Effect of Ozone on the Stability of Solution-Processed Anthradithiophene-Based Organic Field-Effect Transistors

We have investigated the degradation effects of ozone exposure on organic field-effect transistors based on 2,8-difluoro-5,11-bis­(triethylsilylethynyl)­anthradithiophene as the organic semiconducting channel layer, as well as on thin films of this widely used, high-mobility, small molecule semiconductor. Electrical <i>I</i>–<i>V</i> measurements showed a loss of transistor characteristic behavior. We present ¹H Nuclear Magnetic Resonance (NMR) spectroscopy results as well as X-ray Photoemission Spectroscopy (XPS) and Fourier Transform Infrared (FTIR) spectroscopy measurements showing the oxidation of the parent molecule, from which we suggest various possible reaction paths

Dynamic Exchange During Triplet Transport in Nanocrystalline TIPS-Pentacene Films

The multiplication of excitons in organic semiconductors via singlet fission offers the potential for photovoltaic cells that exceed the Shockley–Quiesser limit for single-junction devices. To fully utilize the potential of singlet fission sensitizers in devices, it is necessary to understand and control the diffusion of the resultant triplet excitons. In this work, a new processing method is reported to systematically tune the intermolecular order and crystalline structure in films of a model singlet fission chromophore, 6,13-bis­(triisopropylsilylethynyl) pentacene (TIPS-Pn), without the need for chemical modifications. A combination of transient absorption spectroscopy and quantitative materials characterization enabled a detailed examination of the distance- and time-dependence of triplet exciton diffusion following singlet fission in these nanocrystalline TIPS-Pn films. Triplet–triplet annihilation rate constants were found to be representative of the weighted average of crystalline and amorphous phases in TIPS-Pn films comprising a mixture of phases. Adopting a diffusion model used to describe triplet–triplet annihilation, the triplet diffusion lengths for nanocrystalline and amorphous films of TIPS-Pn were estimated to be ∼75 and ∼14 nm, respectively. Importantly, the presence of even a small fraction (<10%) of the amorphous phase in the TIPS-Pn films greatly decreased the ultimate triplet diffusion length, suggesting that pure crystalline materials may be essential to efficiently harvest multiplied triplets even when singlet fission occurs on ultrafast time scales

Isomerically Pure <i>syn</i>-Anthradithiophenes: Synthesis, Properties, and FET Performance

The synthesis of isomerically pure <i>syn</i>-anthradithiophene derivatives (<i>syn</i>-ADTs) is described. X-ray crystallography is used to compare the solid-state arrangement of <i>syn</i>-ADT derivatives <b>2a</b>,<b>b</b> to the analogous mixture of <i>syn</i>- and <i>anti</i>-ADTs. Single-crystal OFETs based on isomerically pure <i>syn</i>-ADTs <b>2a</b>,<b>b</b> display device performance comparable to those based on a mixture of ADT isomers <i>syn/anti</i>-<b>2a</b>,<b>b</b> with mobilities as high as 1 cm²/(V s)

Direct Observation of Correlated Triplet Pair Dynamics during Singlet Fission Using Ultrafast Mid-IR Spectroscopy

Singlet fission is an exciton multiplication mechanism in organic materials whereby high energy singlet excitons can be converted into two triplet excitons with near unity quantum yields. As new singlet fission sensitizers are developed with properties tailored to specific applications, there is an increasing need for design rules to understand how the molecular structure and crystal packing arrangements influence the rate and yield with which spin-correlated intermediates known as correlated triplet pairs can be successfully separateda prerequisite for harvesting the multiplied triplets. Toward this end, we identify new electronic transitions in the mid-infrared spectral range that are distinct for both initially excited singlet states and correlated triplet pair intermediate states using ultrafast mid-infrared transient absorption spectroscopy of crystalline films of 6,13-bis­(triisopropylsilylethynyl) pentacene (TIPS-Pn). We show that the dissociation dynamics of the intermediates can be measured through the time evolution of the mid-infrared transitions. Combining the mid-infrared with visible transient absorption and photoluminescence methods, we track the dynamics of the relevant electronic states through their unique electronic signatures and find that complete dissociation of the intermediate states to form independent triplet excitons occurs on time scales ranging from 100 ps to 1 ns. Our findings reveal that relaxation processes competing with triplet harvesting or charge transfer may need to be controlled on time scales that are orders of magnitude longer than previously believed even in systems like TIPS-Pn where the primary singlet fission events occur on the sub-picosecond time scale