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Energy Level Regulation,morphology Control,and Optoelectronic Properties Of Conjugated Polymers

Posted on:2022-04-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:L ZhangFull Text:PDF
GTID:1482306569958729Subject:Materials Physics and Chemistry
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In recent years,organic semiconductors represented by conjugated polymers(or conjugated small molecules)have attracted extensive attention due to their unique merits of flexibility,light weight,and large-area solution printability,especially in the applications of optoelectronic devices such as organic/polymer solar cells(PSCs)and organic field-effect transistors(OFETs).Among them,organic/polymer solar cells,as a renewable solar energy device technology,have achieved many breakthroughs recently,approaching the commercial application step by step.This dissertation focuses on the energy level regulation of conjugated polymers and blending morphology optimization,as well as their optoelectronic devices performances,specifically as follows:In Chapter 2,to mitigate the torsion between naphthalene diimides(NDI)unit and adjacent thiophene unit,and to achieve a higher absorption of NDI polymers,a novel Tz NDI unit was constructed through rational molecular design by bridging NDI units with thiazole,and a series of D-A copolymers were synthesized based on this unit.Firstly,quantum calculations were applied to calculate the optimal conformation and energy levels of Tz NDI-polymers,and then their optical,thermal,and electrochemical properties were compared and analyzed.By evaluating the charge transporting performance of OFETs devices,they all showed unipolar n-type transport characteristics,and the highest electron mobility is 0.57 cm2V-1s-1.The n-type OFETs devices based on Tz NDI-polymer exhibit near-ideal transfer characteristic curves,and can suppress unnecessary hole injection and transport phenomena,indicating their practical application potential.In addition,to adjust the aggregation degree and energy level of N2200 and improve charge transportation in all-polymer solar cells(All-PSCs)devices,a terpolymer strategy was adopted to introduce Tz NDI unit into the main chain of N2200 and their photoelectric performance was evaluated by All-PSCs devices.The terpolymer achieved more effective charge transport and better device efficiency(8.7%)than the reference polymer,indicating that Tz NDI unit is an important n-type conjugated polymer building block,which has application potentials not only in n-type OFETs devices but also in All-PSCs devices.In Chapter 3,to reduce the energy loss of photovoltaic devices and obtain higher open-circuit voltages,thiazoleπ-bridge was introduced into the main chain of donor polymer PBDB-T to lower its energy levels and the orientation effect via adjusting the orientation of thiazole bridges was further studied.Quantum computations were first adopted to calculate the optimal conformation and energy levels of the target polymers and then their optical,thermal,and electrochemical properties were compared and analyzed with the prototype.Photovoltaic performances were evaluated by blending polymers with IT-4F.The introduction of thiazole bridges can significantly reduce the energy loss of photovoltaic devices,and further control of thiazole orientation can achieve a higher electroluminescence efficiency and a lower energy loss.Thiazole polymer achieved improved energy conversion efficiencies(PCEs),which are 10.4%and 9.6%,respectively.This study not only provides an effective strategy to reduce the energy loss of PSCs devices but also reveals that adjusting the orientation of asymmetric units in the conjugated backbone is of importance to optimize the device performance of conjugated polymers.In Chapter 4,to regulate the energy levels,after the previous work on regulating the energy levels from the backbones,the energy levels regulation of conjugated polymers from the side-chain engineering was proposed.4-methylthiophene substituted benzodithiophene(BDT)units were designed and synthesized to develop novel conjugated polymer donors.The relevant photovoltaic performance showed that the open-circuit voltages of the novel polymer donors can be significantly improved in fullerene-based PSCs.It is proved that the introduction of 4-methylthio group on the side chain of BDT thiophene is an effective strategy to adjust the energy level of BDT-based conjugated polymers and to improve Vocvalues of photovoltaic devices,which guides the future development of the devices with high Voc.In Chapter 5,to regulate the energy levels,the strategy of side-chain modifications was still adapted and the side-chain engineering of introducing chlorine atom and alkylthio chain onto the thiophene side chain of BDT unit was proposed.Meanwhile,to optimize the blending morphology of the active layer,the molecular weights of the polymer donors were rationally controlled.The aggregation degree of the polymers and the evolution of the blending morphologies with polymer chain growth were studied,as well as their influences on the performance and stability of non-fullerene photovoltaic devices.When mixed with non-fullerene acceptor Y6,the Jsc and FF of photovoltaic devices increase monotonously with the increase of polymer molecular weights,and the device efficiency increases from 11.2%of P-17k to over 16%of P-53k and P-98k.Then,several morphology-characterization methods were applied to analyze the evolution of blending morphologies,and the key effects of molecular weight on the performances of photovoltaic devices from the perspective of micro-morphology were studied.While by comparing the device stabilities,the P-30k blending film achieved the best stability.The relation between stability differences of the blending morphologies and the miscibility of donor and acceptor was revealed.This study not only reveals the evolution of blending morphology with polymer chain growth but also shows the importance of molecular weight optimization for the realization of high-efficient and stable non-fullerene photovoltaic devices,which provides theoretical guidance for the development of practical PSCs in the future.In Chapter 6,to optimize the blending morphology,and thus achieving a breakthrough in All-PSCs,based on the study in Chapter 5,the strategy of molecular weights tuning to optimize the polymer:polymer blend morphology was still adapted and a better device efficiency was achieved.A series of polymer donors PBDB-T with molecular weights from low to high and polymer acceptor PJ1 were selected to study their All-PSCs performances.Among them,the polymer donor with medium molecular weights(PBDB-TMW)and PJ1achieved improved charge transport performance(FF over 75%)and device efficiency of15.4%,breaking the highest All-PSCs efficiency record at that time.By the further study of blending morphologies,it was found that PBDB-TMW:PJ1 blend tends to form the fiber network phase separation morphology,which benefits charge separation and transport.This study guides the future optimization of blend morphology and the development of high-performance All-PSCs devices to some degree and further narrows the efficiency gap between laboratories and commercial applications of All-PSCs devices.
Keywords/Search Tags:conjugated polymers, polymer solar cells, optoelectronic performance, energy level regulation, morphology optimization
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