Research Highlights

This study reveals the crucial impact of microscopic uniformity in lateral energy levels on perovskite film surfaces on device performance, particularly in terms of device lifetime and scalability. The research found that while traditional organic ammonium passivators can suppress electron traps, they lead to microscopic inhomogeneity in surface lateral energy levels, negatively affecting device performance.

In contrast, organic guanidinium passivators can provide flat energy level distribution at the microscopic scale, significantly improving the performance of perovskite solar cells and small modules. Small-area devices and solar modules (27.2 cm²) based on PPAd passivation treatment achieved the highest power conversion efficiencies of 25.5% and 22.5% (with certified active area efficiency of 23.4%) respectively.

The perovskite solar cells maintained 90% efficiency after approximately 6000 hours of continuous operation at 70°C, highlighting the importance of microscopic uniformity in lateral energy levels for enhancing perovskite solar cell performance.

Research Achievements

Research on perovskite solar cells (PSCs) has traditionally focused on suppressing vertical energy disorder to reduce non-productive charge recombination for achieving high efficiency. Professor Rui Wang’s team at Westlake University points out that although significant progress has been made in improving PSC efficiency, lateral energy disorder at large-area module junctions has been largely overlooked.

This study focuses on the characteristics of microscopic inhomogeneity in lateral energy levels on formamidinium (FA) perovskite film surfaces, investigating its impact on device performance, particularly stability and scalability. The research found that while traditional organic ammonium surface passivators can suppress electron trap states across junctions, they create microscopic inhomogeneity in lateral energy levels, affecting PSC stability.

In contrast, organic guanidinium passivators can provide uniform lateral energy level distribution at the microscopic level, significantly improving the performance of PSCs and modules. Experimental results show that devices treated with the organic guanidinium passivator PPAd achieved the highest aperture PCEs of 25.5% for small-area devices and 22.5% for solar modules (27.2 cm²). Furthermore, PSCs maintained 90% efficiency after approximately 6000 hours at 70°C, highlighting the importance of microscopic uniformity in lateral energy levels for PSC performance.

Research Team

Corresponding authors: Professor Rui Wang from Westlake University and Professor Jingjing Xue from Zhejiang University. This research was completed through collaboration among researchers from Zhejiang University, Westlake University, Swiss Federal Institute of Technology Lausanne, Soochow University, Marmara University, Shanghai Advanced Research Institute of Chinese Academy of Sciences, North China Electric Power University, and other institutions.

Research Background

In perovskite solar cells (PSCs), efforts have been focused on constructing favorable energy level distributions in the vertical direction to ensure effective carrier transport across devices. Researchers have invested considerable effort in various techniques to achieve planar energy level landscapes and minimize electronic disorder across junctions, leading to high power conversion efficiencies (PCEs).

These techniques include controlling crystal growth, composition engineering, energy band alignment, and surface passivation. Among these, surface passivation has become the most attention-grabbing and indispensable strategy for achieving stable and high-performance PSCs in recent years.

Many chemicals have been studied for passivating surface defects to reduce energy disorder along vertical carrier transport paths. Through developing various surface defect passivation strategies, the mitigation of electronic disorder at vertical junctions has been extensively studied, focusing on reducing the total density of surface defects. However, lateral surface energy disorder, while harmful to perovskite module performance, has received little attention.

Solution

This study addresses the issue of microscopic inhomogeneity in lateral energy levels caused by traditional organic ammonium passivators by proposing the use of organic guanidinium passivators to construct flat energy level distributions at the microscopic scale. The research selected a series of traditional organic ammonium passivators and organic guanidinium passivators, using alkyl chains and aromatic units as linking groups for comparative study. The molecular structures of these passivators are shown in Figure 1A.

Experimental Process and Steps

To verify the effectiveness of organic guanidinium passivators, the following experimental steps were taken:

1. Perovskite Film Preparation:
   Using a two-step solution method to prepare FAPbI3-based perovskite films (FAxCs1-xPbI3), with surface treatment using the aforementioned organic ammonium and guanidinium passivators.
2. Low-Dimensional Perovskite Single Crystal Growth:
   To better understand the structures formed by different surface treatments, researchers successfully grew low-dimensional perovskite single crystals with PPAm and PPAd.
3. Perovskite Solar Cell Fabrication:
   Small-area perovskite solar cells (0.1 cm²) and large-area solar modules (27.2 cm²) were fabricated using both two-step and one-step methods with PPAm and PPAd surface passivation.
4. Accelerated Aging Test:
   Following the ISOS-L-1 standard protocol, long-term stability of perovskite small-area devices and large-area modules was tested under accelerated aging conditions (70°C, continuous one-sun illumination), tracking PCE changes at the maximum power point (MPP).

Research Characterization

Device Performance Characterization

1. Solar Simulator Characterization:
   Researchers used solar simulators to test the current-voltage (J-V) characteristics of perovskite solar cells and calculate their power conversion efficiency (PCE).
   
   
   • Figure 4A shows the J-V curves of perovskite solar cells prepared using the one-step method, with PPAm treatment, PPAd treatment, and without treatment. The results show that PPAd-treated devices achieved the highest open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF), resulting in the highest power conversion efficiency (PCE). PPAm-treated devices had similar Voc to PPAd-treated devices but lower FF, resulting in lower PCE. Untreated devices had the lowest Voc but similar FF to PPAm-treated devices, with PCE falling between the two.
     
   • Figure 4B presents PCE statistics for 20 perovskite solar cells prepared using the one-step method with PPAm treatment, PPAd treatment, and without treatment. The statistics show that PPAd-treated devices have a more concentrated PCE distribution with the highest average value. PPAm-treated devices show a more dispersed PCE distribution with a lower average value than PPAd-treated devices. Untreated devices also show a dispersed PCE distribution with an average value between the two.
     
   • Supplementary Figure 25 shows PCE statistical data for perovskite solar cells prepared using the two-step method with PPAm, PPAd treatment, and without treatment. Similar to the one-step method devices, PPAd-treated devices showed the highest average PCE, followed by PPAm-treated devices, with untreated devices showing the lowest values.
     
   • Supplementary Figure 27 shows J-V curves of perovskite solar modules (aperture area 27.2 cm²) treated with PPAm, PPAd, and without treatment. The results show that PPAd-treated modules achieved the highest open-circuit voltage (Voc), short-circuit current (Isc), and fill factor (FF), resulting in the highest PCE. Compared to untreated modules, PPAm-treated modules showed slight improvements in Voc and Isc, but a more significant increase in FF, leading to improved PCE.
     
   • Supplementary Figure 49 shows J-V curves of MAPbI3 perovskite solar cells treated with PPAm and PPAd. The results demonstrate that PPAd-treated devices achieved the highest open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF), resulting in the highest PCE.
     
   • Supplementary Figure 53 shows J-V curves of perovskite solar cells treated with PPAm and PPAd. The results indicate that PPAd-treated devices achieved the highest open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF), leading to the highest PCE.
2. Quantum Efficiency Measurement:
   Researchers used quantum efficiency measurement system (EQE) to measure the photoelectric conversion efficiency of perovskite solar cells at different wavelengths and calculate the short-circuit current density (Jsc).As shown in Figure 34, the device exhibits high EQE in the wavelength range of 300 nm to 800 nm, indicating excellent photoelectric conversion capability in the visible light range. The shape of the EQE curve is similar to the device’s absorption spectrum, which also verifies its good light absorption capability. Through integration of the EQE curve, the device’s short-circuit current density (Jsc) was obtained, which matches the Jsc value measured from the J-V curve, further verifying the accuracy of the data.

Enlitech QE-R Solar Cell Quantum Efficiency Optical System helps you measure quantum efficiency

Other Characterizations

The study employed various characterization techniques including:

• Ultraviolet Photoelectron Spectroscopy (UPS):
  UPS measurements of film work function and valence band maximum position revealed similar Fermi levels for perovskite surfaces treated with different molecules, indicating similar energy level distributions for charge carriers crossing vertical junctions.
• Kelvin Probe Force Microscopy (KPFM):
  KPFM measurements showed inhomogeneous Fermi level distribution at the microscopic scale for PAm, PAd, and PPAm-treated perovskite surfaces, while PPAd treatment formed smooth lateral energy level distribution.
• Fourier Transform Infrared Spectroscopy (FTIR):
  FTIR measurements were used to study the interaction between molecules and the PbI2-terminated perovskite surface. Results showed that guanidinium group interactions with the Pb-I framework were stronger than ammonium interactions, attributed to the rich hydrogen bonding provided by two N-H groups in guanidinium.
• Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS):
  GIWAXS measurements revealed low-dimensional perovskite phases formed on the surface of FAPbI3-based perovskite films treated with PAd and PPAd, as shown by diffraction patterns at low qz values.
• Single Crystal X-ray Diffraction (XRD):
  Crystal structures of PPAm and PPAd-based perovskites were determined through single crystal analysis, as shown in Figure 1D.
  
• Scanning Electron Microscopy (SEM):
  SEM images showed that PPAm forms plate-like surface features on top of 3D perovskite, while PPAd forms dot-like surface features, which is more favorable for achieving complete and uniform surface morphology.
• Photoluminescence (PL) Spectroscopy and Imaging:
  Spectral analysis revealed the distribution of PPAd-based 1D phases on 3D perovskite film surfaces, and carrier lifetime spatial distribution was observed for PPAd and PPAm-treated perovskite samples, showing more uniform PL lifetime for PPAd-treated samples.
• Transient Reflectance Spectroscopy (TRS):
  TRS was used to study carrier dynamics at perovskite surfaces treated with both passivators, finding higher surface recombination rates for PPAm, suggesting that local fields induced by energy level inhomogeneity may increase carrier recombination probability.
• Time-Resolved Photoluminescence (TRPL) Spectroscopy:
  TRPL spectroscopy was used to monitor and compare the stability of PPAm and PPAd-treated perovskite films, revealing higher stability in PPAd-treated films.
• In-situ Confocal PL Imaging:
  In-situ confocal PL imaging was used to observe real-time spatial PL information during aging tests, showing PL peak position blue-shifts in PPAm-treated films during aging, while PPAd-treated films maintained stable peak positions.
• Density Functional Theory (DFT) Calculations:
  DFT calculations suggested that guanidinium with phenyl ring linking groups are more likely to arrange in a stacked manner due to their higher stacking energy.

Research Results

Figure 4F shows MPP tracking results for differently treated perovskite solar modules under simulated sunlight at 70°C to evaluate their long-term stability. The graph clearly demonstrates the significant improvement in perovskite solar module stability with PPAd treatment.

PPAd-treated modules retained 90% of their initial efficiency (T90) after 1000 hours, far superior to PPAm-treated modules (losing 20% efficiency after about 300 hours) and untreated modules (losing over 50% efficiency after about 300 hours).

This result directly proves the superiority of PPAd passivator in enhancing the long-term stability of perovskite solar cells, supporting the study’s core argument that organic guanidinium passivators can improve microscopic uniformity of lateral energy levels on perovskite film surfaces, thereby enhancing device performance, particularly in stability and scalability.

The results from Figure 4F align with the film stability analysis in Figure 3. Figure 3 shows that PPAd-treated perovskite films maintained almost constant carrier lifetime during aging tests at 70°C, while PPAm-treated films showed rapid degradation. This further confirms the effectiveness of PPAd treatment in improving perovskite material stability.

Conclusions and Advantages

This study confirms the crucial impact of microscopic uniformity in lateral energy levels on perovskite film surfaces for device performance, particularly in device lifetime and scalability. While traditional organic ammonium passivators can suppress electron traps, they cause microscopic inhomogeneity in surface lateral energy levels, negatively affecting device performance and stability.

Compared to traditional organic ammonium passivators, organic guanidinium passivators offer the following advantages:

• Stronger Chemical Bonding:
  The guanidinium group shows stronger interaction with the Pb-I framework compared to ammonium, due to rich hydrogen bonding provided by two N-H groups in guanidinium.
• More Uniform Surface Coverage:
  The 1D perovskite phase formed by PPAd passivator grows more easily and covers the entire surface more uniformly, reducing surface recombination rates.
• Lower Lattice Strain:
  The 1D perovskite phase formed by PPAd passivator has lower lattice strain when forming on the 3D perovskite surface, promoting more uniform surface coverage.
• More Stable Films:
  PPAd-treated perovskite films show higher stability in accelerated aging tests, attributed to their more uniform surface energy level distribution suppressing ion migration and PbI2 formation.

Based on these advantages, organic guanidinium passivators significantly improve the performance of perovskite solar cells and small modules:

• Small-Area Devices:
  PPAd-treated small-area devices achieved a highest PCE of 25.5%, compared to 24.2% for PPAm-treated devices.
• Solar Modules:
  PPAd-treated solar modules (27.2 cm²) achieved a highest PCE of 22.5% (active area efficiency of 23.4%, certified), compared to 20.8% for PPAm-treated modules.
• Long-Term Stability:
  PPAd-treated devices maintained 90% efficiency after approximately 6000 hours of operation at 70°C, while PPAm-treated devices lost over 20% efficiency after 1000 hours.

Research Conclusion

This study highlights the importance of lateral energy level microscopic uniformity for perovskite solar cell performance, particularly in device lifetime and scalability, providing important guidelines for designing effective passivators in the future.

Original publication: “Micro-homogeneity of lateral energy landscapes governs the performance in perovskite solar cells”, Nature Communications, Published: 09 November 2024, DOI: doi.org/10.1038/s41467-024-53953-4