An X-ray scattering and electron microscopy study of methylammonium bismuth perovskites for solar cell applications

Photovoltaics made from organic–inorganic hybrid perovskite semiconductors are attracting significant interest due to their ability to harvest sunlight with remarkable efficiency. The presence of lead in the best performing devices raises concerns regarding their toxicity, a problem that may create barriers to commercialization. Hybrid perovskites with reduced lead content are being investigated to overcome this issue and here we evaluate bismuth as a possible lead substitute. For a series of hybrid perovskite films with the general composition CH3NH3(PbyBi1−y)I3−xClx, we characterize their optical and structural properties using UV–Vis spectroscopy, scanning electron microscopy and grazing incidence wide angle X-ray scattering. We show that they form crystalline structures with an optical band gap, around 2 eV for CH3NH3BiI3. However, preliminary solar cell tests show low power conversion efficiencies (<0.01%) due to both incomplete precursor conversion and material de-wetting from the substrate. The overall outcome is severely limited photocurrent. With current processing methods the general applicability of hybrid bismuth perovskites in photovoltaics may be limited

Monitoring the Formation of a CH<inf>3</inf>NH<inf>3</inf>PbI<inf>3-</inf><inf>x</inf>Cl<inf>x</inf> Perovskite during Thermal Annealing Using X-Ray Scattering

Grazing incidence wide and small angle X-ray scattering (GIWAXS and GISAXS) measurements have been used to study the crystallization kinetics of the organolead halide perovskite CH3NH3PbI3-xClx during thermal annealing. In situ GIWAXS measurements recorded during annealing are used to characterize and quantify the transition from a crystalline precursor to the perovskite structure. In situ GISAXS measurements indicate an evolution of crystallite sizes during annealing, with the number of crystallites having sizes between 30 and 400 nm increasing through the annealing process. Using ex situ scanning electron microscopy, this evolution in length scales is confirmed and a concurrent increase in film surface coverage is observed, a parameter crucial for efficient solar cell performance. A series of photovoltaic devices are then fabricated in which perovskite films have been annealed for different times, and variations in device performance are explained on the basis of X-ray scattering measurements

Multilayer broadband anti-reflective coatings for bulk heterojunction polymer solar cells

The photocurrent produced by solar cells is limited by reflection losses for all types of photovoltaic devices. The first reflection loss occurs at the glass/air interface of the photovoltaic device. A solar cell without a light trapping mechanism in place loses around 4 % of the solar energy at this interface. To minimise the losses, a broadband multilayer thin film anti-reflection (MAR) coating has been designed and deposited onto the glass surface of a solar cell. The coating consisted of four dielectric layers of alternating thin films of ZrO2 and SiO2. The layers were deposited by using high rate pulsed DC magnetron sputtering using time only for nanometre thickness control. Spectrophotometer measurements confirm that the transmission increased over the spectrum utilized by the bulk heterojunction (BHJ) solar cell (350nm- 700nm). The weighted average reflection reduced from 4.22% to 0.99%. BHJ solar cells with a PCDTBT:PCBM blend serving as the active layer were prepared on a MAR coated soda lime glass slides to verify the effectiveness of the coating. The efficiency increased by 0.18% at STC from 4.98% to 5.17% (a 3.7% relative increase). The gain was achieved by increasing the photocurrent from 11.96 mA/cm2 to 12.36 mA/cm2

High Temporal and Spectral Resolution Interferometric Observations of Unusual Solar Radio Bursts

We report very high temporal and spectral resolution interferometric observations of some unusual solar radio bursts near 1420 MHz. These bursts were observed on 13 September 2005, 22 minutes after the peak of a GOES class X flare from the NOAA region 10808. Our observations show 11 episodes of narrow-band intermittent emission within a span of ≈8 s. Each episode shows a heavily frequency-modulated band of emission with a spectral slope of about −245.5 MHz s−1 [s superscript -1], comprising up to 8 individual blobs of emission and lasts for 10–15 ms. The blobs themselves have a spectral slope of ≈ 0 MHz s−1 [s superscript -1], are ≈200–250 kHz wide, appear every ≈400 kHz and last for ≈ 4–5 ms. These bursts show a brightness temperatures in the range 1012 [10 superscript 12] K, which suggests a coherent emission mechanism. We believe these are the first high temporal and spectral resolution interferometric observations of such rapid and narrow bandwidth solar bursts close to 1420 MHz and present an analysis of their temporal and spectral characteristics.National Science Foundation (U.S.). Research Experience for Undergraduates (Program) (Grant AST-0138506

Anti-topoisomerase drugs as potent inducers of chromosomal aberrations

DNA topoisomerases catalyze topological changes in DNA that are essential for normal cell cycle progression and therefore they are a preferential target for the development of anticancer drugs. Anti-topoisomerase drugs can be divided into two main classes: "cleavable complex" poisons and catalytic inhibitors. The "cleavable complex" poisons are very effective as anticancer drugs but are also potent inducers of chromosome aberrations so they can cause secondary malignancies. Catalytic inhibitors are cytotoxic but they do not induce chromosome aberrations. Knowledge about the mechanism of action of topoisomerase inhibitors is important to determine the best anti-topoisomerase combinations, with a reduced risk of induction of secondary malignancies.<br>As topoisomerases de DNA catalisam alterações topológicas no DNA que são essenciais para a progressão do ciclo celular normal e, portanto, são um alvo preferencial para o desenvolvimento de drogas anticâncer. Drogas anti-topoisomerases podem ser divididas em duas classes principais: drogas anti-"complexos cliváveis" e inibidores catalíticos. As drogas anti-"complexos cliváveis" são muito eficazes como drogas anticancerígenas, mas são também potentes indutores de aberrações cromossômicas, podendo causar neoplasias malignas secundárias. Inibidores catalíticos são citotóxicos mas não induzem aberrações cromossômicas. Conhecimento a respeito do mecanismo de ação de inibidores de topoisomerases é importante para determinar as melhores combinações anti-topoisomerases, com um reduzido risco de indução de neoplasias malignas secundárias