No abstract

Microphase-separated block copolymers composed of electron donor and acceptor blocks may provide morphology control to address many challenges in organic electronics. Crucial to controlling the self-assembly of fully conjugated block copolymers is tuning the interplay between crystallization of the individual blocks and microphase separation between the donor and the acceptor. Thus, we have examined the kinetics of the morphological evolution in P3HT-b-PFTBT block copolymer films during two processes: solution casting and thermal annealing. We use in situ wide-angle and small-angle grazing incidence X-ray scattering to monitor the crystallization of P3HT and microphase separation between the two blocks. We find that during film drying, initial P3HT crystallization happens quickly, before phase separation of the two blocks. However, crystallization is significantly suppressed with respect to neat materials, enabling microphase separation to proceed at time scales after some initial crystallization of the donor block takes place. This enables a mesoscale structure to develop during processes such as thermal annealing because self-assembly of the lamellar structure takes place before the crystallization of the donor block is complete. We also find that significant crystallization of PFTBT blocks after P3HT crystallization is possible at elevated temperatures. Crystallization of both blocks is important to maximize the performance of solar cells and transistors with block copolymer active layers. As a consequence, we exceed 3% average power conversion efficiencies in P3HT-b-PFTBT photovoltaic devices.

Fully conjugated donor–acceptor block copolymers are established as model systems to elucidate fundamental mechanisms of photocurrent generation in organic photovoltaics. Using analysis of steady‐state photoluminescence quenching, exciton dissociation to a charge transfer state within individual block copolymer chains is quantified. By making a small adjustment to the conjugated backbone, the electronic properties are altered enough to disrupt charge transfer almost entirely. Strong intermolecular coupling of the electron donor is introduced by synthesizing block copolymer nanoparticles. Transient absorption spectroscopy is used to monitor charge generation in block copolymer isolated chains and nanoparticles. While efficient charge transfer is observed in isolated chains, there is no indication of complete charge separation. In the nanoparticles, long‐lived polarons are observed as early as ≈15 ns. Thus, aggregation of electron donors can facilitate efficient charge generation.

Fully conjugated donor−acceptor block copolymers can serve as the active layer in organic photovoltaics and other organic electronic devices. Self-assembly into periodic domains and crystallization of the constituent blocks are crucial to enable control of the multiscale structure and consequently electronic properties. Resonant soft X-ray scattering (RSoXS) is an invaluable tool to characterize such materials, where tuning the X-ray energy and polarization reveals molecular orientation and domain spacing. Here, anisotropic soft X-ray scattering data reveal the type and degree of orientation within conjugated block copolymers composed of poly(3-hexylthiophene-2,5-diyl) and various push−pull copolymers. The maximum anisotropy is observed at the scattering vector corresponding to the spacing between block copolymer domains, which scales with the end-to-end distance of the blocks, as expected. Furthermore, the anisotropy in RSoXS data reveals that the crystalline P3HT blocks orient, on average, parallel or nearly parallel to the block copolymer interface; the average tilt angle between P3HT chains and domain interfaces can be extracted from the dependence of the anisotropy with polar angle. We interpret this angle to correspond to the average tilt of rings in the unit cell, potentially within a chiral mesostructure. Results are corroborated with scattering calculations based on simple model structures.

Matrix diagonalization, the key technique at the heart of modern computational chemistry for the numerical solution of the Schrödinger equation, can be easily introduced in the physical chemistry curriculum in a pedagogical context using simple Hückel molecular orbital theory for π bonding in molecules. We present details and results of computations, including both the quintessential examples of polycyclic aromatic hydrocarbons discussed in text books and an interesting extension to a large molecule, C60 (buckminsterfullerene), the first member of the fullerenes to be discovered and synthesized, using a simple Excel spreadsheet-based VBA “application” that we have developed.