A Systematic Investigation Of The Surface Or Interface Modification Of The Photoanodes In Cds/CdSe Quantum Dot Co-sensitized Solar Cells

Developing low cost and high performance solar cells for harvesting and converting solar energy to electricity is one of the most promising technologies to meet the imperative societal need for sustainable clean energy with minimal or no detrimental environmental impact.Over the past decades,quantum dot sensitized solar cells(QDSCs)due to its good stability,easy synthesis and high power conversion efficiency have been investigated to meet the demand for low cost and highly efficient solar power conversion.However,the QDSCs still suffering from high charge recombination and low light harvesting which seriously inhibit the power conversion efficiency.In this dissertation,we focus on enhancing light harvesting and reducing charge recombination via surface or interface modification of the photoanodes in TiO2/CdS/CdSe QDSCs.At the same time,the influence of the surface or interface modification on the behavior of light harvesting,charge separation,transport and recombination as well as the power conversiton efficiency were studied.We provide theoretical and technical support for constructing QDSCs with high power conversion efficiency.The specific research contents and results in chapter 3-6 were described in detail.The major findings are as follows:1.The surface passivation layer in quantum dot sensitized solar cells(QDSCs)plays a very important role in preventing surface charge recombination and,thus,improving the power conversion efficiency.Taking the advantages of a wide light absorption range,superior electron transfer properties,good stability,easy synthesis,CdS/CdSe co-sensitized QDSCs have attracted lots of attention in the research community.We demonstrated the introduction of a ZnSe passivation layer prepared with a successive ionic layer absorption and reaction(SILAR)method in CdS/CdSe co-sensitized solar cells,though not likely in the ideal form of a conformal overlayer,have significantly enhanced the power conversion efficiency.Not only can the ZnSe passivation layer reduce surface charge recombination,but can also enhance the light harvesting.The short-circuit current density,open-circuit voltage,fill factor,and the corresponding photovoltaic conversion efficiency were all significantly improved with the introduction of a ZnSe passivation layer but varied appreciably with the layer thickness.When three SILAR cycle layer was applied,the power conversion efficiency is as high as 6.4%,which is almost doubled the efficiency of 3.4% for the solar cell without ZnSe passivation layer.2.ZnS is the most widely used passivation materials in QDSCs,and we demonstrated ZnSe is also a good passivation material.We give a systematic characterization and comparison of optical and electrochemical properties of passivation layers ZnS and ZnSe and their impacts on the performance of the resultant QDSCs have been investigated.The ZnS and ZnSe passivation layers were all deposited via a reproducible and controlled successive ionic layer absorption and reaction method.QDSCs with ZnSe passivation layer demonstrated with strongly inhibited interfacial charge recombination,with much enhanced light harvesting,resulting in a power conversion efficiency up to 6.4%,which is appreciably higher than 4.9% for the solar cells with a ZnS passivation layer and 3.4% for the solar cells without passivation layer.After carefully contrast,both ZnS and ZnSe have higher conduction band edges than QDs,thus,it can prevent the transfer of electrons from QDs and TiO2 to the oxidized form of the redox couple in the electrolyte.However,the valence band edge of ZnS is lower than ZnSe and QDs,which would undesirably hindering the regeneration of QDs.This work demonstrated that ZnSe would be a good alternative to ZnS as a passivation material.And this work is of great importance for choosing efficient passivation materials.3.Most of the reported power conversion efficiency(PCE)is still lower than 7% due to serious charge recombination and a low loading amount of QDs.Therefore,suppressing charge recombination and enhancing light absorption are still required to further improve the performance of QDSCs.We demonstrated successful design and fabrication of QDSCs with a high efficiency of 7.24% based on CdS/CdSe QDs with two ZnSe layers inserted at the interfaces between QDs and TiO2 and electrolyte.The effects of two ZnSe layers on the performance of the QDSCs were systematically investigated.The results indicated that the inner ZnSe buffer layer located between QDs and TiO2 serve as a seed layer to enhance the subsequent deposition of CdS/CdSe QDs,leading to higher loading amount and covering ratio of QDs on the TiO2 photoanode.The outer ZnSe layer located between QDs and electrolyte behave as an effective passivation layer which not only reduces the surface charge recombination,but also enhances the light harvesting.4.ZnSe deposited via successive ionic layer adsorption and reaction(SILAR)method onto TiO2/CdS/CdSe photoanode has been proven as an effective passivation layer to suppressing charge recombination and enhancing power conversion efficiency in quantum dot-sensitized solar cells(QDSCs).However,the device performance varies appreciably with the deposition process as the chemical identity and the interfacial structure between the passivation layer and the quantum dots and electrolytes have retained quite some unanswered questions.The present paper reports the significant impacts of ZnSe passivation layer with different surface or interface chemistry on the performance of CdS/CdSe QDSCs.The photovoltaic properties show that the performance of assembled cells has a strong dependence on the SILAR immersion sequences started with Zn2+ or Se2-.When Zn2+ was initially deposited,the unintentionally formed QDs/ZnSe/Se/SeO2 structure with a large amount of ZnSe leads to a significant increase in the photovoltaic properties.When Se2-was first deposited,most of the Se2-absorbed on the surface of the photoanode would be oxidized to form Se0 and SeO2,with a small fraction of ZnSe formed.The resulted QDs/Se/SeO2/ZnSe structure leads to a drastic decrease of the solar cell performance.