The Optoelectronic Properties Of CdS/Si Multi-interface Nanoheterojunction

Solar cells have long been thought a promising clean and renewable energysource to replace the traditional fossil fuels in the future. Nevertheless, the realizationof the blueprint depends highly upon the low-cost and environment-friendlyfabrication of solar cells with high energy conversion efficiency and long devicelifetime. Besides the traditional solar cells based on silicon (Si) bulk materials andthin films, some newly appeared solar cells such as those based on polymers,semiconductor quantum dots and various nanostructures also attracted much attention.In recent years, mainly due to the technical maturity and remarkable physical andchemical properties differed from those of bulk Si, various Si nanostructures wereextensively studied as building blocks for high-performance electronic and photonicdevices. In particular, the strong light absorption and efficient charge separationfound in various Si-based heterojunctions, which were constructed by growingcompound semiconductors on Si nanostructures, make the fabrication ofhigh-efficiency Si-based nano solar cells be hopeful. In the previous works, wefabricated the silicon nanoporous pillar array (Si-NPA) by hydrothermally etchingp-type (111) oriented, single crystal silicon (sc-Si) wafers. Si-NPA is a Si hierarchicalstructure characterized by a regular array of micron-sized, quasi-identical and highlynanoporous Si pillars, and characterized by the broader spectrum absorption andmulti-bands emissions. The optical and electrical properties observed in both Si-NPAand the nanoheterosystems based it demonstrated that Si-NPA might be a promising functional substrate in constructing Si-based optoelectronic devices. In thisdissertation, a non-planar n-CdS/p-Si multi-interface nanoheterojunction(CdS/Si-NPA) was prepared by a chemical bath deposition (CBD) method utilizingSi-NPA as a functional substrate, which characterizations of structural properties,photoluminescence, electrical properties, electroluminescence and photovoltaicproperties were systematically studied.The main points in this dissertation are as follows:1. Structure characteristics of CdS/Si-NPACdS/Si multi-interface nanoheterojunction was fabricated by depositing CdS onthe Si-NPA using the chemical bath deposition (CBD), which structure characteristicswere studied. The morphology, structure and components of the interface can becontrolled by tuning the components and concentration of the solution and thetemperature and time of reaction. Through the analysis of XRD and HR-TEM images,the overall structure of CdS multi-interface nanoheterojunction could be concluded asthe follows. The upper layer was a continuous thin film composing of nc-CdS or theiragglomerates, the intermediate layer was the interface region constructed byrandomly and alternately dispersed nc-CdS and Si nanocrystals (nc-Si), and thebottom layer was the unfilled silicon nanoporous layer grown on single crystal Si(sc-Si) substrate. With the increase of the deposition time, the sizes of nc-CdS andnc-Cd were increased. However, the sizes of nc-Cd reach up to the maximum at thedeposition time of50min. It is indicated that the reaction of Cd2+and Si-NPA in theinterface of the solution and Si-NPA was impeded by the deposited CdS films.2. Room-temperature and temperature-dependent PL of CdS/Si-NPAIn order to clarify the emission mechanisms and confirm the radiative andnonradiative recombination processes, we analyzed the the room-temperature and the temperature-dependent photoluminescence of Si-NPA and CdS/Si-NPA. Theultraviolet, blue, orange and red PL bands from Si-NPA were attributed to theradiative recombination through the deep-levels in silicon oxide, oxygen-relateddefect states in silicon nanocrystallites (nc-Si), band-to-band transition within nc-Si,and surface/interface states of nc-Si or between nc-Si and SiOx, respectively.Two emission bands, i. e. the blue band (~446nm) and the green band (~526nm), were observed in the CdS/Si multi-interface nanoheterojunction, whichchromaticity coordination locate in the white light region. After the annealingtreatments with the different temperature, the energy of blue peak do not vary and theenergy of green peak show redshift below the annealing temperature of300Cfollowed by a blueshift above300C. The mimimum green peak energy isobserved in the annealed sample at300C, which is contributed to the appearanceof cubic CdO and the quantum confinement effect. The correlated colortemperature (CCT) decreases with the increase of the annealing temperature.With the increase of the deposition time, the blue energy do not vary,however the intensity of blue peak is decreased compared to the green peak whichenergy decrease. It is indicated that the increasing thickness of CdS films and theincreasing size of nc-CdS result in the redshift of the green emission.The nonradiative recombination processes are very important for the PL ofsemiconductors. The temperature-dependent PL of CdS/Si-NPA annealed at300oCare measured and analyzed. The green and red emissions show redshift withincreasing the temperature, however, the orange emission does not vary. Theintensity of three emissions decrease as the temperature is increasing, and theorange and red emissions nearly vanish above200K. The variation of peakposition and peak intensity should be ascribed to the nonradiative processes. As forthe green emission, the nonradiative processes are the transition from the heavy-hole to light-hole at low temperature and the thermally escape due to scattering with one ortwo LO phonon at high temperature. The main nonradiative processes for the orangeand red emissions are the transition from the level with the different activation energynear the acceptors to the acceptors.3. Electrical properties and electroluminescence of CdS/Si-NPAThe obvious rectifying effect from the CdS/Si multi-interfacenanoheterojunction was observed. A forward current density of~194.7mA/cm2and areverse current density of~0.86mA/cm2was obtained at3.6V, respectively. Therectification ratio is~226.4at3.6V. The onset voltage of~2.9V and thebreakdown voltage above6.0V were obtained, too. The current-voltagecharacteristics from the CdS/Si multi-interface nanoheterojunction at forward biasvolage can be describe by the Ohm’s model (below1.38V) and the space chargelimited current model (SCLC, above1.38V). There exists a very high trapconcentration at the interfacial zone of CdS/Si multi-interface nanoheterojunction.Based on the analysis, one can assume that the trap states at a bias range of1.38-2.0V are partially occupied, while those at V2.0V are completely occupied. Theenhancement of the experimental Poole-Frenkel coefficient compared to thetheoretical one, is due to the presentation of localized electric fields at the vicinity ofthe traps.Electroluminescence (EL) of the as-grown and annealed CdS/Si-NPA at200oC、300oC、400oC and500oC were studied. White EL from as-grown CdS/Simulti-interface nanoheterojunction was obtained at the forward voltage of~6.0V. Thechromaticity coordinate, correlative color temperature (CCT) and color renderingindex (CRI) are (0.24,0.30)、13989K and82.6, respectively. The CCT descreasesand the CRI is improved after annealing treatment at200oC. After300oC annealing treatment, the EL spectrum expands from~380nm to~850nm, which chromaticitycoordinate, correlative color temperature (CCT) and color rendering index (CRI) are(0.34,0.36)、5090K and95.0, respectively. And a new red band is observed. After400oC annealing treatment, the EL spectrum is similar to the one of the sample at300oC. However, the CCT and CRI are decreased and~4223K and~92.8,respectively. After500oC annealing treatment, the chromaticity coordinate, CCTand CRI of EL (~500-800nm) are (0.52,0.46)、2303K and63.7, respectively. It isindicated that CdS/Si-NPA should be a potential material in the field of Si-basedlight-emitting diodes through controlling the annealing treatment.4. Photovoltaic properties of CdS/Si-NPAThe photovoltaic properties of ITO/CdS/Si-NPA/sc-Si/Al were studied. Theobvious photovoltaic effect can be observed, however, the short circuit current andenergy conversion efficiency are very low. In order to improve the energyconversion efficiency, Cd nanocrystals (nc-Cd) are incorporated into the CdS-Siinterface to reduce the series resistance using the in situ deoxidization of Si-NPAin the CBD process. The results show that the series resistance is decreased andthe short circuit current is improved by three orders of magnitude. The energyconversion efficiency is more~530times than the previous result. It is significancethat the incorporation of moderate metal nanoparticles into the interface ofheterojunction is important for decreasing the series resistance of the device, such assolar cells base on multi-interface nanoheterojunction.