UV-Ozone Oxide for Surface Clean, Passivation, and Tunneling Contact Applications of Silicon Solar Cells
Munan Gao, Vibhor Kumar, Winston V. Schoenfeld, Ngwe Zin
Abstract
We demonstrate the versatile use of UV-ozone oxide (UVo) in surface cleaning, surface passivation, diffused junction passivation, and current tunneling applications of crystalline silicon (c-Si) solar cells. A UV-ozone generated oxide is used as a surface clean for random textured c-Si samples and the effectiveness of surface clean is determined by capping with a thin layer of aluminum oxide (AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ). Our developed UVo clean has resulted in a cleaning efficiency almost comparable to that of the benchmarked RCA clean, yielding a saturation current density of 12 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . When planar and textured c-Si samples are capped by a stack of UVo and AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , a UV-ozone growth time of no more than 3 min is found to provide an optimum surface passivation. When tested on phosphorus and boron diffused junctions (with sheet resistance, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> of 110–120 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Omega\!/\!{\scriptstyle\square} $</tex-math></inline-formula> ), the UVo and AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> stack resulted in a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> of 11 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> or lower. The high-resolution transmission electron microscope imaging revealed that UVo structure is stable upon annealing for passivation activation. Last, when applied as a tunneling contact, the UVo realizes a contact resistivity ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ρ<sub>c</sub></i> ) of ∼1 mΩ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and ∼20 mΩ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for boron and phosphorus doped metal-insulator-semiconductor contact structures, respectively, with moderately doped diffusions.