Photovoltaic Properties Of Organic Solar Cells Based On Non-Fullerene Acceptors

As a promising green and renewable energy source technology,organic solar cells(OSCs)have become the focus of research due to their advantages for a wide range of applications,such as semitransparent-OSCs(ST-OSCs)and wearable flexible OSCs.With the continuous development of materials and device processes,the power conversion efficiency(PCE)of OSCs has exceeded 15%,which has reached the standard of practical application.In recent years,the OSCs have developed quickly,benefitted from the rapid development of non-fullerene acceptors(NFAs),which have the advantages of adjustable energy level,broader absorption spectra and stronger absoption coefficient in the visible and near-infrared regions than traditional fullerene acceptors.Therefore,developing the efficient NFAs is an effective way to improve the efficiency of OSCs.Reasonable matching of the donor and acceptor materials is also a necessary condition to improve the efficiency of the OSCs.But sometimes,the D:A-based binary active layers can not meet the full absorption of the spectra and good morphology,so the addition of a suitable third component in the binary system is necessary to improve the efficiency of OSCs,which can fill the spectral absorption defects,smooth the energy levels,and optimize the morphologies.Therefore,choosing a suitable third material to fabricate ternary OSCs is another way to improve the efficiency of OSCs.As we all know,OSCs have a big advantage over traditional silicon solar cells,which can fabricate ST-OSCs with various colors,and NFAs with good absorption in the near-infrared region is undoubtedly an excellent material for preparing semitransparent devices.Based on the above ideas,this thesis not only synthesize the new and high-performance NFAs,but also fabricate efficient ternary OSCs and ST-OSCs through reasonable selection of active layer materials.The main research results are as follows:(1)We reported efficient ternary all-polymer solar cells(all-PSCs)with complementary absorption spectra based on two polymer donors PTB7-Th and PBDD-ff4T and one polymer acceptor N2200.The polythiophene derivative PBDD-ff4T as a hole-cascade material plays a bridging role in energy levels between PTB7-Th and N2200,and thus provides more channels for charge transfer.The ternary all polymer OSCs with 10 wt%PBDD-ff4T content show efficient photon harvesting,enhanced charge mobility and better active layer morphology due to the induced crystallization of PTB7-Th by the inserted PBDD-ff4T in the donor domains.As a result,the device without any extra treatments exhibits an optimized PCE of 7.2%.While the PCEs are 5.9%and 4.2%for the OSCs based on the binary blends of PTB7-Th:N2200 and PBDD-ff4T:N2200,respectively.(2)We fabricated highly efficient ternary OSCs based on two compatible non-fullerene acceptors(IDIC and ITIC)with similar chemical structures and one new D-A-type polyner(PSTZ)donor.By inserting ITIC into the binary PSTZ:IDIC system,the active layer shows smooth and gradient energy levels,improved crystallinity and optimized morphologies,which results in efficient exciton separation,charge transport and collection.As a result,the optimal ternary OSCs based on PSTZ:ITIC:IDIC(1:0.1:0.9)exhibited a higher PCE of 11.1%in comparison with those of binary PSC systems based on PSTZ.IDIC(PCE of 8.06%)or PSTZ:ITIC(PCE of 8.13%).These results indicate that combining two compatible nonfullerene acceptors with similar structures to fabricate ternary OSCs should be a promising strategy to enhance the photovoltaic performance of the OSCs.(3)We designed and synthesized two novel NFAs named POIT-M and MOIT-M by modifying the side-chains of IT-M from para-hexylphenyl to para-hexyloxylphenyl and then to meta-hexyloxylphenyl.Due to the synergistic effects of introducing oxygen atoms and varying substitution positions on the phenyl side-chains,MOIT-M shows a significantly improved absorption coefficient,stronger intermolecular π-π stacking interaction,increased crystallinity and higher electron mobility in comparison with IT-M and POIT-M,which helps to gain higher Jsc and FF in OSCs.These special features combined with the complementary absorption of the narrow bandgap MOIT-M acceptor and wide bandgap polymer PTZ1 donor resulted in a high PCE of 11.6%for the OSCs,which is one of the highest PCEs reported for additive-free PSCs and significantly higher than those of the OSCs based on PTZ1:IT-M(9.1%)or PTZ1:POIT-M(9.7%).Our results indicate that side-chain engineering is an effective way to further improve the photovoltaic performance of NFAs in OSCs.(4)An efficient ST-OSC based on a donor/acceptor electron pair of trifluorinated polymer donor PBFTT and tetrachlorinated acceptor IT-4Cl was fabricated.Due to the halogenation,the photovoltaic materials show higher extinction coefficient,improved crystallinity and higher charge carrier mobility;PBFTT shows lower electronic energy levels,and IT-4Cl show red-shifted absorption spectrum.As a result,the PBFTT:IT-4Cl pair shows the matched energy levels,complementary absorption spectra in NIR region and good blend morphology.Hence,the as-cast OSCs based on PBFTT:IT-4Cl achieved a high PCE of 11.1%with a high Jsc of 19.7 mA cm-2 and a high FF of 73.9%.Owing to the complementary absorption spectra in NIR region,high EQE values between 600 and 830 nm and favourable transparency window between 400 and 600 nm where the human eye has the highest sensitivity in the yellow-green wavelength region(500～600 nm),the ST-OSCs using an ultra-thin(10～20 nm)Au cathode showed the high PCEs of 7.9～9.1%under the high average visible transmittance of 37.3～27.6%.When AVT exceeded 25%,ST-OSCs harvested more than 9%PCE without any post-treatment.