Design And Synthesis Of A-d-a-type Small Molecule Acceptors For Organic Solar Cells

Energy shortage and environmental pollution are the major problems for the global economic sustainable development.Photovoltaic,a novel green technology,has been considered as one of the most promising solutions.Organic solar cells（OSCs）have attracted tremendous interests due to their outstanding properties,such as lightweight,inexpensive,and flexible,and can be fabricated using solution processing techniques.It has been a frontier research field with fierce competition in the world.The recently emerged non-fullerene small molecule acceptors（NF-SMAs）with acceptor-donor-acceptor（A-D-A）-type molecular structures have been extensively investigated in OSCs and have achieved power conversion efficiencies（PCEs）over 14%.Moreover,small molecule acceptors possess well-defined molecular weights and molecular structures and can be obtained at high purities without variation between batches.In this thesis,different series of A-D-A-type SMAs were designed and synthesized with strategies including developing novel donor core units,side chain engineering,and developing new electron-withdrawing end groups.In chapter 2,anthracene unit with good planarity was widely used in the field of organic electronics.A series of SMAs were developed with anthracene as the electron-donating core,and benzothiadiazole（BT）or thiophene as the bridge,and different electron-withdrawing units as the end-groups.Their optical,thermal,electrochemical and photovoltaic properties were studied.As a result,the OSCs with these small molecules as the acceptors achieved a high open circuit voltage(V_OC)of 1.13 V and a best PCE of 1.83%,which demonstrated that the anthracene unit can be used as the donor unit for constructing NF-SMAs.However,most of the anthracene-based acceptors exhibited weak intramolecular charge transfer（ICT）effect.To explain this phenomenon,we developed a series of novel small molecules with anthradithiophene as the donor core and different acceptor units as the end-groups,one can note that these small molecules exhibite weaker ICT effect,which demonstrate that more stronger electron-withdrawing units should be used to enhance the ICT effect of the molecules when synthesized anthracene-based SMAs.This discovery provided an important reference for designing anthracene-based photovoltaic materials in the future.In chapter 3,porphyrin has a planar electron-rich ring structure,which plays a key role in the process of energy conversion in nature.A series of A-D-A-type porphyrin-based SMAs were developed,in which copper atom was introduced into the core and the porphyrin ring was linked to electron-withdrowing units（such as phenyl-fused indanone,INCN）by ethynylene and thiophene bridges.Their optical,thermal,electrochemical and photovoltaic properties were studied.These SMAs are typical narrow band gap compounds with strong ICT effect and wide absorption spectra,which are favorable for harvesting solar photons,and a PCE of 2.44%was obtained.Porphyrin,as an important medium for the energy conversion in nature,has a great significance for scientific research.This study provided an important reference for designing porphyrin-based photovoltaic materials in the future.In chapter 4,two series of newly electron-withdrawing end groups2-（3-oxo-2,3-dihydro-1H-cyclopenta[b]naphthalen-1-ylidene）malononitrile（N）and2-（3-oxo-2,3,5,6,7,8-hexahydro-1H-cyclopenta[b]naphthalen-1-ylidene）malononitrile（HN）were developed and have been used to construct three SMAs（IDT-N,IDT-T-N and IDT-HN）with indaceno[1,2-b:5,6-b’]-dithiophene（IDT）as the electron-donating core.IDT-N and IDT-T-N consisting of the naphthyl-based N group exhibited an expanded plane compared to INCN,which strengthened the intramolecular push-pull effect between the core donor unit and the terminal acceptor units.This strengthened effect resulted in a reduced bandgap that was beneficial for solar photon collection and increased short-circuit current density(J_SC)of the resulting devices.The devices based on the PBDB-T:IDT-N system exhibited a PCE of up to 9.0%,with a J_SC of 15.88 mA cm^-2 and a fill factor of 71.91%.However,the V_OC was only0.79 V,which limited the photovoltaic performances.Cyclic chains adopt preferential conformations which can promote the molecular self-assembly andπ-πpacking properties,and thus lead to an improved J_SC and fill factor（FF）.Moreover,the electron-donating nature of the cyclic alkyl group can elevate the lowest unoccupied molecular orbital（LUMO）level,which can lead to a higher V_OC.By utilizing IDT-HN as the electron acceptor and PBDB-T as the donor,an impressively high PCE of up to 10.22%was obtained with simultaneously improved V_OC,J_SC and FF.In addition,the performance of the devices remains at 91%of its optimum performance with a film thickness of 250 nm,indicating its great potential for future practical application.In chapter 5,a series of near-infrared SMAs were designed and synthesized via side chain engineering.IDT-TN exhibited a red-shifted absorption spectra and a low energy band gap（E_g）of 1.43 eV by changing the side-chains in IDT-T-N from alkylphenyl to alkyl.The fabricated OSCs based on PBDB-T:IDT-TN obtain a PCE up to 5.89%,associated with a relatively high V_OC of 0.97 V and a remarkably low energy loss(E_loss)of 0.46 V,which is one of the lowest values reported for state-of-the-art OSCs and is approaching the losses reported for mature PV technologies.Furthermore,two other SMAs IDT-TO-N and IDT-TON were developed by introducing oxygen atoms into the side chains of IDT-T-N and IDT-TN.The two acceptors exhibited a broad light absorption up to 1013 nm and a very low E_g of 1.22 eV,which are favorable for harvesting solar photons.The fabricated OSCs based on PTB7-Th:IDT-TON obtained a remarkably low E_loss of 0.44 V,which is approaching the losses reported for other conventional photovoltaic technologies,such as Si and CIGS.These findings provide an effective approach for the rational molecular design of SMAs with low V_OC loss.In chapter 6,the device based on PTB7-Th:ITIC has a low V_OC of 0.80 V,and their absorption spectra are overlapped.A novel A-D-A-type SMA,namely ITNO,has been developed with the aim at achieving matched energy levels and complementary light-absorption simultaneously with the donor PTB7-Th.ITNO exhibited an elevated LUMO level and a blue-shifted absorption compaired with ITIC,which are favorable for harvesting more solar photons and achieving higher V_OC when blended with PTB7-Th.As a result,the ITNO-based device exhibited a PCE of 5.45%with a highly elevated V_OC of 0.95 V.In chapter 7,four electronically delocalized molecules,which cotain a non-planar bithiophene（TT）or a planar cyclopentadithiophene（CT）unit as the donor core and INCN or N unit as the terminal groups,were designed as models to understand how molecular shape impacts the photoelectric properties in molecular semiconductors.The SMAs with TT core showed blue-shifted absorption spectra compaired to the CT-based molecule due to the steric interference between two isooctyl groups.Solution-processed OSCs achieved a PCE of 1.59%without any post-treatment when using PBDB-T as the donor.It provides an important reference for further understanding the effect of molecular conformation on their photoelectric properties.