Research Achievements and Highlights

This study, published in Environmental Science: Energy & Fuels (EES), successfully developed a strategy utilizing flowing liquid phase to induce crystallization of cesium lead triiodide (CsPbI3) perovskite, significantly improving solar cell efficiency.

The method employs ammonium formate (AFMS) as an additive, forming a flowing liquid phase during annealing, which alters the reaction pathway, reduces reaction barriers and energy requirements, and achieves uniform film preparation.

The perovskite solar cells achieved a power conversion efficiency (PCE) of 21.85%, with an open-circuit voltage (Voc) loss of only 0.47V, representing the highest efficiency reported in literature for pure CsPbI3 perovskite solar cells.

Research Team

This research was jointly completed by teams from various institutions including the School of Materials Science and Engineering at Shaanxi Normal University and the Dalian Institute of Chemical Physics. The corresponding authors include Shengzhong (Frank) Liu, Fei Gao, and Wangen Zhao.

Research Background

In recent years, all-inorganic perovskite solar cells (PSCs) have attracted significant attention due to their low cost and simple preparation techniques. Compared to organic-inorganic hybrid perovskite solar cells, all-inorganic perovskite solar cells possess more suitable bandgaps and better thermal stability. However, the phase crystallization process of CsPbI3 films has become complex due to the introduction of dimethylammonium iodide (DMAI). While DMAI, as a special phase stabilizer, can greatly reduce the annealing temperature of black CsPbI3, it also presents challenges in controlling the crystallization process.

To better control the crystallization process of CsPbI3 perovskite films, researchers have employed various additive strategies, particularly molten salt-assisted crystallization strategies. Molten salts can modify the crystallization process; for example, formamidinium acetate (FAAc) alters the phase transformation process of CsPbI3 perovskite precursor solution by forming additional intermediate phases.

Furthermore, different alkyl ammonium formates (AAFo) have been used to induce liquid-phase defect passivation, improving open-circuit voltage (Voc) and fill factor (FF), thereby enhancing PCE. Organic ammonium salts such as 1-naphthylmethylammonium formate (NMACOOH) enhance the interaction between HCOO- and Pb2+, reduce the surface reactivity of alkyl salts, and improve device defect passivation and thermal stability. Ion pair stabilizers like dimethylammonium formate (DMAFo) have been introduced into perovskite precursor solutions to suppress iodide ion oxidation and organic cation deprotonation.

Although these molten salts can reduce defects, alter crystallization processes, and produce high-quality CsPbI3 perovskite films, concerns remain about the form of residual counter-ions and anions in the final annealed films and their impact on device performance and stability.

Solution

To eliminate the influence of residual molten salts, this study introduces ammonium formate (AFMS) to modify the crystallization process of CsPbI3 films. Due to its low decomposition temperature, AFMS completely volatilizes during annealing, avoiding residual effects. More importantly, the flowing liquid phase changes the reaction pathway from solid-phase to liquid-phase, reducing reaction barriers and energy requirements, providing mild annealing conditions. Additionally, liquid-phase-based reactions can proceed more thoroughly, producing more uniform films. This method not only improves the crystallinity of CsPbI3 perovskite films but also increases grain size, thereby enhancing solar cell efficiency.

Experimental Process and Steps

1. Material Preparation: Chemical materials including DMF, DMSO, CB, Li-TFSI, TBP, HCOONH4, CsI, PbI2, and DMAI were used without further purification.
2. Functional Layer Preparation:
   • Dense TiO2 layer: Clean FTO substrates were immersed in a 200 mL water solution containing 4.5 mL TiCl4 at 70°C for deposition.
   • Perovskite precursor: 0.6M CsI, PbI2, and DMAI were dissolved in DMSO/DMF mixed solvent (3:17, v/v), stirred for 8 hours at 30°C. Different amounts of HCOOHNH4 (0-8 mg) were added and stirred for 12 hours at room temperature, filtered before use.
   • Spiro-OMeTAD solution: 90 mg Spiro-OMeTAD dissolved in 1 mL chlorobenzene, mixed with 36 μL t-BP and 22 μL Li-TFSI solution.
3. Device Assembly: TiO2 layer annealed at 200°C for 30 minutes and UV-treated, perovskite solution spin-coated at 1000/3000 rpm, annealed at 200°C for 10 minutes, Spiro-OMeTAD spin-coated at 5000 rpm, 80 nm Au electrode thermally evaporated, effective area 0.09 cm².

Research Characterization

Device Performance Characterization

1. Current-Voltage (J-V) Characteristics:
   The research team used Enlitech’s solar simulator (SS-F5-3A) with AM 1.5G spectrum to measure the J-V curves of perovskite solar cells.
   
   

   Results show that after adding AFMS, the device’s Voc increased from 1.17 V to 1.26 V, fill factor (FF) improved from 78.56% to 82.82%, ultimately raising the power conversion efficiency (PCE) significantly from 19.00% to 21.85%.
   
   

   Figure 4a clearly demonstrates the J-V curves of devices with different AFMS concentrations.

   The research team measured forward and reverse scan J-V curves to evaluate the hysteresis effect. After adding AFMS, the hysteresis index decreased from 5.4% to 3.8%, indicating that AFMS effectively suppressed ion migration, passivated interface defects, and reduced charge accumulation.
   

   Figure 4b shows the forward and reverse scan J-V curves.
   

   Figure S15 displays the dark J-V curves of CsPbI3 and AFMS-CsPbI3 solar cells. After adding AFMS, the dark current significantly decreased, indicating that AFMS can effectively passivate interface defects in the perovskite film and reduce dark current leakage.

   Additionally, the research team analyzed the photovoltaic parameters of 25 cells with and without AFMS passivation, finding that after adding AFMS, the average Voc increased to 1.25 V, average Jsc increased to 20.63 mA/cm², average FF improved to 81.72%, and average PCE increased to 21.14%. (Figure 4c and Table S3)
   

2. Maximum Power Point (MPP) Tracking:
   The stability of solar cell output at maximum power point was monitored under continuous AM 1.5G spectral illumination (100 mW cm⁻²) in a nitrogen environment. AFMS-treated solar cells demonstrated excellent stable output characteristics. At a bias of 1.08 V, AFMS-treated cells showed a stable PCE of 21.33% and stable Jsc of 20.12 mA cm⁻², superior to the original cells’ PCE of 19.62% and Jsc of 20.01 mA cm⁻² (at 0.98 V bias). (Figure S17)
   

   It is recommended to use Enlitech SS-X solar simulator as the simulated light source and for J-V curve measurements

3. Quantum Efficiency (EQE):
   The research team used Enlitech’s solar cell spectral response measurement system (QE-R3011) to measure the device’s EQE.
   
   

   Results show that after adding AFMS, the EQE significantly improved in the 350-700 nm wavelength range, attributed to the improved crystallinity of the perovskite film.
   
   Figure S14 shows the EQE spectra and integrated current density.

   It is recommended to use Enlitech QE-R for quantum efficiency measurements

4. Voc Loss Analysis:
   The research team compared the Voc of this study with other CsPbX3 (X = I, Br, and Cl) perovskite solar cells. Through the addition of AFMS, they achieved the lowest Voc loss of 470 mV.

   
   Figure 4d shows the relationship between Voc and optical bandgap (Eg).

   The team measured Voc under different illumination intensities to evaluate defect-assisted recombination. The ideality factor n decreased from 1.798 to 1.108 kBT/q, indicating a significant reduction in non-radiative recombination.

   
   Figure 4e shows the relationship between Voc and illumination intensity.

   It is recommended to use Enlitech REPS for Voc loss analysis

5. Photoluminescence (PL):
   The research team used steady-state photoluminescence (PL) to analyze film defects.
   After adding AFMS, the PL intensity of the films significantly increased, indicating reduced defects and suppressed non-radiative recombination.
   
   

   Figure 3b shows the steady-state photoluminescence diagram.

   The team used time-resolved photoluminescence (TR-PL) spectroscopy to measure carrier lifetime. Results showed that after adding AFMS, the average carrier lifetime (τave) increased from 17.45 ns to 54.12 ns, further supporting the effective suppression of defects.

   Figure S10 and Table S1 provide TR-PL spectra and related parameters.
   

   It is recommended to use Enlitech LQ-100X-PL for PLQY and QLFS measurements

6. Space Charge Limited Current (SCLC):
   The research team used the space charge limited current (SCLC) method to evaluate the defect state density of CsPbI3 films. Through analysis of dark J-V curves of electron-only devices (Figure 3c), they found that after adding AFMS, the defect state density decreased from 2.052 × 10^15 cm⁻³ to 1.184 × 10^15 cm⁻³, indicating that AFMS effectively reduced the defect density in CsPbI3 perovskite films.
   

Other Characterizations

• X-ray Diffraction (XRD): XRD was used to track crystallization process and phase transitions. Figure 1b shows XRD patterns of films with and without AFMS at different annealing times.
  Figure S8 shows XRD spectra of perovskite films with different AFMS concentrations.
  
  
• Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM): SEM and AFM were used to analyze film surface morphology. (Figure S7)
  
• Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC): TGA and DSC were used to analyze AFMS thermal stability. (Figure S3)
  
• Ultraviolet Photoelectron Spectroscopy (UPS): UPS was used to analyze energy levels of perovskite solar cells. (Figure 3d and Figure S11)
  
• Fourier Transform Infrared Spectroscopy (FTIR): FTIR was used to analyze interactions between AFMS and DMAI and PbI2. (Figure 2a, 2b, S5)
  
• Nuclear Magnetic Resonance (NMR) Spectroscopy: 1H and 13C NMR spectra were used to further analyze chemical interactions of AFMS in precursor solutions. (Figure 2c and 2d)
  
• X-ray Photoelectron Spectroscopy (XPS): Used to analyze interactions between AFMS and CsPbI3. (Figure S6)
  
• Water Contact Angle Measurement: The research team measured water contact angles of perovskite films (Figure S7c)
  
• Electrochemical Impedance Spectroscopy (EIS): The research team used EIS to analyze carrier transport processes in PSCs, fitting data with an equivalent circuit including series resistance (Rs) and recombination resistance (Rrec). (Figure S16 and Table S4)
  
• Mott-Schottky Analysis: Mott-Schottky analysis was used to evaluate the built-in potential (Vbi). Results showed that after adding AFMS, Vbi increased, indicating enhanced carrier separation and transport, thereby improving Voc. (Figure 4f)
  

Stability Testing

• Operational Stability Test:
  Continuous operation tests of unencapsulated devices were conducted at room temperature in a nitrogen environment. After 550 hours of maximum power point output, AFMS-passivated perovskite devices maintained over 94.33% of their initial PCE, while original devices degraded to 49.45% of their initial PCE after 500 hours. This indicates that AFMS can significantly improve device stability. (Figure 5a)
  
  
  
• Environmental Stability Test:
  Environmental stability assessment was conducted on unencapsulated solar cells in air under ambient conditions (30% relative humidity, 25°C). After 700 hours of storage, the PCE of AFMS-CsPbI3 solar cells decreased to 92.3%, while CsPbI3 solar cells decreased to 72.5%. (Figure 5b)
  
  
  
• Film Stability Test:
  CsPbI3 and AFMS-CsPbI3 perovskite films were exposed to 30-40% ambient atmosphere, and changes in their morphology and crystallinity were monitored. The original films showed d-CsPbI3 phase after 100 hours, while AFMS-added films maintained pure black g-CsPbI3 phase for 300 hours, with minimal yellow d-CsPbI3 phase appearing only after 350 hours. This indicates that AFMS can improve film air stability. (Figure 5c, 5d, and 5e)
  

Conclusions

• Development of Efficient Strategy:
  This research successfully developed an efficient strategy using ammonium formate (AFMS) to accelerate perovskite film crystallization rate.
• Reaction Pathway Transformation:
  Through the use of AFMS, the research team successfully transformed the reaction pathway from solid to liquid, reducing reaction energy barriers and energy requirements.
• Key Role of Formate Anions:
  The formate anions (HCOO⁻) in AFMS played a crucial role in suppressing anion vacancy defects at grain boundaries and surfaces of CsPbI3 perovskite films. Additionally, the interaction between –HCOO⁻ and Pb²⁺ helped passivate Pb²⁺ defects, thereby improving the crystallinity of CsPbI3 films.
• Performance Achievement:
  Ultimately, AFMS-improved CsPbI3 solar cells achieved a power conversion efficiency (PCE) of 21.85%, with unencapsulated devices maintaining 92.3% of their initial efficiency over 700 hours.
• Future Potential:
  These findings highlight the potential of AFMS in improving the efficiency and stability of CsPbI3 solar cells and show promise for application in a wide range of organic-inorganic hybrid perovskite optoelectronic devices.

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