| Solar cells have attracted a tremendous amount of research activity. Low cost, high efficiency, long life, and environment-friendly new generation of solar cells have become one of the hot spots in the research of solar cells. Silicon quantum dots(Si-QDs) thin films have important application prospects in the field of solar cells, Si-QDs based solar cells are one of the most potential applications for the high efficiency solar cells. All-silicon tandem solar cells were still in the experimental exploration stage, which needs further research work. In this work, we have investigated the preparation and properties of intrinsic and doped Si-QDs thin films, annealing method for Si-QDs thin films, and preparation and performance of Si-QDs/c-Si heterojunction solar cells. In the following,the primary achievements in this work are described.(1) Periodic gradient Si-rich SiNx(G-SRSN) thin films were deposited by combination of magnetron co-sputtering with periodically adjusting the sputtering power and rapid photo-thermal annealing. The structural, optical and electrical properties of the G-SRSN thin films and the single-layer thin films with similar silicon content were compared. The results showed that the QDs density, crystalline fractions, and conductive properties of G-SRSN are significantly superior than that of single-layer thin films.(2) For comparison, two sets of Si-rich silicon nitride films were then treated by microwave annealing(MWA) and rapid thermal annealing(RTA), respectively. It is found that the temperature required for forming Si QDs decreases by 200 ℃ under microwave heating. The crystalline fractions and QDs density of MWA samples are significantly superior than that of RTA samples at the same annealing temperature. We also investigated the SiOx single-layer films after MWA at 1000 ℃ and RTA at 1100 ℃, respectively. According to XPS analysis, found that the Si-Si bond content of MWA sample was higher than that of RTA sample. This phenomenon is attributed to enhanced phase separation caused by the microwave non-thermal effect.(3) In order to study the quantum effects in rapid thermal processing, we propose a new method called light-filtering rapid thermal processing(LRTP). SiNx thin films prepared by magnetron co-sputtering were studied by conventional rapid thermal processing(CRTP) and LRTP, respectively. The study found that the films annealed by LRTP has better microstructure characteristics; LRTP is more conducive to the formation of Si-QDs; the quantum effects in rapid thermal processing have a negative influence on the formation and crystallization of Si QDs in SiNx films.(4) Growth of Sb/SiNx multilayers, followed by high-temperature RTA, was shown to be an effective strategy for synthesizing Sb-doped Si QDs. The doping concentration of Sb(from 0.32 at.% to 1.82 at.%) was controlled by varying the thickness of Sb sublayer. Moderate Sb concentrations were found to enhance the formation of Si QDs. PL results show that the non radiative recombination defects caused by Sb impurities increased with the increase of Sb content, which leads to the decrease of the emission intensity. Hall measurements demonstrate that, the carrier concentration increases, the mobility of Hall decreases, and the conductivity increases at first and then decreases with the increase of Sb content. It is indicated that the excessive high Sb doping will deteriorate the conductive properties.(5) Growth of Si QDs embedded B-doped SiNx films with various doping concentration were fabricated by magnetron co-sputtering combined with RTA. The effects of B content on the structural, optical and electrical properties of the films were studied. The study found that the amount of B dopant has no significant effect on the crystallization characteristics of the films. According to the XPS analysis, it is found that B atoms may be doped in the Si-QDs, or exist in the silicon nitride or the interface between Si-QDs and the matrix. The PL intensity increases with increasing B content, but increases at first and then decreases. The prepared B-doped silicon quantum dots films show the p-type conduction behavior, and the conductivity increases with the increase of B doping content, and at 0.9 at.%, the conductivity reaches a maximum value of 5.834×10-2 S cm-1.(6) The Si-QDs/c-Si heterojunction solar cells based on Sb-doped Si-QDs films were fabricated, and the effect of Sb doping concentration on the photovoltaic properties was studied. It is found that the best Sb doping concentration is 0.85 at.%, corresponding to the device Sb/0.85/3.4(area 3.4 cm2), the conversion efficiency is 0.19%. We studied the devices(Sb doped concentration of 0.85 at.%) with various area, and found that the photovoltaic performance decreased with decreasing area, when the area reduced to 0.3 cm2, the conversion efficiency reached 3.16%. In addition, optimized treatments of device Sb/0.85/3.4 were carried out, including increased AZO conductive layer and hydrogen passivation treatment. It is found that the carrier collection efficiency was improved up to 0.36% after adding the AZO layer. On this basis, the efficiency increased to 0.74% after increased hydrogen passivation step in the process sequence.(7) The Si-QDs/c-Si heterojunction solar cells based on B-doped Si-QDs films were fabricated, and the effect of B doping concentration on the photovoltaic properties was studied. It is found that, with the increase of B doping amount, the photovoltaic performance is improved, when the B doping amount is 0.9 at.%, the efficiency reaches the highest value of 4.26%. SiNx/Si3N4 multilayer films with controllable and uniform Si-QDs size were fabricated, and found that the optical band gap of the films can be adjusted by changing the thickness of SiNx layer. On the basis of this, the Si-QDs/c-Si heterojunction solar cells were prepared. It is found that the larger the band gap is, the higher the cell efficiency is. The best performance device is SL/3.5/1.1(area 0.5 cm2) with 7.05% efficiency. |
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