Engineering topics

Hua, Amandee

Publications and source records attributed to Hua, Amandee.

6 records

DOE OSTI2025

Passivation and Degradation of Sulfur-Treated Silicon Surfaces for Photovoltaics

Sulfur-based passivation for silicon surfaces using H2S gas is an alternative passivation method to reduce the thermal budget for Si photovoltaics. To understand the impact of the high-quality passivation and an observed passivation efficiency decrease after air exposure, we have studied the chemical surface structure by X-ray photoelectron spectroscopy (XPS), X-ray Auger electron spectroscopy (XAES), and S and Si L2,3 X-ray emission spectroscopy (XES). On the S-passivated silicon surfaces, we find the formation of S–Si bonds, in addition to some Si–O bonds. Upon air exposure, sulfur partially desorbs from the Si surface and an increased presence of Si–O and S–O bonds is observed. We identify that well-defined S–Si bonds are crucial to maintain high-quality surface passivation for Si photovoltaics, which allows further optimization of the fabrication process for S-based passivation on silicon.

Hua, Amandee↗

DOE OSTI2023

Hydrogen Sulfide Passivation for p-Type Passivated Emitter and Rear Contact Solar Cells

This work reports on the application of sulfur (S)-passivation to passivated emitter and rear contact (PERC) solar cells. The emitter surface was passivated by hydrogen sulfide (H 2 S) gas phase reaction and capped by a hydrogenated amorphous silicon nitride (a-SiN x :H) layer. The sulfur passivation on a symmetrically n + diffused emitter is shown to lead to an emitter saturation current density (J 0n+ ) of 30 fA/cm 2 at R sheet,n+ ≈ 100 Ω/sq. The application of S-passivation to the emitter surface in the PERC cell structure, with the rear surface passivated by an aluminum oxide (Al 2 O 3 )/a-SiN x :H stack, showed a promising implied open-circuit voltage (iV OC ) of 686 mV before metallization. This iV OC was higher than that for the a-SiN x :H or SiO 2 /a-SiN x :H passivated emitter surfaces (675 and 674 mV, respectively) on PERC cells processed in the same run. However, a significant drop in cell V OC is observed for the S-passivated PERC cell after the completion of device fabrication with laser patterning, screen-printed metal contact deposition, and firing. Nonetheless, an efficiency of ~20% and a V OC of ~650 mV was achieved with an emitter surface passivated by sulfur. We identified that the 760°C contact firing process degrades the S-passivation quality. Furthermore, the surface morphology was studied, and a detailed surface analysis was performed to study the causes of the S-passivated surface degradation.

14 SOLAR ENERGY↗

DOE OSTI2023

Chemical and Electronic Structure at the Interface between a Sputter-Deposited Zn(O,S) Buffer and a Cu(In,Ga)(S,Se) 2 Solar Cell Absorber

The chemical and electronic structure of the interface between a sputter-deposited Zn(O,S) buffer layer and an industrial Cu(In,Ga)(S,Se) 2 (CIGSSe) absorber for thin-film solar cells is investigated with X-ray and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X-ray emission spectroscopy. We find a CIGSSe absorber surface band gap of 1.61 (±0.14) eV, which is significantly increased as compared to the minimal value derived with bulk-sensitive methods (≈1.1 eV). We find no indication for diffusion of absorber elements into the buffer layer. Surface- and bulk-sensitive measurements of the buffer layer suggest the presence of S-Zn and S-O bonds in the Zn(O,S) layer. We find that the naturally existing downward band bending toward the CIGSSe absorber surface is increased by the formation of the interface, likely enhancing carrier separation under illumination. We also derive a flat conduction band alignment, in line with the reported high conversion efficiencies of corresponding large-area solar cells.

14 SOLAR ENERGY↗

DOE OSTI2022

Novel and effective surface passivation for high efficiency n- and p-type Silicon solar cell

The project objective was to develop a novel Si surface passivation method using chalcogens, sulfur (S) and/or selenium (Se), as passivating elements, to withstand industry-standard high temperature contacting and metallization schemes for p-type Si based passivated emitter and rear contact (p-PERC) solar cells. The back surface passivation of PERC cells has been improved drastically with the invention and successful application of an Al 2 O 3 passivation layer. However, the front n + diffused junction surface is still poorly passivated by the standard amorphous silicon nitride (SiNx) anti-reflection coating (ARC) layer. This project sought to address the passivation challenges of both front n+ emitter and undiffused p-Si back surface. Improved p-PERC solar cell performance with open circuit voltage (V OC ) > 680 mV and efficiency of 22% were targeted to validate superior defect passivation properties as compared to standard SiO 2 / Al 2 O 3 passivation. During this project, we systematically investigated process-structure-properties-performance relationships of this novel advanced defect passivation approach. The S/Se passivation was carried out by reacting industrial Czochralski (Cz) Si wafers in H 2 S and H 2 Se gases in a chemical vapor deposition (CVD) reactor at temperatures up to 700°C. After an exhaustive optimization of the process parameters (temperature, time, and gas concentration), we established an optimized process and demonstrated extremely low surface recombination velocities (SRVs) of 1.5 cm/s and 8 cm/s on n-type and p-type Si, respectively, by S-passivation. In-depth surface and interface characterization were performed using soft x-ray and photoelectron spectroscopies (XPS, UPS, XES), combined with capacitance-voltage-frequency (C-V-f) measurements, to decipher the surface chemical/electronic structure and interface defect state densities. These measurements provided critical understanding of the defect passivation mechanism and elucidated the presence of surface S-Si bonds, a reduction of surface dipoles, and low interface state densities (D it ) < 10 11 cm -2 ev -1 . We also found that the Se-passivation is inferior to the S-passivation (by at least one order of magnitude in SRV). Application of the optimized S-passivation to the n+ diffused emitter surface led to a low surface recombination current density, J0 ≈ 40 fA/cm 2 (~ 1/4 of the industry-standard SiNx-passivation), and high implied V OC (686 mV) in p-PERC solar cell structures. The S-passivation process also was found to improve the bulk quality of the p-type Si, better than the SiO 2 or Al 2 O 3 passivation processes. After successful demonstration of efficient passivation of Si surface defects by S, we extensively studied the air, thermal, and illumination stability of the passivation structure. S-passivation itself degrades in air due to competing reactions with moisture and oxygen to form oxides, which can be eliminated by a SiNx capping layer (also acting as a anti-reflective coating). After SiNx process optimization, we demonstrated illumination and thermally stable S-passivation with SRV < 5 cm/s and J 0 < 80 fA/cm 2 . These enhancements in Si passivation, incorporated into p-PERC cells, achieved an efficiency of 19.93% with V OC = 649 mV, using manufacturing metallization and contacting schemes. The low cell performance (cell V OC is much less than the implied V OC = 686 mV, anticipated from surface passivation) was identified due to degradation of S-passivation during the metal firing step (out-diffusion of S from the Si interface to the SiNx surface). The S-passivation of Si surfaces shows significant promise with excellent passivation quality, essential for high performance (high V OC , high efficiency) solar cells. Integration of this innovative defect passivation into devices, however, demands further development of the capping layer, low temperature (<700°C) metallization process, and/or engineering of advanced device structures. Surface passivation-dominated advanced Si solar cells, such as tunnel oxide passivated contacts and Si heterojunctions, are increasingly of interest due to their high-performance potential and will have a growing photovoltaic market share in the near future.

14 SOLAR ENERGY↗

DOE OSTI2021

Efficient passivation of n-type and p-type silicon surface defects by hydrogen sulfide gas reaction

An efficient surface defect passivation is observed by reacting clean Si in a dilute hydrogen sulfide-argon gas mixture (< 5% H2S in Ar) for both n-type and p-type Si wafers with planar and textured surfaces. Surface recombination velocities of 1.5 and 8 cm/s are achieved on n-type and p-type Si wafers, respectively, at an optimum reaction temperature of 550oC that are comparable to the best surface passivation quality used in high efficiency Si solar cells. Surface chemical analysis using x-ray photoelectron spectroscopy shows that sulfur is primarily bonded in a sulfide environment, and synchrotron-based soft x-ray emission spectroscopy of the adsorbed sulfur atoms suggests the formation of S-Si bonds. Furthermore, the sulfur surface passivation layer is unstable in air, attributed to surface oxide formation and a simultaneous decrease of sulfide bonds. However, the passivation can be stabilized by a low-temperature (300oC) deposited amorphous silicon nitride (a-Si:NX:H) capping layer.

14 SOLAR ENERGY↗

DOE OSTI2020

Sulfurization as a promising surface passivation approach for both n- and p-type Si

Sulfur is shown to be an effective and promising surface passivation element for both n- and p-type Si wafers after reacting in dilute hydrogen sulfide gas (2 - 6% in argon) at 550°C. Effective minority carrier lifetimes of > 2 ms and > 0.25 ms are achieved for n- and p-type Si wafers, respectively, comparable to the industry standard thermal oxide and atomic layer deposited aluminum oxide passivation level. Surface characterization by x-ray photoelectron and emission spectroscopy reveals that sulfur is primarily bonded in a sulfide environment.

42 ENGINEERING↗