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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

(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.
Modeling the Effects of Uncertainty and Reliability on the Cost of Energy from PV Systems

(Georgia Institute of Technology, 2005-06) Ristow, Alan ; Begović, Miroslav ; Rohatgi, Ajeet

The cost of energy produced by a photovoltaic system is dependent upon the amount of energy produced by the system and the amortized cost of the system's components. Existing simulation tools either use crude estimators of system reliability or ignore the effects of system downtime on energy production altogether. Furthermore, the costs associated with system components are often not known precisely during system planning. However, it is difficult to reflect this uncertainty in energy cost calculations using conventional deterministic techniques. This work attempts to address these deficiencies by applying a stochastic model of system reliability to the prediction of energy production over a system's life. Similarly, it uses a stochastic model that encompasses the uncertainties associated with system component prices to estimate the uncertainty in the total installed system cost. Finally, using these two results, it computes the uncertainty in the cost of energy produced by the system. Preliminary testing of this approach, using failure data obtained from an actual system, produces cost estimates of $0.300–0.400/kWh, with a mean of $0.349/kWh, consistent with contemporary residential system cost analyses. The link between engineering and economics suggests that the proposed method may be useful as an optimization tool if an appropriate database can be developed from which to draw realistic input distributions.
Study of Direct PECVD SiN(x)-Induced Surface Emitter and Bulk Defect Passivation in P-Type Silicon Solar Cells

(Georgia Institute of Technology, 2005-01) Upadhyaya, A. D. ; Sheoran, Manav ; Rohatgi, Ajeet

This paper shows that direct low-frequency (LF) deposition of SiN films at 425 °C by PECVD followed by a conventional screen-printed contact firing cycle is more effective than a high-frequency (HF) SiN film deposited at 300 °C in passivating both bulk defects and the emitter surface. The emitter saturation current density (Joe), was found to be higher for LF SiN compared to the HF SiN just after deposition. Joe values for LF SiN reduced dramatically after contact firing to 100-200 fA/cm(2), well below the Joe for HF SiN passivated emitters. Solar cells fabricated on float zone (FZ) Si and mc-Si grown by the Heat Exchanger Method (HEM) yielded efficiencies as high as 17.2% and 16.8%, respectively, when coated with LF SiN. The enhanced cell performance is corroborated by a higher short wavelength IQE response in FZ and HEM cells and a higher post hydrogenation lifetime in HEM mc-Si cells coated with LF SiN.
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.
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.
The Effect of the Variation in Resistivity and Lifetime on the Solar Cells Performance along the Commercially Grown Ga- and B-Doped Czochralski Ingots

(Georgia Institute of Technology, 2005-01) Meemongkolkiat, Vichai ; Nakayashiki, Kenta ; Rohatgi, Ajeet ; Crabtree, Geoffrey ; Nickerson, Jeff ; Jester, Theresa L.

A systematic study of the variation in resistivity and lifetime on cell performance, before and after light-induced degradation (LID), was performed along the B- and Ga-doped Czochralski (Cz) ingots. Screen-printed solar cells with Al-back surface field were fabricated and analyzed from different locations on the ingots. Despite the large variation in resistivity (0.57 Ω-cm to 2.5 Ω-cm) and lifetime (100-1000 μs) in the Ga-doped Cz ingot, the efficiency variation was found to be ≤ 0.5%. No LID was observed in the cells fabricated from the Ga-doped ingot. In contrast with the Ga-doped ingot, the B-doped ingot showed a very tight resistivity range (0.87 Ω-cm to 1.22 Ω-cm), resulting in very tight lifetime and efficiency distributions. However, the LID effect reduced the efficiency of these B-doped cells by about 1.1% absolute. Additionally, the use of thinner substrate and higher resistivity B-doped Cz is shown to effectively reduce the LID effect.
Production Viability of Gallium Doped Mono-Crystalline Solar Cells

(Georgia Institute of Technology, 2005-01) Crabtree, Geoffrey ; Jester, Theresa L. ; Fredric, Christian ; Nickerson, Jeff ; Meemongkolkiat, Vichai ; Rohatgi, Ajeet

Results of efforts at Shell Solar to implement the use of gallium dopant as a commercial solar cell production process are presented. Both small area cell results and production related activities and results are discussed. Many researchers have demonstrated that gallium effectively eliminates light induced degradation (LID) of the bulk lifetime, but less effort has been dedicated to implement gallium dopant into a commercial production process. Shell Solar has worked in this direction and expanded past research activities to demonstrate that the full range of resistivity values produced from a gallium-doped crystal can be used to successfully fabricate high efficiency cells. In addition, Shell has produced significant numbers of gallium-doped cells in their production facility and characterized process results from crystal growth to module build. This paper discusses additional subjects essential to production viability, such as gallium metal availability, silicon feedstock availability and management specific to a gallium process and overall cost effectiveness.
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.