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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.
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.
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.
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.
Effective Interfaces in Silicon Heterojunction Solar Cells

(Georgia Institute of Technology, 2005-01) Wang, T. H. ; Page, M. R. ; Yan, Y. ; Branz, H. M. ; Rohatgi, Ajeet ; Wang, Q. ; Yelundur, Vijay ; Levi, D. H. ; Iwaniczko, E.

Thin hydrogenated amorphous silicon (a-Si:H) layers deposited by hot-wire chemical vapor deposition (HWCVD) are investigated for use in silicon heterojunction (SHJ) solar cells on p-type crystalline silicon wafers. A requirement for excellent emitter quality is minimization of interface recombination. Best results necessitate immediate a-Si:H deposition and an abrupt and flat interface to the c-Si substrate. We obtain a record planar HJ efficiency of 16.9% with a high Voc of 652 mV on p-type float-zone (FZ) silicon substrates with HWCVD a-Si:H(n) emitters and screen-printed Al-BSF contacts. H pretreatment by HWCVD is beneficial when limited to a very short period prior to emitter deposition.
Beneficial Impact of Low Frequency PECVD SiN(x):H-Induced Hydrogenation in High-Efficiency String Ribbon Silicon Solar Cells

(Georgia Institute of Technology, 2004-06) Yelundur, Vijay ; Rohatgi, Ajeet ; Hanoka, J. I. ; Reedy, R.

PECVD SiN(x):H-induced hydrogenation of bulk defects in String Ribbon Si during RTP anneal is investigated in this study to enhance the carrier lifetime and understand the role of the plasma excitation frequency and an in-situ NH3 plasma pretreatment before SiN(x):H deposition. The results show that a low frequency SiN(x):H film with a NH3 plasma pretreatment annealed in RTP at 740°C for 60 seconds enhances the lifetime in String Ribbon Si from 5-6 μs to 90-100 μs. Secondary ion mass spectroscopy underneath SiN(x):H films deposited with deuterated ammonia (ND3) and silane shows greater deuterium incorporation in Si under the low frequency SiN(x):H film. Thus, hydrogen incorporated in Si during SiN(x):H deposition may act as an additional source that enhances hydrogen defect passivation during subsequent RTP treatments. In addition, the effect of the anneal time during RTA for hydrogenation is studied in an effort to reduce the hydrogenation time and improve the retention of hydrogen at defects in Si. The RTA time for hydrogenation is reduced to one second without loss of lifetime enhancement and leads to the fabrication of high-efficiency String Ribbon solar cells (17.9%) with photolithography-defined contacts. A rapid belt furnace contact co-firing scheme is developed based on the short RTA and produces screen-printed 4-cm2 String Ribbon solar cells with efficiencies as high as 15.9%.
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.
Cost and Technology Roadmaps for Cost-Effective Silicon Photovoltaics

(Georgia Institute of Technology, 2003-12) Rohatgi, Ajeet ; Ristow, Alan ; Yelundur, Vijay

The cost of photovoltaics (PV) is expected to decrease by a factor of two to four within the next two decades, making PV an integral part of the solution to the problems of fossil fuel depletion and growing energy demand. This paper describes cost and technology roadmaps for achieving 17–18%-efficient crystalline Si solar cells at a competitive manufacturing cost of less than $1/W.
String Ribbon Silicon Solar Cells with 17.8% Efficiency

(Georgia Institute of Technology, 2003-05) Kim, Dong Seop ; Gabor, A. M. ; Yelundur, Vijay ; Upadhyaya, A. D. ; Meemongkolkiat, Vichai ; Rohatgi, Ajeet

We have fabricated 4 cm(2) cells on String Ribbon Si wafers with efficiencies of 17.8% using a combination of laboratory and industrial processes. These are the most efficient String Ribbon devices made to date, demonstrating the high quality of the processed silicon and the future potential for industrial String Ribbon cells. Cofiring PECVD (Plasma Enhanced Chemical Vapor Deposition) silicon nitride (SiN(x)) and Al was used to boost the minority carrier lifetime of bulk Si. Photolithography front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The firing temperature and time were studied with respect to the trade-off between hydrogen retention and aluminum back surface field (Al-BSF) formation. Bulk defect hydrogenation and deep Al-BSF formation took place in a very short time (~1 sec) at temperatures higher than 740 degrees C.
Light Induced Degradation in Promising Multi-Crystalline Silicon Materials for Solar Cell Fabrication

(Georgia Institute of Technology, 2003-05) Damiani, Benjamin Mark ; Nakayashiki, Kenta ; Kim, Dong Seop ; Yelundur, Vijay ; Ostapenko, Sergei ; Tarasov, Igor ; Rohatgi, Ajeet

Light induced degradation (LID) in boron doped Czochralski (Cz) silicon with high oxygen content is known to degrade solar cell efficiency. Multicrystalline Si crystals also have oxygen and use B doping, but LID effects are largely unknown. In this paper, ribbon, Cz, and cast multi-crystalline Si crystals with a resistivity of 1-3 Ωcm were investigated for LID. 15-16% efficient EFG, String Ribbon, and cast mc-Si solar cells, fabricated by manufacturable screen printed technology, show small but measurable LID (0.2% absolute efficiency loss). In less than 15% efficient devices, LID was not detectable in ribbon Si crystals. However, >16% efficient photolithography ribbon Si degraded >0.5% absolute. Analysis of the bulk lifetime using photoluminescence mapping, after cell processing, supports the presence of LID in the good regions of the ribbon materials while the defective regions remained essentially unaffected.