High-efficiency fullerene solar cells enabled by a spontaneously formed mesostructured CuSCN-nanowire heterointerface

Fullerenes and their derivatives are widely used as electron acceptors in bulk-heterojunction organic solar cells as they combine high electron mobility with good solubility and miscibility with relevant semiconducting polymers. However, studies on the use of fullerenes as the sole photogeneration and charge-carrier material are scarce. Here, a new type of solution-processed small-molecule solar cell based on the two most commonly used methanofullerenes, namely [6,6]-phenyl-C61-butyric acid methyl ester (PC 60 BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC 70 BM), as the light absorbing materials, is reported. First, it is shown that both fullerene derivatives exhibit excellent ambipolar charge transport with balanced hole and electron mobilities. When the two derivatives are spin-coated over the wide bandgap p-type semiconductor copper (I) thiocyanate (CuSCN), cells with power conversion efficiency (PCE) of ≈1%, are obtained. Blending the CuSCN with PC 70 BM is shown to increase the performance further yielding cells with an open-circuit voltage of ≈0.93 V and a PCE of 5.4%. Microstructural analysis reveals that the key to this success is the spontaneous formation of a unique mesostructured p-n-like heterointerface between CuSCN and PC 70 BM. The findings pave the way to an exciting new class of single photoactive material based solar cells

Crystallographic, Optical, and Electronic Properties of the Cs2AgBi1–xInxBr6 Double Perovskite: Understanding the Fundamental Photovoltaic Efficiency Challenges

We present a crystallographic and optoelectronic study of the double perovskite Cs2AgBi1–xInxBr6. From structural characterization we determine that the indium cation shrinks the lattice and shifts the cubic-to-tetragonal phase transition point to lower temperatures. The absorption onset is shifted to shorter wavelengths upon increasing the indium content, leading to wider band gaps, which we rationalize through first-principles band structure calculations. Despite the unfavorable band gap shift, we observe an enhancement in the steady-state photoluminescence intensity, and n-i-p photovoltaic devices present short-circuit current greater than that of neat Cs2AgBiBr6 devices. In order to evaluate the prospects of this material as a solar absorber, we combine accurate absorption measurements with thermodynamic modeling and identify the fundamental limitations of this system. Provided radiative efficiency can be increased and the choice of charge extraction layers are specifically improved, this material could prove to be a useful wide band gap solar absorber

Surface and optical properties of phase-pure silver iodobismuthate nanocrystals

The study of surface defects is one of the forefronts of halide perovskite research. In the nanoscale regime, where the surface-to-volume ratio is high, the surface plays a key role in determining the electronic properties of perovskites. Perovskite-inspired silver iodobismuthates are promising photovoltaic absorbers. Herein, we demonstrate the colloidal synthesis of phase pure and highly crystalline AgBiI4 nanocrystals (NCs). Surface-sensitive spectroscopic techniques reveal the rich surface features of the NCs that enable their impressive long-term environmental and thermal stabilities. Notably, the surface termination and its passivation effects on the electronic properties of AgBiI4 are investigated. Our atomistic simulations suggest that a bismuth iodide-rich surface, as in the case of AgBiI4 NCs, does not introduce surface trap states within the band gap region of AgBiI4, unlike a silver iodide-rich surface. These findings may encourage the investigation of surfaces of other lead-free perovskite-inspired materials.publishedVersionPeer reviewe

Crystallographic, Optical, and Electronic Properties of the Cs2AgBi1–xInxBr6 Double Perovskite: Understanding the Fundamental Photovoltaic Efficiency Challenges

We present a crystallographic and optoelectronic study of the double perovskite Cs2AgBi1–xInxBr6. From structural characterization we determine that the indium cation shrinks the lattice and shifts the cubic-to-tetragonal phase transition point to lower temperatures. The absorption onset is shifted to shorter wavelengths upon increasing the indium content, leading to wider band gaps, which we rationalize through first-principles band structure calculations. Despite the unfavorable band gap shift, we observe an enhancement in the steady-state photoluminescence intensity, and n-i-p photovoltaic devices present short-circuit current greater than that of neat Cs2AgBiBr6 devices. In order to evaluate the prospects of this material as a solar absorber, we combine accurate absorption measurements with thermodynamic modeling and identify the fundamental limitations of this system. Provided radiative efficiency can be increased and the choice of charge extraction layers are specifically improved, this material could prove to be a useful wide band gap solar absorber

Risk factors for nasopharyngeal carriage of drug-resistant Streptococcus pneumoniae: data from a nation-wide surveillance study in Greece

<p>Abstract</p> <p>Background</p> <p>A nation-wide surveillance study was conducted in Greece in order to provide a representative depiction of pneumococcal carriage in the pre-vaccination era and to evaluate potential risk factors for carriage of resistant strains in healthy preschool children attending daycare centers.</p> <p>Methods</p> <p>A study group was organized with the responsibility to collect nasopharyngeal samples from children. Questionnaires provided demographic data, data on antibiotic consumption, family and household data, and medical history data. Pneumococcal isolates were tested for their susceptibility to various antimicrobial agents and resistant strains were serotyped.</p> <p>Results</p> <p>Between February and May 2004, from a total population of 2536 healthy children, a yield of 746 pneumococci was isolated (carriage rate 29.41%). Resistance rates differed among geographic regions. Recent antibiotic use in the last month was strongly associated with the isolation of resistant pneumococci to a single or multiple antibiotics. Serotypes 19F, 14, 9V, 23F and 6B formed 70.6% of the total number of resistant strains serotyped.</p> <p>Conclusion</p> <p>Recent antibiotic use is a significant risk factor for the colonization of otherwise healthy children's nasopharynx by resistant strains of <it>S pneumoniae</it>. The heptavalent pneumococcal conjugate vaccine could provide coverage for a significant proportion of resistant strains in the Greek community. A combined strategy of vaccination and prudent antibiotic use could provide a means for combating pneumococcal resistance.</p

Progress on lead-free metal halide perovskites for photovoltaic applications: a review

ABSTRACT: Metal halide perovskites have revolutionized the field of solution-processable photovoltaics. Within just a few years, the power conversion efficiencies of perovskite-based solar cells have been improved significantly to over 20%, which makes them now already comparably efficient to silicon-based photovoltaics. This breakthrough in solution-based photovoltaics, however, has the drawback that these high efficiencies can only be obtained with lead-based perovskites and this will arguably be a substantial hurdle for various applications of perovskite-based photovoltaics and their acceptance in society, even though the amounts of lead in the solar cells are low. This fact opened up a new research field on lead-free metal halide perovskites, which is currently remarkably vivid. We took this as incentive to review this emerging research field and discuss possible alternative elements to replace lead in metal halide perovskites and the properties of the corresponding perovskite materials based on recent theoretical and experimental studies. Up to now, tin-based perovskites turned out to be most promising in terms of power conversion efficiency; however, also the toxicity of these tin-based perovskites is argued. In the focus of the research community are other elements as well including germanium, copper, antimony, or bismuth, and the corresponding perovskite compounds are already showing promising properties. GRAPHICAL ABSTRACT: [Image: see text

Interfaces Between Graphene‐Related Materials and MAPbI3: Insights from First‐Principles

Given the unique and complementary properties of halide perovskites and graphene, it has been a natural step to explore their combination for novel high-performance solar cells. Within this research news the latest theoretical and computational works on interfaces between the prototype perovskite absorber methylammonium lead triiodide (CH3NH3PbI3) and graphene related materials (GRMs) are reviewed. In particular, recent works based on quantum-mechanical calculations from first-principles that probe the structural and electronic properties of interfaces between CH3NH3PbI3, pristine graphene, graphene-oxide, and reduced graphene-oxide are reviewed. How these studies investigate the fundamental mechanisms by which GRMs can improve perovskite based photovoltaics, and how these relate to the several available experimental studies that have clearly shown the positive effects of GRMs on both the charge extraction rates, and the stability of perovskite solar cells are discussed. Finally, the main challenges that remain to be addressed in order to model more realistic systems and further unravel the atomic-scale details that underlie the properties of GRM/perovskite interfaces are discussed

Surface properties of lead-free halide double perovskites: Possible visible-light photo-catalysts for water splitting

Halide double perovskites based on combinations of monovalent and trivalent cations have been proposed as promising lead-free alternatives to lead halide perovskites. Among the newly synthesized compounds Cs2BiAgCl6, Cs2BiAgBr6, Cs2SbAgCl6, and Cs2InAgCl6, some exhibit bandgaps in the visible range and all have low carrier effective masses; therefore, these materials constitute potential candidates for various opto-electronic applications. Here, we use first-principles calculations to investigate the electronic properties of the surfaces of these four compounds and determine, for the first time, their ionization potential and electron affinity. We find that the double perovskites Cs2BiAgCl6 and Cs2BiAgBr6 are potentially promising materials for photo-catalytic water splitting, while Cs2InAgCl6 and Cs2SbAgCl6 would require controlling their surface termination to obtain energy levels appropriate for water splitting. The energy of the halogen p orbitals is found to control the conduction band level; therefore, we propose that mixed halides could be used to finetune the electronic affinity

Interfaces Between Graphene‐Related Materials and MAPbI3: Insights from First‐Principles

Given the unique and complementary properties of halide perovskites and graphene, it has been a natural step to explore their combination for novel high-performance solar cells. Within this research news the latest theoretical and computational works on interfaces between the prototype perovskite absorber methylammonium lead triiodide (CH3NH3PbI3) and graphene related materials (GRMs) are reviewed. In particular, recent works based on quantum-mechanical calculations from first-principles that probe the structural and electronic properties of interfaces between CH3NH3PbI3, pristine graphene, graphene-oxide, and reduced graphene-oxide are reviewed. How these studies investigate the fundamental mechanisms by which GRMs can improve perovskite based photovoltaics, and how these relate to the several available experimental studies that have clearly shown the positive effects of GRMs on both the charge extraction rates, and the stability of perovskite solar cells are discussed. Finally, the main challenges that remain to be addressed in order to model more realistic systems and further unravel the atomic-scale details that underlie the properties of GRM/perovskite interfaces are discussed