(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.
(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.
(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.