Balaji Purushothaman

Synthesis and Structural Characterization of Crystalline Nonacenes

these linearly fused hydro-carbons have made significant contributions to the under-standing of electronic processes in organic semiconductors.Despite the utility of these compounds, the selection ofmaterials suitable for exploration essentially stops at penta-cene. Although numerous studies have predicted enticingelectronic properties for larger acenes,

A survey of electron-deficient pentacenes as acceptors in polymer bulk heterojunction solar cells

We have prepared, characterized and surveyed device performance for a series of electron deficient pentacenes for use as acceptors in polymer bulk heterojunction solar cells, using P3HT as the donor material. All of the materials reported here behaved as acceptors, and variations in the position and nature of the electron-withdrawing group on the pentacene core allowed tuning of device open-circuit voltage. Photocurrent was strongly correlated with the pentacene crystal packing motif; materials with 2D π-stacking interactions performed poorly compared with materials exhibiting 1D π-stacking interactions. The best pentacene acceptors gave repeatable device efficiency in excess of 1.2%, compared with 3.5% exhibited for PCBM-based devices.

π-σ-Phosphonic acid organic monolayer–amorphous sol–gel hafnium oxide hybrid dielectric for low-voltage organic transistors on plastic

A vacuum-free solution processed hybrid dielectric composed of an anthryl-alkyl-phosphonic acid (π-σ-PA) self-assembled monolayer on an amorphous sol–gel processed hafnium oxide (HfOx) is demonstrated for low-voltage organic thin film transistors (OTFTs) on plastic substrates. The π-σ-PA/HfOx hybrid dielectric provides high capacitance (0.54 µF cm−2) and low leakage current (2 × 10−8 A cm−2), and has a chemically and electrically compatible dielectric interface for evaporated and solution processed acene semiconductors. The utility of this dielectric is demonstrated by fabricating pentacene and 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-PEN) based OTFTs with operating voltages under 2 V, subthreshold slopes as low as 100 mV dec−1, and average mobilities of 0.32 cm2 V−1 s−1 and 0.38 cm2 V−1 s−1, for pentacene and TIPS-PEN, respectively.

Role of crystallinity of non-fullerene acceptors in bulk heterojunctions

Understanding the solid-state structure of the bulk heterojunction provides insight into how to improve the performance of nonfullerene acceptors in organic solar cells. We have characterized the self-assembly of three functionalized pentacene acceptors in single crystals, neat films and bulk heterojunctions formed by blending with a diketopyrrolopyrrole-based molecular donor. Atomic force microscopy, grazing incidence wide-angle X-ray scattering and optical spectroscopy indicate that the presence of the donor perturbs the packing and texture of acceptors with smaller substituents. The structural characterization explains the differences in performance among the three acceptors studied and suggests that, unlike fullerenes, disordered domains of molecular acceptors with planar molecular structure have inefficient electron transport in BHJ thin films.

High Performance Organic Thin Film Transistors made Simple Through Molecular Design and Processing

We report on a simple a method of inducing self-isolation of the thin film transistors via manipulation of the chemical interactions between the organic molecules and the surfaces where they are deposited. We use pentafluorobenzenethiol (PFBT) treatment of the contacts and investigate the thin film formation of different types of chemical compounds. In particular, three similar organic molecules: triisopropyl-silylethynyl pentacene (TIPS pentacene), triisopropyl-silylethynyl peri-fluoro-pentacene (TIPS PFP) and triisopropyl-silylethynyl cata-fluoro-pentacene (TIPS CFP), were studied and compared to better understand how the molecular, crystal structure and thin film formation influence the electrical behavior of devices fabricated with them. The molecules exhibit significantly different interactions with the chemically treated substrates. The field-effect mobilities of these devices are directly related to their thin film microstructure.

Crystal design for organic semiconductors: the effects of substitution on crystal packing

Electronic devices based on organic materials have been an area of intense research for the past several years. Pentacene, an oligoacene, is regarded as a benchmark due to its high charge carrier mobility. However it has poor solubilty, unoptimized π-overlap and is highly unstable. We have successfully demonstrated that peri-functionalization with trialkylsilyl ethynyl groups enhances the π-stacking, solubility and stability of pentacene and related compounds. TIPS pentacene exhibits both two dimensional (2D) π-stacking and good solubilty in common organic solvents. However changing the solubilizing groups or functionalizing the acene core results in a disruption of the desired 2D π-stacking. Molecules with one dimensional (1D) π-stacking have poorer mobility than those with 2D π-stacking and hence larger solubilizing groups are required to change the π-stacking to 2D. The emphasis of this paper is to show how small changes in substitution can disrupt π-stacking in these molecules and how one can achieve the desired π-stacking by careful selection of solubilzing groups.

Organic photovoltaic cells based on functionalized pentacenes

We report a study on solar cells using pentacene derivatives with triisopropylsilylethynyl substitution at the 6,13-position and 1,3-dioxolane substitution to the terminal benzenoid rings of pentacene as the electron donor and C60 as the electron acceptor. A significant increase in the open circuit voltage (Voc) was obtained in all the pentacene-derivative based cells with the highest Voc as high as 0.90 V, compared to a 0.24 V value for pentacene. The variation in the Voc of the cells is in qualitative agreement with the larger offset between ionization potential of the electron donor and the electron affinity of C60. The power conversion efficiency (η) at 100 mW/cm2 of EtTP-5/C60 cells reached 0.74%, which is comparable to that of a pentacene/C60 cell (0.82%).