Kirill Zilberberg

Electronic structure of Vanadium pentoxide: An efficient hole injector for organic electronic materials

The electronic structure of Vanadium pentoxide (V2O5), a transition metal oxide with an exceedingly large work function of 7.0 eV, is studied via ultraviolet, inverse and x-ray photoemission spectroscopy. Very deep lying electronic states with electron affinity and ionization energy (IE) of 6.7 eV and 9.5 eV, respectively, are found. Contamination due to air exposure changes the electronic structure due to the partial reduction of vanadium to V+4 state. It is shown that V2O5 is a n-type material that can be used for efficient hole-injection into materials with an IE larger than 6 eV, such as 4,4′-Bis(N-carbazolyl)-1,1′-bipheny (CBP). The formation of an interface dipole and band bending is found to lead to a very small energy barrier between the transport levels at the V2O5/CBP interface.

Low-temperature, solution-processed MoO(x) for efficient and stable organic solar cells.

Sol-gel processed MoO(x) (sMoO(x)) hole-extraction layers for organic solar cells are reported. A Bis(2,4-pentanedionato)molybdenum(VI)dioxide/isopropanol solution is used and only a moderate thermal post deposition treatment at 150 °C in N(2) ambient is required to achieve sMoO(x) layers with a high work-function of 5.3 eV. We demonstrate that in P3HT:PC(60)BM organic solar cells (OSCs) our sMoO(x) layers lead to a high filling factor of about 65% and an efficiency of 3.3% comparable to that of reference devices with thermally evaporated MoO(3) layers (eMoO(3)). At the same time, a substantially improved stability of the OSCs compared to devices using a PEDOT:PSS hole extraction layer is evidenced.

Solution processed metal-oxides for organic electronic devices

Organic electronic devices largely benefit from the smart introduction of inorganic functional materials. Among them, metal-oxide semiconductors have evolved as powerful interface materials that facilitate charge injection/extraction into/out of organic devices. Substantially enhanced device characteristics of organic light emitting diodes (OLEDs), organic solar cells (OSCs), and organic field-effect transistors (OFETs) have been achieved along with a significant improvement in lifetime. In many of these examples, the metal-oxides have been prepared in vacuum processes. To meet the demands of solution processing of organic electronics, solution based methods for functional metal-oxides have been developed. It is the objective of this feature article to provide an overview of the impressive recent progress in finding routes for low temperature solution processing of metal-oxides that in terms of functionality are suitable to replace their vacuum processed analogues as building blocks in organic electronic devices.

Room-temperature solution processed SnOx as an electron extraction layer for inverted organic solar cells with superior thermal stability

Solution processed tin oxide (SnOx) is used as an electron extraction interlayer in organic solar cells. As opposed to devices using TiOx, cells based on SnOx are stable even at elevated temperatures in the presence of moisture. Thus, by using SnOx instead of TiOx the requirements for a costly ultra-barrier encapsulation may be relaxed.

Highly Robust Transparent and Conductive Gas Diffusion Barriers Based on Tin Oxide

Transparent and electrically conductive gas diffusion barriers are reported. Tin oxide (SnOx ) thin films grown by atomic layer deposition afford extremely low water vapor transmission rates (WVTR) on the order of 10(-6) g (m(2) day)(-1) , six orders of magnitude better than that established with ITO layers. The electrical conductivity of SnOx remains high under damp heat conditions (85 °C/85% relative humidity (RH)), while that of ZnO quickly degrades by more than five orders of magnitude.

Plasmonically sensitized metal-oxide electron extraction layers for organic solar cells

ZnO and TiOx are commonly used as electron extraction layers (EELs) in organic solar cells (OSCs). A general phenomenon of OSCs incorporating these metal-oxides is the requirement to illuminate the devices with UV light in order to improve device characteristics. This may cause severe problems if UV to VIS down-conversion is applied or if the UV spectral range (λ < 400 nm) is blocked to achieve an improved device lifetime. In this work, silver nanoparticles (AgNP) are used to plasmonically sensitize metal-oxide based EELs in the vicinity (1–20 nm) of the metal-oxide/organic interface. We evidence that plasmonically sensitized metal-oxide layers facilitate electron extraction and afford well-behaved highly efficient OSCs, even without the typical requirement of UV exposure. It is shown that in the plasmonically sensitized metal-oxides the illumination with visible light lowers the WF due to desorption of previously ionosorbed oxygen, in analogy to the process found in neat metal oxides upon UV exposure, only. As underlying mechanism the transfer of hot holes from the metal to the oxide upon illumination with hν < Eg is verified. The general applicability of this concept to most common metal-oxides (e.g. TiOx and ZnO) in combination with different photoactive organic materials is demonstrated.

Metal-nanostructures – a modern and powerful platform to create transparent electrodes for thin-film photovoltaics

Thin-film solar technology is the subject of considerable current research. The classical material platform of amorphous silicon (a-Si) has been complemented by organic solar cells and more recently by solar cells based on quantum dots or organo-metal-halide perovskites. The majority of effort is focused on the synthesis, characterization and optimization of the photo-active components as well as on the invention of novel device architectures. Low-cost, low-weight, flexibility and the opportunity to create semi-transparent devices are among the most frequently claimed selling points of thin-film solar cells. It is clear that the full potential of this technology and the ability to fulfill its promises are intimately linked with tailored concepts for transparent electrodes beyond established avenues. Transparent electrodes, that can be realized at a large area, at low costs, at low temperature, which are flexible (or even elastic), and which afford a conductivity and transmittance even better than those of indium-tin-oxide, are still vigorously pursued. Even though metal based semi-transparent electrodes have a notable history, there is an ever increasing effort to unlock the full potential of metal nano-structures, especially ultra-thin films (2D) or metal-nanowires (1D) as semitransparent electrodes for thin-film solar cells. This article will review the most recent advances in semitransparent electrodes based on metal-nanowires or metal thin-films. Aside from providing general considerations and a review of the state of the art of electrode properties like sheet resistance and optical transmittance, we aim to highlight the current efforts to introduce these electrodes into solar cells. We will demonstrate that by the use of metal based semitransparent electrodes not only a replacement for established transparent conductors can be achieved but also novel functionalities can be envisaged.

Polyanionic, alkylthiosulfate-based thiol precursors for conjugated polymer self-assembly onto gold and silver.

Anionic, conjugated thiophene- and fluorene-based polyelectrolytes with alkylthiosulfate side chains undergo hydrolysis under formation of alkylthiol and dialkyldisulfide functions. The hydrolysis products can be deposited onto gold or silver surfaces by self-assembly from solutions of the anionic conjugated polyelectrolyte (CPE) precursors in polar solvents such as methanol. This procedure allows solution-based surface modifications of gold and silver electrodes using environmentally friendly solvents and enables the formation of conjugated polymer bilayers. The herein presented alkylthiosulfate-substituted CPEs are promising candidates for increasing the work function of gold and silver electrodes thus improving hole injection from such electrode assemblies into organic semiconductors.