Fabrication And Photovoltaic Effect Of CdS/Si-NPA Nanoheterojunction Array

Solar cells based on silicon nanoporous pillar array (Si-NPA) are of particular scientific importance and have great potential in future application. In this thesis, we firstly report the fabrication of Si-NPA on (100)-oriented single crystal silicon wafers using a two-step etching method. Using Si-NPA as substrate, a continuous film of CdS was deposited on Si-NPA via chemical bath deposition (CBD) method. High light absorption and an obvious photovoltaic effect were observed in CdS/Si-NPA nanoheterojunction array. The main research results obtained in this thesis are listed below.1. Microstructure and surface morphology of CdS/Si-NPASi-NPA is a silicon hierarchical structure consisting of a regular array of micron-sized, quasi-identical, porous silicon pillars with an additional porous layer beneath the array. As can be seen from the SEM images of Si-NPA after CBD process, a continuous film was formed on the surface and the characteristic of the pillar array of Si-NPA was maintained. The XRD results indicated that as-deposited film was CdS with cubic structure, and a phase transition from cubic to hexagonal was observed after annealing at 500℃for 60 min. Based on the experimental results, the average grain sizes of cubic CdS and hexagonal CdS were calculated to be～1.1 nm and～16.7 nm, respectively. EDS measurements carried out at the top sites of the pillars, at the half-height sites of the pillars and the valley sites surrounding the pillars verified that a continuous CdS film was deposited and also revealed the presence of Vs and ICd in CdS film. By comparing these results with those of Si-NPA, it can be noted that O was existed in CdS film. Since both Vs and ICd act as donors in CdS, it can be inferred that the CdS film was n-type, and this was proved later by the electrical measurement. The O signal was believed to come from Cd(OH)2, which was formed at the initial stage of the CBD process.2. Optical and electrical properties of CdS/Si-NPA CdS powder obtain from the solution was yellowish. After the annealing treatment at 500℃for 60 min, a phase transition of CdS from cubic to hexagonal was observed, but XRD results also revealed the presence of cubic CdO. Multi-color photoluminescence (PL) was seen from the room-temperature PL spectra of CdS/Si-NPA. The theoretical studies disclosed that the multi-color PL was attributed to the existence of various trapping states in CdS. It is known that there are two major competing reactions in CBD CdS growth process:homogeneous particle formation and heterogeneous surface reaction. Therefore, a particle sticking growth of CdS is unavoidable during the deposition. So the analysis of CdS powder might provide useful information for improving the quality of CdS films.Properties of light absorption of CdS/Si-NPA. CdS/Si-NPA exhibited high light absorption in the wavelength range of 400～800 nm and the average reflectance was below 7% under different annealing conditions. The optical band gap of CdS was estimated using the plot of (ahv)2～hv, and the Eg value was deduced by extrapolating the linear portion of this plot to the hv axis. It can be seen that the energy band gap values for as-deposited and annealed samples were～2.28 eV and～2.34 eV, respectively. A shift of about 0.06 eV in the value of Eg of the CdS films associated with the lower reflectance was observed the after annealing treatment.Rectification and electron transport mechanism of CdS/Si-NPA. Two-side electrodes were deposited to carry out the electrical measurements, and comb-like electrode pattern was adopted on top to reduce the contact coverage. Here CdS/Si-NPA showed a clear rectification behavior, with an onset voltage of～3.4 V at a current density of～1.3 mA cm-2, a forward current density of～93.6 mA cm-2 at～7.4 V, a leakage current density of～0.63 mA cm-2 at a reverse bias of～7.4 V, a breakdown voltage of～8 V. The forward-to-reverse rectifying ratio was calculated to be～149 at±7.4 V. The conduction mechanism of the device under positive applied voltage can be explained by space-charge-limited conduction (SCLC) model. According to the results, it was the interface states that controlled the carrier transport process.Photovoltaic effect of CdS/Si-NPA. The photovoltaic (PV) properties of the device were measured under an ORIEL solar simulator with 1 sun AM 1.5G illumination. Under illumination, the device exhibited an open circuit voltage (Voc)～265 mV, a short circuit current (Isc)～1.32μA, a fill factor (FF)～32.8%, a series resistance (Rs)～107702Ω, a shunt resistance (Rsh)～296458Ωand an efficiency～1.15×10-4%. The small photocurrent of the device in spite of the high absorbance in Si-NPA can be attributed to the high Rs and the large quantities of interface states at the junction. The Rs generally attributed to the bulk resistance of the semiconductor material, metallic contacts and interconnects. Here the Rs was mainly attributed to the high bulk resistance of the CdS films deposited by CBD process. If the Rs decreased to 100Ω, the efficiency of the device may increased up to～0.12%, which indicated that CdS/Si-NPA might be a promising structure for photovoltaic application.