All-perovskite tandem solar cells gallop ahead

With the goals of “carbon dioxide emissions peak” and “carbon neutrality,” photovoltaic (PV) technology has been showing unprecedented rapid development. As excellent representatives of emerging solar cells, perovskite solar cells (PSCs) have attracted intensive attention over the past decade. Recently, hybrid single-junction PSCs have delivered a certified power conversion efficiency (PCE) up to 26%, approaching the Shockley-Queisser (S-Q) radiative limit. The win-win cooperation of lead-based mixed iodide/bromide wide-bandgap (WBG; approximately 1.7–1.9 electronvolt (eV)) perovskite top subcells with tin-lead (Sn-Pb) low-bandgap (LBG; approximately 1.1–1.3 eV) perovskite bottom subcells to construct all-perovskite tandem solar cells (TSCs) is a promising and cost-effective pathway to surpass the S-Q radiative limit of single-junction PSCs. Moreover, all-perovskite TSCs include a two-terminal (2-T), monolithic configuration and four-terminal (4-T) mechanically stacked configuration. In 2016, Jiang et al.1Jiang F. Liu T. Luo B. et al.A two-terminal perovskite/perovskite tandem solar cell.J. Mater. Chem. A. 2016; 4: 1208-1213Crossref Google Scholar first reported 2-T all-perovskite TSCs with around 7% efficiency by employing identical MAPbI3 as light absorbers in both subcells. This work demonstrated the proof-of-concept success of 2-T all-perovskite tandems by solution processing, accelerating the development of perovskite-based tandems.1Jiang F. Liu T. Luo B. et al.A two-terminal perovskite/perovskite tandem solar cell.J. Mater. Chem. A. 2016; 4: 1208-1213Crossref Google Scholar At the early stage, limited by the unsatisfactory performance and sluggish progress of LBG subcells, the development of 2-T all-perovskite tandems invariably lagged behind that of their 4-T counterparts and single-junction PSCs. To address this dilemma, in 2016, we prepared higher-quality Sn-Pb perovskite films with approximately 1.25 eV by combining individual MAPbI3 and FASnI3 precursors to form (FASnI3)0.6(MAPbI3)0.4 precursors. The corresponding LBG PSCs yielded a PCE higher than 15% for the first time, showing great potential for fabrication of efficient TSCs and representing a significant step toward high-performance all-perovskite tandems. In 2017, we further regulated the (FASnI3)0.6(MAPbI3)0.4 perovskite growth process and fabricated a thicker absorbing layer with 620 nm, reporting efficient Sn-Pb PSCs with an impressive short-circuit current density (JSC) of over 29 mA cm−2, which is very important for application of Sn-Pb subcells in all-perovskite tandems.2Zhao D. Yu Y. Wang C. et al.Low-bandgap mixed tin-lead iodide perovskite absorbers with long carrier lifetimes for all-perovskite tandem solar cells.Nat. Energy. 2017; 217018Crossref Scopus (574) Google Scholar As a result, the best-performing LBG PSCs delivered a PCE of over 17.6% (the first certified 17.01% of Sn-Pb LBG PSCs), enabling efficient 4-T tandems with a steady-state 21.0% efficiency. Subsequently, we demonstrated 4-T tandems with 23.1% efficiency by mechanically stacking 1.75-eV top subcells with the above LBG bottom subcells. This was also the first time for all-perovskite TSCs surpassing the efficiency of single-junction PSCs.3Leijtens T. Bush K.A. Prasanna R. et al.Opportunities and challenges for tandem solar cells using metal halide perovskite semiconductors.Nat. Energy. 2018; 3: 828-838Crossref Scopus (619) Google Scholar Since then, all-perovskite TSCs have made a series of landmark breakthroughs benefitting from preparation of high-performance Sn-Pb LBG perovskite films and devices. Over time, the performance of WBG subcells has also been enhanced significantly, exhibiting obviously reduced open-circuit voltage (VOC) losses and suppressed phase separation. As expected, all-perovskite TSCs gallop ahead with further improvements of preparation techniques, multifunctional materials, and a more in-depth understanding of reducing opto-electronic losses in tandem devices. Recently, Lin et al. reported certified 2-T all-perovskite TSCs with 28% efficiency.4Lin R. Wang Y. Lu Q. et al.All-perovskite tandem solar cells with 3D/3D bilayer perovskite heterojunction.Nature. 2023; Crossref Scopus (3) Google Scholar We believe that all-perovskite tandems will achieve over 30% efficiency in 2 years. Reviewing the development of all-perovskite TSCs, strategies to construct efficient all-perovskite tandems principally focus on enhancing the quality of WBG and/or LBG absorbers as well as optimizing the interconnecting layers (ICLs). Hole-selective layers (HSLs) play important roles in the performance of PSCs. Poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) and poly(bis(4-phenyl)(2,4,6-trimethylphenyl) amine) (PTAA), as the most used HSLs in LBG and WBG devices, always induce unavoidable interface issues. However, development of novel and efficient HSLs to regulate the interfacial behavior of subcells for high-performance TSCs has been largely neglected in previous reports despite commonly employed HSLs with serious interfacial non-radiative recombination losses. Recently, we designed a promising self-assembled monolayer (SAM), (4-[7H-dibenzo(c,g)carbazol-7-yl]butyl)phosphonic acid (4PADCB) and employed it as the HSL in WBG subcells for efficient 1-cm2 all-perovskite tandems with 26.4% certified efficiency.5He R. Wang W. Yi Z. et al.All-perovskite tandem 1-cm2 cells with improved interface quality.Nature. 2023; 618: 80-86Crossref PubMed Scopus (14) Google Scholar A conventional HSL PTAA with hydrophobicity and poor conductivity is actually not suitable for high-performance, large-area, perovskite-based devices. The reported SAMs, including Me-[4-(9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz), MeO-2PACz, and 2PACz, could address these issues to a certain extent; however, their inherent properties of relatively poor solubility in alcohol and deficient surface wettability make preparation of large-scale perovskite films still challenging. Inspired by the result that the self-aggregation behavior for the unfused-ring electron acceptor could be modulated by introducing a local distortion into the molecular backbone, which can enhance solubility and further the homogeneity of the resultant films, two benzene rings were introduced into the carbazole group of 4PACz for precisely designing 4PADCB (Figures 1A, 1B , 1D, and 1E). Notably, 4PADCB displays a dominant face-on orientation distribution parallel to the indium tin oxide (ITO) surface, benefitting the hole transport between the perovskite, HSL, and ITO, but 4PACz does not have any similar feature (Figures 1C and 1F). Moreover, 4PADCB exhibits more uniform anchoring and more complete coverage on the surface of ITO. As expected, the WBG perovskite film deposited on the 4PADCB HSL has enhanced uniformity and optoelectronic quality. Thus, the 4PADCB/perovskite sample has a maximum quasi-Fermi level splitting (QFLS) value of 1.349 eV compared with 1.328 eV of 4PACz/perovskite and 1.300 eV of PTAA/perovskite samples, demonstrating that 4PADCB as an HSL could effectively reduce the interfacial non-radiative recombination losses. By integrating 4PADCB in 1.77 eV FA0.8Cs0.2Pb(I0.6Br0.4)3 devices, we reported centimeter-scale (around 1 cm2) WBG PSCs with a champion efficiency of 18.46% and an impressive VOC of 1.31 V. After integration with LBG subcells, we fabricated 2-T all-perovskite TSCs. As a result, the 1-cm2 tandems yielded a champion PCE of 27.01% with a VOC of 2.11 V, a JSC of 15.37 mA cm−2, and a fill factor (FF) of 83.13%. Notably, the tandem cell also showed a certified stabilized PCE of 26.4%, which is the highest value for certified PCEs of centimeter-scale-area single-junction PSCs and tandems reported (Figure 1G). Finally, as shown in Figure 1H, we discussed the possibility of achieving large-area, all-perovskite tandems with an efficiency of 30% or greater by analyzing optical management strategies with minimized interfacial recombination and transport losses. This work provides a promising pathway to achieve efficient large-area, all-perovskite tandems and modules by developing novel HSLs. In the future, development of efficient HSLs appropriate for WBG and LBG subcells is urgent and meaningful. We believe that these HSLs can be precisely designed by adhering to the following principles: introducing various types of anchoring groups with multiple functions and other functionalized groups, possibly making the HSLs versatile, such as regulating interfacial energy levels, healing surface defects, and even regulating perovskite crystal growth as well as releasing residual stress of perovskite films, applicable to various types of PSCs with different band gaps and components. Considering ultrathin SAMs, higher requirements are made for ICLs. High-performance ICLs with excellent transmittance and low resistance must provide multidimensional protection for the underlying perovskite layer when solution processing the subsequent films to eliminate potential damage to the top subcells. If both subcells in all-perovskite TSCs could use the same HSL, then we could effectively simplify the requirements for capital input, experimental equipment, and preparation technology of devices, which is significant and helpful for accelerating the commercial production process of large-area, all-perovskite TSCs and modules. Upscaling all-perovskite TSCs from laboratory fabrication to commercial production and marketing is still ongoing with challenges. Apparently, solution-processing perovskite films would make it difficult to meet the requirements of high-quality, large-area absorbers. Developing efficient and low-cost scalable solution-coating techniques, such as blade coating, slot die, inkjet printing, evaporation deposition, and so on, is a promising pathway to prepare high-performance perovskite films and devices with an active area greater than 10 cm2. Sputtering or evaporation of functional layers should be compatible with large-scale fabrication techniques. Moreover, for production of large-area tandems and modules, reducing and even avoiding toxic solvents (such as N,N-dimethylformamide (DMF)) and exploring a green solvent system is an urgent topic that needs to be addressed. On top of that, the development of flexible large-area all-perovskite tandems and modules by the industrial roll-to-roll method can significantly expand the application of perovskite-based devices in next-generation vehicles and for building integrated PVs, wearable power supplies, implantable medical electronics, scalable energy storage batteries, and space vehicles. For this goal, we think a lot of works need to be done in the future. This work was financially supported by the National Key R&D Program of China (No. 2022YFB4200303), the National Natural Science Foundation of China (no. 62174112), the Fundamental Research Funds for the Central Universities (no. YJ201955), the Engineering Featured Team Fund of Sichuan University (No. 2020SCUNG102). The authors declare no competing interests.