Design And Photovoltaic Performance Of Wide-Bandgap Conjugated Copolymers Containing Benzotriazole

Synergistic developing of organic photovoltaic materials and devices has boosted the highest calibrated power conversion efficiency to exceed 12%.The significant progress in long-term stability,eco-friendly-solvent processing,and thick-film potential makes organic photovoltaics(OPVs)a promising candidate for commercial application.However,the OPVs containing low-bandgap polymer donors have reached the bottleneck owning to the weak light-absorption of conventional fullerene-based acceptors.Conversely,wide-bandgap(WBG)polymer donors show vast prospect in tandem,ternary,non-fullerene,and all-polymer solar cells although they exhibit moderate performance in single-junction fullerene systems.Currently,the kinds of highly efficient wide-bandgap polymer donors are very limited.Therefore,in this dissertation we are dedicated to develop novel wide-bandgap polymer donors containing difluoro-benzotriazole(ffBTA)or imide-functioned benzotriazole(TzBI)unit and probe their potential in ternary,thick-film,and eco-friendly-solvent processed devices.In chapter 2,we developed a wide-bandgap polymer donor(PBTA-BO)with excellent solubility by integrating ffBTA with benzodithiophene(BDT)units.Based on BDT-ffBTA skeleton,polar pentafluorobenzene(FPh)units were used as the side-chains to construct a novel donor-acceptor type conjugated polymeric additive(PBTA-FPh).PBTA-FPh was designed to improve the miscibility between PBTA-BO and PC71BM,because polar groups favor higher surface energy of polymers.We demonstrated that the increase of the miscibility enhanced the bulky mobility of blend films.The ternary device incorporating a tiny amount of PBTA-FPh showed a much increased short-circuit current(JSC)and fill factor(FF),thus a power conversion efficiency(PCE)approaching 8%was achieved.This is highest efficiency of ffBTA-based polymers in fullerene systems,indicating the positive effect of miscibility control in ternary devices.Inspired by the idea of ternary blending,we added a low-bandgap non-fulllerene acceptor IFBR into the PBTA-BO:PC6iBM system from the points of complementary absorption and matched energy levels.IFBR extends the absorption in the long-wavelength regime.Moreover,the electronic energy levels of IFBR fall in between those of PBTA-BO and PC6iBM,leading to a cascade alignment and thus expanding the charge-transfer channels.The ternary device based on PBTA-BO:IFBR:PC61BM exhibits a PCE of 8.1%,which is much higher than those for binary devices.These results indicate the great promise of benzotriazole-based WBG polymer donors in non-fullerene solar cells.We thus study the use of WBG donors in non-fullerene systems in chapter 3.The recently-developed WBG donor PTzBI containing TzBI and BDT units was selected to match with non-fullerene acceptor ITIC,and another WBG donor PTzBI-DT based on TzBI and benzodithienothiophene(DT-BDT)units was newly created to explore the role of conjugation extension on the photo-electronic properties,the molecular aggregation characters,and the overall device performance.We demonstrated that extension of the backbone conjugation enhanced the crystallinity of polymers.However,the simultaneously increased domain size of PTzBI-DT:ITIC blend film impeded the charge transfer and transport.We used dibenzylether(DBE)as the solvent additive to optimize the blend film morphology and demonstrated that the polymer crystallinity in blend films increased after DBE treatment while the domain size remained nearly unchanged,contributing to much improved fill factors.The newly-developed PTzBI:ITIC system showed a high PCE of 10.24%and the 300 nm-thick-film devices remained a PCE of 7%,which was among the highest efficiency for thick-film non-fullerene solar cells then.The performance of PTzBI in all-polymer solar cells(all-PSCs)was further investigated in chapter 4.A common polymer acceptor N2200 was selected and the role of N2200 molecular weights and processing solvents on the bulky morphology and the device performance were synergistically studied here.We found that PTzBI pre-aggregated in eco-friendly solvent 2-methyltetrafuran(MeTHF),and the absorbance of PTzBI:N2200 blend film spin-coated from MeTHF was much higher than those processed from other solvents like ortho-di chlorobenzene and chloroform.Furthermore,the blend film based on high-molecular-weight(HMW)N2200 showed much higher absorbance.The morphology analysis revealed that PTzBI:N2200Hw blend processed from MeTHF displayed the finest network phase-separation structure,giving rise to over 9%PCE,which was the highest efficiency then.The results provide a new strategy for fabricating all-PSCs.In chapter 5,we explored the strategies for further improving the photovoltaic performance of all-PSCs by tuning molecular structure of polymers.We introduced the siloxane groups onto the side-chain of WBG polymer and demonstrated that the siloxane groups improved the solubility,increased the face-on dominated stacking,enhanced the molecular crystallinity,and ultimately elevated the hole mobility.The resulting polymer PTzBI-Si was used in MeTHF-processed all-PSCs to achieve an impressive FF of 73.76%and an unprecedented PCE up to 10%,indicative of a new level of all-polymer systems.In chapter 6,we focused on developing high-performance thick-film all-PSCs.We introduced siloxane groups onto the BDT-ffBTA conjugated skeleton to obtain another novel WBG donor PBTA-Si.PBTA-Si and PTzBI-Si were integrated into ternary all-PSCs to exploit the respective advantages.The resulting ternary devices showed a very high FF over 76%within a broad range of ternary blending ratios.The device with optimal ratio accomplished a high PCE of 9.17%even in 350 nm-thick-film devices and remained a PCE of approximate 6%when the photoactive layer is 820 nm,implying the excellent compatibility of this system with the future large-scale production.In chapter 7,we aimed to correlate the chemical structures with photo-electronic properties,bulky morphology,charge carrier dynamics,and open-circuit voltage(VOC)losses.By using Fourier transformation photocurrent spectroscopy(FTPS)and electroluminescence(EL),we were able to study the origins of VOC losses precisely and found that the VOC loss mainly came from energy mismatch,i.e.,the decrease of Shockley-Queisser open-circuit voltage(VOC,SQ)induced by reducing bandgap of materials.Therefore,we conclude that the VOC loss could be further lowered by improving the energy level compatibility through fine-tuning the chemical structures of both donor and acceptor.Meantime,the increase of non-radiative open-circuit voltage loss(?VOC,nr)should be concerned when we lower the bandgap in order to improve the JSC.The resulting optimal material combination P2F-EHp:IT-2F shows a PCE approaching 13%in 0.0516 cm2 device,and maintain 12.1%in large-area(1cm2)cells.It is noteworthy that inverted device based on P2F-EHp:IT-2F has remarkable stability(only 7%decay of PCE)under 1100 h continuous AM 1.5G illumination,demonstrating the great promise for future commercialization.