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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Nano Harvest (Flexible nanowire devices for energy harvesting)

Teaser

Summary

Flexible optoelectronic devices provide an amount of 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, for example, rollable displays, wearable intelligent electronics, light sources, bio-medical devices, etc. For energy converters, the main objective is to transform the ambient energy found in the vicinity of the device into usable electrical energy to charge batteries of portable devices. In addition to the device efficiency, the weight and the transportability are also important factors. Therefore, the device integration on light and flexible supports such as plastic or even textile fabric, which can withstand strong deformations without breaking, is desirable. The use of cheap flexible substrates will also significantly reduce the device cost.
The objective of the NanoHarvest 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 photovoltaic (PV) and piezoelectric converters can be realized. The technology can also be applied to other devices such as photodetectors and light emitters. 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 PV device 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 III-nitride and GaAsP nanowires should enable high device efficiency.

Work performed

During the first 18 months, the efforts were focused on the three following points (i) optimization of the nanowire elaboration by molecular beam epitaxy and characterization of their material, electrical and optical properties; (ii) development of the technology for flexible nanowire devices ; (iii) proof-of-concept of a flexible nanowire device.
The main achievement during this period was the development of the technology for fabrication and functionalization of nanowire/polymer membranes and the successful demonstration of optoelectronic devices relying on this technology. As a proof-of-concept demonstrator we have chosen a light emitting diode (LED). We have first demonstrated large area (several square cm) fully flexible blue and green LEDs based on core/shell nitride nanowires. Nanowires were embedded into polymer layers and mechanically lifted-off from their growth substrate. Conductive transparent electrodes to these composite membranes were realized using silver nanowire networks. The LEDs showed bright emission with no performance degradation neither under outward or inward bending down to 3 mm curvature radius nor in time for more than one month storage in ambient conditions without any protecting encapsulation. 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 proposed in the NanoHarvest project.

Final results

NanoHarvest aims at a major breakthrough in terms of 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, we aim to gain in-depth understanding of the conversion mechanisms down to the nanoscale and to engineer original nanowire structures featuring new functionalities. Today, in Photonics Roadmap the word â€œflexible deviceâ€ is used as a synonym to â€œorganic deviceâ€. Indeed, the dominating technology to achieve flexible devices relies today on organic semiconductors. However, the drawbacks of the organic technology are the poor stability in time and a low efficiency when compared to inorganic semiconductors. For example, the maximal luminance of the organic LED displays remains rather limited with respect to inorganic LEDs. In the NanoHarvest project we want to create a new technology combining the advantages of the two worlds. We aim to 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 will be achieved by combining nanowire/polymer membranes as the active element of LEDs, solar cells and piezogenerators.
In particular, NanoHarvest aims to make key advances in the domain of flexible LEDs, solar cells and piezoelectric converters in terms of (i) a deep understanding of conversion mechanisms at the nanoscale and at the device level; (ii) novel designs and architectures; (iii) new technology combining the advantages of the flexible polymer films with the high conversion efficiency of crystalline nanoobjects; and (iv) demonstration of devices going far beyond the present state-of-the-art.
The achievements of NanoHarvest will 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 targeted piezoelectric energy harvesters will also benefit to the development of integrable sensors and piezoelectric actuators. From the point of view of basic research, the strong effort on material optimization of NanoHarvest will allow the fabrication of nanowires with control-by-design properties opening new avenues for fundamental studies in quantum photonics and electronics. In terms of societal and ecological impact, the results of this project will contribute to energy efficiency of devices. The energy harvesters will increase the share of the renewable resources in the total amount of consumed energy. In particular, the proposed portable solar cells have the potential of providing access to electricity in remote regions without grid connection.
It should be emphasized, that this pathfinder project is of high-risk nature and aims at developing a completely new technology. During the first 18 months, the practical feasibility was demonstrated by fabricating a flexible two-colour LED. The work towards demonstration of flexible solar cells and piezogenerators is on-going. This demonstration will justify further research efforts in a more industrial environment, in order to bring the technology to maturity.