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University Center of Excellence for Photovoltaics

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Author: Hilali, Mohamed M. × Author: Meemongkolkiat, Vichai ×

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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.
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    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.
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    Investigation of RTP and Belt Fired Screen Printed AL-BSF on Textured and Planar Back Surfaces of Silicon Solar Cells
    (Georgia Institute of Technology, 2003-05) Meemongkolkiat, Vichai ; Hilali, Mohamed M. ; Rohatgi, Ajeet
    Quality of screen printed (SP) Al-BSF on textured and planar back surfaces was assessed by fabricating and analyzing Si solar cells. BSF was formed by firing SP-Al in a conventional belt furnace as well as in an RTP system. I-V and IQE measurements revealed that the BSF formed on a textured surface by belt firing had the poorest quality. Belt BSF on the planar back resulted in 15 mV higher V(oc). In contrast to the belt BSF, RTP BSF was superior and showed very small difference (≤ 5 mV) between the flat and textured back cells. The positive effect of RTP is attributed to much higher temperature ramp-up rate which produces uniform Al melting and reduces the influence of surface condition. In contrast, slow ramp-up in belt produces non-uniform BSF even on a planar surface. This effect gets worse on textured surfaces.