Research Achievements and Highlights

The research team proposed an innovative Dual-Ligand Passivation (DLP) method that successfully addresses the issue of non-uniform quantum well width distribution in traditional 2D/3D perovskite heterojunctions. The main achievements of this research include:

• Achievement of Pure 2D Passivation Layer:
  Utilizing co-deposition of two ligands, MeCZEAI and mFPEAI, to precisely control the nucleation and growth of 2D perovskite, forming a uniform n=1 2D phase passivation layer, resolving the issue of non-uniform 2D phase distribution caused by traditional single-ligand passivation.
• Device Performance Enhancement:
  Devices employing the DLP strategy achieved efficiency of 25.86% for 0.05 cm² area devices and 25.08% for 1 cm² area devices, while demonstrating excellent operational stability (T90 > 1000 hours) and thermal stability.
• In-depth Understanding of Passivation Mechanism:
  Through various characterization techniques including Transient Absorption Spectroscopy (TA), X-ray Photoelectron Spectroscopy (XPS), and Kelvin Probe Force Microscopy (KPFM), the team thoroughly analyzed how the DLP strategy regulates surface reaction kinetics, homogenizes energy distribution, and enhances charge transport efficiency.
• Improved Surface Potential Distribution:
  DLP-treated perovskite surfaces showed more uniform potential distribution, with a Full Width at Half Maximum (FWHM) of only 68 mV, significantly lower than the 177 mV of single-ligand treatment. Additionally, the surface potential after DLP treatment increased by 80 mV compared to single-ligand treatment.

Research Team

This research was jointly completed by researchers from the National University of Singapore (NUS) and Tianjin University. The corresponding authors are Yi Hou from the National University of Singapore and Wenping Hu from Tianjin University. 

Research Background

In the field of Perovskite Solar Cells (PSCs), surface passivation is a crucial strategy for improving efficiency and stability. Stacking 2D materials on perovskite surfaces, utilizing their quantum confinement effects, effectively passivates defects and adjusts energy level alignment, and is a widely applied method. However, traditional 2D perovskite passivation layers formed using single organic ligands often present the following issues:

• Non-uniform Quantum Well Width Distribution:
  Different ligand molecular sizes and surface reaction kinetics lead to 2D perovskite quantum wells of varying widths, causing non-uniform energy distribution and affecting device photoelectric properties.
• Formation of Multiple 2D Phases:
  Difficult-to-control surface reactions easily generate multiple 2D perovskite phases with different n values, creating additional defects and energy losses.
• Unfavorable Energy Level Alignment:
  2D passivation layers may cause energy level misalignment, hindering charge transport and reducing device efficiency.

Many studies have explored ways to control 2D perovskite quantum well distribution, such as adjusting ligand types or adding various additives. However, for n-i-p type perovskite solar cells, precisely controlling the quantum well distribution and n-value of 2D passivation layers remains a challenge.

Solution

This study addresses the drawbacks of traditional 2D passivation layers by proposing the Dual-Ligand Passivation (DLP) strategy. The core concept involves introducing two organic ligands with different sizes and adsorption capabilities to control their reaction kinetics on the perovskite surface, achieving pure n=1 2D passivation layers. Specifically:

• Dual-Ligand Selection:
  • MeCZEAI (3,6-dimethylcarbazole-9-ethylammonium iodide): Features larger molecular size and lower adsorption energy, preferentially adsorbs on perovskite surface, acting as a reaction modulator.
  • mFPEAI (meta-fluorophenethylammonium iodide): Smaller molecular size, capable of forming 2D perovskite.
• Reaction Mechanism:
  After MeCZEAI preferentially adsorbs on the perovskite surface, it regulates the reaction between mFPEAI and perovskite, ensuring mFPEAI only forms n=1 2D perovskite, avoiding the formation of multiple 2D phases.
• Uniform Energy Distribution:
  Through this mechanism, the DLP strategy successfully achieves a 2D passivation layer with uniform quantum well width distribution, thereby improving device energy distribution.

Experimental Process and Steps

The following details the experimental process, including material preparation, film deposition, and passivation treatment:

1. Material Preparation:
   Using lead iodide (PbI2), cesium iodide (CsI), formamidinium iodide (FAI), and methylammonium chloride (MACl) as precursors for perovskite preparation.MeCZEAI synthesis: Using 3,6-dimethyl-9H-carbazole, sodium hydroxide, 2-chloroethylamine hydrochloride, and tetrabutylammonium hydrogen sulfate as raw materials, synthesized through reflux, filtration, concentration, hydroiodic acid addition, and recrystallization.

   Using mFPEAI as the 2D ligand; Spiro-OMeTAD as the hole transport material; and tin oxide (SnOx) as the electron transport material.

2. Device Structure:
   This study employed an n-i-p structure: ITO/SnOx/Perovskite/Passivation Layer/Spiro-OMeTAD/MoOx/Ag.
3. Film Preparation:
   • ITO glass substrate cleaning: Ultrasonic cleaning with detergent, acetone, and isopropanol.
   • SnOx electron transport layer deposition: Spin-coating diluted SnOx nanoparticle solution on substrate, followed by annealing and UVO treatment.
   • Perovskite layer deposition: Spin-coating mixed precursor solution on SnOx layer, followed by gas quenching. Then heating on a 70°C hotplate and annealing at 150°C.
   • Passivation layer treatment: 
     – Single-ligand treatment: Spin-coating mFPEAI solution on perovskite surface.
     – DLP treatment: Spin-coating mixed solution of mFPEAI and MeCZEAI on perovskite surface.
   • Spiro-OMeTAD hole transport layer deposition: Spin-coating Spiro-OMeTAD solution doped with TBP, Li-TFSI, and FK209 on the passivation layer.
   • Electrode deposition: Thermal evaporation of molybdenum oxide (MoOx) and silver (Ag) electrodes.

Device and Characterization

1. Current-Voltage (J-V) Characteristic Curves:
   The research team used Enlitech’s solar simulator (SS-PST220R) to measure J-V curves under simulated AM 1.5G solar illumination conditions.
   It is recommended to use Enlitech SS-PST solar simulator. The single xenon lamp solar simulator’s spectrum can achieve A++ grade, highly matching the AM 1.5G reference spectrum.
   
   The devices showed higher open-circuit voltage (Voc) and fill factor (FF), leading to improved overall efficiency.Small-area devices (0.05 cm²) treated with dual-ligand passivation (DLP) achieved PCE of 25.86%, with Voc of 1196 mV and FF of 83.7%. (Figure 4c and Table S2)
   
   

   For 1 cm² devices, DLP-treated devices achieved efficiency of 25.08%, directly demonstrating the advantages of the DLP strategy in improving device efficiency. (Figure 4e)
   

2. Transient Absorption Spectroscopy (TA):
   Analysis of quantum well distribution showed that single-ligand mFPEAI-treated perovskite surfaces exhibited characteristic absorption peaks of both n=1 and n=2 2D perovskite (Figure S3), indicating the presence of multiple 2D phases.
   
   
   Figure 1d shows the TA spectra of single-ligand mFPEAI-treated perovskite surface, while Figure 1e shows the TA spectra of DLP-treated perovskite surface.
   
   
3. External Quantum Efficiency (EQE):
   DLP-treated devices achieved an integrated current density of 25.57 mA/cm². Through differentiation of the EQE curve, a valley at 797 nm was observed, corresponding to a bandgap of 1.55 eV, further verifying the DLP strategy’s ability to enhance photoelectric conversion efficiency. (Figure 4d)
   
   It is recommended to use Enlitech QE-R for quantum efficiency measurements
   
   
4. Photoluminescence Quantum Yield (PLQY):
   DLP-treated samples achieved a high PLQY of 12%, significantly higher than the 9% of single-ligand treated samples, demonstrating that the DLP strategy can effectively reduce non-radiative recombination. (Figure 2d)
   
   It is recommended to use Enlitech LQ-100X-PL for PLQY measurements
   
   
5. Time-Resolved Photoluminescence (TRPL):
   Carrier lifetime measurements showed that DLP-treated perovskite films had an average carrier lifetime τave of 1.77 μs, approximately twice that of single-ligand treated films at 0.80 μs. This indicates reduced trap-assisted non-radiative recombination and more uniform energy distribution. (Figure 2c)
   

Additional Characterizations

• Implied Open-Circuit Voltage (iVoc):
  From PLQY data, the research team determined that under one sun illumination, DLP-treated samples achieved an iVoc of 1229 mV. (Figure 2b)
  
• X-ray Photoelectron Spectroscopy (XPS):
  Analysis of surface element chemical states showed fluorine (F) intensity significantly higher than single-ligand treated surfaces, indicating better 2D layer enrichment at the perovskite surface and improved spatial confinement effects. (Figure S6)
  
• Atomic Force Microscopy (AFM):
  Used for surface roughness measurements. (Figure S8)
  
• Scanning Electron Microscopy (SEM):
  Used for surface morphology observation. (Figure S9)
  
• Kelvin Probe Force Microscopy (KPFM):
  Used for surface potential measurements. (Figure 3d, 3e, and 3f)
  
• Ultraviolet Photoelectron Spectroscopy (UPS):
  Used to measure perovskite valence band energy levels. (Figure 3a, 3b)
  
• Space-Charge Limited Current (SCLC):
  Used to calculate trap density and carrier mobility. (Figure S10a)
  
• Transient Photovoltage (TPV) and Transient Photocurrent (TPC):
  Used to analyze charge recombination and charge transport dynamics. (Figure S13)
  
  It is recommended to use Enlitech TPCV Perovskite Solar Cell Transient Photocurrent-Photovoltage Test System
  
  
• Grazing Incidence X-ray Diffraction (GIXRD):
  Used for crystal structure analysis. (Figure S7)
  
  
  
• Nuclear Magnetic Resonance (HNMR):
  Used for molecular structure analysis. (Figure S1 and S2)
  

Summary and Conclusions

This study successfully developed a Dual-Ligand Passivation (DLP) strategy that effectively controls the quantum well distribution in 2D passivation layers of perovskite solar cells, resolving defects in traditional 2D passivation layers and enhancing device performance and stability. The main achievements include:

• Enhanced Device Efficiency:
  Using the DLP strategy, 0.05 cm² devices achieved a maximum power conversion efficiency (PCE) of 25.86%, with open-circuit voltage (Voc) of 1196 mV and fill factor (FF) of 83.7%. Additionally, 1 cm² devices achieved a PCE of 25.08%.
• Improved Interface Properties:
  DLP-treated perovskite surfaces demonstrated more uniform surface potential distribution and enhanced interfacial electric field strength, promoting carrier separation and collection.
• Enhanced Device Stability:
  • After 1000 hours of maximum power point tracking (MPPT) testing, DLP-treated devices maintained 93% of their initial efficiency, demonstrating excellent long-term operational stability.
  • Under thermal stress testing at 85°C, DLP-treated devices maintained 82% of their initial efficiency after 300 hours, showing good thermal stability.

Original Publication

Original Title: A flowing liquid phase induces the crystallization processes of cesium lead triiodide for 21.85%-efficiency solar cells and low-energy loss
Published in: Energy & Environmental Science
DOI: https://doi.org/10.1039/D4EE04051A