Design And Synthesis Of Organic Dopant-free Hole Transporting Materials For Efficient Perovskite Solar Cells

In the past decade,perovskite solar cells(PVSC)have achieved rapid development,and the power conversion efficiency(PCE)improves from 3.9% to 24.2%,which makes the PVSCs the great commercial candidate for the next generation solar cell.To date,most of commonly-used high performance hole transporting materials(HTMs)in PVSCs such as Spiro-OMeTAD not only possess high synthetic cost,but also require a chemical doping to improve their relatively low hole mobility.Unfortunately,the addition of dopants and the corresponding oxidation process significantly deteriorate the long-term stability of PVSCs.As a result,development of efficient and stable dopant-free HTMs is urgently needed.However,few of dopant-free HTMs reported so far can show comparable PCEs to that of doped Spiro-OMeTAD,while most of them also have a costly problem.In this dissertation,a new class of high-performance dopant-free HTMs with facile synthesis has been prepared,while their structure-property relationship in these materials has been systematically studied.The main contents are listed as follows:1.Fluoranthene is a typical polycyclic aromatic hydrocarbon(PAH),which is an ideal building block for dopant-free HTMs due to its several promising advantages including low price,highly planar structure and electron-deficient character.However,few examples on fluoranthene-based HTMs have been reported.We have developed a facile chemistry to prepare unreported 2,3-dicyano-fluoranthene based on the simple Diels–Alder reaction,which can be used as the “core” to construct two new donor–acceptor(D–A)type HTMs,BTF3 and BTF4.It is found that,the introduction of two cyano groups into fluoranthene core not only endows the resulting molecules with suitable energy levels,but also enables highly ordered and strong molecular packing in solid states,both of which could facilitate hole extraction and transport.Base on this,conventional structure and inverted structure PVSCs were fabricated using these molecules as dopant-free HTMs,and the PVSCs based on BTF4 achieved high PCEs of 18.03% and 17.01%,respectively,with enhanced stability.2.Based on the previous chapter,we further developed a class of dopant-free HTMs by using the fluoranthene as the “core”,and the triphenylamine dericatives as the “arm”.Further by tuning the molecular configuration(linear vs.star-shaped),the connection between the electron-donor units and the central fluoranthenecore(single bond vs.double bond)and methoxy substitution sites(para-vs.meta-),the materials structure has been regulated and optimized deeply and carefully with a purpose of studying their impacts on molecular packing behaviors,thermal properties,photophysical properties as well as the device performance.The results demonstrate that: 1)Replacing diphenylamine with triphenylamine as the capping unit of D–A type HTMs can effectively down-shift the HOMO levels and increase the hole mobilities of derived HTMs;2)Introducing ethylene as the connecting π-bridge can effectively increase the hole mobilities of derived HTMs due to enhanced π–π interactions;3)In comparison with the linear structure,the branched materials can considerably increase the glass transistion temperature and film-froming ability on the perovskite layer to improve the moisture and oxygen stabilities of resuiting devices;4)Metamethoxy substitution can be used as an effective structural design strategy to down-shift the HOMO levels and improve the hole mobilities of derived HTMs.Based on this systematic structure-property study,the inherent regularity governing the structure of dopant-free HTMs has been demonstrated preliminarily.In particular,FBA3 with a low lab synthetic cost of 14.75 $ per g can exhibit an impressive PCE of 19.27%,representing one of the best costeffective dopant-free organic HTMs reported thus far.3.For inverted p-i-n device,HTMs not only extract and transfer holes,but also play a vital role in affecting the growth of perovskite layer.However,few of reports have evidenced that the molecular crystallinity of a HTM layer can directly influence the crystal orientation of the perovskite film.Cross-conjugated polymers(CPs)have been demonstrated as an important class of organic semiconductors,due to their facile synthesis and interesting structural,physical and optoelectronic properties.Moreover,although such molecules have a large conjugated structure,they remain transparent in the visible region,which is one of the key requirements for HTMs applied in inverted PVSCs.By simply modifying the linkage positions from para to meta,the resulting iso-poly(triacetylene)s with a side chain group of 2,7-diphenylamine capped fluorene units,PPE1 and PPE2,have been prepared with dramatically distinct PCEs of 11.13% and 19.33%,respectively,in inverted structure PVSCs.Furthermore,through rational surface passivation,the open circuit voltage(Voc)of PPE2-based inverted PVSCs can be effectively improved to afford an impressive PCE of 21.31%.Considering similar optical properties,surface wetting properties,energy levels and charge transport properties of these two polymers,we attribute the significantly enhanced device performance to the enhanced crystallinity of PPE2 compared to PPE1 in solid films,which can endow the resulting perovskite films with enlarged grain size and more oriented crystallization.Not only provides a new design strategy of developing dopant-free HTMs,our study also strongly highlights the important role of promoting a preferred perovskite growth/crystallization for an idea HTM employed in inverted PVSCs.