Energy Materials and Surface Sciences Unit

Changes in the composition of the electrolyte are known to affect the parameters that determine the performance of dye solar cells. This paper describes a robust method for the analysis of the photovoltage in dye solar cells. The method focuses on the study of recombination resistance and chemical capacitance of TiO 2 obtained from impedance spectroscopy. Four dye solar cells with electrolytes producing known effects on photovoltage behavior have been studied. Effects of conduction band shifts and changes in recombination rate in the photovoltage have been evaluated quantitatively.

In the standard solar cell technologies such as crystalline silicon and cadmium telluride, increments of temperature in the cell produce large variations in the energy conversion efficiency, which decreases at a constant rate. In dye solar cells the efficiency remains roughly constant with a maximum at around 30-40 1C and further decays above this temperature. In this work, the origin of this characteristic behavior is explained. Data show that under illumination recombination kinetics in the active layer of the cell is the same between À7 and 40 1C. Consequently, the efficiency of the cell remained virtually constant, with only small differences in the fill factor associated with changes in the series resistance. A further increase in temperature up to 70 1C produces an increase in recombination kinetics yielding lower photopotential and device performance. Finally, it is emphasized that at the normal operating temperatures of solar cells, the gap among the conversion efficiency of different technologies is much smaller than generally acknowledged. 387537 † Electronic supplementary information (ESI) available: Measures of performance of DSC for 500 h lifetime tests, Nyquist plots showing series resistances, values of conduction band shifts, plots of capacitance values vs. V ecb , comparison of capacitance in the dark and under illumination, comments on transport resistance and performance data of DSC after shifting the applied potential the displacement in the conduction band edge (DE c ). See

Silver is a low-cost candidate electrode material for perovskite solar cells. However, in such cells the silver electrodes turn yellow within days of device fabrication. The color change is also accompanied by a dramatic decrease in the power conversion efficiency when compared to otherwise identical devices using gold electrodes. Here, it is shown that the color change results from silver oxidation to silver iodide, due to a reaction with iodine in methyl ammonium lead perovskite. The change in X-ray diffraction and X-ray photo­electron spectroscopy is discussed. Exposure to air accelerates corrosion of the Ag electrodes when compared to dry nitrogen gas exposure. However, iodine not reacted with silver is observed by X-ray photoelectron spectroscopy even for the perovskite solar cell kept in dry nitrogen gas. It is proposed that silver iodide is formed when methyl ammonium iodide migration is facilitated by the small pinholes in the hole transport layer spiro-MeOTAD.

Solution processable perovskite solar cells traditionally use CH3NH3I solid powder as one of the two precursors that requires solvation into a solution and a spin-coating step; resulted films need post-annealing (~1h) for complete conversion to CH3NH3PbI3. Here we describe for the first time the formation of stoichiometric perovskite in ambient air by exposing PbI2 films to simple CH3NH2 gas precursor (as opposed to CH3NH3I solid powders). The reaction completes within a few seconds forming complete-coverage perovskite films with a roughness of 2 nm. The non stoichiometric reaction produces Pb oxides as by-product, which are reconverted by further HI gas exposures. With combined measurements of thin film crystal structure, chemical state, and absorption properties, we elucidate the chemical reaction mechanisms underlying these gas-induced processes. Fabricated solar cell devices show efficiency of 15.3%, which stays almost the same after 133 days. Such a gas-induced reaction also enabled the successful preparation of high quality perovskite films with a size of 100 cm2.

Ultrathin ZnO layer of ca. four nm thick deposited from an aqueous zinc oxide hydrate (ZnO·xH 2 O) solution is developed as the electron transport layer (ETL) in the inverted polymer solar cells (PSCs). Because of the low energy metal−ammine dissociation and hydroxide condensation/dehydration chemistry, a conversion temperature (T A ) as low as 80°C is achieved. With the active layer of poly(N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-3′,2′,1′-benzothiadiazole)) doped with [6,6]-phenyl C 71 butyric acid methyl ester (PCDTBT:PC 71 BM), the average power conversion efficiency of the devices is up to 6.48%, and the short circuit current density reaches 11.4 mA cm −2 . In comparison, the devices with traditional sol−gel processed ZnO electron transport layer has a power conversion efficiency of 5.53% at an annealing temperature of 200°C

In the application of traditional bulk heterojunction polymer solar cells, to prevent the etching of ITO by the acidic poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and thereby improve the device stability, pH-neutral PEDOT:PSS is introduced as the hole transport layer (HTL). After treating the neutral PEDOT:PSS with UV-ozone and with an oxygen plasma, the average power conversion efficiency (PCE) of the device increases from 3.44% to 6.60%. Such surface treatments reduce the energy level offset between the HTL and the active layer, which increases the open circuit voltage and enhances hole transportation, leading to the PCE improvement. Moreover, the devices with the neutral PEDOT:PSS HTL are more stable in air than those with the acidic PEDOT:PSS HTL. The PCE of the devices with the acidic PEDOT:PSS HTL decreases by 20% after 7 days and 45% after 50 days under ambient conditions, whereas the PCE of the devices with the pH-neutral PEDOT:PSS HTL decreases by only 9 and 20% after 7 and 50 days, respectively. X-ray photoelectron spectroscopy shows that the acidic PEDOT:PSS etches the indium from the indium–tin−oxide (ITO) electrode, which is responsible for the degradation of the device. In comparison, the diffusion of the indium is much slower in the devices with the pH-neutral PEDOT:PSS HTL.

Over the past few years, there have been unprecedented advances in solar cells using metal halide perovskite materials as light absorbers. [1-3] A record power conversion efficiency An upscalable perovskite film deposition method combining raster ultrasonic spray coating and chemical vapor deposition is reported. This method overcomes the coating size limitation of the existing stationary spray, single-pass spray, and spin-coating methods. In contrast with the spin-coating method (>90% Pb waste), negligible Pb waste during PbI 2 deposition makes this method more environmentally friendly. Outstanding film uniformity across the entire area of 5 cm × 5 cm is confirmed by both large-area compatible characterization methods (electroluminescence and scattered light imaging) and local characterization methods (atomic force microscopy, scanning electron microscopy, photoluminescence mapping, UV-vis, and X-ray diffraction measurements on multiple sample locations), resulting in low solar cell performance decrease upon increasing device area. With the FAPb(I 0.85 Br 0.15) 3 (FA = formamidinium) perovskite layer deposited by this method, champion solar modules show a power conversion efficiency of 14.7% on an active area of 12.0 cm 2 and an outstanding shelf stability (only 3.6% relative power conversion efficiency decay after 3600 h aging). Under continuous operation (1 sun light illumination, maximum power point condition, dry N 2 atmosphere with <5% relative humidity, no encapsulation), the devices show high light-soaking stability corresponding to an average T 80 lifetime of 535 h on the small-area solar cells and 388 h on the solar module.

enables the vacuum-free OLED fabrication process. [ 5 ] Recently, by applying AAG-EIL and nanoparticle-based silver ink, allsolution processed polymer light-emitting diode displays have been realized. [ 6 ] The interface dipole model is generally accepted to explain the electron injection ability of AAG-EILs. [ 2,3,7 ] The model shows that the AAG-EIL introduces electric dipoles at the organic/metal interface between the light emitting layer and the cathode, which elevates the vacuum level on the metal side, thereby reducing the electron injection barrier. The interface dipole may come from two effects. One is the dipole induced by the electron transfer from the AAG-EIL to the high work function species. [ 2,8 ] The other is the dipole formed by the image charge induced by the AAG-EIL at the organic/ metal interface. [ 7 ] The lone-pair electrons of the nitrogen atom in AAGs are prone to interact with materials with high electron affi nity (EA), such as molecules with deep LUMOs, [ 9,10 ] high work function conductors, [ 3,10 ] and metal oxides. [ 2,8 ] Since the holes in the functional layer of OLEDs during device operation also have high EA, they probably will interact with and be trapped by AAGs. When the holes are trapped at the organic/metal interface, the energy level will bend, decreasing the electron tunneling distance, thereby facilitating the electron injection. In light of the hole-trapping effect, the interface dipole model is not suffi cient to fully describe the electron injection at the organic/metal interface. The holes are injected during the operation of the OLEDs. Unfortunately, the experiments supporting the interface dipole model, such as Kelvin Probe (KP), small-angle X-ray diffraction, photovoltaic study, electro-absorption (signal before charge injection), and photoelectron spectroscopy are carried out under the nonoperating condition of OLEDs in which few holes are present. [ 2-4,7,8,10,11 ] As a result, the experiments failed to reveal the hole-trapping effect by the AAG-EIL. In our contribution, we demonstrate that during the operation of OLEDs, the AAG-EIL not only functions as dipoles, but also traps holes. To confi rm the hole-trapping effect, a spacer layer was employed to minimize the dipole induced by the AAG-EIL, and an opposite dipole was introduced to neutralize the residual dipole. Without the favored vacuum level shift, the holes trapped by www.MaterialsViews.com www.advelectronicmat.de

By treating the organic/metal interface between the light emission layer and the cathode with ether solvent, the device performance of the organic light-emitting diodes with aluminum cathode is significantly improved. The maximum luminous efficiency is not only more than thirty times higher than that of the device without any ether solvent treatment, but also higher than the device with regular low work function metal cathode, such as Ba/Al. The enhanced efficiency results from the reduction of electron injection barrier, which is confirmed by the photovoltaic measurements. X-ray photoelectron spectroscopy study reveals that the formation of a carbide-like layer by the reaction between the thermally evaporated aluminum and the ethylene oxide functional group,-CH 2 CH 2 O-, helps the electron injection.

To address this problem, a class of cathode interfacial material (CIM), usually an organic electrolyte or inorganic salt, has been developed to enable the utilization of more stable high WF conductors like Au, Ag, Al and indium tin oxide (ITO). [ 8-11 ] The key molecular structure of the CIM is the polar component. [ 10,11 ] It is believed that the CIM layer introduces an interfacial dipole at the organic/metal interface to reduce the effective WF of the metal, leading to better energy-level alignment to enhance the device performance. However, there is no agreement in the literatures in regard to the direction of the vacuum-level shift caused by the CIM dipole layer, particularly in devices with a regular device architecture (substrate/ anode/organic/cathode). The confusion comes from the specifi c sample structure employed in the surface potential measurement. To characterize the WF modification effects of CIMs, the most common techniques are the ultraviolet photoelectron spectroscopy (UPS) and the Kelvin probe (KP). Two sample confi gurations are generally used. One is CIM layer on top of the predeposited cathode metal [ 12-16 ] as illustrated in Figure 1 a. The other is CIM layer on top of the organic active layer [ 17-20 ] as shown in Figure 1 b. Generally, the surface potential measurement on the CIM exhibits a decreased WF with respect to the substrates for both sample confi gurations. As a result, the proposed CIM dipole directions are opposite to each other when the measurement results are directly transferred into a complete device. As shown in Figure 1 , the measurement of the CIM on top of the metal gives a negative dipole direction (defi ned as that from cathode to anode) which lifts the vacuum level of the metal, while the measurement of the CIM on top of the active layer gives an positive dipole direction (defi ned as that from anode to cathode) which lowers the vacuum level of the metal. Unfortunately, without further investigation, both are adopted in the interpretation of the CIM's role in a complete device, and the contradiction has yet to be addressed in any report so far. The controversy on the dipole direction is the consequence of only considering one interface of the CIM layer, i.e., either the CIM/metal interface or the active layer/CIM interface. Without taking into account both interfaces, the complete energy-level alignment across the CIM layer cannot be correctly determined.

Shrinking the device dimension has long been the pursuit of the semiconductor industry to increase the device density and operation speed. In the application of thin film transistors (TFTs), all-organic TFT arrays made by all-solution process are desired for low cost and flexible electronics. One of the greatest challenges is how to achieve ultrashort channel through a cost-effective method. In our study, ultrashort-channel devices are demonstrated by direct inkjet printing conducting polymer as source/drain and gate electrodes without any complicated substrate&#39;s pre-patterning process. By modifying the substrate&#39;s wettability, the conducting polymer&#39;s contact line is pinned during drying process which makes the channel length well-controlled. An organic TFT array of 200 devices with 2 μm channel length is fabricated on flexible substrate through all-solution process. The simple and scalable process to fabricate high resolution organic transistor array offers a low cost ...

Large-area lighting panels based on white organic light-emitting diodes (OLEDs) with emission area of 72 Â 72 mm 2 have been fabricated in air through dip-coating process. By studying the effects of the solution concentration, the panel withdrawal speed, as well as improving the panel structure, uniform hole transport layer and emitting layer are deposited with appropriate thickness. The 9-point luminance uniformity and the opto-electrical performance of the dip-coated OLED panel are as same as that of the spin-coated panel. However, the spin-coated panel exhibits dim emission at the corner region and the metal grid region due to panel rotation and material pileup , which the dip-coated panel shows a uniform light emission across the whole panel.

Superhydrophobicity is an important phenomenon in nature that inspires the design of numerous biomimetic functional materials. A superhydrophobic surface is expected to have a three-phase solid-liquid-vapor interface. To directly image the buried air phase in wetted superhydrophobic surfaces, we employed synchrotron radiation-based X-ray phase-contrast imaging to non-invasively probe the surfaces of natural lotus leaves and artificial carbon nanotube films in three dimensions. Reconstructed images of the three-phase distribution surrounding the superhydrophobic surfaces were presented with high resolution and contrast. Further extraction of the phases enabled direct analysis of the relationship between the surface morphology and its wetting property. We found that the protruding micro/nano-structures trapped a layer of air up to 14 μm thick, which played an important role in the observed superhydrophobicity. Direct evidence of the presence of a buffer layer air cushion deepens our understanding of hydrophobicity and opens new opportunities for designing novel bioinspired materials.

Compared to the conventional cell-shaped pixel structure, linear bank structure for solution-processed organic light-emitting diode (OLED) displays has the advantage of simple process, and large aperture-ratio. However, the prolonged drying time of the inkjet-printed ink in the linear bank forms a convex surface profile in the longitude direction. To achieve uniform film surface of the inkjet-printed poly

Solvent treatment has been widely used to improve the device performance of both Organic Light Emitting Diodes (OLEDs) and Polymer Solar Cells (PSCs). One of the proposed mechanisms is the modification of the buried PEDOT:PSS layer underneath the organic active layer by the permeating solvent. By measuring the lateral electric conductivity of the PEDOT:PSS layer, the 3 orders of magnitude's enhancement on the conductivity after solvent treatment confirms that the solvent permeates through the top organic active layer and modifies the PEDOT:PSS layer. Using a "peel-off" method, the buried PEDOT:PSS layer is fully exposed and studied by UV-vis spectra, XPS spectra, and c-AFM images. The data suggest that the permeating solvent dissolves PSS, changes PEDOT:PSS' core-shell structure into a linear/coiled structure, and moves PSS from the bulk to the surface. As a result, PEDOT becomes more continuous in the bulk. The continuous conducting PEDOT-rich domains create percolating pathways for the current which significantly improve electric conductivity.

In organic electronic devices, the interfacial dipole at organic/metal interfaces is critical in determining the carrier injection or extraction that limits the performance of the device. A novel technique to enable the direct measurement of underburied dipoles is developed and demonstrated. By tilting the shadow mask by a small angle, metal atoms diffuse into the opening slit to form an ultrathin metal layer during the evaporation process. As the ultrathin metal layer cannot screen out the dipole-induced surface work function change, the dipole strength and direction at the organic/metal interface can be revealed. It was found that the polarity of the organic material, the Fermi-level pinning and the interface morphology all play important roles in dipole formation. By comparing the energy level shifts at the organic/pre-deposited metal and organic/post-deposited metal interfaces, the dipole formed by molecular interactions could be distinguished from the dipole formed by Fermi-level pinning.