar Cells

We have simulated the rapid thermal firing process using a high-throughput conveyor belt furnace to study the physics of solar cell contact formation in mass production. We show that as sinter dwell time decreases, a lower Ag finger contact resistance is observed. Scanning electron micrographs reveal a correlation between glass thickness at the Ag/Si finger interface and Ag finger contact resistance. Secondary ion mass spectrometry shows that glass-frit and Ag emitter penetration are controlled by sinter dwell time. The observed trends in contact formation lead to lower series resistance, higher fill factors, and greater efficiencies with rapid firing.

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].

Capitalizing on the Glass-Etching Effect of Silver Plating Chemistry to Contact Si Solar Cells With Homogeneous 100–110

Homogeneous high-sheet-resistance emitter (HHSE), excellent surface passivation, and high-quality contacts, along with narrow gridlines, are needed for high-efficiency solar cells. However, HHSE in conjunction with screen-printed (SP) contacts often gives low fill factor (FF) because of high contact resistance. We capitalized on the glass-etching property of light-induced plating of silver to decrease the contact resistance and formed high-quality contacts to 100-110 Ω/sq HHSE. This led to the achievement of 78.5% FF, 38.3 mA/cm2 short-circuit current density (JSC) due to narrow line widths (65 μm), and efficiency of 18.7%.

\Omega/\hbox{sq}

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.

Emitters

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.

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

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%.

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

In this letter, we report on the ink-jet full gridline cells with three different Ag dosages (70, 90, and 130 mg) on 75-Ω/sq emitters. We observed that the open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF ) were highest for the 130-mg dose and decrease thereafter as the Ag dosage decreases. Because of the decreased FF with decreasing Ag dosage, the efficiency followed the same trend. However, we demonstrated, for the first time, fully ink-jetted front Ag gridlines with an FF of 0.765 and efficiency of 18.4% on 239-cm2 commercial CZ wafers with a sheet resistance of 75 Ω/sq . Noteworthy is the achievement of 17.8% efficiency with only 70 mg of Ag, which is ~ 68% less Ag than that used on cells with screen-printed gridlines. This represents a Ag cost saving of ~

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

0.245 per 6-inch wafer compared to the screen-printed counterpart with 220 mg of Ag.

On the Ink Jetting of Full Front Ag Gridlines for Cost-Effective Metallization of Si Solar Cells

Photosystem II (PSII) catalyzes the oxidation of water to O2 at the manganese-containing, oxygen-evolving complex (OEC). Photoexcitation of PSII results in the oxidation of the OEC; four sequential oxidation reactions are required for the generation and release of molecular oxygen. Therefore, with flash illumination, the OEC cycles among five Sn states. Chloride depletion inhibits O2 evolution. However, the binding site of chloride in the OEC is not known, and the role of chloride in oxygen evolution has not as yet been elucidated. We have employed reaction-induced FT-IR spectroscopy and selective flash excitation, which cycles PSII samples through the S state transitions. On the time scale employed, these FT-IR difference spectra reflect long-lived structural changes in the OEC. Bromide substitution supports oxygen evolution and was used to identify vibrational bands arising from structural changes at the chloride-binding site. Contributions to the vibrational spectrum from bromide-sensitive bands were observed on each flash. Sulfate treatment led to an elimination of oxygen evolution activity and of the FT-IR spectra assigned to the S3 to S0 (third flash) and S0 to S1 transitions (fourth flash). However, sulfate treatment changed, but did not eliminate, the FT-IR spectra obtained with the first and second flashes. Solvent isotope exchange in chloride-exchanged samples suggests flash-dependent structural changes, which alter protein dynamics during the S state cycle.

Perturbations at the chloride site during the photosynthetic oxygen-evolving cycle

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.