Novel Cathode Interfacial Materials Of Polymer Solar Cells

Photovoltaics are one of the most effective strategies to utilize sunlight by means of solar cells.Among the known solar cells,polymer solar cells(PSCs)are widely studied by researchers because of their light-weight,low-cost,simple fabrication and flexibility.The main barrier of realizing the commercial application of polymer solar cells is their low power conversion efficiency(PCE).In order to overcome the above issue,several feasible methods to improve device efficiency have been developed,including designs and synthesis of novel donor/acceptor materials,regulation of micro-morphology of the active layer,device structure optimization,and interface engineering.Among them,interface engineering is an effective method to improve the performance of polymer solar cells,and has the advantages of simple operation and low cost.In this thesis,we focused on seeking novel cathode interfacial layer(CIL)materials to improve the performance of polymer solar cells,and carried out the following works:(1)A novel ethanolamine-functionalized fullerene(C60-ETA)was synthesized via a facile one-pot nucleophilic addition reaction and successfully applied as a novel CIL for conventional-structure bulk heterojunction polymer solar cells(BHJ-PSC)devices,leading to obvious efficiency enhancement.Upon incorporating C60-ETA CIL into BHJ-PSC devices based on different active layer systems including PTB7-Th:PC71BM,PTB7:PC71BM and PBDTTT-C:PC71BM,the best PCEs reach 9.66%,8.51%and 7.19%,respectively,which are all higher than those of the corresponding reference devices based on the commonly used Ca CIL,indicating that C60-ETA surpasses calcium(Ca)in terms of CIL performance.The PCE enhancement primarily originates from the increase of short-circuit current(Jsc),which is attributed to the conjunct effects of the improved inter facial contact between the active layer and cathode due to the increase of the roughness of the active layer film,and the improved electron transport facilitating electron extraction by the aluminum(Al)cathode.(2)The MgxAl-NO3-layered double hydroxides(MgxAl-NO3-LDHs)with different Mg:Al molar ratios([Mg2+]+[Al3+]=0.5 M,Mg:Al= 2:1,3:1 and 4:1)were synthesized by co-precipitation method,and ultrathin nanosheets were prepared by ultrasonication-assisted liquid exfoliation.MgxAl-NO3-LDH nanosheets were applied as novel cathode interfacial layers(CILs)in bulk heterojunction inverted polymer solar cell(BHJ-iPSC)devices with PBDTTT-C:PC71BM as active layer,leading to an obvious efficiency enhancement relative to that of the device based on the commonly used zinc oxide(ZnO)CIL.Under the optimized Mg:Al ratio(3:1),PBDTTT-C:PC71BM BHJ-iPSC devices based on Mg3Al-N03-LDH CIL exhibit an average PCE of 6.10%,which is dramatically higher than that of the control device without CIL(3.53%)and surpasses that of devices based on ZnO CIL(5.51%).Such an efficiency enhancement is primarily originated from the increase of fill factor(FF)due to the improved interfacial contact between the indium tin oxide(ITO)cathode and active layer,facilitating the interfacial electron transport.(3)Copper-nitrogen co-doped water-soluble ultrathin carbon nanosheets(Cu,N-CNS)were prepared by thermal polymerization at 300 ℃ using disodium ethylenedi-amine tetraacetate as starting material.The structure of Cu,N-CNS was characterized by Fourier-transform infrared(FT-IR)spectroscopy,X-ray photo-electron spectro scopy(XPS),photoluminescence spectroscopy(PL)and Transmission electron microscope(TEM).Cu,N-CNS was used to replace ZnO as CIL for constructing BHJ-iPSCs with the active layer of PBDTTT-C:PC71BM,affording an obvious enhanced PCE of up to 6.44%relative to that of the device without CIL(-3.39%)or with ZnO CIL(-6.15%).The performance enhancement of BHJ-iPSCs with Cu,N-CNS as CIL was attributed to the factor that Cu,N-CNS interfacial material can effectively reduce the work function of ITO and the corresponding energy level offset between the active layer and the cathode,consequently facilitating the increase of open-circuit voltage(Voc)of the device.Meanwhile,Cu,N-CNS as CIL can effectively reduce the contact resistance and charge transfer resistance of the interface between electrode and active layer,facilitating the interfacial electron transport,and consequently contributing to the efficiency improvement of the device.(4)Two amino-based silane coupling agents as 3-aminopropyltrimethoxysilane(APTMS)and 3-(2-aminoethylamino)propyltrimeth-oxysilane(AEAPTMS)were applied as CILs of BHJ-iPSCs,fulfilled by heating at 100 ℃ to treat ITO surface,resulting in the decrease of the work function of ITO and consequently improved PCE.PCEs of the BHJ-iPSC devices based on AEAPTMS CIL reach 9.06%(PTB7-Th:PC71BM system)and 6.15%(PBDTTT-C:PC71BM system),respectively,which surpass those of the control devices without CIL and the devices with ZnO CIL.The improvement of device performance was mainly attributed to the simultaneous improvements of three photovoltaic parameters including voc,Jsc and FF.Under the same experimental condition,devices based on AEAPTMS CIL exhibit better performance than those based on APTMS CIL,attributing to the larger extent of ITO work function decrease for the case of AEAPTMS CIL.