Tuning Aggregation Behavior of Polymer Donor viaMolecular‐Weight Control for Achieving 17.1% Efficiency Inverted Polymer Solar Cells

Abstract

Main observation and conclusion Recently, the polymer solar cells (PSCs) based on the PM6 and small molecular acceptor (SMA) Y6 have attracted considerable attention in this community for their outstanding photovoltaic performance. As well known, the molecular weight of polymer has dramatic impact on the blend morphology and photovoltaic performance in the polymer solar cells, especially for those polymers with temperature‐dependent aggregation (TDA) behavior. In this work, a series of PM6 polymers with different weight‐average molecular weights (Mw = 41, 67, 74, 84 and 114 kDa) were synthesized, and the effects of the Mw on the morphology and photovoltaic performance were systematically investigated. Notably, molecular weight induces dramatic influences on the aggregation behaviors of the polymers and their corresponding blend morphology, which were explored by the temperature‐dependent UV‐vis absorptions (TD‐Abs), grazing incidence wide‐angle X‐ray scattering (GIWAXS) and resonant soft X‐ray scattering (R‐SoXS) measurements. Increasing Mw promoted the strong face‐on molecular packing and reduced the blend film domain sizes for enhanced donor‐acceptor interfacial areas, resulting in increased short‐circuit current densities (Jsc) and fill factor (FF). However, polymer donor with high Mw value processed stronger tendency of forming excessive pure donor aggregate, which hampered forming the intermixed phase for efficient charge transport, resulting in the inferior FF. Therefore, the optimized OSCs based on PM6 with moderate molecular weight (Mw = 74 kDa) and Y6 exhibited suitable size and purity of the domains, facilitating well‐balanced charge transport, reducing trap‐assisted recombination and leading to significant enhancement of photovoltaic performance with the optimal PCE of 17.1%. This work indicates that proper and precisely tuning the molecular weight of polymer donor is critical to enhance the photovoltaic performance in PSCs.

Cold‐Aging and Solvent Vapor Mediated Aggregation Control toward 18% Efficiency Binary Organic Solar Cells

“…Besides the increased light absorption intensity, the ratio of absorption peaks Ι 0-0 /Ι 0-1 of PM6 molecules in solution show an increment from 1.08 to 1.13 upon cold-aging, further justifying the increased degree of pre-aggregation. [34] Grazing-incidence wide-angle X-ray scattering (GIWAXS) characterization (Figure S4, Supporting Information) of pristine PM6 film cast with or without cold-aging treatment shows a rather similar face-on preferential molecular orientation, presenting a (010) peak at q z = 1.72 Å -1 in the out-of-plane direction and a (100) peak at q xy = 0.31 Å -1 in the in-plane direction. [49] The PM6 film cast upon cold-aging exhibits slightly stronger (010) π−π stacking as well as the (100) lamellar order, suggesting enhanced aggregation and is also consistent with the enhanced light absorption intensity in Figure 3.…”

“…[54] The decreased R g-PM6 suggests the reduced crystalline domain size. [34,55] This is probably because that the polymer chains tend to pack together to avoid contact with solvent molecules in the solution state during cold-aging, and the polymers' radius of gyration decreases, according to Flory's formulation. [13] The corresponding atomic force microscopy (AFM) images in Figure 3a support the GISAXS results, where more fine fibrillar PM6 aggregates can be observed upon cold-aging treatment.…”

“…Besides the increased light absorption intensity, the ratio of absorption peaks Ι 0‐0 / Ι 0‐1 of PM6 molecules in solution show an increment from 1.08 to 1.13 upon cold‐aging, further justifying the increased degree of pre‐aggregation. [ 34 ]…”

“…tuned the molecular weight of PM6 to optimized OSCs with suitable size and purity of the domains to achieve well‐balanced charge transport, and found that PM6 with higher molecular weight possessed a stronger aggregation degree in solutions. [ 34 ] The temperature‐dependent aggregation property of conjugated polymers offers the approach to manipulate the pre‐aggregation through temperature control, which is also a facile and low‐cost approach. [ 35,36 ]…”

“…[33] Zhang et al tuned the molecular weight of PM6 to optimized OSCs with suitable size and purity of the domains to achieve well-balanced charge transport, and found that PM6 with higher molecular weight possessed a stronger aggregation degree in solutions. [34] The temperature-dependent aggregation property of conjugated polymers offers the approach to manipulate the pre-aggregation through temperature control, which is also a facile and low-cost approach. [35,36] Different from previous work on tuning the aggregation in a temperature range from room-temperature to over 100 °C, weThe molecular ordering and pre-aggregation of photovoltaic materials in solution can significantly affect the nanoscale morphology in solid photoactive layers, and play a vital role in determining the power conversion efficiency (PCE) of organic solar cells (OSCs).…”