The authors have fabricated thin film polymer photovoltaics using 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 within regioregular poly(3-hexylthiophene) bulk heterojunction absorbing layers. Using thermal annealing at temperatures approaching the glass transition temperature, they have examined the formation of nanodomains within the matrix. These domains modify charge transport pathways in such a way as to allow for the efficient use of thicker absorbing layers. This results in a nearly 20% gain in overall performance for this polymer system with external power efficiencies exceeding 6%.

We used a combined atomic force microscopic (AFM)/fluorescence microscopic technique to study the mechanical properties of individual, electrospun fibrinogen fibers in aqueous buffer. Fibers (average diameter 208 nm) were suspended over 12 μm-wide grooves in a striated, transparent substrate. The AFM, situated above the sample, was used to laterally stretch the fibers and to measure the applied force. The fluorescence microscope, situated below the sample, was used to visualize the stretching process. The fibers could be stretched to 2.3 times their original length before breaking; the breaking stress was 22 × 106 Pa. We collected incremental stress–strain curves to determine the viscoelastic behavior of these fibers. The total stretch modulus was 17.5 × 106 Pa and the relaxed elastic modulus was 7.2 × 106 Pa. When held at constant strain, electrospun fibrinogen fibers showed a fast and slow stress relaxation time of 3 and 55 s. Our fibers were spun from the typically used 90% 1,1,1,3,3,3-hexafluoro-2-propanol (90-HFP) electrospinning solution and re-suspended in aqueous buffer. Circular dichroism spectra indicate that α-helical content of fibrinogen is ~70% higher in 90-HFP than in aqueous solution. These data are needed to understand the mechanical behavior of electrospun fibrinogen structures. Our technique is also applicable to study other nanoscopic fibers.

Perovskite-based solar cells have attracted much recent research interest with efficiency approaching 20%. While various combinations of material parameters and processing conditions are attempted for improved performance, there is still a lack of understanding in terms of the basic device physics and functional parameters that control the efficiency. Here we show that perovskite-based solar cells have two universal features: an ideality factor close to two and a space-charge-limited current regime. Through detailed numerical modeling, we identify the mechanisms that lead to these universal features. Our model predictions are supported by experimental results on solar cells fabricated at five different laboratories using different materials and processing conditions. Indeed, this work unravels the fundamental operation principle of perovskite-based solar cells, suggests ways to improve the eventual performance, and serves as a benchmark to which experimental results from various laboratories can be compared.

Using poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 bulk-heterojunction blends as the absorbing material, organic photovoltaic devices have been fabricated onto multimode optical fibers. The behavior of the short circuit current density, filling factor, and open circuit voltage as the angle of the incident light onto the cleaved fiber face is varied suggests that the evanescent field at the interface between the fiber and the transparent contact may play a role in coupling light from the fiber into the device. Further, optical loss into the device increases as the fiber diameter decreases.

Poly(3-hexylthiophene):1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 photovoltaic devices were fabricated as a cladding onto large diameter optical fibers. The performance of the devices was dependent on fiber diameter as well as the angle of incidence of light onto the cleaved fiber face. The authors suggest that absorption by the active layer is dominated by different mechanisms at different angles: evanescent coupling of the light at small incident angles and far field scattering of the light from the fiber at higher angles. Further, a comparison of devices that only partially clad the fiber to those that fully clad suggests the formation of confined radiation modes.