Design And Synthesis Of Squaraine-based Semiconductors For Photovoltaic Applications

Squaraines are a kind of organic dyes with a donor-acceptor-donor structure.They are not only convenient to synthesize,but also exhibit intense absorption in the visible and near-infrared region,accompanied with high photostability and chemical stability,making them widely used as organic semiconductors in many optoelectric fields.When used as donor materials in bulk heterojunction organic solar cells?BHJ-OSCs?,a major challenge encountered is their relatively low hole mobilities that always result in low fill factors and thus limited device power conversion efficiencies?PCEs?.In addition,despite great progress made in non-fullerene OSCs,the application of squaraines into this field is still rare,mainly due to their poor compatability with non-fullerene acceptors?NFAs?,leading to that it is very difficult to form well-defined BHJ morphology.On the other hand,squaraines,as a class of zwitterionic p-type semiconductors,ont only present well-matched energy levels with perovskites,but also naturally contain passivation groups such as carbonyl groups,making them suitable material candidates for hole transport materials?HTMs?in perovskite solar cells?PVSCs?,which,however,has been less explored thus far.In this context,squaraines should have great potential for the applications in the fields of OSCs and PVSCs.Thus in this thesis,a series of squaraine-based semiconductors,including small molecules and polymers,have been successfully designed and synthesized through rational molecular design,with the photophysical properties and hole transporting abilities characterized carefully.Furthermore,their applications in OSCs and PVSCs have been systematically studied.?1?A new class of asymmetric squaraines IPSQ1-4 has been synthesized by using isomeric benzoindoline and different functional thiopyranylidene derivatives as the terminal donor units.Organic field effect transistors tests show that IPSQ-3 has the highest hole mobility,which is close to 10^-4 cm²V^-1s^-1.The definement of single crystal structures manifests that different molecules present totally distinct molecular stacking patterns,and herringbone-and“brick-wall”-type can be achieved for IPSQ-1 and-4,respectively.The hole mobilities of these two molecules also behavior very differently;the hole mobility of IPSQ-1 is found to be?50 times that of IPSQ-4,indicating herringbone-type arrangement is more beneficial to charge carrier transport.Thus,our findings in this work can help us to understand the implications of tuning structural properties of squaraines,which will be very useful for designing high-performance squarainebased semiconductors.?2?Butterfly-shaped squaraine dimers DSQ1-3 have been synthesized as donor materials in fullerene-based BHJ-OSCs by using benzoindoline and triphenylamine derivatives as the terminal donor groups,and simultaneously regulating the bridge units with highly planar fluorene or carbazole rings,combined with suitable side-chain engineering.Our results reveal that the modification of connecting bridges between two asymmetric dyes has a minimal effect on their absorption,energy levels and charge transport properties,however,subtle side chain modifications can efficiently improve the charge transport ability in the BHJ films due to enhanced miscibility with fullerene acceptors.By simply replacing the hexyl chains in the fluorene core of DSQ1 by branched2-ethylhexyl chains,the resulting DSQ3 could possess an enhanced hole mobility and thus an improved device PCE based on single-junction BHJ-OSCs?4.02%vs 2.26%?.Moreover,the 2-hexyldecyl substituted carbazole cored DSQ2 can generate the highest PCE of 4.38%among these three dimers,which is benifited from the stronger electron-donating ability of carbazole bridge that can enhance the hole mobility of DSQ2.Thus,our work strengthens the importance of side-chain engineering for designing high-performance squaraine-based donor materials.?3?Based on the design concept of two-dimensional?2D?polymer donors,a pesuedo2D squaraine-based polymer PASQ-IDT has been synthesized by copolymerizing an asymmetric squaraine with the indacenodithiophene?IDT?unit.The resulting polysquaraine not only can effectively weaken the strong exciton coupling that often occurs in the one-dimensional?1D?polysquaraines,but also can exhibit a good compatibility with PC₇₁BM acceptors resulting in favorable nanoscale BHJ morphology.As a result,the PCEs of PASQ-IDT-based single-junction BHJ-OSCs and semi-transparent OSCs are achieved as 6.35%and 4.03%,respectively,both of which are the highest values reported for polysquaraines.Besides,we further explored its application as a dopant-free HTM in inverted PVSCs,delivering an impressive PCE as high as 18.29%that outperforms the PEDOT:PSS-based control devices?16.14%?and most dopant-free HTMs reported at that time.This is mainly due to the zwitterionic structure of sqauraines,which can endow the resulting PASQ-IDT with very good surface wettability to the perovskite precursor solutions that are beneficial to the growth of perovskite films with high quality.?4?Two wide bandgap polysquaraines,PASQ-BDT1 and PASQ-BDT2,have been synthesized successfully by copolymerizing the asymmetric anilinosquaraines with 2D benzo[1,2-b:4,5-b']dithiophene unit.In both fullerene-based and fullerene-free OSCs,PASQ-BDT2 with N,N-diisobutylaniline subunits shows superior device performances compared to PASQ-BDT1 containing triphenylamine subunits,owing to increased light-harvesting ability and more favorable nanoscale morphology in the BHJ films.For the PASQ-BDT2/PC₇₁BM blend,the PCE can be improved from 3.19%to 4.34%after solvent vapor annealing?SVA?treatment,mainly attributed to the optimized film morphology and increased hole mobility.More interestingly,when the processing solvent for fullerene-free devices was changed from chlorobenzene to chloroform,a significant enhancement on PCE from 1.96%to 3.72%can be yielded for the PASQ-BDT2/ITIC blend,due to suppressed charge recombination and enhanced crystallinity in the chloroform-processed BHJ films.Thus,besides the first trial of squaraine-based polymer donors in non-fullerene OSCs,this work also strengthens the importance of solvent effect on sqauraine-based devices.?5?Given the fact that the cyclobutanedione core of squaraines is a naturally good passivator for uncoordinated lead ions,squaraines could be considered as one of ideal candidates for dopant-free HTMs with good passivation effect.By using IDT and asymmetric squaraine isomers as the copolymerization units,two pseudo 2D polysquaraines PSQ1-2 with high hole mobility have been developed as dopant-free HTMs in all-inorganic PVSCs,while their passivation effects have been carefully investigated.The resulting PSQ2 not only exhibits more suitable energy levels and more efficient passivation effects,but also achieves very high hole mobility close to 0.01 cm^-2V^-1s^-1.Thus as the dopant-free HTMs for?-CsPbI₂Br-based all-inorganic PVSCs,the best PCE of PSQ2-based device can reach as high as 15.5%,outperforming those of dopant-free PSQ1?13.6%?and the doped-Spiro-OMeTAD?14.4%?based control devices.This value is also among the best for all-inorganic PVSCs reported so far.