The Design, Synthesis And Photovoltaic Properties Of Two-dimensional Narrow-band-gap Conjugated Polymers Based On Carbazole

Polymer solar cells (PSCs) have attracted much attention due to their unique advantagesof light weight, low cost, flexible and tunable properties. Although the power conversionefficiency of over10%has been achieved for PSCs recently, it is still necessary to furtherimprove the efficiencies for commercialization. One of the main problems of PSCs is themismatch between the solar terrestrial radiation and the absorption spectra of the donorpolymers. In order to overcome this problem and improve PCE, many researchers havefocused on the synthesis of narrow-band-gap conjugated polymers and have proposed a newmolecular design strategy, the so called two-dimensional narrow-band-gap conjugatedpolymers.The two-dimensional narrow-band-gap conjugated polymers were composed ofmain chains with electron-donating units arranged alternatingly, pendantelectron-withdrawing acceptor groups and π-bridges. For this kind of polymers, their opticalproperties and energy levels could be effectively tuned through judicious selection of donorunits and acceptor groups. In addition, their two-dimensional structures would endow themgood solubility and desired isotropic charge transporting abilities. Furthermore, thepostfunctionalization approach led to the resulting polymers with close degrees ofpolymerization and molecular weight distributions, which were favorable for probing therelationships of the structure and property.Compared with other donor units, carbazole exhibited a series of unique merits toconstruct donor materials for PSCs, such as excellent thermal stability due to the rigidbiphenyl structure, high hole mobility, and low energy lying highest occupied molecularorbital (HOMO) energy level, which could result in high air stability and high open circuitvoltage (Voc). In addition, the–NH group could be modified by alkyl to endow the polymerswith good solubility and processablity.In this thesis, we have combined the excellent optoelectronic properties of carbazoleunits and the merits of two-dimensional narrow-band-gap conjugated polymers to get somepromising donor materials with low bandgaps and deep HOMO energy levels at the sametime. Based on the above theories, we synthesized a series of new two-dimensionalnarrow-band-gap conjugated polymers P27CZSDCN, P27CZSDTA, P36CZSDCN and P36CZSDTA based on2,7-carbazole and3,6-carbazole. The thermal, optical andelectrochemical properties of the resulting polymers were systematically studied. On the onehand, the3,6-carbazole-derived polymers exhibited blue-shifted absorption at the shortwavelength region and lower hole mobility compared to their2,7-carbazole-derived analoguesbecause of the conjugation breaks induced by3,6-carbazole units. On the other hand, the3,6-carbazole-derived polymers exhibited much enhanced and slightly red-shiftedintramolecular charge transfer absorption at the long wavelength region. Meanwhile the2,7-carbazole-derived polymers showed higher hole mobility and slightly lower highestoccupied molecular orbital (HOMO) energy levels relative to their3,6-carbazole-derivedanalogues.The bulk heterojunction solar cell devices based on these polymers as donors andPC71BM as acceptor exhibited the highest power conversion efficiencies of2.47%forP27CZSDTA and1.69%for P36CZSDTA. The primary results indicated that P27CZSDTAwas a promising donor for polymer solar cells.