Photoconductivity measurements of CH3NH3PbI3 deposited between two dielectric-protected Au electrodes show extremely slow response. The CH3NH3PbI3, bridging a gap of ∼2000 nm, was subjected to a DC bias and cycles of 5 min illumination and varying dark duration. The approach to steady -state photocurrent lasted tens of seconds with a strong dependence on the dark duration preceding the illumination. On the basis of DFT calculations, we propose that under light + bias the methylammonium ions are freed to rotate and align along the electric field, thus modifying the structure of the inorganic scaffold. While ions alignment is expected to be fast, the adjustment of the inorganic scaffold seems to last seconds as reflected in the extremely slow photoconductivity response. We propose that under working conditions a modified, photostable, perovskite structure is formed, depending on the bias and illumination parameters. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.

In the pursuit to better understand the mechanisms of perovskite solar cells we performed Raman and photoluminescence measurements of free-standing CH3NH3PbI3 films, comparing dark with working conditions. The films, grown on a glass substrate and sealed by a thin glass coverslip, were measured subsequent to dark and white-light pretreatments. The extremely slow changes we observe in both the Raman and photoluminescence cannot be regarded as electronic processes, which are much faster. Thus, the most probable explanation is of slow photoinduced structural changes. The CH3NH3PbI3 transformation between the dark and the light structures is reversible, with faster rates for the changes under illumination. The results seem to clarify several common observations associated with solar cell mechanisms, like performance improvement under light soaking. More important is the call for solar-cell-related investigation of CH3NH3PbI3 to take the photoinduced structural changes into consideration when measuring and interpreting the results.

Bisquert and colleagues demonstrate the impact of positive charge accumulation at the electron contact of perovskite solar cells.

With solar conversion efficiencies surpassing 20%, organometallic perovskites show tremendous promise for solar cell technology. Their high brightness has also led to demonstrations of lasing and power-efficient electroluminescence. Here we show that thin films of methylammonium lead iodide, prepared by solution processing at temperatures not exceeding 100 °C, exhibit a highly nonlinear intensity-dependent refractive index due to changes in the free-carrier concentration and for femtosecond excitation at higher intensities undergo saturation that can be attributed to the Pauli blocking effect. Nonlinear refractive index and nonlinear absorption coefficients were obtained by the Z-scan technique, performed simultaneously in open- and closed-aperture configurations. Both nanosecond- and femtosecond-pulsed lasers at multiple wavelengths were used in order to distinguish between the mechanisms inducing the nonlinearities. The magnitude and sign of the nonlinear refractive index n 2 were determined. For resonant excitation, free carrier generation is the dominant contribution to the nonlinear refractive index, with a large nonlinear refractive index of n 2 = 69 × 10–12 cm2/W being observed for resonant femtosecond pumping and n 2 = 34.4 × 10–9 cm2/W for resonant nanosecond pumping. For nonresonant femtosecond excitation, bound-charge-induced nonlinearity leads to n 2 = 36 × 10–12 cm2/W. These values are equivalent to the best reported metrics for conventional semiconductors, suggesting that organometallic perovskites are promising materials for optical switching and bistability applications.

In this study we systematically explored the mixed cation perovskite Csx(CH3NH3)1−xPbI3.

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Colloidal silver has gained wide acceptance as an antimicrobial agent, and various substrates coated with nanosilver such as fabrics, plastics, and metal have been shown to develop antimicrobial properties. Here, a simple method to develop coating of colloidal silver on paper using ultrasonic radiation is presented, and the coatings are characterized using X-ray diffraction (XRD), high resolution scanning electron microscope (HRSEM), and thermogravimetry (TGA) measurements. Depending on the variables such as precursor concentrations and ultrasonication time, uniform coatings ranging from 90 to 150 nm in thickness have been achieved. Focused ion beam (FIB) cross section imaging measurements revealed that silver nanoparticles penetrated the paper surface to a depth of more than 1 μm, resulting in highly stable coatings. The coated paper demonstrated antibacterial activity against E. coli and S. aureus, suggesting its potential application as a food packing material for longer shelf life.

Band gap localized states and surface states play a dominant role in the application of nanocrystalline metal oxides to photovoltaics and solar fuel production. Electrons injected in nanocrystalline TiO 2 by voltage or photogeneration are mainly located in band gap states. Therefore, charging a nanoparticulate semiconductor network allows one to recover the density of states (DOS) in the energy axis. However, shallow traps remain in equilibrium with the conduction band electrons, while deep traps do not. We show that the characteristic peak of the apparent DOS mixes an exponential DOS and a monoenergetic surface state. A model that incorporates the trap's kinetics proves to be very efficient to assess the important parameters that determine both contributions via variation of charging rate. Contrary to the common theory, we demonstrate that the peculiar capacitance peak of nanocrystalline TiO 2 can be mainly attributed, in some cases, to deep traps in the exponential distribution.