Vijaykumar Upadhyaya

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.

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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Two-dimensional numerical simulations were performed to derive design rules for low-cost, high-efficiency interdigitated back contact (IBC) solar cells on a low-cost substrate. The IBC solar cells were designed to be fabricated using either the conventional screen printing or photolithography metallization processes. Bulk lifetime, bulk resistivity, contact spacing (pitch), contact opening width, recombination in the gap between the p+ BSF and n+ emitter, and the ratio of emitter width to pitch have been used as key variables in the simulations. It is found that short circuit current density (Jsc) is not only a strong function of the bulk lifetime but also the emitter coverage of the rear surface. Fill factor (FF) decreases as the emitter coverage increases because the majority carriers need to travel a longer distance through the substrate for longer emitter width. The simulated IBC results were compared with those for conventional screen printed solar cells. It was found that the IBC solar cell outperforms the screen printed (SP) solar cell when the bulk lifetime is above 50 mus due to higher Voc and Jsc , which suggests that higher performance can be realized on low-cost substrates with the IBC structure

n-Type CZ Substrate

In this paper we report on the design, fabrication and modeling of 49 cm2, 200-mum thick, 1-5 Omega-cm, n- and p-type lang111rang and lang100rang screen-printed silicon solar cells. A simple process involving RTP front surface phosphorus diffusion, low frequency PECVD silicon nitride deposition, screen-printing of Al metal and Ag front grid followed by co-firing of front and back contacts produced cell efficiencies of 15.4% on n-type lang111rang Si, 15.1% on n-type lang100rang Si, 15.8% on p-type lang111rang Si and 16.1% on p-type lang100rang Si. Open circuit voltage was comparable for n and p type cells and was also independent of wafer orientation. High fill factor values (0.771-0.783) for all the devices ruled out appreciable shunting which has been a problem for the development of co-fired n-type lang100rang silicon solar cells with Al back junction. Model calculations were performed using PC1D to support the experimental results and provide guidelines for achieving >17% n-type silicon solar cells by rapid firing of Al back junction

2D-Modeling and Development of Interdigitated Back Contact Solar Cells on Low-Cost Substrates

Formation of a well-passivated boron emitter for mass production of low-cost and high-efficiency n-type silicon solar cells is a major challenge in the photovoltaic industry. In this letter, we report on a novel and commercially viable method, inkjet printing, to create boron emitters. Phosphorus diffusion was used on the rear to form a back-surface held in conjunction with chemically grown oxide/silicon nitride (SiNx) stack on the front and back for surface passivation. Finally, front and back screen-printed contacts were formed through the dielectric stacks to fabricate large-area (239 cm2) n-type cells. This technology resulted in 19.0%-efficient p+-n-n+ cells with a Voc of 644 mV, a Jsc of 38.6 mA/cm2, and a fill factor of 76.3%. This demonstrates for the hrst time the promise of boron-inkjet-printing technology for low-cost and high-performance n-type Si cells.

Rapid Thermal Processing of High Efficiency N-Type Silicon Solar Cells with Al Back Junction

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.

High-Efficiency n-Type Si Solar Cells With Novel Inkjet-Printed Boron Emitters

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.

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

In this paper we report on the fabrication, characterization and analysis of high efficiency planar screen-printed solar cells with high sheet resistance emitter /spl sim/ 100 /spl Omega//square. Three single crystalline materials were used in this study including; boron doped magnetically stabilized Cz (MCz), gallium-doped Cz (GaCz) and float zone (FZ). For these three materials, a wide range of resistivities was investigated including Fz -0.6-4.1 /spl Omega/-cm, MCz 1.2-5.3 /spl Omega/-cm and Ga-Cz 2.6-33 /spl Omega/-cm. Energy conversion efficiencies of 17.7% were achieved on both Fz (0.6-/spl Omega/-cm) and MCz (1.2-/spl Omega/-cm) while 16.9% was obtained on GaCz silicon material. The 17.7% efficiency achieved on these two materials is the highest energy conversion efficiency reported on a planar screen-printed silicon solar cell. These results demonstrate the importance of high sheet resistance emitter in achieving high efficiency manufacturable solar cells.

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

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.

High-efficiency screen-printed planar solar cells on single crystalline silicon materials

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