Perovskite solar cells (PSCs) with an "inverted" structure have better power conversion efficiency (PCE) and operational stability compared to the "normal" device structure, which is a key way to commercialize this emerging photovoltaic technology. Specifically, the PCE of inverted PSCs has exceeded 25% due to the development of improved self-assembly molecules (SAMs) and passivation strategies. However, the poor wettability and reunion of SAM lead to interface loss, which hinders the further improvement of PCE and stability.
Here, Prof. Wei Chen and Assoc. Prof. Zonghao Liu from Huazhong University of Science and Technology and Prof. Nam-Gyu Park from Sungkyunkwan University in South Korea report molecular hybridization at the embedded interface in inverted PSCs to improve the heterojunction interface by co-assembling the polycarboxylic acid-functionalized aromatic compound 4,4',4''-nitrile tribenzoic acid (NA) with the popular SAM[4-(3,6-dimethyl-9H-carbazole-9-yl)butyl]phosphonic acid (Me-4PACz). The molecular hybridization of Me-4PACz and NA can significantly improve the interfacial properties. The resulting inverted PSC showed a record steady-state efficiency of 26.54%. Crucially, this strategy works seamlessly with large-scale manufacturing, achieving the highest certified PCE of 22.74% (pore area: 11.1 cm2) for inverted micromodules. The device maintained 96.1% of its initial PCE after operating for more than 2400 hours in ambient air. The research results were published in Nature under the title "Buried interface molecular hybrid for inverted perovskite solar cells". Sanwan Liu, Jingbai Li, Wenshan Xiao, Rui Chen, Zhenxing Sun, Yong Zhang are the co-authors of this article.
早在2023年7月，华中科技大学陈炜就以题为“Reduction of bulk and surface defects in inverted methylammonium- and bromide-free formamidinium perovskite solar cells”在《Nature Energy》上发文。
The power conversion efficiency of inverted perovskite solar cells (PSCs) based on methylammonium-free and bromination-free formamidine lead iodide (FAPbI3) perovskites still lags behind those with conventional configurations. They improved the bulk and surface quality of the FA0.98Cs0.02PbI3 perovskite film to close the efficiency gap. First, they used dibutyl sulfoxide, a Lewis base additive, to increase crystallinity and reduce the defect density and internal residual stress of the perovskite body. They then treated the surface of the perovskite film with trifluorocarbon-modified phenylethylammonium iodide to optimize energy levels, passivate defects, and protect the film from moisture. The inverted PSC achieves both 25.1% efficiency (24.5% for reverse current-voltage sweep measured by a third-party agency) and higher stability. The unit maintained 97.4% and 98.2% of its initial power conversion efficiency after 1800 hours of continuous operation under 1.5G of sunlight from a solar air mass, and after 1000 hours of continuous operation under hot and humid conditions (85°C and 85% relative humidity).
【Theoretical Screening of Hybrid SAMs】
The authors performed computational studies to evaluate the effectiveness of different SAM combinations (Figure 1). Molecular dynamics (MD) simulations: The authors analyzed the interface configuration of the SAM using MD simulations. The simulation results showed that NA (4,4',4''-nitrile tribenzoic acid) formed a tighter and more uniform layer on the NiO surface when mixed with Me-4PACz (methylcarbazophonic acid) compared to other acids tested, such as benzoic acid and triamitic acid. Density Functional Theory (DFT) calculations: DFT is used to explore the energy benefits of adsorption. The results show that the NA-Me-4PACz combination exhibits higher binding energies, indicating stronger interaction with the NiO surface, which may lead to more efficient passivation and reduced recombination. Hybrid SAM efficiency: The theoretical model predicts that the interface defect state is reduced by about 0.61 eV, thereby increasing the carrier mobility and reducing the energy loss. Theoretical insights support the empirical findings, suggesting that this combination contributes to better energy alignment and charge transfer efficiency at the interface.
Figure 1.Computational simulation of HSL heterojunction
【Quality of Embedded Interface】
SEM images show that perovskite films grown on NA-Me-4PACz hybrid SAMs have fewer nanovoids and more uniform morphology compared to films grown on other SAMs. Specifically, the perovskite film formed by NA-MeSAM has a smooth surface and dense grains, which indicates high-quality film formation. Quantitative analysis showed a reduction of up to 30% in the presence of nanovoids in the NA-MeSAM layer compared to the control. In addition, through molecular hybridization enhancement, the coverage consistency of the entire interface was improved by 15%. Grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis confirmed that the perovskite films on NA-Me exhibited high crystallinity and no preferential orientation, which was conducive to efficient carrier transport and extraction.
Figure 2. Morphology and structure of the perovskite basement region
【Reduced interface loss】
This section details how to minimize energy loss with a hybrid SAM custom interface. This is achieved by improving the arrangement of energy levels at the interface, which facilitates efficient charge transfer and reduces recombination losses. The effectiveness of this method is quantified by enhanced device metrics such as open-circuit voltage (VOC) and fill factor (FF). In detail, photoluminescence (PL) measurements: NA-Me-4PACz samples exhibit extended PL lifetimes and reduced non-radiative recombination. PL lifetime measurements are claimed to be approximately 30% longer than the baseline Me-4PACz sample, indicating a reduction in non-radiative recombination at the interface. Ultraviolet photoelectron spectroscopy (UPS) results show that the valence band shift of NA-Me-4PACz is reduced by up to 0.2 eV, resulting in improved carrier extraction efficiency. The NA-Me SAMs-modified NiO/perovskite interface has a minimal valence band shift, which facilitates efficient hole extraction and reduces the energy barrier, thereby contributing to the increase of open-circuit voltage (VOC).
Figure 3. Reduced interfacial energy loss
【Device Performance】
The authors fabricated inverted PSCs with indium tin oxide (ITO)/NiO/SAMs/FA0.95Cs0.05PbI3/piperazine iodide (PI)/[6,6]-phenyl-C61-butyrate methyl ester (PC61BM)/croton chlor-alkali (BCP)/Ag structures to evaluate device performance. PSC devices with NA-Me SAM exhibit a peak power conversion efficiency (PCE) of up to 26.69% and an impressive VOC of 1.201 V. Stability tests have shown that the PSC retains 96.1% of the initial PCE after more than 2,400 hours of operation under ambient conditions1 sunlight and 97.4% after 500 hours of operation under hot and humid conditions, demonstrating superior durability.
Figure 4. Device PV performance
【Summary】
The integration of NA with Me-4PACz as a hybrid SAM on the NiO/perovskite interface significantly improves the efficiency and stability of inverted PSCs. These findings provide a promising avenue for the development of high-performance solar cells, which are also commercially viable due to their robustness and manufacturability. Future research is likely to focus on further optimizing the SAM component and exploring the scalability of large-area solar panels.
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Source: Frontiers of Polymer Science