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F3.03/EE4.03 - Effect of Surface Dipole Moments in Tuning Band Alignment to Improve Performance of Colloidal Quantum Dot Solar Cells 
April 23, 2014   9:15am - 9:30am

Colloidal quantum dot solar cells (CQDSC) based on lead sulfide (PbS) have drawn much attention due to their ability to deliver a power efficiency of 8%. PbS quantum dots (QDs) exhibit a low and tunable band gap with a high absorption coefficient, thus establishing these QDs as a promising candidate for colloidal quantum dot (CQD) solar cells. CQDSCs consist of a compact/mesoporous TiO2 layer on a transparent conducting oxide (TCO), followed by a PbS QD layer, 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, in the QD layer, both the electron and hole mobility suffers due to the high band gap organic passivating layer around the QDs. The similar energy levels present within the PbS QDs also increase the probability of electron-hole recombination. In this work, we have carried out experimental and theoretical studies of the effect of surface ligands on band positions in colloidal QDs. We have tuned the band alignment of the PbS QDs through the dipole moment of the passivating ligand. The dipole moment creates an induced electric field, which significantly alters the electronic properties of the individual QDs. The variation of the band gap and ionization potential of the same size PbS QDs treated with ligands consisting of differently functionalized thiophenol molecules were experimentally measured by absorption and photoelectron spectroscopy in air (PESA). 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. CQDSCs were fabricated and tested using the ligand-modified quantum dots. We have also considered another approach to tune the band gap as well as the band alignment by tuning the composition of ternary PbSe(x)S(1-x). The results show a correlation between band alignment and solar cell performance metrics. We will discuss how the charge mobility and total power conversion efficiency of the solar cell is affected by band alignment.

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