12 research outputs found

    Efficient All-Printable Solid-State Dye-Sensitized Solar Cell Based on a Low-Resistivity Carbon Composite Counter Electrode and Highly Doped Hole Transport Material

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    Monolithic device architectures provide a route to large-area mesoporous solar cell manufacture through scalable solution-processed fabrication. A limiting factor in device scale-up is availability of low-resistivity printable counter electrode materials and reliable doped charge transport materials. We report an efficient all-printable monolithic solid-state dye-sensitized solar cell (ss-DSC) based on a high-conductivity porous carbon counter electrode and a highly doped 2,2′,7,7′-tetrakis­(<i>N</i>,<i>N</i>-di-4-methoxy­phenyl­amino)-9,9′-spiro­bi­fluorene (spiro-OMeTAD) hole transport material (HTM). A review of current state-of-the-art printable porous counter electrodes in DSC literature was conducted and identified blends of graphite/carbon black as promising composites for high-conductivity electrodes. Direct ex situ oxidation of spiro-OMeTAD produced a stable HTM dopant, and its incorporation with one of the lowest-resistivity graphite/carbon black composite materials reported to date drastically decreases device series resistance, particularly that of the porous insulating spacer. Doping improved all performance parameters, and following optimization we demonstrate scaled-up 1.21 cm<sup>2</sup> (1.01 cm<sup>2</sup> masked) devices achieving a maximum efficiency of 3.34% (average, 3.05 ± 0.23%)

    Near-Infrared Absorbing Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> and Cu<sub>3</sub>SbS<sub>4</sub> Nanocrystals: Synthesis, Characterization, and Photoelectrochemistry

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    Herein, we present the novel synthesis of tetrahedrite copper antimony sulfide (CAS) nanocrystals (Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub>), which display strong absorptions in the visible and NIR. Through ligand tuning, the size of the Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> NCs may be increased from 6 to 18 nm. Phase purity is achieved through optimizing the ligand chemistry and maximizing the reactivity of the antimony precursor. We provide a detailed investigation of the optical and photoelectrical properties of both tetrahedrite (Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub>) and famatinite (Cu<sub>3</sub>SbS<sub>4</sub>) NCs. These NCs were found to have very high absorption coefficients reaching 10<sup>5</sup> cm<sup>–1</sup> and band gaps of 1.7 and 1 eV for tetrahedrite and famatinite NCs, respectively. Ultraviolet photoelectron spectroscopy was employed to determine the band positions. In each case, the Fermi energies reside close to the valence band, indicative of a p-type semiconductor. Annealing of tetrahedrite CAS NC films in sulfur vapor at 350 °C was found to result in pure famatinite NC films, opening the possibility to tune the crystal structure within thin films of these NCs. Photoelectrochemistry of hydrazine free unannealed films displays a strong p-type photoresponse, with up to 0.1 mA/cm<sup>2</sup> measured under mild illumination. Collectively these optical properties make CAS NCs an excellent new candidate for both thin film and hybrid solar cells and as strong NIR absorbers in general

    Surface State Recombination and Passivation in Nanocrystalline TiO<sub>2</sub> Dye-Sensitized Solar Cells

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    The relative role of surface state recombination in dye-sensitized solar cells is not fully understood, yet reductions in the recombination rate are frequently attributed to the passivation of surface states. We have investigated reports of trap state passivation using an Al<sub>2</sub>O<sub>3</sub>-coated TiO<sub>2</sub> photoanode achieved through atomic layer deposition (ALD). Electrochemical characterization, performed through impedance measurements and intensity modulated photovoltage spectroscopy (IMVS), data showed that the Al<sub>2</sub>O<sub>3</sub> deposition successfully blocked electron recombination and that the chemical capacitance of the film was unchanged after the ALD treatment. A theoretical model outlining the recombination kinetics was applied to the experimental data to obtain charge transfer rates from conduction band states, exponentially distributed traps, and monoenergetic traps. The determined electron transfer rates showed that the deposited Al<sub>2</sub>O<sub>3</sub> coating did not selectively passivate trap states at the nanoparticle surface but reduced recombination rates equally from both conduction band states and surface states. These results imply that the reduction in the recombination rates reported in core–shell structured photoanodes cannot be attributed to a modification of surface traps, but rather to the weakened electronic coupling between electrons in the film and the electrolyte species

    Mimicry of Sputtered <i>i-</i>ZnO Thin Films Using Chemical Bath Deposition for Solution-Processed Solar Cells

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    Solution processing provides a versatile and inexpensive means to prepare functional materials with specifically designed properties. The current challenge is to mimic the structural, optical, and/or chemical properties of thin films fabricated by vacuum-based techniques using solution-based approaches. In this work we focus on ZnO to show that thin films grown using a simple, aqueous-based, chemical bath deposition (CBD) method can mimic the properties of sputtered coatings, provided that the kinetic and thermodynamic reaction parameters are carefully tuned. The role of these parameters toward growing highly oriented and dense ZnO thin films is fully elucidated through detailed microscopic and spectroscopic investigations. The prepared samples exhibit bulk-like optical properties, are intrinsic in their electronic characteristics, and possess negligible organic contaminants, especially when compared to ZnO layers deposited by sol–gel or from nanocrystal inks. The efficacy of our CBD-grown ZnO thin films is demonstrated through the effective replacement of sputtered ZnO buffer layers within high efficiency solution processed Cu<sub>2</sub>ZnSnS<sub>4<i>x</i></sub>Se<sub>4(1–<i>x</i>)</sub> solar cells

    In Situ Formation of Reactive Sulfide Precursors in the One-Pot, Multigram Synthesis of Cu<sub>2</sub>ZnSnS<sub>4</sub> Nanocrystals

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    Herein we outline a general one-pot method to produce large quantities of compositionally tunable, kesterite Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS) nanocrystals (NCs) through the decomposition of in situ generated metal sulfide precursors. This method uses air stable precursors and should be applicable to the synthesis of a range of metal sulfides. We examine the formation of the ligands, precursors, and particles in turn. Direct reaction of CS<sub>2</sub> with the aliphatic primary amines and thiols that already constitute the reaction mixture is used to produce ligands in situ. Through the use of <sup>1</sup>H and <sup>13</sup>C nuclear magnetic resonance, Fourier transform infrared spectroscopy, and optical absorption spectroscopy, we elucidate the formation of the resulting oleyldithiocarbamate and dodecyltrithiocarbonate ligands. The decomposition of their corresponding metal complexes at temperatures of ∼100 °C yields nuclei with a size of 1–2 nm, with further growth facilitated by the decomposition of dodecanethiol. In this way the nucleation and growth stages of the reaction are decoupled, allowing for the generation of NCs at high concentrations. Using in situ X-ray diffraction, we monitor the evolution of our reactions, thus enabling a real-time glimpse into the formation of Cu<sub>2</sub>ZnSnS<sub>4</sub> NCs. For completeness, the surface chemistry and the electronic structure of the resulting CZTS NCs are studied

    Cu<sub>2</sub>ZnGeS<sub>4</sub> Nanocrystals from Air-Stable Precursors for Sintered Thin Film Alloys

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    The synthesis of an air and moisture stable germanium complex and its use in the synthesis of ternary and quaternary copper containing nanocrystals (NCs) is described. Through the use of <sup>1</sup>H-/<sup>13</sup>C nuclear magnetic resonance and Fourier transform infrared spectroscopies, thermogravimetric analysis, and powder X-ray diffraction, the speciation and chemistry of this precursor is elucidated. This germanium source is employed in the gram scale, noninjection synthesis of Cu<sub>2</sub>ZnGeS<sub>4</sub> (CZGeS) and Cu<sub>2</sub>GeS<sub>3</sub> (CGeS) NCs using a binary sulfide precursor approach. To demonstrate the versatility of such NCs for fabricating thin films suitable for high-efficiency optoelectronic devices, they are blended with Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS) NCs and selenized to form homogeneously alloyed Cu<sub>2</sub>ZnSn<sub><i>x</i></sub>Ge<sub>1–<i>x</i></sub>S<sub><i>y</i></sub>Se<sub>4–<i>y</i></sub> (CZTGeSSe) thin films. The structural, optical, and electronic properties of such thin films are studied using X-ray diffraction, scanning electron microscopy, UV−vis−NIR spectroscopy, and photoelectron spectroscopy in air. These measurements demonstrate collectively that incorporating Ge into micrometer-sized, tetragonal Cu<sub>2</sub>ZnSnS<sub><i>x</i></sub>Se<sub>4–<i>x</i></sub> (CZTSSe) provides a facile manner in which the conduction band energy can be readily tuned. The strategy developed herein provides a pathway to controlled levels of Ge incorporation in a single step process, thus avoiding the need for intra-alloyed Cu<sub>2</sub>ZnSn<sub><i>x</i></sub>Ge<sub>1–<i>x</i></sub>S<sub>4</sub> nanocrystals

    Probing the Interaction between Individual Metal Nanocrystals and Two-Dimensional Metal Oxides via Electron Energy Loss Spectroscopy

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    Metal nanoparticles can photosensitize two-dimensional metal oxides, facilitating their electrical connection to devices and enhancing their abilities in catalysis and sensing. In this study, we investigated how individual silver nanoparticles interact with two-dimensional tin oxide and antimony-doped indium oxide using electron energy loss spectroscopy (EELS). The measurement of the spectral line width of the longitudinal plasmon resonance of the nanoparticles in absence and presence of 2D materials allowed us to quantify the contribution of chemical interface damping to the line width. Our analysis reveals that a stronger interaction (damping) occurs with 2D antimony-doped indium oxide due to its highly homogeneous surface. The results of this study offer new insight into the interaction between metal nanoparticles and 2D materials

    A New Direction in Dye-Sensitized Solar Cells Redox Mediator Development: In Situ Fine-Tuning of the Cobalt(II)/(III) Redox Potential through Lewis Base Interactions

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    Dye-sensitized solar cells (DSCs) are an attractive renewable energy technology currently under intense investigation. In recent years, one area of major interest has been the exploration of alternatives to the classical iodide/triiodide redox shuttle, with particular attention focused on cobalt complexes with the general formula [Co­(L)<sub><i>n</i></sub>]<sup>2+/3+</sup>. We introduce a new approach to designing redox mediators that involves the application of [Co­(PY5Me<sub>2</sub>)­(MeCN)]<sup>2+/3+</sup> complexes, where PY5Me<sub>2</sub> is the pentadentate ligand, 2,6-bis­(1,1-bis­(2-pyridyl)­ethyl)­pyridine. It is shown, by X-ray crystallography, that the axial acetonitrile (MeCN) ligand can be replaced by more strongly coordinating Lewis bases (B) to give complexes with the general formula [Co­(PY5Me<sub>2</sub>)­(B)]<sup>2+/3+</sup>, where B = 4-<i>tert-</i>butylpyridine (tBP) or <i>N</i>-methylbenzimidazole (NMBI). These commonly applied DSC electrolyte components are used for the first time to fine-tune the potential of the redox couple to the requirements of the dye through coordinative interactions with the Co<sup>II/III</sup> centers. Application of electrolytes based on the [Co­(PY5Me<sub>2</sub>)­(NMBI)]<sup>2+/3+</sup> complex in combination with a commercially available organic sensitizer has enabled us to attain DSC efficiencies of 8.4% and 9.2% at a simulated light intensity of 100% sun (1000 W m<sup>–2</sup> AM1.5 G) and at 10% sun, respectively, higher than analogous devices applying the [Co­(bpy)<sub>3</sub>]<sup>2+/3+</sup> redox couple, and an open circuit voltage (<i>V</i><sub>oc</sub>) of almost 1.0 V at 100% sun for devices constructed with the tBP complex

    Tunable Crystallization and Nucleation of Planar CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> through Solvent-Modified Interdiffusion

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    A smooth and compact light absorption perovskite layer is a highly desirable prerequisite for efficient planar perovskite solar cells. However, the rapid reaction between CH<sub>3</sub>NH<sub>3</sub>I methylammonium iodide (MAI) and PbI<sub>2</sub> often leads to an inconsistent CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> crystal nucleation and growth rate along the film depth during the two-step sequential deposition process. Herein, a facile solvent additive strategy is reported to retard the crystallization kinetics of perovskite formation and accelerate the MAI diffusion across the PbI<sub>2</sub> layer. It was found that the ultrasmooth perovskite thin film with narrow crystallite size variation can be achieved by introducing favorable solvent additives into the MAI solution. The effects of dimethylformamide, dimethyl sulfoxide, Îł-butyrolactone, chlorobenzene, and diethyl ether additives on the morphological properties and cross-sectional crystallite size distribution were investigated using atomic force microscopy, X-ray diffraction, and scanning electron microscopy. Furthermore, the light absorption and band structure of the as-prepared CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> films were investigated and correlated with the photovoltaic performance of the equivalent solar cell devices. Details of perovskite nucleation and crystal growth processes are presented, which opens new avenues for the fabrication of more efficient planar solar cell devices with these ultrasmooth perovskite layers

    Toward Rollable Printed Perovskite Solar Cells for Deployment in Low-Earth Orbit Space Applications

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    The thin physical profile of perovskite-based solar cells (PSCs) fabricated on flexible substrates provides the prospect of a disruptive increase in specific power (power-to-mass ratio), an important figure-of-merit for solar cells to be used in space applications. In contrast to recent reports on space applications of PSCs which focus on rigid glass-based devices, in this work we investigate the suitability of flexible PSCs for low-earth orbit (LEO) applications, where the perovskite layer in the PSCs was prepared using either a Ruddlesden–Popper precursor composition (BA2MA3Pb4I13; BA = butylammonium, MA = methylammonium) or a mixed-cation precursor composition (Cs0.05FA0.81MA0.14Pb2.55Br0.45; FA = formamidinium). The flexible PSC devices display a tolerance to high-energy proton (14 MeV) and electron (>1 MeV) radiation comparable with, or superior to, equivalent glass-based PSC devices. The photovoltaic performance of the PSCs is found to be significantly less dependent on angle-of-incidence than GaAs-based triple-junction solar cells commonly used for space applications. Results from a preliminary test of the robustness of the perovskite film when subjected to LEO-like thermal environments are also reported. In addition, a unique deployment concept integrating printed flexible solar cells with titanium–nickel based shape memory alloy ribbons is presented for thermally actuated deployment of flexible solar cells from a rolled state
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