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Novel processing of colloidal nanocrystals for optoelectronic applications

Periodic Reporting for period 1 - NANOPTO (Novel processing of colloidal nanocrystals for optoelectronic applications)

Reporting period: 2016-09-12 to 2018-09-11

The project NANOPTO focused on the development of colloidal semiconductor nanocrystals showing high photoluminescence and conductivity for the development of efficient solar cells and light-emitting diodes. In particular, the project proposed the development of a “dot-in-matrix” system able to demonstrate these properties.

Colloidal semiconductor nanocrystals have been studied and optimized since the early nineties and they are now finding application in consumer electronics products such as televisions. Nevertheless, in televisions, nanocrystals acts as a colour conversion system where they absorb the light from the TV backlight panel and they emit an individual colour depending on their size and chemical composition. Such system allows for higher brightness and a greater colour volume than before but does not employ true light-emitting diodes (LEDs) based on nanocrystals. In fact, despite the desirable light-emission properties, LEDs based on nanocrystals still struggle in reaching the required performance for application in consumer electronics products due to various issues affecting their performance during device fabrication and operation. Importantly, development of highly efficient LEDs based on nanocrystals for example could pave the way to flexible displays with high colour purity or more efficient large area white-light sources that could lead the development of novel products and put Europe in a dominant position in these technologies.

Similarly, solar cells based on colloidal semiconductor nanocrystals show still limited efficiency compared to other competing materials (for example perovskite and organic semiconductor). Yet, colloidal nanocrystals are very promising for the fabrication of solar cells operating in the infrared spectral region where commonly available silicon solar cells show limited light absorption. Development of a nanocrystal solar cell operating in the infrared spectrum would allow increasing the efficiency of commercially available technologies by creating a so-called “tandem” cell, thus improving energy generation.
In the initial phases of NANOPTO, we focused on the synthesis of different nanocrystals in order to assess which material was more promising to achieve the final aim of the project of fabricating efficient solar cells and LEDs, and test the proposed dot-in-matrix concept. In early 2017, we developed a novel synthesis of perovskite nanocrystals that can be carried out at room temperature. Importantly, our synthetic method demonstrated nearly 100% photoluminescence efficiency following a post-synthesis chemical treatment. This exciting result induced us to focus on this material for developing solar cells and LEDs operating in the visible range. Meanwhile, we started investigating the dot-in-matrix concept in lead sulphide nanocrystals in combination with the functionalization of the nanocrystal surface.

In 2017, we fabricated a solar cell based on our perovskite nanocrystals. Before proceeding with the device fabrication, the pristine perovskite nanocrystals were treated to substitute the bromine atoms in their structure with iodine. This procedure reduces the energy bang gap of the material, thus allowing the full visible spectrum to be absorbed. The fabricated solar cells demonstrated an efficiency above 5%. In parallel with this activity, we studied the surface treatment of lead sulphide nanocrystals and we demonstrated that substituting the native organic molecule found on their surface with a mixture of zinc iodide and an organic acid leads to improved charge transport. Following these results, we focused on the fabrication of LEDs employing the two materials. In particular, we use the perovskite nanocrystals to fabricate a solution processed LED operating in the visible range (green emitting) with an efficiency above 6%. We achieved this result by tuning the synthesis of the perovskite nanocrystals and optimizing the device fabrication procedure. Finally yet importantly, we developed LEDs employing the surface modified lead sulphide nanocrystals. In particular, we observed a strong enhancement of the photoluminescence efficiency when the lead-sulphide nanocrystals are dispersed in a matrix of nanocrystals with the same composition but of smaller size in combination with zinc oxide nanoparticles. In fact, the two latter components acts as a passivating matrix to the emitting large lead sulphide nanocrystals enhancing both the photoluminescence efficiency and the charge transport.

Importantly, we have presented our results in various international conferences both in Europe and in the USA and our findings have been shown to the publics as well, at a workshop organized at ICFO and by participating in the “pint of science” international event in Barcelona. We have now publish a total of 5 peer-reviewed articles concerning our findings during the development of NANOPTO.
We have demonstrated two main improvements beyond the state of the art. First, we fabricated one of the most efficient perovskite nanocrystals LED based on materials processable from solution. This is an important achievement as LEDs fabricated from solution processable materials are compatible with mechanically flexible substrates, and can be fabricated at low cost with less energy consuming facilities. Before NANOPTO, this class of devices was showing an efficiency around 1%. We developed a novel chemical treatment of perovskite nanocrystals and an optimized device fabrication method that increased the LED efficiency above 6%.
The second important progress beyond the state of the art is the demonstration of highly efficient LEDs based on lead-sulphide nanocrystals operating in the telecommunication spectral range (infrared spectrum). We fabricated the LED employing the concepts proposed in NANOPTO: high photoluminescence efficiency combined with increased conductivity in a lead-sulphide dot surrounded by a zinc oxide/lead sulphide matrix. The obtained LED demonstrated an efficiency above 7%, the highest ever reported for this class of nanocrystal devices.
Importantly, the fabrication methods we developed for the two types of nanocrystal LEDs have a direct impact on the future development of this class of devices. In particular, the proposed chemical treatment of perovskite nanocrystals is currently increasingly attracting attention from the scientific community thus leading to faster development of efficient and stable perovskite nanocrystals. On the other hand, the efficient infrared LED based on lead sulphide nanocrystals can be the first step toward further development of this class of devices that can potentially compete with other technologies. Importantly, novel small footprint infrared light-sources are required, for example, in face recognition hardware or telecommunication networks.