Novel Cathode Interlayers For Inverted Perovskite Solar Cells

Inverted perovskite solar cells（IPVSCs）have received great attention due to their easy fabrication through low temperature,suitability for flexible substrates,and compatibility with multijunction solar cells.Recently,the power conversion efficiency（PCE）of single-junction IPVSCs has reached more than 25%,showing great commercialization prospects.In order to improve the performance of IPVSCs,researchers have done a lot of efforts in component optimization,suitable additive selection and surface passivation of perovskite layer.At the same time,the interface research of IPVSCs is also crucial for improving the PCE and stability of devices,especially the modification of electrode interfaces.Because the cathode interface layer（CIL）can adjust the cathode interface energy level arrangement,enhance electron transport,reduce non-radiative combination loss,and even protect the perovskite layer,it is essential to research CIL for improving the PCE and device stability of IPVSCs.To date,bathocuproine（BCP）is the most widely used CIL for IPVSCs.BCP-modified Ag has been commonly utilized as a cathode to achieve record efficiencies in the IPVSCs.However,BCP also has some prominent drawbacks,such as:1.The air stability of the IPVSCs with BCP-modified Al as a cathode is extremely poor;2.BCP film is prone to aggregation;3.Low conductivity of BCP restricts its application in thick films.At present,a severe shortage of low-cost CILs suitable for large-area printing poses a major obstacle to the advancement of the large-scale industrialization of IPVSCs.Therefore,the development of novel CIL materials is extremely urgent for achieving large-scale,low-cost,efficient,and stable IPVSCs.In this thesis,we shall develop two new types of CIL materials suitable for low-cost large-scale production of the IPVSCs,namely surfactant-encapsulated polyoxometalate complexes(TOASi W₁₂ and{Mo₁₃₂}-TOA)and dicyanovinyl substituted quinacridone derivative（DCNQA-Py Br）,focusing on their application and working mechanisms in the devices.The main contents of this thesis are as follows:In chapter 1,we reviewed the development history,working principle,device structure,preparation methods,factors affecting device performance,and strategies to improve device performance of PVSCs.In chapter 2,we employed TOASi W₁₂-modified Al as a cathode in IPVSCs.It was found that the TOASi W₁₂/Al based devices with the PCE of 20.64%can maintain over 80%of the initial PCE after 350 h of storage in an ambient atmosphere with 45%RH.By contrast,the unencapsulated IPVSCs with BCP/Al as a cathode showed a rapid PCE drop from 19.63%to less than 1%within 2 hours.Combined with optical microscopy,X-ray diffraction（XRD）and UV–vis spectra,we found that aluminum electrode corrosion and perovskite degradation occur in BCP/Al devices,while there is no degradation occurring in TOASi W₁₂/Al devices.Water contact angle and X-ray photoelectron spectroscopy（XPS）measurements show that TOASi W₁₂film can effectively block the penetration of moisture and Al atoms into the perovskite layer,thereby effectively avoiding aluminum electrode corrosion and perovskite decomposition.Moreover,the PCEs of 21.06%and 20.52%were realized for the IPVSCs with TOASi W₁₂/Ag and TOASi W₁₂/Cu as the cathode,respectively.In chapter 3,we systematically investigate the evolution of the electrical characteristics of IPVSCs with TOASi W₁₂/Al and BCP/Al as cathodes when exposed to air,revealing the influence of different CILs on device performance.Various electrical characterization techniques including dark current density-voltage（J–V）,photocurrent density versus effective voltage(J_ph–V_eff),capacitance-voltage（C–V）,and transient photocurrent（TPC）measurements were carried out.These measurements show that BCP/Al devices suffered seriously limitations in charge carriers’extraction,transportation,and collection.Moreover,there were increased trap state density and decreased electron mobility during the ageing process of BCP/Al devices,resulting in significant charge recombination losses and poor electron transport abilities.On the other hand,there were little changes in these electrical characteristics for TOASi W₁₂/Al,BCP/Ag and TOASi W₁₂/Ag devices during the ageing process in air.Our results reveal the evolution of the photoelectric properties of IPVSCs in air,which provides a comprehensive understanding of the degradation mechanism of IPVSCs.In chapter 4,we employed{Mo₁₃₂}-TOA as a CIL in IPVSCs.The PCEs of 20.74%,21.23%,and 20.82%were realized for the IPVSCs with{Mo₁₃₂}-TOA/Al,{Mo₁₃₂}-TOA/Ag,and{Mo₁₃₂}-TOA/Cu as the cathode,respectively.Combined characterization of J_ph–V_eff,C-V andμ_e,the results show that{Mo₁₃₂}-TOA as CIL can effectively promote charge-carrier extraction,built-in potential,and electron mobility within the device.Due to{Mo₁₃₂}-TOA has abundant hydrophobic alkyl chains that can effectively shield against moisture infiltration,thus the stability of{Mo₁₃₂}-TOA-based devices are better than the BCP-based devices.In Chapter 5,in order to further develop thickness-insensitive CILs suitable for printing,we embarked on an exploration of DCNQA-Py Br with high conductivity as a CIL in IPVSCs.Notably,as the thickness of the DCNQA-Py Br film expanded from 18 nm to 80 nm,the device’s PCE saw a modest decrease from 20.92%to 18.52%.In sharp contrast,the thickness insensitivity of BCP was evident,resulting in a drastic PCE reduction from 20.22%to 13.13%as the BCP film thickness ranged from 6 nm to 31 nm.More importantly,the thickness of the DCNQA-Py Br film held negligible effects on device stability.In stark contrast,the stability of BCP/Ag devices decreases significantly with the increase of BCP film thickness.Electron paramagnetic resonance（EPR）revealed that DCNQA-Py Br had a paramagnetic signal,while BCP had no EPR signal.It shows that DCNQA-Py Br can be electrically self-doped,contributing to a higher conductivity than BCP.Photo-induced charge extraction by linearly increasing voltage（Photo-CELIV）and TPC characteristics further demonstrated that DCNQA-Py Br/Ag devices have higher charge mobility and faster charge extraction efficiency than BCP/Ag devices.Combined with atomic force microscopy（AFM）,optical microscopy and stylus profilometers measurements revealed that DCNQA-Py Br has excellent film-forming property with this quality remaining intact even as the film thickness increased.Conversely,BCP exhibited a propensity to aggregate,causing a decline in film-forming quality with increasing thickness.Our results provide an efficient,inexpensive,environmentally friendly and thickness-insensitive CIL for the scalable production of IPVSCs by solution method.