Timothy P. Osedach

Improved Current Extraction from ZnO/PbS Quantum Dot Heterojunction Photovoltaics Using a MoO3 Interfacial Layer

The ability to engineer interfacial energy offsets in photovoltaic devices is one of the keys to their optimization. Here, we demonstrate that improvements in power conversion efficiency may be attained for ZnO/PbS heterojunction quantum dot photovoltaics through the incorporation of a MoO(3) interlayer between the PbS colloidal quantum dot film and the top-contact anode. Through a combination of current-voltage characterization, circuit modeling, Mott-Schottky analysis, and external quantum efficiency measurements performed with bottom- and top-illumination, these enhancements are shown to stem from the elimination of a reverse-bias Schottky diode present at the PbS/anode interface. The incorporation of the high-work-function MoO(3) layer pins the Fermi level of the top contact, effectively decoupling the device performance from the work function of the anode and resulting in a high open-circuit voltage (0.59 ± 0.01 V) for a range of different anode materials. Corresponding increases in short-circuit current and fill factor enable 1.5-fold, 2.3-fold, and 4.5-fold enhancements in photovoltaic device efficiency for gold, silver, and ITO anodes, respectively, and result in a power conversion efficiency of 3.5 ± 0.4% for a device employing a gold anode.

Practical Roadmap and Limits to Nanostructured Photovoltaics

The significant research interest in the engineering of photovoltaic (PV) structures at the nanoscale is directed toward enabling reductions in PV module fabrication and installation costs as well as improving cell power conversion efficiency (PCE). With the emergence of a multitude of nanostructured photovoltaic (nano-PV) device architectures, the question has arisen of where both the practical and the fundamental limits of performance reside in these new systems. Here, the former is addressed a posteriori. The specific challenges associated with improving the electrical power conversion efficiency of various nano-PV technologies are discussed and several approaches to reduce their thermal losses beyond the single bandgap limit are reviewed. Critical considerations related to the module lifetime and cost that are unique to nano-PV architectures are also addressed. The analysis suggests that a practical single-junction laboratory power conversion efficiency limit of 17% and a two-cell tandem power conversion efficiency limit of 24% are possible for nano-PVs, which, when combined with operating lifetimes of 10 to 15 years, could position them as a transformational technology for solar energy markets.

Effect of synthetic accessibility on the commercial viability of organic photovoltaics

For organic photovoltaics (OPVs) to become a viable source of renewable energy, the synthesis of organic active-layer materials will need to be scaled to thousands of kilograms. Additionally, the ultimate cost of these materials will need to be low enough to constitute only a small fraction of the cost of the solar cell module. In this study, we present a quantitative analysis, based on published small-scale synthetic procedures, to estimate the materials costs for several promising OPV materials when produced in large quantities. The cost in dollars-per-gram (

Bias-Stress Effect in 1,2-Ethanedithiol-Treated PbS Quantum Dot Field-Effect Transistors

per g) is found to increase linearly with the number of synthetic steps required to produce each organic photoactive compound. We estimate the cost-per-Watt (

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

per Wp) as a function of power conversion efficiency (PCE) for an archetypal OPV structure and find that a relatively simple molecule requiring only 3 synthetic steps will contribute a cost of 0.001 to 0.01

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

per Wp, given a solar module PCE of 10%. In contrast, a relatively complicated molecule requiring 14 synthetic steps will contribute costs in the range of 0.075 to 0.48

Detection of charge storage on molecular thin films of tris(8-hydroxyquinoline) aluminum (Alq3) by Kelvin force microscopy: a candidate system for high storage capacity memory cells.

per Wp. Our findings suggest that the commercial viability of an OPV technology may depend on the synthetic accessibility of its constituent active layer materials. Additionally, this work stresses the importance of optimizing synthetic routes to minimize solvent and reagent usage as well as to minimize the number of required workup procedures in the scaled production of OPV materials.

Multijunction organic photovoltaics with a broad spectral response

We investigate the bias-stress effect in field-effect transistors (FETs) consisting of 1,2-ethanedithiol-treated PbS quantum dot (QD) films as charge transport layers in a top-gated configuration. The FETs exhibit ambipolar operation with typical mobilities on the order of μ(e) = 8 × 10(-3) cm(2) V(-1) s(-1) in n-channel operation and μ(h) = 1 × 10(-3) cm(2) V(-1) s(-1) in p-channel operation. When the FET is turned on in n-channel or p-channel mode, the established drain-source current rapidly decreases from its initial magnitude in a stretched exponential decay, manifesting the bias-stress effect. The choice of dielectric is found to have little effect on the characteristics of this bias-stress effect, leading us to conclude that the associated charge-trapping process originates within the QD film itself. Measurements of bias-stress-induced time-dependent decays in the drain-source current (I(DS)) are well fit to stretched exponential functions, and the time constants of these decays in n-channel and p-channel operation are found to follow thermally activated (Arrhenius) behavior. Measurements as a function of QD size reveal that the stressing process in n-channel operation is faster for QDs of a smaller diameter while stress in p-channel operation is found to be relatively invariant to QD size. Our results are consistent with a mechanism in which field-induced nanoscale morphological changes within the QD film result in screening of the applied gate field. This phenomenon is entirely recoverable, which allows us to repeatedly observe bias stress and recovery characteristics on the same device. This work elucidates aspects of charge transport in chemically treated lead chalcogenide QD films and is of relevance to ongoing investigations toward employing these films in optoelectronic devices.

Near-infrared photodetector consisting of J-aggregating cyanine dye and metal oxide thin films

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 ]

High-density charge storage on molecular thin films - candidate materials for high storage capacity memory cells

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.