Determining the Role of Polymer Molecular Weight for High-Performance All-Polymer Solar Cells: Its Effect on Polymer Aggregation and Phase Separation

Abstract

The molecular weight of a conjugated polymer is one of the key factors determining the electrical, morphological, and mechanical properties as well as its solubility in organic solvents and miscibility with other polymers. In this study, a series of semicrystalline poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) polymers with different number-average molecular weights (M(n)'s) (PPDT2FBT(L), M(n) = 12 kg/mol; PPDT2FBT(M), M(n) = 24 kg/mol; PPDT2FBT(H), M(n) = 40 kg/mol) were synthesized, and their photovoltaic properties as electron donors for all-polymer solar cells (all-PSCs) with poly[[N,N'-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)) acceptor were studied. The M(n) effect of PPDT2FBT on the structural, morphological, electrical, and photovoltaic properties was systematically investigated. In particular, tuning the M(n) induced dramatic effects on the aggregation behaviors of the polymers and their bulk heterojunction morphology of all-PSCs, which was thoroughly examined by grazing incident X-ray scattering, resonant soft X-ray scattering, and other microscopy measurements. High M(n) PPDT2FBT(H) promoted a strong "face-on" geometry in the blend film, suppressed the formation of an excessively large crystalline domain, and facilitated molecularly intermixed phases with P(NDI2OD-T2). Therefore, the optimized all-PSCs based on PPDT2FBT(H)/P(NDI2OD-T2) showed substantially higher hole and electron mobilities than those of PPDT2FBT(L)/P(NDI2OD-T2), leading to a power conversion efficiency exceeding 5%, which is one of the highest values for all-PSCs reported thus far.

Benzocyclobutene-fullerene bisadducts as novel electron acceptors for enhancing open-circuit voltage in polymer solar cells

“…The π-π stacking distance (010) of P3HT in both BCBCBA and bis-PCBM mixed film was calculated to be 0.38 nm (q z ¼1.65 Å À 1 ) from the out-of-plane plot, consistent with the literature values for well-ordered P3HT crystals [47][48][49]. For quantitative analysis of the crystalline order of P3HT, the crystal size of P3HT in each active layer was calculated using Scherrer equation [48,[50][51][52][53]. The detailed calculations are described in Supplementary materials.…”

Structure and design of polymers for durable, stretchable organic electronics

“…100 This trend is also seen in semiconducting polymers, and has the added benefit of improving electrical performance in most systems as well. 101,102 This mechanical toughening arises from the increasing incidence of entanglement of chains with increasing molecular weight. These entanglements act as physical crosslinks, adding energetic barriers to moving chains.…”

Improved All-Polymer Solar Cell Performance of n-Type Naphthalene Diimide–Bithiophene P(NDI2OD-T2) Copolymer by Incorporation of Perylene Diimide as Coacceptor

“…It exhibited properties like high electron mobility >0.85 cm 2 V −1 s −1 in OFET, high PCE efficiency >5% in all-PSCs, solution processability, high crystalline nature, and light absorption capability near visible and infrared region. 15,16,20 However, it exhibited a strong tendency to form aggregates in specific organic solvents that resulted in large-scale phase separation instead of the desirable pure donor/acceptor microphase separation. 28 Consequently, the earlier reports on the all-PSC measurements of P3HT/P(NDI2OD-T2) blend showed very low device efficiency value (PCE, 0.2%).…”