## 1 Introduction  
The vast majority of conjugated polymers dedicated to organic electronics are developed using the donor-acceptor (D-A) strategy, which consists of combining electron-rich (D) and electron-poor (A) units, and in varying their nature through chemical design in order to finely tune their bandgap [1]. Generally, D-A copolymers were mainly used as donor materials (p-type) combined with fullerene acceptors (n-type) for organic solar cells (OSCs). However, since fullerene derivatives suffer from limitations like weak absorption and low stability due to dimerization (burn-in) and unstable morphology [2; 3; 4], non fullerene acceptors (NFAs) were developed for organic electronic devices [5; 6]. With the need of reducing more and more the bandgap, n-type polymers exhibiting a LUMO level at \(-4\) eV have been tested for integration in organic field-effect transistors (OFETs) and OSCs [7; 8; 9; 10]. In this context, a variety of D-A acceptor polymers based on benzothiadiazole (BT), dithienodiketopyrrolo pyrrole (DPP), thienopyrroledione (TPD), naphthalene diimide (NDI), and perylene diimide (PDI) have been developed [11; 12; 13; 14; 15; 16]. Even if they did not provide the same level of performances in OSCs as fullerene-based devices, the use of n-type polymers is of great interest owing to the possibility of tuning their optoelectronic properties to optimize their light-harvesting capability and improve their stability [17]. Among n-type polymers, those based on naphthalene diimide (NDI) and perylene diimide (PDI) are an important series that show good performances in all-polymer solar cells [18; 19; 20; 21; 22]. Performances of OSCs combining PDI-based D-A acceptor polymers with adequate p-type polymers achieved power conversion efficiencies (PCE) up to 7% [23]. It has been demonstrated that D-A copolymers based on DPP combined with thiophene or selenophene could exhibit a high photoresponse with a tuneable absorption edge from 950 to 1200 nm, allowing PCE from 2.9% to 5.3% [24]. A recent review reported ultra-narrow-bandgap D-A copolymers for OSCs, mainly based on DPP and isoindigo units that can be used as donor materials [25]. Some D-A copolymers presenting an absorption in the NIR range based on thiadiazoloquinoxaline [26] or on diazapentalene (DAP) units [27] were efficiently used in organic PDs. Besides D-A copolymers, systems combining two electron-accepting A-A' units have also been reported. They can afford n-type materials more stable under air and with deeper HOMO levels, as reported by the combination of PDI with fluorenone [28]. Dual-acceptor copolymers based on 2-alkylbenzotriazole and 2,1,3-benzothiazole (BT) were reported for OFETs with an electron mobility of 0.01 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\)[29], and all-polymer solar cells with a PCE of 0.31% [30]. Benzotriazole was also copolymerized with other electron poor units, such as quinoxaline and benzobishtiadiazole, and were used as n-type copolymers in OSCs showing a PCE of 0.40%. A-A' copolymers based on isoindigo and BT were also used as acceptor materials in solar cells with a similar PCE (0.47%) [31]. A-A' copolymers based on PDI and BT provided an efficiency of 0.31% [32]. All these copolymers have an optical absorption in the visible range, and showed low performances in solar cells compared with their D-A analogues. A recent strategy is now being developed to polymerize small molecular acceptors (initially used as NFAs), allowing the preparation of n-type polymers for all-polymer solar cells, with PCE up to 16% [33; 34]. Conversely, A-A' copolymers with very low bandgap absorbing in the NIR range have been scarcely considered. Copolymers combining diketopyrrolopyrrole (DPP) and benzo [1,2-c;3,4-c'] bis [1,2,5] thiadiazole (BBT) were obtained with a very low bandgap of 0.65 eV [35]. Used in transistors, they exhibited a clear ambipolar behavior with high n and p-type charge mobilities between 0.5 and 1 cm\({}^{2}\) V\({}^{-1}\) s\({}^{-1}\).