Low-Temperature, Solution-Processed MoO_<i>x</i> for Efficient and Stable Organic Solar Cells

Sol–gel processed MoO_<i>x</i> (sMoO_<i>x</i>) 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 N₂ ambient is required to achieve sMoO_<i>x</i> layers with a high work-function of 5.3 eV. We demonstrate that in P3HT:PC₆₀BM organic solar cells (OSCs) our sMoO_<i>x</i> 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 MoO₃ layers (eMoO₃). At the same time, a substantially improved stability of the OSCs compared to devices using a PEDOT:PSS hole extraction layer is evidenced

Stress Management in Thin-Film Gas-Permeation Barriers

Gas diffusion barriers (GDB) are essential building blocks for the protection of sensitive materials or devices against ambient gases, like oxygen and moisture. In this work, we study the mechanics of GDBs processed by atomic layer deposition (ALD). We demonstrate that a wide range of ALD grown barrier layers carry intrinsic mechanical tensile stress in the range of 400–500 MPa. In the application of these GDBs on top of organic electronic devices, we derive a critical membrane force (σ · <i>h</i>)_crit = 1200 GPaÅ (corresponding to a layer thickness of about 300 nm) for the onset of cracking and delamination. At the same time, we evidence that thicker GDBs would be more favorable for the efficient encapsulation of statistically occurring particle defects. Thus, to reduce the overall membrane force in this case to levels below (σ · <i>h</i>)_crit, we introduce additional compressively strained layers, e.g., metals or SiN_<i>x</i>. Thereby, highly robust GDBs are prepared on top of organic light emitting diodes, which do not crack/delaminate even under damp heat conditions 85 °C/85% rh

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

Thermal Conductivity of Methylammonium Lead Halide Perovskite Single Crystals and Thin Films: A Comparative Study

Thermal management in devices like solar cells, light-emitting diodes, and lasers based on hybrid halide perovskite thin films is expected to be of paramount importance for optimal performance and reliability. As of yet, experimental data of thermal properties of non-iodine-based hybrid halide perovskites is very scarce. Here the thermal conductivity of methylammonium lead halide perovskite (CH₃NH₃PbX₃ X= I, Br, and Cl) single crystals and thin films is analyzed by scanning near-field thermal microscopy. The thermal conductivity of CH₃NH₃PbX₃ single crystals with X= I, Br, and Cl is found to be 0.34 ± 0.12, 0.44 ± 0.08, and 0.50 ± 0.05 W/(mK) at room temperature, respectively. Strikingly, similar thermal conductivities are determined for the corresponding thin-film samples. The thermal conductivity of MAPbI₃ in the cubic phase (<i>T</i> > 55 °C) increases to (1.1 ± 0.1) W/(mK). In addition, the temperature dependence of the thermal conductivities and of thermal expansion coefficients of MAPbI₃ around the phase transition from the tetragonal to cubic phase is presented

Spatial Atmospheric Pressure Atomic Layer Deposition of Tin Oxide as an Impermeable Electron Extraction Layer for Perovskite Solar Cells with Enhanced Thermal Stability

Despite the notable success of hybrid halide perovskite-based solar cells, their long-term stability is still a key-issue. Aside from optimizing the photoactive perovskite, the cell design states a powerful lever to improve stability under various stress conditions. Dedicated electrically conductive diffusion barriers inside the cell stack, that counteract the ingress of moisture and prevent the migration of corrosive halogen species, can substantially improve ambient and thermal stability. Although atomic layer deposition (ALD) is excellently suited to prepare such functional layers, ALD suffers from the requirement of vacuum and only allows for a very limited throughput. Here, we demonstrate for the first time spatial ALD-grown SnO<i>_x</i> at atmospheric pressure as impermeable electron extraction layers for perovskite solar cells. We achieve optical transmittance and electrical conductivity similar to those in SnO<i>_x</i> grown by conventional vacuum-based ALD. A low deposition temperature of 80 °C and a high substrate speed of 2.4 m min^–1 yield SnO<i>_x</i> layers with a low water vapor transmission rate of ∼10^–4 gm^–2 day^–1 (at 60 °C/60% RH). Thereby, in perovskite solar cells, dense hybrid Al:ZnO/SnO<i>_x</i> electron extraction layers are created that are the key for stable cell characteristics beyond 1000 h in ambient air and over 3000 h at 60 °C. Most notably, our work of introducing spatial ALD at atmospheric pressure paves the way to the future roll-to-roll manufacturing of stable perovskite solar cells

Conformal and Highly Luminescent Monolayers of Alq₃ Prepared by Gas-Phase Molecular Layer Deposition

The gas-phase molecular layer deposition (MLD) of conformal and highly luminescent monolayers of tris­(8-hydroxyquinolinato)­aluminum (Alq₃) is reported. The controlled formation of Alq₃ monolayers is achieved for the first time by functionalization of the substrate with amino groups, which serve as initial docking sites for trimethyl aluminum (TMA) molecules binding datively to the amine. Thereby, upon exposure to 8-hydroxyquinoline (8-HQ), the self-limiting formation of highly luminescent Alq₃ monolayers is afforded. The growth process and monolayer formation were studied and verified by in situ quartz crystal monitoring, optical emission and absorption spectroscopy, and X-ray photoelectron spectroscopy. The nature of the MLD process provides an avenue to coat arbitrarily shaped 3D surfaces and porous structures with high surface areas, as demonstrated in this work for silica aerogels. The concept presented here paves the way to highly sensitive luminescent sensors and dye-sensitized metal oxides for future applications (e.g., in photocatalysis and solar cells)