As the composition of the electrolyte is understood to dramatically impact swelling, charging, and transport of organic mixed conductors, we tested the same OECTs in 0.02 M KTFSI, a prototypical electrolyte with a hydrophobic anion, to further probe the effect of molecular weight. The primary purpose for using the same devices in both electrolytes was to keep experiments as comparable as possible; this was possible due to the comparable performance of the transistors when tested in KTFSI with and without prior exposure to NaCl (Table , P3MEEET-4). Device operation in KTFSI displayed lower barriers to hole injection (Vth was less negative) for all polymer batches, which can be attributed to the better doping efficiency of KTFSI as compared to NaCl. While each batch of material showed an increase in performance when operated in KTFSI, those with lower molecular weights in the series (P3MEEET-1, P3MEET-2, and P3MEET-3) only showed a ~ 10-15-fold increase in µC*, whereas those with higher molecular weight showed ~ 39-fold increases, suggesting a molecular weight dependent threshold for charge percolation (Figure ). The C* for each batch were lower when operated in KTFSI than in NaCl, highlighting the observed improvement in µC* is primarily due to an increase in µ. The best performing polymer in this electrolyte too, remains P3MEEET-3, with a µC* of 332 F cm -1 V -1 s -1 due to its µ of 2.14 cm 2 V -1 s -1 , making it the highest performing polythiophene homopolymer OMIEC to-date (Table , Figure ), and is on par with some of the best performing p-type materials, with greater structural complexity. To determine if this batch of P3MEEET was an outlier, an independently synthesized batch with similar molecular weight and dispersity was tested, and also demonstrated higher performance than the lower and higher molecular wight batches (P3MEEET-8, Figure ). To relate the molecular weight dependence of hole mobility to microscopic structure, ex-situ GIWAXS was performed on drop-cast films of each material. P3MEEET-1 through P3MEEET-5 displayed no significant structural differences in the dry state, though a slight decrease in πspacing with increasing molecular weight can be observed (Figure , Table ). Each P3MEEET batch showed isotropic π and lamellar scattering which is likely due to the thickness of the samples (Figure ). However, due to poor resolution of the horizon in in-situ GIWAXS experiments (Figure ), we exclusively focus our discussion on out-of-plane features. The polymers demonstrate dry lamellar (100) spacing of ~20.5 -21 Å and π spacing of ~3.7 Å which are in agreement with previous literature reports. While ex-situ GIWAXS investigations can be informative, since observed differences are due to operation in aqueous media, in-situ and operando characterization are critical. As such, we first investigated passive swelling of the material in deionized (DI) H2O, 0.1 M NaCl, and 0.02 M KTFSI. We performed in-situ GIWAXS using a custom frit cell mount (Figure ), and quartz crystal microbalance (EQCM-D) experiments on sample polymers below 20 and above 25 kg mol -1 , which in the following discussion will be referred to as "low" and "high" molecular weight, respectively. In-situ GIWAXS shows swelling of the lamellae in all three conditions, where polymers in KTFSI swell slightly more than those in NaCl, as seen in Figure . This is corroborated by swelling in EQCM-D experiments however, we observed significantly more swelling of the entire film compared to swelling of the crystallites alone, as seen in the GIWAXS results (Figure ). Crystallite expansion can only accommodate at most a 16% mass increase due to electrolyte uptake into the crystallite free volume (see SI section 4), compared to a >50% total mass increase as determined by EQCM-D, thus the majority of passive swelling occurs in the amorphous regions of the polymer. Regardless of media, high molecular weight polymers swelled ~ 10% more than their low molecular weight counterparts in QCM experiments. Despite the volumetric expansion of the crystallite, we observed only a small decrease in coherence lengths (Table ) and consistent lamellar peak shapes (Figure ), indicating that crystallite structure was maintained in the swollen state, regardless of electrolyte composition. In summary, passive swelling experiments did not reveal molecular weight dependence that immediately explain the trends seen in OECT performance, indicating that observed phenomena are only unveiled under an applied bias. To further investigate the electrolyte-dependent improvement in performance of the higher molecular weight polymers, we performed operando GIWAXS and electrochemical quartz crystal microbalance (EQCM-D) experiments on the "low" and "high" molecular weight polymers during electrochemical cycling (Figure ). Lamellar spacing underwent significant modulation upon electrochemical biasing, whereas π-spacing remained largely unchanged in all samples except for high molecular weight samples in KTFSI (see supporting videos). Comparing low and high molecular weight polymers in NaCl, it can be deduced that doping occurs via anion uptake in both cases, due to the observable lamellar expansion (i), as well as increases in mass (ii) and dissipation (iii), which agrees with literature. While the overall mass uptake is larger in the high molecular weight polymer, the crystallite volume shows higher modulation in the low molecular weight polymer (Figure , Figure ). This suggests that mass uptake in high molecular weight polymers occurs preferentially in the amorphous region. Larger mass modulation within the high molecular weight polymer (Figure , Figure ) agrees with the slightly higher volumetric capacitance observed in the OECTs. It's also noteworthy that, while de-doping occurs predominantly via Cl -expulsion in the high molecular weight polymer (iv), during later stages of the de-doping process (closer to 0 V), low molecular weight polymers show an increase in mass (v), as well as an increase in the lamellar spacing (vi), indicating an uptake of Na + . In comparison, differences between high and low molecular weight in KTFSI are much more subtle (Figure , Figure ). Specifically, the low molecular weight polymer showed smaller modulation of mass, dissipation, and lamellar spacing (Figure ). From the decrease in mass (vii) and lamellar spacing (viii), it can be inferred that doping primarily occurs via cation expulsion, which leads to more "solid-like" polymers, as corroborated by a decrease in dissipation (ix), which could reflect improved hole transport pathways. Enhanced performance of P3MEEET under operation in KTFSI is likely due to doping primarily via cation expulsion rather than anion uptake as in NaCl. During the doping process, polymers in NaCl experience crystalline and amorphous swelling, which is expected to strain percolation, whereas those in KTFSI experience mass expulsion, which improves percolation. The larger modulation of mass for high molecular weight polymers in KTFSI suggests enhanced densification, which may be the source of improved electrochemical performance. The phenomenon of doping via cation expulsion in p-type OECTs has been observed before, and is a direct result of the nature of the electrolyte . We hypothesize that this is particularly pronounced in our materials, since the alkyl spacer near the backbone may improve the ability of the KTFSI anion to reside near the backbone. In conclusion we investigated the relationship between molecular weight and mixed transport in prototypical glycolated polythiophene P3MEEET. When operated within NaCl electrolyte, the batch with intermediate molecular weight P3MEEET-3 was observed to have the optimal µC* (31.4 F cm -1 V -1 s -1 ), with detrimental swelling adversely impacting mixed conduction beyond a molecular weight threshold of 22.4 kg mol -1 , obfuscating the otherwise anticipated enhancement in mobility observed in traditional CPs at higher molecular weights. Therefore, to further investigate the influence of molecular weight on mixed conduction, active swelling was regulated through operation within an electrolyte with a more hydrophobic anion (TFSI -). When operated within KTFSI, enhanced µC* was observed with increasing molecular weight, primarily due to an increase in µ beyond the same threshold observed in NaCl. Through operando GIWAXS and EQCM-D we observed that electrochemical doping of P3MEEET occurred primarily by Cl -uptake in NaCl, leading to significant swelling at high molecular weights, and K + expulsion when operated in KTFSI -both of these behaviors were enhanced at higher molecular weight. Regardless of electrolyte, all polymers experienced significant passive swelling, primarily in the amorphous region, which separated crystalline regions. We hypothesize that this swelling in low molecular weight polymers brought the system to the limit of percolation, whereas in high molecular weight polymers, longer chains were able to maintain connections between crystallites. Since biasdependent swelling further separated crystallites in NaCl at longer chain lengths, percolation is disrupted, thus counteracting the otherwise beneficial effect of increased molecular weight. In KTFSI, however, doping resulted in an overall contraction of the polymer, improving charge transport pathways in both high and low molecular weight polymers, though low molecular weight polymers remain below the percolation threshold. Therefore, comparisons of both mass and lamellar spacing modulation suggest that the significant improvement of performance at higher molecular weights in KTFSI relative to NaCl is due to (1) the greater expulsion of water leading to denser films, potentially benefiting hole transport and (2) detrimental swelling in NaCl.