Synthesis And High Photovoltaic Performance Of Small Molecule Donor-materials For Organic Solar Cells

In this paper, a series of small molecule are designed and synthesized for application as donor materials in solution-processed small-molecule organic solar cells and the relationship of structure-porperties-dvice performance are investigated.In the second chapter, A family of narrow band gap extended π-conjugated D2-A2-D1-A1-D1-A2-D2 type small molecules based on diketopyrrolopyrrole derivatives as the stronger acceptor core(A1) coupled with indacenodithiophene(IDT; D1) and difluorobenzothiadiazole(A2) are synthesized, and their properties as donor materials in solution-processed small molecule organic solar cells are investigated. After THF vapor annealing and the introduction of two fluorine atoms into benzothiadiazole, the fill factor(FF) and power conversion efficiencies(PCEs) of thise dvices can be increased(FF can reach to 71%). In order to further study the influence of structural design on the performance of the final device, the impacts of replacing the thiophene ring by a more electron-deficient thiazole ring and inserting thiophene spacer between electrondonating(D1) and electron-accepting(A1 and A2) aromatic are investigated. The insertion of thiophene spacer at both sides of IDT can improve the planarity of the molecular skeleton significantly and increase the absrption coefficient between 400-600 nm, which increase the short circuit(Jsc) and fill factor(FF), meanwhile replace thiophene ring by a more electron-deficient thiazole ring will increase the open circuit voltage(Voc). Further study revealed that solvent vapor annealing were useful in improving molecular packing, crystallite size, morphological, which lead to the increase of hole mobilities and the PCE of NDPPFBT. Thus makes DPP based small molecules become another high-efficiency(ca.7%) and solution-processed small-molecule donor material.In the third chapter, we are focus on researching the effect of prolonging the π-conjugated backbone length and increasing the number of fluorine atoms in the molecule on the performance of photovoltaic devices. A series of extended π-conjugated D2-A-(T-D-T-A)n-D2(T represents thiophene; for oligomer, n = 1, 2, 3, 4; for polymer, n ≈ 12) type small molecules based on difluorobenzothiadiazole as electron-deficient unit(A) and indacenodithiophene as electron-rich unit(D), with two n-hexyl-substituted bithiophene as the terminal groups(D2) are synthesized, and their properties as donor materials in solution-processed small molecule organic solar cells are investigated. With the extension of the length of π-conjugated backbone, the absrption coefficient and the 0-0 vibration peaks of UV spectrum of the molecules increase gradually, especially for the polymer. However, EQE test found that the extension of the conjugated chain resulted the decrease of the external quantum efficiency between 450-650 nm, meanwhile because of the strong aggregation effect of polymer its short circuit current density and PCE value are very low. It is surprising that the short circuit current density, fill factor and PCE value of BIT-6F-T are very high regardless of whether annealing treatment or not, especially after thermal annealing and vapor annealing, the filling factor reached 75%, and the photoelectric conversion efficiency reach to 8.66%, which is the highest efficiency of organic solar cell based on IDT.In conclusion, we have studied the effect of molecular structure on the performance of photovoltaic devices, the relationship between molecular packing, crystallite size, the morphology of film and power conversion efficiency. Finally, a series of small molecular donor materials with excellent photovoltaic performance are obtained.