Research Achievements and Key Points

Perovskite solar cells(PSCs)Efficiency significantly improved, but open circuit voltage(VOC)Still below the theoretical limit. This is mainly due to energy level mismatch and critical interfaces (especiallySnO₂/Non radiative recombination losses at the buried interface of perovskite. The main issue is the defect of buried interfaces, and precise control of this interface is an improvementPSCsThe key to performance.

This article was published in top international journalsAdvanced Materials (Adv. Mater.)The research, conducted byLi Gang from Hong Kong Polytechnic University（Gang Li）Professor andKuan LiuProfessor team, andThe University of Hong KongJinyao TangProfessor andMingliang LiAssociate Professor TeamJoint leadership. target, aim at, address, focus on, in response toVOCWe have developed an innovative approach to address the issue of lossesBuried interface regulation(Buried Interface Regulation, BIM)StrategyThrough a specific methodSelf assembling amphiphilic phosphonate derivatives(MeOBTBT-POEt), inSnO₂/Constructing a buried interface for perovskitePerovskite crystal driven templateThis template is fixed through-The solid-phase transformation process self assembles to form highly oriented supramolecular structures.

Core concepts and mechanisms of action- Research focuses onMeOBTBT-POEtThe multiple key roles of supramolecular templates in buried interfaces:

•Inducing optimal crystal growthHighly optimized for promoting the formation of perovskite Facing outward direction(100)Crystal orientation is conducive to the vertical transport of charge carriers.DFTCalculations show that it is related to(100)Facial interaction(-0.94 eV).

•Optimize energy level alignment and carrier extraction: Material dipole moment(1.89 D)Adjust the interface energy level to extract the electron potential barrier from0.24 eVdrop to-0.03 eV14Accelerate the transfer of charge carriers.

•Active defect passivationMolecular structure can 'actively' passivateSnO215And perovskite16The inherent defects of the interface effectively suppress non radiative recombination.

Key research achievements

•Efficient power conversion efficiency(PCE): Achieve25.34%The highPCE.

•Extremely low open circuit voltage loss(VOC deficit)：Achieve the highestVOC 1.23 VFor1.536 eVPerovskites with energy gaps,VOCLoss only0.306 VReaching the theoretical limit97.2%.

•Efficient suppression of non radiative recombination:

○ External quantum efficiency of electroluminescence(EL-EQE)Significantly improved to10.29%.

○Photoluminescence quantum efficiency(PLQY)upgrade to27.06%.

○Quasi Fermi level splitting(QFLS)Losses from62 meVSignificantly reduced34 meVConfirmed to be ultra-lowVOCThe losses mainly come from the elimination of non radiative recombination. The trap density is significantly reduced.

•Excellent long-term operational stability: under rigorous testing conditions(ISOS-L-3: MPP, 65 °C, 50% RH)Achieved over1200hourlyT92lifespan

Figure 4e

research team

This article was published in top international journals Advanced MaterialsThe corresponding author of the study isLi Gang from Hong Kong Polytechnic University（Gang Li）Professor andKuan Liu教授And, andThe University of Hong KongJinyao Tang教授.

This important research collaboration brings together the academic strengths of the Hong Kong Polytechnic University and the University of Hong Kong, and combines research teams from multiple academic institutions such as the Chinese University of Hong Kong Shenzhen Campus and the Chinese University of Hong Kong to conduct in-depth and effective explorations on the key issues of improving the performance of perovskite solar cells.

Research Background

•open-circuit voltage(VOC)Excessive lossesDespite the improvement in efficiency,PSCsofVOCStill lagging behind the theoretical limit.

•Non radiative composite losses at critical interfacesThis is causingVOCOne of the main reasons for the loss. especiallySnO₂/Buried interface of perovskiteIts defects can cause serious non radiative recombination.

•Inherent defects in buried interfaces：SnO₂electron transport layer(ETL)Surface uncoordinatedSn2+ (Sn–OH)It will form shallow traps, hindering the interaction between perovskite andETLThe electronic transmission efficiency between them. Defects in perovskite itself (such as uncoordinated)Pb2+）It's also a problem. Deep defects are the main center of non radiative recombination.

•Interface energy level mismatch：SnO₂There may be electron extraction in the energy level arrangement between perovskite and perovskite/The transmission energy barrier is not conducive to the effective transmission of charge carriers.

•The limitations of traditional interface passivationAlthough embedding interface modifications (such as using self-assembled monolayers)SAMsMolecular bridges (or molecular bridges) are considered as collaborative strategies for addressing defects, enhancing interface contacts, and aligning energy levels, and have achieved some results, but there is still much room for further improvementVOC.

•Neglecting the arrangement of interface molecules and the role of templatesPrevious research on interface insertion layers has paid little attention to the arrangement and orientation of these molecular layers, as well as their role as templates in regulating the formation and crystallization of perovskite films, which is often overlooked.

•Interface defects affect stabilityThe composite centers and defects at the interface can also accelerate the degradation of perovskite under heat and moisture pressure.

Figure 3d

Solution

The research team has developed and applied an innovative approachBuried interface regulation(BIM)''The core of the strategy is:

1.Constructing perovskite crystal driven templates: InSnO₂electron transport layer(ETL)The buried interface between the perovskite layer and a specific one is utilizedSelf assembling amphiphilic phosphonate derivatives(MeOBTBT-POEt) Material, through solidification-The solid-phase transformation process forms a highly oriented supramolecular template.

2.The multifunctional effects of supramolecular materialsThis oneMeOBTBT-POEtMolecules play multiple critical roles at interfaces:

○Inducing optimal crystal growthTemplate can drive perovskite to preferentially follow along(100)crystal orientation Growing towards the outer plane direction. This is becauseMeOBTBT-POEtand(100)The crystal planes have interactions and provide thermodynamic driving forces.

○Optimizing energy level alignment and promoting carrier extractionThe dipole moment of a molecule(1.89 D)Regulating interface energy levels reduces electron extraction/The energy barrier for transmission enables more efficient transfer of electrons from perovskite toSnO₂Layer.

○Active defect passivationUnlike traditional passive passivation methods,MeOBTBT-POEtThe functionalized parts on the molecular structure, especially the phosphonate groups, canActively passivateSnO₂Surface uncoordinatedSn2+Defects and defects in the perovskite layer (such as uncoordinated)Pb2+）Chemical interactions occur, effectively suppressing non radiative recombination losses at the interface.

Figure 2c

Experimental process and steps

1.Preparation of special moleculesThe research team first prepared a special amphiphilic phosphonate derivative molecule through a specific chemical synthesis pathway, namedMeOBTBT-POEtThis molecule contains a stiffBTBTCore and a flexible alkyl tail with a phosphonate group.

2.bottom layerSnO₂Preparation of Electronic Transport Layer: Research UseFTO/Glass serves as the substrate. After cleaning treatment, deposit a layer by spin coatingSnO₂Solution. thenSnO₂Thin films are carried out in the ambient atmosphere200 °CAnnealing treatment.

3.MeOBTBT-POEtConstruction of Template LayerAfter annealingSnO₂ Transfer the substrate to the glove box. Prepare itMeOBTBT-POEtMolecules dissolved in chlorobenzene(Chlorobenzene, CB)In the solvent. This molecule is used in the subsequent spin coating of perovskiteDMF/DMSOThe solubility in the mixed solvent is extremely low, ensuring that subsequent steps will not damage the already formedMeOBTBT-POEtLayer. Subsequently, it willMeOBTBT-POEtSolution spin coatingSnO₂ Surface of electronic transport layer. Research has found that these molecules can pass through interfacesself-assemblyThe way, and throughsolid-Solid-phase transformation process, forming aHighly ordered supramolecular templatesEven at room temperature, its weak intermolecular interactions facilitate the formation of ordered structures. By heating (such as60 °C）It can accelerate this phase transition process.

4.Sedimentation of perovskite layers and template induced crystallizationPrepare a specific composition of perovskite precursor solution, includingFAITheMABrTheMAClThePbBr2ThePbI2andCsIDissolve inDMF/DMSOIn a mixed solvent. Spin coating the perovskite precursor solution onto the modified surfaceMeOBTBT-POEtofSnO₂On the substrate. After spin coating, in a controlled humidity environment(20%-30% RH)Perform annealing treatment below(100 °C)During this process, the formedMeOBTBT-POEtSupramolecular templates will serve as aPerovskite crystal driven templateInducing perovskite crystals to move in a specific direction(Highly preferred orientation towards the outer plane direction)(100)Crystal orientationGrowth and crystallization. The study also observed that,MeOBTBT-POEtMolecules andSnO₂Surface presenceChemical interactions, with uncoordinatedSn2+Perform coordination.

5.Sedimentation of subsequent functional layersAfter forming the perovskite layer, transfer the sample back to the glove box. Spin coating hole transport layer in sequence(BABrandspiro-OMeTAD)And conduct annealing.

6.Preparation of metal electrodesFinally, a layer is deposited through thermal evaporation80 nmThick gold electrodes complete the preparation of battery devices.

Figure 1b

Characterization of research results

Electroluminescence external quantum efficiency(EQEEL)Splitting of quasi Fermi energy levels(QFLS)analyze

The research team usesEnlitech LQ-100Photoluminescence quantum yield measurement systemRecord the external quantum efficiency of electroluminescence(EQE)Spectrum. According to being equal toJSCUnder the injection current densityEQEELValue calculation caused by non radiative recombinationVOCLoss. According to the photoluminescence quantum yield(PLQY)Radiation limit open circuit voltage(VOC,rad)Analysis of perovskite semi-structured samplesQFLS.VOC,radFrom the perspective of devicesEQEThe spectrum is calculated through a detailed equilibrium model.

imageS27：EQEELCurve.

image4d：PLQYvalue

image4e：QFLSValue and ΔVOC,rad- QFLS)Value.

optimizedBIMdevicedisplayEQEELsignificantly improve(10.29%), much higher than the control device(2.73%)This indicates non radiative recombination loss(ΔVOC,nr = 58.8 mV)Significantly reduced, compared toJ-VMeasured byVOCThe differences are consistent. The modified perovskite half structure exhibits higherPLQY(27.06% vs 9.09%)This is crucial for achieving high photovoltaic voltage.QFLSAnalysis shows thatBIMThe energy loss of the device Δ(VOC,rad-QFLS)significantly reduce(from62meVto34 meV).These results collectively indicate that ultra-lowVOCThe losses are mainly attributed toSnO₂Effective elimination of non radiative recombination at the buried interface with perovskite.

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current density-voltage(J-V)Characteristic curve and external quantum efficiency(EQE)spectrum

The research team usesEnlitechofSS-X AM1.5GStandard solar simulatorconductJ-VCurve measurement, andQE-RQuantum efficiency measurement systemMeasure external quantum efficiency(EQE)Spectrum. Using source meters to measure devicesJ-VCurve. Perform forward and reverse scans to evaluate hysteresis effects. useEQESystem measurement devices300-850 nmSpectral response within the wavelength range.

image4a：J-Vcurve(current densityvsvoltage).

image4b：EQEspectrum(EQEpercentagevswavelength).

table 1：A table summarizing photovoltaic parameters.

sutraMeOBTBT-POEtmodifiedBIMThe device exhibits significantly better photovoltaic performance than the control device, reflected in higherPCE (25.34% vs 22.81%)TheVOC(1.23V vs 1.197V)andFF(82.28% vs 75.99%).JSCandEQEThe score results are consistent.EQEThe spectrum reflects the response efficiency of the device at different wavelengths. optimizedBIMThe device exhibits the smallestJ-VLag effect. These charts and data demonstrate that buried interface modification significantly improves device conversion efficiency and reducesVOCDirect evidence of losses.

XX-ray diffraction spectrometer(XRD)/Grazing incidence wide-angleXRay scattering(GIWAXS)

monitorMeOBTBT-POEtCrystal structure evolution during hot annealing process, analysisMeOBTBT-POEtInSnO₂The crystal characteristics and orientation of the perovskite film, as well as the analysis of the crystal orientation of the perovskite film.Through various structural characterization techniques,research teamconfirmedMeOBTBT-POEtInSnO₂/The buried interface of perovskite can form a specific crystal structure and effectively induce the preferential orientation growth of perovskite. (Image)1b, picture2a, pictureS17）

XX-ray Photoelectron Spectroscopy(XPS)

研究MeOBTBT-POEtandSnO₂Between andMeOBTBT-POEtVerify the passivation effect of the interface by examining the chemical environment and interactions with perovskite.XPSThe comprehensive results confirmed thatMeOBTBT-POEtNot only withSnO₂ Chemical reactions occur and strongly coordinate with the perovskite layer, leading toEffectively passivatedSnO₂ Oxygen vacancies in perovskite and uncoordinated vacancies in perovskitePb2+Defects.(image3a, b, c)

Ultraviolet photoelectron spectroscopy(UPS)

Evaluate electronic selective interfaces(SnO₂/perovskite)The band structure, measurementSnO₂After modificationSnO₂The work function of perovskite thin films(Work function)And energy level position(As valuable as the topEVBM,conduction band minimumECBM.MeOBTBT-POEtBy modifying its inward negative dipole momentSnO₂ The work function is shifted upwards,Greatly promotes the extraction and transmission of electrons at the interface.(image3d, e)

Time-resolved photoluminescence(TRPL)

Study the dynamics of interface charge transfer and evaluate charge extraction efficiency by measuring carrier lifetime. grow inMeOBTBT-POEtmodifiedSnO₂ The perovskite film on the surface exhibits a shorter average carrier lifetime(183.9 ns vs 381.7 ns)This indicates thatElectrons move from the perovskite layer toSnO₂ETLExtraction and transmission are more efficient.(image3f)

Thermal conductivity spectrum(TAS)Space limited current with pulse voltage(PV-SCLC)

Quantitative estimation of trap state density(Ntrap)Evaluate the suppression effect of interface modification on defect states based on their distribution on energy levels. Multiple characterization results unanimously confirm MeOBTBT-POEtModification effectively reduces the trap state density at the interface(Especially deep level defects)Furthermore, it strongly suppressed non radiative recombination.(image4f )

Long term operational stability testing

Directly verify the long-term stability of perovskite solar cells modified by interfaces in simulating harsh environments.Under testing conditionsT92Exceeding lifespan 1200 Hour.(image4g)

Conclusion

The research team has successfully developed and applied a buried interface control strategy based on supramolecular assembly templates, which utilizesSnO₂ electron transport layer(ETL)Introducing amphiphilic phosphonate derivatives between perovskite layersMeOBTBT-POEtSignificantly improved the performance and long-term stability of perovskite solar cells.

1.MeOBTBT-POEtStructural Evolution and Template FunctionResearch has found thatMeOBTBT-POEtMolecules canSnO₂Surface through solid-Solid phase transformation achieves supramolecular assembly, formingAn ordered and dense near single crystal structureThis self-assembly process serves as a crystallization template,Successfully induced perovskite crystals to exhibit high priority along the out of plane direction(100)oriented growthThis is beneficial for the vertical transport of charge carriers in the device.DFTCalculation verificationMeOBTBT-POEtCompared to perovskite(100)The interaction between crystal planes is(100)Priority oriented growth provides additional thermodynamic driving force.

2.Effective passivation of interface defects:XPSThe results strongly proveMeOBTBT-POEtNot only does success exist inSnO₂On the surface, stillandSnO₂Strong coordination interaction occurred with the perovskite layerThis interaction effectively passivatesSnO₂Oxygen vacancies in the middle(Manifested as inhibitionSn-OHpeak)Uncoordinated with perovskitePb2+The inherent defects significantly reduce the density of interface defects.

3.Optimization of band alignment and promotion of carrier extraction:MeOBTBT-POEtMolecules have distinct dipole moments(1.89 D)The inward negative dipole moment causesSnO₂The work letter is offset upwards(from4.16 eVmove to3.82 eV)This band shift significantly reduces the interaction between perovskite andSnO₂ETLElectronic extraction between/Transmission barrier(from0.24 eVdrop to-0.03 eV)，Greatly promotes electron transfer and extraction at the interface.

4.Non radiative recombination suppression and carrier dynamics improvement:throughTRPLObservation of shortened carrier lifetime in modified perovskite thin films(183.9 ns vs 381.7 ns)This indicates an improvement in electron extraction efficiency.TPVDisplay longer voltage decay time,TPCDisplaying higher non-equilibrium carrier collection and shorter current decay time, all point to effective carrier transfer at the interface and fewer trap assisted recombination.EQEELSignificantly enhanced,PLQYSignificantly improved,QFLSAnalysis shows a significant reduction in energy lossComprehensive defect characterization(SCLCTheTASTheMott-Schottky)Confirm that the density of trap states, especially deep level defects, significantly decreases and the built-in potential increases. theseThe consistent results prove thatMeOBTBT-POEtThe modification strongly inhibits non radiative recombination at the interface.

5.Significant improvement in device performance and stability:Thanks to the interface control effect mentioned above, usingMeOBTBT-POEtModifying buried interfacesPSCsAchieved up to25.34%The photoelectric conversion efficiency(PCE), maximum open circuit voltage(VOC)reach, achieve, attain1.23 V，VOCThe loss is only0.306 VApproaching the theoretical limit97.2%In addition, this strategy significantly improves the long-term stability of the device, even under harsh combined thermal and humidity stresses(ISOS-L-3: MPP, 65°C, 50% RH)，T92Exceeding lifespan1200 Hour.

Reference from Advanced Materials_DOI: 10.1002/adma.202418011

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