Research Achievements and Highlights

This research successfully developed a simple and effective strategy to enhance the efficiency and stability of solar cells by adding trimethylgermanium chloride (TGC) to the FACsPbI₃ (Formamidinium cesium lead triiodide) perovskite precursor solution. The addition of TGC triggers a series of consecutive reactions, including the hydrolysis of Ge-Cl bonds to form Ge-OH groups, which then form hydrogen bonds (O-H···N and O-H···I) with FAI. These continuous interactions effectively protect FA⁺ from decomposition, accelerate crystallization, restrict ion migration, and passivate film defects, ultimately resulting in high-quality perovskite films with superhydrophobic surfaces that maintain their photoactive phase (α-phase) even after 10 days of exposure to high humidity air (relative humidity: 85%). 

Key research achievements include:

• High Efficiency: Traditional (n-i-p) and inverted (p-i-n) FACsPbI₃ perovskite solar cells achieved power conversion efficiencies of 26.03% and 26.38% respectively.
• High Stability: Maintained initial efficiency after 3000 hours of continuous tracking at maximum power point (MPP). Retained 94.8% of initial efficiency after 2200 hours of aging in air (25°C, relative humidity 30±5%); maintained 87.2% of initial efficiency after 450 hours of heating at 85°C in nitrogen glovebox.
• Superhydrophobic Surface: TGC-treated perovskite films exhibited superhydrophobic properties with a water contact angle of 113.89°, contributing to enhanced device stability.
• Defect Passivation: Passivated defects in perovskite films, including reduction of both hole and electron defect densities.
• Enhanced Crystallinity: Accelerated perovskite crystallization rate and improved crystallinity while reducing residual stress in films.
• Ion Migration Suppression: Effectively restricted ion migration, thereby suppressing the formation of vacancy defects.

These achievements have significant implications for advancing the commercialization of perovskite solar cells, particularly in terms of efficiency and long-term stability.

Research Team

This research was completed through collaboration among multiple research institutions, with corresponding authors being Yuwei Duan from Chengdu University of Technology, and Shengzhong Liu and Zhike Liu from Shaanxi Normal University.

Research Background

Perovskite solar cells (PSCs) are on the verge of commercialization due to their excellent power conversion efficiency (PCE), simple preparation process, versatile applications, and low cost.

Formamidinium (FA) perovskites, particularly FACsPbI₃, have been widely used as light-absorbing materials for high-efficiency stable PSCs due to their suitable bandgap and good thermal stability. FACsPbI₃-based PSCs have achieved certified PCEs exceeding 26%, leading the development trend of perovskite photovoltaic technology.

However, optimization strategies for FACsPbI₃ PSCs typically involve either spin-coating organic ligands on top of the perovskite film or introducing molecular bridges at the buried perovskite/charge transport layer (CTL) interface. While these methods effectively passivate surface defects, they may lead to the following issues:

• Surface Passivation Method: Formation of low-dimensional perovskites with uncontrollable phase distribution and thickness, limiting charge transport at heterojunctions.
• Molecular Bridge Method: Limited defect binding sites for molecular bridges at heterojunctions, and narrow processing windows between molecular bridge deposition and perovskite precursor solution spin-coating, potentially creating resistance barriers between perovskite and CTL interfaces.
• Water Molecule Penetration: Existing additives, such as formamidinium formate (FAHCOO) or dodecylammonium formate and other ionic additives, struggle to prevent water molecule penetration into perovskites, leading to device stability issues.

Solution

This research proposes incorporating trimethylgermanium chloride (TGC) into the FACsPbI₃ perovskite precursor solution to address the challenges of traditional methods. TGC triggers a series of interactions in the perovskite solution and film, achieving precise control of the crystallization process and effective defect passivation. The mechanism of TGC action includes several key steps:

1. Ge-Cl Bond Hydrolysis: Ge-Cl bonds in TGC react with water molecules to form Ge-OH groups, initiating subsequent reactions.
2. Hydrogen Bond Formation: Ge-OH groups form hydrogen bonds (O-H···N and O-H···I) with nitrogen (N) and iodine (I) atoms in FAI, stabilizing the perovskite structure and protecting FA⁺ ions.
3. Crystallization and Stability Enhancement:
   – Accelerates crystallization process and reduces nucleation barriers
   – Reduces ion migration through hydrogen bond formation and defect passivation
4. Surface Modification: Forms superhydrophobic surfaces, effectively blocking water molecule penetration and improving long-term stability
5. Defect Control:
   – Simultaneously passivates surface and bulk defects
   – Reduces trap state density and improves charge transport efficiency

Experimental Process and Steps

1. Material Preparation:

• SnO₂ Aqueous Solution: Prepared 0.012 M SnO₂ aqueous solution using urea, hydrochloric acid, thioglycolic acid (TGA), and tin chloride dihydrate (SnCl₂·2H₂O)
• Perovskite Precursor Solution (n-i-p): Prepared 1.5 M FA₀.₉Cs₀.₁PbI₃ perovskite precursor solution using PbI₂, CsI, FAI, and MACl
• Perovskite Precursor Solution (p-i-n): Prepared 1.6 M FA₀.₉₅Cs₀.₀₅PbI₃ perovskite precursor solution using PbI₂, CsI, FAI, PbCl₂, and MACl
• Spiro-OMeTAD Solution: Dissolved Spiro-OMeTAD, 4-tBP, and LiTFSI in chlorobenzene

2. Device Fabrication (n-i-p):

• FTO Substrate Cleaning and SnO₂ Layer Preparation: Cleaned FTO substrates using Hellmanex III, deposited SnO₂ layer using CBD method at 95°C for 6 hours
• Perovskite Layer Preparation: Spin-coated perovskite solution containing different concentrations of TGC, annealed at 150°C
• Device Completion: Sequentially fabricated passivation layer, HTL layer, and finally evaporated Au electrode

3. Device Fabrication (p-i-n): Cleaned FTO substrates and performed surface treatment, fabricated NiOx and Me-4PACz double-layer HTL, spin-coated perovskite layer using anisole as anti-solvent, fabricated passivation layer, ETL layer, and finally evaporated Ag electrode and MgF₂ protective layer.

Research Characterization

J-V Curve (Current Density-Voltage Curve)

Enlitech’s SS-F5-3A solar simulator and Keithley 2400 SourceMeter were used to measure the J-V curves of solar cells.

It is recommended to use Enlitech’s SS-X AM1.5G standard spectrum solar simulator as the simulated light source for J-V curve measurements.

Figure S15 shows the J-V curves of solar cells prepared with different TGC concentrations (Table S5).

Figure 6b displays J-V curves of control and TGC-treated target groups, showing significant PCE improvement after TGC addition.

Figure S17 presents J-V curves of control and TGC-treated groups under forward and reverse scans, with Table S6 summarizing these parameters, indicating lower hysteresis index (HI) in TGC-treated groups.

The J-V curve measurements were conducted under 100 mW cm⁻² illumination, calibrated using an NREL-traceable KG5-filtered silicon reference cell.

EQE (External Quantum Efficiency) Spectrum

Research utilized the Enlitech QE-R system to measure solar cell EQE spectra, verifying JSC measurement accuracy.

It is recommended to use Enlitech’s QE-R PV/Solar Cell Quantum Efficiency Optical System for precise EQE testing data of various solar cells.

Figure 6c shows EQE spectra of control and target groups, with calculated integrated current (Jint) matching JSC values.

Other Characterization Methods

Multiple characterization techniques were employed:

• SEM (Scanning Electron Microscopy): Observed perovskite film surface morphology (Figure 2a and S1)
  
• AFM (Atomic Force Microscopy): Measured perovskite film surface roughness (Figure 2b)
  
• XRD (X-ray Diffraction): Studied perovskite film crystal structure and crystallinity (Figure 2c and S2)
  
• GIWAXS (Grazing-Incidence Wide-Angle X-ray Scattering): Studied perovskite film crystal orientation (Figure 2d, e)
  
• XPS (X-ray Photoelectron Spectroscopy): Studied perovskite film elemental composition and chemical states (Figure 4a-d)
  
• UPS (Ultraviolet Photoelectron Spectroscopy): Studied perovskite film band structure (Figure S13 and Table S4)
  
• PL (Photoluminescence) and TRPL (Time-Resolved Photoluminescence): Studied carrier recombination and charge transport dynamics (Figure 3b, 3c, S4)
  
• TAS (Transient Absorption Spectroscopy): Studied charge transfer dynamics (Figure 3d-f and S5)
  
• ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry): Analyzed TGC distribution in perovskite films (Figure 3g and S6)
  
• NMR (Nuclear Magnetic Resonance): Studied TGC behavior in DMSO-D6 and D2O solutions (Figure S7 and S8)
  
• LCMS (Liquid Chromatography-Mass Spectrometry): Detected TGC hydrolysis products in aqueous solution (Figure S9)
  
• FTIR (Fourier Transform Infrared Spectroscopy): Studied chemical bond changes in TGC and FAI (Figure 4g, h)
  
• KPFM (Kelvin Probe Force Microscopy): Measured surface contact potential of perovskite films (Figure S14)
  
• GIXRD (Grazing Incidence X-ray Diffraction): Analyzed residual stress in perovskite films (Figure 2f, g, h)
  
  
• EDS (Energy Dispersive X-ray Spectroscopy): Studied elemental distribution in perovskite films (Figure S12)
  
• Electrochemical Impedance Spectroscopy (EIS): Measured EIS to study charge transport and recombination in solar cells (Figure 6f and Table S7)
  
• Space Charge Limited Current (SCLC): Measured defect density in perovskite films, calculated using the formula Nt = 2εrε0VTFL/qL² (Figure S18)
  
• Transient Photovoltage (TPV) and Transient Photocurrent (TPC): Studied charge transport dynamics in solar cells (Figure 6i and S19)
  

It is recommended to use Enlitech’s TPCV Perovskite Solar Cell Transient Photocurrent-Photovoltage Test System to enhance perovskite solar cell performance and deepen understanding of cell performance parameters.

• Activation Energy Measurement: Measured the relationship between conductivity and temperature in perovskite films to evaluate ion migration activation energy (Figure 3h)
  

Conclusions

This research successfully developed a simple and effective strategy by adding trimethylgermanium chloride (TGC) to FACsPbI₃ perovskite precursor solution, significantly improving the efficiency and stability of perovskite solar cells. Experimental results confirmed that traditional (n-i-p) and inverted (p-i-n) structures achieved efficiencies of 26.03% and 26.38% respectively, with significant improvements in key parameters including short-circuit current density, open-circuit voltage, and fill factor.

In terms of stability, TGC-treated devices demonstrated excellent performance: maintaining photoactive phase after 10 days of exposure to 85% relative humidity; retaining 94.8% of initial efficiency after 2200 hours of aging in air; maintaining 87.2% efficiency after 450 hours of heating at 85°C; and maintaining 100% efficiency after 3000 hours of maximum power point tracking. These data conclusively demonstrate that TGC can effectively enhance long-term device stability.

The addition of TGC also significantly improved perovskite film quality. Experimental results showed that TGC not only promoted crystallization and increased grain size but also improved film crystallinity, favored α-phase stability, and reduced residual stress and Urbach energy. In terms of carrier transport, TGC introduction reduced non-radiative recombination, extended carrier lifetime, accelerated charge transport, and reduced series resistance while increasing recombination resistance.

Furthermore, TGC demonstrated excellent defect control capability, significantly reducing electron and hole defect densities and increasing ion migration activation energy. Notably, TGC-treated films exhibited superhydrophobic surfaces with a water contact angle of 113.89°, further enhancing device environmental stability.

Original Publication Information

Original Title: Successive Reactions of Trimethylgermanium Chloride to Achieve > 26% Efficiency MA-Free Perovskite Solar Cell With 3000-Hour Unattenuated Operation
Journal: Advanced Materials
DOI: doi.org/10.1002/adma.202414354