| The research field of perovskite solar cells is booming due to its outstanding photovoltaic performance as well as low-cost solution processibility,positioning it as one of the most promising candidate for new generation photovoltaic technology.The perovskite semiconductor can be adopted in diverse types of device architectures including traditional mesoporous structure and n-i-p type planar heterojunction(PHJ)as well as p-i-n type inverted PHJ structure.The latter one is particularly attractive owing to its simple device structure and its compatibility for using low-temperature,large-area manufacturing technique to fabricate flexible devices.Compared to the mesoporous counterpart,however,inverted PHJ perovskite solar cells are mainly suffered from two problems: first,the removal of mesoporous scaffold inevitably leads to a less controllable nucleation and growth of perovskite films with larger morphological deviations.Second,the device interfaces are subjected to energetically unfavourable contact,resulting in the increased chance of charge recombination and decreased photovoltage output.In this dissertation,we focus on developing effective strageties to control the crystallization process and morphology evolution of perovskite film as well as introducing cathode/anode interlayers to modify the respective contact interfaces for high performance inverted PHJ perovskite solar cells.These research works are summarized based on the following four chapters:1.In chapter three,a polymer additive PEOXA with similar molecular structure to that of DMF solvent was introduced into the CH3NH3 Pb I3 perovskite structure to control crystallization process and morphology evolution of perovskite film in two different kinds of solvent systems(DMF & GBL).The results show that the relatively weak interactions among PEOXA,GBL and perovskite materials efficiently suppressd rapid crystal growth of CH3NH3PbI3 perovskite without deteriorating crystal structure,leading to good quality perovskite film with high coverage.An impressive Voc of 1.1 V was achieved for PEOXAdoped devices,which is comparable to that of mesoporous structured perovskite solar cells.We successfully demonstrated inverted PHJ perovskite solar cells with enhanced PCE of 6.35% and 4.35% for rigid and flexible substrates,respectively.2.In chapter four,we introduced a novel amino-functionalized interlayers PN4 N to improve the PCBM/metal cathode contact in the inverted PHJ perovskite solar cells.A flexible amine aliphatic segment in the PN4 N backbone ensures good solubility in IPA solvent.Compared with the typical cathode interlayer,PFN(methanol solvent),the effects of the two cathode interlayers as well as the effects of using different processing solvents on the perovskite underlayer and device performance were systematically studied here.The results show that the efficiencies of PFN-modified devices dropped since its processing solvent,methanol,has high polarity and small molecular size,which could penetrate through PCBM and decompose perovskite crystal to PbI2.However,the processing solvent,IPA,for the PN4 N film has lower polarity and larger molecular size,which can act effectively as an orthogonal solvent to perovskite films.As a result,the PN4N-modified devices exhibited an excellent PCE of 15% and the effective modification suppressed undesired leakage path and improved charge transport properties by removing traps and reducing injection barrier in the cathode interface.3.In chapter five,we developed two alcohol-soluble polymers HSL1 and HSL2 with tailored energy levels to serve as hole selective layers(HSLs)in PEDOT:PSS/perovskite anode interface of inverted PHJ perovskite solar cells.The effects of surface energy of HSLs on perovskite crystallinity and film morphology were systematically studied here.We also investigated the effects of HSLs with high lying LUMO level and deep HOMO level on the charge recombination process and device performance.The results show that the comparable surface energy of HSLs with perovskite realized light-harvesting films with enhanced crystallinity and full coverage.The deep HOMO levels of the HSLs align well with the valence band of the perovskite semiconductors,resulted in increased photovoltage while the high lying LUMO level provides sufficient electron blocking ability to prevent electrons from reaching the anode and reduces the interfacial trap-assisted recombination at the PEDOT:PSS/perovskite interface,resulting in a longer charge-recombination lifetime and shorter charge-extraction time.In the presence of the HSLs,high-performance CH3NH3PbIxCl3-x perovskite solar cells with a PCE of 16.6%(Voc: 1.07 V)and CH3NH3Pb(I0.3Br0.7)xCl3-x cells with a PCE of 10.3%(Voc: 1.34 V)can be realized.This is the highest reported Voc for fullerene/perovskite planar heterojunction solar cells and this finding pave the way for future construction of tandem solar cells since large bandgap perovskite is considered as an important component in tandem devices.4.In chapter six,we simultaneously modified both anode and cathode interfaces of inverted PHJ perovskite solar cells by introducing NiO/DEA HTL and C60(CH2)(Ind)ETL with high lying LUMO level.The surface passivation and recombination process as well as cell performance with dual interfacial modifications were well discussed.The results show that C60(CH2)(Ind)exhibited more efficient PL quenching and electron extraction properties while both NiO/DEA and C60(CH2)(Ind)could provide effective surface passivation of perovskite film.The longer charge-recombination lifetime and shorter charge-extraction time of inverted PHJ perovskite solar cells suggested dual interfacial modifications could more effectively suppress the trap-assisted charge recombination and improve charge extraction efficiency.High performance opaque cell with a PCE of 18.1%(Voc: 1.13 V,FF: 0.8)as well as semitransparent cell with a PCE of 11.0%(AVT: 25.6%)and 12.6 %(AVT: 21.5%)were achieved.These performance combinations are among the best reported for semitransparent perovskite solar cells. |
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