Perovskite-Based Tandem Solar Cells

The recent developments of photovoltaic (PV) have been transformed by the advent of metal halide perovskites. Their unique properties have not only pushed forward the efficiency of single-junction solar cells but also opened new avenues for tandem solar cells. Tandem solar cells combine two or more solar cells with different bandgaps to maximize the conversion of a broad solar spectrum to electrical energy producing higher efficiencies than those of single-junction solar cells. Perovskites, with tunable bandgaps, high efficiencies and ease of fabrication, have emerged as ideal candidates as both top and bottom subcells in a tandem, offering great promise. Perovskite-based tandems involve the stacking or direct fabrication of a wide-bandgap perovskite top absorber onto a silicon (Si), copper indium gallium selenide (CIGS), cadmium telluride (CdTe), the combination of low-bandgap perovskite or an organic bottom absorber.

As we stand on the cusp of a new horizon in solar energy conversion, this special section aims to provide an overview of recent advancements in perovskite-based tandem solar cells disseminated in Solar RRL, highlighting some of the key findings from the scientific community. The contributions cover broad topics, including additive and composition engineering of perovskite subcells, large-area fabrication, mechanical reliability, and interface passivation. This special section on perovskite-based tandem solar cells encompasses 1 review article, 1 perspective, and 6 research articles.

The review that discusses the fundamental and recent progress of perovskite/CIGS tandem solar cells is reported by Zeng Li et al. (10.1002/solr.202301059) titled “A Review of Perovskite/Copper Indium Gallium Selenide Tandem Solar Cells”. The review discusses the recent advancements in perovskite/CIGS tandem solar cells. This review highlights the benefits of perovskite/CIGS tandem configurations, including their high absorption coefficient, tunable bandgap, and potential for flexible substrates. The authors also delve into the performance metrics of two-terminal (2T) and four-terminal (4T) structures. Moreover, this review emphasizes the key technologies and challenges in improving the efficiency and stability of these cells, including optical management, bandgap tuning, defect passivation, all-solution process, interconnecting layer optimization, and mitigation of bottom cell roughness. Lastly, future development and commercialization prospects of perovskite/CIGS tandem cells are discussed.

The perspective focused on the scaling-up of all-perovskite tandem solar cells is written by Juncheng Wang et al. (10.1002/solr.202301066), titled “Development and Challenges of Large-Area All-Perovskite Tandem Solar Cells and Modules”. It analyzes recent advancements in all-perovskite tandem solar cell technology. The perspective discusses the performance of wide-bandgap and low-bandgap perovskites, along with the strategies to improve efficiency and stability. The authors also point out the key challenges in scaling up small-area solar cells to large-area tandem solar modules, focusing on scalable film deposition techniques such as blade and slot-die coating. Additionally, they highlight the issues related to film uniformity, monolithic interconnection technologies, and packaging to ensure the commercial viability of large-area perovskite solar modules.

The research articles in this issue include subcell optimization, stability improvement strategies, and scalable fabrication methods.

The performance of all-perovskite tandem cells is determined by both wide-bandgap and low-bandgap subcells. Weiqing Chen et al. (10.1002/solr.202300896) presented a research article titled “Regulating Interfacial Defect and Stress in Tin-Lead Perovskite Solar Cells”, where they introduced an interfacial engineering strategy to address defects and residual stress in tin-lead perovskite films. By employing 2-aminoterephthalic acid (2-AA) at both upper and lower interfaces, the authors achieved improved film crystallinity and defect passivation for a low-bandgap tin-lead perovskite cell, producing a power conversion efficiency (PCE) of 21.6%. Furthermore, the integration of optimized tin-lead perovskite subcells into a four-terminal tandem solar cell achieved a PCE of 27.5%. The study provides an effective approach to enhance stability and performance in mixed tin-lead perovskite solar cells (PSCs). In the quest for enhanced efficiency and stability of wide-bandgap PSCs, Yue Zhao et al. (10.1002/solr.202301016) developed a passivator-assisted close space annealing (PA-CSA) strategy in “Passivator-Assisted Close Space Annealing for High-Performance Wide-Bandgap Perovskite Solar Cells”. This method enlarges crystal size and passivates defects in wide-bandgap perovskite solar cells with efficiencies over 21.3% (1.68 eV) and 20.2% (1.73 eV) produced by the champion devices. As such, the all-perovskite tandem solar cells achieved efficiencies reaching 27% in both four-terminal and monolithic two-terminal tandem configurations. Xiaojing Han et al. (10.1002/solr.202300648) presented an advancement in the paper titled “Zwitterion Reduces Open-Circuit Voltage Loss in Wide-Bandgap Perovskite Solar Cells with 22% Efficiency and Its Application in Tandem Devices”. They introduced a zwitterionic additive, formamidine sulfinic acid (FSA), which interacts with perovskite components to retard crystallization and improve film quality, resulting in a substantial V_OC improvement and a champion efficiency of 22.1% for a 1.68 eV bandgap PSC. This strategy was also applied to the fabrication of the champion 2-terminal perovskite/silicon tandem solar cell producing a PCE of 28.8%. Kshitiz Dolia et al. (10.1002/solr.202400148) explored the potential of four-terminal perovskite/CdSeTe tandem solar cells in the paper titled “Four-Terminal Perovskite–CdSeTe Tandem Solar Cells: From 25% toward 30% Power Conversion Efficiency and Beyond”. They investigated the impact of transparent back contact and perovskite absorber bandgap on the performance of 4-T perovskite/CdSeTe tandem solar cells, demonstrating 25.1% efficiency. The authors also outlined a pathway for improving perovskite/CdSeTe tandem efficiency to over 30%.

The work by Helen Bristow et al. (10.1002/solr.202400289) highlights the mechanical reliability issues, such as delamination, which must be overcome for commercial viability. In the article “Mitigating Delamination in Perovskite/Silicon Tandem Solar Modules”, they found that the C₆₀/SnO₂ interface has low fracture toughness, leading to delamination risks. By optimizing the SnO₂ buffer layer and reducing sputtering-induced stress, they enhance fracture energy to over 160 J m⁻², thus improving the mechanical stability of the modules. This study is crucial for the commercialization of high-efficiency perovskite/Si tandem solar cells.

In the study “Sputtered NiO Interlayer for Improved Self-Assembled Monolayer Coverage and Pin-Hole Free Perovskite Coating for Scalable Near-Infrared-Transparent Perovskite and 4-Terminal All-Thin-Film Tandem Modules”, Radha K. Kothandaraman et al. (10.1002/solr.202400176) focused on the challenge of scaling up fabrication. They introduced a sputtered NiO_x interlayer to enhance self-assembled monolayer (SAM) coverage, leading to pin-hole free perovskite coating. This modification enabled the fabrication of scalable, efficient PSCs with reduced performance variation. The researchers also demonstrated the potential for upscaling by fabricating near-infrared-transparent mini-modules and achieving 20.5% and 16.9% efficient 4-terminal all-thin-film tandem modules on an aperture area of 2.03 and 10.23 cm², respectively. This work advanced the scalability and performance of PSCs for tandem applications.

Finally, we extend our sincere appreciation to all the contributing authors for their invaluable work in this special section. The thorough and timely assessment of manuscripts, along with the insightful feedback from reviewers, has been greatly valued. Furthermore, we express our deepest gratitude to the Solar RRL editorial team for their exceptional organization, unwavering support, and dedication to advancing scientific knowledge within our community.