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.

Three orders of magnitude is the range over which the grain size (see figure) can be tuned in solution‐processed organic semiconductor thin films for TFTs. Fluorinated triethylsilyl anthradithiophene (FTES‐ADT) is added in fractional amounts to seed crystallization of TES‐ADT. Correlation between device mobility and grain size in the active layer is described by a composite mobility model that assumes charge‐carrier traps are located at grain boundaries.

The mini-slot-die coater offers a simple, convenient, materials-efficient route to print bulk-heterojunction (BHJ) organic photovoltaics (OPVs) that show efficiencies similar to spin-coating. Grazing-incidence X-ray diffraction (GIXD) and GI small-angle X-ray scattering (GISAXS) methods are used in real time to characterize the active-layer formation during printing. A polymer-aggregation-phase-separation-crystallization mechanism for the evolution of the morphology describes the observations.

The device performance of organic polymer:fullerene bulk heterojunction solar cells strongly depends on the interpenetrating network of the involved donor and acceptor materials in the active layer. Since morphology formation depends on the conditions of film preparation, the final morphology varies for different deposition methods. In order to understand and optimize industrial coating processes and, therefore, the performance of the solar cells produced, a deeper understanding of structure formation is important. In situ measurements of slot‐die printed polymer:fullerene active layers are presented that reveal insights into the evolution of the structure. Polymer crystallization and ordering is monitored by in situ grazing incidence wide angle X‐ray scattering (GIWAXS), and in situ grazing incidence small‐angle X‐ray scattering (GISAXS). The development of the morphology exhibits five stages independent of the drying conditions. Two growth rates are observed, an initial slow formation of poly(3‐hexylthiophene‐2,5‐diyl) crystallites in well‐aligned edge‐on orientation followed by a rapid crystal growth. By combining the GIWAXS and GISAXS measurements, a five‐stage growth and assembly process is found and described in detail along with a proposed model of the structural evolution. The findings are an important step in tailoring the assembly process.

We delineate the important role of 2D conjugated alkylthiophene side chains of polymers in manipulating the molecular orientation and ordering at the polymer donor/polymer acceptor (P D /P A ) interface as well as the composition variations in the blend active layer of all-polymer solar cells (all-PSCs). To systematically investigate the impact of 2D conjugated side chains on the performance of all-PSCs, we synthesized a series of polypolymer donors with different contents of alkoxy and alkylthiophene side chains, from 0 to 100% (PBDT-TPD (P1, 100% alkoxy side chain), PBDTT 0.29 -TPD (P2), PBDTT 0.59 -TPD (P3), PBDTT 0.76 -TPD (P4), and PBDTT-TPD (P5, 100% alkylthiophene side chain). The P1−P5 polymer donors produced similar PCEs of ∼6% in fullerene-based PSCs. In contrast, for the all-PSC systems, the changes in the side chain composition of the polymers induced a strong increasing trend in the power conversion efficiencies (PCEs), from 2.82% (P1), to 3.16% (P2), to 4.41% (P3), to 5.32% (P4), and to 6.60% (P5). The significant increase in the PCEs of the all-PSCs was attributed mainly to improvements in the short-circuit current density (J SC ) and fill factor (FF). The 2D conjugated side chains promoted localized molecular orientation and ordering relative to the P D /P A interfaces and improved domain purity, which led to enhanced exciton dissociation and charge transport characteristics of the all-PSCs. Our observations highlight the advantage of incorporating 2D conjugated side chains into polymer donors for producing high-performance all-PSC systems.