Effect of Solar Concentration on the Thermodynamic Power Conversion Efficiency of Quantum-Dot Solar Cells Exhibiting Multiple Exciton Generation

Thermodynamic calculations show that all solar cells can convert solar photons into electricity or fuel with higher theoretical power conversion efficiencies under concentrated sunlight. For conventional (viz, present day) single-junction solar cells that produce at most one electron–hole pair per absorbed photon, the theoretical increase in efficiency is relatively small (absolute values of 38% at 500× vs 33% at 1×). However, when solar concentration is combined with multiple exciton generation (MEG) in semiconductor quantum dots, the increase in theoretical power conversion efficiency is greatly enhanced. For the ideal MEG case, where the threshold for exciton multiplication is twice the bandgap, <i>E</i>_g, the maximum thermodynamic efficiency increases to 75% at 500×, but the optimum <i>E</i>_g shifts to smaller values. If <i>E</i>_g is fixed at the 1-sun optimal level, then the maximum theoretical efficiency still increases markedly, becoming 62% at 500× for the staircase MEG characteristic (defined as producing N electron–hole pairs when the photon energy is <i>N</i> × <i>E</i>_g) and 47% for a linear MEG characteristic that has a threshold photon energy of 2<i>E</i>_g. The bandgaps in these two cases are 0.70 and 0.93 eV, respectively

Photogenerated Free Carrier Dynamics in Metal and Semiconductor Single-Walled Carbon Nanotube Films

Time-resolved THz spectroscopy (TRTS) is employed to study the photogenerated charge-carrier dynamics in transparent films of single-walled carbon nanotubes (SWNTs). Two films were investigated: a film with 94% semiconducting-type tubes (s-SWNTs) and a film with only 7% s-SWNT and 93% metal-type tubes (m-SWNTs). We conclude that charge-carriers are generated with >60% yields at low light intensities in both films. Free-carriers are generated by a linear exciton dissociation process that occurs within ∼1 ps and is independent of excitation wavelength or tube type

Size-Dependent Janus-Ligand Shell Formation on PbS Quantum Dots

We studied the size-dependent Janus ligand shell formation on PbS QDs employing an X-type ligand exchange reaction between native oleate ligands and two substituted cinnamic acid ligands, trifluoromethyl- and dimethyl amino-cinnamic acid, representing electron donating and electron withdrawing ligands. The exchange reactions become significantly more favorable for both electron donating and withdrawing ligands (ΔG becomes more negative) in the smaller QDs compared to the larger QDs likely because the ligand density is smaller on the larger QDs reducing the strength of the ligand–ligand interactions. We found that Janus-ligand shells form more readily on smaller QDs than on bigger QDs with electron donating ligands. We also observed a dependence on the QD concentration that should be considered when forming Janus-ligand shells. Two-dimensional solution nuclear magnetic resonance spectroscopy (2D-NMR) shows evidence of pronounced phase segregation between oleate and electron donating ligands on the smaller QDs consistent with the enhanced ligand–ligand interactions. This study broadens our understanding of how to construct Janus and patchy ligand shell morphologies on small QDs

Pyroelectricity of Lead Sulfide (PbS) Quantum Dot Films Induced by Janus-Ligand Shells

Asymmetry is an essential property to control. To do that in nanocrystalline systems we have developed methods to produce Janus-ligand shells on otherwise symmetric PbS quantum dots (QDs). Here, we demonstrate that control by constructing a system that exhibits pyroelectricity built from spherical PbS QDs. We observed a pyroelectric current in two different configurations. In one configuration, the QDs are self-assembled into close-packed arrays while in the second configuration, the QDs are dispersed into an electro-inactive polymer, polydimethyl­siloxane. Both exhibit a pyroelectric response. In the first configuration we estimate a lower limit of the pyroelectric coefficient to be 1.97 × 10–7 C/m2·K, which is likely limited by the degree of QD alignment during film formation but is already on par with common pyroelectric systems. Compared with inorganic ceramic-like and polymeric pyroelectric materials, pyroelectric films self-assembled from polar QDs are easier to prepare, responsive to light with different energies based on QD exciton energy, and the polarization of each QD could be easily tuned by constructing different Janus-ligand shells

Pickering Emulsions of Self-Assembled Lead Sulfide Quantum Dots with Janus-Ligand Shells as Nanoreactors for Photocatalytic Reactions

Here we prepare Pickering emulsions with semiconductor quantum dots (QDs). Amphiphilic PbS QDs are prepared by constructing Janus-ligand shells comprised of lipophilic oleic acid and hydrophilic 4-(2,2-dicyanovinyl)­cinnamic acid ligands. Upon homogenization, the QDs with Janus-ligand shells self-assemble at the water–dichloromethane interface, forming stable Pickering emulsions. The photocatalytic properties are evaluated by the photodegradation reaction of methyl orange (MO) dye molecules. Under the same conditions, MO was significantly degraded when photocatalyzed by the QD Pickering emulsions, while no degradation was observed when PbS QDs without Janus-ligand shells were employed. This work provides a guide to designing QD-based nanoreactors

Atomically Thin Metal Sulfides

We developed a method to colloidally synthesize atomically thin metal sulfides (ATMS). Unlike conventional 2D systems such as MoS2 and graphene, none of the systems developed here are inherently layered compounds nor have known layered polymorphs in their bulk forms. The synthesis proceeds via a cation-exchange reaction starting from single- and multi-layer Ag2S and going to various metal sulfides. The synthesized ATMS retain their size and shape during the cation-exchange reaction and are either single-layer or a few-layer, depending on the starting Ag2S samples. They have lateral dimensions on the order of 5–10 nm and are colloidally stabilized by Z- and L-type ligands. Here, we demonstrate the synthesis of single-layer and a few-layer ZnS, CdS, CoS2, and PbS. We find that the optical properties of these ATMS are quite distinct from the platelet or quantum-dot versions of the same metal sulfides

Charge Generation in PbS Quantum Dot Solar Cells Characterized by Temperature-Dependent Steady-State Photoluminescence

Charge-carrier generation and transport within PbS quantum dot (QD) solar cells is investigated by measuring the temperature-dependent steady-state photoluminescence (PL) concurrently during <i>in situ</i> current–voltage characterization. We first compare the temperature-dependent PL quenching for PbS QD films where the PbS QDs retain their original oleate ligand to that of PbS QDs treated with 1,2-ethanedithiol (EDT), producing a conductive QD layer, either on top of glass or on a ZnO nanocrystal film. We then measure and analyze the temperature-dependent PL in a completed QD-PV architecture with the structure Al/MoO₃/EDT-PbS/ZnO/ITO/glass, collecting the PL and the current simultaneously. We find that at low temperatures excitons diffuse to the ZnO interface, where PL is quenched <i>via</i> interfacial charge transfer. At high temperatures, excitons dissociate in the bulk of the PbS QD film <i>via</i> phonon-assisted tunneling to nearby QDs, and that dissociation is in competition with the intrinsic radiative and nonradiative rates of the individual QDs. The activation energy for exciton dissociation in the QD-PV devices is found to be ∼40 meV, which is considerably lower than that of the electrodeless samples, and suggests unique interactions between injected and photogenerated carriers in devices

Understanding the Effect of Lead Iodide Excess on the Performance of Methylammonium Lead Iodide Perovskite Solar Cells

The presence of unreacted lead iodide in organic–inorganic lead halide perovskite solar cells is widely correlated with an increase in power conversion efficiency. We investigate the mechanism for this increase by identifying the role of surfaces and interfaces present between methylammonium lead iodide perovskite films and excess lead iodide. We show how type I and II band alignments arising under different conditions result in either passivation of surface defects or hole injection. Through first-principles simulations of solid–solid interfaces, we find that lead iodide captures holes from methylammonium lead iodide and modulates the formation of defects in the perovskite, affecting recombination. Using surface-sensitive optical spectroscopy techniques, such as transient reflectance and time-resolved photoluminescence, we show how excess lead iodide affects the diffusion and surface recombination velocity of charge carriers in methylammonium lead iodide films. Our coupled experimental and theoretical results elucidate the role of excess lead iodide in perovskite solar cells