Synthesis And Photovoltaic Performance Of The Conjugated Polymers Based On Thiophenebenzene-thiophene Derivatives As Electron-donating Units

Polymer solar cells(PSCs) based on bulk heterojunction(BHJ) structure ha ve attracted considerable attention due to their potentially low-cost and easy fabrication of solution processing technology. Short-circuit current density(Jsc), fill factor(FF) and open-circuit voltage(Voc) are very important parameters in determining the power conversion efficiency(PCE) of a PSC. In the past decade, many researchers focused on the study how to improve Voc value of a PSC device. Usually, there are effective optimization method s to improve Voc such as applying appropriate interface buffer layers or tandem structure. Surely, engineering the polymer structure is also a useful strategy to improve Voc and PCE of a PSC device.1. Benzo [1, 2-b:4, 5-b′] dithiophene(BDT) has been proved to be one of the most effective electron-donating moieties(D). Usually, the BDT-based polymers exhibit high HOMO(the highest occupied molecular orbital) energy level, which leads to a low Voc. In order to obtain high-performance photovoltaic polymer with low-lying HOMO energy level, thiophene-2,5- bisalkoxybenzene-thiophene(TBT) has been exploited as electron-donating units, and three novel D-A conjugated copolymers PTBTEH-IDHD、PTBTHD-IDEH and PTBTH-IDEH based on isoindigo(ID) as electron-withdrawing units(A) have been designed and synthesized. PTBTEH-IDHD and PTBTHD-IDEH exhibit low-lying HOMO energy level(-5.47 eV), and the corresponding PSC devices based on [6,6]-phenyl-C61-butyric acid methyl ester(PC61BM) showed the Voc values of 0.8 V for PTBTEH-IDHD and 0.88 V for PTBTHD-IDEH, which are larger than that of the PSC based on the similar polymer PBDTID(0.71 V) with BDT derivatives as electron-donating moieties. The PSC device based on PTBTHD-IDEH with the longer alkyl chain in the D moiety possess higher Jsc(6.21 mA cm-2) and Voc(0.88 V) than those of the PSC device based on PTBTEH-IDHD(Jsc=2.39 mA cm-2, Voc=0.8 V) with the longer alkyl chain in the A moiety. In order to improve the solubility and the conjugation length of the polymer, two hexylthiophenes have been introduced to the ID unit, and the corresponding polymer PTBTEH- HTID showed better photovoltaic performance, the Jsc value of the PSC device was improved to 8.87 mA cm-2 with a slightly decrease of the Voc. As a result, the PC Es of the photovoltaic devices based on the three polymers/PC61 BM were 1.05% for PTBTEH-IDHD, 3.02% for PTBTHD-IDEH and 3.71% for PTBTEH-HTID, respectively.2. In order to broaden the absorption spectra, improve the absorptive capacity of the polymers, and enhance the Jsc values of PSCs, diketopyrrolo-[3,4-c]-pyrroles(DPP) has been introduced to the polymers, and a series of terpolymers PHTID1-DPP1, PHTID3-DPP1 and PHTID1-DPP3 with different molar ratio of HTID: DPP 1:1, 3:1 and 1:3 respectively, have been designed and synthesized. The terpolymers PHTID1- DPP1, PHTID3- DPP1 and PHTID1-DPP3 show smaller lamellar distance and π-π stacking distance than that of PTBTEH-HTID. The active layer based on PHTID1-DPP1: PC61 BM possesses higher hole mobility than those based on the other terpolymers, so the corresponding PSCs device showed the highest Jsc value of 11.13 mA cm-2 and PC E of 4.20%.