Mithun Chowdhury

Solution-Processable Silicon Phthalocyanines in Electroluminescent and Photovoltaic Devices

Phthalocyanines and their main group and metal complexes are important classes of organic semiconductor materials but are usually highly insoluble and so frequently need to be processed by vacuum deposition in devices. We report two highly soluble silicon phthalocyanine (SiPc) diester compounds and demonstrate their potential as organic semiconductor materials. Near-infrared (λEL = 698–709 nm) solution-processed organic light-emitting diodes (OLEDs) were fabricated and exhibited external quantum efficiencies (EQEs) of up to 1.4%. Binary bulk heterojunction solar cells employing P3HT or PTB7 as the donor and the SiPc as the acceptor provided power conversion efficiencies (PCE) of up to 2.7% under simulated solar illumination. Our results show that soluble SiPcs are promising materials for organic electronics.

Effect of Annealing on Exciton Diffusion in a High Performance Small Molecule Organic Photovoltaic Material

Singlet exciton diffusion was studied in the efficient organic photovoltaic electron donor material DTS(FBTTh2)2. Three complementary time-resolved fluorescence measurements were performed: quenching in planar heterojunctions with an electron acceptor, exciton–exciton annihilation, and fluorescence depolarization. The average exciton diffusivity increases upon annealing from 1.6 × 10–3 to 3.6 × 10–3 cm2 s–1, resulting in an enhancement of the mean two-dimensional exciton diffusion length (LD = (4Dτ)1/2) from 15 to 27 nm. About 30% of the excitons get trapped very quickly in as-cast films. The high exciton diffusion coefficient of the material leads to it being able to harvest excitons efficiently from large donor domains in bulk heterojunctions.

Discrete mobility on the surface of glasses

From an applications perspective, the glass transition temperature (Tg)—conceptually, the point at which a liquid transitions into a disordered solid—is perhaps the most important parameter of a glass because it sets the conditions under which it can be successfully exploited as an enabling material. Some time ago, 1991 to be exact, it was discovered that the Tg of the glass former, ortho-terphenyl, when physically confined to the nanometer-length scale, was reduced in comparison with its bulk value (1). A few years later, even more dramatic suppressions in the Tg, because of confinement, were observed for ultrathin films of polystyrene in the supported and freely standing thin-film geometries (2, 3). The implication of these findings was dramatic and twofold. From a technological standpoint, it would be critical to understand these effects to aid in the establishment of new materials’ design rules for the fast-approaching field of nanotechnology that would be partially enabled by confined glasses, including thin films. From a scientific standpoint, the question of “why” became the central challenge. That is, why, when confined to the nanoscale, the Tg can be suppressed so dramatically from the bulk value. Very early, strong evidence pointed to a connection between enhanced dynamics at the free surface of glasses and the suppression of Tg (2⇓⇓–5). However, exactly how this enhanced mobile layer is correlated to the measured changes in Tg and the corresponding underlying molecular dynamics has remained an intriguing, and a far-from-understood question (6, 7). Reporting in PNAS, Zhang and Fakhraai found that the surface diffusivity of ultrathin films of a molecular glass former was enhanced compared with bulk and independent of film thickness, despite a concurrent dramatic reduction in the Tg and enhancement in the relaxation dynamics within … [↵][1]1To whom correspondence should be addressed. Email: rpriestl{at}princeton.edu. [1]: #xref-corresp-1-1

Spatially Distributed Rheological Properties in Confined Polymers by Noncontact Shear

When geometrically confined to the nanometer length scale, a condition in which a large portion of the material is in the nanoscale vicinity of interfaces, polymers can show astonishing changes in physical properties. In this investigation, we employ a unique noncontact capillary nanoshearing method to directly probe nanoresolved gradients in the rheological response of ultrathin polymer films as a function of temperature and stress. Results show that ultrathin polymer films, in response to an applied shear stress, exhibit a gradient in molecular mobility and viscosity that originates at the interfaces. We demonstrate, via molecular dynamics simulations, that these gradients in molecular mobility reflect gradients in the average segmental relaxation time and the glass-transition temperature.

Engineered exciton diffusion length enhances device efficiency in small molecule photovoltaics

In organic photovoltaic blends, there is a trade-off between exciton harvesting and charge extraction because of the short exciton diffusion length. Developing a way of increasing exciton diffusion length would overcome this trade-off by enabling efficient light harvesting from large domains. In this work, we engineered (enhanced) both exciton diffusion length and domain size using solvent vapour annealing (SVA). We show that SVA can give a three-fold enhancement in exciton diffusion coefficient (D) and nearly a doubling of exciton diffusion length. It also increases the domain size, leading to enhancement of charge extraction efficiency from 63 to 89%. Usually larger domains would reduce exciton harvesting but this is overcome by the large increase in exciton diffusion, leading to a 20% enhancement in device efficiency.