Ethan J. D. Klem

Ultrasensitive solution-cast quantum dot photodetectors.

Solution-processed electronic and optoelectronic devices offer low cost, large device area, physical flexibility and convenient materials integration compared to conventional epitaxially grown, lattice-matched, crystalline semiconductor devices. Although the electronic or optoelectronic performance of these solution-processed devices is typically inferior to that of those fabricated by conventional routes, this can be tolerated for some applications in view of the other benefits. Here we report the fabrication of solution-processed infrared photodetectors that are superior in their normalized detectivity (D*, the figure of merit for detector sensitivity) to the best epitaxially grown devices operating at room temperature. We produced the devices in a single solution-processing step, overcoating a prefabricated planar electrode array with an unpatterned layer of PbS colloidal quantum dot nanocrystals. The devices showed large photoconductive gains with responsivities greater than 103 A W-1. The best devices exhibited a normalized detectivity D* of 1.8 × 1013 jones (1 jones = 1 cm Hz1/2 W-1) at 1.3 µm at room temperature: todays highest performance infrared photodetectors are photovoltaic devices made from epitaxially grown InGaAs that exhibit peak D* in the 1012 jones range at room temperature, whereas the previous record for D* from a photoconductive detector lies at 1011 jones. The tailored selection of absorption onset energy through the quantum size effect, combined with deliberate engineering of the sequence of nanoparticle fusing and surface trap functionalization, underlie the superior performance achieved in this readily fabricated family of devices.

Impact of dithiol treatment and air annealing on the conductivity, mobility, and hole density in PbS colloidal quantum dot solids

Crosslinking molecules have recently been combined with colloidal quantum dots to build robust, closely packed, conductive solid-state devices. Ethanedithiol (EDT) has been used in PbS quantum dot photovoltaic devices to assist in film formation during fabrication. However, there is evidence that EDT influences the electronic properties of the colloidal quantum dot (CQD) films. We fabricate thin film field-effect transistors and find that EDT treatment increases the majority carrier mobility by a factor of 10. We attribute this increase to a reduction in interparticle spacing which we observe using transmission electron microscopy. However, this increase is accompanied by a decrease in the majority carrier concentration. Using x-ray photoelectron microscopy, we find that EDT reduces the extent of the surface oxidation which is acting as a p-type dopant in these materials. We find that by lightly reoxidizing, we can redope the CQD films and can do so without sacrificing mobility gains.

Efficient Schottky-quantum-dot photovoltaics: The roles of depletion, drift, and diffusion

PbS colloidal quantum dot photovoltaic devices in a Schottky architecture have demonstrated an infrared power conversion efficiency of 4.2%. Here, we elucidate the internal mechanisms leading to this efficiency. At relevant intensities, the drift length is 10μm for holes and 1μm for electrons. Transport within the 150nm wide depletion region is therefore highly efficient. The electron diffusion length of 0.1μm is comparable to neutral region width. We quantitatively account for the observed 37% external quantum efficiency, showing that it results from the large depletion width and long carrier lifetime combined.

Efficient solution-processed infrared photovoltaic cells: Planarized all-inorganic bulk heterojunction devices via inter-quantum-dot bridging during growth from solution

Solution-processed thin-film organic, inorganic, and hybrid photovoltaic devices have achieved power conversion efficiencies as high as 5%. However, these devices remain limited by their capture of visible energy; more than a half of the sun’s power lies in the infrared. Herein the authors demonstrate photovoltaic devices effective across the visible and all the way out to 1700nm. Only through the use of ethanedithiol as a bridging molecule to affect interparticle linking were they able to achieve fabrication of smooth, continuous quantum dot films on rough, high-surface area transparent metal oxides. This allowed them to increase light absorption while maintaining efficient charge separation and extraction and at the same time avoiding electrical short circuits. They obtained monochromatic infrared power conversion efficiencies of 1.3%, a 50-fold gain over the previous published record of 0.025% in IR solution-processed photovoltaics. The authors demonstrate quantum size-effect tuning of device band gaps...

Solution-processed infrared photovoltaic devices with >10% monochromatic internal quantum efficiency

Large-area, physically flexible, solution-cast photovoltaics are of urgent interest to realize low-cost solar cells. Polymer, polymer-fullerene, and polymer-nanocrystal photovoltaics absorb light only to wavelengths as long as 750 nm, with the exception of one recent report out to 1000 nm. Half of the sun’s power spectrum lies beyond 700 nm; one third beyond 1000 nm; and infrared emitters of growing interest in thermal photovoltaics emit predominantly in the 1–3μm range. We report herein a processible infrared photovoltaic device active beyond 1μm. Our best devices exhibit external quantum efficiencies exceeding 1% and estimated monochromatic internal quantum efficiencies greater than 10%. This represents an improvement by more than 1000 compared to the best previously reported processible >1μm infrared photovoltaics. We employ a novel device architecture in which the infrared-absorbing active layer is based purely on semiconductor nanoparticles with no semiconducting polymer matrix. The replacement of a ...

PbS quantum dot electroabsorption modulation across the extended communications band 1200-1700 nm

We report electric field-induced modulation of absorption in PbS nanocrystal quantum dots across the spectral region 600–2000nm, encompassing the entire telecommunications band. The maxima in the electroabsorption spectra correspond with the positions of the first excitonic peak, confirming the predominance of excitonic broadening as the basis for the observed effect. We estimate the change in dipole moment to be on the order of 20 D in 7nm diameter PbS nanocrystals compared to previously-reported ∼100D in CdSe.

Planar PbS quantum dot/C60 heterojunction photovoltaic devices with 5.2% power conversion efficiency

Of interest for both photovoltaic and photodetector applications is the ability of colloidal quantum dot (CQD) devices to provide response further into the infrared than is typical for other solution-processable materials. Here, we present a simple heterojunction diode structure that utilizes the extended infrared absorption of PbS CQDs. We show that device performance benefits from a discontinuous exciton blocking layer which improves charge separation without limiting charge extraction. By enhancing charge carrier mobility in the CQD layer, we demonstrate a planar heterostructure device with a power conversion efficiency of 5.2% under 1 sun illumination.