Hysteresis-free and highly efficient CH3NH3PbI3 perovskite solar cells employing a compact C60 material as an electron transport layer have been developed for the first time using both rigid glass and plastic substrates.

have already been a lot of studies on bendable conducting electrodes in the fi eld of organic photovoltaics (OPVs) to replace brittle transparent conductive oxides (TCOs) for fl exible solar cell applications, such as graphene, [17][18][19][20] carbon nanotubes, [21][22][23] metal grids, [24][25][26] and conductive polymers. [ 27,28 ] Among them, graphene, a single-layer 2D carbon material, would be the most promising candidate because it is optically highly transparent (about 97% in visible range), mechanically robust, fl exible, and stretchable. TCO-free OPV devices with graphene anode have already been successfully demonstrated showing a PCE of 8.48%, the highest effi ciency for the TCO-free tandem polymer solar cells, [ 20 ] although still lower than 11.0% PCE of the TCOfree perovskite solar cells. [ 29 ] Graphene electrodes also have been recently employed in perovskite devices; [ 30,31 ] however, in these studies, graphene was not used for replacing the conventional TCO electrode but for a top electrode.Here we report highly effi cient TCO-free inverted perovskite solar cells consisting of graphene/molybdenum trioxide (MoO 3 )/ PEDOT:PSS/MAPbI 3 /fullerene (C 60 )/bathocuproine (BCP)/ lithium fl uoride (LiF)/aluminum (Al). A few nanometer thick MoO 3 layers are employed between the graphene and PEDOT:PSS layers, similar to the OPVs adopting graphene as an anode, [ 18 ] which provides hydrophilicity to the graphene surface and elevates its lower work function (4.23 eV) to a higher level (4.71 eV) by hole doping of graphene. The wettability of PEDOT:PSS and the device properties are affected by the thickness of the MoO 3 layer, and, as a result, best PCE of 17.1% is achieved with the graphene-based devices incorporating a 2 nm thick MoO 3 interfacial layer. For comparison, ITO-based perovskite solar cells employing MoO 3 interfacial layers have been also fabricated. Their PCEs also vary with the thickness of the MoO 3 layer, showing the best PCE of 18.8% with a 1 nm thick MoO 3 layer. The effects of the MoO 3 thickness on PCEs of the graphene-and the ITO-based devices are thoroughly investigated by analyzing hydrophilicity of electrode surfaces, electrode work functions, surface morphologies of constitutive fi lms, and device properties.The structure of the devices is schematically illustrated in Figure 1 . We adopted an inverted MAPbI 3 perovskite solar cell structure using PEDOT:PSS and C 60 /BCP as the HTL and the ETL, respectively, because the structure is low-temperature processable and thus suitable for future application on fl exible plastic substrates. A single layer graphene, grown by chemical vapor deposition (CVD), was utilized as a transparent anode rather than a cathode because increasing its work function (≈4.3 eV) by p-doping induced not only an enhanced conductivity but also a desirable energy level alignment with the highest occupied molecular orbital level of HTLs (≈5.2 eV for PEDOT:PSS, for example). Between the graphene and the Organic/inorganic hybrid perovskites are promising materials f...

Recent studies on mobility modeling have focused on characterizing user mobility from real traces of wireless LANs (WLANs) and creating mobility models based on such characterization. However, most of the work does not study how user mobility is correlated in time at different time scales. For example, the future APs with which a user will be associated are predicted without the knowledge of when the association will take place and for how long. In this paper, we build a mathematical model for characterizing both steadystate and transient behaviors of user mobility in WLANs. Specifically, we model user mobility by a semi-Markov process, and obtain the transition probability matrix and the sojourn time distribution from the association history of WLAN users available at Dartmouth college [21]. With the steady-state characterization of user mobility in WLANs, we can estimate the long-term wireless network usage among different access points. By comparing the steady-state distributions of semi-Markov models built based on trace data collected at different time scales, we are able to characterize the degree of correlation in time and location.We also perform a transient behavior analysis of the semiMarkov process (that characterizes user mobility), and devise a timed location prediction algorithm that accurately predicts the future locations of users -both the future access points they will associate themselves with and the association duration. We demonstrate the utility of timed location prediction, by showing how it can be utilized to predict the distribution of future user locations with the time information figured in, and redistributing loads among neighboring APs. An improvement of 80% (in terms of load balance) is observed in a wide spectrum of traffic loads in the simulation.

A moth-eye nanostructured mp-TiO2 film using conventional lithography, nano-imprinting and polydimethyl-siloxane (PDMS) stamping methods is demonstrated for the first time. Power conversion efficiency of the moth-eye patterned perovskite solar cell is improved by ≈11%, which mainly results from increasing light harvesting efficiency by structural optical property.

Micro-fabricated single-layer graphenes (SLGs) on a silicon dioxide (SiO2)/Si substrate, a silicon nitride (SiN) membrane, and a suspended architecture are presented for their use as temperature sensors. These graphene temperature sensors act as resistance temperature detectors, showing a quadratic dependence of resistance on the temperature in a range between 283 K and 303 K. The observed resistance change of the graphene temperature sensors are explained by the temperature dependent electron mobility relationship (~T−4) and electron-phonon scattering. By analyzing the transient response of the SLG temperature sensors on different substrates, it is found that the graphene sensor on the SiN membrane shows the highest sensitivity due to low thermal mass, while the sensor on SiO2/Si reveals the lowest one. Also, the graphene on the SiN membrane reveals not only the fastest response, but also better mechanical stability compared to the suspended graphene sensor. Therefore, the presented results show that the temperature sensors based on SLG with an extremely low thermal mass can be used in various applications requiring high sensitivity and fast operation.