Alkyl side-chain and fluorination engineering in the indeno[1,2-b]fluorene-based small-molecule acceptors for efficient non-fullerene organic solar cells

A series of non-fullerene acceptors based on the indeno[1,2-b]fluorene central moiety, with thiophene or 3-octylthiophene as π-bridge and either non-fluorinated or fluorinated 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile as end-capping groups (namely ICBF-O, ICBF, FICBF-O and FICBF), have been designed and synthesized. The effect of alkylation of the π-bridge and fluorination of the end-capping groups on the absorption spectra, energy levels, active layer morphology and photovoltaic performance were systematically investigated. Alkylation upshifts the molecular LUMO levels and thereby a high open circuit voltage (Voc) of 1.06 V was obtained. However, the larger band gap induced by alkylation led to lower short circuit current (Jsc). Fluorinated acceptors display lower Voc but higher Jsc and FF compared with non-fluorinated acceptors, coinciding with their lower LUMO levels, narrower band gaps and favourable morphology. As a result, the non-fullerene solar cells (NFSCs) based on FICBF showed the highest PCE of 7.41% among these four acceptors. The ICBF based device delivered a comparatively high Voc of 0.99 V and a PCE of 6.07%. The results demonstrated that indeno[1,2-b]fluorene is a promising building block for efficient NFSCs.