Educomunicação, Transformação Social e Desenvolvimento Sustentável

Esta publicação apresenta os principais trabalhos dos GTs do II Congresso Internacional de Comunicação e Educação nos temas Transformação social, com os artigos que abordam principalmente Educomunicação e/ou Mídia-Educação, no contexto de políticas de diversidade, inclusão e equidade; e, em Desenvolvimento Sustentável os artigos que abordam os avanços da relação comunicação/educação no contexto da educação ambiental e desenvolvimento sustentável

Solid-State Mesostructured Perovskite CH₃NH₃PbI₃ Solar Cells: Charge Transport, Recombination, and Diffusion Length

We report on the effect of TiO₂ film thickness on charge transport and recombination in solid-state mesostructured perovskite CH₃NH₃PbI₃ (via one-step coating) solar cells using spiro-MeOTAD as the hole conductor. Intensity-modulated photocurrent/photovoltage spectroscopies show that the transport and recombination properties of solid-state mesostructured perovskite solar cells are similar to those of solid-state dye-sensitized solar cells. Charge transport in perovskite cells is dominated by electron conduction within the mesoporous TiO₂ network rather than from the perovskite layer. Although no significant film-thickness dependence is found for transport and recombination, the efficiency of perovskite cells increases with TiO₂ film thickness from 240 nm to about 650–850 nm owing primarily to the enhanced light harvesting. Further increasing film thickness reduces cell efficiency associated with decreased fill factor or photocurrent density. The electron diffusion length in mesostructured perovskite cells is longer than 1 μm for over four orders of magnitude of light intensity

Integrated optical and electrical modeling of plasmon-enhanced thin film photovoltaics : a case-study on organic devices

The nanoscale light control for absorption enhancement of organic photovoltaic (OPV) devices inevitably produces strongly non-uniform optical fields. These non-uniformities due to the localized optical modes are a primary route toward absorption enhancement in OPV devices. Therefore, a rigorous modeling tool taking into account the spatial distribution of optical field and carrier generation is necessary. Presented here is a comprehensive numerical model to describe the coupled optical and electrical behavior of plasmon-enhanced polymer:fullerene bulk heterojunction (BHJ) solar cells. In this model, a position-dependent electron-hole pair generation rate that could become highly non-uniform due to photonic nanostructures is directly calculated from the optical simulations. By considering the absorption and plasmonic properties of nanophotonic gratings included in two different popular device architectures, and applying the Poisson, current continuity, and drift/diffusion equations, the model predicts quantum efficiency, short-circuit current density, and desired carrier mobility ratios for bulk heterojunction devices incorporating nanostructures for light management. In particular, the model predicts a significant degradation of device performance when the carrier species with lower mobility are generated far from the collecting electrode. Consequently, an inverted device architecture is preferred for materials with low hole mobility. This is especially true for devices that include plasmonic nanostructures. Additionally, due to the incorporation of a plasmonic nanostructure, we use simulations to theoretically predict absorption band broadening of a BHJ into energies below the band gap, resulting in a 4.8% increase in generated photocurrent.8 page(s

Time-of-flight studies of electron-collection kinetics in polymer : fullerene bulk-heterojunction solar cells

The charge-collection dynamics in poly(3-hexylthiophene:[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk heterojunctions are studied in thick (>1 μm) devices using time-of-flight measurements and external quantum-efficiency measurements. The devices show Schottky-diode behavior with a large field-free region in the device. Consequently, electron transport occurs by diffusion in the bulk of the active layer. At high applied biases where the depletion region spans the entire active layer, normal time-of-flight transients are observed from which the electron mobility can be determined. Here, the electron mobility follows Poole–Frenkel behavior as a function of field. At lower applied biases, where the depletion region only spans a small portion of the active layer, due to a high density of dark holes, the recombination kinetics follow a first-order rate law with a rate constant about two orders of magnitude lower than that predicted by Langevin recombination. Erratum can be found in Advanced functional materials, vol. 21, issue 22, p. 4210 doi: 10.1002/adfm.2011900997 page(s

Surface plasmon enhanced infrared absorption in the sensitized polymer solar cell

We have theoretically demonstrated an enhanced infrared absorption of the sensitizer in ternary polymer solar cell by introducing silver gratings at the back metal electrode. A combined model which incorporates the complex optical absorption profile and the electrical transport of the generated charge carriers was successfully developed. Using this model, we considered Si-PCPDTBT as an infrared sensitizer for P3HT:ICBA bulk heterojunction solar cells. A silver grating feature was optimized to produce a highly localized optical field inside the active polymer layer and enhance the infrared absorption of the sensitizer. Finally, an overall short-circuit current enhancement of about 40% is obtained theoretically.3 page(s

Photoinduced charge carrier generation and decay in sequentially deposited polymer/fullerene layers : bulk heterojunction vs planar interface

In this work, we use the time-resolved microwave conductivity (TRMC) technique to study the dynamics of charge carrier generation in sequentially deposited conjugated polymer/fullerene layers. These layers are either fully solution-processed, using orthogonal solvents for the layers of the polymer poly(3-hexylthiophene) (P3HT) and the fullerene phenyl-C₆₁-butyric acid methyl ester (PCBM), or prepared by thermally evaporating a C₆₀ layer onto P3HT films. Our work is motivated by the remarkable efficiency of organic photovoltaic (OPV) devices using a sequentially processed P3HT/PCBM active layer. Here we use an electrodeless photoconductivity probe, so we can photoexcite the sample either through the polymer or the fullerene layer. We use samples with extremely thick P3HT films (2.4 μm) and show that excitation from either side of both as-cast and thermally annealed sample yields virtually identical results, consistent with mixing of the PCBM into the polymer film. We also compare solution-deposited samples to samples made by thermally evaporating C₆₀ on P3HT, and find that we can distinguish between charge generation in bulk-P3HT and at the polymer/fullerene interface. We show that, despite their morphological differences, the carrier dynamics in the sequentially processed samples resemble those of mixed, bulk heterojunction (BHJ) systems. All of this is consistent with the idea that PCBM readily mixes into the P3HT film in sequentially deposited P3HT/PCBM samples, although the total amount of fullerene mixed into the P3HT appears to be less than that typically used in an optimized BHJ. Finally, we discuss the implications for OPV device architectures prepared by sequential deposition from solution.13 page(s

Control of charge separation by electric field manipulation in polymer-oxide hybrid organic photovoltaic bilayer devices

Hybrid polymer/oxide organic photovoltaic (h-OPV) devices have the potential to replace the organic acceptor component with an inorganic, nano-structured oxide. This approach leverages the positive attributes of inorganic materials while maintaining the potential processing advantages of organic electronics. By manipulating the carrier concentration of the oxide acceptor layer in bilayer h-OPV devices, we demonstrate control of the electric field at the planar donor-acceptor interface. The effects of the electric field can be observed in both the J SC and the fill factor of the h-OPV devices. Furthermore, interfacial layers of TiOx are used on ZnO to prevent recombination of geminate electron-hole pairs. It is shown that interfacial TiO x successfully inhibits recombination only when the electric field is strong enough to sufficiently transfer charges to the ZnO layer. If the interfacial electric field is insufficient, then the TiO x instead serves to enhance recombination at the donor-acceptor interface.9 page(s

Integrating nanostructured electrodes in organic photovoltaic devices for enhancing near-infrared photoresponse

We introduce a simple methodology to integrate prefabricated nanostructured-electrodes in solution-processed organic photovoltaic (OPV) devices. The tailored “photonic electrode” nanostructure is used for light management in the device and for hole collection. This approach opens up new possibilities for designing photonically active structures that can enhance the absorption of sub-bandgap photons in the active layer. We discuss the design, fabrication and characterization of photonic electrodes, and the methodology for integrating them to OPV devices using a simple lamination technique. We demonstrate theoretically and experimentally that OPV devices using photonic electrodes show a factor of ca. 5 enhancement in external quantum efficiency (EQE) in the near infrared region. We use simulations to trace this observed efficiency enhancement to surface plasmon polariton modes in the nanostructure.5 page(s

5,10-Dihydroindolo[3,2-b]indole-based copolymers with alternating donor and acceptor moieties for organic photovoltaics

A series of new donor−acceptor π-conjugated copolymers incorporating 5,10-dihydroindolo[3,2-b]indole (DINI) as an electron donating unit have been designed, synthesized, and explored in bulk heterojunction solar cells with diketopyrrolopyrrole and thienopyrroledione as the electron accepting units. A significant effect of the size and shape of the pendant alkyl substituents attached to the DINI unit on the optical and electronic properties of the copolymers is described. Our study reveals a good correlation between the theoretical calculations performed on the selected materials and the experimental HOMO, LUMO, absorption spectra, and band gap energies of the corresponding copolymers. The band gaps of the conjugated copolymers can be tailored over 0.4 eV by the electron-withdrawing nature of the different acceptor units to provide better overlap with the solar spectrum, and the energy levels can be tuned ~0.2 eV depending on the alkyl substituents employed. For the polymers in this study, a nonoptimized power conversion efficiency as high as 3% was observed.11 page(s

Thermal annealing affects vertical morphology, doping and defect density in BHJ OPV devices

We demonstrate that a post-annealing step results in enhanced open-circuit voltage (Voc) and fill factor (FF) and lower reverse saturation current (Js) that consequently increases the power conversion efficiency (PCE) of organic bulk-heterojunction (BHJ) devices by about 40 % as a result of better contact formation, as typically assumed. Although true, we show that additional device properties are affected as well. We found that annealing induces vertical phase segregation and consequently the enrichment of donor and acceptor materials at the correct electrical contact. In addition, a de-doping process and a decrease in defect density also take place and are the major causes for device improvement after post-annealing the OPV devices. Implications for OPV basic research and manufacturing are discussed.6 page(s