Synthesis And Photovoltaic Properties Of Fused Heterocyclic Polymer Donor/acceptor Materials

Improving power conversion efficiency(PCE) of organic photovoltaic devices(OPV) has been considered as the essential way to realize its potential advantages(e.g., low cost, light weigt, flexible and large-area processing). The key to high-efficieny OPV is novel top-performing polymer donor/acceptor materials with low-band gap, sutible electronic energy levels and high charge carrier mobility. Concerning on fundamental research on molecular design and synthesis of donor-acceptor(D-A) copolymer, spectral engineering of D-A copolymer, interface engineering and exploration of structure-property, etc., we developed novel angular shaped benzo[2,1-b: 3,4-b’]-diselenophene based polymer donor materials, and synthesized multifuesd perylene-3,4,9,10-tetracarboxylic tetra(2-hexyldecyl)ester(PTTE) based conjugated polymer PPTTE-Tert with high LUMO level as polymer acceptor material to attain high open circuit voltage of polymer/polymer solar cell. The detailed results are as followings:1. A novel angular shaped monomeric donor of 4,5-bis(2-ethylhexyloxy)benzo[2,1-b:3,4-b’]diselenophene(BDSe) was synthesized and exploited as the “donor” moiety to copolymerize with 4,7-bis(4-ethylhexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole(DTBT) for constructing “donor-acceptor”(D-A) polymer PBDSe-DTBT. The selenium-substituted polymer displayed both a low optical band gap of 1.71 eV and a deep HOMO level of-5.37 eV. With 1% DIO as film processing additive, bulk heterojunction solar cells based on PBDSe-DTBT: [6,6]-phenyl-C71-butyric acid methyl ester(PC71BM) provided a promising average PCE of 5.6%, with an open circuit voltage(Voc) of 0.80 V, a short circuit current(Jsc) of 12.30 mA/cm2 and a fill factor(FF) of 0.57. To the best of our knowledge, this is the highest PCE value among all reported BHJ solar cells based on conjugated polymers containing angular-shaped benzodichalcogenophene derivatives.2. The intrinsic “transmission window” between the high- and low-energy absorption bands(so-called dual-band absorption) of the D-A copolymer restricts more light-harvesting because of weak absorption in this visible region. As reported in this chapter, three novel alternating benzo[2,1-b:3,4-b’]diselenophene(BDSe)-oligothiophene(nT, n=1, 2, 3)-benzo[1,2,5]thiadiazole(BT) copolymers, namely PBDSe-1T-BT, PBDSe-2T-BT and PBDSe-3T-BT, were synthesized. Our work demonstrated changing the number of electron-rich thienyl spacer linking the BDSe donor and BT acceptor served to simply modulate the “transmission window” in the 400-540 nm region. In particular, each addition of electron-rich thiophene spacer into conjugated backbone provides significant bathochromic-shift of the high energy transition and evolvement of the relative intensities of both dual-band peaks towards balancing each other, which leads to the gradually merging of the two absorption bands. Hence, PBDSe-3T-BT exhibits more continuous absorption in the 450 nm-600 nm compared to that of PBDSe-1T-BT and PBDSe-2T-BT. The effects of the thienyl spacer on energy levels, molecular packing and device performance are also investigated. In particular, the optimized PCE of PBDSe-1T-BT, PBDSe-2T-BT and PBDSe-3T-BT based devices were 1.51%, 3.95% and 3.45%, respectively. The difference of device performance is not only related to the modulation of “transmission window” of the dual-band absorption spectra, but also the morphology and carrier mobility of blend films.3. On the basis of modulation of “transmission window” of dual-band absorption spectra, broadening absorption spectra of D-A polymer benefits more light-harvesting, thus possiblily resulting in further improvement of Jsc. In this chapter, we introduced more electron-deficient unit thiadiazolo[3,4-c]pyridine as acceptor unit, and synthesized two novel more narrow low-band-gap D-A copolymers PBDSe-DTPyT and PBDSe-DT2 PyT. PBDSe-DTPyT and PBDSe-DT2 PyT displayed optical band gap of 1.58 eV and 1.53 eV, respectively. Compared to benzo diselenophene-benzo thiadiazole based polymers, both of these polymers exhibited higher Jsc of 12.38 mA/cm2 amd 12.53 mA/cm2, whereas the corresponding devices exhibited broader EQE response(~800 nm). The optimized PCE of PBDSe-DTPyT and PBDSe-DT2 PyT based devices were 4.04% and 4.50%, respectively.4. Besides of fine molecule tailoring of BDSe based copolymer for regulating photon absorption, we further investigated the influence of interfacial modification of PBDSe-DT2PyT: PC71BM/electrode on device performance. In this chapter, a novel low-temperature solution-processed hydrous V2O5·nH2O made from mass-production, highly stable vanadium pentoxide gel through molten-quenching method was utilized to replace PEDOT: PSS as hole-transporting layer(HTL). Encouragingly, the device with V2O5·nH2O displayed simultaneous increase in Voc(0.72 V vs. 0.67 V), Jsc(13.96 mA/cm2 vs. 12.53 mA/cm2) and FF(0.59 vs. 0.54), leading to PCE up to 5.87%. The 30% enhancement in PCE has been afforded with multiple origins including reduced dark current Js and leakge current, improved diode ideality factor n, increased shunt resistance Rsh, reduced carrier recombination and significant enhanced light harvesting of device in wavelength ranged from 400 nm to 550 nm. 5. A new conjugated alternating copolymer, poly([perylene-3,4,9,10-tetracarboxylictetra(2-hexyldecyl)ester-1,7-diyl](PTTE)-alt-5,5’’-(2,2’:5’,2’’-terthiophene))(PPTTE-TerT), was synthesized via Stille coupling reaction and characterized as an electron acceptor. The replacement of perylene diimides(PDI) by PTTE unit raised the LUMO energy level of PPTTE-TerT up to-3.54 eV, which would lead to higher Voc. Polymer/polymer solar cells based on RR-P3HT: PPTTE-TerT at optimized donor-acceptor weight ratio of 1:0.7 achieved the best power conversion efficiency of 0.76%, with a Voc of 0.83 V, a Jsc of 2.38 mA/cm2 and a FF of 0.34. The Voc value is about 0.3 V higher than that of the polymer/polymer solar cell based on blending RR-P3 HT with the acceptor copolymer comprised of alternating perylene diimides(PDI)-terthiophene units. These results indicate that increasing the LUMO level of polymer acceptor is an efficient approach to improve open circuit voltage.