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University Center of Excellence for Photovoltaics

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Understanding the Role of Forming Gas on the Screen-Printed Crystalline Silicon Solar Cell Front Grid

(Georgia Institute of Technology, 2006-09) Ebong, Abasifreke ; Kim, Dong Seop ; Yelundur, Vijay ; Upadhyaya, V. ; Rounsaville, Brian ; Upadhyaya, A. D. ; Tate, K. ; Rohatgi, Ajeet

In this paper we report on the role of forming gas anneal on the fill factor of a small area cell and efficiency loss due to scaling the cell area. Solar cells that are under-fired and those fired at the optimum peak firing cycle showed very marginal response to forming gas anneal. Forming gas anneal is most effective for over-fired cells. The high temperature for the over-fired cells is believed to enhance Ag crystallites growth and the formation of a thick glass layer between the Ag front grid and silicon material. The forming gas anneal aids in reducing the glass to its metal, increase the conductivity of the glass and decrease the contact resistance. Solar cells with four different areas (4-cm(2), 49-cm(2), 100-cm(2) and 156-cm(2)) that were fired at the optimized peak firing temperature showed excellent fill factors without the forming gas anneal treatment. The fill factor was not a strong function of the area even though individually the n-factor and series resistance varied due to edge recombination. The efficiency and short circuit current density showed a quadratic relation with the cell area. The short circuit current density showed a difference of 3.2 mA/cm2 between the 4-cm2 and 156-cm2 cells. The short circuit current density decreased with area due to shading, diffusion length and back surface recombination velocity or Leff, front surface recombination velocity, and area loss due to edge isolation. Improved understanding of these effects coupled with grid design and process optimization can bridge the gap between the small and large area cells.
Development of a Low Temperature Silver Paste for High Efficiency Screen-Printed Solar Cells

(Georgia Institute of Technology, 2006-09) Ebong, Abasifreke ; Zhang, W. ; Bokalo, P. ; Rohatgi, Ajeet

The screen-printing technology provides a low cost high-throughput approach to good contacts for silicon solar cells. However, currently screen-printed contacts are formed at the expense of slight performance and fill factor loss. The front grid contact is particularly important and requires low contact resistance, high shunt resistance, and low junction recombination for high fill factor. Often contacts are fired in the moderate to high temperature range (750-800 degrees C) to achieve low series resistance. However, high temperature firing can lead to junction shunting and recombination, which degrades fill factor. Moreover shallow or higher sheet resistance emitters (50-100 Ω/sq) are desirable for high performance, which makes devices even more vulnerable to high temperature firing. Therefore, in this study, we modify the paste composition by adding some dopants and additives to lower the peak firing temperature for good ohmic contacts. This also reduces the wafer bowing and enhances SiN-induced defect hydrogenation in multicrystalline silicon substrates. The results show that increasing the additives concentration lowered the optimum firing temperature from 780 to 720 degrees C. In addition, the ideality factor is reduced significantly at the lower firing temperature. Thus additives used in this study were able to lower the peak firing temperature and increase the fill factor without hurting the series resistance. Fill factor of 0.774 on textured CZ was obtained at ~720 degrees C peak firing temperature for paste G (SOL9807). These pastes were formulated at Heraeus. Heraeus paste formulation differs in the nature and the amount of additives in the pastes.
18% Large Area Screen-Printed Solar Cells on Textured MCZ Silicon with High Sheet Resistance Emitter

(Georgia Institute of Technology, 2006-05) Ebong, Abasifreke ; Upadhyaya, V. ; Rounsaville, Brian ; Kim, Dong Seop ; Tate, K. ; Rohatgi, Ajeet

In this paper we report on high efficiency screen-printed 49 cm(2) solar cells fabricated on randomly textured float zone (1.2 Ω-cm) and magnetic Czochralski (MCz) silicon with resistivities of 1.2 and 4.8 Ω-cm, respectively. A simple process involving POCl3 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 Ω-cm textured float zone Si, 17.9% on 1.2 Ω-cm MCz Si and 18.0% on 4.8 Ω-cm MCz Si. A combination of high sheet resistance emitter (~95 Ω-/ ) and the surface texturing resulted in a short circuit current density of 37.8 mA/cm(2) in the 4.8 Ω-cm MCz cell, 37.0 mA/cm(2) in the 1.2 Ω-cm(2) MCz cell and 36.5 mA/cm(2) in the 1.2 Ω-cm(2) 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.
2D-Modeling and Development of Interdigitated Back Contact Solar Cells on Low-Cost Substrates

(Georgia Institute of Technology, 2006-05) Kim, Dong Seop ; Meemongkolkiat, Vichai ; Ebong, Abasifreke ; Rounsaville, Brian ; Upadhyaya, V. ; Das, A. ; Rohatgi, Ajeet

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 (J(sc)) 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 μs due to higher V(oc) and J(sc), which suggests that higher performance can be realized on low-cost substrates with the IBC structure.
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.
A Comprehensive Study of the Performance of Silicon Screen-Printed Solar Cells Fabricated with Belt Furnace Emitters

(Georgia Institute of Technology, 2005-06) Ebong, Abasifreke ; Yelundur, Vijay ; Upadhyaya, V. ; Rounsaville, Brian ; Upadhyaya, A. D. ; Tate, K. ; Rohatgi, Ajeet ; Kalejs, Juris P.

ABSTRACT: In this paper we report on the screen-printed solar cells fabricated on three types of silicon materials; float zone (FZ), HEM multicrystalline and EFG ribbon with POCl3 and belt furnace diffused emitters. The belt furnace diffused emitters involved one- and two-side phosphorus spin-on to assess the contaminating effect of the IR belt. The solar cells with POCl3 emitters and co-firing of screen-printed contacts produced efficiencies of 17.3% on FZ, 16.4% on HEM and 15.5% on EFG ribbon silicon. Solar cells with two-side phosphorus emitters diffused on the belt furnace, produced efficiencies of 17.2%, 16.0%, and 15.1%, respectively, on FZ, HEM and EFG ribbon silicon. However, appreciably lower efficiencies of 15.5%, 15.5%, and 14.1% were obtained, respectively, on FZ, HEM and EFG ribbon silicon for belt-diffused emitters with only one-side phosphorus spin-on with the other side on the belt. This difference in efficiency is reflected in Voc loss for the belt-diffused emitters compared to the POCl(3) emitter cells. The IQE measurements supported that solar cells with belt-diffused emitter with two-side phosphorus spin-on and POCl(3) emitter cells had comparable Jsc. However, the cell with phosphorus spin-on on one-side gave much lower IQE because of poor bulk lifetime or the contamination due to direct contact with the belt. These results indicate that the belt emitters can account for appreciable loss in the performance of the many current commercial cells; however, this loss can be regained by applying phosphorus dopant to both side of the wafer.
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.
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.
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.
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.