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University Center of Excellence for Photovoltaics

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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.
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.
High Efficiency Mono-Crystalline Solar Cells with Simple Manufacturable Technology

(Georgia Institute of Technology, 2006-09) Upadhyaya, A. D. ; Yelundur, Vijay ; Rohatgi, Ajeet

This paper describes the analysis and optimization of phosphorus-doped n(+) emitters for Si solar cells with screen-printed contacts to improve the uniformity of contact formation. Analysis of the simulated emitters showed that J(oe) increases with the increase in phosphorus surface concentration. Cells fabricated on emitter having a higher surface concentration and shallower junction depth, were on an average 0.3% (absolute) higher in efficiency and 0.5 mA/cm (2) higher in J(sc) values. Internal quantum efficiency analysis showed that the J(sc) enhancement was due to better short wavelength response in these cells. In addition the fill factors were also slightly higher in the cells with higher surface concentration and shallower junction depth. SEM analysis showed larger (~1.5μm) and more uniformly distributed Ag crystallites on the surface of cells with emitter that had higher surface concentration. This may lead to a more tolerant contact firing process and result in a higher yield of high-efficiency cells. Furthermore, use of emitters with higher phosphorus surface concentration and shallower junction depth reduces the cell processing time appreciably leading to high throughput and cost savings in cell manufacturing. We were able to tailor the emitter profile and the firing conditions of a commercially available front silver paste to obtain good average FF’s of 77.7% in conjunction with short circuit current (J(sc)) of 34.8 mA/cm (2) and an open circuit (V(oc)) of 619 mV and efficiency of ~17% on 149 cm (2) Czochralski silicon wafers.
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.
Decade Performance of a Roof-Mounted Photovoltaic Array

(Georgia Institute of Technology, 2006-05) Begović, Miroslav ; Ghosh, Seema R. ; Rohatgi, Ajeet

The Georgia Institute of Technology’s Aquatic’s Center is equipped with a 342kW roof-mounted photovoltaic array. This array will reach its ten year anniversary in July of 2006. It is therefore an appropriate time to review its performance. This system is closely monitored and studied with the help of a data acquisition system. Data collected from this system is stored and analyzed. Meteorogical parameters such as plane of array insolation are analyzed and compared with predicted values. Additionally, system parameters such as AC energy production, system efficiency, and module temperature are also monitored. The relationship between certain parameters, such as inverter efficiency and inverter loading, are also examined. With the collected data, the reliability of the system may be analyzed using data from system downtimes. From the analysis, we conclude that the system is operating well and in line with expectations.
Investigation of the Effect of Resistivity and Thickness on the Performance of Cast Multicrystalline Silicon Solar Cells

(Georgia Institute of Technology, 2006-05) Sheoran, Manav ; Upadhyaya, A. D. ; Rounsaville, Brian ; Kim, Dong Seop ; Rohatgi, Ajeet ; Narayanan, S.

A low resistivity of 0.2-0.3 Ω.cm has been shown to be optimum for high quality single crystal silicon for solar cells. However, for lower quality cast mc-Si, this optimum resistivity increases owing to a dopant-defect interaction, which reduces the bulk lifetime at lower resistivities. In this study, solar cells fabricated on 225 μm thick cast multicrystalline silicon wafers showed very little or no enhancement in efficiency with the decrease in resistivity. However, Voc enhancement was observed for the lower resistivity cells despite significantly lower bulk lifetimes compared to higher resistivity cells. After gettering (during P diffusion) and hydrogenation (from SiNx) steps used in cell fabrication, the bulk lifetime in 225 μm thick wafers from the middle of the ingot decreased from 253 μs to 135 μs when the resistivity was lowered from 1.5 Ω.cm to 0.6 Ω.cm. This paper shows that solar cells fabricated on 175 μm thick, 1.5 Ω.cm, wafers showed no appreciable loss in the cell performance when compared to the 225 μm thick cells, consistent with PC1D modeling.
Greater than 16% Efficient Screen Printed Solar Cells on 115-170 μm Thick Cast Multicrystalline Silicon

(Georgia Institute of Technology, 2006-05) Upadhyaya, A. D. ; Sheoran, Manav ; Ristow, Alan ; Rohatgi, Ajeet ; Narayanan, S. ; Roncin, Steve

In this paper we report on the impact of mc-Si wafer thickness on efficiency. We have obtained 16.8%, 16.4%, 16.2% and 15.7% efficient screen printed 4 cm(2) solar cells on 280 μm, 170 μm, 140 μm and 115 μm thick cast mc-Si respectively. Analysis of these cells showed that the efficiency of the 115 μm thick cell is limited by a BSRV of 750 cm/s, FSRV of 120,000 cm/s and a BSR of 67%. A module manufacturing cost model for a 25 MW plant was used to demonstrate that 15.7% efficient cells on 115 μm thick wafers are more cost effective than 16.8% cells on 280 μm wafers. The module manufacturing cost reduced from $1.82/W to $1.63/W when the wafer thickness was reduced from 280 μm (efficiency 16.8%) to 115 μm (efficiency 15.7%). A roadmap is developed for 115 μm thick wafers to demonstrate how cell efficiency can be increased to greater than 18% resulting in a module cost of less than $1.40/W.
Investigation of Modified Screen-Printing Al Pastes For Local Back Surface Field Formation

(Georgia Institute of Technology, 2006-05) Meemongkolkiat, Vichai ; Nakayashiki, Kenta ; Kim, Dong Seop ; Kim, Steve ; Shaikh, Aziz ; Kuebelbeck, Armin ; Stockum, Werner ; Rohatgi, Ajeet

This paper reports on a low-cost screen-printing process to form a self-aligned local back surface field (LBSF) through dielectric rear surface passivation. The process involved formation of local openings through a dielectric (SiNx or stacked SiO(2)/SiN(x)) prior to full area Al screenprinting and a rapid firing. Conventional Al paste with glass frit degraded the SiN(x) surface passivation quality because of glass frit induced pinholes and etching of SiN(x) layer, and led to very thin LBSF regions. The same process with a fritless Al paste maintained the passivation quality of the SiN(x), but did not provide an acceptably thick and uniform LBSF. Al pastes containing appropriate additives gave better LBSF because of the formation of a thicker and more uniform Al-BSF region. However, they exhibited somewhat lower internal back surface reflectance (<90%) compared to conventional Al paste on SiN(x). More insight on these competing effects is provided by fabrication and analysis of complete solar cells.
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.