Steric Engineering of Alkylthiolation Side Chains to Finely Tune Miscibility in Nonfullerene Polymer Solar Cells

Abstract

advantages of simple device structure, light weight, and low fabrication cost by solution-processing. [1][2][3][4][5][6][7][8][9][10][11] Despite the great achievement in fullerene PSCs, fullerene acceptors still suffer from several drawbacks, including high-cost purification, poor morphological stability, and weak absorption in visible wavelength range. To solve these problems, considerable research efforts have been dedicated to nonfullerene acceptors, especially smallmolecular acceptors (SMAs). [12][13][14][15][16][17][18][19][20] Recently, emerging SMAs have been demonstrated as alternatives to fullerene acceptors because the excellent photovoltaic performance with power conversion efficiency (PCE) has reached 12-14% recently. [18][19][20][21] To achieve more efficient SMA PSCs, it is essential to design donor polymers that can match with SMAs through favorable morphology as the device performance of PSCs is strongly dependent on the blend morphology. Therefore, morphology controlling has become one of research key points and still faces a great challenge. Some practical physical strategies have been developed such as ternary blend, additives, thermal annealing, etc., to finely tune blend morphology and form miscible donor/acceptor mixture. [22,23] In the meantime, exploitation of chemical tools as an alternative strategy to control blend morphology is showing its advantages in finely molecular-scaled controlling. [24] One of the most commonly used chemical strategies for tuning morphology is through the fluorination or alkylthiolation of the polymeric side chains to tune the solubility, aggregation properties, and thus the good miscibility with acceptors. [25] In comparison with the high cost and complex synthetic route of fluorinated modification on polymer side chains, low cost of alkylthiolation is obviously synthetic superior to the former. However, in recent literatures it is found that, on the one hand, the alkylthiolation of polymer side chains can improve polymeric crystallinity and mobility and lead to a high photovoltaic performance; [26,27] on the other hand, the strengthened crystallinity after the alkylthiolation for high crystalline polymer may form dense and isolated polymer domains, which causes serious phase separation and destroys the miscibility at donor/acceptor interfaces. [24,28,29] As the thermodynamic miscibility between donor and acceptor mainly drives the phase separation degree, therefore, Morphology and miscibility control are still a great challenge in polymer solar cells. Despite physical tools being applied, chemical strategies are still limited and complex. To finely tune blend miscibility to obtain optimized morphology, chemical steric engineering is proposed to systemically investigate its effects on optical and electronic properties, especially on a balance between crystallinity and miscibility. By changing the alkylthiol side chain orientation different steric effects are realized in three different polymers. Surprisingly, the photovoltaic device of the polymer PTBB-m with...

Evaporation‐Free Nonfullerene Flexible Organic Solar Cell Modules Manufactured by An All‐Solution Process

“…ITIC-Th showed longer lamellar packing distance (d 100 ) and shorter π-π stacking distance (d 010 ) than ITIC (in Table S2 in the Supporting Information). [71][72][73][74] As a result, the calculated χ values for PBDB-T and SMD2 showed 0.574 to 0.821 for blended with ITIC, and 0.260 to 0.434 for blended with ITIC-Th. [23,46,56,70] To compare the compatibility the donor polymer and NFA components, the surface tension (γ) was determined and calculated from the contact angles measured using two solvents, water (distilled water) and oil (diiodomethane, DIM) in each pristine film ( Figure S11 and Table S4, Supporting Information).…”

“…[71][72][73][74] As a result, the calculated χ values for PBDB-T and SMD2 showed 0.574 to 0.821 for blended with ITIC, and 0.260 to 0.434 for blended with ITIC-Th. [71,73,77] According to comprehensively the GIWAXS, Flory-Huggins interaction, and DSC results, SMD2 showed a relative high crystallinity compared to PBDB-T, also, ITIC showed a much higher crystallinity than ITIC-Th. A low χ value leads to a too high miscible for the blend films.…”

Wide bandgap polymer donors for high efficiency non-fullerene acceptor based organic solar cells

“…On the other hand, the polymers with relatively weak V7V stacking and less pronounced molecular ordering in solution (such as PX1, PE17 (PTB7), PE1 and PE18 (PBDT-TS1)) tend to form much smaller and random polymer domains upon casting. 163,[236][237][238] The morphology of the donor:acceptor blend can be controlled by pre-and post-deposition treatments of the as-cast film. These morphology optimization methods aim to achieve high average domain purity and highly ordered packing of both donor and acceptor to minimize charge recombination in the NFA-based OSCs.…”

Benzodithiophenedione-based polymers: recent advances in organic photovoltaics

“…P41 with alkoxyl chains and P43 with alkylthio chains showed significantly poor device performance compared with that of P42 with alkyl chains due to the pronounced broader fibril width caused by overaggregated behaviors of the alkoxyl and alkylthio chains compared with the alkyl chains and thus rendered incompatible phase separation. To finely tune the blend miscibility, Huo et al designed a series of polymers P44-P46 by varying the alkylthiol side chain orientations 44 . It is interesting to note that the moderate steric effect of P45 possessed the highest PCE of 12.2% compared with P44 and P46 with side chains in ortho-and para-positions, respectively, which is ascribed to the steric-effect-induced miscibility (SEIM).…”

“…Molecular twist angles between the side chain and backbone of polymers based on three repeat units determined using DFT at the B3LYP/6-31g(d,p) level; b 2D GIWAXS images of P44, P45, and P46 neat films, respectively, and the corresponding IP and OOP line cuts; c proposed mechanism of steric effect on the blend film morphology. Reproduced with permission44 . Copyright 2019 John Wiley and Sons…”