The Transitional Heterojunction Behavior of PbS/ZnO Colloidal Quantum Dot Solar Cells

The nature of charge separation at the heterojunction interface of solution processed lead sulphide-zinc oxide colloidal quantum dot solar cells is investigated using impedance spectroscopy and external quantum efficiency measurements to examine the effect of varying the zinc oxide doping density. Without doping, the device behaves excitonically with no depletion region in the PbS layer such that only charge carriers generated within a diffusion length of the PbS/ZnO interface have a good probability of being harvested. After the ZnO is photodoped such that the doping density is near or greater than that of the PbS, a significant portion of the depletion region is found to lie within the PbS layer increasing charge extraction (p–n operation)

Transfer Printed Silver Nanowire Transparent Conductors for PbS–ZnO Heterojunction Quantum Dot Solar Cells

Transfer-printed silver nanowire transparent conducting electrodes are demonstrated in lead sulfide–zinc oxide quantum dot solar cells. Advantages of using this transparent conductor technology are increased junction surface energy, solution processing, and the potential cost reduction of low temperature processing. Joule heating, device aging, and film thickness effects are investigated to understand shunt pathways created by nanowires protruding perpendicular to the film. A <i>V</i>_oc of 0.39 ± 0.07 V, <i>J</i>_sc of 16.2 ± 0.2 mA/cm², and power conversion efficiencies of 2.8 ± 0.4% are presented

Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics

The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethane­dithiol (EDT) ligands and a lowering of the Fermi level due to oxidation

Influence of Multistep Surface Passivation on the Performance of PbS Colloidal Quantum Dot Solar Cells

The performance of devices containing colloidal quantum dot (CQD) films is strongly dependent on the surface chemistry of the CQDs they contain. Multistep surface treatments, which combine two or more strategies, are important for creating films with high carrier mobility that are well passivated against trap states and oxidation. Here, we examine the effect of a number of these surface treatments on PbS CQD films, including cation exchange to form PbS/CdS core/shell CQDs, and solid-state ligand-exchange treatments with Cl, Br, I, and 1,2-ethanedithiol (EDT) ligands. Using laboratory-based and synchrotron-radiation-excited X-ray photoelectron spectroscopy (XPS), we examine the compositions of the surface layer before and after treatment, and correlate this with the performance data and stability in air. We find that halide ion treatments may etch the CQD surfaces, with detrimental effects on the air stability and solar cell device performance caused by a reduction in the proportion of passivated surface sites. We show that films made up of PbS/CdS CQDs are particularly prone to this, suggesting Cd is more easily etched from the surface than Pb. However, by choosing a less aggressive ligand treatment, a good coverage of passivators on the surface can be achieved. We show that halide anions bind preferentially to surface Pb (rather than Cd). By isolating the part of XPS signal originating from the topmost surface layer of the CQD, we show that air stability is correlated with the total number of passivating agents (halide + EDT + Cd) at the surface

Poly(3-hexylthiophene-2,5-diyl) as a Hole Transport Layer for Colloidal Quantum Dot Solar Cells

Lead sulfide colloidal quantum dot (CQD) solar cells demonstrate extremely high short-circuit currents (<i>J</i>_sc) and are making decent progress in power conversion efficiencies. However, the low fill factors (FF) and open-circuit voltages have to be addressed with urgency to prevent the stalling of efficiency improvements. This paper highlights the importance of improving hole extraction, which received much less attention as compared to the electron-accepting component of the device architecture (e.g., TiO₂ or ZnO). Here, we show the use of semiconducting polymer poly­(3-hexylthiophene-2,5-diyl) to create efficient CQD devices by improving hole transport, removing interfacial barriers, and minimizing shunt pathways, thus resulting in an overall improvement in device performance stemming from better <i>J</i>_sc and FF

Two-Photon Absorption and Two-Photon-Induced Gain in Perovskite Quantum Dots

Perovskite quantum dots (PQDs) emerged as a promising class of material for applications in lighting devices, including light emitting diodes and lasers. In this work, we explore nonlinear absorption properties of PQDs showing the spectral signatures and the size dependence of their two-photon absorption (2PA) cross-section, which can reach values higher than 10⁶ GM. The large 2PA cross section allows for low threshold two-photon induced amplified spontaneous emission (ASE), which can be as low as 1.6 mJ/cm². We also show that the ASE properties are strongly dependent on the nanomaterial size, and that the ASE threshold, in terms of the average number of excitons, decreases for smaller PQDs. Investigating the PQDs biexciton binding energy, we observe strong correlation between the increasing on the biexciton binding energy and the decreasing on the ASE threshold, suggesting that ASE in PQDs is a biexciton-assisted process

Influence of Shell Thickness and Surface Passivation on PbS/CdS Core/Shell Colloidal Quantum Dot Solar Cells

Cation-exchange has been used to synthesize PbS/CdS core/shell colloidal quantum dots from PbS starting cores. These were then incorporated as the active material in solar cell test devices using a solution-based, air-ambient, layer-by-layer spin coating process. We show that core/shell colloidal quantum dots can replace their unshelled counterparts with a similar band gap as the active layer in a solar cell device, leading to an improvement in open circuit voltage from 0.42 to 0.66 V. This improvement is attributed to a reduction in recombination as a result of the passivating shell. However, this increase comes at the expense of short circuit current by creating a barrier for transport. To overcome this, we first optimize the shell thickness by varying the conditions for cation-exchange to form the thinnest shell layer possible that provides sufficient surface passivation. Next, ligand exchange with a combination of halide and bifunctional organic molecules is used in conjunction with the core/shell strategy. Power conversion efficiencies of 5.6 ± 0.4% have been achieved with a simple heterojunction device architecture