Semiconductor nanocrystals, also referred to as quantum dots (QDs), have been the focus of great scientific and technological efforts in solar cells, as a result of their advantages of low-cost, photostability, high molar extinction coefficients and size-dependent optical properties. Due to the multi-electron generation effect, the theoretically maximum efficiency of quantum dots-sensitized solar cells (QDSCs) is as high as 44%, which is much higher than that of dye-sensitized solar cells (DSCs). Thus QDSCs have a clear potential to overtake the efficiency of all other kinds of solar cells.

In recent years, the efficiency of QDSCs has been improved very quickly to around 5%. It is however still much lower than that of DSCs. The low efficiency is mostly caused by the high electron loss between electrolyte and electrodes and the lack of an efficient electrolyte. In this thesis, we have been working to enhance the performance of QDSCs with II-VI group nanocrystals by increasing the electron injection efficiency from QDs to TiO2 and developing new redox couples in electrolyte.

To increase the electron injection, firstly, colloidal ZnSe/CdS type-II QDs were synthesized and applied for QDSCs for the first time, whose photoelectron and photohole are located on CdS shell and ZnSe core, respectively. The spatial separation between photoelectron and photohole can effectively enhance the charge extraction efficiency, facilitating electron injection, and also effectively expand the absorption spectrum. All these characteristics contribute to the high photon to current conversion efficiency. Furthermore, a comparison between the performances of ZnSe/CdS and CdS/ZnSe QDs shows that the electron distribution is important for the electron injection of the QDs in QDSCs. Secondly, colloidal CdS/CdSe quantum rods (QRs) were applied to a quantum rod-sensitized solar cell (QRSCs) that showed a higher electron injection efficiency than analogous QDSCs. It is concluded that reducing the carrier confinement dimensions of nanocrystals can improve electron injection efficiency of nanocrystal sensitized solar cells.

In this thesis, two types of organic electrolytes based McMT-/BMT and TMTU/TMTU-TFO were used for QDSCs. By reducing the charge recombination between the electrolyte and counter electrode, fill factor (FF) of these QDSCs was significantly improved. At the same time, the photovoltages of the QDSCs were remarkably increased. As a result, the overall conversion efficiency of QDSCs based on the new electrolytes was much higher than that with a commonly used inorganic electrolyte.

In addition, CdS QDSCs on NiO photoelectrode were studied which shows a n-type photovoltaic performance. This performance is attributed to the formation of a thin Cd metal film before CdS QDs formation on NiO. Since the CB edge of CdS sits between the Fermi level and the CB edge of Cd metal, a much strong electron transfer between Cd and CdS QD is obtained, resulting in the observed n-type photovoltaic performance of these CdS/NiO QDSCs.

In this thesis, I have been working with the development of nanoparticle sensitized solar cells. In the subarea of quantum dot sensitized solar cells (QDSCs), I have investigated type-II quantum dots (QDs), quantum rods (QRs) and alloy QDs, and developed novel redox couples as electrolytes. I have also proposed upconversion nanoparticles as energy relay materials for dye-sensitized solar cells (DSCs).

Colloidal ZnSe/CdS type-II QDs were applied for QDSCs for the first time. The interesting features of those refer to that their photoelectrons and photoholes are located on the different parts of the dot, namely in the CdS shell and in the ZnSe core, respectively. That spatial separation between photoelectrons and photoholes can so effectively enhance the charge extraction efficiency, thus facilitating the electron injection, and also effectively expand the absorption spectrum. All these characteristics contribute to a high photon to current conversion efficiency. Furthermore, a comparison between the photovoltaic performance of ZnSe/CdS and CdS/ZnSe QDSCs shows that the electron distribution is important for the electron injection of the QDs.

Colloidal CdS/CdSe QRs were applied to quantum rod-sensitized solar cells (QRSCs). They showed a higher electron injection efficiency than the analogous QDSCs. It is concluded that reduction of the carrier confinement dimensions of the nanoparticles can improve the electron injection efficiency of the nanoparticle sensitized solar cells.

Two types of organic electrolytes based on the redox couples of McMT^-/BMT (OS1) and TMTU/TMTU-TFO (OS2) were used for the QDSCs. By reducing the charge recombination between the electrolyte and the counter electrode, the fill factor and the photovoltage of these QDSCs were significantly improved, resulting in a higher efficiency for the studied solar cells than that with a commonly used inorganic electrolyte.

Ternary-alloy Pb_xCd_1-xS QDs used as photosensitizers for QDSCs were found to improve the photocurrent compared to the corresponding CdS and PbS QDs. By considering the effect of different ratios of Pb to Cd in thePbxCd1-xS QDs on the photovoltaic performance it was discovered that the photocurrent increases and the photovoltage decreases with the increase of the ratio in a certain range.

Upconversion (UC) nanoparticles provide a strategy to develop panchromatic solar cells. Three types of UC nanoparticles employed by DSCs were confirmed to work as energy relay materials for effectively extending the light-harvesting spectrum to the near-infrared (NIR) region. They were also found to play a role as scattering centers to enhance the photovoltaic performance of the solar cells.