Fabrication And Properties Of ZnO/CdTe Core-Shell Nanorod Array Solar Cell

Low-cost, high performance photovoltaic device is very important for large-scale application of solar energy. Among various solar cells, well aligned core-shell nanorod array solar cell (NRASC) can enhance the light absorption through anti-reflation and light trapping. Specially, core-shell nanorod array architecture allows the light absorption in the axial direction and charge separation in the radial direction, enabling strong light absorption and fast charge separation. Meanwhile, NRASC provides the direct transport pathways for separated charge carriers for efficient charge transport and collection. Moreover, the NRASC can be produced by the materials with short minority diffusion length, provide an alternative choice for low-cost and high-performance photovoltaic devices. However, the performance of NRASC is not as high as expected yet. As a consequence, the systematical investigations on the interfacial treatment and material optimization are imperatively necessary.N-type ZnO NR array has been widely used as the electron transport component because of the high electron mobility and facile synthesis processing. P-type CdTe is exceptionally promising for photovoltaic devices because of the nearly ideal band gap for single-junction photovoltaics (～1.5 eV) and high optical absorption coefficient. Moreover, a type-II energy band alignment will be formed between CdTe and ZnO, which can nicely facilitate the separation of the photo-induced electron-hole pairs. In this dissertation, we focus on the low-cost fabrication of solid-state ZnO/CdTe NRASC, and improving the performance of ZnO/CdTe NRASC by modifying the interface and optimizing the process of the solar cells.In chapter one, we briefly introduce the basic operating principles and the characterization parameters of solar cell, as well as some factors affecting the photovoltaic performance of the device. We also summarize the advantages and challenges for the nanorod array solar cells.In chapter two, we present the systematically investigations on the improvement of ZnO/CdTe core-shell NRASC using a thin CdS interfacial layer. All-inorganic solid-state ZnO/CdTe core shell nanorod array solar cells have been fabricated by depositing p-type CdTe onto the surface of hydrothermally grown ZnO nanorod using a simple low temperature and low cost solution-based successive ionic layer adsorption and reaction (SILAR) method. We introduce a thin CdS layer at the ZnO/CdTe interface using SILAR method and investigate the influence of the CdS interfacial layer with different thicknesses on the performance of the solar cells. It is found that the overall power conversion efficiency of the ZnO/CdS/CdTe core-shell NRASC with a 4 nm thick CdS interfacial layer can become three times that of the ZnO/CdTe NRASC under AM 1.5G illumination (100 mW/cm2). The improvement of the performance is attributed to the graded energy band alignment of ZnO/CdS/CdTe system and the passivation of the surface defects of ZnO nanorod by CdS layer, which enhance the charge separation and reduce the electron-hole recombination, therefore improve the short circuit current density and the open circuit voltage.In chapter three, we focus on the study of the performance enhancement of the ZnO/CdTe core-shell NRASC through interface passivation with a TiO2 layer. We introduce a thin amorphous TiO2 layer at the ZnO/CdTe interface using atomic layer deposition and investigate its effect on the photovoltaic performance of the NRASCs. A series of experiments show that the thin TiO2 layer, serving as an efficient passivation and blocking layer at the interface of ZnO/CdTe, can remarkably suppress the charge recombination at the interface but negligibly affect the light absorption and the charge separation efficiency, thus leading to significant increases of the carrier lifetime and the open-circuit voltage of the NRASCs. The overall power conversion efficiency of the ZnO/TiO2/CdTe core-shell NRASC becomes six times that of the ZnO/CdTe NRASC under AM 1.5G illumination (100 mW/cm2).In chapter four, we optimize the performance of ZnO/CdTe NRASC. First, we investigate the influence of CdCl2 treatment on the crystallization of CdTe shell and the performance of solar cells. Second, we find that the first cycle sequence of the SILAR process has notably effect on the performance of ZnO/CdTe NRASC and the device fabricated with Te->Cd sequence has much better performance. Third, we investigate the effect of annealing temperature on the solar cell performance and find that ZnO/CdTe NRASC annealed at 350℃ has the best performance, the overall power conversion efficiency can reach up to 3.78%. Lastly, we investigate the time dependence of NRASC performance and find the NRASCs having good stability.In chapter five, we retrospect the failure and success in our investigations, and present brief perspectives on the future research.