Replacing alkyl side chain of non-fullerene acceptor with siloxane-terminated side chain enables lower surface energy towards optimizing bulk-heterojunction morphology and high photovoltaic performance

Towards a good control of the morphology of bulk-heterojunction(BHJ)active layers for polymer solar cells(PSCs),selecting an appropriate side chain for a polymer donor and a nonfullerene acceptor(NFA)is very crucial.In this work,two novel NFAs i-IE-4F and i-IESi-4 F comprising alkyl and siloxane-terminated side chains on the central indacenodithiophene(IDT)core,respectively,were synthesized.Attaching the siloxane-terminated side chain to i-IESi-4 F affords surface energy(γ)of33.32 mN/m,much lower than that of 39.83 mN/m for i-IE-4F,supplying a big chance to tune miscibility with a polymer donor.Two fluorobenzotriazole-based polymer donors J52 and PBZ-2Si bearing alkyl and siloxane-terminated side chains,respectively,showγvalues of 36.08 and 33.10 mN/m,respectively.The estimated Flory-Huggins interaction parameters(χD,A)indicate that the i-IESi-4 F is more miscible than i-IE-4F in pairing with J52 or PBZ-2Si.The resulting i-IESi-4 F-based blend film exhibits low film roughness and accompanies obviously improved BHJ uniformity.In PSCs,the J52:i-IESi-4F and PBZ-2Si:i-IESi-4 F active layers display power conversion efficiencies(PCEs)of 12.67%and 14.54%,respectively,all remarkably higher than PCEs≤7.34%of the i-IE-4F-based active layers.Interestingly,the PBZ-2Si:i-IESi-4 F active layer,a donor:acceptor blend system comprising siloxane-terminated side chains(DSi:ASimatching)with the highest BHJ miscibility due to the combinatory effect of the side chains,shows the highest efficiency,as supported by efficient exciton dissociation,the lowest bimolecular recombination,and the optimal charge transport.Our results demonstrate that attaching siloxane-terminated side chains to NFAs,as a side chain engineering,has big potential in lowering their surface energy towards fine control of BHJ morphology and leading to a better donor:acceptor blend system.

Silaindacenodithiophene-Based Fused-Ring Non-Fullerene Electron Acceptor for Efficient Polymer Solar Cells

In this work, a new A-D-A type nonfuUerene small molecular acceptor SilDT-IC, with a fused-ring silaindacenodithiophene （SilDT） as D unit and 2-（3-oxo-2,3-dihydroinden-l-ylidene）malononitrile （INCN） as the end A unit, was design and synthesized. The SilDT-IC film shows absorption peak and edge at 695 and 733 nm, respectively. The HOMO and LUMO of SilDT-IC are of -5.47 and -3.78 eV, respectively. Compared with carbon-bridging, the Si-bridging can result in an upper-lying LUMO level of an acceptor, which is benefit to achieve a higher open-circuit voltage in polymer solar cells （PSCs）. Complementary absorption and suitable energy level alignment between SilDT-IC and wide bandgap polymer donor PBDB-T were found. For the PBDB-T：SilDT-IC based inverted PSCs, a D/A ratio of 1 ： 1 was optimal to achieve a power conversion efficiency （PCE） of 7.27%. With thermal annealing （TA） of the blend film, a higher PCE of 8.16% could be realized due to increasing of both short-circuit current density and fill factor. After the TA treatment, hole and electron mobilities were elevated to 3.42 × 10-4 and 1.02 × 10-4 cm2·V-1.s-1, respectively. The results suggest that the SilDT, a Si-bridged fused ring, is a valuable D unit to construct efficient nonfullerene acceptors for PSCs.