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University Center of Excellence for Photovoltaics

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High Efficiency Screen-Printed Solar Cells on Textured Mono-Crystalline Silicon

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

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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Investigation of RTP and Belt Fired Screen Printed AL-BSF on Textured and Planar Back Surfaces of Silicon Solar Cells

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.

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

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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Implementation of a Homogenous High-Sheet-Resistance Emitter in Multicrystalline Silicon Solar Cells

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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Understanding and Development of Ag Pastes for Silicon Solar Cells with High Sheet-Resistance Emitters

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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Optimization of Self-Doping Ag Paste Firing to Achieve High Fill Factors on Screen- Printed Silicon Solar Cells with a 100 Ω/sq. Emitter

2002-05 , Hilali, Mohamed M. , Jeong, Ji-Weon , Rohatgi, Ajeet , Meier, D. L. , Carroll, A. F.

Self-aligned selective-emitter cells have been fabricated using a self-doping paste by co-firing the front and back contacts. Good ohmic contacts with ~0.774 fill factor were obtained on 100 Ω/sq. emitters after alloying the self-doping Ag grid by a 900°C spike firing in a belt furnace. Screen-printed selective emitter Fz Si cells gave an efficiency of 16.4%. Selective-emitter cells with effective front-surface passivation produced almost 0.4% higher absolute efficiency than the conventional 45 Ω/sq. homogeneous-emitter cell co-fired at 850°C. IQE data showed a 23% higher spectral response at 400 mm wavelength for the passivated selective-emitter cell over the conventional 40-45 Ω/sq. emitter cell. This is due to lower front-surface recombination velocity and reduced heavy doping effects. Long-wavelength response of the selective-emitter cell was also slightly superior due to the improved back-surface field. As a result, the selective-emitter cell shows a much higher J(sc) and V(oc) than a cofired conventional-emitter cell. Rapid firing of the self-doping paste was found to be more effective than the slow firing process.

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High Efficiency Screen-Printed Planar Solar Cells on Single Crystalline Silicon Materials

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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Record-High-Efficiency Solar Cells on Multicrystalline Materials Through Understanding and Implementation of RTP-Enhanced SiNx-induced Defect Hydrogenation

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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Screen-Printed Back Surface Reflector for Light Trapping in Crystalline Silicon Solar Cells

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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