Effects of Surface Doping of Si Absorbers on the Band Alignment and Electrical Performance of TiO 2 -Based Electron-Sele
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.164
Effects of Surface Doping of Si Absorbers on the Band Alignment and Electrical Performance of TiO2-Based Electron-Selective Contacts Hyunju Lee1, Takefumi Kamioka1, Noritaka Usami2, and Yoshio Ohshita1 1
Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan
2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
ABSTRACT
We have investigated the chemical and electrical properties of a thin SiO2/TiO2 stacking layer deposited on n-Si and heavily phosphorus-doped n++ Si substrates to elucidate effects of phosphorus doping of Si absorbers on the band alignment and electrical performance of a SiO2/TiO2 stack-based electron-selective contact deposited on the differently doped Si substrates. From our XPS study, we show a shift of the TiO2 energy levels up to ~0.13 eV with respect to those of Si as the doping level of Si substrates changes. We also show that the conduction band offset of the SiO2/TiO2 stacking layer at the interface with the n++ Si substrate seems to smaller than that of the SiO2/TiO2 stacking layer at the interface with n-Si substrate. Finally, from our electrical transport measurements, we could conclude that the thinner tunneling barrier, the increased electron density in front of the SiO2 layer in the n++ Si surface, and/or the reduced barrier height by heavy doping, seem to enhance the majority electron transport property of the SiO2/TiO2/n++ Si samples compared to that of the SiO2/TiO2/n-Si samples.
INTRODUCTION Recently, a new alternative to the conventional silicon heterojunction (SHJ) solar cell concept has been suggested and intensively studied. In this new concept, a set of dopant-free carrier-selective contacts (CSCs) is employed to separate and collect photogenerated free carriers in a cell structure [1,2]. One of the most promising materials for dopant-free CSCs is transition metal oxides (TMOs). TMOs were first used in organic solar cells [3]. So far, as dopant-free n- and p-type CSC materials, amorphous
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titanium dioxide (TiO2) [4-9] and sub-stoichiometric molybdenum oxide (MoOx) [10-12] are the most promising material in TMO-based CSC Si solar cells, respectively. Meanwhile, TMO-based CSCs have demonstrated significant progress in Si solar cell performance with an impressive conversion efficiency over 22 % [4,8,11]. However, the performance of TMO-based CSC Si solar cells is still inferior to that of thin tunnel SiOx/phosphorus-doped hydrogenated poly-silicon (n+ poly-Si:H) stack-based CSC Si solar cells (also referred as TOPCon structured solar cells) owing to the higher level of surface recombination and contact resistance of TMO-based CSCs [4,10]. One of the possible reasons for the higher performance of the TOPCon seems to
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