Design And Synthesis Of Tetraphenylethylene-Based Charge Transporting Materials And Their Application In Perovskite Solar Cells

After 10 years development,the energy conversion efficiency（PCE）of perovskite solar cells（PSCs）have soared from the initial 3.8%to 22.7%.Such remarkable increase in the field of photovoltaic devices is unprecedented,thus the research on PSCs has attracted the widespread attention by many scholars.In perovskite solar cells,charge transporting layers can extract charges from perovskite layer and transport them to the electrodes,which is pretty important to improve the performance of PSCs.Charge transporting layers can be divided into electron transport layers and hole transport layers,respectively,which can form a selective contact with perovskite layer,thus increasing charge extraction efficiency,and preventing the charge quenching caused by direct contact with the electrode.Except from the optimization of the perovskite materials and optimization of the device structure,it is also in urgent need to develop efficient charge transport material for perovskite solar cells.In this thesis,we design and synthesize a series of tetraphenylethylene based organic charge transport materials,including hole-transporting materials and electron extraction materials,and successfully apply them in the perovskite solar cells.In addition,the photo-oxidation behavior was discovered in a tetraphenylethylene derivative.The phenomenon was studied systematically,and possible mechanism was provided.The main contents of this thesis include the following three aspects:1.We designed and synthesized four tetraphenylethylene-carbazole based hole-transporting materials（W1,W2,W3 and W4）,and applied them in perovskite solar cells,in order to study the effect of 2,7-carbazole（W1 and W2）and 3,6-carbazole（W3 and W4）toward hole transporting properties.W1 and W2 are molecules based on 2,7-carbazole with two and four substitution positions on tetraphenylethylene moiety,respectively.While W3 and W4 are molecules based on 3,6-carbazole with two and four substitution positions on tetraphenylethylene moiety,respectively.With respect to the structure,2,7-carbazole group is more distorted than 3,6-carbazole group,which results in better solubility of W1 and W2 compared with W3 and W4.This contributes to the formation of film via spin coating method.However,W3 failed to get the corresponding device due to its poor solubility.It is shown that W2 has a higher hole transport mobility,more effective hole extracting ability and more uniform surface among several hole transport materials,therefore PCE of the corresponding device is the highest,which is16.74%.The device based on W1 shows an inferior efficiency of 14.92%,and device with W4 as hole transport layer shows the lowest efficiency of 13.30%.It turns out that tetraphenylethylene could work as efficient core structure for HTMs,and in this series of molecules studied,2,7-carbazole-based HTMs rival the 3,6-carbazole-based one in hole transporting property.2.Two organic electron extraction materials,tetraphenylethylene-diketopyrrolopyrrole（TPE-DPP₄）and tetraphenylethylene-isoindigo（TPE-ISO₄）,were designed and synthesized,and were successfully applied in inverted PSCs.Standard device with fullerenes(C₆₀)as electron transport layer exhibited a PCE of 16.87%.After the introduction of TPE-DPP₄ and TPE-ISO₄ interlayer between perovskite and C₆₀,PCE increased from 16.87%to 18.44%and 18.19%respectively,which mainly came from the improvement in J_sc.The introduction of TPE-DPP₄ and TPE-ISO₄ reduced the charge transfer resistance(R_CT).In addition,DPP moiety contains more heteroatoms than ISO group,which maybe contribute to the trap passivation on the surface and the grain boundary of the perovskite,thus leading to a higher efficiency for TPE-DPP₄-based PSCs.Results in this work indicate that the introduction of TPE-DPP₄ and TPE-ISO₄ interlayer between perovskite and C₆₀ can facilitate charge transport,which is favorable for the improvement of J_sc,thus leading to a high performance in PSCs.3.TPE-4T was designed and synthesized with tetraphenylethylene as the core end-capped with thiophene substituted by alkyl chains.TPE-4T exhibits aggregation induced emission（AIE）and aggregation promoted photo-oxidation behavior.The emission of TPE-4T in tetrahydrofuran（THF）is quite weak and the pristine TPE-4T film exhibits strong bluish-green emission,which turns to quite weak yellow emission after UV irradiation.After solvent treatment,the weakly fluorescent intermediate will become brightly yellow emitted.According to the reported literatures,the photochromic response could be ascribed to be a stilbene-type 6-πelectron ring-closure reaction.However,this phenomenon only occurs in the aggregation state of TPE-4T,and no change can be observed when it is dissolved in dilute solution.As far as we know,aggregation induced emission combined with aggregation promoted photo-oxidation has been reported only in two works quite recently.