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Low temperature microcrystalline silicon prepared by low-frequency inductively coupled plasmas for thin film solar cells
Author
Wei, Deyuan
Supervisor
Xu, Shuyan
Abstract
The innovative LF-ICP has been applied to fabricate high-quality intrinsic μc-Si:H films for thin-film solar cell applications. This thesis mainly consists of two major parts in which the materials properties of μc-Si:H fabricated under direct or remote plasma condition are studied, respectively.
Under direct plasma deposition, the transition zones have been realized by high-density plasma (E-H mode transition), or by hydrogen dilution technique at low-density plasma (E-mode) without intentional substrate heating. Real time monitoring reveals that plasma discharge provides a quickly saturated supplementary substrate temperature between 68 and 200 °C. These μc-Si:H films usually possess small grains (<14 nm), weakly preferred orientation ([111] or [220]), high microstructure factor (>0.8), low hydrogen content (4~8%), and acceptable oxygen and nitrogen contents. Although showing somewhat less dense structure, some μc-Si:H films exhibit the high activations, low dark conductivities, the high photo- conductivities (>10-5 S/cm), and high photo-responses (>100), meeting the basic requirements of device-grade intrinsic μc-Si:H films except the slightly low absorption coefficients, providing the scattered and narrow working windows for photovoltaic applications. In addition, their abnormal features (higher hydrogen content and impurity contents than a-Si:H) can be attributed to the generalized grain boundaries exhibiting the wide bandgap and high capacity for extrinsic atoms. The low-temperature growth of μc- Si:H films are attributed to high atomic H flux and suppression of high- energy ion bombardment due to high density of low-temperature electrons in plasma.
In addition, some abnormal physical phenomena revealed by material properties are also investigated. Besides (SiH2)n multi-hydrides in amorphous phase, Si-H bond configuration consists of mono-hydrides bonded at (111) surface or compact multi-vacancies near crystalline grain/amorphous phase interfaces and grain boundaries. A “plum pudding” model” is presented to interpret the special evolution of compactness in crystallized Si:H films from porous materials with dominant (SiH2)n multi-hydrides to dense material with SiH&SiH3 mixed hydrides. A “four-phase structure” consisting of large crystal, small nc-Si grains, defective grain- boundaries, and a-Si:H matrix has been modeled, so that wider “Tauc’s bandgap” is attributed to quantum confinement of nc-Si grains according to simple effective-mass approximation. The structural instability is represented by the post-oxidation revealed by the distinct Si-O FTIR vibration modes, accompanied with effusion of Si-H bonds and high oxygen contamination, whereas electrical instability shows dramatic increase in dark conductivity, and decrease in activation energy and photo-response. The ultra-shallow donor and high dark conductivity after post-oxidation can be attributed to threefold coordinated O atoms (O+-Si3) and/or thermal donors.
Remote deposition in which plasma discharge and film deposition zones have been separated has also been introduced. Compared with direct deposition, remote deposition decreases the extremely high atomic H flux and annihilate ion bombardments on growing surface, widening operation window for transition zone by at least 2-4 times and shifting up the growth temperature. Remote deposition also provides the longer path for species transport, hence keeping long-lifetime SiH3 and atomic H radicals, and depleting short-lifetime SiH and SiH2 radicals. All these strengthen surface diffusion abilities of precursors, hence decreasing defect density on growing surface and yielding the material quality. In fact, remote deposition pushes a silent transformation of preferred orientation from the random state to the weak [220] preferred orientation, and significantly decrease microstructure factor due to enhanced fraction of SiH hydrides in Si-H bond configuration, revealing the improved structural orders of μc-Si:H films. Furthermore, remote deposition enhances the overall absorption coefficients by ~2-4 times, pulls “bandgap” of high absorption components down to conventional values, decreases dark conductivities without deterioration of photo-conductivities and final photo-responses, leading to the widening of the operation window for device-grade materials. As a result, many μc-Si:H films can exhibit satisfactory absorption coefficients in the red and green light region, and meet the minimal requirements of device quality (low σd (~10-8-10-7S/cm) and high σp (>10-5 S/cm)), leading to several discrete working windows for device-grade materials. Furthermore, the structural order in μc-Si:H material has a positive relationship with its optoelectronic properties reflected by photo-response. However, these μc-Si:H films show low absorption coefficients in the short wavelengths, and display the good but not excellent optical-electrical characteristics.
Solar cells deposited under direct condition exhibits high Voc (584 mV) but low efficiency (< 1%) due to high defect density and resulting high recombination in less dense i-layer. Remote deposition greatly promotes yield rate and efficiency of solar cell. The best solar cell (η: 4.14%; Jsc: 15.5 mA/cm2; Voc: 0.675 V; FF: 0.4) is fabricated on planar ZnO:Al glass substrates. High Voc and low Jsc are due to reduced (interface and bulk) recombination and non-optimized device without light trapping strategies and good contact. Device failure originates from localized micro-punch through, space charge limited current in p-i-p structure, etc. However, low deposition rate should be enhanced in the future.
Under direct plasma deposition, the transition zones have been realized by high-density plasma (E-H mode transition), or by hydrogen dilution technique at low-density plasma (E-mode) without intentional substrate heating. Real time monitoring reveals that plasma discharge provides a quickly saturated supplementary substrate temperature between 68 and 200 °C. These μc-Si:H films usually possess small grains (<14 nm), weakly preferred orientation ([111] or [220]), high microstructure factor (>0.8), low hydrogen content (4~8%), and acceptable oxygen and nitrogen contents. Although showing somewhat less dense structure, some μc-Si:H films exhibit the high activations, low dark conductivities, the high photo- conductivities (>10-5 S/cm), and high photo-responses (>100), meeting the basic requirements of device-grade intrinsic μc-Si:H films except the slightly low absorption coefficients, providing the scattered and narrow working windows for photovoltaic applications. In addition, their abnormal features (higher hydrogen content and impurity contents than a-Si:H) can be attributed to the generalized grain boundaries exhibiting the wide bandgap and high capacity for extrinsic atoms. The low-temperature growth of μc- Si:H films are attributed to high atomic H flux and suppression of high- energy ion bombardment due to high density of low-temperature electrons in plasma.
In addition, some abnormal physical phenomena revealed by material properties are also investigated. Besides (SiH2)n multi-hydrides in amorphous phase, Si-H bond configuration consists of mono-hydrides bonded at (111) surface or compact multi-vacancies near crystalline grain/amorphous phase interfaces and grain boundaries. A “plum pudding” model” is presented to interpret the special evolution of compactness in crystallized Si:H films from porous materials with dominant (SiH2)n multi-hydrides to dense material with SiH&SiH3 mixed hydrides. A “four-phase structure” consisting of large crystal, small nc-Si grains, defective grain- boundaries, and a-Si:H matrix has been modeled, so that wider “Tauc’s bandgap” is attributed to quantum confinement of nc-Si grains according to simple effective-mass approximation. The structural instability is represented by the post-oxidation revealed by the distinct Si-O FTIR vibration modes, accompanied with effusion of Si-H bonds and high oxygen contamination, whereas electrical instability shows dramatic increase in dark conductivity, and decrease in activation energy and photo-response. The ultra-shallow donor and high dark conductivity after post-oxidation can be attributed to threefold coordinated O atoms (O+-Si3) and/or thermal donors.
Remote deposition in which plasma discharge and film deposition zones have been separated has also been introduced. Compared with direct deposition, remote deposition decreases the extremely high atomic H flux and annihilate ion bombardments on growing surface, widening operation window for transition zone by at least 2-4 times and shifting up the growth temperature. Remote deposition also provides the longer path for species transport, hence keeping long-lifetime SiH3 and atomic H radicals, and depleting short-lifetime SiH and SiH2 radicals. All these strengthen surface diffusion abilities of precursors, hence decreasing defect density on growing surface and yielding the material quality. In fact, remote deposition pushes a silent transformation of preferred orientation from the random state to the weak [220] preferred orientation, and significantly decrease microstructure factor due to enhanced fraction of SiH hydrides in Si-H bond configuration, revealing the improved structural orders of μc-Si:H films. Furthermore, remote deposition enhances the overall absorption coefficients by ~2-4 times, pulls “bandgap” of high absorption components down to conventional values, decreases dark conductivities without deterioration of photo-conductivities and final photo-responses, leading to the widening of the operation window for device-grade materials. As a result, many μc-Si:H films can exhibit satisfactory absorption coefficients in the red and green light region, and meet the minimal requirements of device quality (low σd (~10-8-10-7S/cm) and high σp (>10-5 S/cm)), leading to several discrete working windows for device-grade materials. Furthermore, the structural order in μc-Si:H material has a positive relationship with its optoelectronic properties reflected by photo-response. However, these μc-Si:H films show low absorption coefficients in the short wavelengths, and display the good but not excellent optical-electrical characteristics.
Solar cells deposited under direct condition exhibits high Voc (584 mV) but low efficiency (< 1%) due to high defect density and resulting high recombination in less dense i-layer. Remote deposition greatly promotes yield rate and efficiency of solar cell. The best solar cell (η: 4.14%; Jsc: 15.5 mA/cm2; Voc: 0.675 V; FF: 0.4) is fabricated on planar ZnO:Al glass substrates. High Voc and low Jsc are due to reduced (interface and bulk) recombination and non-optimized device without light trapping strategies and good contact. Device failure originates from localized micro-punch through, space charge limited current in p-i-p structure, etc. However, low deposition rate should be enhanced in the future.
Date Issued
2014
Call Number
QC718 Wei
Date Submitted
2014