High Sensitivity of Non‐Fullerene Organic Solar Cells Morphology and Performance to a Processing Additive

Abstract

Although solvent additives are used to optimize device performance in many novel non‐fullerene acceptor (NFA) organic solar cells (OSCs), the effect of processing additives on OSC structures and functionalities can be difficult to predict. Here, two polymer‐NFA OSCs with highly sensitive device performance and morphology to the most prevalent solvent additive chloronaphthalene (CN) are presented. Devices with 1% CN additive are found to nearly double device efficiencies to 10%. However, additive concentrations even slightly above optimum significantly hinder device performance due to formation of undesirable morphologies. A comprehensive analysis of device nanostructure shows that CN is critical to increasing crystallinity and optimizing phase separation up to the optimal concentration for suppressing charge recombination and maximizing performance. Here, domain purity and crystallinity are highly correlated with photocurrent and fill factors. However, this effect is in competition with uncontrolled crystallization of NFAs that occur at CN concentrations slightly above optimal. This study highlights how slight variations of solvent additives can impart detrimental effects to morphology and device performance of NFA OSCs. Therefore, successful scale‐up processing of NFA‐based OSCs will require extreme formulation control, a tuned NFA structure that resists runaway crystallization, or alternative methods such as additive‐free fabrication.

From Solution to Thin Film: Molecular Assembly of π-Conjugated Systems and Impact on (Opto)electronic Properties

“…Song and Baran et al showed that the device performance of PTB7-Th:PCBM[70] based OSCs significantly improved with 3% 1,8-diiodooctane (DIO) additive whereas the same amount of additive deteriorated the device performance of NFA based PTB7-Th:ITIC OSCs due to the formation of coarse morphology . Similarly, Alqahtani and Collins et al have recently shown that NFAs can be highly sensitive to additive contents, forming large crystals in the blend film even when the additive is slightly above the optimal concentration . Nevertheless, solvent additives have been successfully utilized in numerous polymer:NFA systems, suggesting its critical role in achieving favorable morphology and device properties. , Notably, Song and Baran et al demonstrated that an ultrahigh J SC value of 27.3 mA/cm 2 can be achieved by using 4% 1-chloro­naphthalene (CN) additive in PTB7-Th:IEICO-4F system, owing to the increased π–π coherence length of the acceptor, higher domain purity, and improved face-on/edge-on ratio .…”

Recent Advances of Solid Additives Used in Organic Solar Cells: Toward Efficient and Stable Solar Cells

“…Suitable domain sizes are important for device performance and stability due to the limited exciton lifetime. ,− However, owing to the strong self-organization ability of non-fullerene in some blends, crystallization always induces excessive domain size during the film-forming process or continuous growth of domains during the operation process. − Hence, regulating the crystallization of non-fullerene, i.e., crystal nucleation and crystal growth, is an effective method to optimize the device performance and its stability.…”

Surface crystallinity enhancement in organic solar cells induced by spinodal demixing of acceptors and additives

“…[10][11][12][13] Although the research on the process of NFA OSCs is rapidly advancing, the method of solvent additives still has not been replaced. [14][15][16] The non-volatile solvent 1,8-diiodooctane (DIO) was introduced in solution in order to optimize the morphology of the small molecule acceptors, [17][18][19][20][21] e.g. PCBM and ITIC.…”