Research Achievements and Highlights

This research introduces a novel “halide locking” strategy through the incorporation of a multifunctional ammonium salt—thioacetylacetamide hydrochloride (TAACl)—successfully regulating the nucleation and crystal growth process of wide-bandgap perovskites. This strategy not only achieved excellent compositional uniformity during the wet film phase but also induced preferred orientation along the (001) plane, resulting in superior uniformity in both vertical and horizontal directions.

• Record-Breaking Efficiency: Large-area tandem solar cells based on tunnel oxide passivated contact (TOPCon) silicon subcells achieved a record-breaking power conversion efficiency of 31.32%, with an open-circuit voltage (Voc) of 1.931 V and a fill factor (FF) of 81.54%.
• Outstanding Single-Junction Performance: Wide-bandgap perovskite single-junction solar cells achieved exceptional open-circuit voltage-fill factor products (VOC × FF) of 1.074 and 1.040 for small-area (0.0414 cm²) and large-area (1.0208 cm²) devices, respectively.
• Halide Distribution Uniformity: The introduction of TAACl effectively locked halides in the intermediate phase, promoting the formation of highly crystalline perovskite films and homogenizing halide distribution in both vertical and horizontal directions.
• Phase Separation Suppression: This strategy can effectively suppress phase separation under illumination, enhancing long-term device stability.
• Innovative “Halide Locking” Strategy: Through TAACl’s interaction with all cations and anions in the perovskite layer, nucleation and crystal growth are regulated.

Research Team

This research was completed through collaboration among multiple institutions. The first authors are Lina Wang, Ning Wang, and Xin Wu.
The corresponding authors include: Zonglong Zhu, Xin Wu, and Bo Li from City University of Hong Kong; Fang Xu from Shenzhen Technology University; and Xinyu Zhang and Menglei Xu from Jinko Solar Co., Ltd.

Research Background

As single-junction perovskite solar cells (PSCs) continue to improve in power conversion efficiency (PCE), further enhancing device performance has become a research focus. Perovskite materials feature tunable bandgaps (Eg) from 1.25 eV to ~2.3 eV, making them suitable for integration with traditional silicon solar cells and thin-film CIGS solar cells. However, controlling crystallization processes and mitigating phase/composition inhomogeneity when growing perovskite layers on rough silicon substrates remains a significant challenge, ultimately limiting the efficiency and stability of tandem solar cells (TSCs).

• Energy Loss: The actual open-circuit voltage (Voc) values of perovskite subcells (approximately 1.20-1.25 V) are significantly lower than theoretical values (>1.37 V), primarily due to severe non-radiative recombination within and on the surface of mixed-halide perovskite films.
• Halide Inhomogeneity: Bromide ions (Br-) lead to poor uniformity and low crystallinity, creating excessive halide vacancies that form migration channels and recombination centers, unfavorable for device scaling and stability.
• Phase Separation: Thermodynamically unfavorable halide composition mixing forms iodine-rich regions under sufficient illumination, reducing long-term stability.
• TOPCon Technology Limitations: The efficiency of tandem solar cells based on tunnel oxide passivated contacts (TOPCon) barely approaches 30%, far below the simulated practical limit of 39.5%. While optical losses have been minimized through careful structural design and interface layer optimization, electrical losses remain the primary bottleneck for further efficiency improvement.

Solution

This research addresses these challenges by proposing an innovative “halide locking” strategy using thioacetylacetamide hydrochloride (TAACl) as a multifunctional additive to regulate perovskite film nucleation and crystal growth processes.

TAACl Mechanism

• Ion Bonding: TAACl forms strong bonds with all cations and anions in the mixed-halide perovskite layer, effectively locking halides in the intermediate phase, promoting highly crystalline perovskite film formation, and homogenizing halide distribution in both vertical and horizontal directions.
• Nucleation Control: TAACl introduction reduces spontaneous crystal nucleation, promoting uniform halide ion distribution during nucleation. DFT simulations show higher critical nucleation radius and lower nucleation rates compared to control films.
• Crystal Orientation: TAACl induces uniform crystal nuclei growth along the corner-sharing octahedral direction, beneficial for high-performance perovskite solar cells. GIWAXS measurements indicate significantly higher (001) plane integral intensity in target films, suggesting preferred crystal orientation.

Core Issues Addressed

• Compositional Inhomogeneity: TAACl introduction resolves compositional inhomogeneity caused by halide ion migration in traditional methods.
• Non-radiative Recombination: Uniform crystals and composition help reduce film defects, minimizing non-radiative recombination and improving open-circuit voltage.
• Phase Separation: TAACl stabilizes the perovskite lattice, suppressing halide ion migration under illumination, reducing phase separation.

Experimental Process and Steps

Material Preparation

• Perovskite Layer: A 1.68 eV bandgap perovskite absorber layer was integrated with TOPCon silicon cells for optimal current matching in tandem architecture.
• Precursor Solution: 1.4 M perovskite precursor solution containing CsI, MABr, FAI, PbI2 (10% excess), and PbBr2 mixed in DMF:DMSO (4:1 volume ratio).
• Treatment Groups: Target group included 5% TAACl in precursor solution, while control group had none.

Film Preparation

• Spin Coating: Precursor solution was spin-coated on glass/ITO/HTL at 1000 rpm for 10 seconds, followed by 4000 rpm for 30 seconds.
• Anti-solvent Treatment: For control group, 150 μL chlorobenzene (CB) was dropped at center 15 seconds before end of spin coating.
• Annealing: Prepared perovskite films were annealed at 100°C for 15 minutes.

Surface Treatment

CF3-PEAI (2 mg ml−1) was dissolved in IPA and spin-coated at 4000 rpm for 30 seconds, followed by annealing at 100°C for 10 minutes.

Device Fabrication

• Layer Deposition: C60 (25 nm), BCP (6 nm), and silver (100 nm) were thermally evaporated on the surface-treated films.
• Back Contact: A 120 nm thick magnesium fluoride layer was deposited on the back of the glass/ITO substrate to enhance transmittance.

Research Results and Characterization Methods

Solar Cell Performance Enhancement

• Current-Voltage (J-V) Curve Measurement:
  Enlitech SS-F5 solar simulator was used for J-V characteristic measurements and light stability testing, calculating power conversion efficiency (PCE), open-circuit voltage (VOC), and fill factor (FF)
  It is recommended to use Enlitech SS-X solar simulator, which closely matches the AM1.5G standard spectrum.
  
  
  • Figure 3B shows that target group wide-bandgap perovskite solar cells in small area (0.0414 cm²) measurements achieved PCE of 22.95%, VOC of 1.277 V, and FF of 84.08%, while the control group achieved PCE of 20.58%.
    
  • Figure 4B shows tandem solar cells based on TOPCon silicon subcells achieved a record PCE of 31.32%, with VOC of 1.931 V and FF of 81.54%.
• External Quantum Efficiency (EQE):
  Research team used Enlitech QE-R EQE system for measurements to verify solar cell photoelectric conversion efficiency under different wavelength illumination and confirm current density (JSC) accuracy.
  
  It is recommended to use Enlitech QE-R quantum efficiency optical instrument, a high-precision QE/IPCE testing system
  • Figure 3D shows EQE spectra of target and control group single-junction wide-bandgap perovskite solar cells. The target group shows higher EQE across the entire wavelength range, with integrated JSC values matching those measured from J-V curves.
  • Furthermore, research shows integrated current densities of 20.14 mA cm⁻² for top perovskite subcells and 20.02 mA cm⁻² for bottom TOPCon silicon subcells in tandem solar cells, indicating excellent current matching between subcells.
• Steady-State Power Output (SPO): Measurement of solar cell stable power output at maximum power point. (Figure 3C)
  

Composition Uniformity Enhancement

• • Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS): Used to detect ion distribution in films. (Figure S2, Figure 2A)
    
  • Photoluminescence Mapping (PL mapping): Using Enlitech SPCM-1000 laser scanning confocal microscope for 2D PL mapping to analyze film emission characteristics in different regions
    Target group films showed more consistent emission peak positions across different regions, indicating more uniform halide distribution. (Figure 2C, 2E, S16, S17, S18, S19)
    
  • Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS): Analysis of film crystal structure and orientation. (Figure 1F, S7)
    

Crystal Structure Improvement

• X-ray Diffraction (XRD): Analysis of crystal structure changes during annealing process. (Figure 1G, S9, S10, S11)
  
• Density Functional Theory (DFT) Simulations: Calculation of film nucleation radius and nucleation rates (Figure 1D, S6)
  

Additional Characterization Methods

• Photoluminescence Spectroscopy(PL): Measurement of PL spectra changes under illumination to analyze phase separation degree. (Figure 2D, S15)
  It is recommended to use Enlitech LQ-100X-PL photoluminescence and luminescence quantum optical detector to enhance PLQY (photoluminescence quantum efficiency) and QFLS (quasi-Fermi level splitting) measurement capabilities.
• Bias-Assisted Charge Extraction (BACE): Quantification of mobile ion density in films. (Figure 2F)
  
• Activation Energy Determination: Calculation of ion migration activation energy through conductivity measurements at different temperatures (Figure S20, S21)
  

Conclusions

The research team successfully developed a “halide locking” strategy that effectively controls perovskite film nucleation and crystal growth processes on rough silicon substrates, significantly improving perovskite solar cell efficiency and stability.

• Tandem Solar Cell Efficiency Breakthrough: Based on the “halide locking” strategy, monolithic tandem perovskite/TOPCon silicon solar cells achieved a record-breaking PCE of 31.32% with an active area of 1.0208 cm².
• Compositional Uniformity Enhancement:
  • The “halide locking” strategy, through TAACl introduction, combines with all cations and anions in mixed-halide perovskites, achieving compositional uniformity in both vertical and horizontal directions.
  • TOF-SIMS results show uniform distribution of TAACl molecules throughout the film.
  • GIXRD results confirm more uniform composition in both parallel and perpendicular directions.
• Crystal Structure Optimization:
  • GIWAXS results demonstrate that “halide locking” induces preferential perovskite crystallization along the (001) plane.
  • In-situ XRD measurements show more stable crystal structure changes during annealing with TAACl addition, and more uniform initial halide ion mixing.
• Phase Separation Suppression: PL mapping, BACE, and activation energy (Ea) measurements all demonstrate effective suppression of phase separation in perovskite films under illumination using the “halide locking” strategy.
• Single-Junction Solar Cell Performance Enhancement: Single-junction perovskite solar cells achieved PCE of 22.95% with VOC of 1.277 V and VOC deficit reduced to 400 mV. This represents one of the lowest VOC deficits reported for wide-bandgap perovskite solar cells.
• Device Stability Enhancement: After 1000 hours of maximum power point (MPP) tracking, non-encapsulated devices using the “halide locking” strategy maintained 95.43% of their initial efficiency.
  

Original Publication

Title: Highly Efficient Monolithic Perovskite/TOPCon Silicon Tandem Solar Cells Enabled by “Halide Locking”
Source: Advanced Materials
Publication Date: 02 January 2025
DOI: doi.org/10.1002/adma.202416150