Self-Aligned Self-Doping Selective Emitter for Screen-Printed Silicon Solar Cells

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Rohatgi, Ajeet
Hilali, Mohamed M.
Meier, D. L.
Ebong, Abasifreke
Honsberg, Christiana
Carroll, A. F.
Hacke, P.
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A self-aligned selective emitter for screen-printed solar cells is described in which phosphorus dopant is incorporated into a silver paste and diffused into the silicon. This produces an ohmic contact on 70-100 Ω/􀂆 emitter due to the doping of silicon underneath the Ag grid. Alloying is performed in a belt furnace at 900 degrees C for 2 min, above the Ag-Si eutectic temperature of 835 degrees C. SIMS analysis showed a surface doping concentration of 1x10(21) cm(-3) for a fritless paste (Dupont PV167) and 2x10(19) cm(-3) for the fritted paste (PV 168). Sheet resistance due to self-doping alone was quite high 121 Ω/􀂆 and 700 Ω/􀂆 for the PV167 and PV168 pastes, respectively. Therefore, a light diffusion is required underneath the Ag to achieve good ohmic contact. Fritted paste was successfully fired through the SiN(x) AR coating producing a reasonable ohmic contact to the n+ layers doped up to 100 Ω/􀂆 sheet resistance. PC1D model calculations revealed that a selective emitter induced performance enhancement is a function of base resistivity, front and back-surface recombination velocities, and bulk lifetime. For example, if the front-surface recombination velocity (FSRV) is very high (>100,000 cm/s), then the selective emitter under-performs the conventional 40 Ω/􀂆 homogeneous emitter cell. However, if the FSRV is 10000 cm/s the selective emitter gives a 0.6% (absolute) increase in cell efficiency. Selective emitter cells fabricated with 70-80 Ω/􀂆 sheet resistance between the gridlines produced approximately 16% and 15% efficient cells on float-zone and cast multicrystalline Si materials. Series resistance of 0.75 Ω-cm(2) and a fill factor of ~0.76 were achieved. Selective emitter cells were about 0.3% (absolute) more efficient than the conventional cells with 45 Ω/􀂆 homogeneous emitter. Cell analysis revealed that a reduced FSRV could result in greater improvement.
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