One-stone-for-two-birds strategy to attain beyond 25% perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have shown remarkable progress since their inception in 2009, boasting advantages such as low-cost solution processing, tunable bandgaps, high absorption coefficients, and efficient charge carrier transport. Power conversion efficiency (PCE) has rapidly increased to certified values exceeding 25%, and long-term stability has also been improved through interface engineering and molecular passivation. However, the inherent softness and ionic-electronic nature of metal halide perovskites lead to the formation of anion vacancy defects, primarily iodide (I-) vacancies. These defects act as non-radiative recombination centers, reducing both photovoltaic efficiency and operational stability. Iodide vacancies are primarily formed due to iodine loss during film fabrication or ion migration during device operation. The desorption of I- ions can lead to their oxidation into I0 species, initiating chemical chain reactions that accelerate defect formation. This is particularly problematic under high-temperature, continuous light illumination, and electrical bias, negatively impacting long-term stability. Therefore, a crucial step towards highly efficient and stable PSCs involves effectively mitigating the formation and migration of these I- vacancies. Previous attempts to passivate anion vacancies using various additives have shown some success, but often these approaches only address undercoordinated Pb2+ ions after anion escape from the crystal lattice. A more effective strategy would involve in situ pinning of anions within the crystal lattice, preventing their escape in the first place. This requires a material that can strongly bond to the Pb-I framework, localize escaped anions, occupy minimal space to maintain efficient charge transport, remain stable under thermal stress and illumination, and act as a growth-controlling agent to promote perovskite crystallization. While previous studies have explored various additives, many suffer from drawbacks such as degradation upon heating or weak bonding to the crystal lattice, hindering their effectiveness in high-efficiency and stable solar cells. This research proposes a new approach to address these limitations.

The literature extensively documents the challenges related to the stability and efficiency of perovskite solar cells. Significant effort has been dedicated to improving the long-term operational stability of unencapsulated PSCs, with some success in extending operational lifetimes beyond 1000 hours under full sunlight. This has been largely achieved through interface engineering and molecular passivation of the perovskite layer. Numerous studies have highlighted the crucial role of deep-level defects in limiting performance and stability. The elimination of these defects, which serve as non-radiative recombination centers, is essential for achieving high-performance solar cells. Numerous strategies for passivation of these defects have been proposed. These include the use of additives such as quaternary ammonium halide anions and cations, caffeine and theobromine, CsI-DB21C7 complex, and various organic molecules. However, these approaches often suffer from limitations, such as the inability to prevent anion vacancy formation at the source, weak bonding to the perovskite lattice, or the introduction of charge transport barriers. The research gap that this paper addresses is the development of a simple and effective strategy for in-situ anion fixation and defect passivation that overcomes the limitations of existing methods. It focuses on a rational design approach using amidino-based molecules to achieve both high efficiency and stability simultaneously.

This study employed a rational design strategy using 3-amidinopyridine (3AP) molecules as an additive to pin anions to the crystal lattice and suppress the formation of anion vacancies. The strong coordination between 3AP and the Pb-I framework was investigated using various techniques, including density functional theory (DFT) calculations, nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). DFT calculations were conducted to compare the adsorption energies of 3AP with other commonly used organic ligands, as well as to evaluate the formation and migration energies of I- vacancies. NMR and XPS were used to characterize the interaction between 3AP and the Pb-I framework, providing insights into the coordination mechanism. The photovoltaic performance of PSCs with and without 3AP addition was evaluated using standard techniques, including current density-voltage (J-V) curves, external quantum efficiency (EQE) measurements, and stabilized power output (SPO) measurements. Film characterization was performed using grazing incidence wide-angle X-ray scattering (GIWAXS), scanning electron microscopy (SEM), atomic force microscopy (AFM), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and time-resolved photoluminescence (TRPL) spectroscopy. Space charge-limited current (SCLC) measurements were used to determine the defect density and charge mobility of the perovskite films. Capacitance-voltage (C-V) measurements were conducted to analyze the built-in potential. Ultraviolet photoelectron spectroscopy (UPS) was used to study the energy band alignment. In situ PL spectroscopy and photocurrent mapping were employed to investigate the effect of 3AP on stacking defects. Finally, the stability of both perovskite films and solar cells was assessed under various conditions using X-ray diffraction (XRD), SEM, and long-term stability tests, following the ISOS-L-1 stability protocol. The (3AP)PbI4 single crystals were synthesized via a previously reported method.

The study found that 3AP molecules form strong chemical bonds with the Pb-I framework, effectively pinning anions and suppressing I- vacancy formation. DFT calculations showed significantly higher adsorption energy, increased I- vacancy formation energy, and higher migration barrier energy for 3AP compared to other ligands. NMR and XPS data confirmed the strong coordination between 3AP and the Pb-I framework through hydrogen bonding interactions, resulting in an electron-rich environment for Pb. The addition of 3AP led to a significant enhancement in photovoltaic performance. Devices incorporating 3AP achieved a maximum PCE of 25.3% (certified at 24.8%), compared to 22.76% for the control device. This improvement was attributed to increased Voc, Jsc, and FF values. The 3AP-based cells also exhibited significantly improved stability. GIWAXS and SEM analysis showed an increase in grain size and a decrease in surface roughness. TRPL measurements indicated a significant reduction in non-radiative recombination. SCLC measurements revealed a decrease in defect density and an increase in electron mobility. C-V measurements showed a wider depletion region in 3AP-based devices. UPS analysis indicated a better energy level alignment. In situ PL spectroscopy and photocurrent mapping during device operation demonstrated 3AP's role in mitigating stacking defects. Long-term stability tests showed superior performance for 3AP-based devices under various conditions (ambient, thermal stress, and continuous illumination), significantly outperforming the control cells. Similar results were obtained when I- was replaced with Cl- or Br-, indicating that 3AP itself is the crucial factor for defect suppression. The effectiveness of 3AP was also demonstrated across different perovskite compositions.

The findings demonstrate that the strong molecule-perovskite coordination achieved through the use of 3AP is highly effective in suppressing the formation and migration of anion vacancies in FAPbI3-based perovskites. This approach not only improves the efficiency of the solar cells by reducing non-radiative recombination but also dramatically enhances their long-term stability. The improved energy level alignment, reduced trap density, and enhanced charge transport all contribute to the observed performance gains. The in situ passivation of stacking defects during film formation is a key factor that directly contributes to improved charge separation and extraction. The results suggest that this one-stone-for-two-birds approach, addressing both anion fixation and undercoordinated-Pb passivation simultaneously, offers a powerful strategy for developing highly efficient and stable perovskite solar cells and potentially other optoelectronic devices. The simplicity of this method, involving the addition of a single ligand, is particularly attractive for large-scale manufacturing.

This work demonstrates a highly effective strategy for enhancing the efficiency and stability of perovskite solar cells by employing 3-amidinopyridine (3AP) as a multifunctional ligand. The observed significant improvement in PCE and long-term stability highlights the potential of this approach for advancing perovskite photovoltaics. Future research could focus on exploring other amidino-based ligands with different functional groups to further optimize the passivation effect. Investigating the scalability of this method for large-area device fabrication is also crucial. Exploring the applicability of this approach to other perovskite compositions and optoelectronic devices would expand the impact of this finding.

While this study demonstrates a significant improvement in perovskite solar cell performance and stability, it is important to note certain limitations. The long-term stability tests were conducted under specific environmental conditions, and the performance under diverse real-world conditions might differ. Further testing under varied humidity, temperature, and light intensity is needed to fully assess long-term stability. The study primarily focused on FAPbI3-based perovskites; further research is required to evaluate the effectiveness of this approach across a wider range of perovskite compositions. The detailed mechanistic understanding of the ligand's interaction with the perovskite lattice could be further investigated using advanced characterization techniques.