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The behavior of oxide precipitates during solar cell fabrication processes and the resulting effect on device performance have been investigated by transmission electron microscopy (TEM) observation. Samples were prepared with different carbon concentration and under different crystal growth conditions, namely using the conventional and an advanced process. The density of oxide precipitates increased monotonically with the carbon concentration, while the cell efficiency improved with decreasing oxygen precipitate density. When the carbon concentration was reduced to below 10 16  cm −3 , the oxide precipitates grew largely and dislocations were introduced. TEM observations confirmed that the morphology of the oxide precipitates clearly differed depending on the crystal growth conditions. Precipitates grown in platelet form introduced high density dislocations in their surroundings, while the dislocation density was relatively lower around polyhedral-type precipitates. These results thus reveal that oxygen precipitation can be controlled by varying the crystal growth conditions, possibly contributing to the production of high-efficiency solar cells.

The development of high-performance solar cells offers a promising pathway toward achieving high power per unit cost for many applications. As single-junction solar cells are limited to 30–32% conversion efficiency under 1-sun, multi-junction or tandem solar cells are expected to contribute to higher performances. The II–VI compound, chalcopyrite, and kesterite-based tandem solar cells are expected as high-efficiency and low-cost solar cells. This paper presents analytical results for efficiency potential and loss elements of II–VI compound, chalcopyrite, and kesterite-based tandem solar cells. The 2-junction and 3-junction tandem solar cells have potential efficiencies of 36% and 42%, respectively, by improving external radiative efficiency and reducing resistance and optical losses. Some paths for improving efficiencies of tandem solar cells are shown by analyzing their loss elements. Optimization of band-gap energy of sub-cells and photon recycling are also recommended. Graphical abstract

Suppression of the formation of crystal defects is essential for the realization of high-efficiency solar cells. The reactive plasma deposition (RPD) process introduces defects in the silicon crystal bulk and at the passivation layer/silicon crystal interface. This study suggests that oxygen impurities can affect the generation of RPD-induced defects. Although the RPD deposition conditions were the same, the number of RPD-induced recombination centers in Cz-Si was larger than that in the Fz wafer. The increase in 950 °C pre-annealing resulted in increased peak intensity corresponding to defect level E1 in the Cz-Si MOS sample. In the case of Fz-Si, the increase in intensity with increasing pre-annealing time was slight. This indicates that oxygen precipitation might be related to the structure of RPD-induced defects.

Efficiency potential of crystalline Si solar cells is analyzed by considering external radiative efficiency (ERE), voltage, and fill factor losses. Crystalline Si solar cells have an efficiency potential of more than 28.5% by realizing ERE of 20% from about 5% and normalized resistance of less than 0.05 from around 0.1. Nonradiative recombination losses in single-crystalline and multicrystalline Si solar cells are also discussed. Especially, nonrecombination and resistance losses in multicrystalline Si solar cells are shown to be higher than those of single-crystalline cells. Importance of further improvement of minority-carrier lifetime in crystalline Si solar cells is suggested for further improvement of crystalline Si solar cells. High efficiency of more than 28.5% will be realized by realizing high minority-carrier lifetime of more than 30 ms. Key issues for those ends are reduction in carbon concentration of less than 1 × 1014 cm−3, oxygen precipitated and dislocations even in single-crystalline Si solar cells, and reduction in dislocation density of less than 3 × 103 cm−2 in multicrystalline Si solar cells.

Recently, titanium oxide has been widely investigated as a carrier-selective contact material for silicon solar cells. Herein, titanium oxide films were fabricated via simple deposition methods involving thermal evaporation and oxidation. This study focuses on characterizing an electron-selective passivated contact layer with this oxidized method. Subsequently, the SiO2/TiO2 stack was examined using high-resolution transmission electron microscopy. The phase and chemical composition of the titanium oxide films were analyzed using X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The passivation quality of each layer was confirmed by measuring the carrier lifetime using quasi-steady-state photoconductance, providing an implied open circuit voltage of 644 mV. UV–vis spectroscopy and UV photoelectron spectroscopy analyses demonstrated the band alignment and carrier selectivity of the TiO2 layers. Band offsets of ~0.33 and ~2.6 eV relative to the conduction and valence bands, respectively, were confirmed for titanium oxide and the silicon interface.

This study investigated how the SiNx refractive index (RI) and SiO2 thickness, dox, of stacked SiNx/SiO2 passivation layers of the front p+emitters of n‐type crystalline‐silicon (c‐Si) photovoltaic (PV) cells affect their polarization‐type potential‐induced degradation (PID) behaviors. We prepared six n‐type c‐Si PV cells with an RI of 2.0 or 2.2 and with dox of 9, 2, or 1 nm. Then PV modules fabricated from the cells were subjected to PID tests during which a bias of −1000 V was applied to cells with respect to the front cover glass surface. For dox of 9 or 2 nm, rapid polarization‐type PID was observed, irrespective of the RI. However, for dox of 1 nm, the RI markedly affected the degradation behavior, and cells with an RI of 2.2 showed no degradation. These findings are attributable to carrier transport between the high RI (Si‐rich) SiNx and the c‐Si substrates, which can readily occur only when the SiO2 layer is sufficiently thin for electrons to tunnel through the SiO2 layer. These results are important for elucidating polarization‐type PID mechanisms and for developing preventive measures against polarization‐type PID. We investigated how the SiNx refractive index (RI) and SiO2 thickness, dox, of SiNx/SiO2 layers of n‐type crystalline‐silicon photovoltaic cells affect polarization‐type potential‐induced degradation. For dox greater than 2 nm, rapid degradation was observed irrespective of the RI. However, for dox of 1 nm, the RI strongly affected degradation, and 2.2‐RI cells showed no degradation.

We investigated the effect of Fe contamination on the electronic properties of dislocation clusters in relation to oxygen precipitation in multicrystalline silicon (mc-Si). Photoluminescence (PL) spectroscopy and mapping were performed at room and liquid-He temperatures on mc-Si wafers before and after Fe contamination. PL spectra consisted of the band-edge emission, the 0.78-eV emission associated with oxygen precipitates, and the dislocation-related D-lines. The Fe contamination increased the electrically active dislocation clusters. Part of these clusters acted as preferential oxygen precipitation sites, and their electronic properties were not further influenced by the Fe contamination.

The electron-beam-induced-current (EBIC) technique using scanning transmission electron microscopy (STEM) has been applied to the observation of grain boundaries in polycrystalline Si. It was shown that defects in thin regions can disappear or give bright contrast in EBIC images, but are distinct in STEM images. For the sample with a high carrier concentration, the surface effect was shown to dominate the EBIC current for a thin region. The bright contrast of the defects observed for the sample with a low carrier concentration can be attributed to the combination of the diffraction effect and the built-in electric field induced by the depletion of the entire thickness.

This study investigated how the SiN x refractive index (RI) and SiO 2 thickness, d ox , of stacked SiN x /SiO 2 passivation layers of the front p + emitters of n‐type crystalline‐silicon (c‐Si) photovoltaic (PV) cells affect their polarization‐type potential‐induced degradation (PID) behaviors. We prepared six n‐type c‐Si PV cells with an RI of 2.0 or 2.2 and with d ox of 9, 2, or 1 nm. Then PV modules fabricated from the cells were subjected to PID tests during which a bias of −1000 V was applied to cells with respect to the front cover glass surface. For d ox of 9 or 2 nm, rapid polarization‐type PID was observed, irrespective of the RI. However, for d ox of 1 nm, the RI markedly affected the degradation behavior, and cells with an RI of 2.2 showed no degradation. These findings are attributable to carrier transport between the high RI (Si‐rich) SiN x and the c‐Si substrates, which can readily occur only when the SiO 2 layer is sufficiently thin for electrons to tunnel through the SiO 2 layer. These results are important for elucidating polarization‐type PID mechanisms and for developing preventive measures against polarization‐type PID.