| As an important II-VI compound semiconductor, cadmium sulfide(Cd S) has a wide band gap(~2.42 e V), a high carrier mobility(3×105cm2/V·s) and outstanding thermal stability, which make it has a great application value in the fields of electronic and optoelectronic devices. Because the important position of silicon(Si) in the traditional electronic industry, as well as the excellent optoelectronic properties of Si nanomaterials, the construction and the performance study of newly Si-based nanodevices have attracted much attentions. In previous works, the silicon nanoporous pillar array(Si-NPA) was prepared by a hydrothermally etching technique,which is a silicon micro-nanometer structural. Its special physical and chemical properties make it a promising function substrate or template in constructing Si-based optoelectronic nanodevices. Meanwhile, Cd S/Si-NPA was prepared as well, from which a obviously rectification effect and a photovoltaic(PV) effect were obtained.Therefore, it is a new Si-based nanodevice with a significant application prospect.However, the bad optoelectric property of Cd S thin film hindered the improvement of the performance of Cd S/Si-NPA heterojunction, such as the low electroluminescence(EL) efficiency and intensity, the high series resistance(Rs), small short circuit current(Jsc), and low energy conversion efficiency. In this work, the ⅢA elements were doped into the Cd S thin film to improve its optoelectric property and then enhance the performance of Cd S/Si-NPA heterojunction. The main research results achieved in this thesis are listed as follows.1. preparation, characterization and optoelectronic properties of Cd S/Si-NPACd S/Si-NPA nanoheterojunction was fabricated by depositing Cd S thin film on Si-NPA using a chemical bath deposition(CBD) method. Its structure could be described as follows. A continuous grain membrane of wurtzite Cd S is the upper layer.The interface region constructed by alternately and randomly dispersed Cd S nanocrystals(nc-Cd S) and Si nanocrystals(nc-Si) is the intermediate layer. Both the crystal Si and the silicon nanoporous layer constitute the bottom layer. The room and temperature-dependent photoluminescence(PL) results show four emission bands, i.e.blue(~436nm), green(~563nm), red(~688nm) and infrared bands(~810nm), which are attributed to the oxygen-related defect states in nc-Si, the near band-edge emission of Cd S, the radiative recombination from surface defects to Cd vacancies and those to S interstitials, respectively. With temperature increasing, the red band shows redshift,the infrared band does not vary, and the green band shows a ‘Λ’-shape shift, i.e. firstly blueshift(10-100 K) and then redshift(100-300 K). The integral intensities of three bands decrease with temperature increasing, and the red band nearly vanish above 200 K. The ‘Λ’-shape shift of green band is believed to originate from a delocalization process of bounded carriers. The temperature degradation of the PL intensity should be ascribed to the nonradiative processes. The nonradiative processes of green band are the delocalization process of bounded carriers at low temperature and the LO phonon-induced electron escape at high temperature. The main nonradiative processes for the red and infrared bands at high temperature are the same as the green band, and the nonradiative processes at low temperature are the transitions from the localization states near the acceptors to the acceptors.2. preparation, characterization and optoelectric properties of B-doping Cd S/Si-NPAUsing Si-NPA as substrate and boric acid as dopant source, B-doping Cd S/Si-NPA was prepared by growing B-doped Cd S thin film on Si-NPA via a CBD method. After B doping, there is no obvious change on the structure and surface morphology of Cd S/Si-NPA. The morphological feature of Si-NPA is inherited.However, there are obvious effects on the resistivity of Cd S and the photoluminescence(PL), rectification, electroluminescence and photovoltaic(PV)performance of Cd S/Si-NPA. The PL spectra show green, red and infrared emission bands at different temperature. With [B]/[Cd] increasing, the peak intensity of green band firstly increases and then decreases. Meanwhile, the peak intensities of red and infrared bands firstly decrease and then increase, which are due to the change of the incorporation way of B into Cd S. The maximum value of the intensity ratio of green and red band is obtained in the sample B-0.01, where the green emission is greatly enhanced while the red and IR emissions are obviously suppressed. Considering the spectral characteristic of solar spectrum, this would be very beneficial for realizing high-efficiency solar cells. The dark resistivity of Cd S/Si-NPA decreases non-monotonically with [B]/[Cd]. The dark resistivity of the sample B-0.01 with the smallest resistivity is ~98 Ω·cm. Meanwhile, the sample B-0.01 shows the best rectification effect, such as the highest breakdown voltage(>3 V) and the smallest ideal factor(10.6). The EL spectra of Cd S/Si-NPA can be controlled by tuning the [B]/[Cd]. With [B]/[Cd] increasing, the EL spectra could be changed from only one green emission band to two emission bands(green and red emission bands or red and infrared emission bands). The photovoltaic parameters of Cd S/Si-NPA show strong dependence upon B-doping concentration, with optimal characteristics being achieved in sample B-0.01. Its series resistor(Rs) is only 3% of that of the undoped sample, and the short circuit current density(Jsc) is 20 times of that of the undoped sample. The energy conversion efficiency is ~300 times higher than that of the undoped sample.These results indicates that B-doping might be an effective path for promoting the performance of the optoelectronic devices based on Cd S/Si-NPA and the optimal doping amount([B]/[Cd]) is 0.01.3. preparation, characterization and optoelectric properties of Al-doping Cd S/Si-NPAUsing Si-NPA as substrate and Al Cl3 as dopant source, Al-doping Cd S/Si-NPA was prepared by growing Al-doped Cd S thin film on Si-NPA via a CBD method. After Al doping, there is no obvious change on the morphology of Cd S/Si-NPA. However,there are obvious effects on the uniformity and compactness of Cd S thin film. The morphological feature of Si-NPA is inherited. The film of sample Al-0.07 shows the best uniformity and compactness. The room PL spectra of Al-doping Cd S/Si-NPA present blue(~440nm), green(~550nm) and infrared(~800nm) emission bands. The red emission band is not observed and the intensity of infrared emission is very weak,which indicate the Al-doping Cd S thin films have low defect density. The bule emission is ascribed to Si-NPA and the green and red emissions to Cd S. The dark resistivity of Cd S/Si-NPA decreases non-monotonically with [Al]/[Cd]. The dark resistivity of sample Al-0.07 with the smallest resistivity is 2.31×102 Ω·cm.Meanwhile, the rectification effect of Cd S/Si-NPA is improved after Al doping. The turn-on voltage is decreased from ~2.9 V to ~2.1 V, and the breakdown voltage is increased to over 5 V. The EL spectra of Cd S/Si-NPA can be controlled by tuning the[Al]/[Cd]. With [Al]/[Cd] increasing, the EL spectra can be changed from only one green emission band to green and red emission bands. After Al doping, the Rs is decreased and the Jsc is increased, hence the energy conversion efficiency is enhanced.The sample Al-0.07 shows the best PV performance. Its series resistor(Rs) is only0.3% of that of the undoped sample, and the short circuit current density(Jsc) is 30 times of that of the undoped sample. The energy conversion efficiency is ~150 times of that of the undoped sample. There is no doubt that the incorporation of Al element is important for improving the performance of the Cd S/Si-NPA nanoheterojunction and the optimal doping amount([Al]/[Cd]) is 0.07.4. preparation, characterization and optoelectric properties of In-doping Cd S/Si-NPAUsing Si-NPA as substrate and In Cl3 as dopant source, In-doping CdS/Si-NPA was prepared by growing In-doped Cd S thin film on Si-NPA via a successive ionic layer adsorption and reaction(SILAR) method. After In doping, there is no obvious change on the morphology of Cd S/Si-NPA. The morphological feature of Si-NPA is inherited. With [In]/[Cd] increasing, the roughness of Cd S thin film decreases and the uniformity and compactness increase. Only blue and green emission bands are observed from In-doping Cd S/Si-NPA without any defect emission, which indicates that the film has a good stoichiometry and low defect density. With [In]/[Cd]increasing, the dark resistivity decreases from 9.97×103 Ω·cm to 6.53×102 Ω·cm. Both the rectification and photovoltaic effects of Cd S/Si-NPA are improved effectively. The breakdown voltage of Cd S/Si-NPA is increased to over 6.5 V and the energy conversion efficiency of Cd S/Si-NPA nanoheterojunction is improved by two orders of magnitude. A wide emission band(300-700 nm) is obtained from In-doping Cd S/Si-NPA, and the intensity increases with [In]/[Cd]. The value of chromaticity coordinate confirms that it is a white light emission. The chromaticity coordinate and color temperature can be tuned with [In]/[Cd]. It is indicated that In doping Cd S/Si-NPA should be a potential material in the field of Si-based light-emitting diodes through controlling the In doping concentration. |
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