Caitlin McDowell

Toward Additive‐Free Small‐Molecule Organic Solar Cells: Roles of the Donor Crystallization Pathway and Dynamics

The ease with which small-molecule donors crystallize during solution processing is directly linked to the need for solvent additives. Donor molecules that get trapped in disordered (H1) or liquid crystalline (T1) mesophases require additive processing to promote crystallization, phase separation, and efficient light harvesting. A donor material (X2) that crystallizes directly from solution yields additive-free solar cells with an efficiency of 7.6%.

Impact of rotamer diversity on the self-assembly of nearly isostructural molecular semiconductors

Conformational diversity due to different orientations of structural subunits has a complex impact on morphological disorder of organic semiconductors. Here, we isolate the impact of a specific structural change: replacing bithiophene (biTh) units with thieno[3,2-b]thiophene (TT). We compare four molecules with an alternating donor–acceptor structure (D′–A–D–A–D′) composed of a central, electron-rich dithienosilole (DTS) unit flanked by pyridyl-[2,1,3]thiadiazole (PT) or fluorinated benzo[c][1,2,5]thiadiazole (FBT) and end-capped with bithiophene biTh or TT groups. We find that using TT instead of biTh results in an increased degree of order within films cast directly from solution by influencing the self-assembly tendencies of the different molecules. Unlike switching the acceptor subunit, such as FBT for PT, the TT for biTh structural change has little impact on the electronic structure of these molecular semiconductors. Instead, these morphological effects can be understood within the context of the predicted conformational diversity. TT units limit the number of rotational conformations (rotamers) available within this molecular architecture; low rotamer dispersity facilitates self-assembly into ordered domains. As a practical illustration of this greater drive toward self-assembly, we use the TT-containing molecules as donors in bulk heterojunction solar cells with PC70BM. Devices with TT-containing molecules show improved photovoltaic performance compared to their previously characterized biTh analogs (d-DTS(PTTh2)2 and p-DTS(FBTTh2)2) in both as-cast and optimized conditions, with efficiencies up to 6.4% and 8.8% for PT-TT and FBT-TT, respectively. The TT subunit and, more broadly, the strategy of limiting conformational diversity can be readily applied toward the design of solution-processable organic semiconductors with increased as-cast order.

Topological Transformation Reduces Resistance to Crystallization

Two electronically delocalized molecules were designed as models to understand how molecular shape impacts the tradeoff between solubility and crystallization tendencies in molecular semiconductors. The more soluble compound TT contains a non-planar bithiophene central fragment, whereas CT has a planar cyclopentadithiophene unit. Calorimetry studies show that CT can crystallize more easily than TT. However, absorption spectroscopy shows that the initially amorphous TT film can eventually form crystals in which the molecular shape is significantly more planar. Two thermally reversible polymorphs for TT were observed by XRD and grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. These findings are relevant within the context of designing soft semiconductors that exhibit high solubility and a tendency to provide stable organized structures with desirable electronic properties.

Doping Polymer Semiconductors by Organic Salts: Toward High-Performance Solution-Processed Organic Field-Effect Transistors

Solution-processed organic field-effect transistors (OFETs) were fabricated with the addition of an organic salt, trityl tetrakis(pentafluorophenyl)borate (TrTPFB), into thin films of donor-acceptor copolymer semiconductors. The performance of OFETs is significantly enhanced after the organic salt is incorporated. TrTPFB is confirmed to p-dope the organic semiconductors used in this study, and the doping efficiency as well as doping physics was investigated. In addition, systematic electrical and structural characterizations reveal how the doping enhances the performance of OFETs. Furthermore, it is shown that this organic salt doping method is feasible for both p- and n-doping by using different organic salts and, thus, can be utilized to achieve high-performance OFETs and organic complementary circuits.