Co-crystallization of Fibrillar Polymer Donors for Efficient Ternary Organic Solar Cells

Abstract

Ternary strategy is identified as an effective method to fine-tune the optoelectronic properties of the photoactive layer of organic solar cells (OSCs) toward high power conversion efficiency (PCE). Although numerous highperformance ternary OSCs have been established, the underlying fundamentals of ternary OSCs are still not fully understood, and a general rule for the ternary system design is highly appreciated. In this contribution, we demonstrate the construction of high-performance ternary OSCs via the cocrystallization of conjugated host donor PM6 and guest donor D18-Cl. Due to the similar chemical structure, larger planarity, and lower surface energy of D18-Cl, it locates in the PM6 phase and organizes with PM6 during solution casting, leading to the formation of co-crystallized fibrillar donor phase with enhanced structural order and charge transport channels. As a result, a series of binary host systems, including PM6:BTP-4F-C6-16, PM6:Y6, and PM6:Y7-BO obtained enhanced efficiency with the presence of D18-Cl as the guest donor, achieving a superior PCE of 18.5% in the PM6:D18-Cl:BTP-4F-C6-16 ternary OSCs. This work is the first demonstration of co-crystallized polymer donor fibrils to boost the charge transport and power conversion of ternary OSCs.

Bicontinuous donor and acceptor fibril networks enable 19.2% efficiency pseudo‐bulk heterojunction organic solar cells

“…With a similar chemical structure to PM6 and the low surface energy of D18‐Cl, it locates in the PM6 phase and co‐assembles with PM6 into nanoscale fibrils during solution casting, as concluded from our previous work. [ 25 ] Distinctly, for L8‐BO, incorporating only 0.5 wt% of the D18‐Cl can induce long L8‐BO aggregates with a length of 100–200 nm (Supporting Information S1: Figure ), while 1 wt% addition of D18‐Cl leads to the formation of L8‐BO fibril network (Figure 1F). However, with 3 wt% addition of D18‐Cl, the fibrillization of L8‐BO has been depressed and results in more amorphous domains (Figure 1G).…”

“…[ 17–20 ] As discussed in previous works, [ 21–24 ] fibrillation refers to the self‐assembly of materials growing in a specific direction to obtain one‐dimensional structure and has been demonstrated to require different organization dynamics for materials with different structures. For example, with long backbones, conjugated polymers could easily assemble through main chains to form fibrils [ 25,26 ] or ribbons. [ 27 ] On the other side, small molecules such as small molecular nonfullerene acceptors (NFAs) only exhibit weak van der Waals force among their adjacent molecules, [ 28,29 ] and hence enhanced main‐chain interactions (e.g., between the electron‐withdrawing end groups) [ 30 ] or reduced side‐chain interactions [ 31 ] are generally required to promote their growth along the conjugated plane.…”

“…While it is not challenging to realize fibrillation for neat organic components, this situation can be very complicated when binary or ternary components are mixed to construct photoactive layer for OSCs, as their self‐assembly can compete with each other. [ 32,33 ] Although very recent works have demonstrated that the fibrillization of polymer donors or NFAs in mixed films can be enhanced via the co‐crystallization of the conjugated polymers having high compatibility (i.e., D18‐Cl and PM6) [ 25,34 ] or NFAs with similar molecular packing forms (i.e., BTP‐ThMeCl and L8‐BO), [ 35 ] it is still difficult to construct fibrils of donor and acceptor components simultaneously to realize bicontinuous charge transport networks.…”

“…[ 41,42 ] In this work, the fibrillization of state‐of‐the‐art PM6 donor and L8‐BO acceptor was realized by controlling the content of D18‐Cl in each layer. D18‐Cl is highly compatible with PM6 and could induce the fibrillization of PM6 through co‐crystallization, [ 25 ] which is revealed by atomic force microscopy (AFM) and photo‐induced force microscopy (PiFM) techniques. A small content of D18‐Cl also acts as the initiator to induce the fibrillization of L8‐BO.…”

A Fused-Ring Electron Acceptor with Phthalimide-Based Ending Groups for Efficient Ternary Organic Solar Cells

“…Recently, organic photovoltaics (OPVs) with bulk heterojunction (BHJ) received a great deal of attention in the domain of next-generation solar energy technology on account of their low cost, lightness, flexibility, and simple preparation process. − Their power conversion efficiency (PCE) already reached over 19% with the improvement of materials design , and device preparation. , The appearance of fused-ring electron acceptor tremendously boosted the PCE of OPVs . In 2019, with the synthesis of Y6, the OPVs achieved a remarkable PCE of 15.7%, which improved the PCE of OPVs significantly .…”