Alexi C. Arango

Colloidal PbS quantum dot solar cells with high fill factor.

We fabricate PbS colloidal quantum dot (QD)-based solar cells using a fullerene derivative as the electron-transporting layer (ETL). A thiol treatment and oxidation process are used to modify the morphology and electronic structure of the QD films, resulting in devices that exhibit a fill factor (FF) as high as 62%. We also show that, for QDs with a band gap of less than 1 eV, an open-circuit voltage (VOC) of 0.47 V can be achieved. The power conversion efficiency reaches 1.3% under 1 sun AM1.5 test conditions and 2.4% under monochromatic infrared (lambda=1310 nm) illumination. A consistent mechanism for device operation is developed through a circuit model and experimental measurements, shedding light on new approaches for optimization of solar cell performance by modifying the interface between the QDs and the neighboring charge transport layers.

Efficient Titanium Oxide/Conjugated Polymer Photovoltaics for Solar Energy Conversion

[19] Since the mixing ratio of the anions in the solution from which the crystals were grown was revealed to be almost identical to the stoichiometry determined by EPMA of the obtained crystal, the mixing ratio of the anions in the solution was adopted in the chemical formula of the crystal. The results of EPMA of k-(BETS)2FexGa1‐xBr1.0Cl3.0 are: for x = 0.50, Fe/Ga/Br/Cl = 0.51:0.49:1.13:2.85 (0.50:0.50:1.00:3.00); for x = 0.40, Fe/Ga/Br/Cl = 0.40:0.60:0.96:2.87 (0.40:0.60:1.00:3.00); for x = 0.30, Fe/Ga/Br/Cl = 0.32:0.68:1.11:3.07 (0.30:0.70:1.00:3.00); for x = 0.20, Fe/Ga/Br/Cl = 0.24:0.76:1.11:3.16 (0.20:0.80:1.00:3.00); for x = 0.10, Fe/Ga/Br/Cl = 0.12:0.88:1.04:3.22 (0.10:0.90:1.00:3.00). [20] Recent susceptibility measurements revealed that the angle between the easy axis of the antiferromagnetic structure and the c axis is about 35 in k-(BETS)2FeCl4 (E. Ojima, T. Sasaki, private communication). While the easy axis of k-(BETS)2FeBr0.6Cl3.4 is approximately parallel to the b* axis.

Charge transfer in photovoltaics consisting of interpenetrating networks of conjugated polymer and TiO2 nanoparticles

We study the effect of blended and layered titanium dioxide (TiO2) nanoparticles on charge transfer processes in conjugated polymer photovoltaics. A two order of magnitude increase in photoconductivity and sharp saturation is observed for layered versus blended structures, independent of the cathode work function. Using electrodes with similar work functions, we observe low dark currents and open circuit voltages of 0.7 V when a TiO2 nanoparticle layer is self-assembled onto the indium–tin–oxide electrode. Our results for the layered morphologies are consistent with charge collection by exciton diffusion and dissociation at the TiO2 interface.

Photodetectors based on treated CdSe quantum-dot films

We demonstrate photodetectors of sandwich geometry active in the visible spectrum in which the active layer is a 200 nm thick film of CdSe quantum dots (QDs). The solution-phase treatment of the QD film with n-butylamine after casting greatly increases the exciton dissociation efficiency and charge-transport properties of the film. Under 110mW∕cm2 illumination with light at λ=514nm, the photocurrent to dark current ratio, Iphoto∕Idark, is 103 at V=0V, and the 3 dB frequency is ∼50kHz. At room temperature, we observe zero-bias external quantum efficiencies (EQE) from 0.08% to 0.23% in the wavelength range λ=350nm to λ=575nm, corresponding to an internal quantum efficiency (IQE) of 0.6±0.1% across the tested spectrum. At V=−6V, EQE ranges from 15% to 24%, corresponding to an IQE of 70±10%.

Interfacial Recombination for Fast Operation of a Planar Organic/QD Infrared Photodetector

Thin fi lms of organic semiconductors and colloidal nanocrystal quantum dots (QDs) have attracted considerable interest for a variety of electronic device applications due to the tunability of their electronic structure as well as the potential for scalable device fabrication across large-area substrates. QDs are especially interesting due to the freedom available to directly engineer their optoelectronic properties by varying the nanocrystal size [ 1 ] as well as by chemically modifying QD surfaces with oxidation [ 2 , 3 ] or ligand exchange. [ 4–9 ] Of particular interest is the prospect for QD optical response that extends into the short-wavelength infrared (SWIR) part of the spectrum (wavelengths of λ = 1.0 μ m to 2.0 μ m) with QDs of low-bandgap semiconductors such as PbS and PbSe. This wavelength range is largely inaccessible to organic materials yet is critical to effi cient photovoltaics, [ 10 ] night vision, [ 11 , 12 ] biological imaging applications, [ 13 , 14 ] and optical communication. [ 15 , 16 ]

Lateral heterojunction photodetector consisting of molecular organic and colloidal quantum dot thin films

We demonstrate a heterojunction photodetector of lateral geometry that utilizes an evaporated film of the hole-transporting molecular material N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-9,9-spirobifluorene (spiro-TPD) as a charge transport layer and that is sensitized across visible wavelengths by a thin film of colloidal CdSe nanocrystal quantum dots (QDs). High photon-to-electron quantum conversion efficiencies are obtained at room temperature as a result of photoconductive gain. With an electric field of 3.0×105 V/cm applied across the electrodes, we measure the external quantum efficiency at the first QD absorption peak (at wavelength λ=590 nm) to be 13%, corresponding to an internal quantum efficiency of approximately 80%. The operating mechanism of these devices is discussed, noting that the optical response is dominated by the QD absorption spectrum while the charge transport nearly exclusively takes place in the spiro-TPD.

Lateral organic bilayer heterojunction photoconductors

We demonstrate a two-terminal, lateral organic bilayer photoconductor that generates an external quantum efficiency of (12±1)%, with an internal quantum efficiency of (140±2)% indicative of photon-to-electron conversion gain. The photoconductor incorporates a heterojunction between N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (TPD) and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI). Excitons generated with photoexcitation of PTCBI dissociate at the PTCBI/TPD interface and raise the charge carrier concentration in TPD, increasing device conductance. The exposed top surface enables interaction with chemical analytes in the environment, motivating the use of the photoconductor as a chemical sensor that transduces chemical signals into amplified changes in the electrical response.

Mapping Temperature in OLED Displays Using CCD Thermoreflectance

Effective thermal management is essential to engineering organic devices, since heating accelerates their degradation. However, as pixels in organic light-emitting diode (OLED) displays continue to shrink, an important tool for thermal management, infrared thermography, approaches the limits of its spatial resolution. To address this problem, we adapt high-resolution thermoreflectance imaging, which measures minute changes in reflectivity of visible light, to map temperature in a commercial OLED display. We identify a slowly varying thermal signal and find strong evidence of thermal crosstalk, in which a biased pixel heats up its nearest neighbors, even when the neighbors are unbiased. Such crosstalk is indicative of insufficient heat dissipation, which can lead to reduced display lifetime.

High Open-Circuit Voltage in Heterojunction Photovoltaics Using Printed Colloidal Quantum Dots as a Photosensitive Layer

We have successfully printed a thin film of colloidal cadmium selenide (CdSe) quantum dots onto a transparent organic semiconductor, forming a unique solar cell that generates more voltage than previously expected for donor/acceptor photovoltaics. Article not available.