## 2 Experiment  
Regioregular P3 HT (average Mw 50,000-100,000) was purchased from Sigma-Aldrich/Merck and dissolved in anhydrous chlorobenzene, C\({}_{\text{P3 HT}}\)-20 mg mL\({}^{-1}\). The solution was stirred and heated at 70 \({}^{\circ}\)C overnight prior to spincoating at 700 rpm on a glass/ITO/MoO\({}_{3}\) substrates. 10 min annealing on a hot plate (Prazitherm) at 110 \({}^{\circ}\)C ensures complete evaporation of the solvent and crystallization of the film. The resulting film thickness is about 200 nm. The glass substrates with patterned Indium Tin Oxide (ITO) were purchased from Naranjo B.V. (ref. NS1463, \(\text{t}_{\text{ITO}}\) \(\sim\)95 nm, \(\sim\)20 \(\Omega\).\(\square\)). A 10 nm MoO\({}_{3}\) layer was deposited by sublimation in a high vacuum evaporating chamber (Spectros, [PERSON]) at \(\sim\)10\({}^{-6}\) mBar, prior to spincoating the P3 HT. A 10 nm thick and dense Al\({}_{2}\)O\({}_{3}\) encapsulation grown at \(\sim\)70 \({}^{\circ}\)C in primary vacuum (\(\sim\)1 mBar) by atomic layer deposition (Savannah 100, Cambridge Nanotech) completes the structure. Prior to SMM measurements the samples were pasted onto a conducting chuck to which the ITO layer was wired by means of Ag epoxy (RS). Apart from steps in vacuum, the preparation procedure was carried out in inert atmosphere (N\({}_{2}\)) in a glovebox with less than 10 ppm of residual O\({}_{2}\) and H\({}_{2}\)O, to prevent any exposure to air, and possibly alteration of the materials properties prior to measurements. Microwave electrical characterization was carried out using a scanning microwave microscope from Keysight, equipped with a performance network analyzer (PNA). The detection unit was augmented with a home-made Mach-Zehnder interferometer to allow for 50 \(\Omega\) free detection system [14], operating over a 2 to 18 GHz bandwidth and suitable for the study of such high impedance materials as P3 HT. Full platinum-iridium tip probes (Ref. 12 Pt400A) from Rocky Mountain Nanotechnology specifically designed for Keysight's SMM were used. A schematic of the SMM experimental set-up is presented in Figure 1a. Prior to any adjustment of the interferometer, the tip is put in contact with the sample. The deflection setpoint is adjusted to keep contact in the nN range and prevent any damage of the sample and deformation of the tip apex. The Atomic Force Microscope (AFM) is used in spectroscopy mode with no scanning to keep the probe still to perform the measurements. The impinging microwave (Power: 0 dBm) is first split in two waves and coupled back after reflection of one of them at the tip sample contact as shown in Figure 1a and leading to the typical interferometric PNA amplitude and phase profile displayed in Figure 1b. A variable attenuator set in the pathway of the other wave serves to adjust the interferometric profile to lower the S\({}_{11}\) parameter (down to -70 dB typically) as much as possible at the interferometric frequencies. This is required to achieve significant sensitivity. Because of the very low signal coming out of the interferometer, a 2-18 GHz band amplifier (Microsemi AML218L2502, 30 dB gain, 3 dB noise figure and 14 dBm output power at 1 dB gain compression) is added. The interferometric frequencies are detected at minimum PNA amplitude where a 180\({}^{\circ}\) phase shift is observed as explained below. The one at 10.46925 GHz is further highlighted in Figure 1c in semilog scale, and in linear scale in the inset. Practically, near an interference frequency \(f_{0}\), the S\({}_{11}\) parameter measured by the VNA can be approximately modeled by S\({}_{11}\cong A\Gamma_{0}i\sin[\pi(f-f_{0})(t_{2}-t_{1})]e^{-i\pi(f-f_{0})(t_{1}+t_{2})}\), where \(A\) contains average gain and phase shift throughout the circuit, \(t_{1}\) and \(t_{2}\) are the times needed for the microwave signal to travel the two branches of the interferometer and \(\Gamma_{0}\) is the reflection coefficient of the device that allows the interferometer signal to be cancelled (see S6 on interferometer modeling).