Synthesis and Photovoltaic Properties of 2D π-Conjugated Polymers Based on Alkylbenzothiophene Substituted Benzodithiophene Donor Unit with Titanium Sub-Oxide (TiOX) as an Interlayer in the Bulk Heterojunction Device Structure

Chakravarthi, N.,Gunasekar, K.,Jin, S. H.,Lee, J. H. Springer Science + Business Media 2015 Journal of inorganic and organometallic polymers a Vol.25 No.1

<P>A newbenzo[1,2-b:4,5-b']dithiophene (BDT) derivative with conjugated side chain substituent, 5-ethyl-hexylbenzothiophene group, was synthesized as donor unit (D) and copolymerized with two acceptor units (A), 3,3'-diethylhexyl-3,4-propylenedioxythienyl-bisbenzothiadiazole and 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H, 5H)-dione (DPP), respectively, using Stille coupling reaction to afford two new copolymers, P1 and P2. Low bandgaps were successfully accomplished for P1 (1.64 eV) and P2 (1.37 eV) and were attributed to the strong intramolecular charge transfer within the D-A alternating structure. Both P1 and P2 exhibited highest occupied molecular orbital (HOMO) energy levels that were deeper due to the conjugated benzothiophene substituent. The photovoltaic performances of the synthesized polymers were investigated using the device configuration ITO/PEDOT:PSS/polymer:PC71BM/(with or without)TiOx/Al. We employed titanium sub-oxide (TiOx) as an effective interlayer in the bulk heterojunction device structure. The subsequent photovoltaic performances showed high open-circuitvoltages (V-oc) ranging from 0.78 to 0.81 V, whereas the power conversion efficiencies (PCEs) depended strongly on the blend morphologies. The polymer solar cell devices based on the blend of P2 and PC71BM gave the best photovoltaic performance among the series, with a V-oc of 0.78 V, short-circuit current density (J(sc)) of 5.69 mA/cm(2), fill factor (FF) of 58.09 % and PCE of 2.60 %.</P>

The effect of with/without resonance-mediated interactions on the organic solar cell performance of new 2D π-conjugated polymers

Chakravarthi, Nallan,Gunasekar, Kumarasamy,Kranthiraja, Kakaraparthi,Kim, Taeik,Cho, Woosum,Kim, Chang Su,Kim, Dong-Ho,Song, Myungkwan,Jin, Sung-Ho The Royal Society of Chemistry 2015 Polymer chemistry Vol.6 No.40

<▼1><▼1><P>Bis-tolane as an integrated part of the benzodithiophene donor unit for OSCs.</P></▼1><▼2><P>With the goal of discovering a new and plausible approach to utilizing the conjugated side chains (CSCs), other than for the previously reported purpose of two-dimensional (2D) π-conjugation extension in π-conjugated polymers, we report, for the first time, the impact of with/without interactions induced <I>via</I> resonance in CSCs on the molecular weight (<I>M</I>w) and photovoltaic performance of polymers. For this, we designed two donor (D)–acceptor (A) polymers, represented as PBDTBPA(H)-DPP and PBDTBPA(F)-DPP, containing alkoxy-BPA(H) and alkoxy-BPA(F) [BPA = biphenylethynyl] on the benzodithiophene (BDT) unit as novel CSCs, respectively. The introduction of these CSCs generated bis-tolane as an integrated part of the BDT unit, which allowed us to address the difference in the polymerization, photophysical, and photovoltaic properties of PBDTBPA(H)-DPP and PBDTBPA(F)-DPP, as a function of the structural variation of CSCs, which has never been investigated in organic solar cells (OSCs). In contrast to the weak electron-donating nature of BPA(H), BPA(F) exhibited a strong electron-donating ability due to the interaction of the lone pair electrons of the fluorine atom with the triple bond <I>via</I> resonance, which decreased the rigidity of the triple bond, whereas in PBDTBPA(H)-DPP the rigidity of the triple bond was retained with no such interaction. The striking differences in the rigidity and different electron-withdrawing tendencies of the CSCs were well correlated with <I>M</I>w and with the highest occupied molecular orbital (HOMO) energy levels of the polymers. As a result, the inverted OSCs based on PBDTBPA(H)-DPP achieved an open-circuit voltage (<I>V</I>oc) of 0.74 V, and power conversion efficiency (PCE) of 5.58%, which was 38% higher than that of PBDTBPA(F)-DPP-based inverted OSCs. More significantly, the inverted OSC devices were highly stable, retaining 80% of their original PCE after 60-day storage in air, even without encapsulation. To the best of our knowledge, this 5.58% is the highest PCE reported to date for the arylethynyl-substituted BDT donor-based OSCs. These results reveal that bis-tolane [BDTBPA(H)] as an integrated part of the new BDT unit is a promising donor building block for high <I>M</I>w donor polymers in addition to 2D extended π-conjugation for high performance bulk heterojunction (BHJ) OSCs.</P></▼2></▼1>

A FIFTH ORDER NUMERICAL METHOD FOR SINGULARLY PERTURBED DIFFERENTIAL-DIFFERENCE EQUATIONS WITH NEGATIVE SHIFT

Chakravarthy, P. Pramod,Phaneendra, K.,Reddy, Y.N. The Korean Society for Computational and Applied M 2009 Journal of applied mathematics & informatics Vol.27 No.1

In this paper, a fifth order numerical method is presented for solving singularly perturbed differential-difference equations with negative shift. In recent papers the term negative shift has been using for delay. Similar boundary value problems are associated with expected first exit time problem of the membrane, potential in models for neuron and in variational problems in control theory. In the numerical treatment for such type of boundary value problems, first we use Taylor approximation to tackle terms containing small shifts which converts it to a boundary value problem for singularly perturbed differential equation. The two point boundary value problem is transformed into general first order ordinary differential equation system. A discrete approximation of a fifth order compact difference scheme is presented for the first order system and is solved using the boundary conditions. Several numerical examples are solved and compared with exact solution. It is observed that present method approximates the exact solution very well.

Design and Synthesis of New Small Molecule Cathode Interfacial Materials for Organic Solar Cell Application: Effect of Interfacial Dipole Inducing Groups on the Device Performance

Chakravarthi Nallan,Gunasekar. K,Yeong-Soon Gal,Sung-Ho Jin 한국고분자학회 2016 한국고분자학회 학술대회 연구논문 초록집 Vol.41 No.2

A FIFTH ORDER NUMERICAL METHOD FOR SINGULAR PERTURBATION PROBLEMS

Chakravarthy, P. Pramod,Phaneendra, K.,Reddy, Y.N. Korean Society of Computational and Applied Mathem 2008 Journal of applied mathematics & informatics Vol.26 No.3-4

In this paper, a fifth order numerical method is presented for solving singularly perturbed two point boundary value problems with a boundary layer at one end point. The two point boundary value problem is transformed into general first order ordinary differential equation system. A discrete approximation of a fifth order compact difference scheme is presented for the first order system. An asymptotically equivalent first order equation of the original singularly perturbed two point boundary value problem is obtained from the theory of singular perturbations. It is used in the fifth order compact difference scheme to get a two term recurrence relation and is solved. Several linear and non-linear singular perturbation problems have been solved and the numerical results are presented to support the theory. It is observed that the present method approximates the exact solution very well.

Triazine-based Polyelectrolyte as an Efficient Cathode Interfacial Material for Polymer Solar Cells

Chakravarthi, Nallan,Aryal, Um Kanta,Gunasekar, Kumarasamy,Park, Ho-Yeol,Gal, Yeong-Soon,Cho, Young-Rae,Yoo, Seong Il,Song, Myungkwan,Jin, Sung-Ho American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.29

<P>A novel polyelectrolyte containing triazine (TAZ) and benzodithiophene (BDT) scaffolds with polar phosphine oxide (P=O) and quaternary ammonium ions as pendant groups, respectively, in the polymer backbone (PBTAZPOBr) was synthesized to use it as a cathode interfacial layer (CIL) for polymer solar cell (PSC) application. Owing to the high electron affinity of the TAZ unit and P=O group, PBTAZPOBr could behave as an effective electron transport material. Due to the polar quaternary ammonium and P=O groups, the interfacial dipole moment created by PBTAZPOBr substantially reduced the work function of the metal cathode to afford better energy alignment in the device, thus enabling electron extraction and reducing recombination of excitons at the photoactive layer/cathode interface. Consequently, the PSC devices based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl]:[6,6]-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM) system with PBTAZPOBr as CIL displayed simultaneously enhanced open-circuit voltage, short-circuit current density, and fill factor, whereas the power conversion efficiency increased from 5.42% to 8.04% compared to that of the pristine Al device. The outstanding performance of PBTAZPOBr is attributed not only to the polar pendant groups of BDT unit but also to the TAZ unit linked with the P=O group of PBTAZPOBr, demonstrating that functionalized TAZ building blocks are very promising cathode interfacial materials (CIMs). The design strategy proposed in this work will be helpful to develop more efficient CIMs for high performance PSCs in the future.</P>

Synthesis, Characterization, and Photovoltaic Properties of 4,8-Dithienylbenzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-Based Donor–Acceptor Polymers with New Polymerization and 2D Conjugation Extension Pathways: A Potential Donor Building Block fo

Chakravarthi, Nallan,Gunasekar, Kumarasamy,Kim, Chang Su,Kim, Dong-Ho,Song, Myungkwan,Park, Young Geun,Lee, Jin Yong,Shin, Yurim,Kang, In-Nam,Jin, Sung-Ho American Chemical Society 2015 Macromolecules Vol.48 No.8

<P>In all the previously reported 4,8-dithienylbenzo[1,2-b:4,5-b']dithiophene (DTBDT)-based pi-conjugated polymers, the polymerization and two-dimensional (2D) conjugation extension pathways were through the thiophenes fused to the phenyl core of DTBDT and through the thiophenes linked to the benzene core of DTBDT, respectively (BDT-directed DTBDT). Herein, with the aim of discovering another potential way to introduce the DTBDT motif in the donor-acceptor alternating polymer structure, we first report the synthesis of three new p-conjugated polymers, P1, P2, and P3, with a modified DTBDT building block as a donor unit. This modification results in new polymerization and 2D conjugation extension pathways for the polymers through the thiophenes linked to the benzene core of DTBDT and through the thiophenes fused to the phenyl core of the DTBDT, respectively (dithienylbenzene-directed DTBDT). Although these modified polymerization pathways of DTBDT result in less delocalized conjugation along the dithienylbenzene direction, the optical and electrochemical properties reveal that the electron-donating property of dithienylbenzene-directed DTBDT was strong enough to generate strong intramolecular charge transfer (ICT) and maintain low-lying highest occupied molecular orbital (HOMO) energy levels (-5.21 to -5.28 eV) for high air stability. Inverted organic solar cells (IOSCs) were fabricated with the configuration of ITO/ZnO/polymer:PC71BM/PEDOT:PSS/Ag. By systematic optimization of the performance of the IOSCs using polar solvent treatment, the IOSCs based on P1, P2, and P3 displayed promising power conversion efficiencies (PCE) of 6.31, 5.65, and 7.10%, respectively, which compare well with the PCE of already reported BDT-directed DTBDT-based polymers. More importantly, the stability of the IOSCs was demonstrated by their retention of 83% PCE after ambient storage for 30 days. These study results revealed the promising potential of the proposed molecular design strategy for introducing new 2D conjugation extension and polymerization pathways for a DTBDT unit for high performance and stable IOSCs. This strategy can be applied to the judicious molecular design of new polymeric materials for achieving high PCE.</P>

ANALYSIS OF M/M/c RETRIAL QUEUE WITH THRESHOLDS, PH DISTRIBUTION OF RETRIAL TIMES AND UNRELIABLE SERVERS

CHAKRAVARTHY, SRINIVAS R.,OZKAR, SERIFE,SHRUTI, SHRUTI The Korean Society for Computational and Applied M 2021 Journal of applied mathematics & informatics Vol.39 No.1

This paper treats a retrial queue with phase type retrial times and a threshold type-policy, where each server is subject to breakdowns and repairs. Upon a server failure, the customer whose service gets interrupted will be handed over to another available server, if any; otherwise, the customer may opt to join the retrial orbit or depart from the system according to a Bernoulli trial. We analyze such a multi-server retrial queue using the recently introduced threshold-based retrial times for orbiting customers. Applying the matrix-analytic method, we carry out the steady-state analysis and report a few illustrative numerical examples.

A simple structured and efficient triazine-based molecule as an interfacial layer for high performance organic electronics

Chakravarthi, Nallan,Gunasekar, Kumarasamy,Cho, Woosum,Long, Dang Xuan,Kim, Yun-Hi,Song, Chang Eun,Lee, Jong-Cheol,Facchetti, Antonio,Song, Myungkwan,Noh, Yong-Young,Jin, Sung-Ho The Royal Society of Chemistry 2016 ENERGY AND ENVIRONMENTAL SCIENCE Vol.9 No.8

<P>Achieving the state-of-the-art performance of solution processable and flexible organic electronics requires efficient, stable, and cost-effective interfacial layers (ILs). Here, we report an alcohol soluble phosphine oxide functionalized 1,3,5-triazine derivative (PO-TAZ) as an IL, which remarkably tailors the work function of conductors including metals, transparent metal oxides and organic materials, making it an ideal candidate for an interfacial material in organic electronics. Consequently, PO-TAZ thin films enable the fabrication of organic and organic-inorganic (perovskite) solar cells with power conversion efficiencies of 10.04% and 16.41%, respectively, and n-channel organic field-effect transistors with an electron mobility of 8 cm(2) V-1 s(-1). Owing to the low-cost processing associated with PO-TAZ and the tremendous improvement in device performances as compared to the devices without PO-TAZ along with ambient stability, PO-TAZ is a good choice for efficient organic electronics in large area printing processes.</P>

A RETRIAL QUEUEING MODEL WITH THRESHOLDS AND PHASE TYPE RETRIAL TIMES

CHAKRAVARTHY, SRINIVAS R. The Korean Society for Computational and Applied M 2020 Journal of applied mathematics & informatics Vol.38 No.3

There is an extensive literature on retrial queueing models. While a majority of the literature on retrial queueing models focuses on the retrial times to be exponentially distributed (so as to keep the state space to be of a reasonable size), a few papers deal with nonexponential retrial times but with some additional restrictions such as constant retrial rate, only the customer at the head of the retrial queue will attempt to capture a free server, 2-state phase type distribution, and finite retrial orbit. Generally, the retrial queueing models are analyzed as level-dependent queues and hence one has to use some type of a truncation method in performing the analysis of the model. In this paper we study a retrial queueing model with threshold-type policy for orbiting customers in the context of nonexponential retrial times. Using matrix-analytic methods we analyze the model and compare with the classical retrial queueing model through a few illustrative numerical examples. We also compare numerically our threshold retrial queueing model with a previously published retrial queueing model that uses a truncation method.