Abstract 
Polymer solar cells (PSCs) have a photoactive layer based on a bulk-heterojunction that is composed of a mixture of organic semiconductors (usually an electron-donating polymer and an electron-accepting molecule). They represent a promising approach for renewable energy generation and show unique properties such as flexibility, semi-transparency and low energy footprints. Thanks to the rise of non-fullerene acceptors (NFAs), and their used in ternary blends (for example two molecular acceptors blended with a donor polymer), their power conversion efficiency (PCE) within a few years significantly increased up to 19%, thus improving the competitiveness of PSCs. The manufacturing process of this class of solar cells relies on large area printing technologies, which renders their fabrication more sustainable. However, to date, the "record" PCEs are obtained with few square-millimeter sized cells processed under laboratory conditions; i.e inert atmosphere using thin photoactive layers and additional evaporation steps. While the performances of NFA based PSCs have been greatly improved, recent investigations have shown that there are still major issues to accelerate their industrial use. Amongst those, materials and devices stability are still unsatisfactory. Furthermore, for implementing NFA materials into industrial fully printed manufacturing process in air at larger square-cm size, the first pre-requisite is the use of thick active layers. In addition, the blend materials have to be deposited in air using non-halogenated solvents, while record NFA materials are processed under inert atmosphere using halogenated solvents. Finally major performance losses occur when changing device structure and interfacial materials of record cells to industrial inverted devices using PEDOT:PSS for example. Theses major issues in PSCs demand for novel material strategies to overcome limitations towards industrial developments of PSCs.

Until recently, the efficiencies of all-polymer solar cells were clearly below devices based on blends using NFA- molecules and thus the “all-polymer” approach did not attract the interest of the community. However, very recently few polymers resulting from the polymerization of NFA molecules have been reported leading to efficiencies close to 17% using ternary blend. These pioneer works open the doors to overcome morphological instabillity in PSCs. Moreover, a relatively weak thickness dependence of photovoltaic property was observed for all-polymer solar cells.

In this internship, NFA-based polymers will be synthesized using the best NFAs precursors known in the literature. The electronic properties and crystallinity of the NFA-polymers will be controlled by combining the NFA precursors with different co-monomers. The expected outcome of this approach is to improve together with enhanced crystallinity the long-term morphology stability when used in blends. Ultimately, this approach should help us to prepare active layer of higher thicknesses while keeping performance high up to 17%, a pre-requisite for a transfer to industrial roll-to-roll process for instance. 

​References
• Cui, Y. et al., Single-Junction Organic Photovoltaic Cell with 19% Efficiency. Adv. Mater. 2021, 33 (41), 2102420 
• Sun, R. et al., Achieving over 17% efficiency of ternary all-polymer solar cells with two well-compatible polymer acceptors. Joule 2021, 5 (6), 1548-1565
• Avalos-Quiroz, Y. A. et al., Non-Fullerene Acceptors with an Extended π-Conjugated Core: Third Components in Ternary Blends for High-Efficiency, Post-Treatment-Free Organic Solar Cells. ChemSusChem 2021, 14 (17), 3502-3510
• Xu, Y. et al., Ambient Processable and Stable All-Polymer Organic Solar Cells. Advanced Functional Materials 2019, 29 (8), 1806747 


Approaches & materials used
Organic synthesis, characterization (RMN, IR, UV-vis spectroscopy, fluorescence, electrochemistry), Surface characterization (AFM, SEM, GIWAXS), photovoltaic devices (I-V curves, IPCE)

​Desired areas of expertise of the candidate
Organic synthesis, Polymer chemistry, physical comprehension of a pi-conjugated materials, photovoltaic devices

​Send CV, cover letter and grades to Cyril Aumaitre (French or English)
Supervisor: Aumaitre Cyril
17 avenue des Martyrs
38000 Grenoble

​Dates of the internship
January/July 2023