Novel Solution Processible Organic Small Molecules: Designm, Synthesis And Photovoltaic Performance

Bulk-Heterojunction organic solar cells are evolving into a promising cost-effective alternative to the silicon-based solar cells due to their low-cost fabrication through solution processing, light weight, as well as excellent compatibility with flexible substrates etc. In contrast to polymers, small molecules have attracted more and more attentions due to their high purity, high charge carrier mobilities, well-defined molecular structures and definite molecular weights.In this thesis, we discussed principle, structure, development state of bulk-heterojunction organic small molecule solar cells. Then we designed and synthesized a series of solution processible donor-acceptor small molecules as donor materials. We modulated the band gap, energy level, surface morphology, carrier mobility, and photovoltaic properties of the synthesized small molecules by changing the conjugated length and structures. The detailed results of research were described as follows:1. We successfully synthesized a series of symmetrical solution processable small molecules (APPM, AAPM and ATPM) consisting of the electron-accepting moiety (2-pyran-4-ylidenemalononitrile) (PM) and the electron-donating moiety (triphenylamine) linked by different electron-donating moieties (phenothiazine, triphenylamine and thiophene) through a Suzuki coupling reaction. Research results showed that these small molecules possess good solubility, broad absorption and relatively high hole mobility. The open circuit voltage and power conversion efficiencies of 0.80V,0.90V,1.00V, and 0.65%,0.94%,1.31% were achieved for the photovoltaic devices based on APPM/PCBM, AAPM/PCBM and ATPM/PCBM, respectively. The results indicate that it is an effect way to enhance the open circuit voltage and improve the power conversion efficiencies of photovoltaic devices by adjusting the bridge moieties in donor materials.2. We successfully synthesized a series of novel solution processible small molecules (2TAPM,4TAPM and 2BTAPM) consisting of electron-accepting unit (2-pyran-4-ylidenemalononitrile) (PM) and electron-donating unit (Triphenylamine and different thiophene units). Differential scanning calorimetry (DSC) measurement indicates that these small molecules are amorphous. UV-vis absorption spectra show that the combination of PM with moieties having gradually increased electron-donating ability results in an enhanced intramolecular charge transfer (ICT) transition, leading to an extension of the absorption spectral range and a reduction of the band gap of the molecules. Power conversion efficiencies of 1.76% and 2.47% were achieved for the photovoltaic devices based on 2TAPM and 4TAPM respectively, which indicated that it is an effect method for enhancing photovoltaic performance by adjusting molecule structure and thus improving the surface morphology of active layer.3. We have synthesized a series of small molecules with 2-pyran-4-ylidenemalononitrile (PM) as the electron-accepting unit and oligothiophene with different numbers as the electron-donating unit. These small molecules possess good solubility, strong crystallinity, broad absorption range and low band gap properties. Cyclic voltammetry investigation displayed that the highest occupied molecular orbital (HOMO) energy levels of the three molecules rise with the increase of thiophene units. Space charge limited current (SCLC) method reveals that the hole mobility increased with the increase of conjugated chain. Finally, the surface morphology was fine-tuned and the photovoltaic performance was enhanced with the increase of thiophene units (0.03%-1.15%)4. A series of solution processable small molecules based on PM and oligothiophene with different thiophene units were synthesized according to the well-known palladium-catalyzed Suzuki and Still coupling reaction. Furthermore, different length of alkyl chains were introduced for investigating its influences on the properties and photovoltaic performance of these small molecules. UV-vis absorption spectra showed that the optical density was enhanced according to reducing the length of alkyl chains. The photovoltaic device investigations displayed that the surface morphology was improved and thus the photovoltaic performance enhanced gradually from 1.35% to 1.82% and 2.20%.5. we synthesized a new electron-accepting unit 5-(2,6-Dimethyl-pyran-4-ylidene)-1,3-diethyl-2-thioxo-dihydro-pyrimidine-4,6-dione (PD) based on 2,6-Dimethyl- pyran-4-one and 1,3-Diethyl-2-thioxo-dihydro-pyrimidine-4,6-dione by knoevenagel reaction. Furthermore, we designed and synthesized small molecule 8TPDC8 with oligothiophene as electron-donating unit and PD as electron-accepting unit. UV-vis absorption spectra displayed that 8TPDC8 thin film covered the rather broad absorption spectrum from 300 to 900 nm, which indicated that PD is a promising electron-accepting unit for constructing donor materials with broad absorption spectrum. Photovoltaic devices were fabricated with 8TPDC8 as donor and PCBM as acceptor and a relatively high PCE 1.28% achieved.