Thiols Passivate Recombination Centers in Colloidal Quantum Dots Leading to Enhanced Photovoltaic Device Efficiency

Abstract

The use of thiol-terminated ligands has recently been reported to enhance 10-fold the power conversion efficiency (PCE) of colloidal quantum dot (CQD) photovoltaic (PV) devices. We find herein that, in a representative amine-capped PbS colloidal quantum dot materials system, improved mobility following thiol treatment accounts for only a 1.4-fold increase in PCE. We then proceed to investigate the origins of the remainder of the quadrupling in PCE following thiol treatment. We find through measurements of photoluminescence quantum efficiency that exposure to thiols dramatically enhances photoluminescence in colloidal quantum dot films. The same molecules increase open-circuit voltage (V(oc)) from 0.28 to 0.43 V. Combined, these findings suggest that mid-gap states, which serve as recombination centers (lowering external quantum efficiency (EQE)) and metal-semiconductor junction interface states (lowering V(oc)), are substantially passivated using thiols. Through exposure to thiols, we improve EQE from 5 to 22% and, combined with the improvement in V(oc), improve power conversion efficiency to 2.6% under 76 mW/cm(2) at 1 microm wavelength. These findings are consistent with recent reports in photoconductive PbS CQD photodetectors that thiol exposure substantially removes deep (0.3 eV) electron traps, leaving only shallow (0.1 eV) traps.

Molecular Oxygen Induced in-Gap States in PbS Quantum Dots

“…This is in agreement with the previously observed p-type doping of EDT-treated lead chalcogenide QDs. 28,29 Here we focus on another way to measure the energy level of IGS, KPFM, 26,30 which probes the change of surface potential as a function of gate bias in a QD monolayer FET. Since the occupation of states is electrostatically tuned by the gate bias in the FET, we can use KPFM to extract the density of states.…”

Ethanedithiol Treatment of Solution‐Processed ZnO Thin Films: Controlling the Intragap States of Electron Transporting Interlayers for Efficient and Stable Inverted Organic Photovoltaics

“…We note that the EDT treatment has previously been demonstrated to be effective in terms of replacing the original long-chain insulating ligands of lead chalcogenide or cadmium chalcogenide based quantum dots, and the EDT treatment leads to improved inter-particle electronic coupling and enhanced charge transport properties of the quantum dot based films. [24][25][26][27][28][29] Fourier transform infrared spectroscopy (FTIR) analyses on the P-ZnO films reveal the presence of hydroxyl groups and carboxylate groups. As shown in Figure 1a, the broad peak at 3390 cm -1 corresponds to the hydroxyl groups and the peaks at 1560 and 1420 cm -1 correspond to the carboxylate groups.…”

Photovoltaic Devices Employing Ternary PbSxSe1-x Nanocrystals

“…22,26,27 We hypothesize that the better performance of ternary nanocrystals is due to a combination of material properties as well as a redistribution of the trap states. The higher current produced by PbS x Se 1-x , may arise from a significantly larger exciton Bohr radius than PbS due to the incorporation of Se (46nm for PbSe and 18nm for PbS).…”