Diogenes Placencia

Enhanced open-circuit voltage of PbS nanocrystal quantum dot solar cells.

Nanocrystal quantum dots (QD) show great promise toward improving solar cell efficiencies through the use of quantum confinement to tune absorbance across the solar spectrum and enable multi-exciton generation. Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage (Voc) is fundamentally limited due to non-radiative recombination processes in QD solar cells. Here we report the highest open-circuit voltages to date for colloidal QD based solar cells under one sun illumination. This Voc of 692 ± 7 mV for 1.4 eV PbS QDs is a result of improved passivation of the defective QD surface, demonstrating as a function of the QD bandgap (Eg). Comparing experimental Voc variation with the theoretical upper-limit obtained from one diode modeling of the cells with different Eg, these results clearly demonstrate that there is a tremendous opportunity for improvement of Voc to values greater than 1 V by using smaller QDs in QD solar cells.

Titanium dioxide electron-selective interlayers created by chemical vapor deposition for inverted configuration organic solar cells

We demonstrate the use of chemical vapor deposition (CVD) to create unique thin (12–36 nm) and conformal TiO2 interlayers on indium-tin oxide (ITO) electrodes, for use as electron collection contacts in inverted bulk heterojunction P3HT/PC61BM organic photovoltaics (OPVs). Optimized CVD formation of these oxide films is inherently scalable to large areas, and may be a viable non-contact alternative to electron-selective interlayer formation. Oxide-based electron-selective interlayers, such as TiO2, need to be thin, conformal and sufficiently electronically conducting films without sacrificing electron harvesting selectivity. Using volatile titanium-tetraisopropoxide (TTIP) precursors in a flowing N2 gas stream, the CVD process provides nanometer control of film thickness to produce 12–36 nm thickness device-quality films. The best performing CVD films, processed at substrate temperatures of ca. 210 °C, characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were found to be amorphous but stoichiometric TiO2. Solution electrochemistries (voltammetry) of probe molecules were shown to be easily accessible indicators of film porosity and are predictive for electron harvesting selectivity (and hole-blocking) in an inverted configuration OPV platform. Small molecules whose redox potentials placed them energetically above the conduction band edge energy (ECB) were reduced/oxidized at nearly the same rates as for bare ITO. Probe molecules whose redox potentials place them energetically within the band gap region, below ECB, show almost complete blocking of their oxidation/reduction processes, for optimized conformal (and nonporous) TiO2 films. In addition, background oxidation current densities for solution probe molecules correlate inversely with the shunt resistance (RP) measured in OPVs. OPVs with the configuration: ITO/CVD-TiO2/P3HT:PC61BM/MoO3/Ag, using TiO2 films of 12, 24 and 36 nm, were evaluated for short-circuit photocurrent (JSC), open-circuit photopotential (VOC), and fill-factor (FF), versus bare ITO. OPVs using amorphous, conformal 24 nm TiO2 interlayers showed the highest fill factors, lowest RS, highest RP and power conversion efficiencies of ca. 3.7%.

Control of PbSe Nanorod Aspect Ratio by Limiting Phosphine Hydrolysis

The aspect ratio and yield of PbSe nanorods synthesized by the reaction of Pb-oleate with tris(diethylamino)phosphine selenide are highly sensitive to the presence of water, making it critical to control the amount of water present in the reaction. By carefully drying the reaction precursors and then intentionally adding water back into the reaction, the nanorod aspect ratio can be controlled from 1.1 to 10 and the yield from 1 to 14% by varying the water concentration from 0 to 204 mM. (31)P{(1)H} and (1)H NMR show that water reacts with tris(diethylamino)phosphine to create bis(diethylamido)phosphorous acid. It was determined that bis(diethylamido)phosphorous acid is responsible for the observed aspect ratio and yield changes. Finally, it was found that excess oleic acid in the reaction can also react with tris(diethylamino)phosphine to create bis(diethylamido)phosphorous acid, and upon the removal of both excess oleic acid and water, highly uniform, nonbranching nanorods were formed.

Anisotropic Absorption in PbSe Nanorods

We present absorption anisotropy measurements in PbSe nanostructures. This is accomplished via a new means of measuring absorption anisotropy in randomly oriented solution ensembles of nanostructures via pump-probe spectroscopy, which exploits the polarization memory effect. We observe isotropic absorption in nanocrystals and anisotropic absorption in nanorods, which increases upon elongation from aspect ratio 1 to 4 and is constant for longer nanorods. The measured volume-normalized absorption cross section is 1.8 ± 0.3 times larger for parallel pump and probe polarizations in randomly oriented nanorods as compared to nanocrystals. We show that this enhancement would be larger than an order of magnitude for aligned nanorods. Despite being in the strong quantum confinement regime, the aspect ratio dependence of the absorption anisotropy in PbSe nanorods is described classically by the effects of dielectric contrast on an anisotropic nanostructure. These results imply that the dielectric constant of the surrounding medium can be used to influence the optoelectronic properties of nanorods, including polarized absorption and emission, phonon modes, multiple exciton generation efficiency, and Auger recombination rate.

Solution Processed Titanyl Phthalocyanines as Donors in Solar Cells: Photoresponse to 1000 nm.

We report a route to thin-film polymorphs of soluble TiOPc derivatives that exhibit similar near-IR absorptivities as vapor deposited thin-films of the parent TiOPc chromophore (phase-I and phase-II polymorphs) and demonstrate that solution-processed planar and bulk heterojunction solar cells fabricated with one of these derivatives exhibited photoactivity throughout the same near-IR wavelength range without compromising VOC. Solution-processed thin-films of soluble octakis(alkylthio)-substituted TiOPc derivatives 1-3 exhibit absorption extending to 1000 nm. When incorporated into OPV devices, the contributions from the lowest CT excitonic state (QB band) of 1 to device performance were evident in both PHJ and BHJ architectures, indicating sufficient driving force for PIET. This contribution was improved via intimate mixing of donor and acceptor molecules in a BHJ architecture, albeit with a decrease in efficiency. IPCE of the best performing BHJ device revealed a contribution from 1 exceeding that of acceptor PCBM, and extending to 1000 nm.

Synthesis and Optical Properties of PbSe Nanorods with Controlled Diameter and Length

The synthesis of PbSe nanorods with low branching (<1%), high aspect ratios (up to ∼16), and controlled lengths and diameters was demonstrated via the removal of water and oleic acid from the synthesis precursors. It was determined that the proper combination of reaction time and temperature allows for the control of PbSe nanorod length and diameter and therefore control over their electronic states, as probed through absorbance and photoluminescence measurements. Similar to PbSe nanowires, nanorods display higher Stokes shifts than for spherical nanocrystals due to intrananorod diameter fluctuations.

Multijunction organic photovoltaic cells for underwater solar power

A modeling and simulation effort is presented that produces a design of an novel organic photovoltaic (OPV) device specifically tailored for underwater (UW) operation. An analysis of the UW environment is presented which highlights the significant advantages of OPV for UW operation. An OPV multijunction design is presented consisting of two absorber layers with the same spectral response, enabling efficient conversion of the narrow UW spectrum and very high voltages. Novel hybrid organic/inorganic materials are introduced that are capable of forming distributed Bragg reflectors for efficient photon management and improved current. Importantly, the entire solar cell design is based on solution processed materials enabling low cost manufacturing.

Energy Level Alignment of Molybdenum Oxide on Colloidal Lead Sulfide (PbS) Thin Films for Optoelectronic Devices

Interfacial charge transport in optoelectronic devices is dependent on energetic alignment that occurs via a number of physical and chemical mechanisms. Herein, we directly connect device performance with measured thickness-dependent energy-level offsets and interfacial chemistry of 1,2-ethanedithiol-treated lead sulfide (PbS) quantum dots and molybdenum oxide. We show that interfacial energetic alignment results from partial charge transfer, quantified via the chemical ratios of Mo5+ relative to Mo6+. The combined effect mitigates leakage current in both the dark and the light, relative to a metal contact, with an overall improvement in open circuit voltage, fill factor, and short circuit current.

Dark current reduction and bandgap-voltage offset in solution-processed nanocrystal solar cells

In this work, we report dark current reduction in solution-processed PbS nanocrystal-metal Schottky junction solar cells via improved LiF passivation of the interface between PbS nanocrystal films and the metal electrode. For the optimized LiF interfacial layer, the dark saturation current density (J0) is decreased by at least one order of magnitude, resulting in very high open-circuit voltage (Voc) of 692±7 mV under one sun illumination for ~1.4 eV PbS nanocrystals. Using different size of PbS nanocrystals and therefore different bandgaps, we also demonstrate Voc (mV)=553Eg/q-59 as a function of the PbS nanocrystal bandgap (Eg). For different types of junctions employed for solution-processed PbS nanocrystal solar cells, we plot the bandgap-voltage offsets (Eg/q-Voc) under open-circuit conditions, showing strong dependence of the Voc on the Eg regardless of the types of junction used. Similar dependence is also found in solution-processed and sintered CdTe nanocrystal solar cells. These results suggest that suppressing the non-radiative recombination contributions to the dark current, such as improved passivation of nanocrystal surfaces, is more critical to improve the Voc in nanocrystal solar cells, rather than optimizing the device architecture with varying the n-type semiconducting materials.

Small molecule organic solar cells with enhanced near-IR photoactivity: The role of texturing and molecular architecture of the active layers on solar cell performance

Small molecule organic solar cells (OPV), using donor layers based on tetravalent and trivalent metal phthalocyanines (e.g. oxo-titanium TiOPc, or chloroindium ClInPc), are discussed here. As-deposited thin films of these Pcs undergo phase changes to form polymorphs with excellent near-IR photoactivity, accompanied by nano-texturing of the donor layers. In OPVs based on Pc/C60 heterojunctions, this results in a factor of up to 5× increase in short-circuit photocurrents (JSC) and power conversion efficiencies. Fully optimized single junction OPVs based on these materials may ultimately approach 6% efficiencies, however their biggest impact is in providing guidance for the formation of new, solution processable forms of these materials, which retain this near-IR sensitivity and high power conversion efficiencies.