Perovskite solar cells are a promising solar energy conversion technology, but their stability remains a challenge. Compared to n-i-p solar cells, p-i-n geometry perovskite solar cells (PSCs) offer simplified fabrication, better suitability for charge extraction layers, and low-temperature processing. Self-assembled monolayers (SAMs) can enhance the performance of p-i-n PSCs, but ultrathin SAMs may be thermally unstable. Past research has mainly focused on improving the surface and in vivo stability of perovskites, but there have been fewer studies on the degradation effects of self-assembled monolayers under high temperature conditions.
The key issue
Compared with traditional polymer and metal oxide hole transport materials, SAM-based perovskite solar cells have poor thermal stability and have the following key issues:
1. Bonding to the substrate: The thermal stability of SAM-forming molecules depends on their bonding to the chosen substrate. The bond between the anchoring group and the molecular spacer can be broken through temperature-induced desorption, leading to the degradation of the SAM layer.
2. Temperature-induced desorption: Ultra-thin SAM used in perovskite solar cells is prone to thermal desorption at high temperatures. This desorption leads to the loss of the SAM layer and its beneficial properties, such as high hole selectivity and low interface trap state density.
3. Morphological changes: Thermal stress will cause morphological changes in the SAM layer, resulting in a decrease in its performance. These changes affect the uniformity and density of SAM molecules on the substrate surface, affecting charge extraction and overall device stability.
4. SAM-perovskite interface: The stability of the interface between the SAM layer and the perovskite material is crucial to the overall thermal stability of the device. If the interface is not strong, it can lead to performance degradation and loss of performance at high temperatures.
New ideas
Recently, the research team of Zhu Zonglong of City University of Hong Kong and Li Zhongan of Huazhong University of Science and Technology improved the stability and performance of p-i-n perovskite solar cells (PSCs) by using thermally stable hole selective layers (HSL). The HSL consists of a nickel oxide (NiOx) nanoparticle film and surface-anchored (4-(3,11-dimethoxy-7H-dibenzo[c,g]carbazol-7-yl)butyl) Composed of phosphonic acid (MeO-4PADBC) self-assembled monolayer (SAM). SAM has been shown to improve the performance of PSCs, but its thermal stability remains an issue. The authors aimed to improve SAM-based PSCs that are stable at high temperatures and study the degradation effects of SAM-forming molecules.
Technical route:
This study first compared the effects of using MeO-4PADBC self-assembled monolayers (SAMs) and NiOx/MeO-4PADBC as hole-selective layers (HSLs) in p-i-n perovskite solar cells (PSCs). The formation of chemical bonds between MeO-4PADBC and NiOx/MeO-4PADBC was then confirmed using Fourier transform infrared (FTIR) spectroscopy. Finally, the ultraviolet photoelectron spectroscopy (UPS) measurement results show that compared with ITO/MeO-4PADBC, NiOx/MeO-4PADBC has better energy level alignment with different perovskite absorbers. Time-resolved photoluminescence (TRPL) decay data showed that the ITO/NiOx/MeO-4PADBC substrate facilitated hole extraction. Improved hole extraction and energy level alignment lead to enhanced crystallization of perovskites.
Technical advantages
1. Manufacturing advantages: Compared with upright cells, inverted PSCs have manufacturing advantages. The performance of inverted PSCs can be enhanced by using a stable hole-selective layer, making them more efficient and stable.
2. Thermal stability: The self-assembled monolayer (SAM) used to stabilize the charge extraction layer in inverted PSCs is prone to thermal degradation. However, the phosphonate SAM developed in this study is anchored to particles in the nickel film, which optimizes the dipole moment for fast hole extraction and results in low defect density. This improved stability allows the battery to maintain its efficiency even after prolonged operation at high temperatures.
3. High efficiency: The use of stable hole selection layers significantly improves the power conversion efficiency of inverted PSCs. For the 1.53 eV p-i-n PSC, an efficiency of 25.6% was achieved.
4. Long-term stability: The stable hole-selective layer also enhances the long-term stability of PSCs. The battery maintained over 65% efficiency after 1,200 hours of operation at 90°C, showing excellent stability and durability.
Research Content
SAM design and synthesis
The introduction of non-coplanar helical dibenzo[c,g]carbazole (DBC) units as cores into self-assembled monolayer (SAM) molecules improves the dipole moment and interfacial contact in perovskite solar cells, thereby enhancing performance and stability.
First, the DBC unit helps reduce the negative impact on the dipole moment when the OMe group is introduced, because the previous SAM molecule with OMe substitution on the carbazole core leads to a decrease in the dipole moment, causing the HOMO of the SAM molecule to be different from that of the perovskite. The offset between the maximum values of the valence bands. However, the DBC unit in the new SAM molecule, called MeO-4PADBC, alleviates this problem, only slightly reducing the dipole moment compared to the non-OMe substituted SAM molecule (4PADBC). Secondly, the non-coplanar and helical structure of the DBC unit destroys the planarity and symmetry of the SAM molecules, making the interface contact with the perovskite more favorable. This results in ideal energy alignment, fast hole extraction and low defect density at the interface.
Compared with MeO-4PACz, the interfacial binding energy between MeO-2PADBC and perovskite is stronger. The total binding energy (Eb) of MeO-4PADBC is -7.19 eV, while the binding energy of MeO-2PACz is -5.27 eV. This indicates that MeO-4PADBC forms stronger interactions with perovskite materials.
Hole selective layer application
The combination of KPFM analysis, binding energy calculations and thermal stability measurements provides evidence for the formation of chemical bonds between MeO-4PADBC and NiOx/MeO-4PADBC films in p-i-n PSCs.
Kelvin Probe Force Microscopy (KPFM): KPFM was used to analyze the surface potential evolution of self-assembled monolayers (SAM) under heat treatment. KPFM images show that SAM molecules are densely packed on the ITO and NiOx surfaces, indicating that they are bound to the substrate.
Binding energy calculation: Density functional theory (DFT) simulation was used to study the binding energy between MeO-4PADBC and ITO or NiOx substrate. Calculations show that the binding energy of MeO-4PADBC to NiOx (-22.4 eV) is higher than that of ITO (-16.7 eV) at room temperature, indicating that the binding strength between MeO-4PADBC and NiOx is stronger.
Thermal Stability Analysis: Thermal stability of SAM-based PSCs was evaluated through accelerated aging measurements. The NiOx/MeO-4PADBC based device shows negligible change in surface potential after aging, indicating the robustness of the SAM substrate bond. In contrast, ITO/MeO-4PADBC-based devices exhibit fluctuations in surface potential, indicating possible desorption or morphological changes due to thermal stress.
Compared with the ITO/NiOx/2PACz system, the introduction of OMe groups on 2PACz in the ITO/NiOx/MeO-2PACz system causes the work function (Φ) to shift upward. This change in the work function indicates a change in the energy alignment of the system. The decrease in dipole moment caused by the incorporation of OMe groups on 2PACz is responsible for this phenomenon. The highly planar carbazole motif in 2PACz results in a reduced dipole moment. This change in dipole moment affects the interaction between SAM molecules and perovskite, causing the energy arrangement of the system to change.
Solar cell performance and characterization
The photovoltaic performance of PSC using NiOx/MeO-4PADBC as the hole selective layer (HSL) is very excellent, and the PSC achieves a high efficiency (PCE) of 25.6% on a mask area of 0.0414 cm2. The NiOx/MeO-4PADBC strategy is effective for PSCs with different band gaps. It was found that PSCs with NiOx/MeO-4PADBC as the hole selective layer (HSL) are effective for perovskites with band gaps of 1.53 eV, 1.68 eV and 1.80 eV. The absorber achieves high power conversion efficiency (PCE). The PCEs of these devices are 25.6%, 22.7%, and 20.1%, respectively. Furthermore, the steady-state power output (SPO) confirms the reliability of these devices, with stable PCEs of 25.5%, 22.3%, and 19.5% for the respective bandgaps, respectively. These results indicate that the NiOx/MeO-4PADBC strategy is effective for PSCs with different band gaps. The addition of NiOx/MeO-4PADBC as a hole-selective layer results in a lower defect density at the interface, which helps improve photovoltaic performance. The authors also found that the quasi-Fermi level splitting (QFLS) measurements and extracted VOC values of PSCs with NiOx/MeO-4PADBC as the hole-selective layer were comparable. This shows that QFLS measurements indicate that the Fermi level is spatially flat across the device and the energy offset on the hole-selective layer is low, enabling efficient carrier extraction. The VOC values of PSCs with NiOx/MeO-4PADBC are close to higher percentages of their computational potential, with the 1.53 eV device reaching 95% of its computational potential. This indicates that the NiOx/MeO-4PADBC hole-selective layer effectively promotes carrier extraction and minimizes the voltage loss at the interface between the hole-selective layer and the perovskite layer.
PSC stability study
In order to estimate the thermal stability of the ITO/NiOx/MeO-4PADBC substrate, the researchers applied Kelvin probe force microscopy (KPFM) to record the surface potential evolution of SAM under heat treatment. They compared the surface potential of ITO/MeO-4PADBC substrate and ITO/NiOx/MeO-4PADBC substrate before and after thermal aging. The changes in contact potential distribution (CPD) were analyzed to evaluate the ability of SAM to bind to the substrate under heating. In addition, the binding energies of MeO-4PADBC and ITO or NiOx substrates at different temperatures were studied through density functional theory (DFT) simulations.
DFT simulation studies found that the binding energy of MeO-4PADBC to NiOx (-22.4 eV) is higher than that of ITO (-16.7 eV) at 300 K. This indicates stronger binding between MeO-4PADBC and NiOx substrate. At 340 K, the binding energy between MeO-4PADBC and ITO decreases to -11.6 eV, while the binding energy between MeO-4PADBC and NiOx changes less (-20.3 eV). This indicates that MeO-4PADBC on NiOx substrate is more resistant to thermal stress than MeO-4PADBC on bare ITO.
The MeO-4PADBC-based device retained 65% of the initial power conversion efficiency (PCE) after 1200 hours of operation at 65°C. On the other hand, the NiOx/MeO-4PADBC based device retained 90% of the initial PCE after the same duration and temperature.
The temperature-dependent degradation activation energy (Ea) of PSC with NiOx/MeO-4PADBC as hole selective layer (HSL) is approximately 0.389 ± 0.022 eV. This value is almost three times higher than the Ea value of PSC with MeO-4PADBC as HSL, which is approximately 0.150 ± 0.017 eV.
Summary and outlook
In summary, the research team of Zhu Zonglong from City University of Hong Kong and Li Zhongan from Huazhong University of Science and Technology demonstrated an efficient and stable HSL with greatly improved thermal stability, suitable for inverted p-i-n PSCs containing efficient SAMs. The rational molecular structure design and in-depth analysis of MeO-4PADBC show that optimal dipole moment and good contact with perovskite are keys to achieve ideal energy alignment and fast hole extraction to improve device efficiency and stability. In addition, the MeO-4PADBC SAM molecules anchored on the NiOx film can form stronger tridentate bonds with NiOx, effectively reducing voltage loss and further maintaining a strong fixation effect under thermal stress. Our study provides theoretical guidance for the design of efficient and stable HSLs and paves the way for facile access to commercial inverted p-i-n PSCs.
References:
Zhen Li, Xianglang Sun, Xiaopeng Zheng, Bo Li, Danpeng Gao, Shoufeng Zhang, Xin Wu, Shuai Li, Jianqiu Gong, Joseph M. Luther, Zhong’an Li*, and Zonglong Zhu*. Stabilized hole-selective layer for high-performance inverted p-i-n perovskite solar cells, Science. (2023).