Organic photovoltaics have been intensively investigated for their advantages as low cost, low weight and their easy process on large flexible substrates. Much of the attention have been focused on the development of electron donor (D) polymer associated with electron acceptor (A) phenyl-C61-butryric acid methyl ester (PC61BM) or PC71BM. Recently, small molecules have confirmed to be as efficient as polymers with record power conversion efficiencies (PCEs) of 9.60% [1] when used as D-materials and paired with PC71BM [1,2] or as A-materials to replace PCBM [3-4] reaching PCE of 4.1% with P3HT [5]. Moreover they offer a better reproducibly, simpler synthesis and purification than polymers. The development of A-small molecules is a challenge because fullerene derivatives are expensive, difficult to purify, have a poor absorbance in UV-visible spectrum and their optoelectronic properties are hard to engineer. The objective of this study is to identify the influence of small molecule end chains on electrical and morphological properties in pristine and blend films with the well-known donor P3HT. Therefore, we have designed and synthesized new small acceptor molecules for OPV. These symmetrical molecules are designed around a central acceptor fragment, the benzothiadiazole, and have different terminal solubilizing acceptor moieties with linear and non-linear alkyl chains, named CAO and CAEH, respectively. Their synthesis has been developed using a green coupling method: direct heteroarylation that requires neither toxic nor complicated intermediates such as stanyl or boronyl derivatives. Herein, it can be achieved at low palladium-catalyst loading, without ligands and produces less remaining toxic wastes. After detailing different synthesis routes to obtain these molecules of interest, their properties will be investigated. To reach this goal pure molecules and blends were studied in solution and film, processed by spin-coating, and characterized by different techniques. According to DFT simulation, UV-Visible spectroscopy and electrochemistry measurements, these new molecules have an energy gap of 2.1 eV, HOMO-LUMO levels close to PCBM and a better absorption in UV-Visible region than that of PCBM. Moreover, DSC measurements show that the nature of the terminal alkyl chains lead to crystal or liquid-crystal properties. Indeed CAEH exhibits a crystal behavior whereas CAO presents a crystal liquid phase. This is particularly interesting because this property has strong influence on film morphology for both pure molecule and blend. Atomic Force Microscopy measurement emphasizes that CAEH and CAO morphologies are different in pristine film and that the blend morphology is driven by the pristine one. In blend films with P3HT, CAEH exhibits a homogenous morphology whereas CAO shows a “fiber-like” structure and a roughness twice higher than CAEH. Raman and Photoluminescence measurement emphasizes the influence of these different morphologies on P3HT molecular organization in blend.
[1] J. Am. Chem. Soc. 136, 15529 (2014) [2] Adv. Energy Mater. 3, 1161 (2013) [3] Adv. Mater. 26, 4313 (2014) [4] J. Mater. Chem. A, 2, 2657 (2014) [5] J. Am. Chem. Soc., DOI: 10.1021/ja5110602