Maojie Zhang Sci. China Chem.: Asymmetric Non-Fused Electron-Deficient Units Construct Low-Cost Polymer Acceptors for All-Polymer Solar Cells
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The photoactive layer of all-polymer solar cells (All-PSCs) consists of a p-type polymer as the electron donor and an n-type polymer as the electron acceptor, exhibiting good morphological stability and mechanical flexibility. All-PSCs hold great potential for scalable, flexible organic solar cells, garnering substantial research attention. In particular, recent years have witnessed the development of various polymer acceptors using fused-ring small molecule acceptors as copolymer units, enabled by the small molecule acceptor polymerization strategy (PSMAs). This has led to significant improvements in the power conversion efficiency (PCE) of All-PSCs, now exceeding 18%. However, the complex synthesis and purification of these fused-ring small molecule acceptors as key copolymer units greatly increase the production costs of PSMAs. Moreover, the difficulty in precisely controlling the molecular weight (Mn) and polydispersity index (PDI) of polymers results in significant batch-to-batch variations of polymer acceptors, severely impacting the device performance and scalable fabrication of All-PSCs. Therefore, designing and developing structurally simple new key electron-deficient units to prepare polymer acceptors with good batch-to-batch reproducibility and low costs holds great significance.
Recently, Professor Maojie Zhang’s research group at Soochow University/Shandong University designed and synthesized a simple non-fused electron-deficient unit TIC-Br. This unit connects the extensively used monobrominated cyaninated indanedione (IC-Br) end group and a simple alkyl-substituted thiophene unit via just three simple reactions (bromination, formylation, and Knoevenagel condensation) (Figure 1). Studies show that the TIC-Br unit exhibits S•••O noncovalent interactions, enhancing the planarity and rigidity of the molecular backbone. Meanwhile, the asymmetric TIC-Br unit possesses a large dipole moment (4.91 Debye), promoting close π-π stacking between molecules. To explore the application potential of this building unit in polymer acceptors, the authors copolymerized the TIC-Br unit with a stannylated fluorinated thienyl benzodithiophene (BDT-TF-Sn) unit, resulting in the polymer acceptor PTIB. The PTIB polymer films exhibit strong absorption in the visible range (300-800 nm), with a maximum extinction coefficient of 0.64 × 105 cm−1 (Figure 2). In addition, PTIB possesses a high LUMO level (-3.86 eV) and deep HOMO level (-5.70 eV), facilitating matching with donor materials and achieving high open-circuit voltages (Voc). Moreover, PTIB exhibits an excellent electron mobility (1.02 × 10−4 cm2 V−1 s−1), suitable as an electron acceptor.
This study used Enlitech SS-F5-3A products for measurements.
Figure 1. Chemical structures of different polymer acceptors and synthetic routes of TIC-Br and PTIB.
Figure 2. DFT calculation results of TIC-Br and photoelectric characterizations of polymer acceptor PTIB.
All-PSC devices were fabricated using the classic polymer PM6 as the donor material, achieving over 10% PCE and Voc as high as 0.97 V. More importantly, compared to reported representative PSMAs, PTIB exhibits lower synthetic complexity (SC = 35.0%). Further, by evaluating the cost-effectiveness of different All-PSC systems using a figure-of-merit (FOM = PCE/SC), the PM6:PTIB devices reported here achieved a higher FOM value (29.0%), demonstrating excellent potential for commercial applications (Figure 3). Additionally, compared to PYT, PJ1 and PF5-Y5, the device performance of PTIB-based All-PSCs shows weaker dependence on molecular weight, exhibiting very close PCE values across the number-average molecular weight range of 3.5-10.2 kDa and outstanding batch-to-batch reproducibility. This work proposes a new polymer acceptor material design strategy, providing new insights and important approaches towards low-cost, high-performance all-polymer solar cells.
Figure 3. Photovoltaic performance of all-polymer solar cells; statistics of SC and FOM values for high-efficiency polymer acceptors and PTIB; PCE values of All-PSC devices based on polymer acceptors with different molecular weights.
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