Impact of capping agent on the electron transfer dynamics of CdTe QDs with methyl viologen

Abstract

The primary process in quantum dot solar cells is electron transfer between excited state QDs and semiconductor. There are several key factors controlling this electron transfer process including capping agents. Therefore fundamental understanding of capping agent dependent electron transfer dynamics is much needed. Here, we have employed two capping agents namely thioglycolic acid (TGA) and mercapto propionic acid (MPA) in which the later has one additional methylene group in its structure which plays a significant role on electron transfer process. Hence, we have studied the electron transfer dynamics of TGA and MPA capped CdTe QDs with methyl viologen (MV) in aqueous medium by using steady state and time resolved absorption and fluorescence techniques. The results show that MV quenches the fluorescence of CdTe QDs which is capping agent dependent. The obtained quenching rate constant for TGA and MPA capped CdTe QDs are 8.04×1010M−1s−1 and 1.66×1010M−1s−1 respectively. From the quenching rate constant values, it is clear that there is a significant role of capping agent on electron transfer process. This is due to MPA passivate the surface of QDs more than TGA, which slow down the electron transfer process from QDs to MV. Moreover, the observed radical cation (MV•+) from transient absorption measurements confirms that the fluorescence quenching of CdTe QDs by MV is attributed to electron transfer. These experimental results will help to understanding the behavior of QDs with various acceptors towards the applications of quantum dot solar cells.