Intrinsic traps in organic semiconductors can be eliminated by trap-filling with F4-TCNQ. Photovoltaic tests show that devices with F4-TCNQ at parts per thousand concentration outperform control devices due to an improved fill factor. Further studies confirm the trap-filling pathway and demonstrate the general nature of this finding.

Although chemical doping is widely used to tune the optical and electrical properties of semiconducting polymers, it is not clear how the degree of doping and the electrical properties of the doped materials vary with the bandgap, valence band level, and crystallinity of the polymer. We addressed these questions utilizing a series of statistical copolymers of poly­(3-hexylthiophene) (P3HT) and poly­(3-heptylselenophene) (P37S) with controlled gradients in bandgap, valence band position, and variable crystallinity. We doped the copolymers in our series with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) using solution sequential processing. We then examined the structures of the films using grazing incidence wide-angle X-ray scattering, differential scanning calorimetry, and ellipsometric porosimetry, and the electrical properties of the films via the AC Hall effect. We found that the ability of a particular copolymer to be doped is largely determined by the offset of the polymer’s valence band energy level relative to the LUMO of F4TCNQ. The ability of the carriers created by doping to be highly mobile and thus contribute to the electrical conductivity, however, is controlled by how well the polymer can incorporate the dopant into its crystalline structure, which is in turn influenced by how well it can be swelled by the solvent used for dopant incorporation. The interplay of these effects varies in a nonmonotonic way across our thiophene:selenophene copolymer series. The position and shape of the polaron absorption spectrum correlate well with the polymer crystallinity and carrier mobility, but the polaron absorption amplitude does not reflect the number of mobile carriers, precluding the use of optical spectroscopy to accurately estimate the mobile carrier concentration. Overall, we found that the degree of crystallinity of the doped films is what best correlates with conductivity, suggesting that only carriers in crystalline regions of the film, where the dopant counterions and polarons are forced apart by molecular packing constraints, produce highly mobile carriers. With this understanding, we are able to achieve conductivities in this class of materials exceeding 20 S/cm.

Developing novel materials analogous to poly(3hexylthiophene) (P3HT) with increased absorption range and good one-dimensional self-assembly properties should increase photovoltaic performance while taking advantage of the well-established structure−property relationships developed for P3HT. Herein, we have fabricated novel polymer nanowires composed of P3HT doped with varying amounts of selenophene. Doping is accomplished by statistical polymerization and results in the incorporation of selenophene into the P3HT crystal lattice. Selenophene doping increases optical absorption far beyond what can be achieved by simply blending two materials. Polymer nanowire solar cells using selenphene-doped P3HT outperform native P3HT nanowire and corresponding ternary blend solar cells, reaching an overall maximum performance of >4% PCE. These are some of the highest values of any polymer nanowire solar cells and show that the selenophene-doping strategy is important for achieving high-performance devices.

Polymer solar cells are fabricated with systematic variation of the phase purity. Photovoltaic tests demonstrate that devices with ca. 10% of mixed phases outperform pure-phase devices. Photophysical studies reveal the effects of mixed phase on charge generation and recombination. These results show a promising strategy for the optimization of organic electronic materials.

Whereas monodisperse polymers are ubiquitous in Nature, they remain elusive to synthetic chemists. Absolute control over polymer length and structure is essential to imparting chemical functionality, reproducible properties, and specific solid-state behavior. Precise polymer length has proven to be extremely difficult to control. The most successful examples are generally similar to solid-phase oligo nucleotide or peptide synthesis, wherein the polymer is built up one unit at a time with each sequential monomer addition requiring purification and deprotection (or other functional group activation) step. We have discovered a stepwise homogeneous catalyst-transfer polymerization to prepare monodisperse oligo(3-hexylthiophene) using temperature to limit additions to one unit per chain per cycle. This is the first reported example of a one-pot synthesis of monodisperse oligomers that requires no additional purification or intermediate steps. It is our hope that the strategy of temperature cycling to "freeze" intermediates will be generalizable to other living polymerization techniques, such as other catalyst-transfer polymerization systems, and those where a resting state involves an association between the catalyst and growing chain.