The Curious Case of Fluorination of Conjugated Polymers for Solar Cells

Abstract

Organic solar cells (OSCs) have been a rising star in the field of renewable energy since the introduction of the bulk heterojunction (BHJ) in 1992. Recent advances have pushed the efficiencies of OSCs to over 13%, an impressive accomplishment via collaborative efforts in rational materials design and synthesis, careful device engineering, and fundamental understanding of device physics. Throughout these endeavors, several design principles for the conjugated donor polymers used in such solar cells have emerged, including optimizing the conjugated backbone with judicious selection of building blocks, side-chain engineering, and substituents. Among all of the substituents, fluorine is probably the most popular one; improved device characteristics with fluorination have frequently been reported for a wide range of conjugated polymers, in particular, donor-acceptor (D-A)-type polymers. Herein we examine the effect of fluorination on the device performance of solar cells as a function of the position of fluorination (on the acceptor unit or on the donor unit), aiming to outline a clear understanding of the benefits of this curious substituent. As fluorination of the acceptor unit is the most adopted strategy for D-A polymers, we first discuss the effect of fluorination of the acceptor units, highlighting the five most widely utilized acceptor units. While improved device efficiency has been widely observed with fluorinated acceptor units, the underlying reasons vary from case to case and highly depend on the chemical structure of the polymer. Second, the effect of fluorination of the donor unit is addressed. Here we focus on four donor units that have been most studied with fluorination. While device-performance-enhancing effects by fluorination of the donor units have also been observed, it is less clear that fluorine will always benefit the efficiency of the OSC, as there are several cases where the efficiency drops, in particular with "over-fluorination", i.e., when too many fluorine substituents are incorporated. Finally, while this Account focuses on studies in which the polymer is paired with fullerene derivatives as the electron accepting materials, non-fullerene acceptors (NFAs) are quickly becoming key players in the field of OSCs. The effect of fluorination of the polymers on the device performance may be different when NFAs are used as the electron-accepting materials, which remains to be investigated. However, the design of fluorinated polymers may provide guidelines for the design of more efficient NFAs. Indeed, the current highest-performing OSC (∼13%) features fluorination on both the donor polymer and the non-fullerene acceptor.

Phthalimide‐Based High Mobility Polymer Semiconductors for Efficient Nonfullerene Solar Cells with Power Conversion Efficiencies over 13%

“…Sci. Moreover, the stronger acceptor ffBT optimizes the electronic properties of PhI-based polymers, resulting in downshifted highest occupied molecular orbital (HOMO) levels [59] of −5.55 and −5.63 eV and hence yielding high V oc > 0.9 V with small energy losses (E loss , ≈0.60 eV). [57] Compared to the PhI-based alternating D-A type polymer analogues, the incorporation of the stronger electron-withdrawing ffBT unit (vs PhI) enables an intensified intramolecular charge transfer and leads to a broadened and redshifted absorption approaching 700 nm, [58] thus yielding a more complementary absorption with a FREA molecule IT-4F [32] and a high J sc > 19.0 mA cm −2 in the PSCs.…”

Pd-Catalyzed Mizoroki-Heck Reactions Using Fluorine-Containing Agents as the Cross-Coupling Partners

“…Since the fluoroalkyl groups (CF2R and Rfn) have found wide applications in the areas of medicinal chemistry and/or materials science, significant efforts have been devoted to the development of efficient catalytic systems and to the design of versatile reagents for mild, convenient, and direct fluoroalkylation [39][40][41][42][43][44][45]. In 2012, Reutrakul and co-workers described the palladiumcatalyzed Heck-type reactions of [(bromodifluoromethyl)sulfonyl]benzene (187) with styrenes and vinyl ethers (188), furnishing α-alkenyl-substituted α,α-difluoromethyl phenyl sulfones (189) in moderate yields (Scheme 39) [100].…”

“…Introduction of fluorine atom(s) into organic molecules usually brings about a dramatic impact on the physicochemical and biological properties of the molecules [36][37][38]. Fluorine-containing organic compounds have found wide application in the areas of chemistry, biology, and materials science over the past several decades [39][40][41][42][43][44][45]. There have been as many as 25% of pharmaceuticals and 30-40% of agrochemicals on the market containing at least a single fluorine atom [41].…”

“…There have been as many as 25% of pharmaceuticals and 30-40% of agrochemicals on the market containing at least a single fluorine atom [41]. Because only a few naturally-occurring organofluorides have been discovered, most of the fluorinated organic compounds have to be manually synthesized [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51]. It is undoubted that the development of efficient methods to construct fluorine-containing molecules is of great importance [46][47][48][49][50][51].…”

“…It is undoubted that the development of efficient methods to construct fluorine-containing molecules is of great importance [46][47][48][49][50][51]. In general, the fluorinated compounds can be synthesized by direct fluorination or fluoroalkylation, or through reactions with the fluorine-containing building blocks [39][40][41][42][43][44][45][46][47][48][49][50][51]. The incorporation of fluorine-containing fragments into organic frameworks by Pd-catalyzed Mizoroki-Heck reactions has proved to be the simplest and most convenient pathway to build diverse alkenes bearing fluorinated functionalities.…”