Low temperature silicon-based epitaxy for solar cells applications
Lai, Donny Jiancheng
Date of Issue2015
School of Electrical and Electronic Engineering
CNRS International NTU THALES Research Alliance
CNRS International NTU THALES Research Alliance
Epitaxial silicon (Si)-based solar cell technology is an attractive alternative for large-scale and high-throughput manufacturing of cost-effective solar cells through reduced Si consumption. However, due to the optical losses related to reduced Si thickness, it is critical to improve the short-circuit current density (Jsc) of the solar cell. Thus, it becomes imperative to explore a robust scheme to achieve high Jsc for the epitaxial Si solar cells to realize its full potential. The aim of this work is to design, fabricate and characterize epitaxial emitter (epi-emitter) Si solar cells that yield higher Jsc. Three schemes are investigated and compared to determine the most effective scheme to improve the Jsc of the solar cells. Firstly, low temperature Si epitaxy technique is employed to form epi-emitter Si solar cells using bulk crystalline Si substrates, with POCl3 diffused solar cells as the control cell. Next, to lower the contact resistance, the effects of back germanium (Ge) epilayer on an active epitaxial cell performance have been studied; using both highly doped and optimally doped Si substrates. Finally, the effects of architectural and peripheral modifications on the performance of epi-emitter Si solar cells are evaluated. An alternative approach has been demonstrated to grow phosphorus-doped epitaxial Si emitter by ASM 2000 at low temperature (T <700°C). A PCEpseudo of (10.2 ± 0.2)% and Jsc of 28.8 mA/cm2 has been achieved for the solar cell with epi- emitter grown at 700°C, in the absence of surface texturization, antireflective coating, and back surface field enhancement, without considering front contact shading. Secondary ion mass spectroscopy revealed that lower temperature Si epitaxy yields a more abrupt p-n junction; suggesting potential applications for radial p-n junction wire array solar cells. Mechanical twinning observed in the epi- emitter improves the optical absorbance of the cells. Based on the results, a higher PCE can be achieved by increasing the Jsc through optimization of the contact. In order to lower the contact resistance with back aluminum (Al) contact, the epi- emitter Si solar cells have been fabricated using a back Ge epilayer on highly boron (B)-doped Si substrates. The fabrication of these cells involved a two-step epitaxy process to grow the back Ge epilayer, followed by the front side epi- emitter. Control samples are fabricated under identical conditions for comparison. It is found that Jsc of the epi-emitter cell with back Ge epilayer and back B-doped Ge epilayer is ~12.4% and ~16.6% higher than that of the control cell, respectively. The performance of epi-emitter Si solar cells with back Ge epilayer grown on optimally doped Si substrates is compared to the cells with conventional BSF scheme. A maximum PCE of 10.2% and Jsc of 27.2 mA/cm2 have been achieved for the epi-emitter cell with back Ge epilayer. When compared to the control cell, a remarkable relative Jsc improvement of ~24.3% is seen. Moreover, the cells with back Ge epilayer exhibit a significant improvement in EQE response around the infrared region due to enhanced charge separation by the Si/Ge heterojunction, when compared to the cells with BSF epilayer. It is also found that the cell with back Ge epilayer and the cell with BSF epilayer have comparable PCEs. The effect of architectural and peripheral modifications of epi-emitter Si solar cells has been studied. Firstly, the synergistic approach of direct FIB etching and FGA step on the defective epi-emitter layer forms Si nanocone array with Si nanocrystals. An absolute Jsc improvement of 0.3 mA/cm2 is observed with a very small textured surface of ~0.1% and the presence of Si nanocrystals. This suggests the potential of using FIB etch and FGA step to improve the light trapping capability for epitaxial Si solar cells. In addition, such direct patterning technique is ideal for very thin cells that are incompatible with lithography. Secondly, we have demonstrated that the reduced broadband reflectance and the PL property of embedded Si nanocrystals in the Si3N4 layer of the bilayer ARC can improve the PCE of epi-emitter Si solar cells. It is found that the Si nanocrystals could downshift high-energy ultraviolet photons to lower-energy photons to enhance the overall PCE. A relative Jsc enhancement of 12.5% is observed for the epi-emitter cell with bilayer ARC and back Ge epilayer as compared to the control cell with back Ge epilayer. However, front surface recombination, due to poor passivation between the interface of epi-emitter and the Si nanocrystals, may have caused the PCE degradation for the cell with bilayer ARC and back Ge epilayer. To minimize losses due to front surface recombination and enhance the PCE of epi-emitter Si solar cells in future studies related to architectural and peripheral modifications, we recommend using remote hydrogen plasma passivation to passivate the surface of the Si nanocrystals.