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University Center of Excellence for Photovoltaics

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Author: Hilali, Mohamed M. × End date: 2006 × Start date: 2003 ×

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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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    Implementation of a Homogenous High-Sheet-Resistance Emitter in Multicrystalline Silicon Solar Cells
    (Georgia Institute of Technology, 2005-01) Yelundur, Vijay ; Nakayashiki, Kenta ; Hilali, Mohamed M. ; Rohatgi, Ajeet
    Solar cell efficiency enhancement resulting from the implementation of a high-sheet-resistance emitter (95 Ω/sq.) in multicrystalline silicon solar cells with screenprinted contacts is demonstrated in this paper. Solar cells on low-cost String Ribbon Si from Evergreen Solar, Baysix mc-Si from Deutsche Solar, and high-quality float zone silicon with 45 Ω/sq. and 95 Ω/sq. phosphorus-doped n+- emitters are fabricated with RTP-fired screen-printed contacts and characterized to asses the impact of a highemitter-sheet resistance emitter on cell performance. Screen-printed mc-Si solar cells show an improvement in Voc of 4-5 mV in most cases that is attributed to the use of the high-sheet-resistance emitter. An appreciable increase in Jsc by as much as 1.0 mA/cm(2) is also observed due to enhanced blue response identified by internal quantum efficiency measurement.
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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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    Understanding and Development of Ag Pastes for Silicon Solar Cells with High Sheet-Resistance Emitters
    (Georgia Institute of Technology, 2004-06) Hilali, Mohamed M. ; Rohatgi, Ajeet ; Khadilkar, Chandra ; Kim, Steve ; Pham, Tung ; Salami, Jalal ; Shaikh, Aziz ; Sridharan, Srinivasan
    The effect of different properties of the inorganic constituents of the Ag paste used for the front metal grid is investigated for high sheet-resistance emitters. Our results show that both glass frit chemistry and Ag particle size are important for achieving good quality ohmic contacts to high sheet-resistance emitters. The melting characteristics of the glass frit determine the firing scheme suitable for low contact resistance and high fill factors (FFs). In addition, small to regular Ag particles were found to help achieve a higher open-circuit voltage (V(oc)) and maintaining a low contact resistance. P self-doping from the paste was not necessary for good contacts to high sheet-resistance emitters for the rapid firing schemes in this paper. High FFs (>0.78) were achieved on untextured FZ Si cells using rapid firing in a lamp-heated belt-furnace with efficiencies of up to 17.4% on a 100 Ω/sq emitter. This corresponds to an efficiency improvement of ~0.3% absolute over the 45 Ω/sq emitter cells.
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    Record-High-Efficiency Solar Cells on Multicrystalline Materials Through Understanding and Implementation of RTP-Enhanced SiNx-induced Defect Hydrogenation
    (Georgia Institute of Technology, 2004-01) Rohatgi, Ajeet ; Kim, Dong Seop ; Yelundur, Vijay ; Nakayashiki, Kenta ; Upadhyaya, A. D. ; Hilali, Mohamed M. ; Meemongkolkiat, Vichai
    This paper presents results on five record-high-efficiency 4 cm(2) solar cells on three different multicrystalline silicon materials through effective hydrogen passivation of bulk defects during cell processing. Silicon ribbon solar cell efficiencies of 18.2% and 17.9% were achieved on EFG and String Ribbon Si cells fabricated with photolithography front contacts, screen-printed Al-doped back surface field, and double layer anti-reflection coating. In addition, high-efficiency, screen-printed, manufacturable cells were achieved on HEM (16.9%), EFG (16.1%), and String Ribbon (15.9%) Si. It is found that proper implementation of a fast co-firing of front and back screen-printed contacts in a belt furnace can significantly enhance the bulk lifetime to ~100 μs and simultaneously produce high quality contacts with fill factors approaching 0.78. The firing process involves fast ramp-up and cooling rates to enhance PECVD SiN(x)-induced hydrogen passivation of defects and the quality of Al back surface field.
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    Investigation of RTP and Belt Fired Screen Printed AL-BSF on Textured and Planar Back Surfaces of Silicon Solar Cells
    (Georgia Institute of Technology, 2003-05) Meemongkolkiat, Vichai ; Hilali, Mohamed M. ; Rohatgi, Ajeet
    Quality of screen printed (SP) Al-BSF on textured and planar back surfaces was assessed by fabricating and analyzing Si solar cells. BSF was formed by firing SP-Al in a conventional belt furnace as well as in an RTP system. I-V and IQE measurements revealed that the BSF formed on a textured surface by belt firing had the poorest quality. Belt BSF on the planar back resulted in 15 mV higher V(oc). In contrast to the belt BSF, RTP BSF was superior and showed very small difference (≤ 5 mV) between the flat and textured back cells. The positive effect of RTP is attributed to much higher temperature ramp-up rate which produces uniform Al melting and reduces the influence of surface condition. In contrast, slow ramp-up in belt produces non-uniform BSF even on a planar surface. This effect gets worse on textured surfaces.