Synthesis of naphthalene diimide structures for application in polymer solar cells

Author: Jonas Mattiasson Bjuggren

Mattiasson Bjuggren, Jonas, 2019 Synthesis of naphthalene diimide structures for application in polymer solar cells, Flinders University, College of Science and Engineering

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The field of organic photovoltaics (OPV) has made tremendous progress during the 21st century with efficiencies now exceeding 15% for single-junction polymer solar cell (PSC) devices. These efficiencies are highly promising, with the technology set-up to participate in the solar boom that is predicted to constitute a major contribution to the phasing out fossil fuels. Given the urgency to reduce greenhouse gas (GHG) emissions it is imperative that the research conducted is focused toward the commercialisation of PSC. However, complex synthesis routes involving highly toxic chemicals, widespread use of chlorinated processing solvents, and the utilisation of laboratory scale processing methods could delay commercialisation. These methods were necessary to develop a deeper understanding of PSCs, taking the research field from efficiencies of <1% in its infancy to where it is today, but are now counterintuitive for further development of this potentially very green technology. Therefore, further research efforts are needed on green synthesis, environmentally friendly processing, and scalable fabrication to drive the commercialisation of PSC.

This thesis is devoted to the development of n-type naphthalene diimide (NDI) based compounds for use in PSC. Three different material categories were investigated in this thesis work; with i) small molecule non-fullerene acceptors (NFA) presented in Chapter 4, ii) NDI-bithiophene based acceptor polymers introduced in Chapter 5, and iii) cathode interface layers (CIL) discussed in Chapter 6 & 7. Particular attention was given to the green and facile synthesis routes of materials with elevated solubilities in non-chlorinated, greener solvents that are compatible with scalable manufacturing methods.

A number of different NDI based n-type materials were successfully synthesised, ranging from small molecules to polymers. These include series of small molecule NFAs, acceptor polymers, and CILs with variations in crystal forming ability for enhanced morphology control, side chain length along with number for increased solubility in greener solvents, and linker unit as well as pendant side-group moiety for increased doping, respectively.

The small molecule NFAs of varying crystal forming ability, employed in active layer blends in inverted device structures, were found to yield low power conversion efficiencies (PCE) with trade-offs possibly occurring between favourable morphology and the presence of charge carrier pathways. These materials could potetntially fit niche applications, e.g. semitransparent PSCs.

For the acceptor polymers octyldodecyl (OD) side chains were found to yield higher PCEs than their hexyldecyl (HD) counterparts. Moreover, partial substitution of bithiophene in the N2200 polymer backbone yielded high PCEs with tuneable solubilities in greener processing solvents. The novel polymer P(NDI2OD-14T2) achieved PCEs of 4.68% and 4.06% for spin coated (SC) and blade coated (BC) all-polymer devices, respectively.

Significant improvements in PCEs were observed when employing PNDIT10N as a CIL. New record PCEs were achieved for two out of three donor polymers in inverted devices with polymer:fullerene bulk heterojunction (BHJ) layers. This was attributed to a significant reduction of the ITO work function and the formation of an interfacial dipole at the ITO-CIL interface. Moreover, an ultra-thin layer of the CIL experienced a good stability under varied storage conditions.

CILs with a variety of linkers and either tertiary amine or quaternary amine bearing pendant side-groups were studied. Significant reductions of the ITO work function were observed for all CILs. Preliminary results reveal the benzene linker with quaternary amine to experience the highest PCE. Moreover, this polymer also experiences a more suitable absorbance at lower wavelength regions and suitable solubilities for orthogonal solvent processing in inverted PSCs.

Keywords: polymer solar cells, naphthalene diimide, NDI, non-fullerene acceptor, all-polymer solar cell, cathode interface layer, side-group engineering, blade coating, doping

Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Mats Andersson