Molecular Design Synthesis And Characterization Of Conjugated Polymers For Application In Organic Optoelectronic

Solar cells are considered as one of the most promising renewable energy technologies as theyrnharvest the energy of the sun to generate electricity. Organic photovoltaics based on conjugatedrnpolymers are envisioned to generate electricity at very low cost compared with the classicalrnsilicon-based photovoltaics. Up till now, several donor-acceptor alternating polymers wererndesigned, synthesized and utilized for polymer solar cells (PSCs). Power conversion efficienciesrnexceeding 10% have been registered from such devices. However, further optimization of thernconjugated polymer structure is required to attain efficiencies high enough to make thisrntechnology economically attractive and thus, the main focus of this work was to design andrnsynthesize new conjugated polymers and to characterize their photovoltaic performances in solarrncell devices.rnIn order to achieve high performance with organic bulk heterojunction (BHJ) cells, it is veryrncrucial to develop donor photoactive materials which have suitable molecular energy levels.rnDonor conjugated polymers with low-lying highest occupied molecular orbital (HOMO) levelsrnprovide high open circuit voltage (Voc) and thus high efficiencies. In this regard, we designed andrnsynthesized the high bandgap (> 1.9 eV) conjugated polymer PBDTTS-FTAZ with deeprnHOMO energy level from fluorinated benzotriazole acceptor unit and a benzodithiophene-basedrndonor unit incorporating octylthiol side chains. We observed from the electrochemical studiesrnthat PBDTTS-FTAZ had a deeper HOMO energy level compared to the polymer with octyl sidernchain (PBDTT-FTAZ). PBDTTS-FTAZ and PBDTT-FTAZ were used in the fabrication ofrnBHJ polymer solar cells. The device based on PBDTTS-FTAZ:PC71BM showed an enhancedrnVoc of 0.83 V, Jsc of 14.0 mA cm−2, and FF of 0.71, resulting in a much improved PCE of 8.3%,which is among the highest efficiencies recorded for conventional BHJ solar cells based on highrnbandgap polymers and the highest efficiency reported to date for benzotriazole-based polymers.rnAll-polymer solar cells (all-PSC) with conventional and inverted device architectures were alsornfabricated using PBDTTS-FTAZ and PBDTT-FTAZ as donors and low bandgap naphthalenerndiimide (NDI)-based polymers as acceptors. The two donor polymers presented complementaryrnabsorption spectra to the acceptors PNDI-T10 and N2200, which led to better coverage of thernsolar irradiation. The acceptors exhibited high LUMO levels as compared to PCBM. Hence, thernlower-lying HOMO of PBDTTS-FTAZ and the higher-lying LUMO levels of the acceptorrnpolymers would enable higher Voc in the resulting all-PSCs. The inverted all-PSCs using arnPBDTTS-FTAZ:PNDI-T10 blend active layer attained a high PCE of 6.9% with a Voc of 0.89rnV, a Jsc of 12.3 mA cm−2 and FF of 0.63. This PCE was stable without obvious efficiency decayrnover 60 days. However, inverted all-PSCs based on PBDTT-FTAZ:PNDI-T10 showed arnmoderate PCE of 5.0%.rnAll-PSCs were also fabricated form PBDTTS-FTAZ and a new acceptor polymer PIID-PyDPPrnsynthesized from pyridine-flanked diketopyrrolopyrrole (PyDPP) and isoindigo (IID) units. ThernPBDTTS-FTAZ:PIID-PyDPP all-PSC exhibited a more complementary absorption spectrumrnresulting in an encouraging PCE of 4.2% with a very high Voc of 1.07 V due to its low energyrnloss of 0.62 eV. A stability study of this all-PSC revealed that there was no obvious reduction inrnPCE and more than 90% of the initial PCE was retained after two weeks.rnInspired by the results of fluorinated benzotriazole-containing high bandgap polymers, laddertypernhigh bandgap polymers (PIDTT-FTAZ-p, PIDTT-FTAZ-m and PIDT-FTAZ-m) werernsynthesized using indacenodithieno[3,2-b]thiophene (IDTT) and indacenodithiophene (IDT) asrndonors and fluorinated benzotriazole as acceptor and used as electron-donating materials in BHJrnPSCs. A high Voc of 0.95 V was obtained from the meta-substituted IDT polymer PIDT-FTAZm-rnbased PSC. The best PCE recorded was 6.4% for the device made from para-substitutedrnIDTT-based polymer PIDTT-FTAZ-p and PC71BM.Ladder-type high bandgap polymers based on bithiazole and IDTT with (P29) and without (P28)rnthiophene π-spacer were also developed for PSCs. Both polymers showed a similar bandgap (2.0rneV), but the polymer with thiophene π-spacer (P29) exhibited a higher molecular weight.rnAlthough P28-based PSC devices showed higher Vocs due to the deeper HOMO energy level ofrnthe polymer, P29 showed a higher PCE of 4.5% with enhanced Jsc of 10.4 mA cm−2 and FF ofrn0.52.rnBHJ solar cells based on photoactive materials need to have broad absorption to harvest morernphoton flux and thus maximize the photocurrent and PCE. Random terpolymer and ternary blendrnsystems have been used to extend the light absorption of the active layer. In this regard, wernsynthesized random terpolymers from the electron-rich unit thiophene and electron-deficientrnmoieties quinoxaline and isoindigo. PSCs fabricated from these terpolymers were compared withrnthose fabricated from the ternary blends of two alternating polymers to explore the best strategyrnfor extending the light absorption range. The two approaches showed similar Vocs but differentrnJscs. The terpolymer strategy broadened the light absorption range and provided a high PCE ofrn5.8%. This is due to a high Jsc and high hole mobility. The device fabricated from the ternaryrnblend exhibited a lower PCE (3.5%) compared to those fabricated from the terpolymers andrnalternating polymer blends due to the morphological incompatibility of the donor polymers.rnA series of new D-A1-D-A2 random terpolymers were prepared for application in PSCs, in whichrnfluorinated benzotriazole and thienothiophene-capped diketopyrrolopyrrole (TTDPP) were usedrnas electron-accepting moieties and thienyl-substituted benzodithiophene as the electron-donatingrnmoiety. The study revealed that varying the ratio of FTAZ/TTDPP significantly affectsrnabsorption and energy levels of the copolymer. The terpolymer with high content of the strongrnelectron withdrawing units showed strong and broad absorptions between 300–900 nm. The bestrnPCE of 5.7% with Jsc of 15.70 mA cm−2 was recorded from the device fabricated from thernterpolymer with the highest content of strong acceptor unit (TTDPP) and PC71BM.rnIn order to study the effect of donor units on the optoelectronic properties of polymers, D-A typernpolymers based on triazolopyridine acceptor and BDT and IDT donors were designed andrnsynthesized. The optical and electrochemical properties of the polymers were tuned by varyingrnthe donor units. Regioregular and regiorandom polymers based on triazolopyridine were alsornprepared. In contrast to the regiorandom polymer, the regioregular polymer exhibited relativelyrnbroader absorption and deeper HOMO energy level.rnTwo D-A alternating polymers (PIDT-TPD, PIDT-2TPD) based on bithienopyrrolodionern(2TPD) and thieno[3,4-c]pyrrole-4,6-dione (TPD) as acceptor units and IDT as donor unit wererndesigned and prepared for use in optoelectronic devices. In contrast to PIDT-TPD, PIDT-2TPDrnshowed red-shifted absorption spectra and deeper HOMO and LUMO energy levels. High Vocsrnaround 1 V were recorded for PSCs fabricated from these polymers.rnPIDT-2TPD and PIDT-TPD were used as host polymers to fabricate polymer light-emittingrndiodes (PLEDs). PLEDs based on PIDT-2TPD and donor-acceptor donor type NIR-emittingrnunits showed the best performance in the NIR with external quantum efficiency up to 1.16% withrnturn-on voltage of 1.7 V.rnPIDT-2TPD was also used as non-fullerene acceptor to fabricate all-polymer photodetectorsrnwith the donor P3HT. The P3HT:PIDT-2TPD blend exhibited spectral response from 300 nmrnup to the NIR. External quantum efficiency of 34% and high detectivity of 1.3×1012 Jones wasrnrecorded from P3HT:PIDT-2TPD blend BHJ all-polymer photodetectors.rnD–A low bandgap polymers based on unsubstituted benzodithiophene and isoindigo units werernalso designed and synthesized. The polymer P39 with more branched side chains, both on thernthiophene and isoindigo units, showed high molecular weight and deep HOMO energy level. ThernPSC based on P39:PC71BM showed a high Voc of 0.91 V.

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