Morophology Controlled Synthesis Of Nanocrystals And The Investigation Of Nanocrystal Polymer Hybrid Solar Cells

Nanocrystal has become one of the promising alternatives for application in solar cells due to its novel absorption ability in UV visble and near infrared area and its convenient properties for charge transportation. However, the photovoltaic performace of the solar cells based on nanocrystals are still far from that of the conventional silicon based solar cells. Much more attentions should be focused on the morphology controlled synthesis of the narrow bandgap nanocrystals and the optimization of fabrication process for the nanocrystals based solar cells.Based on the facile solvent injection method, high quality PbSe nanocrystals with different morphologies were synthesized in nitrogen atmosphere. The synthetic process of PbSe nanocrystals, including the reaction ligand, reaction time and the ligand concentrations were systematically investigated. Accompanying with comprehensive analysis, the discussion on the possible mechanisms for the shape evolution of PbSe nanocrystals was provided. The microreaction was conducted to obtain the monodispersed spherical CdSe nanocrystals within seconds. Together with the obtained PbSe nanocrystals, CdSe nanocrystals were applied to fabricate hybrid solar cells with inverted structure. The adjustments of the fabrication parameters such as thickness of the active layer and the annealing conditions were used to obtain an optimized solar cell performance. The achieved results are stated as follows:(1) Morphology synthesis of PbSe nanocrystalsThe single ligand (oleic acid (OA)) and coligand (TDPA/TOPO) systems were developed to prepare PbSe nanocrystals; the concentration of the ligand was demonstrated having a key effect on the size and shape of PbSe nanocrystals. When OA was served as single ligand, the high crystallized PbSe nanocrystals with face centre cubic structures were prepared. The evolution of the PbSe from nanospheres to nanoflowers and finally to nanocubes was achieved by increasing reaction time. Furtherly, the shape variation from nanospheres to polyhedrons was readily realized through the increase of OA concentration in the stock solution. More interestingly, the change of anion ligand was proven to be a facile method to control the structure and size of the nanoflowers. When TDPA/TOPO were served as coligand, the different shapes of PbSe nanocrystals such as rice shape, chain like clusters, tripods and nanocubes were obtained by adjusting the TDPA concentration. (2) Design of the inverted structure hybrid solar cellsThe conventional organic solar cell structure includes a transparent indium tin oxide (ITO) layer for hole collecting, a poly(3,4-ethylenedioxythiophene):poly(sty-renesulfonate)(PEDOT:PSS) layer for hole transportation, a bulk heterojunction active layer, and a metal electrode layer for electron collecting(ITO/PEDOT:PSS/Active layer/Al). However, the unstable interface between PEDOT:PSS and ITO, especially under humid conditions, and the degraded conductivity of ITO at high temperature deteriorate the PCE. The inverted structure (FTO/TiO2/Active layer/PEDOT:PSS/Ag) with TiO2as an electron selective layer and with PEDOT:PSS as a hole selective layer was established to provide a stable and enhanced solar cell performance.(3) CdSe based nanocrystal polymer hybrid solar cellsBased on the enhanced heat and mass transfer of the microreactions, high quality spherical CdSe nanocrystals were obtained. The cyclic voltammetry (CV) was used to estimate the HOMO and LUMO values of the CdSe nanocrytals, and the results was consistent with those obtained from optical spectra with a reasonable deviation. After pyridine exchanging, the CdSe nanocrystals were applied to fabricate nanocrystal polymer hybrid solar cells. A ternary system hybrid solar cells that are composed of CdSe nanocrystals, poly (3-hexylthiophene)(P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) with an inverted structure were investigated. The incorporation of CdSe showed increased power conversion efficiency (PCE) compared with that of a binary system with P3HT and PCBM. Through optimizing the solar cell fabrication parameters such as active layer thickness (80nm), and annealing temperature (150℃), an optimized PCE of3.05%was achieved, which is comparable with the record efficiency of shape-tailored CdSe based solar cells. The stability of the fabricated solar cells was studied through exposure in ambient condition without any encapsulation. Over70%of its original efficiency was remained, which shows great potential for future applications.(3) PbSe based hybrid solar cellsBased on the highly crystallized PbSe nanocrystals, inverted hybrid solar cells that composed of P3HT/PbSe and P3HT/PbSe/PCBM were investigated. Through process optimization, the PCE of0.19%was obtained for P3HT/PbSe solar cells. The PCE of2.09%was obtained for P3HT/PbSe/PCBM solar cells, which is a little bit higher than that of without PbSe nanocrystals.