Defects in Epitaxial lift-off Thin Si Films/Wafers and Their Influence on the Solar Cell Performance
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Defects in Epitaxial lift-off Thin Si Films/Wafers and Their Influence on the Solar Cell Performance Bhushan Sopori1, Srinivas Devayajanam1, 2, Prakash Basnyat1, 2, Helio Moutinho1, Robert Reedy1, Kaitlyn VanSant1, T.S.Ravi3, Ruiying Hao3, Jean Vatus3 and Somnath Nag3 1 National Renewable Energy Laboratory, Golden, CO 2 New Jersey Institute of Technology, Newark, NJ 3 Crystal Solar Inc., San Jose, CA ABSTRACT In this paper, we will describe the nature of defects and impurities in thick epitaxial-Si layers and their influence on the cell efficiency. These wafers have very low average dislocation density. Stacking faults (SFs) are the main defect in epi layers. They can occur in many configurations—be isolated, intersecting, and nested. When nested, they can be accompanied by formation of coherent twins resulting in dendritic growth, with pyramids protruding out of the wafer surface. Such pyramids create large local stresses and punch out dislocations. The main mechanism of dislocation formation is through pyramids. Stacking faults degrade solar cell performance. Analyses of the solar cells have revealed that the nested SFs have a controlling effect on the solar cell performance. A well-controlled growth can minimize defect generation and produce wafers that can yield cell efficiencies close to 20%. INTRODUCTION To date, the Si technology has continued to dominate the solar cell/module market over other thin film photovoltaic (PV) technologies. Although, the current price of PV energy is quite low, in part because of the excess production, further cost reductions are needed to reach the SunShot goals. Because the highest-cost item in a Si solar module is the Si wafer, lowering the cost of the wafer itself can be very effective in lowering the overall cost of the module. The current high cost of the Si wafer is due to the fact that there are many process steps involved in making a standard wafer. These include growing an ingot and chopping its seed and tail ends, sawing the ingot into wafers and removing the surface damage due to sawing (kerf loss of about half of the total Si material). One approach to reduce ‘per wafer cost’ is to produce a wafer directly from the gas phase. Epitaxial lift off also called a DGTW (Direct Gas to Wafer) system is a technology aimed at formation of a shaped wafer directly from Si-bearing gases as an epitaxial growth. A brief description of Epi-liftoff technology In this method, a crystalline Si film is grown on a mono-crystalline reusable temporarysubstrate that has a porous surface of suitably tailored characteristics. The epitaxially deposited film (of appropriate thickness) is separated to become a free-standing wafer for solar cell fabrication. Alternately, cell fabrication is completed while the Si film is attached to the temporary substrate, followed by separation of the cell from the substrate. The DGTW epitaxy uses trichlorosilane as a Si bearing gas. Typically the growth is carried out around 1000 °C. Other
details of the growth are available in reference [1]. Typically, the q
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