Strategies for Performance Improvement of Quantum Dot Sensitized Solar Cells

Quantum dot sensitized solar cells (QDSCs) constitute one of the most promising low-cost solutions that currently are explored for the world’s needs of clean and renewable energy. Efficient, low-toxic and stable QDSCs for large-scale applications have therefore formed the subject for much of the solar cell research during recent years. This circumstance also forms the basic motivation for this thesis, where the results of my studies to improve the efficiency and stability of green QDSCs are presented and discussed. The surface condition of quantum dots (QDs) is always crucial to the performance of QDSCs, since surface ligands can influence the loading amount of QDs on photoanodes, and that the surface defects can induce charge recombination in the solar cells. In this thesis work, a hybrid passivation approach was firstly utilized to improve the photovoltaic performance of CdSe QDs. After hybrid passivation by mercaptopropionic acid (MPA) and iodide ions, the loading efficiency of the QDs was increased with the ligands of MPA, and the surface defects on the QDs were reduced by the iodide ions, both aspects contributing to a dramatic enhancement in the efficiency of the CdSe based QDSCs. This hybrid passivation strategy was then employed for lowtoxic CuInS2 QDs, and was also demonstrated as an effective way to modify the surface state of the CuInS2 QDs and improve the performance of the QDSCs based on CuInS2 QDs. To improve the stability of the QDSCs, solid state quantum dot sensitized solar cells (ss-QDSCs) based on CuInS2 QDs were investigated. In addition to the hybrid passivation, increasing the pore size of the TiO2 active layer and changing the composition of the CuInS2 QDs were also found to be useful approaches to improve the performance of the ss-QDSCs based on CuInS2 QDs. Furthermore, for the most used hole transport material2,2 ́,7,7 ́-tetrakis(N,N-di-p-methoxyphenylamine)-9,9 ́-spirobifluorene (Spiro-OMeTAD) in solid state solar cells, silver bis(trifluoromethanesulfonyl)imide was shown to be an effective p-type dopant to increase its conductivity and to improve the performance of the solar cells based on it.