Ligand exchange and photoluminescence quenching in organic-inorganic blends poly(3-hexylthiophene) P3HT:PbS

Abstract

Long alkyl chain ligands such as oleic acid (OLA) which cover the as-prepared PbS nanodots act as an insulating layer that impedes efficient charge transfer in PbS nanodots: polymer hybrid solar cells. The replacement of OLA with tailored ligands of an appropriate chain length is needed to achieve a noticeable enhancement of photovoltaic performance. Several studies have centered on the ligand exchange prior to casting the PbS film(1,2,3). However, this post synthesis approach requires careful consideration for the choice of a ligand as clustering of the nanodots has to be avoided. Recently, a new approach that allows direct chemical ligand replacement in a blended mixture of PbS:P3HT has been demonstrated (4,5,6). In this contribution, the latter approach (post-fabrication) was compared with the post-synthesis ligand exchange. We investigated the effect of the ligand exchange processes to the charge separation dynamics in the P3HT: PbS blends by steady-state and time-resolved photoluminescence (PL). Hexanoic acid and acetic acid were used as a short-length ligand for the post fabrication approach while decylamine, octylamine and butylamine were used for the post-synthesis approach. As expected, decreasing the chain length of the ligand led to an increase of the P3HT fluorescence quenching. The absence of enhancement of PbS luminescence due to energy transfer from P3HT and the dependence of the quenching efficiency on the bulkiness of the ligands coating the QDs suggest that the quenching of the P3HT fluorescence is dominated by electron transfer to PbS quantum dots (QDs). In addition, the fluorescence quenching is also less prominent in the P3HT with higher regioregularity (RR) suggesting an enhanced phase separation in the blend due to more densely packed nature of conjugated polymer with higher RR.