The article provides a comprehensive overview of tandem photovoltaics (PV), highlighting the potential of combining two or more junctions to achieve significantly higher power conversion efficiencies. Tandem PV cells are seen as a key technology to meet the growing demand for clean energy, with projected market share reaching 2% by 2030. The article outlines the current status, challenges, and future development roadmap for tandem PV, focusing on hybrid tandems with two junctions from various PV technologies. Key points include: - **Fundamentals and Status**: The article discusses the principles of tandem PV, including the benefits of combining multiple junctions to absorb a broader spectrum of sunlight and improve efficiency. - **Challenges and Roadmap**: It identifies the technical and manufacturing challenges that need to be addressed, such as current-matching, reliability, and scaling to commercial production. - **Material Selection**: The article reviews the suitability of different materials for top and bottom cells, emphasizing the importance of band gap matching and cost-effectiveness. - **Efficiency Limits**: It presents detailed balance and spectral efficiency models to estimate the efficiency limits of tandem devices. - **Energy-Harvesting Efficiency**: The concept of energy-harvesting efficiency (EHE) is introduced to understand how tandem PV systems perform under varying conditions. - **Cell and Module Demonstrations**: Recent advancements in tandem cell and module demonstrations are highlighted, with perovskite/Si tandems showing promising results. - **Measurement and Metrics**: The article discusses the challenges and methods for accurately measuring tandem cell performance and the importance of metrics like levelized cost of electricity (LCOE) and embodied energy (EPBT). - **Future Development Roadmap**: A high-level roadmap is provided, outlining the activities needed to commercialize tandem PV, including cell-level improvements, reliability, scaling, manufacturing, and deployment. The article aims to encourage collaboration between researchers and manufacturers to overcome these challenges and accelerate the commercialization of tandem PV technology.The article provides a comprehensive overview of tandem photovoltaics (PV), highlighting the potential of combining two or more junctions to achieve significantly higher power conversion efficiencies. Tandem PV cells are seen as a key technology to meet the growing demand for clean energy, with projected market share reaching 2% by 2030. The article outlines the current status, challenges, and future development roadmap for tandem PV, focusing on hybrid tandems with two junctions from various PV technologies. Key points include: - **Fundamentals and Status**: The article discusses the principles of tandem PV, including the benefits of combining multiple junctions to absorb a broader spectrum of sunlight and improve efficiency. - **Challenges and Roadmap**: It identifies the technical and manufacturing challenges that need to be addressed, such as current-matching, reliability, and scaling to commercial production. - **Material Selection**: The article reviews the suitability of different materials for top and bottom cells, emphasizing the importance of band gap matching and cost-effectiveness. - **Efficiency Limits**: It presents detailed balance and spectral efficiency models to estimate the efficiency limits of tandem devices. - **Energy-Harvesting Efficiency**: The concept of energy-harvesting efficiency (EHE) is introduced to understand how tandem PV systems perform under varying conditions. - **Cell and Module Demonstrations**: Recent advancements in tandem cell and module demonstrations are highlighted, with perovskite/Si tandems showing promising results. - **Measurement and Metrics**: The article discusses the challenges and methods for accurately measuring tandem cell performance and the importance of metrics like levelized cost of electricity (LCOE) and embodied energy (EPBT). - **Future Development Roadmap**: A high-level roadmap is provided, outlining the activities needed to commercialize tandem PV, including cell-level improvements, reliability, scaling, manufacturing, and deployment. The article aims to encourage collaboration between researchers and manufacturers to overcome these challenges and accelerate the commercialization of tandem PV technology.