Venkatakrishnan P

We have demonstrated three LY-HTMs for perovskite solar cells in this report. These LY-HTMs embed cyclopentadithiophene (CPT) core that incorporates 2∼4 triphenylamine electron donor units. Designing LY-HTMs with Lewis-basic property of carbonyl or imine, can effectively passivate the defects of insufficiently coordinated Pb2+ in the perovskite layer. The LY-HTMs have good thermal stability and enhanced intermolecular interaction, thereby dense packing, due to the sulfur-sulfur interaction in the dithiophene structure. Moreover, the sulfur-sulfur interaction achieves a deeper HOMO energy level that can be well-matched to perovskite solar cells (PSCs). It also improves the charge mobility to enhance Voc and Jsc values. The best performance using LY-1 as a HTM in PSCs exhibited a Jsc of 23.1 mA∙cm−2, a Voc of 1.06 V, and a fill factor of 0.78, corresponding to an overall conversion efficiency of 19.12% (the control device of spiro-OMeTAD, 17.69%). In addition, the PCEs of the LY-1 PSC devices underwent decays of only 90% and 74.8% of their original values after 10 and 30 days, respectively, under an Ar atmosphere.

While intramolecular Scholl oxidative coupling between two arenes is common, successful C−C heterocoupling between thiophene and arene is scarce. The latter is due to the notorious reactivity of thiophene towards polymerization under oxidative conditions. This report systematically demonstrates how topological variation of electronics and reactivity in thiophene substrates can lead to efficient oxidative heterocoupling. Bis(biaryl)thiophenes having reactive α- and β-positions open are the choice of substrates. The cyclizing arene partners are so electronically tuned for thiophene's reactivity (at α- and β-) as to establish C−C bond oxidatively generating symmetrical as well as unsymmetrical diphenanthrothiophenes which are basic thiananographenes. Depending on the cyclizing-couple's electronics, either arene- or thiophene-centered oxidation initiates C−C heterocoupling. The potential utility of these simple thiananographenes is further unfurled by converting them to functional N−/C-graphene segments that are aza-corannulene precursor and tetrabenzospirobifluorene. Their bright emission and extended electrochemical stability are remarkable that may be potentially important and applicable.

The solution-processed inverted co-planar organic thin film transistor (OTFT) incorporating recessed source-drain-gate and polymer gate dielectric is demonstrated to realize a fully recessed device. The impact of recessed electrodes on the contact resistance ( R-{c} ) and the mobility ( \mu ) is thoroughly investigated by comparing the performance of fully recessed devices, partially recessed (source-drain) devices and standard non-recessed devices. A reduction in R-{c} by two orders of magnitude and 3-fold increase in \mu to attain the values of 60 k\Omega -cm (without any surface treatment) and 1.5\times 10^{-2}cm^{2}/Vs respectively are achieved in a fully recessed device as compared to the standard non-recessed device.

New heteroacenes, named coumarinacenes, have been designed, synthesized and characterized. They possess visible absorption and orange-red emission that are far beyond those of benzo[g]coumarins. They exhibit deep HOMO energy levels, and reveal excellent thermal, photochemical and air stabilities as compared to tetracene/pentacene. Their preliminary thin-film devices display encouraging transistor characteristics, moderate mobility and considerable environmental stability paving way into electronics.

Differently-linked bithienyl-attached fluorenes have been synthesized. While the oxidative photodicyclization of 2,3′-bithiophene-containing fluorene was successful to yield tetrathieno-fused regioisomeric π-expanded fluorenes, the reaction with 2,2′-bithiophene-containing fluorene was very sluggish. Interestingly, the connectivity in bithiophenes appears to determine the (photo)reactivity. On the one hand, under mild Scholl conditions, 2,3′-bithiophene-containing fluorene yielded a highly regioselective product while 2,2′-bithiophene-containing fluorene led to polymerization. The optical and electrochemical properties of tetrathienoannulated π-expanded fluorenes were investigated, and they were found to be quite different from those of the phenanthreno-annulated π-expanded analogues. These newly obtained sulfur-edged π-expanded fluorenes may hold promise in organic electronics.

In this study, we synthesized three simple and inexpensive (34-120 USD/g) 3,3′-bicarbazole-based hole transporting materials (BC-HTMs; NP-BC, NBP-BC and PNP-BC) through a metal-free oxidative coupling, in excellent yields (≥95%). These bicarbazoles contain phenylene or biphenylene substituents on the carbazole N atom, with extended Ï-conjugation achieved through phenylene units at the 6,6′-positions of the bicarbazole. When using NBP-BC as a dopant-free HTM in a p-i-n perovskite solar cell (PSC), we achieved a power conversion efficiency (PCEs) of 12.22 ± 0.54% under AM 1.5G conditions (100 mW cm-2); this PCE was comparable to that obtained when using PEDOT:PSS as the HTM (11.23 ± 1.02%). BC-HTMs showed the large grain size (μm) of perovskite than PEDOT:PSS-based, due to defect passivation on indium tin oxide (ITO) substrate and good hydrophobicity. Furthermore, we realized highly efficient and stable PSCs when using the p-i-n device structure ITO/NiOx/NP-BC/perovskite/PC61BM/BCP/Ag. The bifacial defect passivation effect of the interfacial layer improved the grain size of the perovskite layer and also enhanced the performance; the best performance of the NiOx/NP-BC device was characterized by a short-circuit current density (Jsc) of 22.38 mA cm-2, an open-circuit voltage (Voc) of 1.09 V, and a fill factor (FF) of 79.9%, corresponding to an overall PCE of almost 20%. This device structure has competitive potential because its performance is comparable to that of the record-high-efficiency PSCs. Under an Ar atmosphere, the PCE of the NiOx/NP-BC PSC device decayed by only 4.55% after 168 h; it retained 90.80% of its original PCE after 1000 h. A morphological study revealed that the films of the BC-HTMs were indeed smooth and hydrophobic and that the perovskite films spin-coated upon them were uniform and featured large grains (micrometer scale). Time-resolved photoluminescence (TRPL) spectra of the perovskite films suggested that the hole extraction capabilities of the NiOx/BC-HTMs were better than that of the bare NiOx. The superior film morphologies of the NiOx/BC-HTMs were responsible for the performances of their devices being comparable to those of bare NiOx-based PSCs.

This paper describes the synthesis of an angular coumarinacene. Its molecular structure by single-crystal X-ray analysis is revealed for the first time, which noted a sandwich-dimeric herringbone arrangement. Its solution reveals visible absorption and bright yellow-orange emission. The pyranone-annulated angular coumarinacene is relatively stable, both thermally and photochemically, when compared to popular acenes. Its alignment of frontier orbital energy levels dictates its possible entry into electronics.

Dithieno-annulated benzo[g]coumarins were synthesized in excellent yields using a facile photochemical oxidative cyclization method. These π-expanded benzocoumarins displayed strong absorption in the near-UV/visible region. Moreover, they revealed visible blue to yellow emission in solution when compared to simple benzo[g]coumarins or thienoannulated coumarins. This suggests their possible use in bio-imaging.

Even though there has been a significant progress in organic thin film transistor (OTFT), one of the major limitations that hinders the device performance is contact effect at the junction of semiconductor and source-drain contacts The effect becomes more effective while scaling down the channel length resulting in apparent mobility reduction, hysteresis etc. [1]. Efforts have been made to reduce contact resistance through the reduction of the metal-semiconductor injection barrier by either metal work function modification or by introducing a carrier injecting buffer layer. In this work, recessed drain-source structure on solution-processed polymer gate dielectric is demonstrated to realize bottom gate bottom contact (BGBC) OTFT based on poly[2, 5-bis(3-tetradecylthiophen-2-yl)thieno[3, 2- b]thiophene](PBTTT-C14) as semiconductor.

Various π-extended new bitriphenylene derivatives (both conformationally-free and conformation-locked) have been successfully synthesized by a facile Scholl oxidative cyclodehydrogenation method in good to excellent yields. Their optical properties in solution and film states reveal possibility for self-ordering. The molecular packing in solid-state demonstrates favorable π–π stacking as well as weak intermolecular interactions leading to face-to-face or slipped-stack arrangements. Besides, these butterfly-shaped large bitriphenylenes display near-UV absorption, excellent photochemical, thermal, and electrochemical stabilities. The all-organic anneal-free transparent FETs fabricated from solution-processable bitriphenylenes showcase significant improvement (ca. 4 orders of magnitude higher) in charge transporting abilities (µh: from ca. 10–7 to 10–3 cm2/(Vs)) when compared to model triphenylene. The fabricated FETs unveil excellent air stability (> 1 year under atmospheric conditions) highlighting the utility of these novel link-locked triphenylene skeletons in organic electronics.