Temperature‐Dependent Aggregation Donor Polymers Enable Highly Efficient Sequentially Processed Organic Photovoltaics Without the Need of Orthogonal Solvents

Abstract

The conventional method to prepare bulk-heterojunction organic photovoltaics (OPVs) is a one-step method from the blend solution of donor and acceptor materials, known as blend-casting (BC). Recently, an alternative method was demonstrated to achieve high efficiencies (13%) comparable to state-of-the-art BC devices. This two-step-coating method, known as "sequential processing," (SqP) involves sequential deposition of the donor and then the acceptor from two orthogonal solvents. However, the requirement of orthogonal solvents to process the donor and acceptor constrains the choice of materials and processing solvents. In this paper, an improved version of SqP method without the need for using orthogonal solvents is reported. The success is based on donor polymers with strong temperature-dependent aggregation properties whose solution can be processed at a high temperature, but the resulting film becomes completely insoluble at room temperature, which allows for the processing of overlying acceptors from a wide range of nonorthogonal solvents. With this approach, efficient SqP OPVs is demonstrated based on a range of donor/acceptor materials and processing solvents, and, in every single case, SqP OPVs can outperform their BC counterparts. The results broaden the solvent choices and open a much larger window to optimize the processing parameters of SqP method.

Tuning the Solution Aggregate Structure of a PM7-Based Conjugated Polymer to Enable Green Solvent Processing of Organic Solar Cells

“…This result could be attributed to the temperature-dependent aggregation behavior of PM7 D2 in TL+AN. It is consistent with previous reports that conjugated donor polymers with temperature-dependent aggregation enable morphology control by processing at elevated temperatures, leading to highly crystalline films with desirable domain sizes. ,, …”

“…Interestingly, the CN solution seems to have a shoulder peak at 545 nm; however, the location of this peak is at a much lower wavelength compared to the aggregate peak, denoting that it likely originates from vibronic transitions. The energy separation between this shoulder peak and the main absorption of CN solution at 510 nm is 0.16 eV, which lies in the range of 0.14 to 0.18 eV for vibronic progressions. , …”

“…The energy separation between this shoulder peak and the main absorption of CN solution at 510 nm is 0.16 eV, which lies in the range of 0.14 to 0.18 eV for vibronic progressions. 44,45 Polymer Solution-State Aggregation. To investigate the solution aggregate structures of PM7 D2 in various solvents, we utilized small-angle X-ray scattering (SAXS), which has been widely used to decipher the solution-state aggregate structures of conjugated polymers.…”

Solution Sequential Deposition Pseudo‐Planar Heterojunction: An Efficient Strategy for State‐of‐Art Organic Solar Cells

“…This was also observed by Arunagiri et al. [ 93 ] in PTFB‐O/ITIC‐Th devices with a nearly bilayer structure, where the donor or acceptor enriched regions are initially obvious and the interfaces are sharp. After TA treatment, the donor and acceptor were more uniformly distributed in the vertical direction, indicating that thermal annealing drove intermolecular diffusion and composition intermixing.…”

Vertical Composition Distribution and Crystallinity Regulations Enable High-Performance Polymer Solar Cells with >17% Efficiency

“…Overall, an Y6-enriched layer at the top, a mixed PM6:Y6 layer in the middle, and a pure PM6 layer at the bottom are determined by the NR fitting. The mixed PM6:Y6 layer at the normalized Z axis of 0.4–0.7 is a constituent homogeneous layer due to the interdiffusion effect between the donor and acceptor during the second step solution process, showing a similar pattern of ratio plateau with early LBL-based works . The DIO-induced diffusion of in-depth Y6 molecules among the four LBL samples is strongly exhibited in the transition region between the two layers.…”