Surface modification of semiconductor nanocrystals and their applications in photovoltaic cells

Abstract

Harvesting the sunlight and converting it to electricity using cost-effective and efficient solar cells are challenges of the twenty-first century. Currently, the high cost in monocrystalline Silicon (Si) fabrication and Shockley-Qusisser limit prevent the further development of Si solar cells. One of the approaches to solve these technological problems is incorporation of nanomaterials such as nanoparticles and nanowires into the architectures of solar cells. The motivation of this research work originates in the fact that nanostructured materials are unique with large surface areas, while their surface engineering being important for effective photon adsorption in a wide range of light wavelengths. Hence, introduction of nanomaterials into the photovoltaic (PV) cells leads to new concepts of devices and enhances their performances. Due to the critical roles of nanomaterial surfaces in designed devices, the effect of surfaces for semiconductor nanomaterials, such as silicon nanowires (SiNWs) has been investigated. In this respect, the thesis demonstrates generation of Si-H bondings at SiNW surafces via hydrofluoric acid (HF) treatment. The saturated bondings at nanowire surfaces lead to dye degradations under sonications and have catalyst-like properties, like catalyst. Consequently, controlling the surface to do suitable modifications is a universal way to tune the properties of nanostructures. This is important for surface chemistry and also crucial for further development of electronic devices, including PV cells constructed by nanomaterials. Based on aforesaid study of SiNW surfaces, this thesis has been focused on some surface modifications of semiconductor nanostructures to modulate their properties and utilize them into novel hybrid solar cells. Herein, nanowires of cost-effecive zinc oxide (ZnO) were investigated, and their surfaces were modified with noble nanoparticles. These attached nanoparticles enhance the optical absorption in the range of visible light due to the surface plasmon resonance (SPR). In charge separations, photoexcited electrons are transferred from noble nanoparticles to the ZnO conduction band, while electrons from donor (I-) in the electrolyte compensate the holes left on the nanoparticles. The fill factors (FF) of the cells reach a value of ~0.50. Furthermore, after incorporation of Ruthium (Ru) complex dye into a hybrid solar cell, the open circuit voltage increases to 0.63 V from 0.5 V which is measured for dye-sensitized solar cells (DSSCs) based on the bare ZnO nanowires. The Schottky barrier at the ZnO semiconductor and noble metal nanoparticle interfaces blocks the electron transfer back from ZnO to the dye and electrolyte, and thus increases the electron density at the ZnO conduction band. The photo-conversion efficiency for this type of hybrid solar cells is thus increased from 0.7 to 1.2%. Despite the use of novel hybrid solar cells are constructed based on surface modified ZnO nanowires, the structures still have some disadvantages. Firstly, aggregation of dye molecules may form on the surfaces of ZnO nanowires. Secondly, a part of ZnO exposed to the electrolyte solution may lead to corrosion. An inorganic semiconductor coating can replace conversional dye to form a protective layer and simultaneously work as a sensitizer layer. For this function, we have chosen narrow bandgap semiconductor, cadmium telluride (CdTe), to enhance the optical absorption in wide range of incident light. Photo-excited electrons can be transferred from the conduction band of the CdTe shell layer to ZnO nanowires. The detailed electronic structures of ZnO/CdTe were investigated and compared with ZnO/Dye interface using ultraviolet photoemission spectroscopy (UPS). The analysis shows electric dipole at ZnO/dye interface due to Zn2+/carboxy bonding while no significant bonding existed at ZnO/CdTe interface. However, the excited electrons can be transferred to the conduction band of ZnO nanowires through defect levels at the small heterojunction barrier of ZnO/CdTe. The average stable short circuit current density of the CdTe sensitized photochemical solar cell reaches 4.69 mA/cm2, while it is 3.90 mA/cm2 for in-situ bare ZnO nanowire constructed DSSCs. Based on our investigations, all designed solar cells contain liquid electrolyte. Finally, all solid based solar cells based on one dimensional (1-D) semiconductor nanostructures were constructed. Vertically aligned p-type gallium nitride (GaN) nanowires were grown on n-type Si substrates by a chemical vapor deposition (CVD) method to form heterostructures in solar cell devices. The representative p-type GaN nanowire/n-Si heterojunction cell shows a well-defined rectifying behavior with a rectification ratio larger than 104 in dark. The cell has a high short-circuit photocurrent density of 7.6 mA/cm2 and energy conversion efficiency of 2.73% under AM1.5G illumination (100 mW/cm2). Moreover, the nanowire arrays work as antireflection coatings in solar cells. They effectively decease the energy loss due to the reduced light reflection from Si wafer. This study provides experimental demonstrations for integrating 1-D nanostructure arrays to fabricate new kinds of hybrid photovolatic cells. The further improvement of solar cell efficiency can be realized by combining aforesaid solar cell constrcutions, forming large surface areas with core-shell p/n junctions, like p-GaN/n-Si heterostructures. The ZnO nanowires/nanorods patterned FTO can be employed as cheap substrates. The new hybrid cell structure may lead to breakthrough in conversion efficiencies, while reduce the cost of devices compared with the PV cells based on crystalline Si wafers.