HIT Solar Cell With V20x Window Layer
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HIT Solar Cell With V2Ox Window Layer 1
Erenn Ore1, Gehan Amaratunga1, Stefaan De Wolf2 Department Of Engineering, University Of Cambridge, Cambridge CB3 0FA, United Kingdom. 2 École Polytechnique Fédérale de Lausanne, Breguet 2, Neuchatel, CH 2000, Switzerland.
ABSTRACT In the conventional crystalline silicon heterojunction solar cell with the intrinsic thin layer structure (the HIT solar cell), a p-doped thin film silicon or its alloy (pDTF-Si/A) is used as the hole collecting window layer. However, the parasitic absorbance in the pDTF-Si/A window layer, and the toxic, explosive diborane gas used for p-doping are limiting factors for achieving HIT cells with reduced processing costs and / or higher efficiencies. In this work, pDTF-Si/A is replaced by V! O! , which is deposited by a simple physical vapor deposition technique. Due to the wide band gap of V! O! , the HIT solar cell with the V! O! window layer generates a higher short-circuit current density than the reference conventional HIT cell under 1 𝑠𝑢𝑛, and achieves an open-circuit voltage of 0.7 𝑉. Furthermore, the charge carrier lifetime and pseudo-efficiency values of the HIT solar cell with the V! O! window layer indicate that this cell has the potential to outperform the conventional HIT cell in terms of the power conversation efficiency under the standard test conditions. INTRODUCTION For the conventional crystalline silicon (c-Si) heterojunction solar cell with the intrinsic thin layer structure (the HIT solar cell), the main challenge to improve cell’s performance has been the p-doped thin film silicon or its alloy (pDTF-Si/A) based hole collecting window layer (WL) [1]. Because of the high density of tail states in its forbidden zone, pDTF-Si/A absorbs a significant portion of the incoming light [2][3]. However, the charge carriers generated by the light absorption in pDTF-Si/A do not contribute to the over-all photoconductivity because of the high recombination rates in pDTF-Si/A [4]. Thus, the amount of light absorbed in pDTF-Si/A is wasted. In addition, the high recombination rate in pDTF-Si/A decreases the probability of photo-induced charge carriers generated in the absorption layer reaching the external circuit through WL [5]. pDTF-Si/A is typically processed by a chemical vapor deposition (CVD) process, which involves handling expensive and toxic diborane gas [6] used for boron doping (p-type doping) of silicon. Because of the significant health and safety related concerns, handling diborane requires strict safety regulations and the use of elaborate containment systems [7]. These requirements inevitably increase the solar cell processing costs. The purpose of this work is to replace pDTF-Si/A with a non-silicon based material that can function as a hole collecting WL for the HIT cells. The binary transition metal oxides with wide band gaps and high work functions are identified as the potential WL materials. Since nonstoichiometric molybdenum trioxide (MoO! ) is successfully demonstrated as a hole collecting window layer for silicon based so
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