Molecular surface programming of rectifying junctions between InAs colloidal quantum dot solids.

Heavy-metal-free III-V colloidal quantum dots (CQDs) show promise in optoelectronics: Recent advancements in the synthesis of large-diameter indium arsenide (InAs) CQDs provide access to short-wave infrared (IR) wavelengths for three-dimensional ranging and imaging. In early studies, however, we were unable to achieve a rectifying photodiode using CQDs and molybdenum oxide/polymer hole transport layers, as the shallow valence bandedge (5.0 eV) was misaligned with the ionization potentials of the widely used transport layers. This occurred when increasing CQD diameter to decrease the bandgap below 1.1 eV. Here, we develop a rectifying junction among InAs CQD layers, where we use molecular surface modifiers to tune the energy levels of InAs CQDs electrostatically. Previously developed bifunctional dithiol ligands, established for II-VI and IV-VI CQDs, exhibit slow reaction kinetics with III-V surfaces, causing the exchange to fail. We study carboxylate and thiolate binding groups, united with electron-donating free end groups, that shift upward the valence bandedge of InAs CQDs, producing valence band energies as shallow as 4.8 eV. Photophysical studies combined with density functional theory show that carboxylate-based passivants participate in strong bidentate bridging with both In and As on the CQD surface. The tuned CQD layer incorporated into a photodiode structure achieves improved performance with EQE (external quantum efficiency) of 35% (>1 µm) and dark current density < 400 nA cm-2, a >25% increase in EQE and >90% reduced dark current density compared to the reference device. This work represents an advance over previous III-V CQD short-wavelength IR photodetectors (EQE < 5%, dark current > 10,000 nA cm-2).

Ambient-processed colloidal quantum dot solar cells via individual pre-encapsulation of nanoparticles.

We report colloidal quantum dot solar cells fabricated under ambient atmosphere with an active area of 2.9 mm(2) that exhibit 3.6% solar power conversion efficiency. The devices are based on PbS tuned via the quantum size effect to have a first excitonic peak at 950 nm. Because the formation of native oxides and sulfates on PbS leads to p-type doping and deep trap formation and because such dopants and traps dramatically influence device performance, prior reports of colloidal quantum dot solar cells have insisted on processing under an inert atmosphere. Here we report a novel ligand strategy in which we first encapsulate the quantum dots in the solution phase with the aid of a strongly bound N-2,4,6-trimethylphenyl-N-methyldithiocarbamate ligand. This allows us to carry out film formation and all subsequent device fabrication under an air atmosphere.

Site-selective optical coupling of PbSe nanocrystals to Si-based photonic crystal microcavities.

A novel method for patterning optically active colloidal PbSe nanocrystals on Si surfaces is reported. Oleate-capped PbSe nanocrystals were found to adhere preferentially to H-terminated Si surfaces over oxide and alkyl-terminated Si surfaces. Scanning probe lithography was used to oxidize locally a dodecyl monolayer on the Si surface of a silicon-on-insulator wafer prepatterned with photonic crystal microcavities. Aqueous HF was then used to remove the oxide and expose H-terminated Si areas, yielding patterned PbSe nanocrystals on the Si surface after exposure to a nanocrystal solution. This patterning technique allows for the selective deposition of PbSe nanocrystals at the main antinode of the silicon-based microcavities. More than a 10-fold photoluminescence enhancement due to the cavity-nanocrystal coupling was observed.

Emission spectrum of electromagnetic energy stored in a dynamically perturbed optical microcavity.

An ultrafast pump-probe experiment is performed on wavelength-scale, silicon-based, optical microcavities that confine light in three dimensions with resonant wavelengths near 1.5 mum, and lifetimes on the order of 20 ps. A below-bandgap probe pulse tuned to overlap the cavity resonant frequency is used to inject electromagnetic energy into the cavity, and an above-bandgap pump pulse is used to generate free carriers in the silicon, thus altering the real and imaginary components of the cavity's refractive index, and hence its resonant frequency and lifetime. When the pump pulse injects a carrier density of ~ 5 x10(17) cm(-3) before the resonant probe pulse strikes the sample, the emitted radiation from the cavity is blue-shifted by 16 times the bare cavity linewidth, and the new linewidth is 3.5 times wider than the original. When the pump pulse injects carriers, and thus suddenly perturbs the cavity properties after the probe pulse has injected energy into the cavity, we show that the emitted radiation is not simply a superposition of Lorentzians centred at the initial and perturbed cavity frequencies. Under these conditions, a simple model and the experimental results show that the power spectrum of radiation emitted by the stored electromagnetic energy when the cavity frequency is perturbed during ring-down consists of a series of coherent oscillations between the original and perturbed cavity frequencies, accompanied by a gradual decrease and broadening of the original cavity line, and the emergence of the new cavity resonance. The modified cavity lifetime is shown to have a significant impact on the evolution of the emission as a function of the pump-probe delay.

Depleted-heterojunction colloidal quantum dot solar cells.

Colloidal quantum dot (CQD) photovoltaics combine low-cost solution processability with quantum size-effect tunability to match absorption with the solar spectrum. Rapid recent advances in CQD photovoltaics have led to impressive 3.6% AM1.5 solar power conversion efficiencies. Two distinct device architectures and operating mechanisms have been advanced. The first-the Schottky device-was optimized and explained in terms of a depletion region driving electron-hole pair separation on the semiconductor side of a junction between an opaque low-work-function metal and a p-type CQD film. The second-the excitonic device-employed a CQD layer atop a transparent conductive oxide (TCO) and was explained in terms of diffusive exciton transport via energy transfer followed by exciton separation at the type-II heterointerface between the CQD film and the TCO. Here we fabricate CQD photovoltaic devices on TCOs and show that our devices rely on the establishment of a depletion region for field-driven charge transport and separation, and that they also exploit the large bandgap of the TCO to improve rectification and block undesired hole extraction. The resultant depleted-heterojunction solar cells provide a 5.1% AM1.5 power conversion efficiency. The devices employ infrared-bandgap size-effect-tuned PbS CQDs, enabling broadband harvesting of the solar spectrum. We report the highest open-circuit voltages observed in solid-state CQD solar cells to date, as well as fill factors approaching 60%, through the combination of efficient hole blocking (heterojunction) and very small minority carrier density (depletion) in the large-bandgap moiety.

Quantum dot photovoltaics in the extreme quantum confinement regime: the surface-chemical origins of exceptional air- and light-stability.

We report colloidal quantum dot (CQDs) photovoltaics having a approximately 930 nm bandgap. The devices exhibit AM1.5G power conversion efficiencies in excess of 2%. Remarkably, the devices are stable in air under many tens of hours of solar illumination without the need for encapsulation. We explore herein the origins of this orders-of-magnitude improvement in air stability compared to larger PbS dots. We find that small and large dots form dramatically different oxidation products, with small dots forming lead sulfite primarily and large dots, lead sulfate. The lead sulfite produced on small dots results in shallow electron traps that are compatible with excellent device performance; whereas the sulfates formed on large dots lead to deep traps, midgap recombination, and consequent catastrophic loss of performance. We propose and offer evidence in support of an explanation based on the high rate of oxidation of sulfur-rich surfaces preponderant in highly faceted large-diameter PbS colloidal quantum dots.

Heavy-metal-free solution-processed nanoparticle-based photodetectors: doping of intrinsic vacancies enables engineering of sensitivity and speed.

Photodetection in semiconductors enables digital imaging, spectroscopy, and optical communications. Integration of solution-processed light-sensing materials with a range of substrates offers access to new spectral regimes, the prospect of enhanced sensitivity, and compatibility with flexible electronics. Photoconductive photodetectors based on solution-cast nanocrystals have shown tremendous progress in recent years; however, high-performance reports to date have employed Pb- and Cd-containing materials. Here we report a high-sensitivity (photon-to-electron gain >40), high-speed (video-frame-rate-compatible) photoconductive photodetector based on In(2)S(3). Only by decreasing the energetic depth of hole traps associated with intrinsic vacancies in beta-phase In(2)S(3) were we able to achieve this needed combination of sensitivity and speed. Our incorporation of Cu(+) cations into beta-In(2)S(3)'s spinel vacancies that led to acceptable temporal response in the devices showcases the practicality of incorporating dopants into nanoparticles. The devices are stable in air and under heating to 215 degrees C, advantages rooted in the reliance on the stable inclusion of dopants into available sites instead of surface oxide species.

Thiols passivate recombination centers in colloidal quantum dots leading to enhanced photovoltaic device efficiency.

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.

Efficient, stable infrared photovoltaics based on solution-cast colloidal quantum dots.

Half of the sun's power lies in the infrared. As a result, the optimal bandgaps for solar cells in both the single-junction and even the tandem architectures lie beyond 850 nm. However, progress in low-cost, large-area, physically flexible solar cells has instead been made in organic and polymer materials possessing absorption onsets in the visible. Recent advances have been achieved in solution-cast infrared photovoltaics through the use of colloidal quantum dots. Here we report stable solution-processed photovoltaic devices having 3.6% power conversion efficiency in the infrared. The use of a strongly bound bidentate linker, benzenedithiol, ensures device stability over weeks. The devices reach external quantum efficiencies of 46% in the infrared and 70% across the visible. We investigate in detail the physical mechanisms underlying the operation of this class of device. In contrast with drift-dominated behavior in recent reports of PbS quantum dot photovoltaics, we find that diffusion of electrons and holes over hundreds of nanometers through our PbSe colloidal quantum dot solid is chiefly responsible for the high external quantum efficiencies obtained in this new class of devices.