The Study On Indene Fluorene Fused Derivative Wide Band-Gap Small Molecule Acceptor And High-efficiency Device Construction

Organic solar cells(OSCs)have attracted extensive attention in recent years due to their advantages such as light weight,low cost,and the ability to be prepared by printing.However,their power conversion efficiency(PCE)is still inferior to inorganic solar cells.In order to improve the efficiency of OSCs,researchers have done a lot of work in new material design,morphology control,interface engineering,and physical mechanisms.In addition,tandem device structure in which two sub-cells with complementary absorption are connected in series through an interconnecting layer(ICL)can effectively reduce the thermalization loss and transmission loss simultaneously,which are unavoidable in single-junction devices,and is an effective strategy to improve the PCE of OSCs.Furthermore,as the PCE of OSCs keep increasing,developing high performance large area devices towards commercial application becomes a hot topic in this field.The research work of this dissertation mainly focus on the construction of high-efficiency tandem and large-area devices based on the wide band-gap indene fluorene fused derivatives non-fullerene acceptors,which are mainly divided into the following four parts:The first part of the work is based on the wide band-gap(WBG)small molecule acceptor Tf IF-IC developed by our group in the early work.Fluorine atoms are introduced into the end-group to control the absorption and energy level of the material,and a new WBG acceptor Tf IF-4F has been synthesized.Compared with Tf IF-IC,the molecular energy level of Tf IF-4F is reduced and its absorption spectrum is red-shifted.In addition,the introduction of fluorine atoms also makes Tf IF-4F have a more ordered molecular packing,and therefore exhibits higher electron mobility.When blended with the donor PBDB-T-2F to prepare single-junction devices,the optimal PCE reached 13.1%,and the open circuit voltage(V_OC)was as high as 0.98V,corresponding to a low energy loss of 0.63 e V.This is the best performing acceptor with such a wide bandgap larger than 1.6 e V.Due to the WBG absorption characteristics of the PBDB-T-2F:Tf IF-4F,it is very suitable for the construction of tandem OSC devices.When PBDB-T-2F:Tf IF-4F and PTB7-Th:PCDTBT:IEICO-4F are used as the front cell and the back cell to construct tandem solar cells,PCE of 15%is obtained.These results demonstrate that Tf IF-4F is a promising WBG non-fullerene acceptor for application in the front cell of tandem solar cells.On the basis of preliminary work,the second part of the work designed a new non-fullerene acceptor Tf IF-4Cl through end-group chlorination.Compared with Tf IF-4F,the combination of the Cl atom in Tf IF-4Cl not only leads to red-shifted absorption,but also improves the molecular packing ability.When Tf IF-4Cl is blended with the polymer donor PM7to fabricate OSC,the optimal PCE reaches 14.0%,the corresponding short-circuit current density(J_SC)is 18.5 m A cm^-2,the fill factor(FF)is 78%,and the V_OC is as high as 0.97 V.Through the photoluminescence(PL)and time-resolved photoluminescence(TRPL)tests,it is found that the PM7:Tf IF-4Cl system presents efficient charge transfer and exciton dissociation.In addition,due to its efficient charge transport capability,the device efficiency based on PM7:Tf IF-4Cl shows a low dependence on the thickness of active layer.The high V_OC but moderate photocurrent of PM7:Tf IF-4Cl based small area device are beneficial to achieve large area device with high scalability according to the Joule law,thus we manufacture 1.01 cm²devices and achieve a remarkable PCE of 13.3%,which retains 95%in efficiency when device area is scaled up from 0.04 cm² to 1.01 cm².These results indicate that the acceptor with both broadened absorption and high V_OC is attainable via chlorination and also demonstrate the superiority of the PM7:Tf IF-4Cl based OSCs for printed large-area PSC application.The third part of the work focuses on the design of interconnecting layer in tandem OSCs.A series of organic-inorganic composite interface materials(Zn O NPs:PEI)were prepared by mixing different proportions of PEI into the solution of zinc oxide nanoparticles(Zn O NPs).The introduction of PEI can effectively adjust the work function of the composite interface without introducing additional parasitic absorption.By applying Zn O NPs:PEI as the electron transport layer(ETL)in OSCs,the FF has been greatly improved when compared with the Zn O NPs-based device,which can be attributed to the reduction of the interface barrier and thereby improving the electron extraction ability.After that we applied Zn O NPs:PEI as the ETL in tandem OSCs and constructed an ICL with low optical loss and low electrical loss successfully.When choosing the predeveloped PM7:Tf IF-4Cl as the WBG front cell and PCE10:COi8DFIC:PC70BM as the low band-gap back cell to construct tandem devices,the PCE reaches to 17.42%and its FF reaches up to 79%,which indicates the excellent interconnecting property of this ICL.To the best of our knowledge,79%is the highest FF value reported thus far.In addition,the tandem devices showed good repeatability as the PCE of 72individual devices were distributed between 16.8%and 17.4%,and the average value was17.09%.This work proves the potential of organic-inorganic composite interface in OSCs and its superiority in constructing ICL.On the basis of previous work,this part of work develops a new strategy to regulate the charge recombination inside the sub-cell of the tandem devices.The front sub-cell used in this work is PM7:Tf IF-4Cl,as reported in the second part of the work.The non-fullerene acceptor shows a much higher absorption coefficient than the donor in the front sub-cell,and the absorption reduction of donor can be well complemented by that of acceptor when increasing the acceptor proportion,thus leading to a significant overall absorption enhancement even with thinner film.As a result,the optimal thickness of the front sub-cell is reduced and its charge recombination is suppressed,which leads to a high fill factor in the tandem device.Ultimately,fine-turning of the composition in the front sub-cell enables an efficient tandem OSC with a very high fill factor of 78%and an excellent PCE of 18.71%(certified by an accredited institute to be 18.09%)to be obtained,which is the highest PCE value for organic tandem devices reported to date.Furthermore,PBDB-T-2F:IDT-N is used as the front sub-cell applying in the construction of tandem devices and a similar result was obtained,thus proved the universality of this strategy for different active layer.This work therefore presents the effectiveness for elevating the overall performance of tandem OSCs by suppressing the charge recombination in sub-cells.It also demonstrates that regulating the donor and acceptor ratios in sub-cells is an effective strategy for simultaneously obtaining high J_SC and FF.