Lead sulfide (PbS) quantum dots (QDs) exhibit a low, tunable band gap and a high absorption coefficient, establishing PbS QDs as a promising candidate for colloidal quantum dot (CQD) solar cells and allowing PbS CQD devices to deliver an unprecedented power efficiency of 8%. These types of solar cells consist of a compact/mesoporous TiO2 layer on a transparent conducting oxide (TCO), followed by a PbS QD layer deposited by spin coating/drop casting methods, and finally a thermally evaporated metal which serves as a back contact. In these devices, it is important to control the band gap as well the band position of the QDs to efficiently inject electrons into TiO2 and holes into the metal electrode. However, the similar energy levels present within the PbS QDs increase the probability of electron-hole recombination. In this work, studies were performed to tune the band alignment of the PbS QDs through the dipole moment of passivating ligand. The dipole moment creates an induced electric field, which significantly alters the electronic properties of the individual QDs. We have studied the variation of the band gap and ionization potential of the same size PbS QDs treated with ligands consisting of differently functionalized thiophenol molecules by absorption and photoelectron spectroscopy. Our results show that the ionization potential of PbS QDs, having a diameter of 3 nm, can be tuned from -5.5 eV to -4.8 eV by changing the ligand from nitrothiophenol to methylthiophenol, whereas the band gap varies from 1.25 eV to 1.37 eV, respectively. DFT calculations were performed to evaluate the dipole moment of the ligands, and a comparison of the calculated and experimental results shows that the ionization potential varies linearly with the dipole moment of the ligand. CQD solar cells were fabricated and tested using the ligand-modified quantum dots.  We will discuss how the charge mobility and total power conversion efficiency of the solar cell is affected by band alignment.

Tuning band alignment by surface dipole moments to improve performance of colloidal quantum dot solar cells

247th National Meeting (2014)

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