Research Achievements and Highlights

This research developed a multifunctional Lewis base, methyl (methylsulfinyl)methyl sulfide (MMS), to regulate crystal growth and band edge boundaries of MA- and Br-free CsFA-based perovskite, significantly reducing non-radiative voltage loss (𝑉𝑙𝑜𝑠 𝑛𝑜𝑛−𝑟𝑎𝑑). Specifically, this study has several important highlights:

• High-Efficiency Inverted Perovskite Solar Cells: Through MMS regulation, high-efficiency inverted perovskite solar cells were successfully fabricated, achieving a certified efficiency of 26.01%, which is currently the highest efficiency for MA/Br-free CsFA double-cation perovskite inverted solar cells.
• Minimodule Application: The technology was successfully scaled up to 12.96 cm² minimodules, achieving a conversion efficiency of 22.67%, demonstrating its enormous potential for practical applications.
• Outstanding Stability: The devices exhibited excellent thermal and operational stability, maintaining over 75% of initial efficiency after 1000 hours of thermal aging at 85°C, and retaining over 82% of initial efficiency after 1650 hours of maximum power point tracking (MPPT).
• Band Edge Redshift: MMS treatment leads to a redshift in perovskite band edges, increasing perovskite absorption of incident light and thereby improving short-circuit current density (JSC).
• Reduced Non-radiative Recombination: MMS effectively passivates defects in perovskite, reducing non-radiative recombination and significantly improving device voltage and fill factor.

Research Team

This research was jointly completed by teams from the School of Chemical Engineering at Sichuan University and the Frontier Science Center for Flexible Electronics at Northwestern Polytechnical University: The corresponding authors are Qiang Peng and Yihui Wu.

Research Background

• Efficiency Limitations: Compared to regular structure perovskite solar cells, inverted structures have long had lower efficiencies. Although in 2023, inverted PSCs’ efficiency first surpassed regular structures, reaching 26.1%, there is still room for improvement, especially in MA-free perovskites.
• Non-radiative Voltage Loss (𝑉𝑙𝑜𝑠 𝑛𝑜𝑛−𝑟𝑎𝑑): Non-radiative recombination is a key factor limiting inverted PSCs efficiency improvement, particularly in CsFA double-cation perovskites, where uneven distribution of Cs and FA cations easily forms traps, leading to severe voltage losses.
• Crystal Growth Control: Regulating perovskite film crystal growth, reducing defects, and improving uniformity are key challenges for enhancing device performance.
• Stability Issues: Perovskite materials tend to degrade in efficiency under thermal aging and continuous illumination.
• Band Edge Limitations: Traditional perovskite materials have limited band edge absorption ranges. Expanding absorption range to improve photocurrent is also an important research direction.

Solution

Addressing the above research background and challenges, this study proposes using multifunctional Lewis base MMS to regulate CsFA perovskite crystal growth and band boundaries. The working mechanisms of MMS are as follows:

Halide Oxidation Suppression:

MMS effectively suppresses halide (iodide) oxidation, reducing FAI decomposition into FA0 and I2, stabilizing the perovskite precursor solution. The FAI/DMF solution with MMS remains transparent and colorless after 9 hours, while the control group turns yellow.

Figure S2b shows an absorption peak at 366 nm in the control group, indicating FAI decomposition, while this phenomenon is absent in the MMS group.

Optimization of Perovskite Phase Transition:

MMS accelerates perovskite intermediate phase transition through strong coordination with PbI2 and FAI. Figures 2c, 2d show faster δ-phase to α-phase conversion in the MMS group compared to the control group.

Additionally, MMS can inhibit the formation of inactive δ-phase, as confirmed by UV-Vis absorption spectra in Figure 2g, S19.

Improvement of Electronic Properties:

Space-charge-limited current (SCLC) measurements indicate that the MMS group has lower electron and hole trap densities, effectively passivating defects. The MMS group shows a 5 nm redshift in perovskite absorption edge, with bandgap decreasing from 1.555 eV to 1.551 eV, confirmed by high-sensitivity external quantum efficiency (HS-EQE) spectra. KPFM measurements show increased contact potential difference (CPD) in the MMS group, with upward shift in Fermi level, facilitating charge carrier separation and transport.

Experimental Process and Steps

Material Preparation:

Using unpurified lead iodide (PbI2), formamidinium iodide (FAI), cesium iodide (CsI), lead chloride (PbCl2), Lewis base MMS, with DMF and DMSO as solvents.

Device Fabrication:

1. FTO substrates were UV-ozone treated, followed by spin-coating of NiOx aqueous solution (annealed at 120°C) and Me-4PACz ethanol solution (annealed at 110°C) as hole transport layer and self-assembled monolayer.
2. Perovskite layer preparation used DMF:DMSO as solvent, dissolving precursors (PbI2, FAI, CsI, PbCl2) to prepare experimental groups with and without MMS. First, PEAI/DMF solution was spin-coated to increase wettability, followed by perovskite solution spin-coating with benzyl methyl ether as anti-solvent. After 110°C annealing, PDAI2/(IPA:CB=1:1) solution was spin-coated and re-annealed.
3. Finally, C60, BCP, and silver electrodes were thermally evaporated, and MgF2 anti-reflection layer was evaporated on the glass side.
4. Minimodules used vacuum-assisted method (10 Pa) for large-area film preparation, with two-step spin-coating and 110°C annealing for 20 minutes. Other processes remained identical to small-size devices, completing six series-connected sub-cells module.

Research Characterization

This research employed various advanced characterization techniques to analyze the effects of MMS on perovskite films and device performance enhancement mechanisms. The following details each characterization result:

Photoelectric Performance Characterization:

1. Current-Voltage Curves (J-V Curve):
   The research team measured device J-V curves using a source meter under standard AM 1.5G solar simulator (100 mW/cm²).
   
   Figure 3c shows MMS-treated devices achieving 26.12% conversion efficiency in reverse scan, with open-circuit voltage (Voc) of 1.192 V, short-circuit current density (Jsc) of 26.00 mA/cm², and fill factor (FF) of 84.27%. Control group devices achieved 24.43% efficiency.

    

   Table 1 summarizes key photoelectric parameters for control and MMS-treated devices.
   

   
   Figure 3f shows MMS-treated minimodules achieving 22.67% conversion efficiency in reverse scan, with open-circuit voltage of 7.000 V and fill factor of 78.62%. Control group minimodules achieved 19.49% efficiency.

2. External Quantum Efficiency (EQE):
   The research team used Enlitech QE-R3011 equipment to record monochromatic external quantum efficiency (EQE) spectra, verifying solar simulator reliability and calculating integrated photocurrent density.
   Recommend using Enlitech QE-R Quantum Efficiency System for spectral mismatch calculation and solar cell conversion efficiency uncertainty assessment

    

   Figure 3d shows MMS-treated devices achieving integrated photocurrent density of 25.62 mA/cm², control group at 25.20 mA/cm², consistent with Jsc values measured from J-V curves.

   Figure S25 shows redshift in EQE spectra edge for MMS-treated devices, consistent with UV-Vis absorption spectra results.

3. Stabilized Power Output:
   The research team tracked devices’ steady-state output efficiency at maximum power point to verify device stability.
   Figure 3e shows MMS-treated devices achieving 25.6% steady-state output efficiency at 1.04 V, with no significant degradation over 400 seconds.

    

   Figure S30 shows MMS-treated minimodules achieving 21.6% steady-state output efficiency at 5.92 V.

   Figure S26 shows steady-state output efficiency for control group devices.

4. Voc Loss Analysis:
   This research successfully reduced non-radiative voltage loss (𝑉𝑙𝑜𝑠 𝑛𝑜𝑛−𝑟𝑎𝑑) through MMS introduction, significantly decreasing to 67 mV after MMS treatment.
   Figure 4c shows spectral efficiency when using solar cells as light-emitting diodes (LEDs).
5. High-Sensitivity External Quantum Efficiency (HS-EQE):
   The research team used Enlitech PECT-600 (now FTPS) equipment to record high-sensitivity external quantum efficiency (HS-EQE) spectra, extracting Urbach energy (EU) and analyzing material structural quality and electronic properties.
   
   Recommend using Enlitech FTPS Fourier Transform Photocurrent Spectroscopy System to measure high-sensitivity external quantum efficiency (HS-EQE)

    

   
   Figure 2h shows redshift in HS-EQE spectra band edge for MMS-treated devices, with Urbach energy (EU) decreasing from 17.58 ± 0.39 meV to 16.23 ± 0.25 meV, indicating improved crystal quality and electronic properties.

   The research team measured TPC and TPV curves using spectrophotometer. Figures 4d and 4e show longer TPC and TPV decay times in MMS-treated devices, indicating effective charge recombination suppression and improved carrier extraction efficiency.
   

6. Electroluminescence (EL) Measurements:
   The research team used LED photoluminescence (PL) quantum yield measurement system with Enlitech REPS-VOC light source measurement system.
   Recommend using Enlitech REPS Perovskite and Organic Photovoltaic Voc Loss Analysis System to improve your solar photovoltaic efficiency and published results

    

   
   Figure S32 shows enhanced EL emission peak and redshift in MMS-treated devices.

Other Characterizations:

• Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS):
  
  Figure 2d shows α-phase formation 20 seconds faster in MMS group with higher α-phase signal intensity, indicating improved crystal quality.

   

  Figure S9 shows final film GIWAXS patterns, with MMS group showing better crystallinity.

• X-ray Photoelectron Spectroscopy (XPS):
  Study of MMS interaction with perovskite precursors.(Figure S5)
  
• Scanning Electron Microscopy (SEM):
  Top-view and cross-sectional SEM images measurement. (Figure S12)
  
• Nuclear Magnetic Resonance (NMR):
  Liquid 1H NMR spectra measurement to study MMS interaction with perovskite precursors. (Figure 1e)
  
• Dynamic Light Scattering (DLS):
  Increased colloidal size in perovskite precursor solution after MMS addition. (Figure S4)
  
• UV-Visible Absorption Spectroscopy (UV-Vis):
  Film and solution absorption spectra measurement. (Figure S18)
  
• Contact Angle Measurement:
  
  Figure S33 shows larger contact angle for MMS-treated perovskite films, indicating lower surface energy.
• Kelvin Probe Force Microscopy (KPFM):
  Film surface contact potential difference measurement. (Figure 2f)
  
  
• Ultraviolet Photoelectron Spectroscopy (UPS):
  Determination of perovskite film Fermi level. (Figure S16)
  
• Hall Effect Measurements:
  
  Figure S17 shows negative slopes for both control and MMS groups, indicating n-type semiconductors, with MMS group showing slightly higher slope, indicating enhanced n-type behavior.
• Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS):
  Confirmation of MMS distribution in films. (Figure S6)
  
• Time-Resolved Photoluminescence Spectroscopy (TR-PL):
  TR-PL spectra measurement. (Figures S20, S21 and Table S2)
  
• Electrochemical Impedance Spectroscopy (EIS):
  EIS spectra measurement using electrochemical workstation. (Figure 4f and Table S5) 
  
• Space-Charge-Limited Current (SCLC):
  Quantification of defect density in perovskite films using SCLC method. (Figure S22)

Research Results Summary

This research successfully utilized MMS (methyl (methylsulfinyl)methyl sulfide) as a multifunctional Lewis base to effectively regulate crystal growth, band structure, and photoelectric performance of MA (methylammonium) and Br (bromine) free CsFA (cesium formamidinium) based perovskite films, significantly improving the efficiency and stability of inverted perovskite solar cells.

Record-Breaking Efficiency Improvements:

• Certified Efficiency: Through MMS introduction, the research team successfully developed the highest efficiency MA/Br-free CsFA double-cation perovskite inverted solar cells to date, achieving certified efficiency of 26.01% (reverse scan) and quasi-steady-state output efficiency of 25.30%.
• Minimal Non-radiative Voltage Loss: The research achieved an extremely small non-radiative voltage loss (𝑉𝑙𝑜𝑠 𝑛𝑜𝑛−𝑟𝑎𝑑) of 67 mV.
• Minimodule Performance: Extending this strategy to 12.96 cm² minimodules achieved 22.67% conversion efficiency.

Multifunctional Effects of MMS:

• Crystal Growth Regulation: MMS can suppress halide oxidation, reduce δ-phase formation, and accelerate α-phase transition.
• Band Structure Adjustment: MMS causes a 5 nm redshift in perovskite absorption edge, enhancing light absorption while improving n-type characteristics.
• Defect Passivation: MMS reduces defect density in perovskite films, decreasing non-radiative recombination.
• Carrier Transport Optimization: MMS promotes charge transport, carrier extraction efficiency, and suppresses charge recombination.

Stability Enhancement:

MMS-treated devices demonstrate excellent thermal and operational stability. After continuous thermal aging for 1000 hours at 85°C, they maintain over 75% of original efficiency. Under 1 sun illumination at 30 ± 5°C, they retain over 82% of original efficiency after 1650 hours of maximum power point tracking.

Original Publication

Original Title: Crystal phase and band edge modulation of MA- and Br-free CsFA-based perovskite for efficient inverted solar cells and minimodules
Published in: EES Energy & Environmental Science
DOI: doi.org/10.1039/D4EE05860G