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Flexible nanowire devices for energy harvesting

Periodic Reporting for period 4 - Nano Harvest (Flexible nanowire devices for energy harvesting)

Reporting period: 2019-10-01 to 2021-03-31

Flexible optoelectronic devices provide new functionalities and have the potential to open up a new branch of industry. In particular, flexible and energy efficient devices such as light emitting diodes (LEDs) but also photovoltaic and piezoelectric energy harvesters are today a topic of intense research, motivated by their applications such as portable battery chargers, rollable displays or light sources, bio-medical devices, etc. In addition to the device efficiency, the weight and the transportability are also important factors together with the device cost. Therefore, the device flexibility and integration on light cheap supports such as plastic or even textile fabric, which can withstand strong deformations without breaking, is desirable. These flexible energy harvesters and light emitters have the potential of providing access to electricity and lighting in remote regions without grid connection.
In this context, the main objective of the Nano Harvest project is to develop a new class of flexible optoelectronic devices combining polymer films with semiconductor nanowires. The idea is to enable substrate-free devices by encapsulating semiconductor nanowires into polymers, removing them from their growth substrate, functionalizing and assembling the membranes. With this versatile transfer approach flexible light emitters, photovoltaic and piezoelectric converters can be realized. The technology can also be applied to other devices such as photodetectors. One advantage is to combine nanowire/polymer membranes with different functionalities by stacking them. Thus material combinations unavailable with monolithic nanowire growth can be achieved. By stacking together free-standing polymer-embedded nanowires, a multi-bandgap solar cell can be realized and applied to almost any supporting material such as plastic, metal foil or fabrics. Multi-layered flexible and compact piezo-generators based on ordered arrays of nanowire heterostructures can be produced. Multi-colour light emitting diodes combining Red, Green and Blue emitters can be fabricated. The crucial ingredient and also the common basis for all these devices are the advanced nanowire heterostructures with new control-by-design functionalities. Nanoscale engineering and in-depth understanding of physical phenomena in nanowires open the way to reach high device efficiency.
In the Nano Harvest project, the efforts were focused on the 4 main directions: (i) optimization of the nanowire elaboration by molecular beam epitaxy; (ii) characterization of their material, electrical and optical properties; (iii) development of the technology for flexible nanowire devices and (iv) fabrication of proof-of-concept devices.
First, we addressed the physical mechanisms governing the energy conversion from the single nanowire level up to the macroscopic device level. We analysed the carrier generation and collection in nanowire solar cells using nanoscale tools such as the electron beam induced current microscopy and cathodoluminescence. In particular, these investigations were successfully used to optimize the surface passivation in these nanomaterials. We also applied a modified Atomic Force Microscope to quantify the piezo-conversion, namely we established the relation between the output generation and the nanowire stiffness, which allowed us to quantify the electromechanical coupling coefficient of GaN nanowires.
The nanoscale analyses guided the optimization of the device architecture, of the material growth and of the fabrication process. One major achievement is the development of the technology for fabrication and functionalization of nanowire/polymer membranes and the successful demonstration of optoelectronic devices relying on this technology. It is to be noted that the membrane technology has pushed us to initiate a new research direction on van der Waals epitaxy using graphene to facilitate the nanowire lift-off.
For the proof-of-concept demonstrators, we focussed on light emitting diodes (LEDs). We have first demonstrated large area fully flexible blue and green LEDs based on core/shell nitride nanowires. The LEDs showed bright emission with no performance degradation neither under outward or inward bending down to 3 mm curvature radius. Fully transparent flexible LEDs with a high optical transmittance above 60% were realized to demonstrate the integration of green and blue LED membranes into a two-layer bi-color nanowire-based flexible LED. The two layers emitting different colours could be either separately driven to generate green or blue light or simultaneously biased to generate a broad electroluminescence spectrum. This constitutes the first successful demonstration of nanowire/polymer membrane assembly and proves the viability of the approach of the NanoHarvest project for optoelectronic devices. Next, white flexible nanowire LEDs were demonstrated using nanophosphor doping of the polymer membrane and the color quality was optimized by tuning the phosphor type and concentration. Finally, a stretchable blue LED was fabricated by using a dedicated stretchable transparent contact consisting of carbon nanotubes. Nanowire-based piezo-generators were also achieved delivering power densities in the mW/cm3 range and output voltages of about 1 V for a single layer, which can be further increased by stacking the piezogenerators.
The results of the project were disseminated in international journals, specialized conferences, but also through outreach activities (e.g. “Fête de la science” of the Paris Saclay University). Nano Harvest results have also initiated new research lines : nanowire piezosensors and micro-LEDs enabled by van der Waals epitaxy on graphene.
Nano Harvest focusses on both fundamental and technological aspects of design and fabrication of nanowire flexible devices, such as light emitting diodes, solar cells and piezogenerators. In terms of scientific impact, Nano Harvest made advances in the domain of flexible LEDs, solar cells and piezoelectric converters namely in (i) understanding of conversion and passivation at the nanoscale and at the device level; (ii) novel designs containing heterostructured nanowires and new contact architectures; (iii) new technology combining the advantages of the flexible polymer films with the high conversion efficiency of crystalline nano-objects; and (iv) demonstration of devices going beyond the present state-of-the-art for flexible emitters and harvesters.
Today, despite the drawbacks, the dominating technology to achieve flexible devices still relies on organic semiconductors. In the Nano Harvest project, we struggle to change this paradigm and to create a new technology combining the advantages of the two worlds. We build devices characterized by high efficiency and long time stability typical for inorganic materials, but exhibiting mechanical flexibility and allowing for modularity like organic devices. This is achieved by combining nanowire/polymer membranes as the active element of LEDs, solar cells and piezogenerators.
The achievements of NanoHarvest also contribute to the development of other domains of flexible devices such as high-sensitivity photodetectors, which have a large palette of applications, such as biodetection, imaging, industrial inspection, etc. The investigated piezoelectric energy harvesters also benefit to the development of integrable sensors and piezoelectric actuators.