Self-assembled monolayers as emerging hole-selective layers enable high-performance thin-film solar cells

Since human society has been rapidly industrializing over the past century, excessive energy consumption and environmental damage have raised awareness of the need for clean, renewable energy sources. Especially after the outbreak of the Russian–Ukrainian war, the development of alternative energy issue has been elevated to an unprecedented strategic level. Solar energy, as one of the clean and renewable energies, is experiencing a historical stage of changing its role from supplementary energy to alternative energy. The exploration of photovoltaic (PV) cells with new materials and structures is urgent to meet the demand of achieving carbon-peak and carbon-neutralization goals. Beyond traditional PVs based on crystalline silicon, solution-processed thin-film solar cells (TFSCs) demonstrate significant benefits in simple, cost-effective procedures compatible with various substrates.1Li F. Jen A.K.-Y. Interface engineering in solution-processed thin-film solar cells.Acc. Mater. Res. 2022; 3: 272-282Crossref Scopus (6) Google Scholar Recently, the most well-known developed solution-processed TFSCs are organic solar cells (OSCs) and organic–inorganic hybrid perovskite solar cells (PSCs), whose power conversion efficiencies (PCEs) have been recorded exceeding more than 19% and 25%, respectively. While the theoretical consumption of PCEs of solution-processed TFSCs can be achieved larger than 30%, it still faces great challenges in optimizing functional materials and device structures, especially in designing ideal functional layers and maintaining charge transport at interfaces. One efficient method to improve PV performance is to adjust the chemical and electrical properties at interfaces with electron-selective layers and hole-selective layers (HSLs).1Li F. Jen A.K.-Y. Interface engineering in solution-processed thin-film solar cells.Acc. Mater. Res. 2022; 3: 272-282Crossref Scopus (6) Google Scholar It should be sensed that the progress of exploring new-type HSL materials is relatively slow, which seriously limits the breakthrough of PV performance. Notably, there are only few options for HSL materials, such as PEDOT:PSS, PTAA, and metal oxides (Figure 1A). However, PEDOT:PSS has always been criticized for its corrosivity, hygroscopicity, high cost, and parasitic absorption, whereas PTAA suffers from energy mismatching and wetting issues. For metal oxides, electronic traps usually induce interface Schottky barriers, charge recombination, and further energy loss. Though these inevitable drawbacks have not yet produced a serious impact, they are still veiled threats for further industrialization and commercialization. To deal with these problems, self-assembled monolayers (SAMs) have emerged as a class of promising HSLs to construct high-performance PSCs and OSCs since 2018.2Magomedov A. Al-Ashouri A. Kasparavičius E. et al.Self-assembled hole transporting monolayer for highly efficient perovskite solar cells.Adv. Energy Mater. 2018; 8: 1870139Crossref Google Scholar SAMs are a class of ultrathin films assembled by small molecules, which usually consist of an anchor group, a functional head, and a spacer. When they are constructed in TFSCs, the strong covalent bonding between anchors and substrate can allow the minimum thickness and the effect of defect passivation of HSLs. In order to ensure that SAM molecules adhere to metal-oxide substrates, carboxylic acid, phosphonic acid, and silyl groups are frequently used as active anchors to bond with hydroxyl-rich substrates. The functional head group endows SAMs with certain properties or functions, and mainstream SAMs are obtained on the basis of carbazole and phenothiazine derivatives for their suitable energy alignment and wide commercial availability.3Bin H. Datta K. Wang J. et al.Finetuning hole-extracting monolayers for efficient organic solar cells.ACS Appl. Mater. Interfaces. 2022; 14: 16497-16504Crossref PubMed Scopus (5) Google Scholar,4Ullah A. Park K.H. Lee Y. et al.Versatile hole selective molecules containing a series of heteroatoms as self-assembled monolayers for efficient p-i-n perovskite and organic solar cells.Adv. Funct. Mater. 2022; 32: 2208793Crossref Scopus (6) Google Scholar,5Al-Ashouri A. Köhnen E. Li B. et al.Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.Science. 2020; 370: 1300-1309Crossref PubMed Scopus (723) Google Scholar Their small conjugation system provides good affinity and low parasitic absorption to OSCs,3Bin H. Datta K. Wang J. et al.Finetuning hole-extracting monolayers for efficient organic solar cells.ACS Appl. Mater. Interfaces. 2022; 14: 16497-16504Crossref PubMed Scopus (5) Google Scholar,4Ullah A. Park K.H. Lee Y. et al.Versatile hole selective molecules containing a series of heteroatoms as self-assembled monolayers for efficient p-i-n perovskite and organic solar cells.Adv. Funct. Mater. 2022; 32: 2208793Crossref Scopus (6) Google Scholar but for PSCs, an additional chemical modification on the functional heads is needed to further address the issues in practical situations, like surface wettability and defect passivation.2Magomedov A. Al-Ashouri A. Kasparavičius E. et al.Self-assembled hole transporting monolayer for highly efficient perovskite solar cells.Adv. Energy Mater. 2018; 8: 1870139Crossref Google Scholar,5Al-Ashouri A. Köhnen E. Li B. et al.Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.Science. 2020; 370: 1300-1309Crossref PubMed Scopus (723) Google Scholar The flexible alkyl chain is frequently selected as the spacer, and it works synergistically with the head group to optimize the molecular conformation by restricting the movement of the heads, preserving the molecular flexibility and offering interchain van der Waals force. Even under situations of light irradiation or heating, benefitting from the superior flexibility of SAMs, the thermally activated molecular motion can be released without undermining the device performance. All three components work together to optimize the optoelectronic properties and to modulate the electrode/active layer interfaces for better device performance. Summarily, the SAM design should follow the guidelines including (i) energy alignment, (ii) regular and dense packing, and (iii) good interaction with substrate and active layer. Generally, the properties of SAMs as HSLs can be optimized by modulating the energy level, wettability, and surface interaction. Different from the transport mechanism of PTAA and PEDOT:PSS, charge carriers are extracted by SAMs from the active layer hopping to the corresponding electrodes, which require interface energy alignment and electrode with suitable work functions. A proper SAM in TFSCs helps to create a desirable energy-level alignment for charge selection, decreasing the energy loss of charge transport, which can improve the current density, the open-circuit voltage, and the fill factor. The dipole of SAMs can lead to a work-function shift of substrates, facilitating efficient charge injection, which is usually affected by both the molecular structure and the packing quality. Recently, co-SAM packing conformations have been optimized by regulating the ratio of carbazole-derived and n-butyl-derived SAM molecules to achieve efficient hole selection.2Magomedov A. Al-Ashouri A. Kasparavičius E. et al.Self-assembled hole transporting monolayer for highly efficient perovskite solar cells.Adv. Energy Mater. 2018; 8: 1870139Crossref Google Scholar In PSCs, the control of nucleation and crystal growth is pivotal for obtaining a high-quality active layer. In order to modulate the interactions with perovskite precursors, it is crucial to graft substituent groups onto the functional heads,2Magomedov A. Al-Ashouri A. Kasparavičius E. et al.Self-assembled hole transporting monolayer for highly efficient perovskite solar cells.Adv. Energy Mater. 2018; 8: 1870139Crossref Google Scholar,4Ullah A. Park K.H. Lee Y. et al.Versatile hole selective molecules containing a series of heteroatoms as self-assembled monolayers for efficient p-i-n perovskite and organic solar cells.Adv. Funct. Mater. 2022; 32: 2208793Crossref Scopus (6) Google Scholar,5Al-Ashouri A. Köhnen E. Li B. et al.Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.Science. 2020; 370: 1300-1309Crossref PubMed Scopus (723) Google Scholar such as heteroatoms (like S and O) and substituent groups (like amines and halogens). It not only solves the wetting issues, passivates defects at the active layer/SAM interface, and suppresses the interfacial recombination but also facilitates the wide applications of SAMs in PSCs. Taking advantages of these, monolithic perovskite/silicon tandem solar cells were fabricated with SAMs, showing an impressive certified PCE of 29.15%.5Al-Ashouri A. Köhnen E. Li B. et al.Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction.Science. 2020; 370: 1300-1309Crossref PubMed Scopus (723) Google Scholar The development of SAMs in OSCs seems to fall far behind that in PSCs, which is due to the limited choice of SAMs in complementing shortcomings of less carriers in OSCs. In this regard, carbazole-based SAMs were applied as HSLs in OSCs based on PM6:BTPeC9 to investigate the evolution of hopping barriers with increasing alkyl chains from C2 to C4. Surprisingly, the champion device with a PCE 17.4% was achieved by C3, indicating that hopping barriers could be modulated by spacer length.3Bin H. Datta K. Wang J. et al.Finetuning hole-extracting monolayers for efficient organic solar cells.ACS Appl. Mater. Interfaces. 2022; 14: 16497-16504Crossref PubMed Scopus (5) Google Scholar Utilizing SAMs is not only a cost-effective way to significantly improve the PV performance (Figure 1B) but also offers a compatible platform for diverse substrates, simpler dopant-free processing protocols, and green-solvent processability. However, before their practical applications, SAMs as advanced materials for TSFCs still have to face challenges in terms of molecular design, film fabrication, and device fabrication. For molecule design, high-efficiency and versatile SAMs for both OSCs and PSCs are expected to reduce the reagent cost and streamline the synthetic procedure of SAM-based PV devices. However, even with the exact same SAM molecule, devices may exhibit completely different performances due to the variant SAM qualities caused by diverse fabrication methods, which is often neglected in current reports of SAM-based TFSCs. We suggest that the SAM fabrication and the characterization method should be carefully described and stated in research articles. Moreover, the commercialization requires more lab experiments in device investigation. Long-term stability and sustainability are the main barriers for commercialization beyond efficiency. Attention should also be paid to instability issues caused by external factors including heat, UV radiation, and environmental moisture. Despite these unresolved problems, it is a fact that SAMs are opening a new era of material development for high-efficiency TFSCs due to their flexible structural modification and great processing properties, which cater to various needs and practical conditions. Particularly, their potential flexibility, biocompatibility, and ultrathin quasi-2D structure enable the fabrication of complicated electronic systems with superior performances, redefining the clean energy suppliers for wearable and healthcare electronics. We are pleased to find that some carbazole-based SAMs have already been used in commercialized devices. The unique features of SAMs also trigger the functional revolution in other fields of related electronics, including but not limited to field effect transistors and ultrasensitive sensors based on SAMs. Z.L. acknowledges support from the National Science Foundation (nos. 62275076, 92163135, and 11904098) and the Shanghai Pilot Program for Basic Research (22JC1403200). A.K.-Y.J. is thankful for the sponsorship of the Lee Shau-Kee Chair Professor (Materials Science) and the support from the APRC Grant of the City University of Hong Kong (9380086); the TCFS grant (GHP/018/20SZ) and the MRP grant (MRP/040/21X) from the Innovation and Technology Commission of Hong Kong; the Green Tech Fund from the Environment and Ecology Bureau of Hong Kong (202020164); GRF grants from the Research Grants Council of Hong Kong (11307621 and 11316422); Shenzhen Science and Technology Program (SGDX20201103095412040); Guangdong Major Project of Basic and Applied Basic Research (2019B030302007); Guangdong-Hong Kong-Macao Joint Laboratory of Optoelectronic, Magnetic Functional Materials (2019B121205002); the US Office of Naval Research (N00014-20-1-2191); and the CRF grant from the Research Grants Council of Hong Kong (C6023-19GF). The authors declare no competing interests.