Effects Of SAMs Interface Modification On The Performance Of CdS Quantum Dots Sensitized ZnO Nanorods Solar Cells

Quantum dots sensitized solar cells(QDSSCs) become one of the third generation solar cells due to their low cost and high theoretical conversion efficiency. The power conversion efficiency of QDSSC is always lower than that of dye-sensitised solar cells(DSSCs) due to its low quantum dots loading amount and high interface electronic-hole recombination rate. The surface modification of photoanode is an effective approach to improve the photoelectric conversion efficiency of QDSSCs. In this thesis, we chose different kinds of self-assembled monolayers(SAMs) to modify the interface of ZnO photoanode and investigated their effects on the optical properties of ZnO and photovoltaic performance of QDSSCs. The relative influence mechanism was revealed. The innovative research results are as follows:Firstly, we fabricated the primary and secondary growth of ZnO nanorods by a two-step CBD method, which were used as photoanode to fabricated QDSSCs. The results show that the power conversion efficiency of QDSSCs with secondary growth of ZnO nanorods exhibits a 41% enhancement compared to the ZnO nanorods with primary growth. That is because: the length of ZnO nanorods increases and the surface area of ZnO nanorods array on unit area substrate turns larger so that the loading amount of CdS QDs on the surface increases. Thus, more photoelectron can be produced under the same sunlight and the short circuit current density increase as well. As a result, the photoelectric conversion efficiency of QDSSCs exhibits an enhancement.Secondly, we used the soaking method to deposit 3-PPA SAMs on the surface of ZnO nanorods and study the effect of 3-PPA ethanol solutions concentration and immering time on the photovoltaic performance of QDSSCs. The best photovoltaic performance comes from the CdS/3-PPA(10mM,1min)/ZnO solar cell, which shows the higher power conversion efficiency of 1.36%. This is because: the 3-PPA modification suppresses the recombination process of the electron-hole due to its passivation to the surface defects. The results of UPS reveal that the 3-PPA layer can form an energy barrier between ZnO and CdS to efficiently retard the back transfer of electrons.Thirdly, we employ 3-PPA, BPA and APPA as SAMs, who owns the same phosphonic acid headgroup but different tail group, to modify the surface of ZnO nanorods photoanode. The results reveal that the power conversion efficiency of CdS/3-PPA/ZnO is 1.41%, but the power conversion efficiency decreased by using BPA and APPA as SAMs. This is because the SAMs has different tailgroups, the tailgroups of 3-PPA, BPA and APPA are-COOH,-CH3 and-NH2, respectively. The-COOH is electron-withdrawing groups, so the effective work function of ZnO modified with 3-PPA was reduced, so that an energy barrier has been formed between CdS and ZnO, which is very efficient on retarding the back transfer of electrons. However, the-CH3 and-NH2 are the electron-donating groups, increased the work functions of the surface of ZnO, which is not good for the transmission of photoelectron.Finally, we employed three SAMs derived from benzoic acid such as MBA, BBA and FBA, which owns the same carboxylic acid headgroup but different tail group, to modify the surface of ZnO nanorods. The interface dipole of MBA/ZnO and BBA/ZnO are directed toward the ZnO surface and the work function of MBA/ZnO and BBA/ZnO are decreases, so that a energy barrier has been formed between CdS and ZnO, which is very efficient on retarding the back transfer of electrons, which results in the enhancement of conversion efficiency of solar cells. But the interface dipole of FBA/ZnO is shifted away from the ZnO surface, so the work function of FBA/ZnO is increased, the band offsets between ZnO and CdS is increased, which is not good for the transmission of photoelectron, which results in the lower photovoltaic conversion efficiency.