donors possess the advantage of having a well-defined molecular weight, easy purification steps, and little batch-to-batch variations [10, 11]. Among the myriad of available molecular structures, diketopyrrolopyrrole (DPP) is a typical industrial pigment chromophore with a high optical density. It has been widely used as a building block in the design of donor molecules due to its facile synthesis from low-cost starting materials, strong electron affinity that enables lower frontier energy levels, and ease of structural modification, which allows for tailored optoelectronic properties. The DPP unit can be combined with other molecular units to form a \"donor-acceptor-donor\" structure [12] that enhances its potential for photovoltaic applications. This is the case, for example, of the 3,6-bis(5-benzofuran-2-yl) thiophen-2-yl) -2,5-bis (2-ethylhexyl) pyrrole [3,4-c] pyrrole-1_4_-dione molecule (DPP(TBFu)2) that can form part of solar cells when mixed with PC\({}_{71}\)BM as an acceptor, giving 4.4% power conversion efficiencies (PCE) [13]. The combination of the morphology at the nanoscale, phase separation, moderate mobility of the holes, and the relatively high V\({}_{\text{oc}}\) account for the obtained PCE value.  
Due to the opportunities offered by the DPP, the commercial availability of the components, and the simplicity of the device composition, we have selected the DPP(TBFu)\({}_{2}\):PC\({}_{61}\)BM system (see Figure 1 for the molecular structures) as a benchmark model to study the effects of three different post-deposition treatments on the PCE of the PV devices. As-cast photoactive films often exhibit poor device performance and require subsequent processing to improve their efficiency. Post-deposition treatments are vital for molecular ordering at the nanoscale and are a key factor in the achievement of high fill factors (FFs) and high PCEs. The post-deposition treatment processes influence the intra- or intermolecular weak interactions (essentially \(\pi\)-\(\pi\) stacking) leading to the formation of well-ordered crystalline domains [14]. The molecular ordering increases the degree of donor and acceptor phase separation, enhancing charge transport and reducing charge recombination. The post-deposition treatment determines the length scale of phase separation as well, which is critical for exciton dissociation and charge transport. If the length scale is too large, excitons may recombine before reaching the interface between donor and acceptor phases, while reduced phase separation impedes efficient charge transport. In fact, domain sizes of 20 nm in bulk heterojunction (BHJ) materials are optimum for charge separation and transport because the exciton diffusion length is about 10 nm [13]. In this work, we have selected well-known post-treatment strategies, such as thermal annealing (TA), exposure of the organic layer to a saturated solvent vapor (solvent vapor annealing, SVA), and the less known joule-heating annealing (JHA). To the best of our knowledge, JHA, consisting of applying a current bias to the device, has been mainly applied to improve the electrical and thermal properties of inorganic materials but has not been previously used to improve the crystallinity of the active layer [15, 16]. Finally, we selected polyethylene terephthalate (PET) as a flexible substrate. Flexibility and lightweightness are two of the major advantages that OSCs present over traditional silicon solar cells. Moreover, in comparison to other existing substrates, such as polyimide or polyethylene napthalate, PET is the substrate of choice for most solar cell manufacturers due to its mechanical properties, excellent transparency, and solvent resistance combined with its low price [17]. Consequently, we have also studied the impact of using PET on the performance of the DPP(TBFu)\({}_{2}\):PC\({}_{61}\)BM system.  
Figure 1: Chemical structures of the active layer compounds DPP(TBFu)\({}_{2}\) and PC\({}_{61}\)BM and schematic representation of the device structure used in this work.