Although it is known that molecular interactions govern morphology formation and purity of mixed domains of conjugated polymer donors and small-molecule acceptors, and thus largely control the achievable performance of organic solar cells, quantifying interaction-function relations has remained elusive. Here, we first determine the temperature-dependent effective amorphous-amorphous interaction parameter, χ(T), by mapping out the phase diagram of a model amorphous polymer:fullerene material system. We then establish a quantitative 'constant-kink-saturation' relation between χ and the fill factor in organic solar cells that is verified in detail in a model system and delineated across numerous high- and low-performing materials systems, including fullerene and non-fullerene acceptors. Our experimental and computational data reveal that a high fill factor is obtained only when χ is large enough to lead to strong phase separation. Our work outlines a basis for using various miscibility tests and future simulation methods that will significantly reduce or eliminate trial-and-error approaches to material synthesis and device fabrication of functional semiconducting blends and organic blends in general.

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.

The influences of various processing parameters and polymer molecular weight on the morphology and properties of poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′′′‐di(2‐octyldodecyl) 2,2′;5′,2″;5″,2′′′‐quaterthiophen‐5,5′′′‐diyl)] (PffBT4T‐2OD)‐based polymer solar cells (PSCs) are investigated. High spin rate/high temperature conditions are found to significantly reduce polymer crystallinity and change polymer backbone orientation from face‐on to edge‐on. Most surprisingly, it is found that the median domain sizes of PffBT4T‐2OD:PC71BM blends processed at different temperatures/spin rates are nearly identical, while the average domain purity and the molecular orientation relative to polymer:fullerene interfaces can be significantly changed by the processing conditions. A systematic study is carried out to identify the roles of individual processing parameters including processing temperature, spin rate, concentration, and solvent mixtures. Furthermore, the effect of molecular weight on morphology control is also examined. These detailed studies provide important guidance to control and optimize various morphological parameters and thus electrical properties of PffBT4T‐2OD‐type materials for the application in PSC.

A 7.3% efficiency non-fullerene polymer solar cell is realized by combining a large-bandgap polymer PffT2-FTAZ-2DT with a small-bandgap acceptor IEIC. The complementary absorption of donor polymer and small-molecule acceptor is responsible for the high-performance of the solar-cell device. This work provides important guidance to improve the performance of non-fullerene polymer solar cells.

Silicon phthalocyanines (SiPcs) have shown great potential as n-type or ambipolar organic semiconductors in organic thin-film transistors (OTFTs) and organic photovoltaics. Although properly designed SiPcs rival current state-of-the-art n-type organic semiconducting materials, relatively few structure–property relationships have been established to determine the impact of axial substituents on OTFT performance, hindering the intelligent design of the next generation of SiPcs. To address this omission, we have developed structure–property relationships for vapor-deposited SiPcs with phenoxy axial substituents. In addition to thorough electrical characterization of bottom-gate top-contact OTFTs, we extensively investigated SiPc thin films using X-ray diffraction, atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and density functional theory (DFT) modeling. OTFT performance, including relative electron mobility (μe) of materials, was in general agreement with values obtained through DFT modeling including reorganization energy. Another significant trend observed from device performance was that increasing the electron-withdrawing character of the axial pendant groups led to a reduction in threshold voltage (V T) from 47.9 to 21.1 V. This was corroborated by DFT modeling, which predicted that V T decreases with the square of the dipole induced at the interface between the SiPc pendant and substrate. Discrepancies between modeling predictions and experimental results can be explained through analysis of thin-film morphology and orientation by AFM and GIWAXS. Our results demonstrate that a combination of DFT modeling to select prospective candidate materials, combined with appropriate processing conditions to deposit molecules with a favorable thin-film morphology in an “edge-on” orientation relative to the substrate, yields high-performance n-type SiPc-based OTFTs.