Design,Synthesis And Application Of Non-fullerene Small Molecule Acceptors For Organic Solar Cells

Bulk heterojunction(BHJ)polymer-small molecule solar cells have been widely studied for their ability to fabricate curved,folded and semi-transparent devices.At present,the power conversion efficiency of single-junction organic solar cells(OSCs)with binary blend of polymer and small molecules has reached up to 18.22%,which makes the commercialization of organic solar cells no longer out of reach.This milestone leap originated from the development of the design concept of fused ring"acceptor-donor-acceptor"(A-D-A).Small molecule acceptors(SMAs)can be symmetrical or asymmetrical.Asymmetric non fullerene SMAs not only have the advantages of symmetrical acceptors such as the diversity of chemical structure and good regulation of photoelectric properties,but also show stronger intermolecular binding energy and larger dipole moment.These unique features could enhance the interaction between molecules and improve OSCs filling factor(FF)and power conversion efficiency(PCE).The molecular skeletons of acceptors can be constructed with fused or non-fused rings.Unfused acceptors(UFAs)refer to cutting the central fused ring into several simple aromatic pieces.UFAs are relatively easy to synthesize and the overall cost is low.The solar radiation energy in the infrared and near-infrared regions(NIR)has reached 50%of the total radiation.Therefore,it is of great significance to design acceptors with the absorption more than 900 nm by the optimization of the molecular structure.Narrow band gap(NBG)acceptors usually show very high short circuit current due to increased radiation absorption in NIR region.According to VOC=(?)E-Eloss,it can be seen that the open circuit voltage(VOC)of a single junction photovoltaic device can’t be too high,thus resulting in the loss of device efficiency.Tandem solar cells can effectively solve this problem.Tandem solar cells can effectively utilize the radiation energy in the UV-visible and near infrared regions by putting a wide band gap solar cell and a narrow band gap solar cell into the same device.In addition,the Voc of the tandem OSCs is the sum of VOCS of sub-cells.The power conversion efficiency can exceed the thermodynamic limit of single junction device.In photovoltaic devices,the chemical structures of acceptors can affect their frontier orbital energy level,absorption spectra,intermolecular stacking patterns and the active layer morphology.Therefore,in order to match the energy level of donor polymers and complement their spectra,it is very important to select aromatic units with appropriate electronic structure and stereochemical structures in constructing the acceptor materials.The first four chapters of this dissertation were to control the structural rigidity,energy level,and absorption spectra of the acceptors via the tuning of central fused-ring skeleton and end groups.A series of fused non-fullerene acceptors were designed and synthesized.The fifth chapter of this dissertation focused on the manipulation of side chains to optimize the intermolecular stacking pattern and crystallinity of the acceptors.The BZT-series non-fused SMAs were designed and synthesized.The influences of acceptors on their optical and electro-chemical properties were investigated.The device performances based on these acceptors were studied with binary and ternary blended single-junction photovoltaic devices and tandem solar cell devices.AFM measurement was carried out to study the morphology of the photosensitive layer and to analyze the correlation between the phase separation and the device performance.The optimization of photovoltaic devices by adjusting the morphology of the photosensitive laye was carried out.During the photoelectric conversion process of solar cells,effective charge extraction and carrier transfer are very important,which directly determines the output efficiency of solar cells.Within the process of carrier transfer,there are various forms of recombination,resulting in the energy loss and inferior photovoltaic efficiency.The recombination mechanism of carriers was studied and the main types of energy loss were clarified.The specific work mainly includes the following parts:1.The central donor unit of A-D-A fused acceptor material is generally a rigid planar aromatic fused ring system,which will enhance π-π stacking between molecules,improve carrier mobility and reduce the non-radiation energy loss.Most high-performance small molecular acceptor materials used benzene group as the central fragment.However,there were few reports with the expansion of phenyl group to naphthalene or anthracene.In chapter 2,we creatively introduced dibenzanthracene(DBAR)unit into the molecular skeleton of A-D-A fused system.Four kinds of non-fullerene acceptor materials DBAR1-4 were designed and synthesized.The single-junction binary solar cells based on DBAR:PCE-10 and DBAR:PBDB-T was prepared.At the same time,the effects of additives and thermal annealing on device performance were studied.The experimental results showed that the VOCS of OSCs devices based on DBAR acceptors were greater than 1.0 V and PCEs were about 2-3%.The reason for the poor efficiency is that the central donor part of acceptor molecule has relatively weak electron-donating capability.The lack of electron-rich group limits the intramolecular charge transfer(ICT),which provides some guidelines for the subsequent design of non-fullerene receptor materials.2.Asymmetric non-fullerene SMAs can adjust the band gap and tune photoelectric properties through various chemical modification.In chapter 3,we synthesized a new asymmetric small molecule receptor IDT-Cl-R with electron-rich IDT as the central skeleton,4-octylphenyl groups as the side chains and 3-ethyl rhodanine and 2Cl-IC as the end groups.A single junction binary solar cell based on PM6:IDT-Cl-R photosensitive layer was prepared.At the same time,the effects of solvent and donor-acceptor weight ratio on the performance of OSCs devices were discussed.Research findings indicated that when the mass ratio of donor to acceptor was changed to 1:1.5,the maximum photoelectric conversion efficiency of 8.1%was achieved with VOC of 1.0 V and FF of 56.2%.Compared with the DBAR series compounds in chapter 2,the efficiency of the photovoltaic devices based on IDT-Cl-R molecules has been significantly improved,indicating that the increase in the electron-donating capability of the central core was crucial to the device performance.The effective intramolecular charge transfer in the A-D-A structure can lead to a redshift in the absorption spectrum,and improve the short-circuit current and device efficiency.3.Quinoxaline(Qx)is strong electron-withdrawing group and can be stabilized with its resonance structures.The introduction of Qx into the conjugated heteroaromatics can reduce the disordered rotation of single bonds around the aromatic ring and reduce the recombination energy generated by free σ bonds of the target molecule.These features could promote charge transfer and efficient accumulation of molecules.Adjusting the substituents on the conjugate skeleton is an effective strategy to regulate the crystallization behavior and phase separation scale of the blends.Based on the above considerations,we designed two fused acceptors TFQ-F and TFQ-Cl based on quinoxoline core.In addition,we attached 4-trifluoromethylphenyl unit to quinoxaline structure to enhance the solubility of the material.Halogen atoms can effectively reduce the energy gap by enhancing intramolecular charge transfer.Binary single junction solar cell devices based on PM6:TFQ-F and PM6:TFQ-Cl were fabricated.The performance of these solar cells was optimized by changing the donor concentration,additives and annealing temperatures.The results showed that the champion cell performance was obtained with the power conversion efficiency of 13.51%for PM6:TFQ-F,and 13.28%for PM6:TFQ-Cl.The recombination of charge carriers and charge collection in these device were studied by measuring the light intensity dependence of short-circuit current density and open-circuit voltage.The experimental results demonstrated that the carrier recombination in PM6:TFQ-F and PM6:TFQ-Cl cells were mainly bimolecular recombination,and there were less monomolecular recombination and Shockley-Read-Hall recombination.Moreover,most of the charges can be collected effectively before the occurrence of bimolecular recombination,which results in the high efficiency of these devices.Quinoxaline acceptor based cells showed higher external quantum efficiency and lower energy loss,which could provide the insight on the design of efficient non-fullerene small molecule acceptors.4.In this work,two-dimensional conjugated N-heterocyclic ring structure was introduced into the molecular skeleton to design and synthesize BEIT-4F molecule.There was a strong ICT effect between the strong electron-donating core and the electron-deficient terminal group,and the rigid two-dimensional conjugated skeleton showed a strong tendency to form a π-π stacking between molecules,resulting in the absorption of the thin film up to 1000 nm.The photoelectric conversion efficiency of binary single-junction solar cells based on PCE-10:BEIT-4F was as high as 9%.According to the principle of spectral complementation,a ternary blend battery based on PCE-10:BT-ClC:BEIT-4F was prepared.After thermal annealing at 120℃,the short-circuit current and filling factor of the device were enhanced,and the device efficiency reached 11.6%.Photo-generation mechanism of ternary blends was systematically studied by photoelectron spectroscopy.The OSCs based on ternary blend with PCE-10:BT-ClC:BEIT-4F was approximately equivalent to the two parallel binary heterojunctions.To enhance the absorption of active layers,we fabricated the tandem solar cells based on BEIT-4F.The tandem solar cell can realize wide range absorption from ultraviolet band to near infrared band,which is very important for photovoltaic devices to realize high PCE.The power conversion efficiency of tandem organic photovoltaic devices reached 15.2%under simulated 1.5G illumination.The efficiency was further improved to 15.9%after the anti-reflection coating was deposited on the surface of the glass substrate.Two-dimensional conjugated acceptors showed efficient photovoltaic performance,which also lays a foundation for molecular design from one-dimensional conjugation to two-dimensional conjugation.5.Fused rings have been widely used in the construction of OSCs,but the long multi-step syntheses of the fused system are very complicated.However,the synthesis of non-fused ring acceptors is handy and facile.We designed and synthesized three non-fused acceptors BZT-C8,BZT-C16 and BZT-O with different side chains.The weak interaction between O…S and S…F ensured the planarity of non-fused ring acceptors.Binary single-junction solar cells based on polymer PM6,BZT-C8 and BZT-O molecules were prepared,and the device performance was optimized by using additives and different thermal annealing temperatures.The PCE of battery devices based on PM6:BZT-C8 and PM6:BZT-O reached 5.87%and 2.89%respectively.A photovoltaic device with ITO/PEDOT:PSS/PM6:BZT-C16/PFN-Br/Al structure was fabricated.The experimental data show that the maximum cell rate of PM6:BZT-C16-based devices was 4.73%when chloroform was used as processing solvent to prepare the photosensitive layer.The morphology of photosensitive layer was characterized by atomic force microscope.As revealed by AFM measurement,the phase separation scales of PM6:BZT-C16 film was very large,and the existence of obvious aggregation state resulted in a high degree of carrier recombination during the transfer process,so the internal loss of photo-generated charge was severe.There is still much room to improve the efficiency of BZT-series compounds.In order to reduce the recombination loss and enhance the utilization rate of excitons,further optimization of the morphology of the photosensitive layer is needed.The thin film photovoltaic devices based on polymer and small molecule are becoming increasingly prominent in the field of optoelectronic devices,and the development of SMAs is also ongoing with each passing day.However,the balance between rigid framework and side chain has always been a dilemma in the design of acceptor.Although there is still a lack of perfect guiding theories in the design of narrow band gap receptors with the balance between molecular packing and charge transfer,various methods for adjusting the morphology are constantly being developed.The efficiency of OSCs continues to break through from less than 1%to 18.2%,and is constantly approaching the efficiency of silicon cells.In addition,OSCs show great potential in flexible and semi-transparent photovoltaic applications.They can be combined with architecture design,long-range Communications,bioelectronics and cross-complement commercial crystalline silicon cells.In summary,OSCs have a bright future.