Synthesis And Photovoltaic Properties Of Diketopyrrolopyrrole-Based Samll Molecules With Naphthyl Or Fluoronaphthyl As End-groups

Compared to the inorganic semiconductor solar cell, organic bulk heterojunction（BHJ）solar cell has the potantial to enable mass production of low cost, large-area solar energyconversion devices at low temperature, thus becoming a research hotspot in recent years. Asan electron donor in the active layer of a BHJ solar cell, small molecular organicsemiconductor needs to have broad light absorption, high hole mobility, appropriate HOMOand LUMO levels and good solubility favorably for purificatio n and device processing.Diketopyrrolopyrrole （DPP） has many advantages, such as excellent chemical stability, co-planar molecular structure and facile synthesis. Moreover, DPP can reduce theband gap effectively as a result of its strong electron-withdrawing ability.Naphthalene is an aromatic fused ring with planar structure, and the π electrons aredelocalized within the molecule plane. Small molecular donors containing appropriatelymodified naphthalene are expected to get potentially high mobility.The main points of this thesis are to design and synthesize DPP-based small moleculeswith naphthyl and fuoronaphthyl as end groups, and to investigate their photovoltaicperformance as electron donors in the active layer of BHJ solar cell.In chapter II we prepared organic semiconductor materials with tetrathiophene as thecore and naphthylor fuoronaphthylas end groups. The results show that the introductionoffuoronaphthyl greatly increases the hole mobility.Based on the above results, solution-processable donor materials CP9and CP9withtwo dithienylDPP as core and naphthyl or fuoronaphthyl as end groups to widen theabsorption are prepared in Chapter III.CP9is a crystalline solid with a Tmof approximately216C. X-ray diffractionexperiments reveal that thermal annealing increases crystallinity of the as-cast film, thusbeneficial to the absorption and charge-transport properties. CP9exhibits two reversibleone-electron oxidation waves at0.86and1.16V vs. Ag/AgCl reference electrode （and0.39and0.69V vs EFc+/Fc）, respectively. Fitting the space-charge-limited current characteristics in a hole-only device results in a hole mobility of².7×10^-4cm²V^-1s^-1at low-voltagesfor CP9. A preliminary characterization of the solar cell（ITO/PEDOT:PSS/CP9:PC61BM（solution processed）/Al） yields a power conversionefficiency of approximately3.0%under simulated AM1.5G illumination (66.4and100mW cm^-2, respectively). The fluorine effects on material properties such as morphology,absorption, electrochemistry, charge transport and the resulting device performance arediscussed.Due to the low solubility of CP8and CP9at room temperature, the compounds togetherwith PC61BM have been dissolved in CHCl3by constant heating. The films of the blend arethus obtained by spin-casting the hot solution. In this context, we prepared compoundsCP12and CP13by replacing2-ethylhexyl in compounds CP8and CP9with2-butyloctylgroups in order to improve solution processability.CP12and CP13are crystalline solids with a Tmof189and178oC, respectively. Thetwo compounds exhibit two reversible one-electron oxidation waves at⁰.85V and¹.14Vvs Ag/AgCl reference electrode （and0.39and0.68V vs EFc+/Fc）, respectively. Fitting thespace-charge-limited current characteristics in a hole-only device results in a hole mobilityof3.36×10-5cm²V^-1s^-1for CP12and9.76×10-5cm²V^-1s^-1for CP13at low voltage. Thecharacterization of the unoptimized solar cells （ITO/PEDOT:PSS/CP12orCP13:PC61BM/Al） reveals a PCE of3.37%for CP13and3.00%for CP12under simulatedAM1.5G illumination with an intensity of100mW cm^-2.