Research On The Zinc Oxide Nanoparticles And Their Application In The Organic Solar Cells

The organic solar cells (OSCs) are attractive as a new renewable energy source for the low-cost, flexible, lightweight, large-area, and facile manufacturing. In recent study, inverted organic solar cells (IOSCs) with a transparent electron-collecting electrode have received a lot of attention due to its high air stability and free vacuum coating processes. In the structure of IOSCs, an n-type metal oxide film such as zinc oxide (ZnO) nanoparticles is inserted as an electron transport layer. Especially, ZnO nanoparticles are considered as a promising candidate due to its relatively high electron mobility, environmental stability, high transparency and facile preparation.However, ZnO nano-structured materials have large specific surface area and high density of defects. These defects may affect the life of the carriers and the transfer of electronics, which will affect the device performance. This paper studies the effects of ZnO nanomaterials defects on solar cells (ITO/ZnO/P3HT:PCBM/Mo03/Ag) performance.Firstly:we fabricated ZnO nanoparticles films annealing at different temperature (80℃,120℃,150℃,200℃) to study the effect of ZnO surface states on the cell performance. Experimental results show that hydroxyl groups can be effectively detached from ZnO film by annealing. Hydroxyl groups detach more with increasing annealing temperature, resulting in less degradation of the active layer, which will help to the improvement the device performance. But annealing at a very high temperature, such as200℃, the contact barrier between ZnO nanoparticle film and ITO glass becomes larger, which reduces the performance of device. Therefore, an appropriate annealing temperature is important to improve the cell performance.Secondly:reaction at different reaction temperature (0℃,30℃,60℃) lead to different sizes of the ZnO nanoparticles, which result in the change of surface defect density. The experimental results show that ZnO nanoparticle size becomes large with increasing reaction temperature. Therefore, the defect density decreased with the increasing reaction temperature. That reduces the number of electrons trapped so that the electrons can be effectively exported. We can conclude that the increase of ZnO nanoparticle size can effectively improve the device performance.Thirdly, the type of defect on ZnO nanoparticles surface are changed when altered the concentration ratio of zinc acetate ((CH3COO)2Zn-2H2O) and lithium hydroxide (LiOH).The Ratio of reactants are1:1,1:2,1:3.5, respectively.The experimental results showed that the type of defect is considered oxygen interstitial (Oi) which is shallow acceptor defect when the [Zn2+/Li-] concentration ratio is1:3.5.The other two are regarded as oxygen vacancies (Vo), which is deep donor defect. Experiments show that shallow acceptor defect is possible to directly capture electron, while the deep donor defect captures electrons after ionization. Ionization energy of deep level is very large, so their ability of capture electrons is weaker. The experiment results demonstrate the shallow acceptor (Oi) is disadvantage to the transfer of the carriers.