Study On Two-layered Structure Solar Cell Based On The ZnO Nanocrystal/polymer Composites

Heterojunction solar cells with the structure of inorganic-nanocrystal/ polymer not only have inorganic nanocrystals` advantages of high carrier mobility, outstanding chemical stability,but also have the characteristics of polymer,which is flexible,processable as well as high absorption coefficient.As a result,it has attracted more and more attention.In this thesis,we focused on ZnO nanocrystals and polymers and studied the impact of ZnO nanocrystalline layer on the performance of photovoltaic devices with the bilayered structure of ZnO/MEH-PPV and ZnO/P3HT.Results are listed as follows.1.The stucture and size of the ZnO nanocrystals were characterized using X-ray powder diffraction(XRD) and transmission electron microscope(TEM).The absorption spectrum of pure MEH-PPV film and the composite bilayer MEH-PPV/ZnO film were compared.It is found that the absorption spectrum of MEH-PPV/ZnO bilayer film is significantly broadened,and the peak of ZnO nanocrystals is also observed.In addition, we fabricated heterojunction solar cell with bilayered structure of ITO/PEDOT: PSS/MEH-PPV/ZnO/Al.It is found that the short-circuit current density(J_SC) and power conversion efficiency(η) of the bilayered solar cell are improved significantly as compared to that of the pure polymer solar cell.This indicated that the induction of ZnO nanocrystals layer could enhance the exciton dissociation and carrier transportation.For the sake of comparision,the device with the structure of ITO/PEDOT:PSS/ZnO/ MEH-PPV/Al was also fabricated.However,the photovoltaic properties of the device with such a structure were very poor.Which may be resulted from the formation of a dipole layer at the interface between MEH-PPV and ZnO nanocrystals.2.The effects of different annealing temperatures on the photovoltaic performances of MEH-PPV/ZnO bilayered solar cells were studied.The results indicated that higher annealing temperature can improve the photovoltaic performance of the bilayered devices.The improvement can be attributed to the crystallization of the polymer at higher temperature and formation of a lot of ordered regions,which make the formation of better composite film.The Atomic Force Microscopy(AFM) results of the films under different annealing temperatures further confirmed the conclusion above. The results showed that the film annealled at higher temperature became more smooth.3.We used P3HT and ZnO nanocrystal as the active layer to fabricate the bilayered solar cell device with the structure of ITO/PEDOT:PSS/P3HT/ZnO/Al.It is also found that the photovoltaic performances(J_SC andη) of the device was improved as compared to that of the pure P3HT solar cell device,which further demonstrates that the introduction of ZnO nanocrystal layer can enhance the photovoltaic performance. Furthermore,it is also observed that the power conversion efficiency of the device based on P3HT/ZnO is higher than that of the device based on MEH-PPV/ZnO.This may result from the difference of the polymer chains,and this indicates that polymers with different structure have much effects on the efficiency of heterojunction solar cell device.