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University Center of Excellence for Photovoltaics

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Now showing 1 - 7 of 7
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    High Efficiency Screen-Printed Solar Cells on Textured Mono-Crystalline Silicon
    (Georgia Institute of Technology, 2005-10) Rohatgi, Ajeet ; Ebong, Abasifreke ; Hilali, Mohamed M. ; Meemongkolkiat, Vichai ; Rounsaville, Brian ; Ristow, Alan
    In this paper we report on high efficiency screen-printed 4 cm(2) solar cells fabricated on randomly textured float zone, magnetic Czochralski (MCz) and Ga-doped Cz silicon. A simple process involving POCl(3) emitters, low frequency PECVD silicon nitride deposition, Al back contact print, Ag front grid print followed by co-firing of the contacts produced efficiencies of 19.0% on textured float zone, 18.2% on MCz and 17.7% on Ga-doped Cz. A combination of high sheet resistance emitter (~100 Ω-/sq.) and the surface texturing resulted in short circuit current density of 37.3 mA/cm(2) for 0.6 Ω-cm float zone cell, 38.2 mA/cm(2) for 4.8 Ω-cm MCz cell and 37.4 mA/cm(2) for 1.5 Ω-cm Ga-doped Cz cell. Open circuit voltages were consistent with the base resistivity of the three materials. However, FF was highest for float zone (0.784) followed by MCz (0.759) and Ga-doped Cz (0.754). Model calculations performed using PC1D showed that, once the lifetime exceeds 200 μs for this cell design, the efficiency no longer has a strong dependence on the bulk lifetime. Instead, the performance is limited by the cell design including contacts, base resistivity, doping profiles, and front and back surface recombination velocities. Detailed analysis is performed to explain the high performance of these screen-printed cells and guidelines are provided for ≥20% efficient screen-printed cells.
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    Investigation of High-Efficiency Screen-Printed Textured SI Solar Cells with High Sheet-Resistance Emitters
    (Georgia Institute of Technology, 2005-01) Hilali, Mohamed M. ; Nakayashiki, Kenta ; Ebong, Abasifreke ; Rohatgi, Ajeet
    In this study it is found that the efficiency enhancement (Δη) resulting from the use of a 100 Ω/sq emitter instead of a conventional 45 Ω/sq emitter is substantially enhanced further by surface texturing. This enhancement is greater for textured cells by at least ~0.4% absolute over the enhancement for planar cells, and is mainly due to the greater difference in the front-surface recombination velocity (FSRV) between the high and low-sheet-resistance emitter textured cells. A FSRV of 60,000 cm/s resulted in a reasonably good V(oc) of ~642 mV for the 100 Ω/sq emitter textured cell. Our investigation of the Ag-Si contact interface shows a more regular distribution of Ag crystallite precipitation for the textured emitter (mainly at the peaks of the texture pyramids). The high contact-quality resulted in a series resistance of 0.79 Ω-cm, a junction leakage current of 18.5 nA/cm(2) yielding a FF of 0.784. This resulted in a record high-efficiency 4 cm(2) screen-printed cell of 18.8% (confirmed by NREL) on textured 0.6 Ω-cm FZ, with single-layer antireflection coating.
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    High Efficiency Screen-Printed Planar Solar Cells on Single Crystalline Silicon Materials
    (Georgia Institute of Technology, 2005-01) Ebong, Abasifreke ; Hilali, Mohamed M. ; Upadhyaya, V. ; Rounsaville, Brian ; Ebong, I. ; Rohatgi, Ajeet
    In this paper we report on the fabrication, characterization and analysis of high efficiency planar screen-printed solar cells with high sheet resistance emitter ~ 100 Ω/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 Ω-cm, MCz - 1.2-5.3 Ω-cm and Ga-Cz 2.6-33 Ω-cm. Energy conversion efficiencies of 17.7% were achieved on both Fz (0.6-Ω-cm) and MCz (1.2-Ω-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.
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    Screen-Printed Back Surface Reflector for Light Trapping in Crystalline Silicon Solar Cells
    (Georgia Institute of Technology, 2001-10) Ristow, Alan ; Hilali, Mohamed M. ; Ebong, Abasifreke ; Rohatgi, Ajeet
    Evaporated metal back surface reflectors have been shown to yield high values of internal rear reflectance, and are particularly eff ective when combined with a thin dielectric layer between the silicon and the metal. However, evaporated metals are not compatible with low-cost solar cell fabrication processes and generally do not scatter light well, resulting in inefficient trapping of light. In this work, a ffordable screenprinted metal pastes have been employed to fabricate eff ective low-cost back surface reflectors. The best of these, fabricated from screen-printed silver paste on a thin silicon nitride dielectric layer, yield back surface reflectance values similar to those of evaporated metal reflectors. Furthermore, the screen-printed back surface reflectors in this study are shown to be highly diff use, thus enhancing light trapping in planar silicon solar cells. PC1D simulations suggest that a solar cell with a screen-printed metal/dielectric back surface reflector should outperform one with a high-quality aluminum back surface field.
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    Self-Aligned Self-Doping Selective Emitter for Screen-Printed Silicon Solar Cells
    (Georgia Institute of Technology, 2001-10) Rohatgi, Ajeet ; Hilali, Mohamed M. ; Meier, D. L. ; Ebong, Abasifreke ; Honsberg, Christiana ; Carroll, A. F. ; Hacke, P.
    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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    Rapid Thermal Technologies for High Efficiency Silicon Solar Cells
    (Georgia Institute of Technology, 2001-06) Ebong, Abasifreke ; Cho, Y. H. ; Hilali, Mohamed M. ; Rohatgi, Ajeet ; Ruby, D. S.
    This paper shows that rapidly formed emitters (≤ 6 min) in a conveyor belt furnace or 3 minutes in an RTP system, in conjunction with a screen-printed (SP) RTP Al-BSF and passivating oxide formed simultaneously in 2 minutes can produce high efficiency cells with no surface texturing, point contacts, or selective emitter. It is shown for the first time that an 80 Ω/ emitter and SP Al-BSF formed in a high throughput belt furnace can produce 19% FZ cells, 18.4% MCZ cells and greater than 17% CZ cells with photolithography (PL) contacts. Using PL contacts, we also achieved 19% efficient cells on FZ, >18% on MCZ, and ~17% boron-doped CZ by emitter and SP Al-BSF formation in less than 10 minutes in a single wafer RTP system. Finally, a manufacturable process with 45 Ω/ emitter and screen-printed (SP) Al-BSF and Ag contacts formed in the conveyor belt furnace gave 17% efficient cells on FZ silicon. Compared to the photolithography cells, the SP cell gave ∼2% lower efficiency along with a decrease in Jsc and fill factor (FF). This loss in performance is attributed to a combination of the poor blue response, higher series resistance and higher contact shading in the SP devices.
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    Rapid Photo-Assisted Forming Gas Anneal (FGA) for High Quality Screen-Printed Contacts for Silicon Solar Cells
    (Georgia Institute of Technology, 2000-09) Ebong, Abasifreke ; Hilali, Mohamed M. ; Rohatgi, Ajeet
    Formation of low-cost high-quality contacts is the key to cost-effective silicon solar cells. Screen-printing is widely used in Industry because it is simple, low-cost and rapid. However, cost and throughput gains are attained at the expense of performance. Fill factors of most commercial cells are in the range of 0.68-75 for single crystalline material. This paper shows that a rapid 400°C/0.5-3 min photo-assisted anneal in a forming gas ambient can raise the fill factor (FF) of screen-printed (SP) single and multicrystalline (mc) Si cells from - 0.70 to - 0.77 and 0.76, respectively. Dark I-V analysis showed that this results from a decrease in series resistance by a factor of 2 to 4. Thus initial belt firing conditions can be tailored (~ 700DC) to first prevent the junction shunting, which generally results in high series resistance (Rs), and then the rapid photo-assisted anneal in forming gas ambient can be used to reduce the resistive losses for achieving high FF without much junction shunting. The LBIC analysis on multicrystalline silicon shows that a 30-second forming gas anneal in RTP not only reduces the glass frit at the silicon/silver interface but also enhances hydrogenation of bulk defects.