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Radiation detection demands new scintillators with high quantum efficiency, high energy resolution, and short luminescence lifetimes. Nanocomposites consisting of quantum dots and Ce3+ doped nanophosphors may be able to meet these requirements. Here, we report the luminescence enhancement of LaF3:Ce/CdTe nanocomposites which were synthesized by a wet chemistry method. CdTe quantum dots in LaF3:Ce/CdTe nanocomposites are converted into nanowires, while in LaF3/CdTe nanocomposites no such conversion is observed. As a result, the CdTe luminescence in LaF3:Ce/CdTe nanocomposites is enhanced about five times, while in LaF3/CdTe nanocomposites no enhancement was observed. Energy transfer, light reabsorption, and defect passivation are the likely reasons for the luminescence enhancement. [ABSTRACT FROM AUTHOR]

Interfacial electronic properties of the CH3NH3PbI3 (MAPbI3)/MoOx interface are investigated using ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy. It is found that the pristine MAPbI3 film coated onto the substrate of poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate)/indium tin oxide by two-step method behaves as an n-type semiconductor, with a band gap of --1.7 eV and a valence band edge of 1.40 eV below the Fermi energy (EF). With the MoOx deposition of 64 A upon MAPbI3, the energy levels of MAPbI3 shift toward higher binding energy by 0.25 eV due to electron transfer from MAPbI3 to MoOx. Its conduction band edge is observed to almost pin to the EF, indicating a significant enhancement of conductivity. Meanwhile, the energy levels of MoOx shift toward lower binding energy by --0.30 eV, and an interface dipole of 2.13 eV is observed at the interface of MAPbI3/MoOx. Most importantly, the chemical reaction taking place at this interface results in unfavorable interface energy level alignment for hole extraction. A potential barrier of --1.36 eV observed for hole transport will impede the hole extraction from MAPbI3 to MoOx. On the other hand, a potential barrier of --0.14 eV for electron extraction is too small to efficiently suppress electrons extracted from MAPbI3 to MoOx. Therefore, such an interface is not an ideal choice for hole extraction in organic photovoltaic devices. [ABSTRACT FROM AUTHOR]

In a little over a year, there has been an unexpected breakthrough and rapid evolution of highly efficient solid-state hybrid solar cells based on organometal trihalide perovskite materials. This technology has the potential to produce solar cells with the very highest efficiencies while retaining the very lowest cost. The authors have measured the electronic density of states of CH3NH3PbI3 using ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and x-ray photoemission spectroscopy (XPS). The valence band maximum and conduction band minimum positions are obtained from the UPS and IPES spectra, respectively, by linear extrapolation of the leading edges. The authors investigate the Au/perovskite and C60/perovskite interfaces by UPS and XPS. An interface dipole of 0.1 eV is observed at Au/perovskite interface. The energy levels of perovskite shift upward by ca.0.4 eV with Au coverage of 64Å upon it, resulting in band bending, hence a built-in field in perovskite that encourages hole transport to the interface. The XPS results show a strong initial shift of core levels to lower binding energy in the perovskite, which indicates that electrons transfer from the perovskite film to fullerene molecules. Further deposition of fullerene forms C60 solid, accompanied by the reduction of the electron transfer. The strongest electron transfer happened at 1/4 monolayer of fullerene. [ABSTRACT FROM AUTHOR]

The thickness dependence of fullerene on CH3NH3PbI3 perovskite film surface has been investigated by using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital and highest occupied molecular orbital (HOMO) can be observed directly with IPES and UPS. It is observed that the HOMO level in fullerene shifts to lower binding energy. The XPS results show a strong initial shift of core levels to lower binding energy in the perovskite, which indicates that electrons transfer from the perovskite film to fullerene molecules. Further deposition of fullerene forms C60 solid, accompanied by the reduction of the electron transfer. The strongest electron transfer happened at 1/4 monolayer of fullerene. [ABSTRACT FROM AUTHOR]

We report the enhancement of the photoconductive gain of nanocomposite near-infrared photodetectors by a zinc oxide nanoparticles (ZnO NPs) rich surface at the nanocomposite/cathode interface. An argon plasma etching process was used to remove polymer at the surface of nanocomposite films, which resulted in a ZnO NPs rich surface. The other way is to spin-coat a thin layer of ZnO NPs onto the nanocomposite layer. The ZnO NPs rich surface, which acts as electron traps to induce secondary hole injection under reverse bias, increased hole injection, and thus the external quantum efficiency by 2-3 times. The darkcurrent declined one order of magnitude simultaneously as a result of etching the top nanocomposite layer. The specific detectivity at 800 nm was increased by 7.4 times to 1.11 × 1010 Jones due to the simultaneously suppressed noise and enhanced gain. [ABSTRACT FROM AUTHOR]

We report the observation of self-doping in perovskite. CH3NH3PbI3 was found to be either n- or p-doped by changing the ratio of methylammonium halide (MAI) and lead iodine (PbI2) which are the two precursors for perovskite formation. MAI-rich and PbI2-rich perovskite films are p and n self-doped, respectively. Thermal annealing can convert the p-type perovskite to n-type by removing MAI. The carrier concentration varied as much as six orders of magnitude. A clear correlation between doping level and device performance was also observed. [ABSTRACT FROM AUTHOR]

The fullerene-based organic Schottky-junction solar cells have recently attracted intensive research interest because of their unique electrical performance, such as significant photocurrent generation from excitons created in fullerenes and large open-circuit voltage (VOC) output induced by high Schottky-barrier height between the anode and the fullerene acceptor. This manuscript reports another remarkably appealing advantage that the fullerene-based Schottky-junction solar cells are more stable than the bulk heterojunction counterparts. The better stability is likely due to mitigative polymer photo-oxidation and/or little morphological change of active film in the aged Schottky-junction devices. The transition from Schottky-junction to bulk heterojunction appears at polymer donor loading ratio of 20-25 wt.% by examining the variation in the VOC with increased loading ratio of the poly(3-hexylthiophene) donor. Multiple experimental evidences, including the absorbance spectrum measurement, photoluminescence study, active film morphology characterization, and charge mobility measurement, conclusively reveal that the transition from Schottky-junction to bulk heterojunction is correlated to the polymer donor aggregation in the active films. [ABSTRACT FROM AUTHOR]

Grazing-incidence x-ray diffraction and atomic force microscopy were performed on bulk heterojunction regioregular poly(3-hexylthiophene) (RR-P3HT) [6,6]-phenyl-C71-butyric acid methyl esters spin-cast films with different film processing conditions to correlate the crystalline nanostructure of P3HT with the corresponding solar cell performance. The increase in long wavelength absorption for solvent annealed films is related to highly conjugated crystal structure of RR-P3HT phase-separated in the active layer. Upon thermal annealing, the solvent annealed 50-nm-thick device shows high solar cell performance with fill factor up to 73% and power conversion efficiency of 3.80%. [ABSTRACT FROM AUTHOR]

Recently, electrical bistability was demonstrated in polymer thin films incorporated with metal nanoparticles [J. Ouyang, C. W. Chu, C. R. Szmanda, L. P. Ma, and Y. Yang, Nat. Mater. 3, 918 (2004)]. In this letter, we show the evidence that electrons are the dominant charge carriers in these bistable devices. Direct integration of bistable polymer layer with a light-emitting polymer layer shows a unique light-emitting property modulated by the electrical bistability. A unique negative differential resistance induced by the charged gold nanoparticles is observed due to the charge trapping effect from the nanoparticles when interfaced with the light-emitting layer. [ABSTRACT FROM AUTHOR]

Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends have been demonstrated to form highly efficient polymer photovoltaic devices. In this letter, the time-of-flight technique is used to investigate the influence of composition and heat treatment on charge transport properties of P3HT and PCBM blends. The transport of electrons and holes both display a transition from dispersive to nondispersive and return to dispersive again as the percentage of PCBM increases. A balanced mobility of both electron and hole is obtained at a composition of 1:1 weight ratio, and it is nearly independent of the electrical field in the range of our test. The increase in carrier mobility is attributed to the formation of a more-ordered structure in the blend. This structural ordering is further enhanced by slowly evaporating the solvent during film formation which results in additional increase in carrier mobility. However, no such effect is observed in thick films (∼200 nm), indicating the presence of such high-degree ordering due to heat treatment. [ABSTRACT FROM AUTHOR]

The thickness dependence of fullerene on CH3NH3PbI3 perovskite film surface has been investigated by using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital and highest occupied molecular orbital (HOMO) can be observed directly with IPES and UPS. It is observed that the HOMO level in fullerene shifts to lower binding energy. The XPS results show a strong initial shift of core levels to lower binding energy in the perovskite, which indicates that electrons transfer from the perovskite film to fullerene molecules. Further deposition of fullerene forms C60 solid, accompanied by the reduction of the electron transfer. The strongest electron transfer happened at 1/4 monolayer of fullerene. [ABSTRACT FROM AUTHOR]

We report the observation of self-doping in perovskite. CH3NH3PbI3 was found to be either n- or p-doped by changing the ratio of methylammonium halide (MAI) and lead iodine (PbI2) which are the two precursors for perovskite formation. MAI-rich and PbI2-rich perovskite films are p and n self-doped, respectively. Thermal annealing can convert the p-type perovskite to n-type by removing MAI. The carrier concentration varied as much as six orders of magnitude. A clear correlation between doping level and device performance was also observed. [ABSTRACT FROM AUTHOR]