Study on fundamentals of colloidal quantum dots for highly efficient photovoltaics

Abstract

Colloidal quantum dots (CQDs) are good candidate materials for next generation solar cells because of bandgap tunability in broad range, solution processibility, and suppressed thermal energy loss of carriers. However, the state of art power conversion efficiency (PCE) of CQD photovoltaics (PVs) is around 11%, which is lower than PCE of commercialized solar cells. Poor electrical property of CQDs compared to bulk materials is one of the biggest reasons for relatively low PCE of CQD PVs. PCE of CQD PVs can be improved by controlling the surface since surface of CQDs with high surface to volume ratio strongly affects electrical properties. More specifically, based on understanding the effect of surface to optoelectronic properties in CQDs, their energy band structures, doping level, mobility of electrons and holes, and photoelectric conversion stability can be modulated. In this thesis, CQD PVs with high PCE were realized by investigating electrical and photoelectric conversion properties. It has been revealed that the properties of CQD PVs are dependent on solvent used during the formation of active layer and surface molecules. In particular, it turned out that the number of surface traps, the mobility of electrons and holes, and the photoelectric conversion properties are highly sensitive on acidity of protic solvents which are treated on CQD surface. Moreover, hysteresis-free and photostable CQD PVs were obtained by investigating the correlation between the hysteresis in current-voltage curves and the mobile ions from organic ligands on the surface of CQDs.