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Towards Radically Innovative Materials for Better and Sustainable Organic Solar Cells

Periodic Reporting for period 1 - TRIBOSC (Towards Radically Innovative Materials for Better and Sustainable Organic Solar Cells)

Reporting period: 2018-10-01 to 2020-09-30

To address the increasing energy demands, solar light harvesting is regarded as an environmentally benign green approach in the future energy market. Since its inception, the bulk heterojunction organic solar cells (BHJ-OSCs) displayed a huge leap in the power conversion efficiency from 1% to >17% recently. To date small molecule non-fullerene acceptors (SM-NFAs) have emerged as promising molecular frameworks in replacing electron-deficient fullerenes from the BHJs in OSCs. Unlike fullerenes, facile functionalization of these molecular architectures via low-cost synthetic routes affords excellent tunable optoelectronic and electrochemical properties. Thus the main problem being addressed in this proposal was to find an alternative renewable energy resource which can replenish at a much faster rate than it is being consumed. The submitted proposal aims at the design and synthesis of photofunctional donor-acceptor molecular architectures which can find potential applications in organic photovoltaic devices.
As these kinds of structural motifs can be synthesised and purified in bulk in contrast to the inorganic semiconductors, they can serve as potential alternatives in addressing increasing energy demands. The overall objectives of the proposal includes (i) Design, synthesis of donor-acceptor-donor triad in which DPP (acceptor) was flanked between napthothiophene donors, (ii) To impart better solubility and thereby solution processability the alkyl side chains appended on the DPP subunit was varied, (iii) Solution-state photophysical and electrochemical measurements along with the theoretical calculations were performed on these novel organic materials, (iv) thin films of donor-acceptor-donor triads were obtained by spin coating them on to ITO or glass substrate from solvents such as dichloromethane, toluene, o-dichlorobenzene etc. The preliminary characterization of the above obtained organic thin film was done using optical and atomic force microscopy.The following training objectives were achieved by the applicant: (i) Design and synthesis of donor and acceptor precursor materials and novel donor-acceptor-donor triad, (ii) Morphological analyses of aggregates of donor-acceptor-donor triad, preparation of their thin films via spin-coatong and their characterization using optical and atomic force microscopy, (iii) Supervision of junior research scholars and PhD students, and (iv) Developing innovative ideas to build independent research career.
WP1.1: Synthesis of N-heterotriangulene (NHT)-Diketopyrollopyrrole (DPP) conjugated D-A materials
-Results: (i) Synthesis of the NHT acceptor precursor. (i) Two-fold Ullmann coupling of methylanthranilate with methyl-2-iodobenzoate gave the key precursor as triester, (ii) iodination of the precursor in presence of silver sulphate in ethanol yielded the tri-iodinated derivative and (iii) saponification, followed by the cyclization of the in situ generated acyl chloride, employing standard Friedel-Craft’s acylation conditions using SnCl4 as a Lewis acid furnished NHT. (ii) Synthesis of the thiophene appended DPP donor precursor. (i) Modified Reformatsky reaction between succinate ester and 2-thiophenecarbonitrile in presence of t-BuOK and t-amyl alcohol, (ii) N-alkylation at the amide nitrogens using 1-iododhexane and 1-iodooctane yielded soluble dialkyl-DPP-thiophene conjugates, (iii) Bromination with N-bromosuccinimide (NBS), followed by stannylation using t-butyl tin chloride afforded the donor subunit.
-As part of our collaboration with Prof. Dario Pasini, University of Pavia, Italy I was successful in synthesising novel donor-acceptor-donor triad sandwiching DPP between naphthothiophene donors. (i) Synthesis of the naphthothiophene (NT) donor precursor. (i) 3-thiopheneacetic acid to generate NT-acid, (ii) the acid was esterified in presence of a base such as K2CO3, (iii) bromination of the ester using NBS in CHCl3 afforded monobrominated NT derivative which was subsequently stannylated using t-butyl tin chloride in presence of LDA in THF to afford the stannyl derivative of the donor subunit.
-(ii) Synthesis of the thiophene appended DPP acceptor precursor. (i) Modified Reformatsky reaction between succinate ester and 2-thiophenecarbonitrile in presence of t-BuOK and t-amyl alcohol, (ii) N-alkylation at the amide nitrogens using 1-iododhexane and 1-iodooctane yielded soluble dialkyl-DPP-thiophene conjugates, (iii) Bromination with NBS in CHCl3 yielded the dibrominated DPP acceptor subunit.
-Furthermore, Pd-catalyzed Stille cross-coupling of NT-SnBu3 (donor) and DPP-Br2 (acceptor) under standard conditions afforded the symmetrical donor-acceptor-donor triad (NT-DPP-NT).
WP1.2: Photophysical and electrochemical measurements of D-A-D conjugate:
-Results: (i) The preliminary photophysical measurements such as absorption, fluorescence, fluorescence lifetime and quantum yield measurements of novel NT-DPP-NT conjugate was carried out in dichloromethane. (ii) Concentration dependent measurements were performed to estimate the extinction coefficient of the novel donor-acceptor-donor triad. (iii) Electrochemical measurements such as cyclic voltammetry and spectroelectrochemical measurements. (iv) To map the frontier molecular orbitals and electron density of the highest occupied and lowest unoccupied molecular orbitals theoretical calculations were performed. The results obtained as part of the proposal will be disseminated in the form of publication in peer reviewed science journals: (i) a first draft of the NT-DPP-NT triad manuscript is submitted to the supervisor which has three collaborators, 1. Prof. Dario Pasini, University of Pavia, Italy,(2) Prof. Steven Davies, School of Chemistry, University of Nottingham, UK and (3)Dr. Mahesh Hariharan, School of Chemistry, IISER-TVM, Kerala, India., (ii) A tutorial review entitled Rational Design Principles for Highly Efficient Small Molecular Fullerene Free Acceptors is under preparation, (iii) the manuscript compiling the results obtained for water soluble DPP polymers is under preparation, (iv) a book chapter entitled Chirality in Hierarchical supramolecular architectures is accepted for publication.
As mentioned in the objectives of the proposal we anticipate that the current proposal will lead to the fabrication of organic solution processable solar cells employing the SM-NFA designed and synthesised by the applicant in the host laboratory at the university of Nottingham, UK under the supervision of Prof. David B Amabilino. As stated in the technical part of the proposal the research proposal submitted to the Ambient Processing Cluster Tool (Henry Royce Institute facilities), entitled the Development of Organic-based Bulk-heterojunction Solar Cells with High Efficiency Using Novel Semiconductive Small Molecules, Additives and Conductive Monolayers on Top of the Active Layer for Enhancing Interphase Connectivity at the University of Cambridge to access the instrument for constructing organic solar cell was granted (1200 GBP, awarded 80 hours, Ongoing). We anticipate that the solar cells fabricated conjoining the SM-NFA and a wide bandgap polymer donor will exhibit power conversion efficiencies comparable to the best reported OSCs.
Photophysical measurements of NT-DPP-NT in DCM
Crystal structure of DPP and NT-NT