Synthesis And Photoelectric Properties Of Materials Based On 4,8-Bis(thienyl)-benzo[1,2-b:4,5-b′]dithiophene Derivatives

Small molecule organic semiconductors for use in organic solar cells(OSCs) have been attracted broad attentions because of their ease of synthesis, high purity, definite molecular structure, low batch-to-batch variation, and good reproducibility in device performance. Ideal donor material should have a narrow band gap and a deep HOMO energy level, which are crucial for achieving high JSC and VOC, respectively. To date, the power conversion efficiency of near 10% for small molecule OSCs have been achieved, which shows promising application prospect. Therefore, it will provide important guidance for further development of highly efficient organic small molecule semiconductor materials to explore the relationship between molecular structure and properties of semiconductor materials.To deeply understand the structure-property relationship of organic conjugated molecules based on 4,8-bis(5-alkylthiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene(TBDT) as core, we synthesized two series of symmetrical A-π-D-π-A structure organic small molecules with oligothiophene units as π-bridge 2-cyano-3-octyloxy-3-oxo-1-propenyl(COOP) and 2,2-dicyano-1-propenyl(DCV) as terminal acceptor units. We completely studied the influence of the length of the conjugated oligothiophene units on their optical, electrochemical, and photovoltaic properties. The structure-property relationship is summarized and the results are as follows:1. We synthesized four A-π-D-π-A structure organic small molecules(COOP-n T-TBDT) based on TBDT as building block, COOP as end-capped unit, and extended by monothiophene, bithiophene, terthiophene, and quathiophene π-bridges, respectively. The structures of the molecules were characterized by means of 1H NMR, 13 C NMR and MS. We examined their optical properties by ultraviolet- visible absorption spectra and results showed that COOP-n T-TBDT has broad absoption over 350- 700 nm. Interestingly, the optical band gaps changed little(~ 1.8-1.9 e V) with the increase of the conjugated chain length. Then, their electrochemical properties were measured by cyclic voltammetry and the HOMO and LUMO energy levels were calculated. According to our resrarch, the four molecules possess relatively low HOMO levels(-5.4--5.6 e V), which was enhanced with the increase of the number of thiophene unit. Finally, the OSCs based on the four small molecules as donor and PC61 BM as acceptor were fabricated and their photovoltaic properties have been investigated. Results indicate that their photovoltaic properties were promoted with the extension of the thiophene π-bridges. The OSCs based on COOP-3T-TBDT:PC61BM(1:0.4, w/w) and COOP-4T-TBDT:PC61BM(1:0.4, w/w) achevied the PCEs of 4.73% and 5.62%, respectively.2. By changing the terminal acceptor moity, a series of four A-π-D-π-A structure organic small molecules(DCV-n T-TBDT) based on 2,2-dicyano-1-propenyl(DCV) as end-capped unit, TBDT as central block, and flanked by different oligothiophene units. The structures of the compounds were characterized by means of 1H NMR, 13 C NMR and MS. We completely examined their optical and electrochemical properties by ultraviolet-visible absorption spectra and cyclic voltammetry, respectively. The devices based on the four small molecules as donor and PC61 BM as acceptor were fabricated and their photovoltaic properties have been investigated. It turned out that DCV-n T-TBDT also has broad absoption over 350 – 750 nm, which is similar to COOP-n T-TBDT. Their band gaps((~ 1.8 e V) had no obvious change with the increase of the number of thiophene unit. In addition, the low-lying HOMO levels of the compounds is benefical for obtaining high Voc. Surpringly, different from COOP-n T-TBDT, the photovoltaic properties of DCV-n T-TBDT shows little dependence on the molecular structure, which may result from the difference of molecular packing with the introduction of DCV unit.3. The difference between the DCV unit and COOP unit on the molecular optical, electrochemical, and photovoltaic properties have been discussed in details. The results showd that the DCV-ended compounds exhibited longer absorption wavelength and narrower band gaps. Besides, the DCV-ended molecules have relarively lower HOMO energy levels. Since terminated with different units, the two series compounds differed in the photovoltaic performace. The power conversion efficiency of the DCV-ended compounds didin’t improve with the extension of the thiophene π-bridges, among which, the optimized DCV-1T-TBDT and PC61 BM blended solar cell achieved a highest performance with a PCE of 4.48%, a high Voc of 0.93 V, a Jsc of 8.54 m A cm-2, and a FF of 0.56. Neverthless, the OSCs based on COOP-4T-TBDT:PC61BM achieved the highest PCE of 5.62% with a high Voc of 0.93 V, a Jsc of 9.60 m A cm-2, and a FF of 0.63.