The Design,Synthesis And Photovoltaic Properties Of D-A Conjugated Copolymers Based On Thiophene-Fused Benzo-heteroaromatic Ring

As a typical of clean energy,polymer solar cells(PSCs)have attracted wide attention because of their relatively low-cost,light weight,and large area flexible devices with high-throughput roll-to-roll fabrication,in which the electron donor material and the electron acceptor material for active layer used in PSCs devices is the key factor to affect the photovoltaic performances of PSCs devices.In recent five years,the rapid development of small molecular receptor materials has made great breakthroughs in photoelectric conversion efficiency(PCE)of PSCs.Comparatively,the development of organic and polymer donor materials has been very slow in recent years,and the excellent performance of donor materials is still very limited.Therefore,it is important to design and synthesize more polymer donor materials with excellent comprehensive performance and explore effective synthetic strategies for the further development and application of organic solar cells(OSCs).Based on this,a series of D-A copolymers based on thiophene-fused benzo-aromatic heterocycles have been designed and synthesized via the synthetic strategy of the“Fusion of Aromatic Heterocyclic Ring"to stabilize the quinone structure of the conjugated main chain.The structure,photophysical,electrochemical properties and device performances of these copolymers were characterized,and the impact of structural changes of polymer donor materials on the opto-electronic properties and the performances of corresponding PSCs devices was systematically discussed.The main content of this thesis is divided into the following seven chapters:The chapter 1 is the introduction,which briefly introduces the different types of solar cells as well as their advantages and disadvantages,and then mainly focuses on the development of organic solar cells(OSCs),working principles,and the electron-acceptor material and the electron-donor material for active layer materials widely used in OSCs recently.Finally,the molecular design ideas and research contents of this thesis are presented.In Chapter 2,a new and simpler synthetic route for the thiophene-fused benzotriazole(BTAZT)unit containing an ester group has been designed.And then BTAZT as an acceptor has been applied to construct a series of D-A copolymers with FBDTT unit as a donor(P1-P3).In the molecular design of polymers,two different alkyl chains were introduced into BTAZT and FBDTT units,respectively,to regulate the crystallinity,intermolecular interaction and aggregation morphology of conjugated main chains.The influence of different alkyl chains on the photophysical and electrochemical properties of polymers and the performance of polymer solar cell devices were investigated and discussed.The results show that the polymer(P1)containing the same alkyl chain with smaller steric hindrance both at the acceptor and donor units exhibits more regular main-chain structure and better crystallization.The blended film with ITIC shows strong interaction and ordered intermolecular chain aggregation,which effectively improves the charge mobility of the active layer blended film and increases the current density of the PSCs device.Therefore,PSCs based on P1and ITIC blended film have the highest J_SC and FF,thus leading to the highest photoelectric conversion efficiency(PCE).In Chapter 3,the structure of D-A copolymer synthesized in Chapter 2 has been modified so as to match well with ITIC acceptor materials.Considering the strong electron-pulling ability of ester group,a fluorine substituted thiophene-fused benzotriazole(f BTAZT)unit was synthesized by introducing a fluorine atom with relatively weak electron-pulling ability to replace the ester group.The corresponding D-A copolymers(P4 and P5)were synthesized by combining f BTAZT with BDTT and FBDTT donor units,respectively.The photovoltaic properties of the PSCs were evaluated with P4 or P5 as a donor material and ITIC as an acceptor material.The impact of fluorine atoms introduced into BTAZT or BDTT units on the photophysical and electrochemical properties of the polymers as well as the photovoltaic performances of the PSCs were investigated.The results indicated that coexistence of fluorine atoms both in BTAZT and BDTT units resulted in relatively low HOMO energy level of the corresponding polymer,thus leading to a relatively high open-circuit voltage and photoelectric conversion efficiency.In Chapter 4,in order to further improve the open-circuit voltage of PSCs,two D-A copolymers(P6 and P7)were constructed based on BTAZT acceptor unit with two different electron-withdrawing groups(ester group and fluorine atom)and FBDTT donor unit with an alkylthiol group for PSCs.It is found that P6 with a long alkyl chain ester group shows a more redshifted intramolecular charge transfer absorption,and also exhibits a better complementary absorption spectrum with a small molecular acceptor IHIC than P7.Moreover,P6:IHIC blended film shows higher and more balanced hole/electron charge mobilities as well as better morphologies with more suitable phase separation scale for efficient charge separation and transport than P7:IHIC blended film.Thus,the P6:IHIC based PSC device exhibits the higher power conversion efficiency of 8.35%with higher short-circuit current density(J_SC=16.77m A cm^-2)and fill factor(FF=64.89%)compared with the P7:IHIC based PSC device,indicating that the different electron-withdrawing side groups at BTAZT unit exerts a significant influence on the properties of the polymers as well as the photovoltaic performances of the PSCs.In Chapter 5,based on the previous work in Chapter 2 and 3,two D-A copolymers(P8 and P9)containing the intramolecular"F…S noncovalent bond conformational lock"have been constructed with BDTT unit containing two F atoms at?-sites of thiophene ring to combine an ester substituted BTAZT or a F atom substituted BTAZT,respectively.The influence on the photophysical properties of polymers and the photovoltaic performances of corresponding PSCs was investigated by the introduction of multiple fluorine atoms in the conjugate backbone and the intramolecular"F…S noncovalent bond conformational lock",and the photovoltaic performances of PSCs devices constructed with ITIC acceptor material was evaluated.It is found that the"noncovalent bond conformation lock"is not much stable in solution.Thus,the effect of electron-withdrawing of F atom weakens the ICT effect and makes the absorption blue shift.However,the strong intermolecular aggregation in the thin film results in the hindrance of single bond rotation and strengthens the intramolecular"noncovalent bond conformation lock"effect.By stabilizing the conjugated main chain conformation,the ICT effect is enhanced and the absorption spectrum is significantly red-shifted,which effectively improves the charge mobolity and current density of PSCs.At the same time,the HOMO level of conjugated main chain can also be reduced by introducing multiple fluorine atoms to enhance the voltage of OSCs.device.At the same time,the stability of quinoid structure for conjugated mainchain can also influence the photophysical properties of polymers and the photovoltaic performances of corresponding PSCs.Therefore,the performance of polymer PSCs devices is ultimately affected by the above factors.Among them,the device based on P9 exhibits higher comprehensive performance than that based on P8.Its maximum PCE reaches 7.75%,while is much higher than that of P8 based device with PCE of 6.05%.In chapter 6,in order to broaden the absorption range of D-A polymers in visible light and further improve the short-circuit current density of the PSCs,four D-A₁-D-A₂terpolymer(P10-P13)were constructed by using BTAZT or f BTAZT and BTAZ-2F as co-acceptors to combine with BDTT or FBDTT unit,respectively.Their photophysical and electrochemical properties were studied and compared.Then,the corresponding PSCs devices were fabricated based on the polymer as a donor material and ITIC as an accepting material,and their performances were evaluated.Also,the impact of fluorine atoms introduced into BTAZT or BDTT units on the photophysical and electrochemical properties of the terpolymers and the photovoltaic performances of the PSCs were detailed discussed.The results showed that the introduction of fluorine atoms into the BTAZT acceptor unit causes a small blue-shift in the absorption spectra and widens the bandgap of the polymer,while the introduction of fluorine atoms into the BDTT donor unit has little effect on the optical bandgap and spectral absorption of the ternary polymers.Among them,the PSCs based on P10 without F atom on BTAZT and BDTT units has the lowest V_OC and FF,while P11 and P13 containing a F atom on BTAZT show relatively higher J_SC and FF,respectively.Meanwhile,the introduction of fluorine atoms both into BTAZT and BDTT units shows the best overall performance of PSCs device for corresponding polymer(P13)with the highest PCE value reaching 7.31%.In Chapter 7,in order to well match the absorption spectra of the classical narrow bandgap small molecule acceptor material(ITIC),an ester group substituted thiophene fused benzothiadiazole(BTT)was firstly decarboxylated and then fluorinated to obtain two corresponding thiophene fused benzothiadiazole unit(BTT and f BTT),and then they have been acted as a acceptor unit to construct the corresponding D-A copolymers with FBDTT(P14 and P15),in which hexyl group was introduced into a thiophene bridge to ensure good solubility of polymers.Their photophysical properties and thermal stability were characterized.The corresponding PSCs devices were fabricated using polymer as donor material and IHIC as acceptor material and their performance was evaluated.The impact of introducing fluorine atoms into thiophene ring on the photophysical and electrochemical properties of the polymers as well as the photovoltaic performance of the devices were detailed investigated.It is found that that the presence of F atom on BTT results in blue shift of the absorption spectrum of P15with poorer absorption complementarity to the acceptor material IHIC in the range of650-800 nm than that of P14,which leads to the decrease of J_SC,while the P15 has a lower HOMO level of P14 that is favorable for the enhancement of the V_OC of the corresponding PSCs.Finally,the PCE of PSCs device based on P14 showed higher PCE of 7.43%than that based on P15(PCE=6.41%).