The Design, Synthesis And Applications Of Novel Low Band-gap Donor Materials For Polymers Solar Cells

With the development of human civilization, the demand of energy is growing fast. As theglobal petrochemical resources are limited, it is difficult to solve such a contradiction betweensupply and demand of human developing. So looking for new, non-polluting, renewableenergy resources have become a worldwide issue. Solar energy received further attention insolving the energy crisis because of its inexhaustible feature. Although the history of humanuses of solar energy can be traced back to ancient times,due to low area density of solarradiation, it accounts only a small proportion of industrial and daily life energy supplyes.Converting the solar energy directly into the electricity at a low cost is an important way forthe effective use of solar energy. Polymer solar cells (PSC) have attracted much attention, dueto its low cost, roll-to-roll printing process, the easy fabrication on flexible, thin and large areasubstrates compared to silicon-based solar cells. Power conversion efficiency of the PSC canexceed10%, but there is still a long way to go for the commercial applications. There are twomajor problems of polymer solar cells, one is the low efficiency and another is the lifetime.Both of them are related to the materials in the PSC active layer. How to design andsynthesize polymers with a narrow band gap, with a broad absorption spectrum and a highcarrier mobility to improve the efficiency, is currently a research priority of the PSC.In this thesis, we have started from the design principle of the narrow band gap polymers.We have designed and synthesized several donor-acceptor (D-A) altinating conjugatedpolymers to obtain narrow band gap polymers. The electron-deficient structure units, such asbenzothiadiazole and naphthothiadiazole, were used as the electron acceptors; theelectron-rich structure units, such as fluorene, carbazole, benzodithiophene and so on, wereused as the electron donors, respectively. Those low band gap polymers were characterizedand have achieved high energy conversion efficiency.In the second chapter, the traditional fluorene-benzothiadiazole copolymer (PFDTBT) hasachieved a high performance. We have optimized this structure by increasing a thiophene unitbetween the fluorene and benzothiadiazole. Alkyl chains were also introduced to improve thesolubility at three positions, named PFO-M1, PFO-M2and PFO-M3respectively. All thespectra of polymers were red-shiftted significantly. PFO-M3have obtained an efficiency of2.6%, which better than PFDTBT.In the third chapter, we have studied the famous electron acceptor-benzothiodiazole, andtried to optimize this structure. Then we designed a new electron acceptor structure–naphthothiadiazole. We have studied the synthesis and reactivity of naphthothiadiazole. Naphthothiodiazole have shown better electron withdrawing ability and a greater molecularplanarity when compared to benzothiodiazole, which makes a greater potential in the designof the polymer solar cell materials. Because the solubility of naphthothiadiazole is not goodenough, which is new challenges of molecular design. We need to balance the molecular planeand solubility, to ensure the solution-processibility for high performance polymer solar cells.In the fourth chapter, we combine the material design concepts of Chapters2and3. Wehave used a bithiophene unit between donors and naphthothiadiazole unit to build D-A typelow band gap polymers. These polymers have shown good device performance whencompared to the similar structure of benzothiodiazole-base polymers. The highest deviceefficiency was close to5%.In the fifth chapter, we have synthesized D-A type conjugate polymers-PBDT-DTNT andPBDT-DTBT, which benzodithiophenewas used as the electron donor, benzothiadiazole andnaphthothiadiazolewere used as the acceptor respectively. Both polymers have comparedthrough spectroscopy, electrochemistry, XRD and mobility tests. Those material analyseshave shown huge advantages of naphthothiadiazole for polymer solar cell materials. The finaldevice performance has verified our designconcepts, and for the device with PBDT-DTNT asdonor phase and PC71BM as an acceptor, an energy conversion efficiency of6%has beenachieved, much higher than the polymer PBDT-DTBT.In the last Chapter, we have used the high-performance electron donor units, such ascyclopentadithiophene, silodithiophene and indencanodithiophene, naphthothiadiazole as theacceptor, to build D-A type low band gap conjugated polymers. These polymers have shownhigh performance in their solar cell devices, which proved naphthothiadiazole is an excellentelectron acceptor unit for PSC donor materials. This indicated a great potential ofnaphthothiadiazole-based polymers for PSC in the future.