Investigating Working Mechanism Of Cathode Interlayer In The Polymer Solar Cells By Photoemission Spectroscopy

Polymer solar cells(PSCs)have attracted considerable attention due to their key advantages of light weight,synthetic variability,full-solution processing,large area roll-to-roll fabrication and the lucrative possibility of direct integration into flexible devices.Currently,power conversion efficiency(PCE)of single PSCs have rapidly increased to over 13% with good ambient stability.Many efforts have been made in terms of new material synthesis,device structure optimization,controlling the morphology of the active layer and interfacial engineering to improve device performance.In particular introducing interfacial modification layers in the PSCs is an effective approach to improve interfacial contact between the active layer and electrodes,which can increase the efficiency of electron/hole extraction at cathode/anode.Although we have done a lot of work on the interfacial modification at cathode in the PSCs,there is still a lack of clear understanding for electronic structures and chemical structures at cathode/cathode interlayer(CIL)/active layer interface.Ultraviolet photoemission spectroscopy(UPS)and X-ray photoemission spectroscopy(XPS)have been proved to be the effective experimental methods to study the energy level alignments and chemical structures at metal/organic interface and organic/organic interface.In this thesis,we focus on improving the PCE of the PSCs by using the CIL materials and systematically investigating the electronic structures and chemical structures at cathode/CIL/active layer by using UPS and XPS.More details are listed as follows.In the introduction section,the basic concepts,development history,working mechanism,device structures and influencing factors on the performance of the PSCs are described.In addition,the recent work specially materials is reviewed as well.In the second chapter,scientific principles and equipment of UPS and XPS are introduced in brief.on the device performance.When the three MPc Xs were applied as a CIL in the PTB7:PC71BM based PSCs,the PCE values reached 8.1% for the devices with Ag as cathode while the PCE values could reach 8.2% for the devices with Al as cathode.Moreover the three MPc Xs led to similar device performance in the PSCs with either Ag or Al as cathode.It indicates that central metallo cores in the MPc Xs have a negligible effect on the device performance.UPS measurements demonstrated that the MPc Xs decreased not only the work function(WF)of metal cathodes but also barrier of electron extraction from PC71 BM to cathode.XPS results demonstrated that inserting the MPc Xs between the active layer and cathode can limit,even avoid the damages of the Al or Ag atoms to the PTB7:PC71BM during the thermal deposition of the metals.In the fourth chapter,we studied the applications of dicyanomethylenated quinacridone derivatives(DCNQA-Py Br and QA-Py Br)as the CILs and working mechanism in the PSCs.In the PTB7:PC71BM based conventional PSCs with Al as cathode and DCNQA-Py Br as the CIL,the PCE values reached 8.25% and the device still has a high PCE(7.31%)when the thickness of DCNQA-Py Br over 30 nm.However,when the QA-Py Br as CIL,the PCE decreased rapidly with increase of QAPy Br thickness.It indicated that the DCNQA-Py Br film had higher conductivity than QA-Py Br film.UPS measurements found that there was a strong charge transfer state between DCNQA-Py Br and Al,namely,DCNQA-Py Br is doped with N-type by Al.Electron paramagnetic resonance(EPR)measurements showed that the DCNQA-Py Br film had the property of self-doping,while the QA-Py Br film did not.In the PTB7:PC71BM based inverted PSCs with ITO as cathode,DCNQA-Py Br and QA-Py Br In the third chapter,we investigated the influence of the metal core of three metallophthalocyanine derivatives(MPc Xs: Zn Pc(OC8H17OPy CH3I)8,VOPc(OC8H17OPy CH3I)8,Ti OPc(OC8H17OPy CH3I)8)as cathode interface modifiers as the CIL,the PCE values reached 5.26% and 4.73%,respectively.UPS measurements demonstrated that the DCNQA-Py Br and QA-Py Br decreased not only WF of ITO but also barrier of electron extraction from PC71 BM to cathode.Therefore DCNQA-Py Br is a promising CIL material in both of conventional and inverted PSCs.