## 2 Materials and Methods  
Materials:  
DPP(TBFu)2 and PC61 BM were purchased from Luminescence Technology Corp. and from SOLENNE (99.5%), respectively. The solvents used are: CHCl3 (99.8%, Sigma-Aldrich, St. Louis, MO, USA) for the active layer deposition, THF (>99.9%, Sigma Aldrich) and CH2 Cl2 (>99.8%, DCM, Sigma-Aldrich) for the SVA treatment. Aluminum (99.99%) and LiF (99.995%) were purchased from Kurt J. Lesker-Materials Group and Sigma-Aldrich, respectively. Pre-patterned Indium Tin Oxide (ITO) on glass and on PET were purchased from PsioCTec and Visiontek, respectively. Device preparation:  
First, the substrates that were treated identically were rinsed with acetone to remove the residual photoresist layer. The substrates were then placed in a glass holder and sequentially sonicated in acetone for 10 min and in isopropanol twice for 10 min. Finally, the substrates were ozone-treated in a homemade system for 45 min, and subsequently spin coated in air with a layer of sonicated and filtered (0.45 \(\upmu\)m, cellulose acetate) solution of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Orgacon JJ\(-\)1005) at 8000 rpm for 90 s and using an acceleration of 4320 rps. The PEDOT:PSS film was dried at 120 \({}^{\circ}\)C under an inert atmosphere for 30 min. The active layer was dissolved in CHCl3 and spin coated in air over the PEDOT:PSS layer from a 20 mg\(\cdot\)ml\({}^{-1}\) (total concentration in CHCl3) solution of DPP(TBFu)2 and PC61 BM (6:4 \(w/w\) ratio) in a bulk heterojunction (BHJ) way [18]. The spin-coating conditions were adapted to obtain a layer thickness of 100 nm. The post-fabrication treatment was carried out straight after deposition of the active layer:  
* Solvent vapor annealing (SVA) was achieved by exposing the films to a saturated vapor of solvent of THF or DCM in an airtight vessel [19]. The vessel (200 mL) was filled with 7 mL of an organic solvent and left sealed for 5 min prior to the SVA step to ensure that the headspace was saturated with the solvent. The substrates were exposed to the solvent vapors until the color of the films changed (between 1 and 2 min). * Thermal annealing (TA) was done on a heating plate at 65\(-\)110 \({}^{\circ}\)C until the color changed. * Joule-Heating-Annealing (JHA) was also explored. This treatment is based on the heat released by the flow of an electric current generated after application of a constant voltage through the metallic contacts of the devices. The domains are formed by means of increasing the temperature due to the current flow through the active layer. The constant voltage applied was 5 V, and the current observed was 30-40 mA. Finally, the cathode layer was deposited by thermal evaporation in an ultra-high vacuum chamber (<1 \(\times\) 10\({}^{-6}\) mbar). Metals were evaporated through a mask leading to devices with an area of 25 mm\({}^{2}\). LiF (1 nm) and Al (100 nm) were deposited at a rate of 0.1 A\(\cdot\)s\({}^{-1}\) and 0.5 A\(\cdot\)s\({}^{-1}\), respectively. Characterization:  
The UV-Vis absorption of the films was measured using a PerkinElmer spectrophotometer (Lambda 950). The J-V characteristics of the devices were measured in an ambient atmosphere using an ATLAS Solar Simulator without the use of encapsulating layers. The illumination intensity was measured to be 100 mW\(\cdot\)m\({}^{2}\) with a calibrated Pyranometer CMP6 with a sensitivity of 18.76\(\cdot\)10\({}^{-6}\) V/W\(\cdot\)m\({}^{-2}\) and a METEON Irradiance Meter. The applied potential and cell current were measured with a Keithley 2602A System digital source meter. The photocurrent and irradiated light intensity were measured simultaneously and processed with home-built Labview(r) software. The thickness and roughness of the films were measured with a stylus profilometer Veeco (Dektak 150) from a scratch applied in the center of the film. Atomic Force Microscopy was used mainly to study the roughness of the active layer but also to observe the different morphology of the donor/acceptor domains.