Sample preparation

Substrate cleaning: The z-cut quartz substrates were cleaned with Hellmanex solution followed by a thorough rinse with deionised water. The substrates were then washed with acetone, isopropanol and ethanol. In the last cleaning step, the substrates were plasma-etched in O₂ for 10 min.

Polymer films: These were prepared by dynamic spin coating from solution. For each sample, 40 μL of the polymer solution were deposited on pre-cleaned quartz substrates for the polymer-only films and on evaporated MAPbI₃ for the polymer:MAPbI₃ films. The spin speed was varied for each polymer film deposition in order to achieve a difference in thickness (see further details for each polymer below), while the duration of the spinning process was kept at 45 s for all samples. All steps apart from the cleaning of the substrates were carried out under nitrogen atmosphere. The films were kept in a nitrogen glovebox before and in-between measurements.

F8BT: (Poly(9,9-dioctylfluorene-alt-benzothiadiazole)) with an average molecular weight of M_w > 20,000 (CAS number 210347-52-7) was purchased from Sigma-Aldrich. The “thick” F8BT films used for this study were spin-coated from a solution of F8BT in anhydrous toluene with a concentration of 10 mg mL⁻¹ at a spin speed of 2000 rpm, the “thin” films from a solution with a concentration of 5 mg mL⁻¹ of F8BT in anhydrous toluene at 5000 rpm.

Super Yellow: (Merck poly(1,4-phenylenevinylene)-based copolymer) with M_w ≥ 1,300,000  (CAS number 26009-24-5) was purchased from Sigma-Aldrich. The Super Yellow films were spin-coated from a solution of 5 mg mL⁻¹ of the polymer in anhydrous toluene, the “thick” films at a spin speed of 1000 rpm and the “thin” films at 3000 rpm. An additional film was spin-coated at 4000 rpm, which was of comparable thickness to that of the films spin-coated at 3000 rpm, and all measurements from this film were in excellent agreement with the data collected for the 3000-rpm film sample.

P3HT: (Poly(3-hexylthiophene-2-5-diyl)) was purchased from Sigma-Aldrich (CAS number 104934-50-1) and dissolved in anhydrous toluene at a concentration of 15 mg mL⁻¹. The solution was then deposited onto either quartz substrates, or onto the MAPbI₃ thin film on quartz by dynamic spin coating with rotation speeds of 2000, 3000, 4000, 5000 and 6000 rpm, to vary the thickness (see Supplementary Table ³) of the P3HT layers. For films termed “thick” and “thin”, the spin speeds 2000 rpm and 6000 rpm were used. The films were prepared on a thin MAPbI₃ (65-nm) layer on a z-cut quartz substrate in order to allow for the separation of absorption components arising from the polymer and the MAPbI₃. In addition, two control samples of different P3HT thickness were prepared on a 260-nm MAPbI₃ film, showing the same photophysical behaviour, compared to those of the films on the thinner MAPbI₃ layers on quartz.

PTAA: (Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) was purchased from LUMTEC (CAS number 1333317-99-9) and dissolved in anhydrous toluene at a concentration of 15 mg mL⁻¹. The solution was then deposited by dynamic spin coating with different spin speeds. The spin speeds used were 2000 and 6000 rpm for the “thick” and “thin” films, respectively.

MAPbI₃: (Methylammonium lead iodide) layers were fabricated using thermal evaporation⁷¹. In brief, MAI and PbI₂ were placed in separate crucibles, and the substrates were mounted on a rotating substrate holder to ensure that a uniform film was deposited. The temperature of the substrates was kept at 21 °C throughout the deposition. The chamber was evacuated to reach a high vacuum (~10⁻⁶ mbar), before heating of the PbI₂ and the MAI. The substrates were then exposed to the vapour with an average deposition rate of 0.4 Å s⁻¹. The rates of both the MAI and PbI₂ deposition were monitored using quartz-crystal microbalances. The thickness of the MAPbI₃ thin film was set by controlling the exposure time of the substrates to MAPbI₃ precursor vapour. Three film thicknesses of MAPbI₃ were used in this work: 450 nm (F8BT and Super Yellow samples), 260 nm (PTAA and P3HT control samples) and 65 nm (P3HT samples). Since all of these thicknesses significantly exceed the Förster radii determined for ET in polymer:MAPbI₃ dual layers, the thickness of the MAPbI₃ layer has no effect on the ET dynamics or efficiency.

Film thicknesses: These were measured using a Veeco 150 Dektak profilometer and were cross-checked with the respective absorption data. All samples were scratched with a sharp razor blade down to the z-cut quartz substrate, and the total thickness of each sample was determined. For the polymer:MAPbI₃:quartz samples, the known thickness of the MAPbI₃ layer was subtracted to obtain the polymer film thickness. The relative error in the determined film thickness value is estimated from repeated Dektak measurements on different spots on the films to be of the order of 10%. Full listing of values of the thicknesses of all polymer films fabricated for different substrates and spin speeds is provided in Supplementary Tables ² and ³.