DTffBT sub-cell J sc = 7.86; V oc = 0.89 DTPyT sub-cell J sc = 6.99; V oc = 0.83 PBHJ cell J sc = 13.5; V oc = 0.87 Figure 4 | J-V curves of representative ternary PSCs. a, A P3HT:PCBM:SiPc system with (solid lines) and without dye (broken lines) before (thin lines) and after annealing (thick lines). Reproduced from ref. 36, ACS. b, A DTffBT:DTPyT:PCBM system. Reproduced from ref. 45, ACS. c, A PTB7:PID2:PCBM system. Reproduced from ref. 33, NPG. d, A α-6T:SubPc:SubNc system. Reproduced from ref. 73, NPG.

Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Here we report that a single parameter, hole mobility, determines the fill factor of several hundred nanometer thick bulk heterojunction photovoltaic devices based on a series of copolymers with varying amount of fluorine substitution. We attribute the steady increase of hole mobility with fluorine content to changes in polymer molecular ordering. Importantly, all other parameters, including the efficiency of free charge generation and the coefficient of nongeminate recombination, are nearly identical. Our work emphasizes the need to achieve high mobility in combination with strongly suppressed charge recombination for the thick devices required by mass production technologies.

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.

A consensus is emerging that mixed phases are present in bulk heterojunction organic photovoltaic (OPV) devices. Significant insights into the mixed phases have come from bilayer stability measurements, in which an initial sample consisting of material pure layers of donor and acceptor is thermally treated, resulting in swelling of one layer by the other. We present a comparative study of the stability of polymer/fullerene bilayers using two common OPV polymer donors poly(3-hexylthiophene), P3HT, and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, and four fullerene acceptors phenyl-C61-butyric acid methyl ester, phenyl-C71-butyric acid methyl ester, [60]PCBM bis-adduct, and indene C60 bis-adduct. Using in situ spectroscopic ellipsometry to characterize the quasi-steady state behavior of the films, we find that the polymer glass transition temperature (T g) is critical to the bilayer stability, with no significant changes occurring below T g of the high T g PCDTBT. Above the polymer T g, we find the behavior is irreversible and most consistent with swelling of the polymer by the fullerene, constrained by tie chains in the polymer network and influenced by the rubbery dynamics of the mixed region. The swelling varies significantly with the nature of the fullerene and the polymer. Across the eight systems studied, there is no clear relationship between swelling and OPV device performance. The relationship between the observed swelling and the underlying fullerene–polymer miscibility is explored via Flory–Rehner theory.