Fabrication Of Ordered ZnO Nanorod Arrays,Interface Modified And Their Application In ZnO/Cu₂O Heterojunction Photovoltaic Cells

The separation and collection efficiency of photon-generated charge carriers play an important role in the performance of thin film photovoltaic（PV）cells.With the development of nano-technology,scientists hope to enhance the carrier separation and collection efficiency through constructing nano-structure PV cells.Constructing nano-structure heterojunction,for example,introducing nanorod arrays,nano-cone array or nano-cavity array composite structure,can significantly increase the carrier separation and collection efficiency in the PV cells.Among them,the nanorod composite structure can construct a radial heterojunction which reduce the carrier separation distance without sacrificing optical absorption,therefore,it has a potential application in poor PV materials.In this paper,the Zn O nanorod arrays were chosen as the study objects,and the ZnO/Cu₂ O composite structure PV cells were prepared for studying and understanding the influence of nanorod geometric parameters and surface properties on the device performance.The main research contents can be divided into the following sections:（1）Position-controlled hydrothermal growth of ordered individual ZnO nanorod arrays on Indium Tin Oxide substrate.Ordered individual ZnO nanorod arrays on indium tin oxide（ITO）substrate were fabricated combining both top-down and bottom-up technologies.The traditional patterning mask method was modified by employing a thicker poly（methyl methacrylate）（PMMA）aperture template.Ordered Zn O nanorod arrays were hydrothermally grown with the assistance of patterned PMMA template,and the final ZnO nanorod arrays showed perfect alignment.Finally,we studied the electrical properties of the ZnO nanorods.The high conductive properties between ZnO nanorods and ITO substrate suggest that these ordered ZnO nanorod arrays on conductive ITO substrate may offer important advantages for ZnO nanorod-based photoelectric devices.（2）Constructing ZnO nanorod-Cu₂ O composite thin film PV devices.In this section,the Cu₂ O films are prepared by electrodepostion.The morphology,structure,optical and surface photovoltaic properties of the Cu₂ O films are systematically studied.Through contrasting the optical absorption depth and carrier diffusion length of Cu₂ O,we can learn the restricting factors for the performance of bilayer structure ZnO/Cu₂ O cells.Through the introduction of random ZnO nanrod arrays in composite structure ZnO/Cu₂ O cell,a high carrier collection region aroud the ZnO nanorods can be constructed,where Cu₂ O and ZnO nanorods could form a radial heterojunction with a short carrier collection distance.The random ZnO nanorod-Cu₂ O composite cell can obtain a higher short-circuit current and power conversion efficiency.However,a lower open circuit voltage also displayed,this is mainly due to the introduction of large interface charge carrier recombination.In order to weaken the influence of interface recombination on open-circuit voltage,at the same time to increase the absorbing material filling volume ratio in high carrier collection region.Therefore,the ordered Zn O nanorod arrays are employed to build ordered nanorod composite film PV device.The ordered nanorod cell show a greatest short-circuit current and a close open-circuit voltage to bilayer cell,finally can get the highest power conversion efficiency.Therefore,we think that the optimization of nanorods spacing is an effective way to raise performance of nanorod composite thin film PV devices.（3）Improving the performance of ZnO-Cu₂ O PV cells by modifying heterojunction interface quality.ZnO surface etchinig in alkaline solution can be inhibited by dissolving ZnO powder in precursor electrolyte.This can produce a improved heterojunction quality,and reduce the interface state density.The cells with Cu₂ O prepared in buffered electrolyte have a higher short-circuit current,open-circuit voltage and fill factor,the power conversion efficiency can reach 0.76 %.The analysis about heterojunction ideal factor shows that the device performance improvements are caused by the optimization of heterojunction interface quality.Therefore,we think that this is a effective way to optimize the electrodeposited Cu₂O/ZnO interface quality,and further to improve the performance of PV devices.The main innovation points of this thesis are:（1）The successful preparation of ordered ZnO nanorod arrays on non-epitaxial conductive polycrystalline conductive substrate;（2）A detailed study of the optical and surface photovoltaic properties of electrodeposited Cu₂ O films;（3）Constructing ordered Zn O nanorods-Cu₂ O heterjunction PV cell,reducing the influence of random morphology on open-circuit voltage;（4）Reducing the etching on ZnO surface in the electrodeposition Cu₂ O process by employing buffered electrolyte,enhancing the heterojunction quality.