研究目的
To investigate the factors responsible for degradation of device performance of silicon solar cells featuring MoOx front contacts upon annealing.
研究成果
A combination of HRTEM and SR-EELS studies were performed in this work on hole-selective MoOx contacts. The HRTEM and SR-EELS revealed that no hole-blocking interlayer forms at the MoOx/ITO interface following annealing, ruling out this as a possible cause for performance degradation. The primary difference observed between as-deposited and annealed samples in the HRTEM and SR-EELS was a homogenization of the MoOx film upon annealing. In the as-deposited state, a slightly reduced MoOx interlayer is observed at the MoOx/SiOx interface that disappears upon annealing. Device simulations were performed to investigate the role of this interlayer’s work function, as well as that of the bulk MoOx. It emerged that as long as the interlayer work function is greater than or equal to Ev, c-Si, the bulk MoOx work function ultimately dictates the band bending and therefore hole-selectivity of these contacts.
研究不足
The study of these materials by SR-EELS analyses is complicated. MoO3 and sub-stoichiometric MoO3 tend to suffer from electron beam damage, the signal/noise ratio of the Mo-M2,3 edges makes this kind of study challenging and the interpretation of the EELS edges of molybdenum oxides is not straightforward.
1：Experimental Design and Method Selection:
A combination of HRTEM and SR-EELS was used to study the compositional changes at nanoscale before and after thermal annealing at ≈ 200°C.
2：Sample Selection and Data Sources:
p-type {100} Si was used as substrate. A ≈ 5-10 nm thick MoOx film was deposited on the Si substrate under vacuum by thermal evaporation using a solid MoO3 source. Subsequently, a ≈ 50 nm thick indium tin oxide (ITO) layer was deposited over MoOx by sputtering in Ar/O2 atmosphere at room temperature. Select samples were then annealed in air at 200°C for 30 minutes.
3：List of Experimental Equipment and Materials:
Cross-sectional TEM specimens were prepared by a focused ion beam (FIB) using a FIB FEI 200 TEM. Bright field (BF) and high resolution TEM (HRTEM) images of p-Si/MoOx/ITO stacks were obtained in conventional TEM (CTEM) mode with an FEI TEM Tecnai F30 operated at 300 kV. Spatially-resolved electron energy loss spectroscopy (SR-EELS) studies were carried out in STEM mode using a JEOL ARM 200CF operated at 200 kV and equipped with a cold field-emission gun, double-aberration correctors, high-angle annular dark-field (HAADF) detector and a GIF Quantum EELS spectrometer with dual-EELS.
4：Experimental Procedures and Operational Workflow:
During SR-EELS analyses, the energy resolution was about
5：5- 6 eV with a dispersion of 1 eV/ch. For most of the EELS experiments, the convergence and collection angles were 28 and 88 mrad, respectively. Spectra were collected in STEM mode by using spectrum line scans. Core-loss and low-loss spectra were acquired simultaneously in dual-EELS with the an acquisition time-setting of 1 s to avoid electron beam damage. Data Analysis Methods:
The core loss spetra’s chemical shifts were calibrated by simultaneously acquiring zero-loss spectra to ensure accurate measurement of the shifts of the Mo M3 edge relative to the zero-loss peak.
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