Overcoming Disordered Preaggregation in Liquid State for Highly Efficient Organic Solar Cells Printed from Nonhalogenated Solvents

Abstract

efficiencies (PCEs) of OSCs have been approaching 20%, [5][6][7][8][9] lifting its limitations for industrialization. These encouraging efficiencies are usually obtained from toxic solvent by spin coating process, which is not compatible to production upscaling. On switching to scalable printing techniques and environmentally friendly solvents, there is usually a notable drop in PCEs, which deserve further research attentions to bridge the gap. Strategies including modifying the molecular structures of photoactive materials, [10,11] incorporating additives, [12,13] adopting new device structures, [14] adjusting film formation kinetics with third component [15,16] or substrate temperature [17] or gas flow [18] have been attempted to achieve high PCEs with scalable and halogen-free solvent processing. Revealing the molecular design rules and film formation dynamics is vital to control the thin-film microstructures and finally the photovoltaic performance.In this work, we systematically investigate the fundamental processingmicrostructure-function relationship during transforming from spin coating with halogenated solvent to doctor blade coating with halogen-free solvent for high-performance photoactive materials based on polymer donor D18 [19] and two representative nonfullerene acceptors (NFAs), i.e., BTP-eC9 [11] and Y6. [20] The current power conversion efficiencies of laboratory-sized organic solar cells (OSCs), based on the spin-coating process with halogenated solvents, have exceeded 19%. Environmentally friendly printing is needed to bridge the gap between laboratory and industrialization by being compatible with rollto-roll large-area production. Here, the molecular design rules are revealed for enhancing the green printing potential of the state-of-the-art photovoltaic martial systems by investigating the detailed structure formation dynamic and the key determining factors. By comparing two model systems based on D18:Y6 and D18:BTP-eC9, it is found that disordered preaggregation in liquid state can result in over-sized domains with reduced crystallinity and disordered molecular orientation, which significantly limits device performance. By systematically tuning the length of the inner alkyl side chains with multiple Y-series materials, the authors demonstrate that molecular side-chain engineering can effectively supress the detrimental disordered preaggregation in liquid state during environmentally friendly printing process, leading to enhanced crystallization with preferential faceon molecular orientation, more efficient exciton dissociation and charge carrier transport, and finally high upscaling potential. The work provides deeper insights into molecular engineering and structure formation dynamics toward environmentally friendly production of OSCs.

Modulating the Growth of Nonfullerene Acceptors Toward Efficient and Stable Organic Solar Cells Processed by High‐Boiling‐Point Solvents

“…In contrast, the RMP film of L8-BO demonstrates only one 𝜋-𝜋 stacking diffraction peak (1.72 Å −1 ), indicating the reduction in heterogeneity. [22] As for PTFBDT:L8-BO blend films processed by RMP, the intensity of 𝜋-𝜋 stacking peak (1.67 Å −1 ) in the OOP direction and the lamellar peak (0.34 Å −1 ) in the IP direction is enhanced (Figure 3g). The 𝜋-𝜋 stacking distance decreases from 3.90 Å (reference) to 3.76 Å (RMP).…”

Fully Printed and Industrially Scalable Semitransparent Organic Photovoltaic Modules: Navigating through Material and Processing Constraints

“…A processing constraint of Y6 has been its preaggregation in certain solvents, including the nonchlorinated solvent o-xylene, leading to large Y6 domains in the film and thus to reduced performance. [48,49] However, this challenge has been overcome by modifying the side chains of Y6. [23][24][25][26] One of those tailored materials is Y6-C12 (BTP-4F-12), which enables favorable processability in o-xylene and thus provides excellent device performances.…”

Green‐Processed Non‐Fullerene Organic Solar Cells Based on Y‐Series Acceptors

“…Poor solubility tends to result in significant aggregation in BHJ thin films processed with non-halogenated solvents and then solubility enhancement is a potential strategy to address this issue for Y6. One approach to achieving this is to lengthen the alkyl side units of electron acceptors, [31] as demonstrated in previous discussion about the work of Du et al and Li et al [29,30] In addition, Kim et al examined the impact of outer side chains on the photovoltaic performance of Y-series acceptors, utilizing XY to fabricate photoactive layers. [32] To enhance crystallinity, selenophene was utilized instead of thiophene at the outermost core of Yseries acceptors, allowing for the use of single-component XY in the photoactive layer fabrication.…”

“…Furthermore, the simple synthetic procedures resulted in a much improved average figure of merit (aFOM) when compared to other systems, suggesting the great potential for practical applications. [29] Copyright 2023, WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim.…”

“…b) AFM images of the BHJ blends based on D18:Y6 fabricated from CF or XY. Reproduced with permission [29]. Copyright 2023, WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim.…”