Igal Deckman

Spontaneous interlayer formation in OPVs by additive migration due to additive–metal interactions

The presence of interlayers between the active layer and the electrode are known to modify the metal work-function and enhance carrier extraction, consequently improving OPV device performance. Spontaneous formation of interlayers by surface-enrichment of suitable additives eliminates separate processing steps and hence is technically advantageous and cost effective. However, surface enrichment is limited to additives with low surface energy. Here we show that additive migration to the organic/electrode interface could be induced by additive–metal interactions, modulated by the interactions between the additive and the underlying substrate. In this study, additive migration induced by metal evaporation is studied by blending P3HT with PEG, an established interlayer material with a surface energy higher than that of P3HT. XPS analysis reveals that, as expected, PEG is not present on the surface of the organic spun film. However, Ca or Al evaporation induces a significant migration of PEG to the organic/metal interface. In contrast, Au evaporation does not induce such migration. The comparison between Al, Ca and Au, metals with significantly different reduction potentials, revealed that the driving force for PEG migration is its chemical interaction with the deposited metal atoms. The extent of PEG migration was also found to depend on the type of underlying substrate, ITO/PEDOT:PSS or ITO. Finally, the PEG interlayer is shown to reduce the Al work function confirming that spontaneous additive migration induced by metal–additive interactions could be harnessed for charge extraction in organic electronic devices.

Flexible and stretchable power sources for wearable electronics

Compliant battery design strategy for wearable power sources with high degree of flexibility and stretchability. Flexible and stretchable power sources represent a key technology for the realization of wearable electronics. Developing flexible and stretchable batteries with mechanical endurance that is on par with commercial standards and offer compliance while retaining safety remains a significant challenge. We present a unique approach that demonstrates mechanically robust, intrinsically safe silver-zinc batteries. This approach uses current collectors with enhanced mechanical design, such as helical springs and serpentines, as a structural support and backbone for all battery components. We show wire-shaped batteries based on helical band springs that are resilient to fatigue and retain electrochemical performance over 17,000 flexure cycles at a 0.5-cm bending radius. Serpentine-shaped batteries can be stretched with tunable degree and directionality while maintaining their specific capacity. Finally, the batteries are integrated, as a wearable device, with a photovoltaic module that enables recharging of the batteries.

Atomic layer deposition of zinc oxide onto and into P3HT for hybrid photovoltaics

Hybrid organic–inorganic bulk heterojunction (BHJ) photovoltaic devices continue to be a promising alternative for present semiconductor solar cell technology. However, to become competitive hybrid devices must improve their currently low efficiencies. A major challenge to overcome is the control over the hybrid morphology, and more specifically, directing interpenetrated nano-scale phase separated continuous networks through the active layer. Here we demonstrate that atomic layer deposition (ALD) can be used to deposit hybrid BHJ photovoltaic films with exceptional control over film composition and morphology. The BHJ is prepared by exposing a pre-formed conjugated polymer film to an ALD alternating sequence of a metal oxide precursor and water. In this study ZnO was grown from cycles of diethyl zinc (DEZ) and water inside pre-formed P3HT films. We find that DEZ diffuses into the amorphous regions of the P3HT film, followed by oxidation by water to form ZnO crystalline particles (5–10 nm). Importantly, the inorganic crystalline phase is formed within the polymer amorphous regions while the ordered polymer domains are maintained. Investigation of the growth mechanism and control over the number of ALD cycles allowed us to direct a BHJ morphology with: (I) a continuous ZnO network through the P3HT film; (II) a descending concentration gradient of ZnO from the top surface down to the substrate; and (III) a dense ZnO electron transporting layer on the polymer film surface. The successful morphology-control is manifested in efficient photocurrent generation, evident from time resolved microwave-photoconductivity measurements and device performances that are similar to those reported for intensely optimized conjugated polymer/metal oxide solar cells.

Screen printed passive components for flexible power electronics.

Additive and low-temperature printing processes enable the integration of diverse electronic devices, both power-supplying and power-consuming, on flexible substrates at low cost. Production of a complete electronic system from these devices, however, often requires power electronics to convert between the various operating voltages of the devices. Passive components—inductors, capacitors, and resistors—perform functions such as filtering, short-term energy storage, and voltage measurement, which are vital in power electronics and many other applications. In this paper, we present screen-printed inductors, capacitors, resistors and an RLC circuit on flexible plastic substrates, and report on the design process for minimization of inductor series resistance that enables their use in power electronics. Printed inductors and resistors are then incorporated into a step-up voltage regulator circuit. Organic light-emitting diodes and a flexible lithium ion battery are fabricated and the voltage regulator is used to power the diodes from the battery, demonstrating the potential of printed passive components to replace conventional surface-mount components in a DC-DC converter application.

A robust, gravure-printed, silver nanowire/metal oxide hybrid electrode for high-throughput patterned transparent conductors

High-throughput patterning and enhanced mechanical stability are key to enabling large-area applications of metal nanowire mesh transparent electrodes. In this work, hybrid transparent conductors based on silver nanowires embedded in an indium zinc oxide matrix were prepared by high-speed gravure-printing (1.0 m s−1) from a single, stable liquid precursor. These gravure-printed films demonstrate excellent conductivity (9.3 Ω □−1) and transparency (T550nm ∼ 91%), as well as robust mechanical properties. The encapsulating indium zinc oxide matrix dramatically improves adhesion, surface roughness (Rq < 5 nm), film uniformity, and thermal stability (up to 350 °C) of the embedded silver nanowires. These properties of the hybrid films make them a suitable electrode material for a variety of printed electronic devices, such as flexible OLEDs and solar cells.

The effect of thermal annealing on additive migration to the organic/metal interface in OPVs

The power conversion efficiency of solar cells based on conjugated polymer:fullerene derivative donor:acceptor bulk heterojunctions is not yet sufficient for commercialization. The two most common techniques used to enhance cell performances are thermal treatments and utilization of interlayers. In this work we investigated the effect of the sequence of thermal annealing and the metal evaporation on interlayer formation induced by additives migration toward the metal/organic interface. For this purpose we chose to study P3HT:PCBM:PEG blends, on which we performed thermal annealing before or after the Al cathode deposition. We further characterized the device performances and determined, by XPS, the blend/Al interfacial compositions. We conclude that thermal annealing before Al deposition inhibits the migration of PEG to the organic/metal interface in the P3HT:PCBM:PEG system, while annealing after the Al deposition enhances it. Thus, our study reveals that there is a great significance in the sequence of which the thermal annealing and the cathode deposition are performed in additive-containing organic blends, on the interlayer formation, and as a result, on the device performance.

Interlayers Self-Generated by Additive-Metal Interactions in Organic Electronic Devices

The fundamental structure of all organic electronic devices is a stack of thin layers sandwiched between electrodes, with precise intralayer morphology and interlayer interactions. Solution processing multilayers with little to no intermixing is, however, technically challenging and often incompatible with continuous roll-to-roll, high-speed manufacturing. Here, an overview of a recently developed methodology for self-generation of interlayers positioned between the active layer and metal contact is presented. The interlayer material is blended as an additive in the active layer and migrates to the organic/metal interface during metal deposition. The driving force for this migration is additive-metal interactions. The generated interlayer positions an interfacial dipole that reduces barriers for charge transfer across the organic/metal interface. This methodology is generic and, as reported here, the self-generated interlayers significantly improve the performance of many devices. Importantly, this approach is compatible with printing and reel-to-reel processing. Directives toward additive selection, processing conditions and integration in future applications are also discussed.