Transforming the molecular orientation of crystalline lamellae by the degree of multi-fluorination within D–A copolymers and its effect on photovoltaic performance

Besides lowering the HOMO energy level of polymer donors to promote higher open circuit voltages (V_oc) of polymer solar cells, fluorination of their conjugated backbones has been widely demonstrated to improve the overall device performance as well by inducing face-on orientated polymer crystalline lamellae, which is profitable for vertical charge transport. Nevertheless, over-fluorination (up to 4 fluorine atoms) causes severe solubility reduction and serious segregation, leading to large phase separation of PC₇₁BM and subsequent low and unbalanced charge transport, thus deteriorating J_sc, FF and the overall PCE. The underlying impact of multi-fluorination of polymer donors on their interlinked optoelectronic and morphological properties thus remains beyond reach. Guaranteed with sufficient solubility, two D–A copolymers, poly[(5,5′-(2,5-bis((2-octyldodecyl)oxy)-1,4-phenylene)bis(4-fluorothiophene-5,2-diyl))-alt-((5-fluorobenzo[c][1,2,5]thiadiazole)-4,7-diyl)] (PDTBBT-3F) and poly[(5,5′-(2,5-bis((2-octyldodecyl)oxy)-1,4-phenylene)bis(4-fluorothiophene-5,2-diyl))-alt-((5,6-difluorobenzo[c][1,2,5]thiadiazole)-4,7-diyl)] (PDTBBT-4F) with 3 and 4 fluorine substituents on the same backbone, respectively, were therefore designed and synthesized. Although lower-lying HOMO energy levels and stronger intermolecular interactions were observed as before, more interestingly, the molecular packing of PDTBBT-3F and PDTBBT-4F in neat films was found to be diametrically opposite. PDTBBT-4F exhibits a preferred face-on orientation whereas the edge-on orientation is dominant for PDTBBT-3F. When mixed with PC₇₁BM to form BHJ blends, the molecular orientation of PDTBBT-3F crystals was transferred to uniform edge-on/face-on intermixing with better crystallinity. Oppositely, the face-on oriented crystals were suppressed to be comparable with edge-on orientated ones with less crystallinity for PDTBBT-4F from the neat film to the BHJ blend. Such distinctive morphology improved the charge generation efficiency, balanced the charge transport and suppressed the charge recombination in the PDTBBT-3F:PC₇₁BM device, eventually leading to an approximate 40% increase of the maximum power conversion efficiency (PCE, 8.33%) in comparison with the PDTBBT-4F:PC₇₁BM device (5.92%). Although the maximum PCE is modest compared with the highest state-of-the-art polymer:fullerene blends, we clearly reveal the effect of the degree of multi-fluorination of polymer donors on their self-assembled thin film structure and photovoltaic properties. Attractive molecular orientation transformation of the crystalline lamellae triggered by the degree of multi-fluorination in this case is likely to percolate into related organic semiconductor fields beyond polymer solar cells.