We report here an efficient method for preparing high efficiency CH 3 NH 3 PbI 3 perovskite solar cell under high relative humidity, where morphology of PbI 2 was found to be of crucial importance. CH 3 NH 3 PbI 3 was formed on mesoporous TiO 2 layer by two-step spin coating method. During the first-step spin-coating procedure to form PbI 2 layer, FTO glass substrate was pre-heated at temperature ranging from room temperature (without pre-heating) to 60 o C.An average power conversion efficiency (PCE) showed 11.16 % without pre-heating, which was improved to 15.31% as the temperature of substrate (T sub ) was raised to 50 o C. Preheated substrate led to higher photocurrent and voltage than non-pre-heated one. When T sub increased to 60 o C, a PCE was declined to 10.49% due to large portion of unreacted PbI 2 .Compared to the non-pre-heated substrate, unreacted PbI 2 was presented on the pre-heated substrates after the second-step spin-coating of CH 3 NH 3 I as confirmed by X-ray diffraction and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) depth profile analyses. Improved crystallinity of PbI 2 induced by substrate pre-heating was responsible for incomplete conversion of PbI 2 to CH 3 NH 3 PbI 3 . Nevertheless, increases in photocurrent and voltage by pre-heating was attributed to better pore filling and surface coverage of perovskite layer, as observed by focused ion beam assisted scanning electron microscopy (FIB-SEM) images, which was associated with morphology of PbI 2 layer. According to the study on effect of CH 3 NH 3 PbI 3 thickness controlled by concentration of PbI 2 , substrate temperature was found to play predominant role in determining photovoltaic performance rather than thickness. A best PCE of 15.76% was achieved along with photocurrent density of 21.27 mA/cm 2 , voltage of 1.033 V and fill factor of 0.718 from the perovskite solar cell prepared in 50% relative humidity.
We investigated the surface potential of the ferroelectric domains of the epitaxial PbTiO3 (PTO) films using both Kelvin probe and piezoresponse force microscopy. The surface potential changes as a function of applied biases suggested that the amount and sign of surface potentials depend on the correlation between polarization and screen charges. It also suggested that the trapped negative charges exist on the as-deposited PTO surfaces. Injected charges and their resultant surface potentials are investigated by grounded tip scans. The results unveiled the origin of surface potential changes during ferroelectric switching in the epitaxial PTO films.
Intersaccadic periods of fixation are characterized by incessant retinal motion due to small eye movements. While these movements are often disregarded as noise, the temporal modulations they introduce to retinal receptors are significant. However, analysis of these input modulations is challenging because the intersaccadic eye motion is close to the resolution limits of most eyetrackers, including widespread pupil-based video systems. Here, we analyzed in depth the limits of two high-precision eyetrackers, the Dual-Purkinje Image and the scleral search coil, and compared the intersaccadic eye movements of humans to those of a non-human primate. By means of a model eye we determined that the resolution of both techniques is sufficient to reliably measure intersaccadic ocular activity up to approximately 80 Hz. Our results show that the characteristics of ocular drift are remarkably similar in the two species and that a clear deviation from a scale-invariant spectrum occurs in the range between 50–100 Hz, generally attributed to ocular tremor. The amplitude of this deviation differs on the two axes of motion. In addition to our experimental observations, we suggest basic guidelines to evaluate the performance of eyetrackers and to optimize experimental conditions for the measurement of ocular drift and tremor.
Studies in the mouse retina have characterized the spatial distribution of an anisotropic ganglion cell and photoreceptor mosaic, which provides a solid foundation to study how the cortex pools from afferent parallel color channels. In particular, the mouse's retinal mosaic exhibits a gradient of wavelength sensitivity along its dorsoventral axis. Cones at the ventral extreme mainly express S opsin, which is sensitive to ultraviolet (UV) wavelengths. Then, moving toward the retina's dorsal extreme, there is a transition to M-opsin dominance. Here, we tested the hypothesis that the retina's opsin gradient is recapitulated in cortical visual areas as a functional map of wavelength sensitivity. We first identified visual areas in each mouse by mapping retinotopy with intrinsic signal imaging (ISI). Next, we measured ISI responses to stimuli along different directions of the S- and M-color plane to quantify the magnitude of S and M input to each location of the retinotopic maps in five visual cortical areas (V1, AL, LM, PM, and RL). The results illustrate a significant change in the S:M-opsin input ratio along the axis of vertical retinotopy that is consistent with the gradient along the dorsoventral axis of the retina. In particular, V1 populations encoding the upper visual field responded to S-opsin contrast with 6.1-fold greater amplitude than to M-opsin contrast. V1 neurons encoding lower fields responded with 4.6-fold greater amplitude to M- than S-opsin contrast. The maps in V1 and higher visual areas (HVAs) underscore the significance of a wavelength sensitivity gradient for guiding the mouse's behavior. Two elements of this study are particularly novel. For one, it is the first to quantify cone inputs to mouse visual cortex; we have measured cone input in five visual areas. Next, it is the first study to identify a feature map in the mouse visual cortex that is based on well-characterized anisotropy of cones in the retina; we have identified maps of opsin selectivity in five visual areas.
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