Achieving 20% Toluene-Processed Binary Organic Solar Cells via Secondary Regulation of Donor Aggregation in Sequential Processing

Abstract

Sequential processing (SqP) of the active layer offers independent optimization of the donor and acceptor with more targeted solvent design, which is considered the most promising strategy for achieving efficient organic solar cells (OSCs). In the SqP method, the favorable interpenetrating network seriously depends on the fine control of the bottom layer swelling. However, the choice of solvent(s) for both the donor and acceptor have been mostly based on a trial-and-error manner. A single solvent often cannot achieve sufficient yet not excessive swelling, which has long been a difficulty in the high efficient SqP OSCs. Herein, two new isomeric molecules are introduced to fine-tune the nucleation and crystallization dynamics that allows judicious control over the swelling of the bottom layer. The strong non-covalent interaction between the isomeric molecule and active materials provides an excellent driving force for optimize the swelling-process. Among them, the molecule with high dipole moment promotes earlier nucleation of the PM6 and provides extended time for crystallization during SqP, improving bulk morphology and vertical phase segregation. As a result, champion efficiencies of 17.38% and 20.00% (certified 19.70%) are achieved based on PM6/PYF-T-o (all-polymer) and PM6/BTP-eC9 devices casted by toluene solvent.

1. Introduction

Organic solar cells (OSCs) have been extensively studied owing to the charming features of light-weight, flexibility, semitransparency, compatibility with roll-to-roll solution-based production, etc. [1–6]. Non-fullerene acceptors with high absorption coefficients in the red and near-infrared region has significantly improved of the performance of OSCs [7–10]. The combined effort of materials innovation and device engineering leads to the ~ 20% power conversion efficiency (PCE) of single-junction OSCs [11–13].

However, a high fraction of the top-of-the-line PCEs are realized through the use of highly vaporizable solvents such as chloroform to ‘lock’ the morphology of the active layer in a relatively instable thermodynamic non-equilibrium state. Along with its unneglectable toxicity, the potential of chloroform in large-scale production is very limited [14, 15]. In this context, employing greener solvents with a high-boiling point to achieve high efficiency OSCs is essential for the industrialization of the technology. However, in order to balance charge transport and film morphology, the state-of-the-art OSCs are composed of polymer donors and non-fullerene acceptors with a complex fused-ring framework and lengthy side chains, which typically results in a low solubility in most non-halogenated solvents [15, 16]. Without extra post-treatments, the aggregation, crystallization and phase segregation of the active layer blend casted by non-halogenated solvents, strongly affected by the drying kinetics, are difficult to regulate. The overall nanoscale morphology produced this way is typically in an unfavorable state, resulting in much lower efficiency than chloroform-casting devices [17–20]. Therefore, it is crucial to broaden the methods that can effectively control the morphology of active layer when high-boiling point non-halogen solvents are used to prepare the active layer. However, it is difficult to effectively adjust the molecular stacking and donor–acceptor phase segregation through targeted solvent-material interaction in a conventional blend-cast device, i.e., making the solvent selectively interact with the donor (or acceptor) alone.

This makes the sequential processing (SqP) approach even more promising, as it allows independent optimization of donor and acceptor layers, potentially overcoming the limitations of blend casting with green solvents.