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Science's Latest Inverted Perovskite!
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Research Background
Perovskite solar cells, as an emerging photovoltaic conversion technology, have huge development potential. However, their stability remains challenging. Compared to conventional n-i-p structured solar cells, p-i-n geometry simplifies fabrication processes, is more suitable for charge transport layers, and also reduces process temperatures. Self-assembled monolayers can enhance the conversion efficiency of p-i-n structured cells, but ultrathin self-assembled monolayers may be unstable at high temperatures. Current research mainly focuses on enhancing the stability of perovskite materials themselves, with less attention on the degradation effects of self-assembled monolayers under high temperature conditions.
Key Issues
Compared to conventional polymers and metal oxide hole transport materials, the thermal stability of perovskite solar cells based on self-assembled monolayers is poorer, with several main issues:
- The chemical bonding between self-assembled monolayer molecules and the substrate relates to its thermal stability. High temperatures can cause cleavage of chemical bonds between anchoring and spacer groups, leading to degradation of the self-assembled monolayer.
- The ultrathin self-assembled monolayers used in perovskite solar cells are more prone to thermal desorption under high temperature conditions, causing loss of their beneficial properties such as high hole selectivity and low interface state density.
- Thermal stress affects the morphology of the self-assembled monolayer, reducing its performance. This disrupts the uniformity and coverage density of monolayer molecules on the substrate surface, which in turn impacts charge extraction and overall stability.
- The interfacial connection between the self-assembled monolayer and the perovskite absorption layer relates to the overall thermal stability of the device. Weak interfaces lead to performance decay at high temperatures.
Recently, the research teams of Prof. Zonglong Zhu at City University of Hong Kong and Prof. Zhongan Li at Huazhong University of Science and Technology have improved the stability and performance of inverted p-i-n perovskite solar cells (PSCs) by using thermally stable hole-selective layers (HSLs). The HSL consists of a nickel oxide (NiOx) nanoparticle film with a surface-anchored (4-(3,11-dimethoxy-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (MeO-4PADBC) self-assembled monolayer (SAM). SAMs have been shown to enhance PSC performance but their thermal stability remains an issue. The authors aim to improve the stability of SAM-based PSCs at high temperatures and investigate the degradation effects of the SAM-forming molecules.
Technical Approach
This study first compared the effects of using MeO-4PADBC self-assembled monolayer (SAM) and NiOx/MeO-4PADBC as hole-selective layers (HSLs) in p-i-n perovskite solar cells. The results showed that compared to using only MeO-4PADBC, NiOx/MeO-4PADBC had better hole extraction effects. We tested the chemical bonds between MeO-4PADBC and NiOx/MeO-4PADBC using Fourier transform infrared spectroscopy, confirming the formation of chemical bonds between them. We also tested the energy level alignment between different materials and perovskite absorbers using ultraviolet photoelectron spectroscopy. The test results showed that compared with ITO/MeO-4PADBC, NiOx/MeO-4PADBC had better energy level alignment with different perovskite absorbers. Time-resolved photoluminescence decay curves indicated that ITO/NiOx/MeO-4PADBC substrates helped improve hole extraction efficiency. Taken together, the NiOx/MeO-4PADBC layer improved hole extraction efficiency and energy level alignment, thereby enhancing perovskite crystallinity.
Technical Advantages
- In terms of manufacturing process, the inverted structure is easier to achieve in mass production compared to conventional regular cells. This study used a stable nickel oxide modified phosphonic acid self-assembled monolayer as the hole extraction layer, which not only improved cell efficiency, but also made the cell more stable and reliable.
- In terms of thermal stability, traditional self-assembled monolayers tend to degrade at high temperatures. The phosphonic acid self-assembled monolayer anchored on nickel oxide nanoparticles in this study optimized the interface dipole moment, achieved rapid hole extraction, and greatly reduced the interface defect density. Therefore, even under high temperature and long-term working conditions, the cell efficiency can still be well maintained.
- In terms of conversion efficiency, the stable hole extraction layer in this study can significantly improve the power conversion efficiency of inverted perovskite solar cells, achieving a high efficiency of 25.6% at a working voltage of 1.53V.
- In terms of long-term stability, the cell in this study maintained an efficiency above 65% after continuous operation for 1200 hours at 90°C, exhibiting excellent stability and durability.
Research Content
This study designed a self-assembled monolayer (SAM) molecule MeO-4PADBC with a spiral dibenzo[c,g]carbazole (DBC) as the core unit, which optimized the performance of perovskite solar cells in multiple aspects.
Firstly, the addition of the DBC unit mitigated the negative impact of the MeO substituent on the dipole moment of the SAM molecule. Previous studies found that MeO substitution on the carbazole core led to a decrease in the dipole moment, resulting in a large offset between the HOMO of the SAM molecule and the valence band maximum of the perovskite. In the newly synthesized MeO-4PADBC, the addition of the DBC unit improved this problem, and the dipole moment of MeO-4PADBC was only slightly lower than the non-MeO substituted 4PADBC.
Secondly, the non-coplanar spiral structure of the DBC unit disrupted the planarity and symmetry of the SAM molecule, which facilitated contact with the perovskite. This led to better energy level alignment, faster hole extraction rate, and reduced interfacial defect density.
Compared with MeO-4PACz, MeO-2PADBC had higher interfacial binding energy with the perovskite. Measurement results showed that the total binding energy of MeO-4PADBC was -7.19 eV, while that of MeO-2PACz was -5.27 eV. This indicates that MeO-4PADBC can form stronger interactions with the perovskite.
The tests in this study used products from Enlitech.
Hole Selective Layer Application
Kelvin probe force microscopy (KPFM), binding energy calculations, and accelerated thermal aging experiments together confirmed the formation of chemical bonds between the MeO-4PADBC self-assembled monolayer and the NiOx substrate.
KPFM images showed that MeO-4PADBC molecules were densely adsorbed on ITO and NiOx surfaces, indicating their binding with both substrate materials. Calculations showed that the binding energy between MeO-4PADBC and NiOx (-22.4 eV) was higher than that with ITO (-16.7 eV), so the chemical bond with NiOx was more robust. In accelerated thermal aging experiments, the surface potential of NiOx/MeO-4PADBC substrate remained stable, while that of ITO/MeO-4PADBC fluctuated, further verifying the stability of NiOx/MeO-4PADBC bonding.
Additionally, introducing OMe groups on the carbazole ring led to a decrease in the dipole moment of the self-assembled monolayer. This was due to the highly coplanar carbazole ring structure exacerbating the negative impact of OMe groups on the dipole moment. The reduced dipole moment changed the interaction between the self-assembled monolayer and the perovskite, resulting in shifts in the system energy level structure and upward movement of the work function.
In summary, multiple experimental techniques have jointly confirmed the stable chemical bonding between MeO-4PADBC and NiOx. Introducing OMe groups reduces the dipole moment of the self-assembled monolayer, changing its interaction with the perovskite.
Solar Cell Performance and Characterization
Perovskite solar cells using NiOx/MeO-4PADBC as the hole extraction layer exhibited excellent photovoltaic performance. With an effective area of 0.0414 cm2, the cell efficiency reached as high as 25.6%. This strategy was effective for perovskite absorbers with different bandgaps. For cells with bandgaps of 1.53 eV, 1.68 eV and 1.80 eV, their efficiencies reached 25.6%, 22.7% and 20.1% respectively. Steady state power output results further verified the high efficiency and reliability of these devices.
The lower defect density at the NiOx/MeO-4PADBC interface helped improve photovoltaic performance. The consistency between quasi-Fermi level splitting and open circuit voltage values indicated continuous spatial energy levels in the device enabled by the hole extraction layer, resulting in high extraction efficiency. The open circuit voltage of the 1.53 eV device reached 95% of its theoretical value, demonstrating the NiOx/MeO-4PADBC layer effectively reduced interfacial voltage losses and achieved highly efficient carrier extraction.
In summary, with NiOx/MeO-4PADBC as the hole extraction layer, perovskite solar cells with different bandgaps all achieved relatively high power conversion efficiencies. Interface quality optimization was the key mechanism.
Figure 2 Photovoltaic performance of PSCs with different HSLs.
PSC Stability Research
Researchers used Kelvin probe force microscopy to test the interaction between the self-assembled monolayer MeO-4PADBC and ITO and NiOx substrates under heated conditions. Changes in the contact potential distribution indicated that the interfacial binding force of NiOx/MeO-4PADBC was superior to ITO/MeO-4PADBC.
Density functional theory calculations found that at room temperature, the binding energy between MeO-4PADBC and NiOx was -22.4eV, higher than that with ITO (-16.7eV). When the temperature rose to 340K, the binding energy between MeO-4PADBC and ITO decreased significantly to -11.6eV, while that between MeO-4PADBC and NiOx changed relatively little (-20.3eV), maintaining higher stability.
In accelerated thermal aging experiments, the cell based on NiOx/MeO-4PADBC maintained 90% efficiency after operating at 65°C for 1200 hours. Meanwhile, the cell based on MeO-4PADBC only maintained 65% efficiency.
The activation energy for degradation of the NiOx/MeO-4PADBC cell was about 0.389eV, nearly triple that of the MeO-4PADBC cell at 0.150eV, indicating superior thermal stability of NiOx/MeO-4PADBC.
Figure 3 PSC degradation mechanism analysis
Figure 4 Long-term stability evaluation of PSCs at different temperatures.
Summary and Outlook
In summary, the teams of Prof. Zonglong Zhu at City University of Hong Kong and Prof. Zhongan Li at Huazhong University of Science and Technology designed and demonstrated a highly efficient and stable hole selective layer material MeO-4PADBC. Its thermal stability was significantly enhanced, making it highly suitable for inverted perovskite solar cells. The molecular structure of MeO-4PADBC was optimized to have a moderate dipole moment and good contact with the perovskite, achieving ideal energy level alignment and rapid hole extraction, enhancing device efficiency and stability. Additionally, anchoring MeO-4PADBC on NiOx enhanced chemical bonding, effectively reducing interfacial voltage losses and improving thermal stability. This study provides theoretical guidance for designing efficient and stable hole selective layer materials, and paves the way for practical applications of perovskite solar cells.
