Molecular Design, Synthesis And Photovoltaic Performance Of Solution Processible Organic Small Molecules With Donor-acceptor Structure

Due to the growing global energy crisis, the exhaustion of the traditionalresources and alternative energy sources not yet forming, mankind is faced with adilemma of the development or survival. Therefore using most abundant solar energyto replace fossil energy becomes a very promising solution. To date, organic solar cellsgradually become a research focus, due to the advantages like production of low-cost,simple preparation, flexibility and the preparation of large area, compared withinorganic solar cells. Meanwhile, relative to the polymer material, small molecules fororganic solar cells receive more and more researchers’ attention, due to its advantage offixed structures and molecular weights, high purity.In this thesis, we reviewed the development of small molecules for the activelayers of bulk heterojunction organic solar cells, the structures and principle of bulkheterojunction organic solar cell devices, and designed and synthesized a series ofsolution processable small molecule material with donor–acceptor structure. By theadjustment of molecular structure, we achieve a series of small molecule materialswith designed properties like energy levels and absorption spectra. And we designednew pendant small molecular structure, therefore, we obtained a series of solutionprocessable small molecule organic solar cell materials. The detailed results ofresearch were described as follow:1. Utilizing donor-acceptor (D-A) structure, we have been synthesized two smallmolecules, BvT-DCN and BT-C6, based on the vinylene-linkedbenzothiadiazole-thiophene (BvT) moiety via Heck coupling and Knoevenagelcondensation reaction. In addition to the base moiety, BvT-DCN and BT-C6has twoelectron-accepting groups, malononitrile and2-(3,5,5-trimethylcyclohex-2-en-1-ylidene)malononitrile groups, to further increasetune the energy levels to the appropriate region. Both molecules showed goodsolubility, and broad absorption spectra due to the existence of the intramolecular charge transfer. And the molecules showed red shift absorption and low-lying energylevels because of the increasing ability of the electron-accepting groups. The two smallmolecules were used as donor materials in solar cells with PCBM as the acceptor andthe photophysical and morphological properties of the photoactive layers have beeninvestigated. Under the illumination of AM1.5,100mW/cm2, the device based onsmall molecules showed different PCE because of their different morphology:BvT-DCN0.07%, BT-C60.55%.2. Following the design ideas of the second chapter, we designed andsynthesized two linear molucules, BT-CNOC8and BT-DCN, usingbenzothiadiazole-thiophene (BT) moiety and donor-acceptor (D-A) structure. And wedesigned a pendant type small molecule utilizing the three arm-shaped structure oftriphenylamine group. We chose the appropriate electron-withdrawing group to adjustthe absorption spectra and energy levels of these three molecules. Keeping the sameabsorption spectra and energy levels, we could discuss how the different molecularstructures impacted on the performance of photovoltaic devices. These smallmolecules were used as donor materials in solar cells with PCBM as the acceptor andthe photophysical and morphological properties of the photoactive layers have beeninvestigated. Under the illumination of AM1.5,100mW/cm2, the device based onsmall molecules showed different PCE because of their different molecule strutures:TPA-TBT1.13%.3. Based on the pendant molecular structure, we designed and synthesized twopendant molecular TPA-BT-C8and TPA-3Th. We could compare them with TPA-TBT,and study how the conjugate lengths and structures of molecules impact onphotovoltaic performance. From the absorption spectra, we found that the absorptionpeaks went red-shift by increasing the conjugate length, and the donor-acceptorstructure can increase the absorption property. Because of the excellent absorptionproperty to spectrum, Jsc of the photovoltaic device based on TPA-BT-C8is twice asmuch as that of TPA-3Th. Therefore, PCE of the photovoltaic device based onTPA-BT-C8is also twice as much as that of TPA-3Th.4. Two non-fullerene small molecules, BT-C6and BT-C12, based on thevinylene-linked benzothiadiazole-thiophene (BT) moiety flanked with 2-(3,5,5-trimethylcyclohex-2-en-1-ylidene)malononitrile have been synthesized viaHeck coupling and Knoevenagel condensation reaction. The two molecules showedgood thermal stability, intense absorption bands that covered a wide range from300to700nm, which attributed to the intramolecular charge transfer between electron richmoieties and the electron withdrawing moieties, and low optical bandgaps of~1.60eV.The lowest unoccupied molecular orbital (LUMO) levels of both molecules wererelatively higher than that of [6,6]-phenyl C61butyric acid methyl ester (PCBM),which promised high open circuit voltage (Voc) for photovoltaic application. Bulkheterojunction (BHJ) solar cells using poly(3-hexylthiophene)(P3HT) as the electrondonor and the two molecules as the acceptors were fabricated. Under the100mW/cm2AM1.5illumination, the devices based on P3HT: BT-C6(1:1, w: w) showed thepower conversion efficiency (PCE) of0.67%, Jscof1.63mA/cm2, Vocof0.74V, andFF of0.56.