Research Achievements and Highlights

Through the introduction of multifunctional isonicotinamide (IA) derivatives, this research significantly improved the performance of wide-bandgap (WBG) perovskite solar cells and successfully applied them in all-perovskite tandem solar cells. The key achievements include:

• Crystal Orientation Optimization: Optimized (100) crystal orientation of wide-bandgap perovskites
• Defect Density Reduction: Reduced defect density in perovskite thin films
• High Efficiency Achievement: Achieved 19.34% power conversion efficiency (PCE) and 1.342 V open-circuit voltage (VOC) for 1.77 eV wide-bandgap perovskite solar cells, leading performance among similar devices
• Tandem Cell Performance: Successfully fabricated all-perovskite tandem solar cells with PCE up to 28.53% (certified 28.27%) with excellent operational stability

The highlight of this research lies in its simple molecular design strategy that effectively addresses long-standing efficiency and stability issues in wide-bandgap perovskite solar cells.

Research Team

This research was jointly completed by research teams from Soochow University and Sichuan University in China. The corresponding authors are Dewei Zhao from the School of Materials Science and Engineering at Sichuan University; and Changlei Wang, Tianshu Ma, and Xiaofeng Li from the School of Optoelectronic Science and Engineering at Soochow University.

Research Background

• Performance Bottleneck of WBG Perovskite Subcells: Wide-bandgap perovskite solar cells (WBG PSCs) suffer from low open-circuit voltage (VOC) and severe non-radiative recombination losses. High bromide content in wide-bandgap perovskites leads to phase separation, bulk and surface defects, and significant band offset issues.
• Crystal Orientation Control Challenges: While studies have shown that crystal faces affect cell performance and attempts have been made to control grain growth through additives, achieving precise control of wide-bandgap perovskite crystal orientation remains a significant challenge.
• Multifunctional Additive Development: Researchers have attempted to use Lewis bases with electron-donating properties as passivators to reduce halogen vacancies, but developing multifunctional additives that can simultaneously regulate crystal orientation and passivate defects requires further investigation.
• Low-Cost Solution Processing Methods: All-perovskite tandem solar cells show high efficiency potential and low-cost solution processing methods, making them highly competitive in the solar market. They consist of wide-bandgap (WBG, Eg ≈1.77 eV) perovskite top subcells and low-bandgap (LBG, Eg ≈1.25 eV) mixed tin-lead (Sn-Pb) perovskite bottom subcells.

Recent advances in low-bandgap subcells and interconnecting layers (ICLs) have rapidly improved all-perovskite tandem solar cell efficiency, achieving certified PCE exceeding 30%.

Solution

This research proposes a multifunctional additive strategy, centered on using isonicotinamide (IA) and its derivatives to control perovskite crystal orientation and defect passivation. Specifically:

• Molecular Design: The research team selected three isomers: pyridinecarboxamide (PA), nicotinamide (NA), and isonicotinamide (IA). These compounds share the same pyridine nitrogen (C5H4N-), carbonyl (C═O), and amino (-NH2) functional groups but differ in their spatial configurations, leading to variations in adsorption methods and passivation mechanisms.
• Crystal Orientation Control: The addition of IA promotes preferential growth of perovskite crystals along the (100) plane, beneficial for vertical charge transport.
• Defect Passivation: IA molecules can bind with defect sites in the perovskite, reducing trap density and thereby improving device open-circuit voltage.

Experimental Process and Steps

Different nicotinamide derivatives (PA, NA, IA) were used as additives to improve the efficiency and stability of wide-bandgap perovskite solar cells (WBG PSCs), and their effects on perovskite film crystallization, defect passivation, and photoelectric performance were investigated.

Precursor Solution Preparation

• WBG Perovskite Precursor: FAI, CsI, FABr, PbI2, and PbBr2 were dissolved in DMF/DMSO mixed solvent, with addition of Pb(SCN)2, MASCN, KSCN, and varying ratios of PA, NA, or IA additives.
• LBG Perovskite Precursor: Separate FASnI3 and MAPbI3 precursor solutions were prepared and then mixed.

Film Preparation

• WBG Perovskite Film: WBG precursor solution was spin-coated on the substrate, first at low speed then high speed, with diethyl ether dripped during high-speed spinning, followed by annealing. After annealing, EDAI2 solution was dripped and spin-coated, followed by another annealing step.
• LBG Perovskite Film: LBG precursor solution was spin-coated on the substrate, with diethyl ether dripped during high-speed spinning, followed by annealing. Subsequently, EDAI2 solution was dripped and spin-coated, followed by another annealing step.

Solar Cell Device Fabrication

• Single-Junction WBG Perovskite Solar Cells: Sequential deposition of 4PADCB, WBG perovskite layer, and thermal evaporation of C60, BCP, Cu electrode layers on ITO substrate.
• Single-Junction LBG Perovskite Solar Cells: PEDOT:PSS spin-coating, LBG perovskite layer deposition, thermal evaporation of C60, BCP, Cu electrode layers on ITO substrate, with cover glass and epoxy encapsulation.
• All-Perovskite Tandem Solar Cells (TSC): Sequential deposition of 4PADCB, WBG perovskite layer, C60, SnO2/IZO, PEDOT:PSS, LBG perovskite layer, C60, BCP, Cu layers on substrate, followed by encapsulation.

Characterization Methods

Device Performance Analysis

• Current-Voltage (J-V) Curve Measurements:
  The research team used Enlitech’s SS-F5-3A solar simulator for measurements under 100 mW/cm² AM 1.5G solar illumination, with J-V curves measured in inert atmosphere.
  

  It is recommended to use Enlitech SS-X as the simulation light source for precise J-V curve data measurement.

  Figure 4a shows J-V curves of WBG PSCs treated with different additives.

  Figure S21 exhibits J-V curves of control and differently treated WBG PSCs under forward and reverse scans.

  Figure S25a shows J-V curves of large-area WBG PSCs.

  Figure S27 displays J-V curves with different treatments.

  Figure S29a presents J-V curves of single-junction LBG PSCs.

• External Quantum Efficiency (EQE) Spectrum Measurements:
  The research team used Enlitech’s QE-R system, utilizing white light as external source with 550nm and 850nm filters for bottom and top subcell measurements respectively.
  
  

  It is recommended to use Enlitech’s QE-R for external quantum efficiency measurements.

  Figure 4c shows EQE spectra of WBG PSCs treated with different additives.

  Figure 5c displays EQE spectra of top and bottom subcells in tandem solar cells.

  Figure S25b shows EQE spectra of large-area WBG PSCs.

  Figure S29b displays EQE spectra of single-junction LBG PSCs.

Additional Characterization Methods

• Scanning Electron Microscopy (SEM): Used to observe film surface morphology and microstructure, analyzing the effects of additives on grain size and surface smoothness. (Figures 2a-d, S3, S4)
  
• X-ray Diffraction (XRD): Analysis of crystal structure and crystallinity. (Figures 2e, S8, S7)
• X-ray Photoelectron Spectroscopy (XPS): Analysis of chemical states and energy levels, studying the passivation effect of additives on Pb2+, Br−, and I−. (Figures 3h, S12)
• Time-Resolved Photoluminescence (TRPL): Evaluation of carrier lifetime and analysis of non-radiative recombination levels, with average carrier lifetime (τave) calculated from TRPL decay curves. (Figure S9, Table S1)
• Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS): Analysis of additive distribution in films, particularly IA molecule enrichment at interfaces. (Figure 2g)
• Ultraviolet Photoelectron Spectroscopy (UPS): Measurement of material work function and valence band maximum (VBM), understanding how additives adjust energy levels and optimize perovskite-hole transport layer (4PADCB) energy alignment. (Figures S15, S16)
• Dynamic Light Scattering (DLS): Analysis of colloidal particle size effects in precursor solutions, particularly the increased particle size caused by IA additives, indicating the formation of prenucleation phases. (Figure S5)
• Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS): Analysis of film crystal orientation. (Figures 2h-k, S10)
• Space-Charge-Limited Current (SCLC): Measurement of hole-type and electron-type devices to analyze how different additives affect trap density. (Figures 3i, S13, Table S2)
• Electrochemical Impedance Spectroscopy (EIS): Analysis of device interface resistance to understand how additives affect charge recombination. Series resistance (Rs) and recombination resistance (Rrec) can be analyzed from Nyquist plots. (Figure 4f)
• Photoluminescence Mapping (PL mapping): Detection of film uniformity and defect distribution. (Figures 3a-d, S26)
• Temperature-Dependent Photoluminescence: Measurement of PL spectra at different temperatures to analyze carrier recombination mechanisms. (Figures 3e-f, 3g)
• Urbach Energy: Analysis of film defect density and electronic disorder. (Figure S14)

Conclusions

This research successfully applied multifunctional nicotinamide derivatives to wide-bandgap perovskite solar cells (WBG PSCs) and thoroughly investigated how these additives affect perovskite film crystallization, defect passivation, and photoelectric performance. The focus was on isonicotinamide (IA), which demonstrated superior performance in improving crystal orientation and passivating defects compared to pyridinecarboxamide (PA) and nicotinamide (NA), due to its unique planar spatial configuration.

Key Findings:

• Crystal Orientation Control: IA, through its unique spatial structure, promoted preferential orientation of (100) crystal planes in perovskite films. This improvement in orientation helped enhance charge transport efficiency and reduce crystal defects.
• Defect Passivation: The pyridine nitrogen (C5H4N-), carbonyl (C═O), and amino (-NH2) groups in IA molecules can coordinate with Pb2+ ions, effectively passivating uncoordinated Pb2+ and I- vacancies. Additionally, the -NH2 group participates in electrophilic reactions, filling FA+ and Pb2+ vacancies, further reducing defect density. IA treatment increased the formation energy of iodine vacancy defects.
  
• Film Quality Enhancement: IA-treated films exhibited larger grain sizes and smoother surfaces, reducing film roughness. Dynamic light scattering analysis showed that IA precursors led to increased colloidal particle size, accelerating perovskite nucleation kinetics and improving film quality.
• Photoelectric Performance Optimization: IA-modified WBG PSCs showed higher photoluminescence (PL) intensity and longer carrier lifetime, indicating improved film quality and suppressed non-radiative recombination. Temperature-dependent PL measurements showed smaller exciton binding energy in IA samples, helping reduce carrier recombination and enhance carrier separation. XPS analysis indicated favorable bonding between pyridinecarboxamide reagents and uncoordinated Pb2+. SCLC measurements showed that IA effectively suppressed electron and hole traps, thereby enhancing device performance.
• Solar Cell Performance Enhancement: IA-modified WBG PSCs achieved 19.34% power conversion efficiency (PCE) and 1.342 V open-circuit voltage (VOC), one of the highest VOC values reported for WBG PSCs. Additionally, IA-modified cells maintained good stability under continuous illumination.
• Tandem Cell Performance: Using IA-optimized WBG PSCs as top subcells, all-perovskite tandem solar cells (TSCs) were successfully fabricated, achieving 28.53% PCE (certified at 28.27%), maintaining over 90% of initial efficiency after 600 hours of continuous illumination.
  

Original Publication

Title: Enhancing Photovoltaically Preferred Orientation in Wide-Bandgap Perovskite for Efficient All-Perovskite Tandem Solar Cells
Source: Advanced Materials
Publication Date: January 7, 2025
DOI: doi.org/10.1002/adma.202412943