(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.
(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.
(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.